Frequently Asked Questions (FAQs)

Q1: My Sound City amp's/cab's serial number is nnnnnnn; what year was my amp/cabinet made?

A: This is a tough one, and short of various inspection stickers, and reliable transformer codes and pot codes, there is currently no definitively reliable way to determine an accurate date based on serial numbers alone, with the possible exceptions of the DMI-era products and beyond. The serial numbers during this era appeared as 74001234 or 78115422, as just two examples. (Here's an example of a speaker cab's ID plate.) The two-digit prefix in these serial numbers seems to indicate the year made, and in the latter example, which was seen on a DMI-Sound City F115 bass cabinet, it appears the model number might have been included in the serial number as well.

For the earlier Dallas Arbiter-era Sound City products, however, the best you can legitimately determine is a range of years during which an amp or cabinet was made based on the eras in which these products appeared. (See also "Q3" below.)

These eras and their approximate and relative date ranges are as follows:
- Arbiter Electronics-era Mark 1: mid-60s,
- Dallas Arbiter-era Mark 2: mid to later-60s,
- Dallas Arbiter-era Mark 3: very late 60s to early 1970,
- Dallas Arbiter-era Mark 4: ~1972--1973 in the US and as early as 1970 in England,
- DMI-era Mark 4, and the B150 and L150: ~1974--1976,
- DMI Tour Series: ~1977--1978, and
- DMI SMF Tour Series: ~1978 to early 80s.

The Mark 4 series 50 Plus, 120, and 200 Plus heads first appeared in around 1972 to 1973 in the US (see the "Dallas Arbiter" catalog pages) and as eary as 1970 in England (see "September 1970" advertisement on the Related Links page), and continued to be sold by DMI.

Prior to that, Sound City made the Mark 3 100 and 200 models and the ST 50 PA. The first Sound City amp was probably the "One Hundred," which was, unofficially, the Mark 1 model, and which was designed by Dave Reeves of Hiwatt fame. The Mark 2 designation saw the inclusion of the 200 model along with the L100 Mark 2. Various Mark 3 and Mark 4 PA heads also appeared, as did a few Mark 4-era "slave" heads.

It's easiest to determine the range of years of manufacture by the models themselves and by specific cosmeticdifferences between them (changes in grill cloth, piping and vein, trim, handles, corners, wheels, etc.). Finally, remember also that transformers and pots could have been replaced, so accurate date codes go out the window with the old/replaced parts.

Q2: How much is my SC amp/cab worth?

A: As much as you can get for it. Seriously, the best way to determine value is by watching how much amps and cabs fetch on eBay. There really is no hard and fast method of appraising Sound City equipment; they've spent far too much time as poor stepchildren to Hiwatts for anyone to take them seriously as collectibles, but they are collectible in their own way.

To a great extent, condition, originality, and, especially, model determine price. For example, A) a10-out-of-10 condition, unmodded 120 head will probably sell for less than a 5-out-of-10 condition Mark 3 100 with Chinese tubes; B) a clean Dallas Arbiter-era 4x12 cab will fetch less with Eminence speakers than it will with Fanes; C) a Mark 3 200 will probably get more than a Mark 4 200 Plus in identical condition.

Finally, a Dave Reeves-modified "One Hundred" (Mark 1) formerly owned by Pete Townsend, recently sold in the UK for $10,000! (See the "Model One Hundred" link on Photo Page 5.)

Q3: What's the difference between Mark 4 amps and earlier models?

A: Sound City amps have gone through several design eras, which are designated as follows: Arbiter-Sound City; Mark 1 (unofficial terminology); Mark 2; Mark 3; Mark 4; DMI; Mark 5 (which apparently never left the prototype stage and might have followed the SMF/Tour Series); Tour Series; and SMF Tour Series. Following is a listing of each era:

