Biasing Question

[Firestorm]

So what was the problem? The other way should work too.

I wonder if he was measuring pin 6 rather than pin 3?

[martin manning]

So what was the problem? The other way should work too.

I wonder if he was measuring pin 6 rather than pin 3?

That possibility crossed my mind, and is the reason I asked for the voltage on pin 3.

[Celery_Strat]

Operator error, I believe. I think I was measuring the voltage the wrong way earlier. Now, I have a hum in the reverb that most DEFINITELY WAS NOT THERE before. It gets louder when the reverb knob is turned up. Reverb works, but there's still the hum. Sould I start another thread, or continue here?

[Firestorm]

Did you change anything or just measure the bias? Reverb cables on good and solid? Not reversed? Footswitch (if there is one) plugged in or not)?

[Celery_Strat]

all I did was bias the power tubes. All cables are just as they were. This is SUPER frustrating. This thing sounds like absolute heaven, ...until you turn the reverb up past 2. Its more of a "buzz" than a "hum" like a distant basketball halftime buzzer...

[Drumslinger]

This is just one example of what is on the market just to simplify things. I can see you want to understand the workings of the measurement but this can come in handy.

http://www.amp-head.com/product_info.ph ... id=30{8}23

[tubeswell]

I'd like to see the actual results on a given amp under all 3 methods.

Well I'd be glad to, if I could get this to work. (I feel really stupid, here.) When you say "drop", in relative to what measurement? I'm missing that part. Drop from what? With the tubes in and the amp on, I get 250 mV across pin3 and the CT.

Voltage drop = another way of saying 'amount of voltage' (in this case - the voltage dropped between the plate and the CT of the high-tension winding.

Was that 250mV measured on the AC setting on your meter? (I ask this because 250mV would be way too much, you should be looking for around 20-28mV for a 6V6, or 28-35mV for a 6L6, in fixed bias Class AB1 mode) - you need to measure the DC mV when doing this

[Celery_Strat]

Yeah, the bias issue is sorted. I placed some RCA jacks on the back plate with 1ohm resistors in line to plug into to bias from now on. It's idling at 29mA. Now I'm trying to sort out this reverb issue.

all I did was bias the power tubes. All cables are just as they were. This is SUPER frustrating. This thing sounds like absolute heaven, ...until you turn the reverb up past 2. Its more of a "buzz" than a "hum" like a distant basketball halftime buzzer...

[martin manning]

Yeah, the bias issue is sorted. I placed some RCA jacks on the back plate with 1ohm resistors in line to plug into to bias from now on. It's idling at 29mA. Now I'm trying to sort out this reverb issue.

all I did was bias the power tubes. All cables are just as they were. This is SUPER frustrating. This thing sounds like absolute heaven, ...until you turn the reverb up past 2. Its more of a "buzz" than a "hum" like a distant basketball halftime buzzer...

I don't know what your plate voltage is, but 29 mA is pretty hot, isn't it?

Re the hum, why don't you try to isolate the problem a little more: Pull the reverb driver tube. If it still hums then you know the problem is between the reverb driver's plate and the reverb pot. Next pull the reverb recover tube. If the hum stops, then you have it narrowed down to the loop and the recover stage. A hum like this is often a grounding problem, so are the reverb send/return jacks isolated from the chassis? Where is the ground for the loop? It should be grounded only once, at the cathode of the recover stage. Maybe post a layout?

[Cliff Schecht]

29 mA is great for 6V6's. I usually estimate about 35mA plate and screens per 6V6 when designing with them.

[martin manning]

29 mA is great for 6V6's. I usually estimate about 35mA plate and screens per 6V6 when designing with them.

At what plate voltage?

[Celery_Strat]

Plate voltage is 377. I've seen most guys run theirs at that setting or higher. Infact, they both have "29" written on them. (matched set from tubedepot.com) Regarding the buzz, I pulled the driver tube (the 12at7 next to the RT) Still buzzes louder the higher you turn up the reverb knob. Checked the cables, all are solidly plugged in. Removed the tank just long enough to see, (still there). Tried another footswitch, (still there). Pulled the 12aX7 (the recovery) and i have no buzz.

[martin manning]

Try a different tube for the reverb recovery?

For 14W max dissipation, 0.029*377/14=78% The screen current is in there too, so it's not quite that high, but still very warm. I'm pretty sure the "29" on the box is not a recommended plate current.

[Jana]

you didn't mix up the cables, did you?

[Celery_Strat]

Swapped out the recovery tube... Same sound...