Cathode Bias Question on 5E3

[bal704]

I'm currently working on a project that is a modified 5E3, with the preamp a bit different, but the power amp side the same as a 5E3 with the exception that I'm using a dedicated 330 Ohm/25uf combination for each Cathode of my 6V6's, rather than the 250/25uf that's shared by both.

I've got 375VDC between pins 3 and 8 on each 6V6, and 17VDC between Pin 8 and ground for each. According to the Weber Bias Calculator, that makes about 18W per tube, which seems awfully high. Also, that's about 50mA for the plate current, which seems high to me.

Do these 2 values appear to be out of range for the 6V6? The amp sounds fine BTW.

[10thTx]

I often use 330ohm/10w shared cathode resistor for push pull 6V6 tubes and 5881 tubes. This is with a cathode biased amps of course.

With respect, 10thtx

[martin manning]

Sounds too hot to me. A shared 250 ohm cathode resistor would be replaced by a pair of 500 ohm.

[Randall]

How did you come to the 330 ohm cathode resistors? You are pulling about 19 watts each, and will cook those suckas in short order. 500 - 600 ohms ea would be a better choice. The math:

E = IR
E/R = I
17v /330 ohms = 0.0515 or 51.5 mA per tube

P=I*E or
E*I=P
375v * .0515 = 19.3 watts

or

17v / 500 ohms = 0.034 = 34 mA per tube
375 * 0.034 = 12.75 watts per tube

These voltages will shift a bit and not stay stagnant as you adjust the cathode resistor, but you can see how you need higher resistances in your cathode to bring your 6V6s into safe range of 12 - 13 watts maximum dissipation.

[Firestorm]

Too hot I think. But you also lose "feedback" when you split them. It's nice to have the pairs hanging off the same ground reference.

[pdf64]

I'm a big fan of an individual cathode bias resistor for each power tube; by that, more effective self regulation seems to be achieved, resulting in poorly matched tubes settle to a good balance.
Each resistor can then be fully / partially / un bypassed, for some interesting effects.

Also the VB+ seems a bit high and will rise if the current draw is lowered; is a proper 5Y3 being used? If not, a USA made one may help.
Pete

[bal704]

I don't have any big 500 ohm resistors lying around, so I ordered some. In the meantime, I hooked both Cathodes to one of the 330 ohm/25 uf pairs. I'm getting 383 VDC plate voltage and 24VDC at pin 8. If my math is correct, that's about 13 W per tube. Also, to clarify, I did go with a SS rectifier rather than a tube.

Also, now that I've had a chance to turn my volume up a bit, I'm noticing this phenomena I've not heard before. When I hit a chord, as the notes are decaying, I hear what sounds to be a buzz in my speaker. It's hard to describe, and I don't think I can record it. Almost sounds like a rattle in my speaker cab as the chords are decaying. The amp is not sitting on cabinet, and I've tried three different cabs in 2 different rooms with 3 different guitars, so I'm pretty sure it's amp related.

Has anybody heard of this, or am I just hearing something non-amp related?

[tubeswell]

The buzz could be a vibration from something loose (chassis bolt, speaker mount etc). It could be 'cone cry'

[bal704]

It could be 'cone cry' I guess, from what I can read about it. It's happening on three different cabinets (2- 2x12's and 1-1x12).

I was more concerned about it being a trait of the amp I'm working on, and something I need to fix. Sounds like maybe not.

[martin manning]

I don't have any big 500 ohm resistors lying around, so I ordered some. In the meantime, I hooked both Cathodes to one of the 330 ohm/25 uf pairs. I'm getting 383 VDC plate voltage and 24VDC at pin 8. If my math is correct, that's about 13 W per tube. Also, to clarify, I did go with a SS rectifier rather than a tube.

Also, now that I've had a chance to turn my volume up a bit, I'm noticing this phenomena I've not heard before. When I hit a chord, as the notes are decaying, I hear what sounds to be a buzz in my speaker. It's hard to describe, and I don't think I can record it. Almost sounds like a rattle in my speaker cab as the chords are decaying. The amp is not sitting on cabinet, and I've tried three different cabs in 2 different rooms with 3 different guitars, so I'm pretty sure it's amp related.

Has anybody heard of this, or am I just hearing something non-amp related?

24V/330R/2=0.036A, times 359V = 12.9W. That is 108% of the 12W Pa max rating for a 6V6, but includes the screen current so the plate is likely under 100%. That is probably about as high as I would want to go. A shared 330R means you would need a 660R resistor on each tube; 680R is a standard value and should work.

A quick look at Fender's '57 Deluxe schematic shows they are running at 14.8W (250R shared Rk, 349V Va-k), which works, I guess. That seems a bit hot, but then again, they sell tubes, don't they?

The buzz phenomenon is not likely to be cone cry if it occurs with different cabs, and almost certainly not if these cabs don't make the noise when driven by other amps.

[bal704]

Is there anything in the amp that would cause the buzz? I realize it could be any number of external (non-amp related) issues, but if it's something in the amp, I'd like to fix it.

[pdf64]

See http://www.valvewizard.co.uk/cathodyne.html it may be the cathodyne overdriving unpleasantly, if so a big grid stopper eg 470k could help.
Pete

[bal704]

I hooked up a 470k resistor to pin 7 of the PI, and it seemed to help a bit. Definitely a step in the right direction.

Any other suggestions?

[Firestorm]

What's the layout look like? Original 5E3s have a notorious flaw in proximity of input to output.