I acquired a near-mint Red Knob Twin for $450. Opened it up to verify that it had no mods and original filter caps. The jacks work, the 1ohm power tube cathode resistors read 1.1ohm, the voltage divider power resistors look clean and read within 10%. Other than the reverb running away above 2 o'clock, everything works. Tubes have all been replaced with Sovteks except the PI and reverb driver which are EI Yugos. I have some other 12AT7s to try and see if that fixes the runaway reverb. There is no noticeable hum in full-power mode and just barely existent hum in low-power.
Should I get the 30+ year old IC filter caps replaced soon?
Near Mint Red Knob Twin - replace filter caps?
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Stevem
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Re: Near Mint Red Knob Twin - replace filter caps?
Yes, while you have it apart replace all the electro type caps and filters.
So don’t forget the cathode bypass caps and the bias filter.
Many times when I have had run away reverb conditions the springs in the pan where very shot and dangling too loose, this assumes it’s the right reverb pan and has not been swapped out and that the recovery / mixer gain stage is not producing too much gain.
The cathode resistor in that stage may have drifted up high in value.
I have also had tarnished rca connections on the pan side and the amp side create a high resistance and make for reverb feedback and a condition like your stating.
On a side note, I have two Univox amps that use 6BM8s for the PI and the output section .
I love these all of 8 watt amps!
They sound like a little Marshall metal face amp when cranked up and feeding a non Jensen 12” driver.
So don’t forget the cathode bypass caps and the bias filter.
Many times when I have had run away reverb conditions the springs in the pan where very shot and dangling too loose, this assumes it’s the right reverb pan and has not been swapped out and that the recovery / mixer gain stage is not producing too much gain.
The cathode resistor in that stage may have drifted up high in value.
I have also had tarnished rca connections on the pan side and the amp side create a high resistance and make for reverb feedback and a condition like your stating.
On a side note, I have two Univox amps that use 6BM8s for the PI and the output section .
I love these all of 8 watt amps!
They sound like a little Marshall metal face amp when cranked up and feeding a non Jensen 12” driver.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Near Mint Red Knob Twin - replace filter caps?
Thanks for the tips. I'll see if I can troubleshoot that reverb issue tonight. My first scratch build amp using the triodes from the 6BM8 in a LTPI, fed by just a parallel triode 12AX7 into a gain control with no tone stack. It was a beast. I can't recall what happened to it so I should build it again.