New tubes, bias current at 5mv

[frusciante89]

Hi guys, I have been away from home for almost one year, and I came back a couple of days ago with a new set of tubes for my 1987 amp... I removed the preamp tubes and followed all the steps in the manual: all regular. I then installed the power tubes (Mullards EL34), but the current reading I'm getting is on the order of 4,6 mA when the pot is turned all the way counterclockwise... The tubes are getting pretty hot but I don't get why the bias is not working (it worked before, although I remember that I had to add the extra resistor to get to 35mA)... Note: I have the LarMar mod, and when I'm biasing the volumes are at zero, the controls at 5 and the LarMar gain is at 10 (LarMar off)...
These are the voltage reading I'm getting:

1 2 3 4 5 6 7 8 9
V1 202 / 1,2 2,97 ac 2,97 ac 197 / 1,2 2,97 ac
V2 176 / 0,8 2,97 ac 2,97 ac 292 / 177 2,97 ac
V3 244 / 32 2,97 ac 2,97 ac 233 / 32 2,97 ac
V4 GND 2,97 ac 455 454 -50 457 2,97 ac
V5 GND 2,97 ac 455 454 -50 457 2,97 ac

Any idea?

[frusciante89]

I also tried to put back the old preamp tubes but I still can't go past 4.5 mA...

[neikeel]

Whose 1987 is it - original or clone?

Pics of the bias circuit?

What is your plate voltage (pin 3 of outputs)?

What method ar eyou using to bias?

What is the negative voltage at the junction of the two 220k resistors where the wire comes form the last bias cap?

[danman]

Check everything that Neikeel mentioned first. I just wanted to ask if you are trying to bias a new set of power tubes that you have never used or, are they the same power tubes that were in the amp previously? If you have never tried to bias these tubes before, it may be that you got a pair that are way out of spec and will not bias properly. Do you have another amp that you can try them in?

Edit: You also mentioned the preamp tubes. Why did you remove those to bias the amp?

[ivan H]

Hi, rather than removing all preamp tubes while biasing, just remove the phase inverter tube. This prevents any noise generated in the preamp/PI from being presented to the output tubes (a signal). Cheers

[frusciante89]

Ok guys, I solved the problem, basically the multimeter for some reason was bad... I bought a new one and everything went good... However, I'm having a problem now after two weeks of using it... It sounded really amazing in this last days, however I have to use the past: yesterday I had the impression that it was sounding dull, with no personality... Today I had the same feeling, and at one point the amp started making a quite loud mixture of cracking sounds and a swish like someone blowing on a microfone... This is volume independent, I tried tapping the tubes lightly but nothing changes in the sound... What could it be?

[frusciante89]

I tried also to change all the 3 preamp tubes with the old ones but the noise reappears 30 s after I turn the amp on...

[neikeel]

A failing plate resistor or another in the B+ line will do that (but you re right a duff pre makes those noises too).

[frusciante89]

But if a pre is bad, wouldn't the noise be influenced by rolling the volume pot? In this case it isn't..

[ivan H]

Rolling back the volume will only stop the noise if its the 1st tube going bad, but like u said, u swapped them so its not a tube. The treble cap in the tone stack going bad can make swooshy type noise. Hope u get it sorted. Cheers