Probably a really simple bias question

[boxofknobs]

Hi there,
I really appreciate this forum....just saying...

I recently built a "50w" head based loosely on a hiwatt power section...well. This one to be exact. Fired it up and it worked the first time. After doing some initial calculations I realized it is only putting out about 29w with bias supposedly at "max" for the tubes.

I've installed individual bias controls on the amp and it works great. I seem to be getting proper bias voltage for the amp. I guess my question is: what is the proper bias setting for individual controls like the ones pictured here?
Used separate 1ohm resistors from pin1/8 to GND. Measuring mV across the resistor to determine bias point. right now on each tube I have 40mV with plates @500dcv. I'd like to take screen current into account when doing this calc (6mA roughly). Using JJ 6CA7's or whatever those really are.

[Enzo]

40ma current sounds fine to me, especially if you reduce it by your 6ma. if you wonder what the screen current is, just measure voltage across the screen resistor and calculate.

How are you driving it, and how are you measuring. GUys often say you have to measuer it at onset of clipping, but if you run the signal through the preamp, it may well clip before it gets to the power amp. Feed a signal right into the PI and see how hard you can drive it. And remember, the whole amp goes into feeding those power tube grids, so how much signal is reaching them. it is like the gas pedal on your car, if you don't push it all the way down, the car will not hit top speed.

[boxofknobs]

Which brings me to my next question...glad you mentioned it.

I've got 22k grid stoppers on the input grid of the power tubes. I'm also not using the 4 preamp tubes that hiwatt uses so I was going to ask if maybe I need to lower the value of the grid stoppers so it actually pushes the tubes to the proper voltage?

Also, how much is too much screen current for these tubes?

[Chuck H]

I've got 22k grid stoppers on the input grid of the power tubes. I'm also not using the 4 preamp tubes that hiwatt uses so I was going to ask if maybe I need to lower the value of the grid stoppers so it actually pushes the tubes to the proper voltage?

The grid stopper value, higher or lower (within reason) won't affect voltage drive to the grids significantly. You don't need to change those or consider them for this issue.

Also, how much is too much screen current for these tubes?

Well, the data sheet shows typical operation at 11mA at idle (seems high-ish), but not a max spec.

You mention that you aren't using the stock tubes. Are you using different model number tubes than spec'd? This could significantly alter preamp performance WRT reaching maximum output. If you like the tone now, and don't want to change to stock tube models you can always do as Enzo suggested and just inject a signal at the PI to determine if the amp is making full power. Regardless of whether or not the preamp is "under performing" (which is a misnomer if it's safe and you like the tone )

I'm a little confused by your stated bias calculation since your idle bias is at 40W.?. The current should increase and the plate voltage should decrease when the amp is conducting a signal. You're still biased AB1 (albeit a little hot) so the net wattage when conducting should be higher than idle. I think there may be a mistake or misunderstanding in your calculations.

[boxofknobs]

I've been looking at the datasheet all day and I missed that.....11mA...there it is

Geez...thanks Chuck.

I'm not using any odd tubes. 3 12ax7 tubes and 2 6ca7 tubes. thats it.

Bias is at 40mV across a 1 ohm resistor at idle with no input. I measured AC voltage across a speaker load to come up with 29w output I was talking about earlier.

I am using 12ax7 and 6CA7 tubes for this build. I'll throw the signal gen Monday and try re-measuring the output....


All suggestions appreciated though....Anything else?

[g1]

I'm a little confused by your stated bias calculation since your idle bias is at 40W.?.

From his numbers, I'm figuring 20W per tube.

How are you calculating the amp's power output?

[boxofknobs]

Thats exactly what I got.

Measured AC voltage across speaker load, ohms law.

[Chuck H]

From his numbers, I'm figuring 20W per tube.

Sane here. I meant total watts dissipated by the amp at idle. Sorry that wasn't clear.

[Chuck H]

Thats exactly what I got.

Measured AC voltage across speaker load, ohms law.

How do you know that the amp was being driven to maximum clean signal reproduction?

[boxofknobs]

How do you know that the amp was being driven to maximum clean signal reproduction?

I was actually measuring full output power while driving the amp from the front end with a signal gen with gain and volume maxed

[g1]

At what amount of signal from the generator and were you looking at output with a scope?

[boxofknobs]

At what amount of signal from the generator and were you looking at output with a scope?

About 200mV as close to A as I can get.

[g1]

Still not sure if you're measurement if full clean power out or grossly clipped. Without a scope you won't know. It's possible your clean power level is even less than the 29W due to some fault in the amp.

[Chuck H]

Enzo raised the possibility that the preamp may be distorting prior to the power tubes being driven to their full potential. This may be the case. It's probably best to confirm this though. If you like the amp with alternative tubes in it that's fine and it won't hurt a thing, but we should confirm that that's the problem rather than assume. Just so we can be sure the amp is indeed working properly otherwise. Are you using alternative tube models in the sockets, or just alternative brand tubes? If you're using alternative tube models, do you have the correct tubes on hand for the purposes of this test?

[boxofknobs]

Enzo raised the possibility that the preamp may be distorting prior to the power tubes being driven to their full potential. This may be the case. It's probably best to confirm this though. If you like the amp with alternative tubes in it that's fine and it won't hurt a thing, but we should confirm that that's the problem rather than assume. Just so we can be sure the amp is indeed working properly otherwise. Are you using alternative tube models in the sockets, or just alternative brand tubes? If you're using alternative tube models, do you have the correct tubes on hand for the purposes of this test?

I'm using 12ax7 tubes and 6CA7 tubes, all JJ. At this point I'm trying to explain that I'm about to try enzo's method on Monday. I think we are in agreement here. you are right. we should not assume.