5F2A gain questions

[Groover]

What should my target figures be for the 6V6 cathode current and screen voltage with the plate voltage at 394 Volts?

[Groover]

Changed Rk to 550 ohms, B+ at plate is now 398v, at screen 382v, and at cathode 23v.
Using the Weber bias calculator it appears that I have 40ma plate current and 15 watts plate dissipation.

Is this reasonable?

FWIW I think the amp is starting to sound good.

[Bruce / Mission Amps]

What should my target figures be for the 6V6 cathode current and screen voltage with the plate voltage at 394 Volts?

12-14 watts / 394 v = 28ma to +33ma.
But that's if your B+ rail can stay at 394vdc and the 394v was measured from plate to cathode.... I bet it wasn't.

So, 27ma to 31ma

[Groover]

No, the 398 volts is the B+ to ground. subtract the voltage at the cathode of 23v and you are left with 375 volts plate to cathode.

See my note about the figures using the Weber bias calculator. It appears I am running at 15w plate dissipation? Is that in the safe range?

[tubeswell]

No, the 398 volts is the B+ to ground. subtract the voltage at the cathode of 23v and you are left with 375 volts plate to cathode.

See my note about the figures using the Weber bias calculator. It appears I am running at 15w plate dissipation? Is that in the safe range?

IMHO that might be good for 5881s or even okay for 6L6GCs, but is a bit too hot for 6V6s. However the JJ6V6S might last longer than other 6V6s in that situation. Can you get the plate voltage down to around 360 (with zeners)?

[Bruce / Mission Amps]

No, the 398 volts is the B+ to ground. subtract the voltage at the cathode of 23v and you are left with 375 volts plate to cathode.

See my note about the figures using the Weber bias calculator. It appears I am running at 15w plate dissipation? Is that in the safe range?

Guess you didn't notice my 27ma to 31ma numbers.... my message seems to have been cut short from my thoughts.

.027a x 375 = 10.125 watts
.031a x 375 = 11.600 watts
You can keep doing this if you want,
375v/12 watts, 375v/13 watts, 375v/14watts... etc etc

[Groover]

So I may put a 6L6 in it, and I have some data about setting it up for that tube, but suppose I wanted to stick with a 6V6 and 5Y3 in this amp. What should I do to calm it down to a safer range?

Instinct tells me to add resistance, either in the power supply to lower the B+, or in the cathode circuit to bias it colder?

[tubeswell]

So I may put a 6L6 in it, and I have some data about setting it up for that tube, but suppose I wanted to stick with a 6V6 and 5Y3 in this amp. What should I do to calm it down to a safer range?

Instinct tells me to add resistance, either in the power supply to lower the B+, or in the cathode circuit to bias it colder?

A 6L6 (or 5881) will draw 900mA on the heater winding (vs 450mA for a 6V6). If your PT can supply enough juice, try it and see.

[Groover]

I have made some more changes in my little 5F2a experimental amp.

1) swapped in SE 5k:8ohm OT from AES
2) reduced screen dropping resistor to 2k
3) 1st stage is now partially bypassed with 11mFd cap in series with 1k5 resistor
4) added 2nd volume control after 2nd stage

unchanged from before: switchable bypass cap on 2nd stage, variable NFB

I like the flexibility that I get with the two volume controls. What is puzzling me now is whether to have the tone control wired to the first or 2nd volume control. So here is the question - do I want treble boost more at clean or at overdriven settings? (because of the interactivity of the tone & volume control the treble boost function doesn't work at high settings of the volume control.) Not that anyone else can really answer for me.

I suppose I could add a dpdt switch.... Did I hear someone say "Drill Baby Drill"?

[Groover]

I still think I may be getting too much plate voltage, quiescent current, and dissipation in the 6V6 in this amp.

measurements at idle: plate to gnd - 385 V, screen to gnd - 383 V, cathode to gnd -23 V

according to the Weber Bias calculator tool I am running at ~ 40 mA plate current and ~ 14 watts dissapation.

What is the best way to bring this tube into safe operating range? How about the easiest way? How about the cheapest way?

Note: This amp has a Mercury Magnetics Tone Clone VC PT. It may be called tone clone, but I think it puts out more volt-amps than a stock Fender. I think that is why I have this excess dissipation problem.

[tubeswell]

I still think I may be getting too much plate voltage, quiescent current, and dissipation in the 6V6 in this amp.

measurements at idle: plate to gnd - 385 V, screen to gnd - 383 V, cathode to gnd -23 V

according to the Weber Bias calculator tool I am running at ~ 40 mA plate current and ~ 14 watts dissapation.

What is the best way to bring this tube into safe operating range? How about the easiest way? How about the cheapest way?

Note: This amp has a Mercury Magnetics Tone Clone VC PT. It may be called tone clone, but I think it puts out more volt-amps than a stock Fender. I think that is why I have this excess dissipation problem.

If the tubes aren't red-plating and it sounds good, then why worry?

But if you want then either;

1) Up the cathode resistor into the next biggest available size; or

2) Put 2 x 12V 5W zeners in series between the High Tension centre tap and the ground return. This should take the B+ to around 360, and your plate current will drop accordingly. The cathodes (banded ends) of the zeners point towards the ground return.