UL power amp

[Hallingamp]

Hi all.

I have tried googling for an answer on this, but the sites I found on ultra linear amps suggests my voltage-readings are so totally way off I thaught I'd better check with you guys.

I'm working on my fourth guitar amp and have decided to make the power amp on this one ultra linear. (First time for everything.)

Long story short;
2x6L6, B+ is 510V, cathode biased with 820ohms/47mF, comon cathode resistor, no screen grid resistors, 12AX7 driver (typical Marshall 50W), presence and resonance controls from the 8ohm tap (both works, but not as much as expected).

I've hooked the whole power amp up, and it works. It sounds good too, using a seperate preamp. Very warm and deep sounding.

My problems/worries:

I would expect more power from this setup. I have driven it to distortion, and it sounds more like 10-15W than 40-50W. And I am not really comfortable with the voltage and current readings.

My voltage and current(shunt) readings on the power tubes:

Anode 1: 504V, 31,5mA
Screen grid 1: 506V, 31,2mA

Anode 2: 503V, 32,4mA
Screen grid 2: 506V, 32,2mA

Cathode voltage is 54,2V.

Does this look right?

Shouldn't the plates have a higher voltage then the screen grids? Should I add screen grid resistors?

I'm using Italian Novarria iron for both the PT(360V-0-360V) and OT(4k primary), and though my italian is questionable I'm pretty sure "Grigio: A2" translates to Grey: Anode 2, and "Bianco: G1" must be White: Grid 1. I must have checked ten times I have hooked it all up right, and I have tried two sets of known-to-be-good tubes with comparable results.

I tried to switch phase from the PI, and the presence and resonance controls seemed to totally stop working, so I switched back. I don't get any feedback squeel or howl of any kind in either position.

I'm sorry for the long rant, but I'm trying to give you as much information as possible. Hope someone can give me some insight on this.

TIA

Jan

[Enzo]

Without screen resistors, I don't know how you measured the current there. You can't shunt the center tap to the screen tap, that includes the plate current passing through .

What color are the wires? Usually if the plate lead is blue, its screen tap will be blue with a stripe, same for the brown one - brown with a stripe for the screen.

Transformer windings have resistance. SInce the UL taps are inboard from the plate taps, they will have lower resustance to the center tap than the plate leads. Use your ohm meter to sort it out.

Your cathode voltage and the measured plate current agree and seem reasonable.

How much signal is getting to the control grids of the power tubes?

Disconnect the feedback line for now, any help?

You have driven it to distortion, but we don't know that that is power tube distortion do we? You might have low gain somewhere and are overdriving things to get more out. Your phase splitter could be holding you up for example.

[Hallingamp]

Thanks Enzo.

Of course you're right about my screen grid current readings. And I really should have thaught of measuring the OT pri myself. Oh well, it was around midnight here in Norway when I posted.

I checked everything you mentioned here, and the driver tube was responsible for the volume loss. Kinda embarrassing. Still, nice to be sure everything is hooked up right.

But those plate/grid volts still puzzle me. From what I've read the screen grids should have a slightly lower voltage than the plates. Even with this design. Am I right? I feel my plate/grid voltage readings should be "the other way around". Or do I just worry too much?

Jan

[DirtyGeorge]

Your voltage readings are okay. As Enzo pointed out the resistance from center tap to the screen taps is less than the resistance center tap to plate. So when idle, B+ drops less at the screens than at the plates.

[Hallingamp]

OK.

Thank you.

Jan