  • As far as I know, t he Arbiter-Sound City era had two flavors: 100 watt and 200 watt.
  • The Mark 1 era included just the Model "One Hundred," which apparently came in a Lead version and a Bass version. I've found no evidence of a 200-watt (or any other model of the) Mark 1, nor that the term "Mark 1" was ever officially used by Sound City. (I use it here only for clarity.)
  • The Mark 2 era included the L/B100 and L/B200, with some additional designations that included the suffix "M" and the suffix "W." I have no information about what these suffixes meant or that there were ever Mark 2 PA versions.
  • The Mark 3 era included the L/B100, L/B200, PA 100, PA 200, Concord Combo (lead and bass models), and ST50 PA.
  • The Mark 4 era included the 50 Plus, 50R Plus, 50 PA Plus, 50R PA, "50 Watt Combination Amplifier," Concord Combo (lead and bass models), GT80 Combo, the 120, 120R, 120PA, and 120 Energizer Slave Unit, the L/B200 Plus, the PA 200 Plus and the PA 200R. (Please Note: All the 50-watt heads and the "Plus" designation for them and for the 200-watt models, are exclusive to the Mark 4 era, and were never used/seen before.)
  • The DMI era included many of the models mentioned in the preceding paragraph, but also included the introduction of the Bass 150 and Lead 150 heads, which were exclusive to the DMI era. Aside from these two amps, the DMI era was really more of a name change, from Dallas Arbiter to DMI.
  • The Mark 5 era included a 120R; no evidence has been found A) that this amp ever made it to production or B) that there were any other Mark 5 models.
  • The Tour Series era had one model, which was called "Tour Series."
  • The SMF Tour Series era had one model as well, called "SMF Tour Series."

Among the cosmetic and model-naming changes along the way, most of the Mark 4 models have what many refer to as an "active preamp." The exceptions to this are the Mark 4-era 50R PA, 50 PA, 50 PA Plus, and 120 PA, all of which have passive preamps. The Mark 3-and-earlier models also typically have passive preamps, as well as In/Out jacks for reverb, etc.

As a rule, and regardless of era, the 50-watt models and the combos all used two EL34s; the 100-watt models used four EL34s; the 120-watt models and the SMF/Tour Series models used six EL34s; and the 150-watt and 200-watt models used four KT88s.

Finally, the rear-panel "Sensitivity" switch was introduced with the Mark 4 series and was never used again.

Q4: How do I know if my amp has a "passive" preamp?

A: If you have a Mark 1, 2, or 3 guitar or bass amp, or a Mark 4 50R PA, 50 PA, 50 PA Plus, and 120 PA head, you can be reasonably assured that its preamp is passive. But to know for certain, perform this simple test:

1. Set all tone controls to zero (off).
2. Gradually turn up the volume control(s).
If your amp still makes sound when you turn up the volume control(s), you have a passive preamp. (This is exactly opposite of the Sound City "active" (Mark 4) preamp, which provides no sound with its tone controls set to zero [off].)

Note: The terms "active" and "passive" are meaningless for slave heads, which have no tone controls.

Q5: What kind of speakers are in my Sound City speaker cabinet?

A: Sound City (Dallas Arbiter and Dallas Music Industries) never made its own speakers; instead, Sound City used several speaker makes in its cabinets. Celestion, Fane, Eminence, and Vega (in the B118v bass cab, only) were used. Among the 12-inch speakers, the least common were Celestions and the most common were Eminences, but Eminences are often the most difficult to identify because the name does not appear on them; you need to find the speaker's EIA code.

Eminence speakers have large square magnets (with three diagonally placed screws) and stamped frames. And because Eminences can sometimes be confused with CTS speakers (which Sound City never used, but which also sometimes had large square magnets), you need, ultimately, to refer to the speaker's EIA code, which is usually stamped somewhere on the speaker's frame or magnet and will begin with "67" if the speaker is an Eminence or "122" if the speaker is a cast-frame Fane, but believe me, there's no confusing an Eminence for a Fane, or vice versa.

Finally, early in the Sound City/DMI-era (~1974/1975), the Sound City GT80 combo amp (a hot-rodded Concord combo) was available with 12-inch Electrovoice SROs as custom-order speakers.

If the speakers in your Sound City cabinet or combo are not one of the manufacturers mentioned above, they are most probably not the cab's or combo's original speakers. Check out the "Related Links" page for more information on EIA codes.

Q6: I want to see if my amp powers up, but I don't have a speaker cabinet. Can I just turn on the amp and see if the tubes light up?

A: You can do this, but only very briefly, a few seconds at most. According to Mark Huss, " As long as nothing is plugged into the output jacks, you should be fine. Most tube amps automatically short the output jacks if nothing is plugged in. It's much worse (and potentially fatal for the OT) if you have a speaker cable plugged into the head but not the speaker. Unlike solid state amps, tube amps are much happier with their output shorted then open-circuited."

