6G2 Brown Princeton Overvoltage Issues

[jjasilli]

Finally gotten to restoring my Brown Princeton.  Both trannies date to the 46th week 1961; so the amp looks to have been manufactured in late 1961 or early 1962.


Your thoughts are appreciated.  I guess I can live with the power tube voltages. Do you agree to install new dropping R's to drop the small bottle voltages?   My voltages are way high:


*  6V6 Plates & Screens, loaded: 420 VDC; schematic calls for 315VDC
*  Bias voltage -40VDC; schematic calls for -35VDC. 
*  Preamp tube plates unloaded are each about 416VDC.  Schematic calls for 135 -to- 230VDC depending on which plate.  So, I'm reluctant to install the preamp tubes at these voltages.

Do you think it's OK in this vintage amp to install 1 Ohm R's to calculate plate diss?


Pertinent info:
* Wall supply voltage = measures 120VAC from my variac.
*  3-prong power cord installed.  Polarity SW & death cap are entirely bypassed.
* The cap can is replaced with 30-30-30.  (Original cap can was 20-20-20 which deviates from the schematic.)
* Bias cap is replaced.  Other bias supply components measure to spec.
* Both cathode bypass caps are replaced in the small bottle tubes.


Thanks for your input!

[HotBluePlates]

Finally gotten to restoring my Brown Princeton.  Both trannies date to the 46th week 1961; so the amp looks to have been manufactured in late 1961 or early 1962.
...
Pertinent info:
* Wall supply voltage = measures 120VAC from my variac.
...

Set your variac for 117vac, and repeat your measurements (especially checking heater voltage).

My testing of a couple brown Deluxes indicated 117vac was the correct outlet voltage for these amps.

... I guess I can live with the power tube voltages. Do you agree to install new dropping R's to drop the small bottle voltages?   My voltages are way high:


*  6V6 Plates & Screens, loaded: 420 VDC; schematic calls for 315VDC
...
*  Preamp tube plates unloaded are each about 416VDC.  Schematic calls for 135 -to- 230VDC depending on which plate.  So, I'm reluctant to install the preamp tubes at these voltages.

From a number of vintage Fender (and Vox) amps, I've come to believe, "The Amps are Right, the Schematics are Dubious."

Fender installed 3 different power transformers in the brown Deluxe amps.  The 1962 Deluxe I've kept has the "low voltage" 125P17A power transformer, and puts 405v volts on the 6V6 plates with 177vac from the wall outlet.  That's well above the schematic's voltages.  Imagine what I'd think if I had the "high-voltage" 125P2A that puts 448v on the 6V6 plates!


I strongly suspect your amp is fine, and shouldn't be tinkered.  We don't know when we're looking at schematics & building clones, but the real vintage amps don't necessarily conform to the schematic.

-------------------

Doing some sleuthing, the 6G2 Princeton appears to continue the circuit that had been the 5F11 Vibrolux during the tweed years.

The 5F11 Vibrolux used the Triad 6069-B power transformer.  According to some photos I've saved from online listings, that PT seemed to be present in the "tweed 5F11 Vibrolux" made as late as 1961.


Meanwhile, the 6G2 Princeton had the new Schumacher 125P1A (the later Princeton Reverb would get the 125P1B power transformer).
While the schematics show slightly lower voltage for the 6G2 Princeton, that seems like a draftsman's error, or a bad guess about the differences between the two transformers.  And the 6G2 Princeton schematic shows larger filter caps that would tend to make DC Volts higher, even if the PT output AC Volts are the same.


Reiterating, I think the schematic gave you wrong expectations.  Trust the amp.

[jjasilli]

Thanks, Hotblue!  I just bought a pair of bias tester kits from Doug.  I don't want to alter this vintage amp too much. 


117VAC drops the 6V6 plates & screens to 411VDC, loaded.  Small bottle plate pins now at 405VDC, with power tubes in, but small bottles unloaded. 


Do you think I should drop the small bottle plate voltages to spec?

[AlNewman]

Is it possible to calculate plate dissipation across the OT windings?

[HotBluePlates]

Do you think I should drop the small bottle plate voltages to spec?

What "spec"?

What is written on the schematic, but doesn't derive from the real amps?


I would personally leave the amp stock, and accept that the real-world voltage of a 1961-1962 6G2 Princeton never matched the numbers on the schematic.  Especially the way the 6V6 plate voltage has settled down to basically the same as found on the lower-voltage 6G3 Deluxe amps, I don't think there's a voltage-issue.

[HotBluePlates]

Is it possible to calculate plate dissipation across the OT windings?

Yes, though the winding resistance is likely to be a little higher when the amp is on & the transformer is warm.

These days I'd rather spend the money on some Eurotubes Pro One bias probes.  Yes they are expensive, but they measure just the plate current & the plate-to-cathode voltage.  If you add the Watts option they will also calculate plate dissipation.

Yes, they duplicate meters others may already have, but I've found the convenience gets them used more often than would have happened if I had to pull the chassis every time.

[jjasilli]

It's alive!


It's passing good guitar signal into my test speaker cab & the harmonic tremolo works.  Will checkout tone tomorrow when no one else is around.     


Still need to measure plate diss, and the blue signal caps for mF value & DC leakage; and verify the original spkr works.

[jjasilli]

Assembled a pair of Doug's Bias Testers.  Only getting 6mA for ea 6V6 of Cathode Current.  A little more work to be done there.


All the vintage blue signal caps test good for value & leakage.

[jjasilli]

Amp is restored, re-biased & ready for eBay.  Here's some photo's.