Metropoulos Forum

hey folks,
i recently built at 2550 style amp and i noticed that the plate voltages on either side of the phase inverter are very different. by about 100 volts! I'm not in front of my readings now to give you exact numbers but they are pretty far off. I know the resistors are slightly different but this seems wrong. I've tried a new PI tube and no change. The other voltages seem fine.
The amp sounds OK but it seems like it should be much louder. It sounds quieter than my other 50 watters and maybe quieter than my 18 watt. Also a bit buzzy in the distortion department but that me be break in or just the nature of the amp. It certainly doesn't sound bad, but I'm wondering if those weird voltages may be affecting the volume and distortion quality (I'm sure they are). Any ideas and/ or solutions. Thanks in advance!

Matt

You really shouldn't have more than a 10v difference between the PI plates.

So either the wiring is wrong or the tubes you have tried are not very well balanced.

A trick used on some amps is to put a trimmer pot between the plate resistors.

Or you can adjust the plate resistor values to get it closer.
But ideally you do want some difference for best tone.

Then you adjust the trimmer for maximum sustain and harmonics which is generally about 5-7 volts difference.
You can use a 5K-25K trimmer. I like a 10K on the amp I have.
This is how I adjust it, I turn the volume up quite loud then lightly brush the strings on a single coil guitar.
I like using the neck pickup on a strat.

I adjust the trimmer waiting a few seconds each time because it takes a little time (caps) to hear the change.

When I get it at the right point, it will feedback into a sweet harmonic and sustain like crazy.

Thanks for the great and quick post! Would this have any affect on overall volume of the amp?

matta wrote:Thanks for the great and quick post! Would this have any affect on overall volume of the amp?

Yes, the more balanced the PI is, the louder it will seem, at the expense of some good harmonic distortion, which is very subjective anyway.

100V difference with two different PI tubes is quite suspicious, sounds like something is amiss. Can you post the PI pin voltages, also your B+? Maybe a pic of you PI section?

That said, 2550 or 2555 amps do not push the PI as hard as older more traditional Marshalls could, so overall volume will typically seem less with Jubs than their earlier 50 or 100 watt kin.

the B+ is 455
i have 471 on pin 3 of el34s

v3 voltages:
pin1: 216
pin3:33.8
pin6:363
pin8:33.8

matta wrote:the B+ is 455
i have 471 on pin 3 of el34s

v3 voltages:
pin1: 216
pin3:33.8
pin6:363
pin8:33.8

Those voltages look fine, and about what I would expect, except for pin 6, it looks suspicious..... there's no drop, that looks like the full B+. I would look there.

Should be more like 245V pin 1, 235V pin 6, 43V pins 3 and 8, given your B+ in a Jubilee.

Given the numbers you show, looks like perhaps pin 8 of V3 (cathode) is not making contact, would give you the plate voltages you show on both triodes and actually the cathode voltages too (though you'd expect them to be a little off from each other by a couple/few volts).

Either that or pin 7 of V3 has a bad connection/V3B triode has no grid reference. Same results there, but cathode voltages will match exactly if pin 3 to 8 jumper is in place, so this may be the issue here.

Gotta be one of the two above.

So prime suspects are:
-You ommited the jumper on V3 socket from pin 3 to 8.
-If there, that jumper has a bad connection at pin 8, resolder if in any doubt.
-Check resistance of wire on pin 7 to board, should be a dead short. If not, resolder its ends on pin 7 and board/replace wire if still reads open.
-Check the 1Meg resistor that the wire on pin 7 connects to on the board. Resolder both ends of resistor if in doubt of its connections. Replace resistor if it appears to be damaged or defective (open).

Can verify the 1 Meg pin 7 grid reference integrity here by measuring resistance from pin 7 of V3 to ground, should read about 1015K (a little over 1 Meg). If you cannot get that reading, reads open/no continuity, you know the issue concerns pin 7 and its grid reference.

Less likely:
-The tube socket itself is corroded/bad/pin broken and not letting pin 7 or 8 make contact.
-Bad tube, since you tried two though, very unlikely.

Resurrecting this topic just to say thanks (mainly to SDM), because I was having the same issue (no voltage drop across 100k plate resistor of the PI), and it turned out to be a defective 100nF cap, the one between the feedback line and the grid pin (pin 7).

I was going crazy about this, because the amp started to sound thinner and more harsh, and Presence and Resonance controls started to affect soooo little to the sound.

So I'm sharing this to advice that this 100nF Capacitor can be another suspicious when something like this happens!

I took it out of the circuit and it measured 184 Ohm...

Great post! I've had this happen to me before, and I'm sure you have just saved a bunch of people some bench time.
Thanks