Scratch build brown Vibroverb 6G16 woes - bias related?

[seveneves]

Hi all,

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

Firstly, schem/layout here.

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

[xtian]

The schematic has no bias adjustment pot. Do you want to sketch what you have now? Anyway, you need to REDUCE bias voltage closer to zero (like to -40v instead of -45) in order to get more current flowing through your power tubes. And yes, figure this out before worrying about the tremolo.

[seveneves]

The schematic has no bias adjustment pot. Do you want to sketch what you have now?

I initially wired up the bias pot like this.

Pulled that out and wired as stock but with a 10k in there (as my original post) in an attempt to increase the plate current (went from 4W to 7W - still too cold, it seems).

[sluckey]

The mojo circuit is good, but you need to fine tune the 1K/3W bias range resistor. Wire the bias circuit exactly like the mojo circuit using the 10K pot and 15K tail resistor. Set the bias pot to half rotation. Now increase the value of the 1K (bias range) resistor on the bias board until you have your desired plate dissipation. Now you will be able to adjust hotter or colder than your desired plate dissipation.

[Helmholtz]

PT HT voltage (from your thread on the TGP) and B+ are considerably lower than original.
So will be the screen voltage.
The lower the screen voltage, the higher (i.e. less negative) the grid voltage needs to be for the same idle current.

[seveneves]

PT HT voltage (from your thread on the TGP) and B+ are considerably lower than original.
So will be the screen voltage.
The lower the screen voltage, the higher (i.e. less negative) the grid voltage needs to be for the same idle current.

Hi Helmholtz.

Apologies, I forgot to mention I am using a PT for a Tremolux/Vibrolux so seems voltages will be a little lower...

https://cdn.shopify.com/s/files/1/0519/ ... ecb6ff.pdf

[seveneves]

The mojo circuit is good, but you need to fine tune the 1K/3W bias range resistor. Wire the bias circuit exactly like the mojo circuit using the 10K pot and 15K tail resistor. Set the bias pot to half rotation. Now increase the value of the 1K (bias range) resistor on the bias board until you have your desired plate dissipation. Now you will be able to adjust hotter or colder than your desired plate dissipation.

Thanks Steve for your helpful reply.

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 also lowered the bias resistor to 10k. Now I'm getting 367V plate voltage (Tremolux/Vibrolux PT), 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...

[seveneves]

One more thing/FWIW... 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! One small victory. Still need to get the bias sorted, however...

[sluckey]

Seems I'm getting closer... do I just keep upping the bias range resistor and/or lowering the bias resistor?

Yes, keep increasing the bias range resistor, but leave the tail resistor alone. I suggest going back to the 15K value like mojo used.

[Helmholtz]

Apologies, I forgot to mention I am using a PT for a Tremolux/Vibrolux so seems voltages will be a little lower...

Measurements from my original 1970 Vibrolux Rev. (with GZ34):
- PT HT winding: 340V - 0 - 340V in Standby,
- B+: 422V at idle,
- Bias voltage: -41V @ 38mA idle cathode current,
- Clean output: 35W.

[danman]

Some pairs of tubes will naturally bias up lower than others. You may pop in a different pair and find that the dissipation is too high. If you want to keep using that pair of tubes, keep following Sluckeys advice. FWIW, a colder bias setting will give you a much richer tremolo effect if that's important to you.

[seveneves]

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. Yes, the trem sounds nice with the bias set here!