Looking for 18W schematic using 6973's

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BillHarder
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Looking for 18W schematic using 6973's

Post by BillHarder »

I salvaged an organ power amp which was stock 12AX7pre, 12AU7PI and a pair of 6973's. It has a 5y3 rectifier. PT is 380-0-380 and I am seeing 440VDC rectified. Has a bias tap. 6973'a are currently biased at -35V with 400V on the plates. Also has a third position which was for some off chassis function which could be used, currently occupied by a 12AT7. Seems like a nice candidate for a 18W conversion as the son has been wanting a Marshall clone for a long time. Wondering if anyone has built one of these. A search yields a couple people talking about it but can't find any schematics from anyone who has successfully built one. Anyone useful guidance or links appreciated. Thanks in advance.
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geoff 1965
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Re: Looking for 18W schematic using 6973's

Post by geoff 1965 »

check out this plexi 6V6, very similar B+ and plate voltages to yours.
Plexi6V6.gif
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Re: Looking for 18W schematic using 6973's

Post by JMPGuitars »

6V6 and 6973 are interchangeable using an adapter. My suggestion would be to do a 6V6 build, and use adapters to convert to 6973. That will be much more versatile, and there are adapters for other tubes as well.
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crgfrench
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Re: Looking for 18W schematic using 6973's

Post by crgfrench »

I have some EL84 to 6973 adapters I use in the Fullerwell.
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geoff 1965
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Re: Looking for 18W schematic using 6973's

Post by geoff 1965 »

anyone know the specs of the T2 output transformer that schematic refer's to?
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BillHarder
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Re: Looking for 18W schematic using 6973's

Post by BillHarder »

I was hoping someone had actually built one tweaked for the 6937 but beggars can't be choosers right :mrgreen: ? That 6v6 plexi scheme does look very close to mine as far as the PT output and layout goes though and it might just do the trick as a basis for the build. Thanks for the tip.

I hate to drill out the chassis and put in octal sockets but the sockets are wafer style and do need to be replaced and it is a good point that it would be more versatile that way. HMMM. decisions decisions...

Geoff 1965,
My OT should be pretty close to whats pictured in schematic. I believe it is an old Woodward/Schumacher. I can give you the numbers off it and an ohm reading but not sure how far that will get you as far as specs.
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Re: Looking for 18W schematic using 6973's

Post by geoff 1965 »

the T2 on the schematic is traynor but there is 2 YBA transformers,one is 25W 5K1 C.T. paper/oil and the other is 20W 10K C.T. paper/oil,yours should be in the ballpark.
i've seen good reviews of those 6973's but you don't see them here in the U.K. in fact my valve data book has no reference to them.
good luck with the project.
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ellis2699
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Re: Looking for 18W schematic using 6973's

Post by ellis2699 »

Cool organ amp! What model is it? The tube layout looks cool, and those transformers are huge!

Have you looked at the old Supro Model 24 schematics? It's got three 12AX7s, two 6973s, and a 5Y3 rectifier, with about 350 volts at the power tube plates. These things sound incredible when cranked- like a brown deluxe, but a bit sweeter in the mids.

I've been looking for a 6973 chassis like this for a Model 24 conversion, but I keep finding Hammond AO-35s with original tubes at prices I can't pass up. The AO-35s are great 18 watt conversion candidates, but it's a lot easier to just turn them into Dr. Z Carmen Ghia clones.

I suspect you'll have a lot of clean headroom with those gigantic transformers and 400 volts on those 6973 plates. I'm curious as to how this thing is going to sound- maybe like some kind of Supro/Hi-Watt hybrid monster.
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BillHarder
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Re: Looking for 18W schematic using 6973's

Post by BillHarder »

The amp came out of a Thomas Model "M". Weird because the the rest of the organ is all transistorized.

Thanks to Sluckey on EL34 forum I have an "as was" schematic before I pulled it apart. It is now stripped down, the sockets have all been replaced, the bell housings got a fresh coat of paint along with the chassis. Here is what it looks like now and the planned schematic. Working on wiring the heaters now.
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ellis2699
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Re: Looking for 18W schematic using 6973's

Post by ellis2699 »

Wow! I didn't think Thomas Organ made anything with tubes in it, at least not in the mid-sixties when they were building the 'American Vox' amps (Super-Beatle, Baby Beatle, etc.). They're certainly known mostly for their transistorizing of the classic Vox line, which we all know to be less than spectacular. I do give Thomas credit for influencing Dick Denny to design the Vox UL series. Without those amps, we might be a tad bit bored by songs like Dr. Robert, And Your Bird Can Sing, Paperback Writer, and most of the Sgt. Pepper and Led Zeppelin II albums, or at least I might be (a little maybe).

