Tube Amp Archive

Helmholtz wrote: ↑Tue Oct 07, 2025 10:01 pm What is the voltage at TP13 (V BIAS)?
What is the voltage "V LTP"?

Helmholtz, both points are -97VDC, which is also the same as I have at the grids of the EL34s

Thanks a bunch for the responses guys. Hemholtz and RG I pushed the amp off my bench in the effort of progress on other things, but will revisit soon. I totally misread that other thread , in the sense that the OP has the same symptoms as me, then another poster stated he had the same technical prob...

I need some help with this one. Amp sounds great, all channels, but super low output. After checking the EL34 grids, I've got -92VDC, so they're barely on. Bias pot does nothing. 500+VDC rail I read in another thread that Jon Snell says the FETs are known issue. https://www.tdpri.com/threads/blackst...

That was a success. Note for any future techs reading, the ACL and ACN on new unit was opposite of what the original was. TBH I'm not even even sure if this is the original unit, or if it was replaced in the past. Nonethless, the new unit is clearly labled on the underside, and the Black wire feedin...

Yep that's the one, apparently it's a semi-common issue w the amp if Rheinhardt himself tested it and found it to be suitable. As I understand , this decision was made in some point in production to lighten the entire amp, as originally there was just a 12V winding on the PT, which made the amp heav...

Ok thanks. I went with the one I linked as I saw it mentioned in a repair vid, and it has the exact same connections as the original failed one.

Found one : https://www.mouser.com/ProductDetail/709-RPS120-12

What are you guys using for replacements? I've got one here with a burnt up screen resistor(unrelated??), but proper DCV everywhere. However the amp has no 12V supply, which apparently is for switching, heaters, pilot. I see one on a UK site but figured I'd check here first. Also of note, this scree...

I went with the Hammond 156L, and it works perfectly in the amp. This was 100% the cause of the hum, as it hummed badly with a 20W (overkill I know) 150r resistor there. Even with the PI tube pulled, the ripple would have passed right through the Cathodyne's Rp and through the coupling cap to the ou...

Martin! Good to hear from you. As always, that makes perfect sense

So my choices are the
Hammond 156L, 5H 75mA,
Hammond 155H , 5H 50mA

The 155H would be an EXACT drop in on the chassis, given the center to center mounting holes already there, which is 2" .
The 156L is 2.4" CTC, so would require shoehorning.

is 50mA enough?

Did you replace the cap can (C25A, B, C)? And the original cap can is no longer connected to anything? A 6H 75ma choke would be sufficient. Much better than a resistor. Don't overthink the stacked voltage doublers too much. I like to think of it as a four cell flashlight. Yep I replaced the capcan ...

And further, lifting C13 from the grid of the PI, the hum remains, so it HAS to be in the cathodyne node, eh? Sounds good to me. I've cleaned up the PI/PA schematic to make it easier to follow. I suggest checking all four resistors in the PI. Be sure R30 has a good connection to ground. Check for c...

If hums with the PI (V3) removed, and it goes away on standby and returns when standby is on "operate" then it is coming from C14, C15, and R33, R30, R31. The resistors you can easily measure in circuit. R33 goes to the screen power rail. R30 and R31 go to ground. I would tend to suspect C14 and C1...

So I pulled C12, the splitload coupling cap feeding the grid of the stage driving the PI. Hum remains. Given that , and that the hum disappears when I lift C14 and C15 couplers to the 6L6 grids, we can say without a doubt the hum is generated in the driver/PI tube. Edit : And further, lifting C13 fr...