NZGuitars.com : Filling you with GAS since 2003

Here’s a good discussion of this topic:
http://www.valvewizard.co.uk/Triodes_at ... encowe.pdf

It gets pretty technical and is focused on pedal design, but you can get a sense of the benefits and limitations.

Divide and conquer! Pull out all of the preamp tubes - just leave the power tubes in and power it up to see if it hums. If it does, the issue is in the power amp section. If the hum disappears it's the preamp, start adding one preamp tube at a time, starting from the socket closest to the power tube...

If it was properly rated (maybe 10W?) it shouldn’t get too hot to touch. Certainly not as hot as a rectifier tube!

If you want more sag just get Clarry to add some series resistance to the power supply. A GZ34 has an internal resistance of about 175ohms, so a resistor in the same ballpark will produce similar response. It’s way cheaper than rewinding a transformer, installing another valve socket, buying a recti...

Does it have the problem at lower and higher volumes? Could be a bad or extremely microphonic preamp valve. Try tapping them lightly with the amp on and the volume up a bit. You could also try swapping the two preamp valves around just to see if that changes anything. Also, give your guitar lead a w...

With hum, it’s usually down to grounding issues or the heater circuit. You could try elevating the CT of the heater windings by 20-30VDC. Attaching it to the output valve cathode usually does the trick but in your case I’m not sure how the zener would affect things. It’s an easy thing to try in any ...

Jay wrote: ↑Wed Jun 07, 2023 7:10 pm Amps with PCBs can GTFOML, Especially the ones with double sided PCB tracks.

It doesn’t help that they often use lead-free solder on those, so you have to roast the board to remove components.

Sounds like you’re connecting to the wrong side of C1 and C3! Do you have a multimeter? Always a good idea to test for voltage before you do anything.

That actually looks virtually like the Trainwreak type 3 MV? Based on a quick google on this site and looking at the schematic, are the bits where I’ve circled the phase inverter outputs? Yep, actually that would work quite well. I had something else in mind but the trainwreck option is simpler. An...

You can’t use R25 as a MV - at least not easily. It has voltage across it originating from the top of the 56k cathode resistor and acts to establish the grid biasing. It would only have one gain stage between it and the previous volume control, so it probably wouldn’t get you where you wanted to be ...

Lowering the sag resistors will not have a huge effect on preamp voltages at idle. The reduction in voltage is proportional to how much current the power amp is sucking through them - at lower volumes it’ll be small but once you start to overdrive the power amp it’ll have a more noticeable effect. T...

10w resistors will do the job provided they’ll fit.

This amp doesn’t have a negative feedback loop as far as I can tell.

C2 and C19 are the caps to reduce to make the switching less bright and dark (respectively).

Reg18 wrote: ↑Fri Jun 02, 2023 9:11 pm So in this pic R22 and R30?

Yep! They’re the ones.

I’d forget about the caps - they’re not the reason for the sag. Just off the left of your snipped schematic image are two 270 ohm 7W resistors between the bridge rectifier and the 1st cap (c28). Change those resistors out for something lower, depending on how much less sag you want. You could drop t...

jeremyb wrote: ↑Wed May 10, 2023 8:45 am Found a friend in our dishwasher yesterday morning…