Fender Bassman 1962 6G6-B Bias Supply Voltage

[DrGonz78]

Well I got this amp where the plate voltage is 509V and the bias supply voltage is 59v. Luckily despite the two 6L6GC tubes being different brands they match up nearly perfect right about 20ma a tube. The cabinet says this is a 6G6-B where the printed B+ is 428v and my circuit matches the 6G6-B. I guess I am a bit surprised to see 509v plate voltage and I would be less surprised if it was the 6G6-A model where the printed B+ was 470v. Not a worry though since I installed 600v filter caps.

So I am wanting to get the bias supply closer to 55v or so. At this point I would just parallel another resistor onto the 27K bias resistor (shunt resistor right?). I know enough that 100k-150k resistor paralleled to the 27k would bring the total resistance to around 21-22k. Can anyone help me understand how to perform the math to be able to accurately approximate what the bias supply voltage would be after altering the 27k resistor to a lower value? Thanks for any help and guidance. Also do I have this correct that lowering the 27k resistor will indeed bleed some more voltage to ground in return lowering overall bias supply voltage?

[potatofarmer]

You want to bleed *less* to ground, making the bias voltage more negative. It's a simple voltage divider comprised of the 1k and 27k resistors. https://learn.sparkfun.com/tutorials/voltage-dividers

My two cents, I'd replace the bias supply with the one from the 6G6A; you could even replace the 10k resistor in that with a pot. Might be unacceptable for an amp this old though, but maybe start with replacing the 1k with a 470R.


Whoops, I had it backwards too; thought the raw supply was 59V and that he wanted to cool the bias off from 50V to 55V.

[Chuck H]

Potatofarmer has it. The voltage divider formed by the 1k/27k resistors is really not going to give much control manipulating the load resistor. Voltage division is currently 1/27th (bias wind resistance not accounted for though). A parallel resistor across the 27k can certainly get you there. Or you could use a 100k pot and a 28k (or similar value, 22k, 33k) resistor in series and then tacked in parallel to the 27k resistor and have adjustable bias.

I think changing the 1k series resistor to 470r will cut your voltage division in half (bias wind resistance not accounted for though) and heat your bias too much.

[Leo_Gnardo]

My solution for this kind of bias supply is to add a 10K or 20K trimpot in series with the 27K resistor. In your case I'm sure 10K would be plenty. Set the trimmer up as a rheostat & you're good to go. FWIW I use Bourns half watt "blue cube" trim pots, you can get 'em for about a buck and a quarter from Mouser, DigiKey and I'm sure other sources.

[Chuck H]

But that would only allow for MORE bias voltage. Gonz wants LESS.?. So maybe change to a 18k resistor and then add a 20k trim pot.

[Enzo]

How about increasing the 1k instead, that would make the voltage divider more responsive in general as a bonus.

[Leo_Gnardo]

But that would only allow for MORE bias voltage. Gonz wants LESS.?. So maybe change to a 18k resistor and then add a 20k trim pot.

Whups, correctamundo... I was just waking up when I typed that. 22K another choice. 18K fixed & 20K variable would give you a very wide range bias adjust, nothin' wrong with that.

Gonz, where do you get your 600V caps?

[DrGonz78]

It's all makes so much more sense to me now thanks! I will probably change the 1k to 2k and then drop a 18k into the circuit. At that point I should have about 55v or so. From there I will add a 20k trimmer pot to be able to adjust the bias on the amp better.

I got the 600v 20uf caps here.... Part #EL-CAP-20/600 They are ELUM branded and I have read some good feedback from Antique Radio guys who have been using them for years.
Capacitors

[pdf64]

A high VHT may indicate that the PT is being fed more voltage than its primary was intended for.
What are the actual, measured mains and heater Vac?

[DrGonz78]

A high VHT may indicate that the PT is being fed more voltage than its primary was intended for.
What are the actual, measured mains and heater Vac?

When I am working on the amp at the bench I have had it metered at 120vac on the variac. I have double checked the accuracy of the 120vac Line setting on the variac and it is pretty good - 119-121vac when set on variac rotation to 120vac. While double checking the mains voltage I measure 6.72vac for the heaters. My guess is that this might be a 6G6-A transformer that got fit to a 6G6-B chassis. Well that is my guess why the B+ is so high. I imagine when I set the tubes upwards to 30ma the B+ voltage might reflect the 6G6-A voltages to a tee.

[pdf64]

Many of the 60s Fender drawings note a mains of 117Vac, eg http://el34world.com/charts/Schemati...a165_schem.pdf
So you may still end up a bit higher than the schematic values.

[Chuck H]

When I am working on the amp at the bench I have had it metered at 120vac on the variac. I have double checked the accuracy of the 120vac Line setting on the variac and it is pretty good - 119-121vac when set on variac rotation to 120vac. While double checking the mains voltage I measure 6.72vac for the heaters. My guess is that this might be a 6G6-A transformer that got fit to a 6G6-B chassis. Well that is my guess why the B+ is so high. I imagine when I set the tubes upwards to 30ma the B+ voltage might reflect the 6G6-A voltages to a tee.

Well someone might want to correct me on this, but...

Without knowing better it's been my experience with Fender models that "A" always preceded "B" in model designations. It's also been my experience that Fender very often had crossover models because they used up their old inventory making the newer model deigns. Not that you implied otherwise, but the higher voltage PT could have been installed at the factory.

[Leo_Gnardo]

I imagine when I set the tubes upwards to 30ma the B+ voltage might reflect the 6G6-A voltages to a tee.

I'm sure your B+ will be closer to the mark but still a little on the high side. I find this to be the case on practically all the larger 60-63 Fenders I've worked on.

[Enzo]

And...

They are just guitar amps.


Nothing is precision here. If the voltage is off, so what?

[DrGonz78]

Without knowing better it's been my experience with Fender models that "A" always preceded "B" in model designations.

Still it is strange that the "B" model has the very simple bias supply circuit and the "A" model has the much more robust version.

My PT is 125P7A with 606-145 stamped on it, 45th week of 1961. The other transformers in the amp are 1962 and the serial number puts the amp as produced in 1962, serial #BP04562. I have come across one thread online where a guy says his 63 Bassman has a 125P7B PT. The 125P7A seems to be the same PT in the Super & Super Reverb 6G4A models. Cannot find any charts or direct info on the 125P7B transformers.
Fender XFMR chart
My '63 Bassman has a 125P7B power transformer rather then a 125P7A? | Harmony Central