So, always, always, ALWAYS turn on a tube amp ONLY when there is a speaker cable connected between the amp and a cabinet! Nothing will trash an output transformer like NOT doing this. Be nice to your Partridges and they'll be nice to you.

Q7: One of the speakers in my 4x12 cab buzzes when I play; do I have a bad speaker?

A: Maybe... but before you assume the "buzzing" means a bad speaker, check some things first.

Check all the bolts securing that speaker and make sure they're snug. A loose bolt can cause the speaker's frame to make all sorts of funny sounds. If, alternatively, you have wood screws securing the speaker, check if they're loose. Wood screws lose their grip after a while, and bolts and T-nuts are the best for securing speakers. Also, make sure the speakers have gaskets between their frames and the baffle board. Metal-to-wood contact can cause noises as well.

After you check these things, make sure there's nothing lying/wedged/stuck in the space between the speaker's surround and the speaker's frame, such as a small piece of wood, or a pebble, or a small section of stripped wire insulation, or something like this. It's amazing what strange buzzes and sounds a tiny piece of stray junk can cause when it's stuck inside this space. Also check for a small tear in (or a separation between) the surround or (and) the cone, which can cause a buzzing sound.

If the buzzing is still there after you've checked all these things and everything is okay, tey moving the speaker to a new position (up, down, left, or right) and see if the sound travels with the speaker.

If the buzzing goes away, problem solved. If the buzzing travels with the speaker, this speaker might have a problem. And if the buzzing doesn't travel with the speaker, there's a good chance the buzzing is coming from something else and not that speaker... like a separation between the plywood's laminations, for example.

Q8: When I play through my amp and cabinet, I hear a "ringing" sound coming from amp head and the speakers; what is this?

A: This might be caused by a microphonic tube. As tubes age, their internal components loosen a bit and can rattle. To check this, isolate the head from the cab by putting the head on the floor or putting something between the head and the cab, like foam or a folded blanket.

If the ringing sound goes away when you isolate the head from the cab, this probably means the tubes are microphonic, and you have three choices: 1) do nothing and just live with it; 2) replace the tubes with new tubes; 3) isolate the amp from the cab when you play. A good solution for No. 3 is to get Sorbothane footers that virtually eliminate vibrations between the head and the cab. They aren't inexpensive, but if you want your rig to sound good, and don't want to replace your "vintage" tubes, they're a great solution.

(You can also check for microphonics by very gently tapping a tube's glass bulb; if the tube is microphonic, you'll hear it when you tap the tube.)

Q9: Can I use a Sound City L100 or L120 for bass guitar?

A: Sure... but remember that bass guitar requires a good deal of headroom in order to reproduce the low-end transients of which basses are capable. And if you're using a 5-string bass with a low B-string (31 Hz), your headroom requirements are even greater.

Because of this headroom requirement for bass guitar, some modern bass amps provide as much as two kilowatts (2000 watts) of power, so what can you expect from a 100-watt or 120-watt tube head that is, perhaps, 30-years old!?

You can expect a very good sound IF you remember that you can't get water from a stone. Very efficient bass speakers (and an efficent cabinet design) can provide more output for a tube head, but only to a point.

Also, the inherently and relatively poor damping capability of tube amps can cause excessive speaker excursion (i.e., the speaker will seem to move in and out almost uncontrollably; solid-state amps provide much a better speaker damping capability, but who wants to use a S-S bass amp?). You can reduce this effect by using less low-end EQ. Adding a great deal of low-end EQ can needlessly rob your amp of its vital headroom: a simple 3-dB increase on your Bass control is the equivalent of asking your amp to provide twice its rated power, so if you crank your Bass control, you're just asking for problems (really bad sound, distortion, blown speakers, etc.).

Above all, use the right tool for the right job; you wouldn't bring a bicycle to a motorcycle race, would you?

Q10: I keep hearing and reading horror stories about amp heads being ravaged by shipping companies. Any suggestions for shipping amp heads?

A: Please note that these are just my opinions; nonetheless, here they are...

