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I wanted to take on a high gain, low watt project, and chose the AX84
Single Ended Lead amp. And, although documentation of this particular amp is not as extensive as some of the other projects, it came out great!

It has a pretty good clean sound, also.
Two gain controls for two 12AX7 preamp tubes...a clean power section
using a KT88 (I'm using a 6550, it's what I have). Master, Bass, mid, and treble. No other frills. SS rectifier section. I used a bridge with a Weber tranny.

I have changed this amp to run a 6SN7 as the power tube for about 2 watts of output. The tube is wired in parallel, with the plate and cathodes tied together. Problem is, the tube is running very cold!

So I have a 1 ohm, 10 watt resistor in linebetween the cathodes (tied together) and a 5K bias pot.
The meter is reading "3" on the 200m DC range.

This reading doesn't change if I turn the pot.

I attached the meter from ground to the cathode, and I get a reading of 12VDC. This does reflect change when I dial the pot.

So,what the heck am I looking at here?
(Not sure how to read the millivolt range on my meter)

My B+ is showing 345 volts at the plate.

If you're using the paralleled 6SN7 as a power tube, the 5K cathode bias pot is way too much resistance, I'd go with a 1K W-W pot, and experiment from there.

If you're measuring across a 1 ohm resistor, just use the "voltage" mode rather than the "current" mode, and apply ohm's law, for example, 25 mA across 1 ohm would read as 0.025V or 25 millivolts. Using the current mode adds in extra shunt resistance and will seriously skew the readings.

A somewhat better tube coice might be the 6BL7GTA, it has a bit more "heft" and lower plate impedance.

SE CAD shows a paralleled 6BL7GTA to be nearly identical to a triode-strapped EL84, and should be good for 2 to 2.5 watts, using around 400 ohms for the cathode(s) resistor, similar to an EL84's requirements. The 6BL7 has the same pinout as 6SN7, so it is an easy plug 'n' prayswap.

HTH

/ed B[/i]

Thank you.
I'm using the 6SN7 because I have quite a few to play with.
NOS GTA's and GTB's.
Also...using a 5K because I simply don't have a 1K. I bridged the
terminals with a 4.7K resistor to bring it within some range of sweep.

When I'm done, I'll measure the total resistance and substitute with
a fixed resistor.

In addition to the 6BL7GTA you could try a 6BX7GT. I built a G*S*G using three 6BL7s using three halves for each channel. The 6BX7 and 6BL7 was for the most part interchangable. I got about 6 or 7 watts out of the three halves so 2 watts per triode is doable.

Again...thank you!

But I have some NOS tubes, and as advice might go...why not design the amp around the available tubes?

I'm a newbie, trying to figure out if what I have wrought here is in a safe range. I guess so, even though I really am not measuring things properly.
I put the 1 ohm/5Watt resistor in there...but the number doesn't make sense.

What I do know is this...it sounds good for what I am trying to use it for.
It is still running cool as a a proverbial cucumber to the touch. But I would like to know how it is biased, what I am looking at...and why the MV reading looks like it is at 3MV. Is this even possible?

And...isn't the cathode supposed to be reading -10V, not positive?

Shrug...I am confused. Sounds great, but the math looks wrong.

The cathode will be positive in reklation to the "ground" buss, effectively making the grid voltage negative in relation to the cathode.

-10V G to K voltage will give you a pretty low plate / cathode current with a paralleled 6SN7, I'd SWAG that 1K or eevn less, would get you closer to ideal operating conditions.

The 6BL7 can handle a combined plate dissipation of 12W for both sections, or 10W for a single section in TV vertical deflection usage, which is serving "hard time"... , though this can be pushed somewhat higher for audio use. The 6SN7 is good for about half of this rating.

I chose 6BL7GTA only because I have a quantity of them, 6BX7 is also good, but then I only have one on my collection, and it has been used hard and long, so it is near being "comatose". These seem to be harder to find at the fleas and swaps.

Another possibility is the older metal-shell 6N7, but the pinout is different, as it has a shared cathode for both sections. 6N7 is the ancestor of 6SN7 and 6BL7.

/ed B

Update...

Shiznitz! This amp sounds awesome!
Then, something odd happened...it began to eat preamp tubes, then it decided to fry a cathode resistor on V2, then burn one up on B+2 to the output transformer.

Cause?

Best I can see is the filament pilot light was bad...it was interrupting the current for the heaters. Replaced the light, and things seem fine now.

Another oddity to stick in the mental toolbox.