Fender tube amp question:

Ivan Funk · Post by **Ivan Funk** » 8 Jun 2023 9:03 am

1972 Fender Bassman Ten has a SS rectifier.
3 diodes in series per leg of the HT.

Would there be advantages or disadvantages to using 4 diodes per side or more per side?
I’m trying to drop the B+ a little without resorting to CT zener diode or dropping resistors etc…

Bill Sinclair · Post by **Bill Sinclair** » 8 Jun 2023 10:15 am

I don't think a couple more rectifier diodes is going to drop your B+ by more than a couple volts. My 70's Bassman 10 chassis actually has a knockout for a rectifier tube and taped off 5V windings on the transformer. You might see if yours has the same. Switching to a tube rectifier should drop the voltage a bit and also give you a little sag if that's what you're after. If I ever do that to mine, I'll probably just wire it according to a blackface bassman schematic. What's your reason for wanting to lower the B+?

Ivan Funk · Post by **Ivan Funk** » 8 Jun 2023 10:28 am

Thanks Bill,

Yes mine is the same as yours - has the 5v windings for a tube rectifier.
Can the PT handle the current draw of a tube rectifier?

Either way - I don't want sag - I like the stiffness of SS.

Reason for wanting to lower B+:
My B+ is 525v with all tubes in and standby off (or closed). I have JJ 6L6gc as power tubes in it. 516v on the plates.

I've been reading forums for the last few days and some people say don't worry about it.

Some attribute it to higher line voltage compared to 40 years ago. I have 375v on the PT secondary HT where the schematic says it should be 365v.

Any ideas for lowering it or should I leave it?

Thanks,

Raybob Bowman · Post by **Raybob Bowman** » 8 Jun 2023 11:05 am

Fender schematics have note votlages are + or - 20 percent. You are fine.

Dave Grafe · Post by **Dave Grafe** » 8 Jun 2023 1:16 pm

The higher line voltage is a myth. I was there then and 120vac was the norm for most of the USA in 1960.

Your B+ voltage is within Fender spec, and if your amp sounds fine there is no problem, but if it ain't broke we can fix that too.

Tim Marcus · Post by **Tim Marcus** » 8 Jun 2023 1:30 pm

voltage is headroom - let it soar!

the one thing I would check is your heater voltage - it should be 6.3vac and if that drifted high it can shorten tube life and also does make things sound less good than they would at 6.3 or lower. Adding a 1-3ohm 5W resistor after the pilot light will solve that problem for you should you need to solve a problem

Ivan Funk · Post by **Ivan Funk** » 8 Jun 2023 7:03 pm

Thanks guys.
That's good to hear.
The amp does sound good as it is now.

Hi Tim - yeah the heater voltage is a bit high at 6.9v
Do I add 1-3ohm 5 watt resistor in series with the heater AC line? Like a dropping resistor?

The heater AC already has the two 100 ohm resistors to ground as the schematic shows.

Also - the bias is at 44ma. Is this too high?
Should it be around 36ma?

Would bringing the bias down raise the plate voltage?
Where's a good safe balance?

Raybob Bowman · Post by **Raybob Bowman** » 8 Jun 2023 7:33 pm

6.9v is within the plus/minus 20%. Good.
For bias adjusting, shoot for 70 percent plate dissipation.
https://www.tedweber.com/bias-calc/

Ivan Funk · Post by **Ivan Funk** » 8 Jun 2023 7:56 pm

Thanks Raybob.
Very cool.

Dave Meis · Post by **Dave Meis** » 8 Jun 2023 11:24 pm

Bias seems a little high at 77% dissipation (525x.044=23.1 watts, 23.1/30=.77).

Ivan Funk · Post by **Ivan Funk** » 8 Jun 2023 11:34 pm

Thanks Dave.

New numbers after rebuilding the rectifier board:

523v B+

517v on the plates

42ma bias

What's that 72% dissipation?
Still a bit high?
I'll bring the bias down a bit.

Ivan Funk · Post by **Ivan Funk** » 9 Jun 2023 2:57 am

On a whim I re-flowed every solder joint along the heater AC circuit. Now the heater voltage is down to 6.7v
Good enough.

Final numbers:

516v on plates

38ma bias

I'll leave like that. It sounds really good.

Thanks everyone for your help!

Dave Meis · Post by **Dave Meis** » 9 Jun 2023 9:01 am

That's a safe 65%. The 70% number is the maximum, not necessarily the target. Under 70%, let your ears be the guide!

My Twin sounds great at 55%-60%. Sounds like you got it figured!

Ivan Funk · Post by **Ivan Funk** » 9 Jun 2023 11:52 am

Thanks Dave.

Yes it's nice to have it running safe.

