The Amp Garage

If I'm understanding correctly, this will do what you want...

I use this step bit...
https://www.amazon.com/Greenlee-GSB04-S ... 08TVF22W4/

Much cheaper at Harbor Freight
https://www.harborfreight.com/search?q=step%20bit

Your cathode follower can't work because the grid has no bias voltage. See https://www.valvewizard.co.uk/accf.html.

Be aware that regardless of how you bias the CF (either by direct coupling, fixed bias, or bootstrapping) you will end up with a pretty big dc voltage on your relay.

martin manning wrote: ↑Sat Jun 07, 2025 12:46 am Looks like your HV input lead is coming from the DC side of a safety silicon diode. It needs to be connected directly to the HV AC.

BINGO!

Yes. Work fine, last long time.

Layout will work. I suggest using a filter cap rated for 150V. You may need to increase the value of the 100K resistor to get the desired bias adjustment range.

I'm using Hammond iron (290AX + 1750E). The PT has two high voltage taps, 550-0-550 and 650-0-650 The 290AX is 275-0-275 or 325-0-325. Your voltages seem reasonable with that 750Ω sag resistor. If you want more voltage, decrease that sag resistor. https://www.hammfg.com/files/parts/pdf/290AX.pdf?v=...

Brown-Black-Black-Silver-Brown is 1Ω 1%.

See for yourself... https://el34world.com/charts/resistor/5 ... isitor.htm

#1. I doubt there is a sonic difference. Probably measure a bit more ripple on the bias voltage in your amp. Hiwatt liked to use 100µF bias caps in all their amps. I suggest you change the 10µF in your amp to 100µF just because Hiwatt was fond of 100µF. #2. The added resistors on your power tube soc...

OK, now I understand. Sorry for my confusion.

I doubt there's any sonic change by replacing the original 22K with a different brand 22K. A schematic would show exactly where that resistor is in the circuit. Heck, it may not even be in the signal path.

The resistor next to the cap is 22K. And the resistor to the left of the 22K is 2.2K. They are not both 22Ks. Maybe I'm misunderstanding what you said? Are these the two resistors you are talking about?

Stevem wrote: ↑Sun Jun 01, 2025 11:01 am Would not the use of a Zener there add in switching noise right where we don’t want it?

The zener will only switch on once then it remains on until you turn the amp off. It's working with dc voltage unlike the rectifier diodes that work with AC voltage and switch on/off at a 60Hz rate.

ronin wrote: ↑Sat May 31, 2025 8:13 pm If I am reading correctly, both are 22k resistors. Please correct me if I am wrong.

Wrong. red/red/red = 2,200Ω. red/red/orange is 22,000Ω

Look for an old Akaia/Roberts R2R carcase on eBay. I found this one for cheap. It is just what you need...

In lieu of having a scope I use the DMM, as I think many hobbyists do, but it’s good to know the results are inaccurate. It's not that the results are inaccurate. You're just using the wrong tool for the task. You would not use a screwdriver to cut a board, would you? :mrgreen: Your DMM measures DC...