‘81 Princeton Reverb Bias Question

[Helmholtz]

Oops, too late, I changed the resistor but here's the B1270 schematic. Enzo requested V3 cathode, which I listed earlier as 1.6

Referring to this schematic, what is the voltage at point B?

V4a)jumpy 190-350

That is suspicious. Might be a bad V4 socket contact, bad V4 tube or a leaky filter cap section. But V4a plate voltage can't be higher than the supply voltage at point D.

V3a)104

With a supply voltage of 205V this as well as the 1.6V cathode voltage shows a plate current of around 1mA as opposed to specified 0.8mA, corresponding to +20%. Not critical.

[Enzo]

V4 is jumpy because it is the trem oscillator.

Perk, you said the amp was from 1981, so I put up the 1981 schematic. Your AB1270 schematic is a 1970 schematic. eleven years earlier. The schematics don't match of course, but what matters is which one is closer to the actual amp?

[Perkinsman]

B voltage on my schematic is the same as A voltage on the one Enzo posted, 408vdc. I retested V1,2&3, all within 10% of my schematic. V4 is now 0v at both pins 1 (plate1) & pin 3 (cathode) but pin 6 (plate2) is good at 200v. I check pins & replace V4 & report back.

[Helmholtz]

V4 is jumpy because it is the trem oscillator.

Would make sense with V4b, not V4a (acc. to B1270 schematic).

Tube sections confused?

[Enzo]

V4a)jumpy 190-350 b)160.

This is how it was reported back in post #15.

I posted the 1981 schematic to be helpful, but if it does not match the amp and your 1970 one deos, we will use the 1970 drawing. But PLEASE answer that question, which one matches the actual circuit in the amp? it only confuses things to be discussing both drawings, and especially when we start changing parts from one value to the other.

[Helmholtz]

And please clean (Deoxit) and retension socket contacts.

[Perkinsman]

Aside from the difference IÂ’ve pointed out, I donÂ’t see any other differences. Enzos posted ‘81 schematic makes more sense with the 4 supply voltages, A-D listed & the 4.7k resistor in place of the 18k since it brought voltages up to within spec so let’s use it.
I tightened V4Â’s pins & replaced with a new tube but still no difference in pins 1 or 3. They both are still 0.

[Helmholtz]

the 4.7k resistor in place of the 18k since it brought voltages up to within spec.

It's no good idea to change an amp's design during troubleshooting. How did the replaced 18k resistor measure?

[Perkinsman]

It brought the voltages up to spec except V4, which is why I thought it is most likely the correct schematic. There don’t appear to be other differences.

[g1]

Aside from the difference IÂ’ve pointed out, I donÂ’t see any other differences. Enzos posted ‘81 schematic makes more sense with the 4 supply voltages, A-D listed & the 4.7k resistor in place of the 18k since it brought voltages up to within spec so let’s use it.

Look at the cap and resistor coming out of the bias diode. They are different. Which set do you have?
Voltages we can expect to be somewhat different. You should not change parts to match a different schematic just to make voltages agree with a schematic.
Did the 18K you replaced look like it was not original?

[Perkinsman]

Yep, you’re correct. The original bias cap was 80/70 & resistor was 22k, which matches Enzos schematic. I replaced them with a 33/160 & 27k because I read 50/70 & 27k on the B1270 schematic I was using. I’ll replace them with as close to original value as I have & report back.

[Enzo]

You don't decide one schematic is the correct one because the voltages match. The schematic is the correct one because the parts and circuit on it are the same as what is in your amp. If we THEN need to change something, fine,

A schematic is like a road map of a city. If we are in Cleveland, a map of Toledo is not right, regardless of how nice the streets line up.

[Leo_Gnardo]

Yep, you’re correct. The original bias cap was 80/70 & resistor was 22k, which matches Enzos schematic. I replaced them with a 33/160 & 27k because I read 50/70 & 27k on the B1270 schematic I was using. I’ll replace them with as close to original value as I have & report back.

100 uF 100V is common, and as close as you'll find to the original these days. I keep a supply on hand. Who knows where Fender found those odd values back in the day. There's nothing magic about them, that's for certain.

[Perkinsman]

I replaced the bias cap to 80/160 & put in a 22k resistor, Bias is now -31, supply & plate voltages within spec except for V3plates (126v) & V4 b plate & cathode which are still 0.

[Perkinsman]

[QUOTE=Enzo;552747]You don't decide one schematic is the correct one because the voltages match. The schematic is the correct one because the parts and circuit on it are the same as what is in your amp. If we THEN need to change something, fine,

A schematic is like a road map of a city. If we are in Cleveland, a map of Toledo is not right, regardless of how nice the streets line up.[/QUOTE

I had been using what I thought was the correct schematic. Once you posted one, I realized there were differences that actually made more sense as to what I was looking at, and corrected because it more accurately represented what I was seeing both component wise & voltage but thanks for the road map analogy...and if we're in Cleveland please just shoot me now