6BM8 reverb amp

[Paul1453]

I'm trying to figure this amp out.

I hope some of you experts can give me some clues.   

I wire traced and have attempted to create a real schematic for this thing.

Do you think it could possibly work the way it is drawn?

Before I traced the circuit, I tried to test it.
Here's what I did to test it.
I put an 8 ohm speaker on the output.
I took the speaker output from another amp and put it on the inputs of this amp.
I powered it up and got audio through it.   
Adjusted the pot and got more reverb and volume.

Still not much volume, but I called it good and attempted to flip it on Ebay.
It didn't sell on 1st listing, so I'm trying to figure it out before trying to sell it again.
I'm wondering if I could possibly use the reverb tank on a different PP 6L6 build or not?

If I did that, I think I might be able to use the chassis and PT on a Hoffman Stout build.

Does the circuit I've drawn look like it could work with the test conditions I gave it?
Now I see they were not the best conditions for this amp, but it did pass audio and respond to the pot adjustment.

Oh, this is the 1st full circuit I've drawn with this software.
So I did my best.   

[Paul1453]

A few more pics.   

[eleventeen]

I don't see any means of getting plate B+ on either 6BM8 triode.

[Paul1453]

I don't either, and was perplexed about that.

I'll double check my wire tracing scratchpad, but I thought I got everything traced.   

I am also confused about the tank, with a L and R channel and Gnd input.
Output only has output and Gnd.

[TIMBO]

Hey Pual, your amp should look something like this, with the 6gw8 output section and the 12ax7 would be the driver/recovery for the reverb.
 

[Paul1453]

I'm trying to figure out what I got 1st,
then figure out what it could be.

It definitely appears the 12AX7 is the reverb recovery tube, Timbo.
But what is supposed to be driving the tank is still a mystery to me.

The way it is does not meet the needs of a guitar amp.

Some voltage on the plates of the 6BM8s triodes sure would help.
Added those connections on this version of the attempted reverse engineered schematic.

Does this circuit look like it would function now?

How do I figure out the reverb tank inputs and what should be driving them?
Since the OT measures 2 ohms on the secondary, does that mean I should also have a 2 ohm speaker load?

[PRR]

R10 is surely not 780K.

Why? We want V3b to have gain of -1. It has gain of maybe 35-40, except the cathode resistor is not bypassed, so pencil 18-20. The R11+R10 divider needs ~~20:1 loss. Assuming R11+R10 should be similar to R13, so R11 may be 550K, we expect to find 29K (27K, 33K) at R10.

R23 is surely not 106K. Cathode resistors are 1K-2K.

Audio winding resistance should be "much less" than load impedance. Otherwise the transformer steals half our power. Ideally zero winding resistance, but that can't happen. Winding R is often ~~1/10th of load to keep losses "small". That may be a 16 Ohm tap. Or it may be 8 Ohms and a don't-care or penny-pinch approach to power efficiency.

> what is supposed to be driving it is still a mystery

A reverb tank.

And probably not a hard-driven tank, because it appears the tank is driven with a mix (ugh) of two speaker sources. The tank output must not be more than a few mV, because V1ab is wired for gain of 3,000 (won't be that much, but over 1,000) so is too sensitive for guitar. I think the whole V1 section may as well be re-designed (but keep the parts that make sense, R1 R2 R3).

[Paul1453]

R10 is surely not 780K.

Why? We want V3b to have gain of -1. It has gain of maybe 35-40, except the cathode resistor is not bypassed, so pencil 18-20. The R11+R10 divider needs ~~20:1 loss. Assuming R11+R10 should be similar to R13, so R11 may be 550K, we expect to find 29K (27K, 33K) at R10.

R23 is surely not 106K. Cathode resistors are 1K-2K.

Audio winding resistance should be "much less" than load impedance. Otherwise the transformer steals half our power. Ideally zero winding resistance, but that can't happen. Winding R is often ~~1/10th of load to keep losses "small". That may be a 16 Ohm tap. Or it may be 8 Ohms and a don't-care or penny-pinch approach to power efficiency.

> what is supposed to be driving it is still a mystery

A reverb tank.

And probably not a hard-driven tank, because it appears the tank is driven with a mix (ugh) of two speaker sources. The tank output must not be more than a few mV, because V1ab is wired for gain of 3,000 (won't be that much, but over 1,000) so is too sensitive for guitar. I think the whole V1 section may as well be re-designed (but keep the parts that make sense, R1 R2 R3).

You are right of course, PRR.   

2 missed components added, others slightly modified.
See snip-it.

Does this finally look like something that would work?

I was worried about using my normal procedure to find the Z of the OT.
I put 6.6VAC on the OTs secondary and measure the VAC across the whole primary.
With this OTs CT connected to supply voltage A and it's E-cap.
Would I possibly blow that cap connecting 6.6VAC to the OT's secondary?

Oh and none of the Data sheets I can find for the 6BM8 show the data for AB1 PP plate load resistance.

[PRR]

> put 6.6VAC on the OTs secondary

What is the OT power rating?

