8-15w Donor Amp/Chassis Suggestion

[jordan86]

I’d like to try my hand at converting/repurposing an old amp into a guitar amp. Not against gutting most everything. I’ve toyed with a Hammond reverb unit, ala Dr Z Carmen Ghia.

I really like the idea of a cranked lower wattage amp though, maybe something with 6K6 or 6AQ5 power section. For sure push pull. Got one SE and that’s enough.

Anyone know a good donor chassis or brand I can be feeding into the Google to help me narrow down something to purchase? 

[shooter]

my last build was inspired by this one, I used 6K6's
Silvertone1482.pdf (el34world.com)
I think they sold for $49.99 new 

[bmccowan]

I've done quite a few conversions of PAs and organ amps - its fun, and the bones are usually good. I was just playing a couple of them today, mostly to shake out the cobwebs. There are more 2x6V6s out there than there are 6K6 or 6AQs. But typically the voltage on the old PA amps is low enough to use 6K6s; The brands that I have found to be robust are:
Bogen; Stromberg-Carlson, Grommes (also goes by Precision), Webster, Hammond, Conn, Eico, Thordarson.
If you want a simple amp, the Grommes Little Jewel amps show up often - model numbers are LJ-1 through 7. LJ-7 is 2x EL-84 the earlier ones are all 2x6V6. They were sold assembled and as kits. Bogen Challenger series are really nice and Stromberg-Carlson amps are built like tanks. There are also some funky no-name amps out there. I think most were made by hobbyists doing a Heathkit type thing. If you can spend some time at flea markets and garage sales you'll make out better than the Bay.

[PRR]

...cranked lower wattage amp though.... For sure push pull. .....

Under 8 Watts, push-pull made no sense. Over 8 Watts, SE made no sense. (I did have a "10W" SE but an oddball.)

If you want a part-watt (any more will annoy the neighbors) P-P you may have to be creative.

[bmccowan]

I can't argue with PRR. I recently completed a P-P 6K6 amp and its pretty darn loud. But I do not play cranked; too old. It sounds great at the jazzy/bluesy levels I like. I also just completed a SoLow ECL84 P-P that is probably 4 watts. So that's creative, Tubenit's creative, not me. Its also loud but I can touch the breakup without ear plugs. Our parents were right, electric guitars and amps are too freakin loud.

[jordan86]

Under 8 Watts, push-pull made no sense. Over 8 Watts, SE made no sense. (I did have a "10W" SE but an oddball.)

If you want a part-watt (any more will annoy the neighbors) P-P you may have to be creative.

I don’t completely disagree with you PRR. I’ve never played a 1W amp though. I know Mojo sells a PP 1W. And Robinette has one as well. 12Au7 I think. Bedroom volume isn’t exactly what I’m going for. Just want that cranked amp tone at more manageable volumes. I guess I have no experience with an 8W PP for that matter either. I’ve heard YouTube samples that the D-Lab guy put out on a few conversions recently with 6K6 and 6AQ5, so that’s what sparked the original idea. I do have a 10-20w SE, so that’s why the preference toward PP.

Maybe it’s not the right direction. Or maybe it’d be easier to just build from scratch? Just kinda starting down this road and wondering what I might be looking for to salvage. I assume radios from the 50s or 60s at this point.

[Stuff4bikes]

PP 6AQ5 isn't a bedroom amp.....pretty good gain with that combo...

I just finished a SE,  55 in V1,  57 in V2 driving a 2A5 with about 250v on the plate, somewhere around 3w I guess.
Through a single V30 or rajin cajun it's a little more than bedroom volumes.
Through alnico 12"rola or Jensen its not bad.

Look for old record players, they have some pretty decent "bones" to them...usually the record players are beyond sensible repair.
You might luck out and find a decent speaker/speakers in some also.

[sluckey]

Hammond amps are easy conversions. I've used AO-39, AO-43, and AO-44. The AO-44 is a small 6GW8 amp. The others are EL84 amps but can easily do 6V6s too. The Hammond power amps are usable as is. Just need to add a guitar friendly preamp. Lot of fun. Some challenges. Look on my website for some ideas. I've also converted an AO-63 but that may be bigger than what you are looking for. Old tape decks are good too.

[Williamblake]

The question - at least for me - is, what output pentode to use. SE EL95 is nice but too loud, EL91 is rare, ECL80 looks promising and appearantly was used in a lot TV gear.

[shooter]

FWIW;
the PP 6K6 I did was "living room loud", you had to talk pretty loud to get over the amp.
I mixed n matched speakers in a 2 X 12.  I did get the sound you're talking about, but just barely.  I reconfigured to 6V6 which got well into that sound, but also bumped the loudness enough to be "house loud".


Look at Steve's conversions
look at the library for simple like GA-8, 5E3
pair that type amp with good speakers, enough preamp signal to spank the PA hard and you'll be close I believe
don't overlook a pedal or 2 for "your sound"
those amps WILL annoy the neighbors but just won't make it over a loud drummer.