1. Always ship FedEx; offer to a seller to pay extra for FedEx shipping. It's worth it.

2. Label tubes and tube sockets in pencil (so they can be erased later), and remove all tubes prior to shipping. Ship tubes separately. If selling an amp, make it clear you'll do this in your ad and/or before you sell it, and especially do this for combo amps where tubes can be sheared off by a flying speaker. This JUST happened to a friend here in ABQ: the 12-inch speaker in his combo amp broke loose and smashed his tubes.

3. Double box your amp and surround the inner box with a) bubble wrap... the larger bubbles, or b) foam or foam peanuts. NEVER send a heavy amp in just one box, unless it's the original box with the original packing material. ALWAYS write FRAGILE on the box's top and sides.

4. Insure the amp (and the separately shipped tubes) for "true value" if this is more than what you paid. Insuring only for the price you paid can get you into trouble. A good deal is only a good deal if the amp arrives unscathed, whereas a $500 amp that you paid $250 for and insured for $250 will only get you half of what it's worth.

5. Put the shipping label on the side of the box you want to face up. Label this side of the box "TOP" and also write "THIS SIDE UP." I've seen way too many boxes with labels on the sides; remember, delivery guys will place a box so they can read the label easily. And for what it's worth, on the bottom of every box I ship I also write "IF YOU CAN READ THIS THEN THIS BOX IS UPSIDE DOWN."

Q11: I noticed my Sound City amp has output transformer taps for 4, 8, and 16 ohms as well as a tap labeled "100V." Can I use the 100V output as a line output?

A: The short answer is no. The 100V line setting was intended to be used to drive PA speakers installed in factories, churches, schools, etc.; 100V line speakers have transformers mounted on them to maintain the proper voltage over the long cable runs and multiple speaker installations usually associated with such in-house PA systems. The high transmission voltage minimizes cable losses. This is not a "line out" and should not be connected to mixers, etc., under any circumstances.

Q12: I keep hearing people say Sound City amps don't have much gain but are capable of lots of volume. I thought gain and volume were the same thing...?

A: What you've heard is sort of correct: Sound City amps are theoretically capable of providing some gain, but are definitely capable of providing lots of volume. But what you thought about gain and volume needs a little clarification, because gain and volume are not exactly synonymous.

Gain is the amplifying factor of one or several stages of an amplifier, which is applied to the input signal. In an amplifier, the gain is subject to attenuation by means of the signal path's components. What is referred to as gain in an amp is the overloading of one stage by a preceding stage. Subsequently, a control that can cause this effect is called a Gain control and can affect several stages at once or just a single stage.

Volume is the overall loudness of the sound waves put out by the speakers. In an amp, the Volume control is used to attenuate the gain in any stage of the amplifier. This is usually done by means of a variable resistor (i.e., a potentiometer) shunting part or all of the signal to ground. An amp that has a Gain control can make a clean signal louder or quieter or, conversely, a distorted signal louder or quieter.

To address the first part of your question, in SC Mark 4 heads, which do not have a Gain control, per se, the tone controls were intended to provide the potential for stage-to-stage overload (i.e., gain). According to its designer, the Mark 4 active preamp was intended to provide more gain than previous SC heads by making each tone control a sort of gain control for its particular band of frequencies, which in turn could overload the following tone stage. So, you could theoretically boost one, two, or all three of the tone controls, adjust the Normal and/or Brillint Volume controls accordingly, and achieve some gain (some overdriven signal). Some feel this is possible with the Mark 4 circuit while some don't. (See also "Active Tone Controls" on the Descriptions of Amplifier Features page.)

Having said all this, a Volume control can become a Gain control when you consider how it functions in a chain of devices leading to an amp's input. Consider that you might have some number of effects pedals connected to your amp. Each effects pedal might have some type of Volume control and as the Volume control on each pedal in this chain is increased, that effect adds gain to the next stage in the chain (i.e., to the next effects pedal), and eventually, this signal reaches the amp. You could also achieve this same thing with an instrument that has an active preamp and, resultingly, some amount of gain potential.

(Many thanks to Satamax for technical aspects posted here.)

Q13: I recently bought a Sound City amp without its tubes. What types of tubes does this amp use and where do they go in the amp's tube sockets?

A: Locate your amp, as follows...

