64 Princeton Amp Power Tube Dissipation

[pdf64]

31mA plate current seems rather high.

[Pedro Vecino]

In my opinion 25mA and 18mA is a more suitable setting. Although I would particularly look for a closer approximation between the tubes.
If dŽont have an adjustable bias circuit you can replace the 27K resistor that is usually connected above the electrolytic by a line of 15K (fixed) y 25K (adjustable) resistors in series.

[g1]

Resistance Primary-Tube3: 166.9
Resistance Primary-Tube4: 166.9

I'd double check those resistance readings you took of the OT that you did your calculations with. Exact same on both sides seems very suspicious.
Maybe try it cold with the power tubes removed.

[caesparza]

Checked again today without tubes. 157.9 and 158.

[caesparza]

Yes I'm thinking on putting a variable BIAS control as I have on my Silverface Princeton Reverb and as you show on the picture.

Thanks.

[Chuck H]

Just to be clear... You originally had higher current on 6v6 (2). Then after a tube swap you still had higher current on 6v6 (2). So I asked you to swap the tubes and see if the higher current followed the tube or stayed with the socket. You said you did that and it followed the tube. Does that mean that the higher current is now on 6v6 (1)? Or is the current on 6v6 (2) still higher and you interpreted "follow the tube" as 6v6 (1) and 6v6 (2).

Which tube socket has the higher current at this time?

[caesparza]

Hi Chuck, the current stays with the tube. If I put the higher current on socket one, then there is the higher current, if I swap it to the other socket, then the higher reading goes to that socket.

[Chuck H]

Ok. I was following up on what g1 was saying about the strangely symmetrical OT primary DCR. That is very unusual and, strange as it seems, the symmetry might be what indicates an imbalance in a case like that. But... I did some research and found other references to BF Princeton OT's that also report the same symmetry. There was one reference to the primary halves being wound with different gauge wire to balance the DCR. Though I can't be sure that wasn't a DIY project. Still, that along with the current following the tube is good info to have clarity on at this point.?. I think.

[caesparza]

Ok, checked my box of tubes and found some 6V6GTs.

After trying some of them, the best results I get are with a set that generate the following values:

Plate Voltage Tube3: 418
Plate Voltage Tube4: 418

Voltage Drop Tube3: -3.12
Voltage Drop Tube4: -3.66

Plate Current Tube3: 19.76mA Dissipation: 8.25W (68%)
Plate Current Tube4: 23.16mA Dissipation: 9.68W (80%)

Assuming a Max dissipation of 12W.

So my guess is that I need to match the correct tubes to balance the BIAS.

Right now I'll use a variable resistor to determine the correct value on the BIAS circuit to lower a little the current on Tube4. Amp is sounding good with no noises or hiss. I also made the negative feedback loop switcheable, the effect is subtle but I think it would do wonders when applying drive pedals to the amp.

A question, can I mix 6V6GT with a 6V6GTA?

[Leo_Gnardo]

Ok, checked my box of tubes and found some 6V6GTs.

After trying some of them, the best results I get are with a set that generate the following values:

Plate Voltage Tube3: 418
Plate Voltage Tube4: 418

Voltage Drop Tube3: -3.12
Voltage Drop Tube4: -3.66

Plate Current Tube3: 19.76mA Dissipation: 8.25W (68%)
Plate Current Tube4: 23.16mA Dissipation: 9.68W (80%)

Assuming a Max dissipation of 12W.

So my guess is that I need to match the correct tubes to balance the BIAS.

Right now I'll use a variable resistor to determine the correct value on the BIAS circuit to lower a little the current on Tube4. Amp is sounding good with no noises or hiss. I also made the negative feedback loop switcheable, the effect is subtle but I think it would do wonders when applying drive pedals to the amp.

A question, can I mix 6V6GT with a 6V6GTA?

That pair of tubes, close enough already. As Enzo reminds us constantly, these are guitar amps not precision lab gear. Sure it's nice to have an exact match, but when you're dealing with a random batch of tubes "close is good enough."

One of the nice things with closely matched output tube pairs is cancellation of power supply hum. With unbalanced tubes, there's a hum level that will not go away. If the hum from your amp is not objectionable, go with what you got. Granted some people need minimum/no hum, they play generally jazz in quiet environments and want background noise to fade to black. At the other end of the performance spectrum, a little bit of hum quickly fades far into the background once the drummer starts smackin' that snare. Twang away and smile!

I'm sure you can mix GT with GTA no problem.

[caesparza]

Yes Leo.

I gave it a second thought and decided to leave the BIAS circuit untouched. The amp is sounding good and I it's not going to see a lot of playing time, so I think I can get away with the current mismatch.

Thanks a lot to everybody for the good information and kind help.