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

[dehughes]

Hey there,

I have a friend's transitional '64 Princeton (6G2) circuit and while I love the vibe....it is a super dark amp, even with the tone pot maxed. I noticed that the preamp stages are seeing 220v...WAY higher than the schematic, even given Leo's schematics are notorious for being "ballpark". I subbed in a 5AR4 to see if the (even higher) voltages caused the amp to darken further, and it indeed did...so now I suspect that something's up in the power supply string that's causing excessively high voltages and thereby darkening it up.

Of course I tried a different speaker, known good tubes throughout, etc...

What do you guys think?

[orrong65]

Are the voltages upstream of the plate resistor in line with the schematic? Have you disconnected and checked the 10k and 1k 1W power resistors? These components do a lot of work in these amps and can change value over time, although they usually drift higher.

The first thing I do in these amps (I have a 6G2) is to check the value of all the power train resistors and replace them if they have drifted off spec, and replace the filter caps as well.

[dehughes]

Are the voltages upstream of the plate resistor in line with the schematic? Have you disconnected and checked the 10k and 1k 1W power resistors? These components do a lot of work in these amps and can change value over time, although they usually drift higher.

The first thing I do in these amps (I have a 6G2) is to check the value of all the power train resistors and replace them if they have drifted off spec, and replace the filter caps as well.

The voltages upstream are NOT in line with the schematic...well, sort of, actually:

The PT secondary is putting out about 343vac, which is to spec with the PT (125P1A) for the 6G2 and the AA964 NON-reverb Princeton. However, the voltages POST 5Y3 are totally not to spec whatsoever with the 6G2 schematic. I believe the schematic suggests 315-ish vdc on the OT CT, whereas I'm getting a solid 400vdc on the plates. I believe that 400v figure is about right given the voltage drop of a 5Y3 against a dual 343vac PT secondary.

No, I've not checked those resistors....I'll do that for sure tomorrow, but I think the problem lies elsewhere given the super high voltages upstream in my amp vs. the 6G2 schematic.

As for the caps....really? I'm not a collector (far from it!), but would swapping out the caps fundamentally shift the vibe of the amp? I mean, it's dark, but I can tell the FEEL is there.... The amp has zero hum and no odd noises....so I'm hesitant to swap electrolyitcs if that's not for sure the issue. And this is coming from a guy who de-solders first and asks questions later....

Would you say your 6G2 is appreciably "duller" than a Blackface non-reverb Princeton?

[Alexo]

How's the bias? Cold power tubes = minimal voltage drop over the rectifier.

[dehughes]

How's the bias? Cold power tubes = minimal voltage drop over the rectifier.

You know, that's a good point...I'll have to test that out later today. The resistors are all stock, so I wager the bias is in the ballpark, and the tremolo works well....so I doubt its too far off, but I'll check it anyway.

[dehughes]

The bias is nearly exactly to spec....at least according to the schematic. -35 volts. Just over 400v on the plates (which is the main discrepancy, aside from preamp plate voltages). The dropping resistors test out good...1k, 10k, and 100k to the preamp plates. Hmm... I'm still getting upwards of 232v on the preamp plates...and yes, I swapped out tubes.

Are my voltages in reality just "normal"? That is, should I just ignore the schematic's voltages? Can anyone with a 6G2 comment on whether or not my voltages are correct? Perhaps I'm looking in the wrong direction? I still can't shake the feeling that 400v on 6V6 plates in a Princeton with a bias voltage of -35v is correct....

[Blind Lemon]

Wall voltage is a little higher now than in 1962.

BL

[tubeswell]

I doubt the higher voltages will make it 'darker' sounding. If anything, higher voltages make a tube amp chimier. Have you tried subbing in different/newer tubes?

[dehughes]

Yeah, the wall voltages are higher...but would going from 110v to 120v on a primary cause the secondary voltages to rise nearly 100v? Maybe....but that seems a bit much. AND, we're talking about 100v difference on the 12AX7 plates.

And yes, I subbed in all new tubes. No change.

Hmm...

[dehughes]

You know, I wonder if because of this being a "transitional" 6G2 (parts of it are actually right out of the AA964 schematic...), it's running higher preamp voltages because it has the filter values and bias setup of the Blackface but with the dropping resistors of the Brown? Still though, the PT is the same on both models (according to the schematic), AND the bias voltage of mine is spot on for the 6G2 schematic...

[breakfastbuddy]

change the 0.02 cap after the first stage to a 0.01 cap or change the 0.1 caps to ,047 before the output tubes ,

[dehughes]

change the 0.02 cap after the first stage to a 0.01 cap or change the 0.1 caps to ,047 before the output tubes ,

Yeah, that would change the tone, for sure, but I want to keep it stock and make sure it's "correct". If there's nothing wrong with it, then I'll assume I just don't like the tone, but I think there's more to it.

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?

[tubeswell]

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

[dehughes]

BINGO! Maybe they were distracted at the factory, but yeah, 250k ain't right in there. 1M = sweetness...man, this amp is killer. I'm going to put the original speaker back in (currently has a Weber 10A125...original is an old Oxford) and see how it was "meant to sound".

Thanks guys!

However, I'm still up in the air about 220v on the V1 plates. Again, according to the 6G2 spec sheet, the PT putting out 340v on the secondary and the bias being -35v is perfect, but really? 220v on the V1 plates? That's way over the sheet's 135v measurement....

But then, this IS a transitional Princeton, so who knows.

[physikmatze]

Hi,

could you please post the spec sheet of the 6G2? The original schematic shows no AC voltages for the HT winding.
The early 60s Princeton 6G2 used the 125P1A power tranny, which was later replaced with the more powerful 125P1B also used in the Blackface Princeton (AA964).

I think your transitional model uses the Blackface tranny but the Brownface circuit design. So your voltages are spot on according to the AA964 schematic.


Mathias