1973 Fender Twin Reverb - Tinny Sharp sound

[Yorkie]

Help required to solve the following fault.

My son purchasd a 1973 Silverface twin reverb about a year ago, it work perfectly okay for about six month, but it has developed the following fault. This is his discription: The fault increases with volume, setting at halfway with or without reverb there is a tinny sharp sound when playing 5th B, 8th top B, 13th B and 18th top E, it is far worth on single coil guitars.

Preamp and driver tubes and reverb unit have been replaced.

Any suggestions most welcome.

Thanks

[hollisdevillo]

that sounds like bad caps. this is definitely the case if the caps have not been changed in the past 20 years. check the electrolytics. if they have bubbles on the positive end then they're done. and the coupling caps and bypass caps most likely then need to be replaced. if the caps have been chenged recently, then you have to isolate the problem. does this happen on both stages, in the vibrato circuit as well as the normal channel. try pulling a tube and see if this cuts anything out. it also king of sounds like a microphonic tube. pulling one and seeing what happens will tell you. or swapping them around. if this is the case, (not the caps) but the caps are still bad, then you should still replace them, your tone and response will improve 10 fold. should also get rid of the balance adjustment bias and change it back to blackface. is that what that acdc song is about, back in black? no i guess not since they used marshalls.

[Twist]

Remove the back cover. I'm assuming you didn't replace the output tubes. Turn the amp on and let it warm up for about 15 minutes. Flick each output tube with your fingernail or use something like a popsicle stick. Don't tap too hard, you could break the glass. Many times old output tubes can rattle or make noise from particular frequencies coming from the speakers. This may not show up till the amp has been on for up to an hour.

[Dave Curtis, dB AudioTech]

and the coupling caps and bypass caps most likely then need to be replaced.

I'd leave the coupling caps alone unless you find a bad one.

-DC

[Chuck H]

Those aren't the typical notes responsible for "ghosting". The symptoms don't sound like bad filter caps to me. Let alone bypass and coupling caps. DO NOT REPLACE THE COUPLING CAPS.

But... At that amps age I'm sure the filter and bypass caps are due for replacement anyhow. Those caps have a lifespan of about 10-15 years.

As for the problem... I'll bet it's dirty tube sockets or a microphonic tube. When these problems start and the cabinets "resonant frequency" is hit it can be very confusing.

Replace all the electrolytic caps in the amp. Clean the sockets and jacks with contact cleaner and mild abrasion. Try a replacement set of power tubes. Also do a "tube swap" on the preamp tubes, by replacing the first tube with a known good one. Then, if the problem persists replace the next tube with the one you just pulled. Etc...

All this is standard amp care that's overdue anyhow. And I'll bet you the amp is fine (not just repaired, but sounding it's best) when your done. Unfortunately tubes and good filter caps are expensive. But that's the brakes.

Chuck