Metropoulos Forum

Hello everyone!
I have a strange problem:
The Bias voltage is about -50V and as i switch on the standby, measuring the current on the the 1Ohm resistor, it appears that the tube is draining 140 mA, which i believe is way too much. The current don't changes if i set the pot into a different position (the voltage does).
the voltages on the power tubes are:
3: 368V
4: 343V
5: -57V (if standby on)
7: 360V

voltages on the pre-tubes seem to be good

both tubes the same...
If i plug the amp into speakers just a low motorboat sound comes out (loud as hell).

Does anybody has a clue where i made a mistake?
Tryed different Kt66 but it's the same problem.

Thanks for the help

I've had problems like this if I have the output transformer wires to the power tubes switched. You get positive feedback through the amp, which would sound like a huge buzz if you had speakers plugged in, but if you don't have speakers plugged in your only clue is the huge current draw at any bias setting. Swap the transformer wires to pin 3 of the output tubes, or another sometimes quicker way is to disconnect the negative feedback from the speaker jacks.

ahhhh thanks that was the problem!
had no chance now to plug it into the speakers, but the currentflow is now normal and thats a good sign.

Tallic wrote:ahhhh thanks that was the problem!
had no chance now to plug it into the speakers, but the currentflow is now normal and thats a good sign.

Your not turning that amp to ON without a speaker and/or a load"attenuator"?.
You need a load at the output of the amp when in operating mod, you probably already know that but am telling it just in case.. It can save costly repairs...

na i won't do it, thanks i know, but i don't have speakers at home, so at home 50 Watts resistor and in the practice room the cab.

Men but the sound is amazing!

The only thing i noticed is that if i turn the presence about 6 or 7 up, the amp gets a very high squeal if i turn the treble above 6. I think its the feedback resistor because normaly you take the 16 ohm tap but with the Mercury RS it's not possible. Or do you think i'm looking on the wrong place?

Try moving the negative feedback (NFB) wires around. Keep both as short as possible while staying away from other wires, especially on the pots side of the board. Try to keep them away from the OT primary wires (twisted pair going to pin 3) or cross them at a 90 degree angle if necessary.

Is 8 ohm the highest impedance you have? If so, you way want to use a smaller NFB resistor like 15k. That would be similar to 27k/16 ohms, which is already pretty clean sounding. Just depends how clean you want the power section. (This would have little to no effect on the squeal; just a general comment.)

No i could switch to 16 ohm but the radiospare OT is not able to produce more than one impedance configuration at a time (and my speaker requires 8 ohm) ok thank you i'll try it.

so the smaller the value of the feedback resistor the cleaner the powersection stays? maybe the higher the value the more prone to noise, probably, or how does it react?

Tallic wrote:...so the smaller the value of the feedback resistor the cleaner the powersection stays? maybe the higher the value the more prone to noise, probably, or how does it react?

Lowering the NFB resistor creates more negative feedback, which lowers the gain in the PI and on into the power tubes. Also, the higher impedance tap you use, the more negative feedback. The stock JTM45 27k/16 ohm is already clean sounding to me so I probably wouldn't lower the resistor. Just wanted to point out the options.

Here's a recent discussion about NFB tone and noise effects in a 100w: http://forum.metroamp.com/viewtopic.php?f=4&t=36695" onclick="window.open(this.href);return false;

So i've been testing the amp now for some weeks, and it still sounds pretty awesome. The squeal i mentioned before does not come from the speakers, it comes from the amp itself. Maybe some powertube oscillation which comes from bad wiring?
the squeal comes if i turn up the precences about 8 and then turn up either the treble about 6 or the mid about 8 so it has something with the high freq to do i think...
Ever heard of something like this?

Two things do that;

1) Head cab not shielded
2) Lack of 5k7 or 4k7 swamp resistors on the output valve grids