6G2 Princeton voltages....dark amp...too dark?

[dehughes]

Yeah, the AA964 schematic has the voltage measurements for the PT secondary, whereas the 6G2 schematic does not. However, both schematics show the same PT, so I used that as a reference point. My amp does indeed have the bias circuit values of the AA964 as well as the filter values (20uf) of that circuit, albeit with only three filter stages (as per 6G2 specs). Very unique amp here...

[physikmatze]

However, both schematics show the same PT, so I used that as a reference point.

No, the 6G2 shows a 125P1A while the AA964 shows A 125P1B!

Mathias

[dehughes]

However, both schematics show the same PT, so I used that as a reference point.

No, the 6G2 shows a 125P1A while the AA964 shows A 125P1B!

Mathias

Hey, you're right! I definitely have a 125P1A in my amp....but the secondaries are at 343v or so.

So, if the spec sheet on the 6G2 is correct, then a rough guess as to the secondary winding on a 125P1A should be 260v per side....if you want to run that through a 5Y3 and end up with about 315vdc. This seems unusually low for a Fender secondary PT winding....even the 5E3 runs higher secondary voltages than this. I wonder if the difference between the 125P1A and B just has to do with rectifier current winding and ma for the extra preamp tubes?

Can anyone speak to the proper secondary voltage rating on a 125P1A?

[martin manning]

UPDATE: Looking over pictures of the amp last night I noticed that there is a 250k volume pot instead of a 1M! This is clearly original from the factory.....I think someone grabbed the wrong pot and never thought anything of it! Changing that would make it a bit brighter, yes?

I think you have it. That would change the input impedance of the following stage - the input impedance interacts with the inter-electrode capacitance of the stage to shape the frequency. A lower 'R' to 'C' ratio means more bass and vice versa

Even though this swap worked, I don't think the pot element interacting with the Miller capacitance of the following stage is the reason why. The larger pot will actually reduce the high frequency response since it goes like 1/(RC). I suspect it's the response of the volume/tone network itself.

What brand is your 5Y3? Sovtek's are not really 5Y3GTs, and known to have a lower voltage drop than vintage tubes, that would account for as much as 30V.

Modern wall voltages are ~10% higher now relative to when the schematic was drawn, so expect everything to increase by that factor. If you're at 343VAC on the secondaries now, it would be around 309 with the primary reduced by 10%; a 34V difference.

[dehughes]

Thanks for the post!

The rectifier is a re-branded GE 5Y3...definitely old school 5Y3 goodness. I swapped with a JAN 5Y3 Phillips and there was no discernible difference in tone, vibe, or voltage, so I wager the original 5Y3 is still good.

I think you're spot-on regarding wall voltages....hmm.... Would it be worth it in your opinion to have Heyboer make a lower-voltage PT for this amp so as to get the voltages more to spec? Being as putting in the correct pot made such a huge difference in the tone and feel of the amp, I'm inclined to get it "back to spec"...

[Alexo]

I would not recommend replacing the original PT with an MM in order to get back to original spec.

If you're 30 volts off, just get a 30 volt zener diode, place it in series with your rectifier tube, striped side facing the rectifier. Definitely use a diode rated for at least 20 watts, or use smaller zeners in series.

Reversible, easy and a lot cheaper than Mercury Magnetics!

(although personally, I would not worry about 30 volts)

[dehughes]

I would not recommend replacing the original PT with an MM in order to get back to original spec.

If you're 30 volts off, just get a 30 volt zener diode, place it in series with your rectifier tube, striped side facing the rectifier. Definitely use a diode rated for at least 20 watts, or use smaller zeners in series.

Reversible, easy and a lot cheaper than Mercury Magnetics!

(although personally, I would not worry about 30 volts)

Elaborate on the Zener diode, please. I thought one usually put voltage dropping Zeners between the secondary CT and ground.

And, it's not the secondary voltages that make me wonder so much as it's the 220v on the V1 plates. That's about 80-90v over 6G2 spec, which even considering increased wall voltages today, seems to be excessive...

[martin manning]

I tend to agree with Alexo viz. leaving the original PT in place. If you want to reduce the voltages on the preamp tubes, you could just replace the 10k dropping resistor with a larger one. A 22k would likely drop an additional 30V, and it may indeed sound better. How are the filament voltages looking?

On the 250k vs. 1M pot, I got curious and modeled the tone stack. The smaller pot shifts the whole frequency response curve up about two octaves, and since it has a +6dB/octave ramp from about 1kHz on up, you lose about 12dB of treble. This is only true at low volume though, there is no difference when the volume is maxed out (except at the low end, < 100Hz).

[dehughes]

Nice. I knew the 1M did something at low volumes.

About the voltages....yeah, I'd like to do something that keeps the amp "stock" but puts the voltages back where they should be, a la 1964 wall voltages. A zener seems to be the easiest way to do it, but I don't want to mount it to the chassis, and am not sure if suspending it inside the chassis would allow it to dissipate enough heat...