Metropoulos Forum

Guys, I have a PDF of this Metroamp kit, based on the 1979 JMP 2203. I would like to build this kit over time — leisurely with a very knowledgeable friend who happens to be an electrical engineer.

I only have a loose understanding of certain sections of these amps. I'm an owner of a Marshall DSL-100, which has it's merit, and I do like the tone I get from it with my guitar and bridge pickup. One of the cool features about the DSL, is that there are 3 pins on the back of the amp, and 2 bias trim pots for easy access. It makes biasing very straight forward, if you know what the plate voltage is, and what the voltage drop over the screen grid resistors are. Many people often use this as a guideline, and to give the tubes ample buffer:

65% OF PLATE DISSIPATION BASED ON MY PLATE VOLTAGE OF 466:
25 watts ÷ 466vdc X 0.65% of plate dissipation, + 0.005 screen current
= 0.0398, OR 39-40 mA PER TUBE
BIAS RANGE PER SIDE = 78mV OR 80mV.

Regarding the TESTING SECTION: PART 5 of the METROAMP 100 WATT MASTER VOLUME assembly PDF, I notice that a resistor is used on each tube socket. The biasing instructions in the PDF illustrate the following:

• Carefully measure voltage in mV at PIN 1 of V4
• Use a small screwdriver to adjust the bias pot clockwise
• Re-check the reading at PIN 1
• Continue this process until you have dialed in the desired voltage

When you have the correct reading on V4, measure mV on PIN 1 of V5, V6 and V7. They should all measure approx the same as V4. But will vary slightly depending on how closely matched your output tube set is. You can adjust the bias so each of the output tubes are in the correct range.

My Question: Why is a single bias trim pot used in this build, and not a dual trim pot?

I'm interested in this, because it would be very convenient to integrate a 3 pin molex like the DSL's, and have dual trim pots. You don't have the pull the amp out to bias, and everything is somewhat external so you don't have to probe INTO the amp. I'm not sure how it would be done on a Metroamp type kit, but is installing a dual trim pot less desirable than a single trim pot? And if so, why?

Much thanks.

It is single pot because vintage marshalls only had one.. You can install two pots pretty easily if you want, but the 1 pot works in so many amps so why bother? Tube matching isn't THAT important when aiming for vintage tones. Just bias to the hottest tube, move coldest to V4 position and rock on!

Unless using really crappy pairs of tubes that are over 8mA out, it's really not worth it. A 5 or 6mA mismatch can actually be desireable in a push-pull guitar amp.

Ahh. Much thanks for that guys. I was under the impression that a single pot was more desirable to get very closely matched bias ranges.

But, you see no flaw, or fault in using a dual bias trim pot — with a matched quad?
In other words, it's not LESS accurate than a single pot, or more prone to drift etc?

Some guys have actually modded their DSL's to get rid of the dual trim pot, and incorporate a resistor and single pot like on the older amps. I just never understood the why behind it.

Thanks.

Well, two pot isn't at least worse.

What I think would be cool in a Metro build, or JCM clone build, is a custom panel that you cut out in the back of the rear panel, or back of the cab's head.

Then, install a wire from the OT to some kind of pin connector that faces out of the back panel - with a sliding cover. In the same area, you could have the 3 pins to measure bias, and the left and right bias trim pots. — Like everything you'd need to measure and set bias in one convenient place.

Some people do that. For me it's too much of a PITA, I prefer biasing the standard way. You don't anyway do it every day so it isn't that big of a deal to open the chassis to bias.

julkke, I just found this today by accident. This mounted inside a clone build would be amazing. No more biasing, ever. It's like auto pilot for biasing.

http://tubesync.co.uk/index.php?url=buy ... ductsID=22

http://www.tubesync.co.uk/

Now that's a cool idea.

Very cool idea indeed! I couldn't find the price for the unit, do you know it?