Scratch build brown Vibroverb 6G16 woes - bias related?

[fazeka]

Hi all,

Have a scratch build brown Vibroverb 6G16 I am having an issue with. Well, really two issues.

Firstly, schem/layout here: https://el34world.com/charts/Schematics/files/Fender/Fender_vibroverb_6g16_schem.pdf

The first order problem is that I can't seem to get the 6L6GCs to bias up to anything over 7w dissipation.

I have a 10k bias resistor in there now. Plate voltages around 394-395V, plate current at 16-17mA. Pin 5 voltages are -45V.

When I had a 10k pot with a 15k resistor in there originally, I had plate voltages of 406-407V, plate current at 11-12mA.

Of course the amp has an ugly type of distortion when I hit a chord moderately hard. Kinda like a splat... I guess this is some kind of crossover distortion, possibly due to too cold of a bias?

I've never seen a Fender with a bias resistor lower than 10k... but do I just keep lowering the bias resistor to get it to bias warmer (he asked, rhetorically)? That doesn't feel right...

The other problem is that the amp "chirps" with the pulse/speed of the trem when I turn up the intensity. I'm thinking this could be related to the above as the trem works off the bias? And/or it could be a lead dress issue...?

Any leads (pun intended? ) would be greatly appreciated.

TIA,
Chris

[tubeswell]

Firstly, schem/layout here: https://el34world.com/charts/Schematics/files/Fender/Fender_vibroverb_6g16_schem.pdf

The first order problem is that I can't seem to get the 6L6GCs to bias up to anything over 7w dissipation.

I have a 10k bias resistor in there now. Plate voltages around 394-395V, plate current at 16-17mA. Pin 5 voltages are -45V.

When I had a 10k pot with a 15k resistor in there originally, I had plate voltages of 406-407V, plate current at 11-12mA.

It would seem like your bias output voltage is too negative. You've given us the Fender schematic but it sounds like you built the bias circuit differently - can you post the 'as-built' schematic please?

[fazeka]

It would seem like your bias output voyage is too negative. You've given us the Fender schematic but it sounds like you built the bias circuit differently - can you post the 'as-built' schematic please?

I initially wired up the bias pot like this:

https://www.mojotone.com/Amp_Kits/Brownface/Vibroverb_6G16_WD.pdf

Pulled that out and wired as stock but with a 10k in there (as my original post).

[tubeswell]

Do you mean you replaced the stock 33k resistor with a 10k resistor? - No wonder it's cold, that's a bias output voltage about 90% of what the bias tap can deliver (which is too cold). You need more voltage division because you want to get the bias voltage to approx 1/10th of the B+ voltage (albeit that the bias voltage has to be negative)

[Esquirefreak]

I'd increase the 1k R on the bias board to get you in the ballpark mid rotation on the bias pot. I'd also wire the pot as a variable resistor in case the wiper fails.

/Max

[Esquirefreak]

About the trem, there's a trick where you put a diode across the INT pot to get rid of pulse/thumping. It can be a bit finicky to tune the bias trem with bigger output tubes, like 6L6GC.

[fazeka]

Do you mean you replaced the stock 33k resistor with a 10k resistor? - No wonder it's cold, that's a bias output voltage about 90% of what the bias tap can deliver (which is too cold). You need more voltage division because you want to get the bias voltage to approx 1/10th of the B+ voltage (albeit that the bias voltage has to be negative)

I was getting more current by lowering this resistor. Or so I thought.

[fazeka]

I'd increase the 1k R on the bias board to get you in the ballpark mid rotation on the bias pot. I'd also wire the pot as a variable resistor in case the wiper fails.
/Max

Hi Max,

FWIW, I'm using this PT which granted appears to be for a Tremolux/Vibrolux:

https://cdn.shopify.com/s/files/1/0519/5956/2405/files/PAM-PT-40-18R_Spec_Sheet_448e5883-4e88-4d87-9622-8c76e2ecb6ff.pdf

I increased the "bias range" resistor from 1k to about 2.3k. I was initially using a 5R4G but switched to a GZ34/5AR4. I lowered the bias resistor to 10k. Now I'm getting 367V plate voltage, 35mA plate current which is 43% dissipation and still seems much too cold.

Seems I'm getting closer... do I just keep upping the bias range resistor and/or lowering the bias resistor? Seems a little strange to have to increase/decrease either/both by so much relative to the schem values... 

[fazeka]

About the trem, there's a trick where you put a diode across the INT pot to get rid of pulse/thumping. It can be a bit finicky to tune the bias trem with bigger output tubes, like 6L6GC.

When I increased the "bias range" resistor and decreased the bias resistor, this brought the bias less cold. But then the amp started squealing. I flipped the OT primary leads on each power tube socket (pin 3) and the oscillation disappeared. So the trem works and sounds pretty great now without me resorting to a diode across the intensity pot. One small victory. Need to get the bias sorted still, however...

[Esquirefreak]

367v plate voltage seems low, to me. No shorts or wrong value dropping resistors? I think the last Vibrolux Reverb i worked on had just above 400v on the plates. And my old 6G9B Tremolux had 399v biased at 32w idle.

I'd rather keep increasing the bias board resistor than keep lowering the shunt resistor on the pot. If the pot is wired as a variable resistor and loses contact with the wiper, you will get "max" available negative voltage. Cheap insurance.

/Max

[fazeka]

I'd rather keep increasing the bias board resistor than keep lowering the shunt resistor on the pot.

To confirm, this is the one that is listed as 1k on the schem, yes?

[Esquirefreak]

Correct 👍

[Esquirefreak]

Your output tubes could possibly be worn if you can't bias them properly 🤷‍♂️

[fazeka]

Update: I have a "bias range" resistor of 2k35 (two 4k7 in parallel) with a 10k pot and 15k resistor for the sweep. Proper GZ34. Got it to 402V @ 35mA for ~14W, which is nearly 50% of max dissipation. Stopped and listened to the amp and like what I hear, so I am leaving it there.

[Esquirefreak]

Those figures looks to be spot on for the Blonde Tremolux 👍👍👍

/Max

[Willabe]

Got it to 402V @ 35mA for ~14W, which is nearly 50% of max dissipation.

That's on the cold side, did you try closer to 60% dissipation? You might like it a little better.