Metropoulos Forum

Hi all, I have a '77 Marshall master volume superlead 100 with EH 6550s in it. It is actually a European model with the bias feed the original spec 220k resistors intended for EL-34s. However, I got a new quad of JJ EL-34s, installed them, but they won't conduct more than about 20mA per side with the bias pot fully cranked!?!? Are the conduction points on new JJ EL-34s that much different than EH 6550s from the early 2000s?? The board is all original and I really don't want to have to lift the board and start swapping resistors to change the -v point of the bias circuit just for these tubes. It's both a hassle and a risk to the solder traces, not to mention having to possibly swap back if I change my tube brand/type. Anyone run into this or note a huge difference in required -v bias voltage to make these newer tubes conduct in a proper range or am I missing something here?

It's not unusual to come across a pair of tubes with a bias point that falls outside of the average. You could try a different pair that draws a slightly higher idle current or, find the resistor that is inline with the bias pot and reduce it's value a bit. I have had to swap my 47k in my 50 watter down to a 33k to bias up certain sets of tubes properly.

If you purchased your tubes from a seller that rates their bias points, you could return those and ask for a set that idles a bit hotter.

It’s a common mode to change the pot to a 50k but you have to be careful doing this as any shunt impedance to the grid of the triode adds to the maximum shunt impedance allowed by tube datasheets. Just keep this in mind if you’re changing these resistors or making your own design. Make the total shunt impedance as small as possible to give the performance you need. The grid bias supply doesn’t draw much current ~3 to 4mA max.

Thanks for the replies. Well, it seems silly to me to have to start changing the bias supply now when I swap tubes. I mean, owners (myself included) of this amp have been able to put in new EL-34s since 1977 without doing so, and the pot is there for a reason. The last ones I was able to bias up normally were Winged =C= (used to be Svetlana). Now that JJ are one of the only robust EL-34s around, looks like I'm forced to modify an otherwise 100% stock amp to work with them.

The resistor after the bias pot is stock and a value of 56K. Some have referred to this resistor as the "range resistor", but I'm not sure if this is correctly named. I also see a typical value used for the range resistor on factory EL-34 JMPs of 47K. I found that if I piggybacked a 100K in parallel with the existing 56K after the bias pot, I got in the correct voltage range to bias the JJ EL-34s. This new value according to my calculations makes the resistor value for this position 35.897k ohms. So would that keep the total shunt impedance small enough?

Thanks again!

Yes that will also work!

Cheers,
-Matt