Beginner Amp Repair Questions

MarcInMemphis · Post by **MarcInMemphis** » Mon Jul 04, 2005 2:10 pm

Hey, I've got a mid-late 70's 50 watt Marshall mkII JMP (4 input model with 6550 tubes). The amp is seriously damaged (both the power and output transformers are trashed and some of the components on the circuit board are damaged as well). Pretty much I've got a nice looking chassis and box and all the pots, sockets etc. but none of the real guts of the amp. I just bought new transformers on ebay (Drakes from a '82 50 watt JCM 800, not perfect replacements but they aught to work). I plan on buying a new board from the Metroamp website, they look way nicer than the printed PCB that came stock in the amp. So here's my questions:

1. What is a safe way to drain the filter caps in this amplifier? Also, what are the three contact points on a filter cap (anode, cathode, gound?) they seem to be color coded. I don't want to go poking around in the amp until I'm sure it's safe, so any help here would be greatly appreciated.

2. What type of solder and what wattage soldering iron should be used for repairing / building amplifiers.

3. I want to switch this amp from 6550 to EL34 tubes. I assume that the Metroamp boards are built for EL34s. Are there any changes that I need to make to the wiring of the tube sockets or do I just drop in the new board?

4. I was told that the amp was damaged because the power transformer failed and high voltage was put across the circuit. Are there any other components besides the transformers and the board that are likely to be ruined. I plan on replacing the filter capacitors anyway because they are quite old. What about tube sockets, switches, etc???

I'm new to working on amplifiers and I would really appreciate any advice that people have. Thanks.

Flames1950 · Post by **Flames1950** » Mon Jul 04, 2005 3:27 pm

There are actually two capacitors in each of those cans, so what you're seeing is two positive terminals and a common ground terminal. The usual quick-n-dirty cap draining method is to take a jumper wire with alligator clips on each end, clip one end to the chassis and the other to pin 3 on the first preamp tube (a blue wire if it's stock.) You can leave the jumper while you work for added safety, just don't forget to yank it before you flip the switch.
It's hard to say what all may have been damaged. Why didn't the fuses stop damage from occuring, did someone try the tin-foil-on-the-fuse trick to get it running at a gig? Any chance that all you need are fuses? (Well, it was a nice thought.)
You shouldn't try anything much bigger than a 25-30 watt soldering iron for the most part. It may take more heat than that for connecting grounds solidly to the backs of potentiometers or to a chassis but most work will be done with the lower wattage. (Do as I say not as I do, I regularly go at PCB's with my iron set to 700+ degrees....)

MarcInMemphis · Post by **MarcInMemphis** » Mon Jul 04, 2005 4:00 pm

Which preamp tube is the first preamp tube. Also, do I need to have a tube in the socket in order for the capacitors to drain?

MarcInMemphis · Post by **MarcInMemphis** » Thu Jul 07, 2005 4:06 pm

Can anyone suggest a good tube amp repair / building book for beginners. I took physics in college so I know the basics of calculating resistance, capacitance etc, and i have a little soldering experience (i've made simple stompboxes before). I've heard that a series of manuals called The Ultimate Tone by O'Connor are very good, but I'm wondering if they would be over my head or not. Any suggestions would be greatly appreciated. thanks.

NY Chief · Post by **NY Chief** » Thu Jul 07, 2005 4:11 pm

TUT books are real detailed and very "techie". You can learn a shit load just exploring this board. Read everything. These guys are GOOD!!!

Flames1950 · Post by **Flames1950** » Thu Jul 07, 2005 11:54 pm

The first preamp tube is the 12AX7 on the outside of the chassis (we often refer to it as V1, and its halves as V1a and V1b.) They are, in order, V1 (first stage for the Bright channel on one half, and first stage for the Normal channel on the other), V2 (a gain stage common to both channels on one half, and a "cathode follower" for the other half that changes the circuit impedance for the tone controls), V3 (the phase inverter, splitting your signal into an "upper" and "lower" half), and lastly V4 and V5 (power tubes for a fifty watter) or V4, V5, V6, V7 in the case of a 100 watter.

BrownSound1 · Post by **BrownSound1** » Sat Jul 16, 2005 5:09 pm

Flames1950 wrote:(Do as I say not as I do, I regularly go at PCB's with my iron set to 700+ degrees....)

Actually that is a fairly common practice in Mil-Std manufacturing facilities. They are more worried that you have a grounded tip iron, and I've seen them recommmend to their workers to set their irons at 720 degrees F. So don't feel too bad about it.