London City DEA70 mkIV

[dahond]

I have a LC mkiV 50w.
It's kind of a Marshall Super Bass clone from '70.
Schematic here : http://www.drtube.com/schematics/londoncity/dea70.gif

There seems to be a problem with bias.
If i set one tube to 35mA the other reads 27mA.
the tubes are matched, if I change them, the problem stays with the socket, so I changed the sockets. No result.
I changed the bias resistors, I changed the couple capacitors. nothing changes, I still have an unbalanced bias.

I'm running out of ideas.

[Albert Kreuzer]

Hi dahond,

maybe you just have an unbalanced OT. Have you measured the DC resistance from OT center to pin 3 of each EL34?
How does it sound? If everything else is OK, I'd just leave it alone at a safe bias setting or find a set of "accurately mismatched" tubes.

Cheers,
Albert
www.albertkreuzer.com

[dahond]

OT

from OT to pin 3 i have 1,7V on one side and 1V on the other.

[Albert Kreuzer]

OK, so turn the amp off, drain (discharge) the filter caps and measure DC resistance (ohms) from the OT center to pin 3 of each EL34 (exactly where you measured those volts).
If one side is 1.7x the other, you have a seriously imbalanced OT. Maybe one side has shorted turns (hopefully not) or it just was made this way.
To get a balanced power stage with this OT you will need separate adjustable bias pots.

Hope this helps,
Albert

[dahond]

double bias pot

So I can simply ad one bias pot in parallel with the other and match unmatched tubes?
I do not have to ad a second bias circuit?

That would be great!

[Albert Kreuzer]

Well, it's almost that simple.
You'll have to use two 50k (or 47k) trimpots instead of the one 22k. Then the wiper of each trimpot goes to one of the 470k resistors (of course you have to separate them at their junction). Yes, that's about it.

Cheers,
Albert

[dahond]

double bias pot

I doubled bias pots, everything works fine.
Just to be clear,
Can I use an old philips with a new JJ?

[Albert Kreuzer]

Yes.

Cheers,
Albert