6g3 preamp voltages needed.. the real ones

[Jasonpatrick]

I use a 5V4 in my clone and it gets it "in the ballpark".

Which ballpark? Seems to be a few haha

 

[misterdontmove]

Which ballpark? Seems to be a few haha

Between 60 and 70% dissipation and voltage near or below 400v.

 

[slider313]

That is surprising.

You have your power tube plate voltage well above the schematic value, 365V, but the pre amp voltage match the schematic.

I know fender schematics need some interpreting, but that is hard to make sense of.

Nothing to worry about however.

The negative bias voltage is also listed as -26. This may be a mistake on their part, as at that setting, the power tubes would run very, very hot, which would also drop the plate voltage.

 

[Pete Farrington]

I’m on the fence about upping the last dropping resistor from 27k to 47k since I’m missing a tube or even maybe looking at the 10k and making it a 22k but you let me know what you all think

Another option would be to replicate the current draw of the missing normal channel input stage by adding a resistor of about 270k to 330k between B+4 and ground.
That should keep the time constants and voltages in the HT supply about the same as stock.

 

[Jasonpatrick]

Another option would be to replicate the current draw of the missing normal channel input stage by adding a resistor of about 270k to 330k between B+4 and ground.
That should keep the time constants in the HT supply about the same as stock.

You rock. Great idea !

 

[Jasonpatrick]

Another option would be to replicate the current draw of the missing normal channel input stage by adding a resistor of about 270k to 330k between B+4 and ground.
That should keep the time constants and voltages in the HT supply about the same as stock.

So AnB w/270k and without. It dropped the voltages a bit. V1 is down to 167 and 184 pin 1-6. bias also dropped on both, now they are 1.4 and 1.36. B+4 is now 286vdc. PI voltage changed a little, now 256v and 251v. Everything else dropped a volt or less. Sounds great. Maybe a tad better? lol I doubt it. Probably all in my head. Thanks for the suggestion. Learned something new today.

 

[Esquier]

My G63 clone uses the Weber W130something "Deluxe power transformer" of Chinese origin. I was pleased that when I chose the lower 275volt secondaries and genuine British GZ34 I ended up with 375VDC of B+. But if yours is too high, that's where a 5V4 would come in handy

 

[2L man]

My G63 clone uses the Weber W130something "Deluxe power transformer" of Chinese origin. I was pleased that when I chose the lower 275volt secondaries and genuine British GZ34 I ended up with 375VDC of B+. But if yours is too high, that's where a 5V4 would come in handy

Food for thoughts There are "no volume/idle voltages" and there are playing/volume voltages and the louder amp is played the HV sag begin to effect more.

Rectifier type has significant effect. Silicon diodes make and keep voltages highest. SS diodes also do nit mind high filtering capacitance and it has significant effect. Tube rectifiers drop High Voltages a lot more and between rectifier types there are differences which some come from filter capacitor specification.

When playing is dynamic having mix of loud notes/chords and no or quiet parts (sag and bloom playing) there come nice possibility to "tune" the amplifier

Feeding 275VAC thru GZ34 should make idle HV lower than feeding 325VAC thru 5Y3 but when 6V6 push pull outputs 12W voltages should be other way.

I have lost measures and can't trust my memory but when I had install new 6V6GT or JJ6V6S to 5E3 and bench test driving it to huge distortion about 370VDC idle HV did sag almost 100V down. Most likely there were resistors in rectifier anode circuits which increase HV sag and also protect tube rectifier for high current surges.

GZ34 filament curent is about the same what 5Y3 has. GZ34 can be made to simulate 5Y3 sag installing series resistors to both anodes. Then its operating life also come much longer. Filter capacitance can be higher but it effect to HV sag speed and also recovery speed. Not much but that is good to remember if "feel" still needs something

 

[misterdontmove]

Between 60 and 70% dissipation and voltage near or below 400v.

Just rechecked out of curiosity. 390 volts at just about 20ma. That's around 60% for 14 watts, or just under 70% for 12 watts.

 

[peteb]

The negative bias voltage is also listed as -26. This may be a mistake on their part, as at that setting, the power tubes would run very, very hot, which would also drop the plate voltage.

If the plate voltage or B1 drops then B2 drops and B3 drops.

They move up or down as a set.

That’s why I said that it is hard to make sense of your power tube plate voltage being 80 volts above the schematic and your pre amp plate voltages matching the schematic.


My 6G2 follows a more regular pattern

With standard bias, all of the plate voltages are above the schematic. With a cold bias, they all moved even higher than that.

 

[slider313]

That’s why I said that it is hard to make sense of your power tube plate voltage being 80 volts above the schematic and your pre amp plate voltages matching the schematic.

I've yet to see any 6G3 that matches the plate voltage on the schematic (365V). That said, my amp used the 125P2A and using the stock GZ34 it ran the power tubes in the 448v range, biased at 20mA per tube. The pre-amp voltages were also higher.

My statement concerned me using a 5Y3 in place of the GZ34, with the 125P2A, and seeing the pre-amp voltages in the range of the schematic.

The later 6G3 used a 125P17A power transformer, which gave approximately the same voltage, with a GZ34, as my amp did with the 5Y3.

 

[peteb]

My statement concerned me using a 5Y3 in place of the GZ34, with the 125P2A, and seeing the pre-amp voltages in the range of the schematic.

Thank you for explaining.

That is the part I misunderstood.

I thought with the stock circuit you were getting +80 volts on the power tube plates and +0 volts on the pre amp plates, compared to the schematic.

 

[Jasonpatrick]

Here some voltages from a freshly serviced 1961 6g3 using the original 125p17A and a GZ34 @117vac all original everything but E caps.
B+1 415vdc
B+3 356vdc
B+4 298vdc
V1P1 190 vdc (and yeah I know)

 

[NTC]

Three other things that would be interesting:

1. Heater voltage
2. Output tube bias voltage.
3. Bias current/Pd.

 

[Jasonpatrick]

Three other things that would be interesting:

1. Heater voltage
2. Output tube bias voltage.
3. Bias current/Pd.

Ahh yeah. Since it had the original bias resistor and tubes, that would have been a good number to have

 

[peteb]

Here some voltages from a freshly serviced 1961 6g3 using the original 125p17A and a GZ34 @117vac all original everything but E caps.
B+1 415vdc
B+3 356vdc
B+4 298vdc
V1P1 190 vdc (and yeah I know)

Nice results !


This is what I see

Voltage > schematic > measured > difference

B+1, 375, 415, +11%.

B+3, 325, 356, +10%

B+4, 270, 298, +10%

V1P1, 165, 190, +15%


Those results look very good to me.