Guild Thunderbass Questions

[drewl]

Just wondering if anyone has worked on at least one of these over the years.


Re-furbing one of these, got pretty much everything sorted out, new filter caps, output tubes etc.
The schematic has 6L6's for the output tubes, this one had 8417's, possibly just thrown in by someone or did these come with anything other than 6L6's?

Bias voltage was really low, only -40v at the source with about -20 at the tubes. I did replace bad bias filter caps and bad/leaky PI coupling caps, but bias was still the same so I added a doubler circuit.
I'm going to swap out the 2.7K bias ground with a 10K pot for better adjustment.
Somebody had removed the output tube cathode resistors, I'm trying to decide whether to use one or ten ohm resistors for easy bias measurement.

Power was also low, one problem being the 10K high voltage dropper at B3 was replaced with a 39K
The other was the 7247 phase inverter was replaced with a 12AX7.
I don't know why they elevated the heaters, seems like overkill.

So it's nice and quiet now, putting out about 50-60 watts

[Helmholtz]

Schematic?

Not familiar with the amp, but generally bias voltage should not be measured at the power tube grids.
Depending on meter input resistance, the reading will always be lower than actual.

Of course measuring idle current always is best.

[drewl]

Doh!
forgot to link like an idiot

[g1]

When you mentioned missing cathode resistors, I thought hybrid bias might account for low bias voltage, but at 6R8, they wouldn't be doing that much.
There is supposed to be -73V on tap there though.

[Helmholtz]

What cathode voltages do you measure at the bias test points?
That is with original value 6.8R cathode resistors.
Supposed to be 0.414V per side(see schematice notes) meaning a very high idle current of 60mA.

[drewl]

Like I mentioned, somebody removed the cathode resistors
also mentioned the bias supply being about half what is listed on the schematic
So I was just curious if someone had worked on one and seen similar readings.
Transformers look original with no sign of replacement.
Anyway I installed 10 ohm cathode resistors, doubled the bias voltage and replaced the output tubes
Amp is stable and quiet putting out about 50 to 60 watts.
Hope this helps someone in the future

[Helmholtz]

What is the voltage drop across the 10R cathode resistors?

[Leo_Gnardo]

I've seen several of these, factory installed 8417 output tubes in place. They required a fairly low bias voltage in the -25V range, even lower than EL34. Voltage doubler is a good solution to raise bias to usable levels for 6L6 types.

8417 looked like it might be a winner back in the day, early 60's, delivering high power and requiring that unusually low bias voltage. Like 7591 we don't much find them outside audio applications. Some manufacturers gave them a go but neither caught on. Tube manufacturers ceased making what wasn't selling. AFAIK typical military versions / alternate type numbers seem absent. Examples I find tend to be rattly. Or very rattly. I have a Bogen MO200A mono boat anchor that ran eight of them. Good luck finding a matched set, I told myself long ago. It might someday get 6L6's and be used as a room heater in cold weather.

[Helmholtz]

The schematic of post #3 actuall seems to be for 8417s though it calls for 6L6GCs.
The bias supply voltage of -44V is voltage divided via the 2.7k from the bias pots to ground.
A higher resistor value would result in more negative grid bias, but maybe still not enough for 6L6s.

[drewl]

What is the voltage drop across the 10R cathode resistors?

I dialed it in to about .3 volts each, 30mA each

I would have kept the 8417's, but one was bad. I tried 6550's but power was about the same, so I used 6L6's

Bogen, ha!
I have a few of those great amps, one little MO100 uses four 8417's
The other MO100's get 100watts out of a pair of 6550's
Talk about overkill, separate high voltage and heater transformers, two GZ34 rectifiers and one EZ81 rectifier.

Anyway, thanks everyone
I've only worked on three of them over the years, and that was at least ten years ago

[g1]

The schematic of post #3 actuall seems to be for 8417s though it calls for 6L6GCs.

This one seems to be for a 8417 version. Bias voltage much less. Has added regulator tube.

[Helmholtz]

This one seems to be for a 8417 version. Bias voltage much less.

Just looked up the 8417 data.
This is really a very special tube. Has around 4 times the gm of a 6L6.
Typical grid bias in class AB is -15V.

https://frank.pocnet.net/sheets/168/8/8417.pdf

[Helmholtz]

One of the consequences of the high gain of a 8417 is that it needs much less grid signal than a 6L6.

[Leo_Gnardo]

Bogen, ha!
I have a few of those great amps, one little MO100 uses four 8417's
The other MO100's get 100watts out of a pair of 6550's
Talk about overkill, separate high voltage and heater transformers, two GZ34 rectifiers and one EZ81 rectifier.

Those 100's are bricks. Imagine two on one chassis. That's the 200, it's two amps in parallel with a common input circuit also common feedback, output transformers joined together in parallel too. Bizarre.

[drewl]

Yeah, I've seen pix of the 200's
they rival any expensive high end tube audiophile amps

The 100's can be paralleled for 200 Watts.
I used to drive one with my '73 Marshall 50w for really LOUD gigs!