High frecuency noise on high notes

[passfan]

I've been playing a newly wired Rocket Reverb for a couple of weeks now with nothing more than an intermittent output cable (jack corrosion) on the tank. I pulled her down today and changed the bias resistor, as I was pushing my output tubes pretty hard, and now when I fire it up I have what seems like 15 khz riding subtly on my high notes. You don't really hear it as much as it rips into your head on both sides like someone dragging their fingernails across a chalkboard. Bias puts me at 36.5 volts at 360 ohms with a plate of 425 volts. Bias resistor ties to ground at the same point as the reverb driver and I've re flowed that point. My bias change did drive up the B+ by 25 volts. Anyone ever run across this before ?

[passfan]

I think I have discovered what has happened here though it will be a couple of days until I can check it out. An output tube swapping mistake appears to have allowed the full B+ of 400 volts to hit my cathode resistor and capacitor. The cap , which is rated at 160 volts could very well be responsible for the difference in fhz on the output. I swapped out a set of 7591's for a set of 6L6's thinking the pin out was the same. The cap looks fine but 3 times the normal rating could have blown a hole in the dielectric so fast that the cap didn't have time to swell ??? Does that sound feasable ?

[Reeltarded]

I think that is a good bet. It's a decent line of thought.

Try it and if it works, you get it! (better than before )

[husky]

I think I have discovered what has happened here though it will be a couple of days until I can check it out. An output tube swapping mistake appears to have allowed the full B+ of 400 volts to hit my cathode resistor and capacitor. The cap , which is rated at 160 volts could very well be responsible for the difference in fhz on the output. I swapped out a set of 7591's for a set of 6L6's thinking the pin out was the same. The cap looks fine but 3 times the normal rating could have blown a hole in the dielectric so fast that the cap didn't have time to swell ??? Does that sound feasable ?

Yes but where in your circuit is a 160V cap?

[martin manning]

How are the sockets wired? Are you using any lugs as tie points? Do you have 1 & 8 tied together as for EL34?

[passfan]

That was it. New cap and she sounds great again. Yes pins 1 and 8 are tied together and pin 6 is used to bounce the screen resistor to 4. Pin 4 is what got me as on a 7591 pin 4 and 8 are common internally which put my 400 volt screen on my 160 volt bias cap tied to pin 8. Never swap out different tubes without looking at the pinouts. I was absolutely sure and I wound up wasting a couple hours. It could have been much worse.

[Reeltarded]

Congrats! You did it!

[cbass]

Glad to hear you got it goin.It might be worth swapping a wires on the sockets and trying those 7591's especialy if they're old stock damn good sounding tubes.I've been wanting to build a 7591 rocket so you could be the guinea pig.

[passfan]

First I have to test them to see if they're still alive. It would be a shame to as they came out of an older Hammond E100 series organ chassis and tested almost new. The Rocket is for sale now as a chassis.