  • Sound City 50 Plus: A stock/unmodified Sound City 50 Plus typically uses four preamp tubes--three ECC83s (12AX7s) and one ECC81 (12AT7). It also has two EL34 output tubes. The layout/positions of/for all the 50 Plus's tubes can be clearly seen in the following two schematics:
    • 50 Plus 1 (w/ all four preamp tubes in line)
    • 50 Plus 2 (w/ three ECC83s in line and ECC81 between the EL34s)
  • Sound City 120: A stock/unmodified Sound City 120 typically uses five preamp tubes--four ECC83s (12AX7s) and one ECC81 (12AT7). It also has six EL34 output tubes. The layout/positions of/for all the 120's tubes can be clearly seen in this schematic.
  • Sound City Bass 150: A stock/unmodified Sound City Bass 150 typically uses three preamp tubes--two ECC83s (12AX7s) and one ECC81 (12AT7). It also has four KT88 output tubes. The layout/positions of/for all the Bass 150's tubes can be clearly seen in this schematic.
  • Sound City 200 Plus: A stock/unmodified Sound City 200 Plus typically uses five preamp tubes--four ECC83s (12AX7s) and one ECC81 (12AT7). It also has four KT88 output tubes. The layout/positions of/for all the 200 Plus's tubes can be clearly seen in this schematic.
  • Sound City Concord ("Lead" Model): A stock/unmodified Sound City Concord Combo ("Lead" Model) typically uses five preamp tubes--three ECC83s (12AX7s) and two ECC81s (12AT7s). It also has two EL34 output tubes. The layout/positions of/for all the Concord Combo's tubes can be clearly seen in this schematic

Q14: My phase inverter (PI) tube is an ECC801s, but I was told it should be a ECC81; is this correct?

A: Yes. The ECC801s also goes by the following designations: 12AT7, 12AT7WA, E81CC, ECC81, CV455, CV4024, and 6201.

Q15: What sonic benefits will I realize from a cap job (will it reduce hum and noise)?

A: Allow me to answer your question by offering what a friend of this site had to say on the topic.

The first thing I do when I buy a vintage amp is is give it a cap job, and replace every resistor in the power tube sockets (if they are at least 10% out of spec). Whe n I got my Sound City amp it sounded terrible; it was noisy and weak for a 50-watt head, and it hummed really bad. One of the problems was a bad capacitor on the ground selector (the .047uf X 600V cap).

Usually when I get an amp, it has been sitting around 10 years or more (at least the last three I bought hadn't been turned on for 10 years from what the former owners told me; this one had been sitting for over 20 years!), so the caps are all dried up and useless. To reduce hum, I cut out the old AC cord and install a new 3-prong modern AC cord and grounded the third prong to the chassis. That usually helps out a lot. New filter caps will greatly reduce hum but will not get rid of it 100%. Next to your tubes, the caps are the biggest part of your tone; if you have dried-out old caps, they will not pulse DC current fast and strong enough and will fail to filter the 60-cycle hum. On top of that you run the risk of blowing a transformer if the caps completely fail (the DC current will start leaking into the AC current, and that's when you see amps with new transformers).

The effect of the new caps is that the amp sounds healthy, the humming is mostly knocked out, the highs and lows are crisper, the amp sounds like it was when it was new (at least I think that's what it sounded like), and it's much louder. If you do a cap job, do not let any tech or vintage amp guy talk you into using IRC NOS caps. Webber's rule is that caps have a shelf life of 5 years, so its best to keep it close to 1 year. IRC went out of business in 1998, and you don't want to start out a new cap job with 7-year old caps, right? The JJ caps have always been good to me and they are all fresh and new; F&T are also very good. These two makes are your best bet, they are readily available, and they are made for guitar amps. The first thing you should change on a vintage amp amp is the bias-supply capacitor. If that cap goes say goodbye to your nice power tubes!

Another thing with these amps is the cloth-braided wire. Cloth won't keep the electro-magnetic fields (EMFs) contained, especially 35 years later when these wires are brittle. Another way of reducing hum is by taking all the cloth-braided wire out of the amp and installing better solid-core lead wire. When these wires are being strung over different circuits (especially with almost no protection) it's a hum fest! Redress the wires over the top of the circuits instead of tightly tucking them underneath all the other circuits; this should take care of a large part of the hum. Also remember that Sound Cities have active tone controls, which means your preamp tubes better be good; any sign of weakness or microphonics will be 10 times worse than with passive tone controls.