I love these conversions of old non-guitar tube amps, and judging by the prices these raw examples go for, it appears I'm not alone. These organ/reverb/PA/film projector amps were probably mostly mediocre at what they were designed for, but man, they all seem to make outstanding guitar amps. They're the rare exception to the rule that most of us have of not 'molesting' vintage equipment. They relieve us of our temptation to mess around with the historic Fender, Gibson, Marshall, Vox, and Valco amps (a few of which would benefit from a mod or two), while still kind of 'honoring' these other engineers who designed and built these multi-purpose amps with exceptional attention to detail and quality. Otherwise, these amps would be relegated to a dusty basement shelf for all of eternity, or end up as scrap. So this is pretty cool.

And nice job so far on the restoration and conversion!

Bryan
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Re: Looking for 18W schematic using 6973's

Post by colossal »

Hi Bill,

In reviewing your 6973 Plexi schematic, the plate voltage is quite high and the screen voltage is well above the stated limits in the datasheet. Now, that's never stopped Fender from exceeding datasheet limits for screens, grid leaks, etc in virtually every amp they made, but 6973s running at 440V on the plates are going to need a rather large drop to keep the screens under control, so that screen resistor might need to be increased quite a bit.

I built a 6V6 Plexi a long time ago, much to those specs with 440V on the plates, but with 5k in series with a choke to tame the screens. It was a fun amp.
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BillHarder
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Re: Looking for 18W schematic using 6973's

Post by BillHarder »

I am operating under the assumption that if it worked on the thomas organ without issue it will continue to do so in a a guitar amp application but your point is well taken. I may need to further revise. The power amp section is pretty much all stock and the only real changes made are in the pre and pi.
I considered the Supro dual tone/24 but my pt was a bit on the hot side for stock and my son doesnt use tremolo so seemed like the plexi was a better use of the chassis with less rework.
Appreciate the feedback and compliments.
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Re: Looking for 18W schematic using 6973's

Post by Pressed_Rat »

Is your Bass pot really 100 Meg audio?
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BillHarder
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Re: Looking for 18W schematic using 6973's

Post by BillHarder »

Good catch! No, that is supposed to be 1M-A. Typo on my part.

Finished up the heater wiring yesterday. Here is where I am at so far.

Starting on the power switch now.
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BillHarder
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Re: Looking for 18W schematic using 6973's

Post by BillHarder »

Got the PT wires all terminated, power cord, power switch and pilot light all in yesterday.
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Re: Looking for 18W schematic using 6973's

Post by JMPGuitars »

Why did you run two separate sets of leads for the heaters? That's normally done with one chain to keep the leads shorter, and reduce potential heater noise.
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BillHarder
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Re: Looking for 18W schematic using 6973's

Post by BillHarder »

This will be a point to point build and the pre-amp heater wires will be buried under all the resistors and capacitors making rewiring nearly impossible in the future. Therefore, I wanted them independent of the output tube heater string so if/when I make changes to the output tube wiring, it has no impact on the preamp heater wires. At least until I am done experimenting with output tubes.

If I find it is causing hum, it is pretty simple to go back and attach the pre-amp string to the output tube string, shield them, twist them, etc.

I reversed the twist in the two heater strings to try and optimize noise cancelling where they run in parallel back to PT. At least that is my theory. Will see if it works or not.
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Re: Looking for 18W schematic using 6973's

Post by Brianmac »

I'm late to this topic. but wondering how the project is coming along. I have built a Supro 24 Lite into a Hammond AO-39 chassis, and then rewired V1 for a 5879 pentode so now its kind of a Gibson GA-40/Supro 24 mashup. A very cool sounding amp. I just finished an 18W TMB and after I play it awhile I intend to rewire the power section for 6973/6CZ5/6CM6. A jumper across pins 1 and 3 allows you to use all three of those tubes, and the 6CM6 gives you the 6V6 tone without needing socket adapters. I'm not a real fan of EL-84 amps.
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Re: Looking for 18W schematic using 6973's

Post by JMPGuitars »

Brianmac wrote:
Fri 01/31/20 4:31 pm
I'm not a real fan of EL-84 amps.
Most EL84 amp circuits aren't very good. None of them IMO are anything as good as these 18 watter circuits. Do a lot of playing with that amp, and maybe record something with it before you change it. Play with it cranked if you can. The tubes don't matter as much as the circuit.
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Re: Looking for 18W schematic using 6973's

Post by Brianmac »

JMP - I'll take that advice and try to be patient. I've been tweaking the preamp cathode resistor and bypass cap values a bit, and upping grid stopper to the PI as it seems to me that there is too much preamp gain in the TMB channel (extra gain stage) for the power tubes to shine.
For other EL-84 circuits I have built a Spitfire, a C ghia, and a simple AC-15 - so far I do like the 18W better. The Spitfire in particular does not get much character until my ears are bleeding. Some like that I know, but I like my amps to have some grind when used without a pedal.
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