Help diagnose my 1959RR, she went "POP"...

[cjshaker]

I was just trying out a new amp, a 1959RR (Randy Rhoads model) Marshall. At first the tone was so-so, but after playing about an hour I started hearing crackling sounds. Wiggling the cord did nothing, as did turning any knobs to check for dirty pots.

I decided to check the stock Svet tubes, and lo and behold, one of the EL34s was showing a short on my TV-7 tube tester. I thought problem solved. I stuck in a new set of matched EH tubes I had on hand after checking them out also. I did not bias, just wanted to check if the problem was gone.

Turned the amp back on, waited for it to warm up and hit the standby. EXCELLENT TONE!! It made a huge difference with the new tubes. I had also stuck in NOS vintage 12AX7s' (2 RCAs' and 1 Mullard) to get rid of the cheap Marshall tubes. I was happy as heck and blown away by the sound I was getting.....................for about 1 minute.

POP...........nothing.

I quickly checked the output tubes and they were all lit.
Next, I pulled it apart. Chassis out, and found 3 burnt resistors.
R11
R31
R32

R31 & R32 are on both sides of a "Hum Balance Control" circuit (a schematic can be found at MetroAmps, I cant post PDF to Photobucket). Im not even sure what this circuit is but it directly involves a secondary side of the Output transformer so Im thinking this is NOT a good sign.

My concern also is R11, in the preamp circuit. Why would this one go in the preamp section. I dont know how to post a PDF document, but I have a schematic for the RR.

Can anybody tell me what would/could have caused this? Right now Im thinking the OPT could be fried. That will flat out suck.

Heres a couple of shots of the resistors.
The preamp resistor.
http://i43.photobucket.com/albums/e363/cjshaker/RR2.jpg

The Hum circuit resistors.
http://i43.photobucket.com/albums/e363/cjshaker/RR1.jpg

[donnyjaguar]

Your output tube's filament is/was shorted to the cathode killing R31/32. Seen this before. The others R11/12 I don't know because i have no idea where they are going. :)

[cjshaker]

Your output tube's filament is/was shorted to the cathode killing R31/32. Seen this before. The others R11/12 I don't know because i have no idea where they are going. :)

Thanks for the input (pun intended). Do you know what and how the Hum Balance circuit works? Seems its just a singular secondary circuit that goes directly to ground through the resistors and a pot. Im guessing it affects the PT output operation in some way to alter PT hum characteristics? Im not sure how this would be caused by the filament shorting unless it caused a surge in the PT that directly affected or caused a surge in the Hum circuit.
Heres a link to MetroAmps and the PDF file for the schematic.

http://forum.metroamp.com/viewtopic.php?f=25&t=22308

As you can see in the schematic, R11 and R12 go directly to either side of the cathode of V2. Only R11 seems to have been blown. Even though R12 is black, it wipes clean, although obviously it will need to be checked before trusting. Could the shorted tube have caused a surge through the whole tone circuit to blow these?

Thanks for any info you guys can give me.

[Mr. SD-1]

Thanks for the input (pun intended). Do you know what and how the Hum Balance circuit works? Seems its just a singular secondary circuit that goes directly to ground through the resistors and a pot. Im guessing it affects the PT output operation in some way to alter PT hum characteristics? Im not sure how this would be caused by the filament shorting unless it caused a surge in the PT that directly affected or caused a surge in the Hum circuit.
Heres a link to MetroAmps and the PDF file for the schematic.

http://forum.metroamp.com/viewtopic.php?f=25&t=22308

As you can see in the schematic, R11 and R12 go directly to either side of the cathode of V2. Only R11 seems to have been blown. Even though R12 is black, it wipes clean, although obviously it will need to be checked before trusting. Could the shorted tube have caused a surge through the whole tone circuit to blow these?

Thanks for any info you guys can give me.

http://www.schematicheaven.com/faq/humbalance_pros_cons.pdf

I suspect a heater/cathode short related to V2 (cathode follower). Some 12AX7 types, especially those with spiral filaments, cannot handle elevated cathode voltage.

[cjshaker]

http://www.schematicheaven.com/faq/humbalance_pros_cons.pdf

I suspect a heater/cathode short related to V2 (cathode follower). Some 12AX7 types, especially those with spiral filaments, cannot handle elevated cathode voltage.

Thanks for the link. It doesnt seem to jive with the Marshall circuit though. They are talking about the circuit being between the heater filaments. The Marshall schematic does not show it interacting with any other circuit, just going straight from secondary winding to ground through the resistors and pot.

Before, I mistakenly talked about the Output Transformer, when I should have been saying the Mains transformer. The schematic shows the Hum Balance secondary winding coming off the mains, through the two 100R resistors, then through the pot to ground. Im still not sure how a short in the output tube would have caused these to blow, unless like I said, there was a surge in the primary winding, causing a voltage "spike" in the secondary winding of the Hum Balance circuit causing the resistors to blow.

I could be way off base, or just full of wind. Im not an amp tech, but I know a little about amps and I pick stuff up quickly. My Dad taught me how to find my way around a schematic and checking things like PT, OT, chokes, proper voltages, biasing etc, just not all the details about theory behind designs.

[jay42]

Some 12AX7 types, especially those with spiral filaments, cannot handle elevated cathode voltage.Notably, the Sovtek 12AX7LPS.

Those 100 ohm resistors hanging off the Hum Balance control sure look like fuses...sigh. Very strange notion -- they either shouldn't be there and the pot s/b 2W, or they should be 2W to 3W wire-wounds.

OP -- replace all of those resistors. If there's enough heat to discolor them and they don't go open, they've still been compromised.

[cjshaker]

Ok, after reading and checking the schematic more, I see that the Hum Balance circuit is a center tap on the heater winding. Now I can see how the resistors could blow with a short in the tube and how the circuit works.

Do you guys think that I can just replace these resistors and be on my way, or do I need to get into it farther and check things out? Thanks guys for the input.

[cjshaker]

Notably, the Sovtek 12AX7LPS.

Those 100 ohm resistors hanging off the Hum Balance control sure look like fuses...sigh. Very strange notion -- they either shouldn't be there and the pot s/b 2W, or they should be 2W to 3W wire-wounds.

OP -- replace all of those resistors. If there's enough heat to discolor them and they don't go open, they've still been compromised.

Thanks Jay, good advice to replace them all. Be safe.
Yes, I should have replaced the cheap tubes before I turned the damn thing on. I know from experience that these tubes suck, but I wanted to hear the amp, and it cost me. Next time, if I get a Marshall with stock tubes, I think Ill just re-tube and bias before doing anything.