Thomas Tube amp rebuild

Hi all, Merry Chrismas, Kawanza, Hannaka, seasons greetings.. whatever :wink:

Was hoping to get a little guidance on a project of mine. I have limited experience, most significant, built a LA2a. Anyway been doing alot of reading on tube amp design and picked up a Thomas Lesile amp on Ebay as a trial ground for a guitar amp. Seen many designs I could copy to get some more experience, this seemed like a great test bed. Wanted to be able to use the chassie, tube sockets, Pxr trans and output trans.

I made up a schematic the best I could. The amp just had a fragmented harness so the controls are missing. Consequently I'm not clear on parts of the circuit. (and I'm pretty green at this)

www.demofactory.biz/Thomas_BT-1.bmp

Any thoughts on what I have, or what my next steps would be? What do I need to know about the transformers to use them in a gtr design? Just needed a push in the right direction.

save it as a .gif or .jpg and re-upload it. that thing is huge man. 7.7MB, thats 5 and a half old floppy disk for just one image. seem a little strange?

> save it as a .gif or .jpg and re-upload it. that thing is huge man. 7.7MB

BMP files are un-compressed. This maybe made sense for local images on Windows 2.0, but is choking my DSL more than a tube-amp scematic should.

I know some MS-Paint programs don't give you any option other than BMP.

GIF is always your best bet for clean line drawings (fresh schematics). GIF compresses repeated pixels of the same color into a color+count byte. The de-compressed version is exact.

JPG is good for things that do not have long sequences of identical color. It encodes repeats but also knows when to round-out small differences that the eye won't see. Great for landscapes and girly-pics, generally terrible for scematics, but sometimes the best option for old yellowed faded schematics that can't be cleaned-up in the scanning.

PDF also has built-in zoom to arbitrary size, which is handy when the scematic is four times bigger than my monitor.

PDF, 170KB

> What do I need to know about the transformers to use them in a gtr design?

"gtr" = guitar?

No change. The voltages and ratios are already a good match for each other, the "quality" is a little "too good" for guitar but not stinky-clean. The circuit is fine, except the parts you have not notated, and for most guitar work we use little to no negative feedback. If you are doing a bare-chassis re-build, just steal the design from a Fender or Ampeg of similar power.

Oops sorry about that

www.demofactory.biz/Thomas_BT-1.gif

Yes for one of those gitars

Right off the bat I just wanted to be able to plug in a guitar to see what it sounds like. Many of the gtr amp schematics showed a 68k(+) and 1 meg(-) resistor on the input jack. Anything special about this? I assume its something that's needed to interface with a gtr pickup.

I'm also assuming that the circuit coming off of V2B is a tone stack of sorts because of the expression control inputs and the missing parts on the other end of the harness that I don't have. Or is this for negative feedback? (need to read up on what that is)

Can I just drop in a volume pot on the grid of V1B, wire up an input jack and give it a ride?

Too early...


www.demofactory.biz/Thomas_BT-1.gif

:roll:

> Can I just drop in a volume pot on the grid of V1B, wire up an input jack and give it a ride?

Looks good.

Guitar wants a hi-Z input, but you don't have to go crazy. Odds are that V1a's input resistor is already well over 100K, good enough. I thought the main purpose of the 68K in series was so you could have two jacks and mix two instruments without big interaction. It may also soften the clipping if you overdrive V1a. But I'm sure it will work without it.

V2b does not make sense as drawn. (Neither does the cathode return on V3 V4.) If I had to guess, it is "tremulo", a low frequency oscillator driving the output grids into cutoff for a wah-wah-shake effect.

Well my knowledge on this isn't much more than being able to match up schematics I have seen to what I drew for this one and apply the concepts I've picked up so far. I did check the V2B circuit a few times because it didn't match up to anything I've come across.

I have no idea what a Leslie expression control does. Maybe the circuit will explain itself when I get it plugged in. Or maybe I'll just rip a bunch of it out and attempt to wire up a more well known configuration.

This will be interesting. Now where is that circuit breaker? :roll:

Well wired it up and fired it up. It be a tube amp. Kinda flabby Hiwatt sound but I'm definately operational. Took some voltage reading, saw V2B voltage oscilating about 20v, expected a tremelo when I turned it on but it played straight. Still don't know what that circuit is actually doing.

Now I can start playing around some.

Thanks for the push :wink:

Was hoping to get a little more help. I'm not clear on what the configuration for V2A is all about. I haven't come across a schematic showing a pretube cathode connected to output tubes grid.

I disconnected V2B (at the resistor connecting the circuit to the output grid) and everything seemed to work fine. Guess this was something not used (tremelo?)

I also found an additional resistor to ground coming off V3 & V4 cathode

If I wanted to swap out output tubes, what do I need to know? I found that an EL84 is a substitute (changing the pinout), is there anything else I need to knwo to do this?

> I disconnected V2B

There is logically a tremulo on/off/speed control in the harness you are missing. Try to find the schematic for that.

> If I wanted to swap out output tubes, what do I need to know?

Yes, EL84/6BQ5 is the "same" tube except re-pinned, probably to lock-up the replacement market. Not sure why you'd want to re-wire... thetubestore.com has good old-production 6GK6 for less than new EL84/6BQ5. The very best of today's tubes are maybe as good as the stuff turned out routinely in the 1960s, but you can end up with some real sick new tubes too.

There is nothing in a 9-pin that will beat the 6GK6/EL84/6BQ5 series. (Well, you could run 7189 with a silicon rectifier power supply in fixed-bias for a couple more watts, but try finding healthy 7189s today.) 6AQ5 (or 6V6 or 6F6 with octal socket) can be run in an amp like this, but you will HAVE to re-figure the cathode resistor (larger), 6AQ5 will probably be over its voltage rating, and you will get a little less power. 6L6-family will make the heater winding run warm, and won't make significantly more power, though the distortion curve will be different.

Might need an extra driver stage to make up for the 6V6 transconductance difference if switching to the octal.

You guys are great - BTW green over here. :wink: I guess I'll stick with the output tubes I got. I really appreciate the "hey think about this for a minute" insight, I need to be reined back and challenged on my thinking.

Well I just got done pulling out the stuff for V2B (Should of thought maybe I'd want to play with a tremolo, except I havn't been able to find schematics anywhere for this thing). Anyway I've just been playing around with coupling caps and volume controls to get a feel for what does what.

Right now I'd really like to know how to go about introducing more gain/drive and moderating the output volume. This thing is loud. For learning I tried to put another vol pot in before V2. It allowed me to get some great gain/drive but it also gave me lots of hum as I turned down the V2 pot and turned up the V1 pot.

Just throw out some Ideas and I'll be off figuring it out.

Been reading more on this stuff (I figured out that it is a fixed bias design) and wondered if removing the V2B circuit would effect the bias on the output tubes.

I also think I answered my own question on the cathode hooked to the output tube grid. Push/pull output where the one pre tube supplied each phase of the signal. yea?