I would guess 6 Watts output. In (guess) 8 Ohms that is 7V RMS. But a reverb-only channel is usually weak-bass. And 60Hz is pretty deep bass.

I think you may be up in iron saturation, strain your 6VAC source, and give erroneous readings.

Find some 3.1VAC.

Or use the fact that today's $3 DMMs will reliably read AC mV (up to a few hundred Hz). Put 6VAC on one side of the Primary (that should be way-safe for 300V DC iron). Assuming 15:1 from 1/2 Pri to Sec, expect 0.4VAC at Sec. That is a readable number on a DMM. (With older multimeters it could be the low-ACV crunch-zone where anything 0-0.5VAC was bunched-up in a small span.)

[Paul1453]

> put 6.6VAC on the OTs secondary

What is the OT power rating?

Or use the fact that today's $3 DMMs will reliably read AC mV (up to a few hundred Hz). Put 6VAC on one side of the Primary (that should be way-safe for 300V DC iron). Assuming 15:1 from 1/2 Pri to Sec, expect 0.4VAC at Sec. That is a readable number on a DMM. (With older multimeters it could be the low-ACV crunch-zone where anything 0-0.5VAC was bunched-up in a small span.)

Thank you PRR!   

I will try putting the 6.6VAC across 1/2 the OT primary and see what my DMM makes of it.
Even if I have to snip the OT CT wire and reconnect it after testing.
That should give me the info I need on the OT.

How about the last modifications to the circuit?
I missed a few connections at first.
I think I got them all on there now.
Do you think I might have an accurate schematic now?

The values were the actual component readings.
It appears I got some bogus readings from multiple paths on some things.
Decided to just go across 1 resistor and verify the reading jives with the color codes.

So my initial testing procedure wasn't that bad after all?
Yes, the reverb tank inputs appear to be driven by L & R speaker outputs.
Kind of strange.  Maybe this was just used as a separate amp to add a reverberation effect to a bigger amp?
Is this tank a dud for conversion to a guitar reverb tank?

[PRR]

> just used as a separate amp to add a reverberation effect to a bigger amp?

Yes. It is obviously an add-on. Not even as sexy as a  Fisher Space Ex-Pander.

> Is this tank a dud for conversion to a guitar reverb tank?

It can be made to work. You have everything from tank out to small speaker. You don't have a reverb driver or a guitar preamp.

With that tube-set, it could be a fine little amp (no reverb), or part of a clunky add-on to put boiing onto a reverbless amp. You could even repurpose one 6BM8 as a reverb driver, one as a mini-Champ 3 Watt loudspeaker amp, with the four triodes as preamps and glue. This does lead to *two* new OTs: one 3W for speaker, one 1/2W for reverb drive.

The two 68uFd(?) caps to the tank input would logically come from stereo main loudspeakers (of a main amplifier). What bothers me is that these will also kill all stereo separation from the main amps above a few hundred Hz. (Unless your drawing is confusing and they don't both go to the same point.) It is an oddball, and maybe rare for a good reason.

[Paul1453]

1st I want to Thank You for your astute guidance, PRR.   

I don't regret purchasing this amp.
I paid $60 including shipping for it.
It had 3 Bugleboy tubes, and the EZ81 and the 12AX7 pretty much cover my costs.
Someone else has this exact amp without Bugleboy tubes listed for $350 BIN free shipping.
I doubt his will sell, but who knows?
Mine did not sell with 3 Bugleboy tubes and a Miniwatt 6BM8 for $150 + shipping.

I did some research before purchasing.
When I saw the 6BM8 and reverb tank, I expected that tube would drive the tank.
In no way did I expect this to be an add on Boiing amp. 
That is where experience helps.

You obviously have tons of experience that you freely share.
I sincerely appreciate that.   

I wonder what you think about this plan.
If I change to SS rectifier, there are the 4 sockets I need to build a Hoffman Stout.
I have a functional component board for the Stout that I made on my breadboard.
I don't know if my PP EL84 OT will fit where the old OT was.
Or if the layout with those placements would be OK.
It seems to me, that by moving the EL84s over one spot towards the PT/OT.
That I could add the other 12AX7 on the far end with inputs and controls down there.
It would then have the input and preamp tubes as far away from the PT/OT as this chassis will allow.
I could then put the output jack on the back, far from the input.

Do you think this would be worth a try?

It would leave me with a Bugleboy rectifier and 6BM8 I might be able to use as reverb on another amp.
I'm not sure this tank will work with a transformerless 6BM8 used as a driver and recovery tube.
Or if this tank would work with Tubenit's 1 tube reverb if I got a reverb transformer.

I respect and value your opinion.
If this were your amp, what kind of plan would you pursue?

I attached what I believe is very close to what the schematic for this amp is.

[Paul1453]

> just used as a separate amp to add a reverberation effect to a bigger amp?

Yes. It is obviously an add-on. Not even as sexy as a  Fisher Space Ex-Pander.

If you say so, but I like what I got for $60 delivered much better than this.   

http://www.ebay.com/itm/Fisher-Dynamic-Spacexpander-Tube-Reverberation-Unit-w-Original-Mullard-Tubes/131746848531