Now to put channel 1 / bass normal channel back to stock.
Someone had "modded" it and the tone controls are backwards and don't do much. Channel 2 sounds really good.

Henry Brooks · Post by **Henry Brooks** » 10 Jun 2023 11:12 am

The reason why Fender used three diodes in series is because at that time solid state diodes didn't have the very good reverse voltage capabilites. Modern diode are much better so only one diode is needed today. Also, you often find resistors in parallel with the diodes back in the day. The resistors equalize the voltage drop across the diodes.
Henry

Ivan Funk · Post by **Ivan Funk** » 10 Jun 2023 5:21 pm

Thanks Henry.

Bob Sigafoos · Post by **Bob Sigafoos** » 10 Jun 2023 7:13 pm

Does the bias pot on this amp serve as a bias balance between the two tubes? Not the bias voltage supply. I've fiddled with one and it seems to be a hum adjustment for the power amp tubes. Which resistor would affect the bias voltage to both tubes?
The 3.3K or the 15K one.

Ivan Funk · Post by **Ivan Funk** » 10 Jun 2023 8:38 pm

Yes Bob that is correct - the pot is a bias balance pot.
(1972 Bassman Ten)

The center lug 15k resistor to ground sets the bias.

You can either swap out resistors and check the bias like I did (I ended up at 16k for 38ma bias) or, some people convert the balance pot to work as a bias set pot instead. Or another option is to change the 15k resistor to 10k and add a 25k pot wired to the 10k which goes to ground. Or something like that.

The experts can probably explain it better.

I just noticed that my wall AC voltage is around 120v mid day and goes up to 127v or more at night.
So I've been chasing my tail obsessing over voltages.
I think I need to just go play!

Pat Chong · Post by **Pat Chong** » 13 Jun 2023 12:22 pm

Hi Folks,

It is a bias adjustment, not a hum adjustment. If your bias is set too low, the tubes pull too much current and your amp will hum (I'll bet the tube plates glow a little bit, too.)

Anyway, the reason behind the bias adjust is to keep the tubes from pulling too much current while they aren't being used: The less current, the more headroom. If the bias is set too high, though, it could turn the tubes off.

Tube technology is interesting!
.........Pat.

Bill Sinclair · Post by **Bill Sinclair** » 13 Jun 2023 12:45 pm

Pat,

On a stock Bassman 10, the pot is a balance adjustment of the bias between two tubes. The way to tell when they are balanced is the point at which there is the least hum.
Fender changed the function of that pot to a balance adjustment on some of their tube amps in the 70's. Perhaps to allow you to use mismatched output pairs? I changed the function of the pot on my Bassman 10 back to a true bias pot with wiring similar to a blackface Bassman.

Ivan Funk · Post by **Ivan Funk** » 13 Jun 2023 2:39 pm

Thanks Bill.

Someone had messed up channel 1 so I rebuilt it to a Super Bassman circuit. This brought all voltages down to spec so maybe there had been a short somewhere...?
Sounds really good.
I like the odd-ball deep switch that some people don't like.

There's a very faint minuscule hum at idle with master vol on 10 and channel volumes all the way off. Maybe I only notice it because I recently moved out to the countryside and it's quieter than any place I've ever lived before.
I cleaned up lead dress layout and moved twisted heater wires away form signal wires as much as possible and that helped a little bit. Replaced all caps and resistors in both preamps. The hum is barely noticeable but I have had other Fender tube amps I was able to get dead silent. (Twin, Pro-Reverb, etc..)

If I pull one preamp tube there is less hum.
If I pull both preamp tubes there's is zero hum.
Do some 12ax7 tubes hum more than others?
Should I try different tubes?
Any ideas?

Pat Chong · Post by **Pat Chong** » 13 Jun 2023 2:51 pm

Hi Bill,

I did not know it was changed to a "balance" adjust, it would help "mismatched output tubes", as you said.
Being familiar with bias controles and not having seen a "balance" control as such, somehow it looked like a misprint.

Thanks, ..........Pat.

Dave Grafe · Post by **Dave Grafe** » 13 Jun 2023 3:21 pm

By all means try different preamp tubes, preferably the low-noise 7025 spec over stock 12AX7.

Ivan Funk · Post by **Ivan Funk** » 13 Jun 2023 10:50 pm

Thanks Grafe.
I will do that.

Ivan Funk · Post by **Ivan Funk** » 1 Jul 2023 8:14 pm

Final update:

I added a 0.1 ohm 5 watt resistor to the heater line.
Brought it down to 6.4 volts.
Put it back together and played it all day.
Sounds great.
Dead quiet at idle with master vol wide open.

Thanks again everyone for you input!
Very helpful.
A fun rebuild.