Is this amp "redplating"?

[Sirca]

Just got this old 4x6L6 Montgomery Ward head and the tube plates are looking pretty hot. Is this what is referred to as "redplating"?

[img:920:690]http://img228.imageshack.us/img228/6214/redplate.jpg[/img]

[Gaz]

Yep, turn that off if you haven't already

[guitardude57]

Yeah.......don't do that!

You need to check things well, before you power that up again.........
definitely some problems there.

Bias is out.

Check resistors, diode, and grid leaks. If you have worked on high voltage amps before, you should know what to check.

If not let us know, and we will walk you through it. In meanwhile wait to power it up again.

Model name and number? Innards pix would help, and schematic if you have/or can find one.

[Roe]

that's some pretty serious redplating

[Sirca]

Yeah.......don't do that!

You need to check things well, before you power that up again.........
definitely some problems there.

Bias is out.

Check resistors, diode, and grid leaks. If you have worked on high voltage amps before, you should know what to check.

If not let us know, and we will walk you through it. In meanwhile wait to power it up again.

Model name and number? Innards pix would help, and schematic if you have/or can find one.

Thank you. The amp is a Montgomery Ward GIM-9171A, circa 1970. 2 channels, 4 inputs, reverb and tremolo (both of which are working on this amp).

Here's a link to the schematic:

www.el34world.com/charts/Schematics/War ... -9171A.pdf

and some pictures:

[img:593:254]http://img651.imageshack.us/img651/7841/9171a.jpg[/img]

[img598]http://img15.imageshack.us/img15/2751/gim9171aback.jpg[/img]

I'm not experienced in troubleshooting tube amps and would very much appreciate any guidance. I'm in a rural area with no amp techs within 200 miles. I'm handy with a DMM and soldering iron, fwiw, and understand that I'd need to discharge the caps first to safely work inside it.

I'm going to take some gut shots of this amp and will post them later today.

Thanks!

[Sirca]

hmmm...where to start? you should be able to click for a full size image...

[img1345]http://img684.imageshack.us/img684/2599 ... 1aguts.jpg[/img]

[Structo]

Yeah, once you get bit by charged caps you will always remember to discharge them.There are a couple different ways to go about it.

You can use a test lead with insulated alligator clips at each end.
Clip one end to a chassis ground, the other to a plate resistor.

Since you are using an amp with 12ax7's that makes it easier.

Clip the other end of the test lead to pin one of V1.
You want to do this where the wire from the socket connects to the board.
There should be a 270K resistor there.
Clip the lead on the eyelet or turret where the wire and resistor joint.

This drains the filter caps through that plate resistor to ground.

Always double check the filter caps with a volt meter to be sure they are drained.
Clip the black probe to the chassis and touch the red probe to the + end of the filter caps.
On some amps you have to switch the standby switch to discharge all the caps.

The other way I use is to take a test lead and cut it in the middle then solder in a 2 watt 100K resistor between the two pieces.
Then heat shrink it.
With this device you clip one end to the chassis and the other to the first big filter cap.

Double check with a meter to be sure there is no voltage on the caps.

[Structo]

I circled pin one of the tube with a yellow circle that you can drain the caps through. Look at the full sized picture below by clicking on it.
Clip a test lead to a chassis ground (bare metal on chassis) then clip the other end to that pin I circled.
Double check by clipping the black probe to the chassis and touch the red probe to all the lugs on the cap can. Which is those things on the left side of the chassis where the green power cord wire connects.

If that doesn't work then you should make a discharge tool as I described above.

[mbeldyga]

Take power tubes out of their sockets and check what's voltage have you got at pin 5 of each 6l6s. This is your bias voltage which should be negative (respect to ground)

[Sirca]

I circled pin one of the tube with a yellow circle that you can drain the caps through. Look at the full sized picture below by clicking on it....

Thanks *very* much. Caps are drained.

[Sirca]

Take power tubes out of their sockets and check what's voltage have you got at pin 5 of each 6l6s. This is your bias voltage which should be negative (respect to ground)

I pulled all the tubes (preamp, power, and reverb) and measured across pin 5 to ground (chassis) and got 1.12 V across all the 6L6 sockets. Then I measured again got 1.43 V consistently. Will check again.

[Sirca]

not really topical, but here's a pic of the reverb tank label.

[img718]http://img683.imageshack.us/img683/8169 ... bplate.jpg[/img]

[Cliff Schecht]

Your amp is cathode biased. In the schematic I see two 150 Ohm resistors in parallel with 35uF, 50V capacitors but in your pictures I do not see this capacitor. Add this in and also, for safe measure, you probably want to recap the entire amp (just the electrolytics, not the ceramics/others).

Once you recap the amp, go through with a volt meter with NO tubes installed and make sure the high voltage rails are relatively close to what the schematic shows. This is where you need to be careful not to shock the crap out of yourself, I prefer using clip style leads to prevent from ever having to reach into the chassis. The numbers will be slightly higher than what the schematic says because the power transformer is unloaded but if these voltages are off then you need to fix this before you try to play the amp.

[Sirca]

Your amp is cathode biased. In the schematic I see two 150 Ohm resistors in parallel with 35uF, 50V capacitors but in your pictures I do not see this capacitor. Add this in and also, for safe measure, you probably want to recap the entire amp (just the electrolytics, not the ceramics/others).

Once you recap the amp, go through with a volt meter with NO tubes installed and make sure the high voltage rails are relatively close to what the schematic shows. This is where you need to be careful not to shock the crap out of yourself, I prefer using clip style leads to prevent from ever having to reach into the chassis. The numbers will be slightly higher than what the schematic says because the power transformer is unloaded but if these voltages are off then you need to fix this before you try to play the amp.

Thanks for the info...I didn't notice the discrepancy till you pointed it out. I'll sort thru this and order some new electrolytics. You've all been very helpful and it's very much appreciated. Will let you know how it turns out.

[mbeldyga]

I didn't even looked a schematic. My bad, sorry for that. It's cathode biased indeed, so about 1V looks fine - check voltages with schematic like Cliff Schecht said. Your bias caps are in place i belief. These silver electrolytic near to chassis's top.