Guyatone Bassman as D-clone donor amp - Project update

[Darkbluemurder]

A few months ago I purchased a Guyatone Bassman. It was made in Japan and looks very similar to a SF Bassman. Little information was available on that amp. It has

- 2 channels with 2 inputs, Vol, T, M, B and a Bright switch each
- Master Vol and Presence controls
- 3 preamp tubes but further holes for 3 additional preamp tubes (seems like Guyatone used the same chassis for all models)!
- 4 EL 34 tubes which I am not sure that they were original

Transformers are normal size for a 100W amp although I don't know if they are any good.

Is anybody here familiar with these amps? It has a decent clean sound although it is not as loud as one would expect from a 100W amp. From front panel layout and chassis space this should be a good platform even though it is PCB (pots, jacks and tubes chassis mount). Of course I could lose the PCB and use an eyelet board.

As always any input is most welcome.

[Luthierwnc]

It certainly looks like a good candidate. Make sure you can get some B+ out of the thing. If you aren't sure what tubes originally came with it, check the OT for impedance:

1 disconnect the primary leads
2 put wall voltage on the outside primary leads (use a variac, if possible)
3 measure the volts on the secondary taps
4 divide the primary volts by the secondary
5 square the results.

This gives the ratio of what the tap reflects per one ohm of secondary Z.

Example: 120v in, 4.8v across the common and one of the speaker hots.
120/4.8 = 25
25 X 25 = 625
If that was the middle tap of what you think are 4, 8 and 16, multiply the 625 X 8ohms and you get 5k which is right in the range for a pair of 6L6's.
Check the others for confirmation although they aren't always precise.

This will help you figure out what tubes to use and where to put the negative feedback loop. If the amp was actually built for 4 EL34's, you should get a calculation of around 2K.

Good luck, Skip

[Darkbluemurder]

Skip,

Thanks a lot. I don't think the B+ is a problem - while I did not measure all the voltages the B+ at the phase inverter is around 400V so this should be good enough.

What worries me though is that the heater voltage is only 6.0V and the amp has a loud hum that does not go away. Need to do some trouble shooting I'm afraid. But I believe I should rip out the EL 34s and put in 6L6s.

DBM

[Luthierwnc]

6 volts on the heaters shouldn't be a problem - not a humming problem, anyway. Usually they measure over on older amps because wall voltage has gone up. If it was originally a two-prong cord and the filament supply has a center-tap, tape off the latter and hang a 100R resistor off each leg.

After you have heard lots of different hums, you can identify them a little better. It could be a bad cap, tube or a ground loop (my favorite). An intermittant grid leak resistor is also a possible culprit. Usually you would want to fix the amp before you mod it but with such a massive overhaul, you will probably shotgun replace most of the suspect components anyway. Remember the ones you didn't change if you have the same problem.

Pay really close attention to how you want to ground your components. On some amps you can put down grounds almost anywhere but some (Ampegs in particular) are very touchy about that. I have gone to a star ground off one of the PT bolts and have a length of bare romex behind the pots as a preamp buss.

Good luck, Skip

[Darkbluemurder]

Tracked the humming problem. When I took the cap can off to check the caps again I noticed that the caps still held 50V. This was weird since the amp has bleeder resistors. Turned out that the ground connection of the caps was a cold solder joint ... amp works and sounds fine now, although it has not as much power as my Concert or Bassman. I guess it needs new power tubes but now I can start building.

[skyboltone]

But I believe I should rip out the EL 34s and put in 6L6s.
DBM

Stephan,
Don't these tubes need to look into a different load impedance? I know it's frequently done but I think most of the tonal difference can be attributed to a mismatch at the OT.

Dan

[Darkbluemurder]

Hmm I don't know. I believe I read somewhere that a typical Fender 40W OT has a primary of 4k2. A Marshall OT has 3k4. This does not seem like they are far apart. At least they appear closer than those for 6V6 tubes - and quite a few people change from 6V6 to 6L6 without problems. I will let my ears decide. If it's not right I can still go back to EL 34s.

[Darkbluemurder]

Good news - I was able to implement the power supply, the power amp and the basic ODS architecture in this amp.

B+ is 500V, screen voltage close to that. I used 10k - 20k - 2k2 as the dropping string. V1 has 210V, V2 220V.

70s values, F-tone stack (to be changed to D-tone stack) 10uF on V1 cathodes, 0.047 mid cap.
OD channel has still the stock 250k pots (to be changed to 100k - need to order more parts).

Sounds very promising so far. It likes humbuckers and my Strat but somehow does not like my Firebird as much. Too trebly. Touch responsiveness is there.

cheers Stephan

[Darkbluemurder]

D-tone stack implemented. Values are treble cap 330pf, bass cap 0.1uf, midcap 0.047uf (later changed to 0.01uf - the jury is still out on that), slope resistor 150k, all pots 250kA).

The next thing I will do is try 0.022 or 0.033 as mid caps before revisiting the OD snubbers.

I also installed an OD tone control (1MA pot with a 0.022uf cap). I will reduce this cap and I will most likely also reduce the pot to get a wider spread. Right now it works between 0 and 4, and the tone at 0 is too dark.

I will also replace the choke with a resistor since the choke has open connections - not a good idea with a B+ of 500Vdc ... The choke has 170 ohms. I have a large watt resistor of 330 ohms. This will also bring the voltages from the screens onwards down a little.

Finally I measured heater voltage with only 2 EL 34s in it - 6.3V exactly. With 4 EL 34s it was only 6.0V. This was the last evidence that this amp should really have 4 6L6s. Do you think -49V bias is sufficient for 500Vdc on the plates or should this be more? I know Fender sets this at -52V.

As always thanks for your input.
Stephan

[Darkbluemurder]

Most recent changes:

1. Changed mid cap to 0.022uf. This is my favorite and it is going to stay.

2. Replaced the choke with a 270 ohms resistor.

3. Put a 220k resistor in parallel to the last filter cap.

4. Changed the tone cap of the OD tone pot to 0.01uf. This doesn't make the tone as dull as the 0.022uf. I may reduce it even further.

The amp blooms a lot more than before. The clean sound is OK but still lacks sparkle. The OD sound is very nice. Good smooth sustain and cleans up well with the guitar volume. Tomorrow I will take a shot at the channel switching (major challenge for me!).

I will also relocate the master volume pot. Right now it is at the rightmost position and there are long shielded wires going to and from this pot. By relocating it more to the left side I am confident that I can shorten the wire and probably do away with the shielded wires.

Cheers Stephan

[Darkbluemurder]

Yesterday I installed the switchable midboost (2200pf in series with 390pf, switch shunts the 390pf) and a switchable NFB at CL2. I must say I prefer the sound without the NFB - it's a bit fuller in this amp. In the Bassman it was exactly the opposite - the NFB extended lows and highs.

The midboost is just what the doctor ordered for my Firebird.

I also swapped the 250kA Drive pot for a 100kB (the last one I found in my box - new parts ordered). My ear tells me that 100k has a better overall tone and no nasal honk even though the gain is a little less. After having tried a linear pot for the first time I must say it has a lot going for it. Since I had the log pot on 8-9 most of the time I can live with a relatively narrow range of clean/crunch tones. I consider it a benefit of the linear pot that I can have good gain at 5-6 but the bass is reduced compared to a setting of 8-9 on a linear pot.

By now I like this amp better than my Concert - I definitely need to go back and refine that amp!