And finally, don't buy into the hype that an old used set of Mullards or Telefunken are always better than a fresh set of Sovteks. It's true that if both were new the Mules and Tele's would be much, much better; but it comes down this: would you rather have the 70's Goldie Hawn or the 2005 Goldie Hawn? Keep a fresh set of EL34's and 12AX7's (EH's work best for me) around all the time. I just went through some problems on one of my Hiwatts; it turned out that I had a bad Telefunken ECC83 that tested good, but it was definitely a dud. A fresh Sovtek in there made it sound better than I've ever heard it.

Hope this helps.

Q16: Must I use two speaker cabinets with a 200 Plus or an SL 300+ head? Or can I somehow get around the A-B feature?

A: No and yes! The A-B feature on the SL300+ and 200 Plus heads was a "safety feature" intended to keep players from destroying their surprisingly delicate Sound City speakers by forcing players to use two speaker cabinets with these 200-Watt beasties. 

Here's how the A-B feature works and what you can do to bypass it (fool the amp).

How the A-B Feature Works
The four speaker connections on the rear of your amp are separated into two parallel groups: Group A and Group B, with two jacks (speaker outputs) in each group.  These two jacks are, in turn, parallel with each other.  A stock (i.e., unmodified) amp with this feature requires that you connect two speaker cabs to the amp --- one cab in A and the other cab in B --- AND that both speaker cabs be the same nominal impedance, which allows you to set the amp's impedance selector: four 16-Ohm speakers in parallel = 4 Ohms; two 16-Ohm speakers in parallel = 8 Ohms; two 8-Ohm speaker cabs in parallel = 4 Ohms. So you would set the impedance selector accordingly.

Two Ways to Bypass the A-B Feature
Following are two ways to do this:

1. Take the amp to a qualified tech and have all four speaker outputs rewired in parallel to one another; this way you won't have to observe the A-B requirement and can connect one cab to any of the four parallel output jacks, or the appropriate combination and number of speaker cabinets to any of these jacks.  (If you feel up to doing this yourself, have at it, BUT PLEASE realize dangerous voltages exist inside the amplifier!!! Exercise EXTREME caution!!!)

Warning: I can't possibly stress strongly enough just how dangerous the inside of an amp is even when it's turned off and disconnected from AC power and how you should exercise caution when poking around inside!!

2. Or, to fool the amp, make and use a dummy plug*, as follows:

2.1. Remove the jacket from a male 1/4-inch plug.

2.2. Remove any wire attached to the plug's leads and separate the two leads so there is no possibility they can touch (short).

Note: You could place shrink tubing or electrical tape on each lead to insulate them and put the jacket back on the plug; otherwise, you won't be able to put the jacket back on the plug.

2.3. Insert the dummy plug into either of the two jacks in A OR either of the two jacks in B.

2.4. Connect your 16-, 8-, or 4-Ohm speaker cabinet in either of the two jacks in the unused side.

Note: In other words, if your speaker cab is connected to A, put the dummy plug in B, or vice versa

2.5. Set the amp's impedance selector to the appropriate impedance: 16, 8, or 4.

Note: If you plan to connect multiple cabs (two or four) of the same impedance, calculate the combined parallel impedance and set the impedance selector accordingly. Refer to this page at the SC site for how to measure DC resistance (Re) and calculate impedances. Remember that you cannot connect four 8-Ohm speaker cabinets to the amp, because four 8-Ohm cabinets in parallel = 2 Ohms, which is too low for the amp!

That's it! Good luck!

* The dummy plug looks like an open to the amp, and an open is (theoretically) an infinite impedance. Any real impedance in parallel with infinity is darned close to that real impedance.

Q17: Can you help me find the specifications for a Partridge power transformer used for the mains in the Sound City B200W Mk III Custom Built? The numbers on it ( the ones I can see without pulling it off the chassis) are:TG9182 and G9662. I have had this amp since I was 18 and would really like to get her back up and running again. I know it is the mains transformer because, after checking everything else that could possibly blow the mains fuse, I threw a 10 amp fuse in and looked for smoke(last resort). Well, as you might expect, smoke began emanating from the mains transformer, thus indicating a short in the mains transformer. I only did this after disconnecting the transformer from the rest of the amplifier and the fuse continued blow out. Anyway, the transformer has two HT secondary windings and, not wanting to change the entire power supply (in order to keep it as original as possible) I have been attempting to find a suitable replacement which I believe I may have. However, not knowing the output voltage and current requirements, I am unable to settle on the proper replacement part. Any help you could give me would be greatly appreciated. Thank you for your time.