[mresistor]

I'm going to build a simple 5F2a and see what that sounds like.  SE with one 6V6 and might be able to go SE with a 6K6. Might someday try one of the Gibson SE amps. I'm only building this one to use up some parts I have laying around.

[Lectroid]

I second (edit: third) sluckey's suggestion of the AO-43 Hammond chassis if you're okay with 2-6BQ5s in push-pull.  It would be hard to fit two octals into the space but otherwise that chassis has a lot to recommend it.  If you can find one from an organ tech with the reverb tank, you're 70% home

[PRR]

Triode power stages are not the sound of rock n roll.

Kitchen-radio power pentodes 35C5 25L6 are not quite right because the least of them makes a couple Watts P-P.

6AU6(12AU6) is a good cheap little RF pentode which will do about 1/3rd Watt SE, maybe a slim Watt in P-P. Keep down under 180V of B+: at practical (findable) OT impedances you don't need the voltage. Tie the screens to a 100k 2Watt mil-spec pot across the B+, 0.5uFd bypass, and you can trim power down below tenth-Watt.

[bmccowan]

I like Hammond chasses too. The iron is always good and surprisingly I have found the cap cans to be functioning ok. But 2xEL84 is where Jordan began the discussion - wanting less volume than a Carmen Ghia. I think you could go as low as 6K6 with that iron - but not sure. I think you do not want to go much over 300V with that tube, but I also think Fender went way over that in the reverb unit. The AO-39 with a 5Y3 rectifier will get down close to 300V. I have built a few amps with that chassis and/or iron. Sluckey's Dual Lite is built with that iron.
Anyways, some not so unique ideas for getting the overall volume down while still visiting crunchland: VVR - very easy to install and I find they work pretty well; an attenuator; an inefficient speaker; high quality musician's earplugs (neighbors? what neighbors?); and as others suggest, a pedal. My recent favorite is really cheap - Caline Mellow Drive. I also built a Klon clone and that is nice too.   

[bmccowan]

PRR - along the lines of your suggestions, I just purchased a few EF80 pentodes. Rob Rob suggests them as a pentode alternative to triodes in micro amps - I was targeting a bit under 200V for those. Any thoughts on those tubes?

[jordan86]

Well....its NOT a low wattage amp, but a friend as offered me a FREE chassis. Hammond H1 he pulled from a Hammond DR20 tone cabinet. He's had them sitting for a few years and was kind enough to offer me one at no charge. Seems to have some interesting things. Fields coils that affect bias, two chokes, condensers (seem like filter caps?), and then no real preamp. OT is 4x6V6 to 4 ohms. I'd probably pull two 6V6's and have 2x6V6 into an 8 ohm load, with comfortably oversized iron. Although the B+ seems quite low, maybe enough for a 6K6 with a 5Y3 rectifier?

Any thoughts on a good circuit to build into this? With the

Attached some sample photos from another website and a schematic.

[PRR]

You don't have those field coils.

This PT will kick-up 600 Volts DC at start-up or no-load. Don't trash that cap-bank, it is the best solution.

285V is plenty for 6V6 (which are just better 6F6).

The preamp was in the organ console. But you have plenty of power and space.

But really, "free" may not be a bargain. You are not a 1939 church.

[sluckey]

I would fix it as is. Probably nothing wrong with it. Those oil filled condensers last a looong time. Gonna need a 5K/30W and a 250K/25W to simulate the field coils. They will provide the necessary current load to make voltages agree with the schematic. Then you can use PM speakers.

BTW, that 6SN7 is not the PI tube. Still need a proper PI of your choice to drive the 6SN7 driver tube. Then just add your favorite one tube 6SL7 guitar friendly preamp. I already have two 6SL7 preamps that would work good with this. Look at my Rocky and my RCA radio conversion. You could even use the PI circuits from either of those. Also look at the Ampeg B15N.

Condensers is just the old man's name for capacitors. Same thing.

[PRR]

> 250K/25W to simulate the field coils

250 (no K), isn't it?

[jordan86]

Yeah I will have to look into field coils. But my friend did mention the replacement with high wattage resistors. He’s an EE that works with Texas Instruments here in North Texas. He did offer me the field coil speakers, but they would likely need to be reconed.

Would I be correct to assume that if I replaced the field coils with resistors of lower values, it would raise the voltages of the B+ rail? Maybe get me somewhere closer to 400V? Possibly with a 5AR4 and a pair of 6L6’s?

Would it be too crazy to scrap the schematic entirely and just use the iron for like a single channel AB763?

[PRR]

Stripped of frills:

[PRR]

Why do you want 400V? A 4-pack of 6V6 at 280V is good for 28 Watts, far in excess of the "8-15w" in the thread title. And the Hammonds are notoriously musical, moreso than Fenders.

[jordan86]

Why do you want 400V? A 4-pack of 6V6 at 280V is good for 28 Watts, far in excess of the "8-15w" in the thread title. And the Hammonds are notoriously musical, moreso than Fenders.

I’m just vacilating on whether to do something totally different PRR. I still like the idea of something low wattage.

