Mitchell Pro-100

[dcowger]

I bought an old Mitchell Pro-100 a couple days ago for 60 bucks. Felt like it is probably a pretty good deal if I can get it working. Right now the amp turns on and works but in general produces a very quiet signal that is distorted and kind of cuts out (for example if I turn it to 10 it sounds like I would expect it to be if only turned to 2... and the signal is distorted and kind of cuts out). Voltages all check out (except the heater voltage is a little low 5.5V... but seems unlikely that is the problem). Based on what I've heard thus far I think maybe it is a bias problem. This looks like a fixed bias. I'm thinking about playing with that and possibly installing a variable bias

Thoughts?

But before I do anything I think I need to replace the electrolytic capacitors. I noticed Aaron with MonkeyMatic looks like he did a killer restore/mod of a Mitchell pro-100:

http://amps.monkeymatic.com/mitchell-pro-100/

The picture of the insides of his show less E.caps than mine. I attached a picture of the insides of mine and the cap replacements and some questions.

Thoughts?

[xtian]

Hi, Daniel. Welcome to TAG, and thanks for posting. Best if you post your gut shot as JPEG so it's easy to view. I'll post my schematic, below.

I does look like you have extra filtering. You can figure out what's going on by tracing the connections.

Yes, definitely do a cap job, including the orange cathode bypass caps on the preamp board. I suggest going with the schematic for the amount of filtering--mine is in great shape with caps as shown on schematic.

After you've got new caps in, but BEFORE you put any tubes back in, start it up (best with a lightbulb limiter in case of wiring error) and observe unloaded voltages, especially for the heaters. Should be 6.3-7.0 volts with no tubes.

If all that goes well, install tubes and check bias. We'll mess with that if we need to!

[dcowger]

Thanks, xtian!
I uploaded a jpeg of the insides on this post, but it looks like it looses the comments I had; but hopefully everyone can see what I'm looking at.

Sounds, good. I'm going to follow your schematic. you must have put in a variable bias as well?

BTW, I pulled the tubes and measure 6.9Volts on the heaters. So, I'm assuming that part is ok.

Was it difficult to desolder these connections and pull apart? Looks like you have to get at a lot of these from underneath?

[xtian]

I did not bother with the adjustable fixed bias, because factory bias was close enough, if a bit cool. I'm getting about 65 watts clean.

Not too bad to get under the filter board--just remove the screws and pry the board up. Looks like you'll have to unsolder the CT (red yellow) that goes over the board to the standby switch.

[dcowger]

Looking at the inside of your amp I don't see a bias resistor(s) in parallel with the 50/70 Ecap? Am I missing it?

[xtian]

It's on the filter board, hidden by the big caps in my photo. Not sure what value I have in there.

[drew]

Right now the amp turns on and works but in general produces a very quiet signal that is distorted and kind of cuts out (for example if I turn it to 10 it sounds like I would expect it to be if only turned to 2... and the signal is distorted and kind of cuts out).

Maybe a poor mechanical connection somewhere, or a dirty or oxidized connection or set of contacts? Have you gone through the amp with Deoxit, and tightened up the jacks, pots, ground points, etc?

[dcowger]

Drew - thanks for the comment. No, I haven't done that and looking at the amp that could definitely be an issue. I will work on that tonight. Hopefully that helps. Haven't used deoxit.

xtian - what is the board in the center of your amp? Mine doesn't have that board and judging from the schematic it doesn't look like I'm missing something. Is it something custom for your build? Just curious. I'm ordering up some new caps tonight, including the cathode bypass caps per your suggestion. Also going to order some new resistors to follow the changes you made. Thanks again for all the help.

[xtian]

That's a custom FX loop.

I suggest you wait to swap preamp resistor values until you've got this thing running and sounding good, otherwise you won't know what the changes sound like. My two cents.

[dcowger]

Yeah good point. I will get it working then will mess with resistors.

By the way, I checked voltages on the tubes. Seems like everything looks pretty normal to me. Any thing look off you to you?

POWER TUBES (4 Ruby 6L6GC tubes)
Pin# #1 #2 #3 #4
1 0 0 0 0
2 0 0 0 0
3 497V 493V 495V 495V
4 492V 489V 490V 490V
5 -54V -53V -54V -53V
6 0 0 0 0
7 0 0 0 0
8 0 0 0 0

PREAMP TUBES (3 Ruby 12AX7 tubes)
Pin# #1 #2 #3
1 283V 268V 0.3V
2 0 0 0
3 2V 2V 45V
4 0 0 0
5 0 0 0
6 284V 262V 267V
7 0 0 20V
8 2.3V 2.0V 45V
9 0 0 0

Also, since this might be a bias issue with the amp, a friend of mine recommended I use the attached bias circuit revision; he's had good luck with it on other amps. Think it would be a good fit?