1968 Fender Bassman refreshing some components.

[Lydian]

My buddy has a ’68 Bassman in need of some new lytics and a 3 prong chord. I have a couple of questions about some things that are new to me.

The bias potentiometer section looks like AB165 but the rest of the amp looks either AA270 or AA371. I’m wondering how to deduce the reasons for this hybrid appearance. Also, from the output tubes there is a 5mfd 25v Mallory cap tying both pin 8’s together. What is the job of this cap? And should I just cut it out.

The bias resistor is 10k ohm, should it be replaced with a 470ohm 1 watt?

There are also 150 ohm 7w resistors on the same pin 8’s to ground. I was considering putting 1 ohm 1 watt resistors here in order to bias the amp in the way I am comfortable with. Any issue there?

All the old Mallory lytics will come out and replaced with the proper substitutes. Any resistors that are toast, or out of spec, will just be replaced. The death cap will be cut out and a 3 prong chord needs to be installed, which I would put common ground underneath the lower left bolt of the power transformer correct?



Its been an amazing year learning countless hours of material on these little boogers. This project will only continue to challenge and educate me in this hobby. If need be I can provide pictures potentially. Thanks for any help and advice you guy have.



-Dalton

[The Dude]

Some confusing info there fo sho! Does the amp still have the tube chart inside? If so, there should be a chassis number on it. Maybe that will help you determine what you have and what's been modified?

[Randall]

There are also 150 ohm 7w resistors on the same pin 8’s to ground. I was considering putting 1 ohm 1 watt resistors here in order to bias the amp in the way I am comfortable with. Any issue there?


-Dalton

Those 150 ohm 7 watt resistors suggest it is a cathode bias circuit, and so you cannot replace with typical 1 ohm resistors. It sounds like a Super Reverb of the same vintage I had in a while back, which had cement cathode bias resistors and a "bias" pot. I never found any documentation for it.

[Enzo]

So it sounds more like this one:
http://bmamps.com/Schematics/fender/bassman_ac568.pdf

The very first question is this: is the guy happy with the sound of his amp, other than wanting maintenance? If so, why would we even consider changing the circuits?

When a circuit works, we should not approach every part we do not understand as something to change. Often it is there for a reason. One thing to always keep in mind is that it is all about the CIRCUIT, not the parts. Looking at a single part ignores all the rest of the circuit around it.

5mfd cap tying both pin 8’s together...should I just cut it out.
bias resistor is 10k ohm, should it be replaced with a 470ohm 1 watt?
(remove) 150 ohm 7w resistors on the same pin 8’s to ground

Why?

The 5uf cap helps stabilize the output stage, with the separate cathode bias resistors.

In this circuit revision, I'd suspect the two cathode resistors were there to add 4v of bias without having to change much else, especially the power transformer bias winding. Alternatively, maybe they reduced the bias supply a few volts and got them back with the cathode resistors, which left un-bypassed added a little desirable sag.

If you want to use a 1 ohm resistor to measuer idle current, you know that Ohm's Law works just as well across 150 ohm resistors? Just a few different strokes on the calculator. Or if you are just zeroed in on 1 ohm resistors, put them in series with the 150 ohm resistors.

Before you go wholesale replacing resistors that are out of spec, first make sure you are measuring them out of circuit. But beyond that, sometimes the off center resistance is part of the sound of a particular amp. Look at the notes upper right in the schematic I linked. It clearly states that readings are +/-20%, and that goes for the 20% resistors they used back then. Many electrolytic caps had as wide as -20/+80% tolerance. These are just guitar amps, not precision lab gear.

[g1]

The bias potentiometer section looks like AB165 ...

The bias resistor is 10k ohm, should it be replaced with a 470ohm 1 watt?

I think that's a typo, and you mean the 1K that feeds the bias diode?
As you now see with the AC568 schematic Enzo linked, the 1K is the correct value for the stock circuit, so no need to change it to 470R.

The pot is actually for "matching" the output tubes, and is a bias-balance rather than the standard bias adjustment. This enables use of unmatched tubes and is adjusted for minimum hum.
You can adjust the actual bias by changes to the 15K resistor connected to the pot, and the 1K feeding the diode.

[Lydian]

Does he like the sound? He did, the filter caps are so old I can barely get sound from it. There is apparent leaking from the second cap that is in series. The 70uf 350v

Great stuff! Ill make sure to really look over the scheme Enzo. I assume people like 1 ohm resistors from cathode to ground because this places a "1" in the denominateR of ohms laws simplifying the theory. So after measuring plate voltage, what is the procedure that I need to do on the 150 ohm cathode resistor in order to know tube dissapation? Measure voltage across the resistor then divide by 150? Then multiply with the plate voltage?

Thanks guys.

P.s. 10k was a typo, should have been 1k.

[Leo_Gnardo]

So after measuring plate voltage, what is the procedure that I need to do on the 150 ohm cathode resistor in order to know tube dissapation? Measure voltage across the resistor then divide by 150?

Yes, stop there. That's the plate current + screen grid current. sg typically is 1 to 5% of plate current = negligible. So you can take that figure as plate current.

FWIW I often, almost always, change these amps from combination bias to fixed bias.

[Enzo]

it is just Ohm's Law. Current = volts/ resistance. 1 ohm is convenient to do the arithmetic in your head, but "1-5-0" plugs into a calculator pretty easily too. So yes, volts over 150. If you want to be anal, measure the resistors, they might be 151 or 148 or something.


If he liked the sound of it, then OK update the worn parts, but resist the urge to change it. No snipping out this and changing the value of that.

[Lydian]

The pot is actually for "matching" the output tubes, and is a bias-balance rather than the standard bias adjustment. This enables use of unmatched tubes and is adjusted for minimum hum.
You can adjust the actual bias by changes to the 15K resistor connected to the pot, and the 1K feeding the diode.

Changing the 15 k and the 1k feeding the diode. Which direction would I go to increase tube dissipation? I'm at about 14 watts per tube right now. I could use a tad more volume and clean tone. Any guesses to get to 16-19 watts per tube? Must both resistors be changed or can 1 or the other produce a bias difference independentalty?

Thanks,
Dalton

[g1]

You can reduce the 15K to increase dissipation. Either by trying smaller value resistors, or by tacking a resistor in parallel with the 15K.
But I have my doubts that this will get you more volume or clean tone (many Fenders are biased this cold). Only one way to find out though .

[Justin Thomas]

A "tad" more clean volume might be doable. But Bassmans are not your typical "Fender Clean" amps... Just sayin'... Crunch by 4 is perfectly normal.

Justin

[The Dude]

I doubt it will matter. This is idle current/bias. It's not like a MV adjustment. It's not going to radically effect max SPL. IMO, you'd be better off maybe trying less NFB if you want to open it up a bit.

[Lydian]

It's nice to know what I'm hearing is normal, I'll spread the news to my buddy, amp owner. I'll let him hear it and show him the NFB mod I can throw on the ground switch.

Thanks, until next time... Cheers!
-Dalton

[Justin Thomas]

Oh my... my 67 Bassman had a -ve FB switch when I bought it. With none, it went from cranked (not "loud," cranked) tweed to raging beast monster from hell, along with a lot of hiss and noise. Completely out of control. Kinda awesome, actually, but not very practical. I <DO> use it for bass, after all...

Justin

[The Dude]

Well, I did say less NFB, not no NFB.