[shooter]

I’m just vacillating

I would fix it as is. Probably nothing wrong with it.

make it work, then oscillate, rotate, and yell YIPPIE   

[sluckey]

> 250K/25W to simulate the field coils

250 (no K), isn't it?

Of course.

[sluckey]

Would I be correct to assume that if I replaced the field coils with resistors of lower values, it would raise the voltages of the B+ rail? Maybe get me somewhere closer to 400V? Possibly with a 5AR4 and a pair of 6L6’s?

Wrong. Smaller value resistors would reduce the B+ even more. Study the schematic a bit more.

Would it be too crazy to scrap the schematic entirely and just use the iron for like a single channel AB763?

Yes. That 440-0-440 would produce over 600V B+ in a standard AB763 power supply. Really need to use that choke input filter as is to get sensible voltages. That's why I suggested fix it as is and just add a guitar preamp. Plus all your groupie amp builder followers will think you are a real hero for having such a unique amp.

[PRR]

Reducing the 250r will raise the B+ a whole 55V; but also become real confusing since (IIRC) the three filter caps are hard-wired this way (physically in series! not-quite parallel electrically).

Reducing the 5k will reduce the B+ the hard way, by straining the PT.

It is what it is. Not amiable to different output.

[jordan86]

But really, "free" may not be a bargain. You are not a 1939 church.

Wow. Just looked up 6SL7 and 6SN7 tubes. Quite pricey for preamp valves!!!

[shooter]

maybe put a line-out on your SE build and use that pre for testing phase of this amp. 

[sluckey]

But really, "free" may not be a bargain. You are not a 1939 church.

Wow. Just looked up 6SL7 and 6SN7 tubes. Quite pricey for preamp valves!!!

Yeah, but what a unique amp you would have! You know me. I love oddball Hi-Q stuff. Wish I had one of those.

[jordan86]

Could anyone give me a quick explanation of what's what's going on between the 6SN7 and the power tubes?  I'm particularly confused by C1 and C2.  I believe R4 and R5 are the plate load resistors. C3 is a coupling cap? Which makes R6 and R7 are grid stoppers?

Also, not really sure what R8 is doing. I guess that's the issue with look at schematics of separate preamps and poweramps with their companion?

C4, C5, and C6 seem like really low values for filter caps too.

Attaching schematic again for reference.

[shooter]

I'll start 
C1 and 2 are your couplers from the 6sn7 to the PA
R4 & 5 are the plate load resistors for the 6sn7



I get confused on leaks n stops so.....next 

[PRR]

Is very old thinking. C456 are OIL caps because electrolytics are not reliable enuff for church. Together with TWO chokes, the filtering is excellent.

R8 is a dropper to feed/filter an external preamp.

C3 filters the bias, against the B+ of the driver. It is just another way to do it.

C1 C2 are obviously coupling caps from driver to power tube grids.

[sluckey]

What shooter said... plus, C3 is the bias supply filter cap. Yes, it's connected to B+ rather than ground, but remember, B+ is AC ground because of those big honking 6 µF filter caps. Yes, 6µF does seem small by modern power supply design. But at the time this amp was built 6µF was a pretty big filter cap. And they are hi-Q oil filled caps. Probably still outlive you.  Also consider the design of this PS. It's a choke input filter feeding a C-L-C filter and finally another 4µF filter. I bet the B+ is smoother than today's typical Fender amps.

About that 6SN7. It is just a simple differential driver for the output tubes... differential input and differential output. Notice there are two signal inputs at P1. Pin 6 feeds the top grid and pin 1 (180° out of phase) feeds the bottom grid. These two signals get amplified by the 6SN7 and coupled to the 6V6 grids. THE 6SN7 IS NOT A PHASE INVERTER.  The phase inverter lives on another chassis. Given the age of this amp, there's a good chance that the PI is actually a transformer.

I told you this is a unique power amp. IMO it's worth keeping intact.

[Stuff4bikes]

I picked up a JR20 amplifier....similar to yours but with 6SJ7, 6SC7 and 6SN7's....I removed the cap and the 15H choke but kept the 3H choke...

I decided to make it a simple amp so it is basically a cathode bypassed Harvard with 6l6gb's in push pull.
12ax7 into the triode of a 6BN8....I made a "Marshall style" head cab to put it in....it looks and sounds pretty good...

I can't get pics on here or I would put some up.....

[jordan86]

Thanks all. That helps tremendously and all makes sense to me now. It will be interesting to feed a preamp into this. Never seen a guitar amp with a stage between the PI and the power amp, so I’m greatly intrigued to see how that sounds in real life.

[jordan86]

I assume removing R8 is the best path forward, or would it make more sense to use that as a part of a “node” to power my added preamp and PI tubes?

[thetragichero]

Thanks all. That helps tremendously and all makes sense to me now. It will be interesting to feed a preamp into this. Never seen a guitar amp with a stage between the PI and the power amp, so I’m greatly intrigued to see how that sounds in real life.

https://www.ampwares.com/schematics/marshall/major_200w.pdf
svt as well but i suppose that's a bass amp

[jordan86]

Took possession of the actual amp today. Photos attached. Came with glass and working speakers.

[jordan86]

Inside and one speaker