'65 Fender Twin NOT REISSUE Popping on Power Up and Power Down

[keysnleslie]

A brief history; I restored this amp 12 years ago with new power and output transformers (Kendrick products) had both speakers reconed,new grillcloth,tubes,etc... Last year the owner replaced all tubes (with J and J's) and then brought it to me because it kept blowing fuses. I checked it out and did find it blowing fuses and it had one power tube that would flash and take out the fuse. I replaced the bad tube and all was fine except for a "multiple popping sound" when switched from standby to play,and also when power was shut off. I replaced all filter caps with Sprague Atoms,brought them up slowly on my variac,did everything right to the best of my ability,and it seemed fine,but the popping was still there,and he played it regularly for the year. I now have it on the bench again to replace a carbon tracked octal socket,and to re-tube with Groove Tubes. Having completed this work,it sounds fine,runs well,and STILL HAS THE POPPING SOUND !!! I removed tubes one at a time from right to left,kept them out (not remove and replace each) and condition dissapears only when driver tube is removed.I am begining to hear voices telling me to shave the neighbors dog.....Do I need professional help,or does any one have any ideas that don't involve a hammer?

[Don Moose]

Have you been through R.G.Keen's Tube Amp Debug Page?

[keysnleslie]

I went through Kean's Debug page tonite;

I don't see any arcing,I do not know how to test the choke or how to determine "break over" as described. Could you "dumb it down" a bit for me?
Thanx

[Enzo]

One simple way to see if the choke is faulty, aside from RG's procedures, would be disconnect it and tack a resistor in its place. Oh, I don't now, a couple hundred ohms or something. it doesn't matter if the amp is not exactly happy that way, ALL we are looking for is if the choke was the source of that noise.

Pulling the PI tube disconnects all signal input from the entire amp to the power tubes. Since the noise stops that way, all we know is that the power tubes are not the souce of it.

REmovng and replacing each tube might provide more insignt than leaving them out.

Do any of the controls affect the noise? Does turning the volume down reduce the noise DO the tone controls affect the tone of the noise? ANy contol that affects it is after its source.

Have you swapped a known good preamp tune into each preamp position? Including the PI, which is technically not a preamp tube.

If it is not a tube, you could have a noisy resistor, a borderline coupling cap, a cracked solder joint, etc.

[TD_Madden]

Standbye switch going out......heard this is a problem with the older amps.

[Bruce / Mission Amps]

Standbye switch going out......heard this is a problem with the older amps.

Right, Check the Standby switch and or a bad screen resistor... these are the components I'd be looking now.

[TD_Madden]

Found this also on http://www.premierguitar.com/archive...dda/01_05.htm:

Hi,
The standby switch on my Marshall JCM 800 2210 sometimes pops LOUD when I turn it off. Do you know what could be causing this?
Thanks
Aksel Ornes

Askel,
Well, there are probably more than a few reasons why some amps will "pop" when the standby switch is thrown. First off, the standby switch itself could be bad. The internal contacts could be worn causing the switch to excessively arc internally. The only real solution here is to replace the switch. Next, I've seen this problem caused by a questionable output tube. Again, the only way to tell is to substitute a different set of tubes and see if the problem disappears. If you have another amp with the same output tube configuration, go ahead and borrow them to temporarily install in your JCM 800. Don't worry about having them properly biased; you're just using them to troubleshoot your amp. If you determine that it is the tubes, purchase a new set for your 800 and have them installed and biased by a reputable tech in your area.

Next, if you still haven't found the cause of the problem, try having a technician install 0.047uf/630V caps from each leg of the stand-by switch to ground. In some amps they may work better hooked to the "line" side of the switch, and in other amps they may work better attached to the "load" side of the switch. Your tech will have to find this out by trial and error. Of course, there's always the possibility that a filter cap in the power supply is causing the problem, and again, the only way to know for sure is to substitute new caps for the old ones. This, of course should only be done by someone who is well versed in the servicing of tube amps because... "It's deadly in there"! OK, one of these tips should hopefully have alleviated the "popping" symptom, but if not, then it just may be something you'll have to live with, and if the amp is working fine otherwise, I wouldn't worry too much about it.

[TD_Madden]

Personally I'd stay away from the .047 cap idea, unless they fail OPEN.....

[keysnleslie]

Eureka I Found It !!!

Thank You gentlemen,I have found the problem. When we removed the extra gain stage and returned the components to their original use (tremolo) I never soldered the one lug to the back of the pot,and on my 3rd attempt to check for arcs I noticed the photo cell flashing in time with the popping,and began to work backwards from there. We now have a happy,healthy,great sounding amp. Thank You again.