First MAJOR amp repair

[Quencho092]

I've read volumes about the causes and effects of a toasted screen grid resistor and after some testing, i have figured out that my buddy's 74 bassman must have fizzed out because of power tube failure, taking the 470 ohm resistor with it.

I've biased plenty of times before so after the soldering is done it'll be smooth sailing. I just setup an order at mouser for a couple of ohmite 3 watt 470 ohm resistors and 3 watt 100 ohm resistors, and i'll be ordering sprague atoms for the cap job. TAD 6l6wgc's will be going in there once all is done.

Any amp gurus out there that could give any suggestions to why else that screen resistor might have toasted? If not, as soon as those resistors arrive, it'll be show time.

 

[Quencho092]

Re: First MAJOR amp repair

i guess to most of you real techs this is no sweat, but i guess im an aspring tech/amp builder and it's a step forward to being independent from the 'douche techs' in my area.

 

[Scott_F]

Re: First MAJOR amp repair

congrats... the good news is there was no transformer damage when the tubes blew out their aorta. Those resistors take the hit. 1 watt would have been sufficient on a Fender amp. (6V6 or 6L6) On an EL34 amp, you'd want to bump that up a bit, rating and value. When that resistor is gone, tube no workie. If you ordered the resistors, order a few extra so you can check them on your meter before you install them. Try to match them up if possible. some amps didn't use to use them at all, marshalls were all over the place for several years, some had them, some didn't.

If you have really good ears, I've heard you can tell the difference with and without. I'd rather not run any of my amps without them, so I'll never find out. Matching them up makes sense though, so I would do it.

 

[Scott_F]

Re: First MAJOR amp repair

Also, you might want to hop over to the Kendrick website and look at this little light bulb gizmo that Gerald Weber sells for some ungodly amount.

The idea being, you should power up your caps slowly when you put new ones in. How? Most of us do not have a variac, but that'd be the quickest way. Weber takes a 60 watt lightbulb and wires the socket in line with the power going to the amp. For the inital turning on, it sucks some of the current from going to the amp giving you that slow power up everyone recommends.

110v wall hot-----------bulb------------amp
110v wall neutral------------------------amp

My weak drawing will probably not make much sense, but it works.

 

[Benji2204]

Re: First MAJOR amp repair

it's a step forward to being independent from the 'douche techs' in my area.

+1

 

[Quencho092]

Re: First MAJOR amp repair

damn douche techs!