A: Hope I can help. Refer to the LB200 schematic, which I assume is similar to yours. The schematic tells us that you have 620V DC B+ to the KT88 plates and 345 VDC to the KT88 screens. Working backwards, the AC input to the rectifier would need to be NOMINALLY 0.707 times the DC output.

So for the screen supply you would need a transformer secondary of 345 x 0.707 = 244 VAC. However the screens are supplied through a choke, so I would add another 10 VAC to compensate for that - say a nominal 250 VAC winding. It does not really matter if the screens are running at less than the specified voltage.

For the plate supply you would need 620 x 0.707 = 438 VAC. However the plate supply is in series with the screen supply, so by subtracytion, the plate transformer would need to deliver 438 - 244 = 194 VAC.

Since the plate and screen windings are in series, it follows they both contribute to plate current. Both must have a rating of at least 400 mA DC continuous. In this circuit, this means 560 mA AC (this is because the output voltage is increased by 1.414); therefore, the input current must increase proportionately to maintain the same power in (VA) as power out ( Watts). Best results will be achieved by using separate transformers for plate, screen, bias and heater supplies. This is because when plate current increases to a high level, the internal losses in the transformer cause all secondary windings to sag. Ideally, bias, screens, and heaters should stay at a constant voltage.

Various 6.3 VAC heater supply configurations can be used, but it is best practice to use a separate supply for the preamp to the main power amp.

I would use 4 x 470 uF instead of 4 x 160 uF. Note that the effective capacitance in the plate supply is only 160 uF, which is not much "grunt" for a pair of KT88s. This would be OK as is for guitar or PA but not so good on bass guitar. If you have the cash and the physical space you can use even 4 x 1,000 uF to advantage. Make sure they have adequate surge rating (i.e., 450 VDCW for the plate caps and 400 VDCW for the screen caps).

In any event, to protect the transformer during capacitor inrush charging current, you should consider installing a surge reducing resistor of say 10 ohms in each of the plate and screen supplies, between the transformer and the rectifier. One resistor in each circuit is sufficient. The filter cap in the screen supply after the choke is also shown as 160 uF. This should be as large as you can physically fit into the chassis (1,000 uF or more), because the screen supply voltage should remain as constant as possible throughout all duty cycles. Classy amps of yesteryear had regulated screen supplies as standard.

This is problematic for us because there are no voltages specified for the power supply output, but I would hazard a guess and say about 100 VDC. There are a couple of ways you can tackle the bias. One is to supply the grids direct (i.e., -44 VDC) and dispense with the adjusters. While this might seem a backward step it improves reliability out of sight because the adjustable pots are always a risk.

In my stereo I am feeding 12 tubes from a common non-adjustable bias source and it works fine. The only reason for having adjustable bias is to accommodate unmatched tubes. So if your tubes are matched you do not need adjustable bias. However, to allow for variation in tubes, it would pay to have a little more voltage than specified. The correct bias can be fixed by using a series dropping resistor before the -44 VDC point The main advantage from using direct coupling in the bias supply is that when grid current flows in the KT88s, the bias will tend to stay negative longer (i.e., at higher power output levels).

Whatever you do, significant improvement in performance will be realized by using a full-wave rectifier.

To check for internal shorts in the transformer, do the following:
1. Disconnect the transformer from the mains supply.
2. Disconnect the rectifiers in both plate and screen circuits.
3. Measure the DC resistance between all of the windings.

Notes: There should be no measurement at all between separate windings. Be sure to check all the windings. Note the circuit specifies a "slow-blow" fuse. I presume this is to accommodate the significant capacitor charging current at switch-on. You can try a higher rated fuse (e.g., 5A).

If a full-on short occurs it will blow anyway and do its job. Even though the circuit does not specify it, it is obvious the mains supply fuse also needs to be a slow-blow type. I would use a 5 or 10A fuse there.

Finally, don't forget the fuse is there to protect the mains from the amp and not the other way around.

Good luck!!!!


To be continued...

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