Tired old Marshall JCM900 4100 problem

[jmaf]

drat!
My excuse is that I am home with a chest infection - I don't even smoke!
NOT 470K, or even 470 ohms - got the number wrong, not the power - they are 1K - sorry, typo in haste. Hence statement - "I can't trust anything till I put 4 same valves in, and I'll try to run with the 1k's and 5881's."
Anyway, 79mA = -46V, 467 plate.
Remember I am mixing the 2nd lot 5881's (2) with new 6L6's (2) 'cos I aint got a matched quad till tomorrow- but they will be 5881's!
Dave

I have a hunch your matched quad's gonna work a dandy

The 1k resistor is ok for 5881's. What happens is the screen sags more and the tone is more compressed. If you feel it lacks punch, then you'll need to reduce it to 470 ohms.

Get well soon from the chest infection.

[Dave Mac Farlane]

I must be tired.
-51V is the lowest I can get - 512V plate.
I already have 56K, think I may change it for 47K, see how it does - I am so out of practice with audio - I'm gonna have to dig out all my old college books.
Dave.

[Dave Mac Farlane]

Thank you. You are most encouraging - I shall keep you posted.
This forum is tops.
Dave.

[Dave Mac Farlane]

OK, sovtek 5881's arrived.
Place in holders, knowing I have plate V - 510V no load, max available -52V on 5.
First Valve - Mains transformer side of amp - 35.9 mA. Looks good.
Reconnect - move on to next in line - wow! 45.7 mA.
Next, same again, but 47 mA.
Hmmm, any takers?
I'm going to alter the divider circuit, but I don't have a 47K pot - any suggestions on a configuration to get me - 55 up volts
Dave

[Dave Mac Farlane]

This isn't interim if it continues to work - I haven't thought it all the way through, but I strapped in a 100K in place of a 56K on the top of the bias pot - I've now got a healthy -68V with no tubes in.
Need a second brain on this one, for mine feels as if it's malfunctioning - stress of having no amp all week - my Peavey envoy is no substitute.
Help guys....... I need to put the tubes in now - am I doing anything stupidly dastardly wrong (shakes and sweats, reaches for the bottle, willing the screen to offer him a reply...........

[Dave Mac Farlane]

Told you I was getting desperate - couldn't get the bias current up to 20 mA - Hee hee!
Replaced that with three 22K in series - got good vibes so far.
Dave.

[g1]

First Valve - Mains transformer side of amp - 35.9 mA. Looks good.
Reconnect - move on to next in line - wow! 45.7 mA.
Next, same again, but 47 mA.

Not sure here whether you are trying one tube in different sockets or you have all the tubes installed and are checking each tube?
A 10mA difference between tubes in a matched set is not acceptable. Please refer to Enzo's post #4. Try all your new tubes in the same socket to see if they are unmatched, then try one of the tubes through the 4 sockets to see if there is a mismatch in the circuit.

[Dave Mac Farlane]

Many thanks, G one, I checked my notes - 39 and 45 were the differences - and what you say is wisdom indeed. checked 'em just like you said - perhaps I didn't hang about too long at that current!
I have altered the resistor above the bias pot to 66K (yeah, three 22k's in series - it was all I had here) and I am now running all new 5881's at 34.9 to 35.1 as it heats up (checked all four) - just had a play and am delighted.
Was concerned I might have gone to the wrong part of the voltage divider, but it seems to be working without too much heat.
This now means I have a spare set.
Thanks to you and all of our friends for your help and support.
Dave

[Dave Mac Farlane]

Better than that guys, its going and sounding a treat!
Dave.