Ampeg VL-502 EL34 bias voltage

[The Wease]

Hi, I'm new to the forum.

Working on a 91 VL-502. Just replaced all the electrolytics, have done the Heater Mod, have reseated all the other Molexes (Boy! aren't they crap!).
The Ampeg schematics leave me yearning for more Info as so does Lee Jackson's one "Master Series" series video where he is servicing a VL-1002,
biasing that at -42vdc for a quad of EL34s.
Does anyone out there know what it should be for the VL-502 with just a pair EL34s? The range of adjustment on it seems to only be -32vdc to -41vdc.

Also... are these amps just inherently hummy? With all the controls set to zero there is still a low cycle background hum that might be from the PT.
I can make it disappear at 117vac with my variac, but when I dial it back up to 124vac (typical of today's supply) it gradually increases.
Any thoughts?

Thanks

[jmaf]

First, don't set the bias by voltage.

But to answer your question unless you've changed anything else the bias voltage is the same for 2 or 4 tubes, it doesn't vary. If it was set correctly for 4 tubes, it should remain correct for 2. You need to compensate the output impedance though, use twice the speaker impedance for half the tubes.

[The Wease]

Thanks jmaf,

Electrolytics and heater mod (as per Lee, removing hum balance resistor and moving CT to ground) is all I've done.
This amp is a stock 2 tube VL502, never had 4 tubes, and don't know if the bias was set correctly. Not my amp.
I can tell you that there is 500vdc on the Plates, and my Fluke is reading 25ma current on the OT with a 16 ohm load, with -40vdc on the Grid.

[g1]

Can you be more specific about how you are doing that current measurement? In series with the CT of the OT ?
If so, that value will be for 2 tubes.

[The Wease]

Hi G1, I believe they may call it shunted. Meter is set to 200mA dc, one lead clipped to CT and other to either Primary on Pin 3, the plate. I'm reading 24.9mA.
So... are you saying the current draw is half or double?

[jmaf]

You want to see somewhere around 30 to 32mA for each tube.

[g1]

No, if you are using shunt method, then the reading is for one tube.
So, .025 amps times 500V at the plates gives 12.5 watts idle dissipation. For a 25W rated EL34 you are at 50%.
Some would call that biased cool, but I prefer the extra life and reliability of cool bias. If you don't hear any objectionable crossover distortion, I would leave it. If you want to try warmer, you could raise it up, maybe try 60%. That would be about 30mA with 500V on the plates.

[The Wease]

That's what I was thinking! I guess it's time to start checking some resistors?

[The Wease]

I'm with you on that G1! That was what I was coming up with as well, I would like to get it up a bit more.

[g1]

I think your adjustment pot should have enough range. Less negative voltage will increase the idle current. So you will be moving from -40V toward -35V at the grids while monitoring the current.

[The Wease]

Thanks, I'll see what we can do with it tomorrow and let you know how we make out.
If that adjustment pot doesn't have enough range for some reason, I'm thinking that maybe a resistor has drifted out of tolerance?
I think we all know what a fresh new day can do for us sometimes!

[jmaf]

Thanks, I'll see what we can do with it tomorrow and let you know how we make out.
If that adjustment pot doesn't have enough range for some reason, I'm thinking that maybe a resistor has drifted out of tolerance?
I think we all know what a fresh new day can do for us sometimes!

The pot has enough range. EL34's are nervous tubes, their current varies a lot with little voltage change on the grid.

[The Wease]

Thanks g1 & jamf. Bias is adjusted to 32mA, amp sounds good. With 492vdc on the plates, coming in at just about 63% dissipation.
There was enough on the adjustment pot! Thanks Guys!
Now... if I can only trace down that Hum! Cheers!

[jmaf]

Now... if I can only trace down that Hum! Cheers!

Since the hum is still there with the volumes set to zero, it might be mismatched power tubes. First thing I'd do is remove the power tubes and use a signal follower to see if the hum is arriving at pin 5 of the EL34's. If it is, it's coming from the phase inverter region. If not, then there's a good chance the EL34's are mismatched and letting hum through.