Tube Amp Archive

I hate those little Piher pots.

All these small digital scopes seem to have a max input of 300v RMS with a 10x probe. Won't scoping nominal waveforms (DC + AC) inside a tube amp push this limit?

Agree, it seems the 4M7 on the input and the plate resistors would be a better noise-reducing target then, say, the 470R cathode resistor.

Maybe they were ahead of their time anticipating the "carbon comp distortion is good" fad.

Re-amplifying to isolate the actual speaker from the attenuating network makes calculations simpler and avoids having your network depend on load/speaker impedance. You just have to make sure the re-amp stays in its linear range, which doesn't seem to be an issue in your use case. Also, ideally the ...

The symbol drawings in that schematic look awfully familiar.

The attenuator seems very complicated, is there no easier way to accomplish what you want?

Is the re-amp a completely separate product/build? Why not combine the two?

A single point of failure.

The Marwatt "HIWATT" channel has a HIWATT tone stack but still a normal unregulated PI. The regulated PI is part of the sound IMO. Also, you're not going to get a lot of headroom (another HIWATT trademark) using EL84s.

I agree, the switch on the speaker jack is not needed and adds complexity.

I like sluckey's diagram, but I'd add a 220ohm 5w resistor across the output jack as insurance against contact corrosion or bad cables.

The most immortal amp I know of is the Peavey CS-800. We used a pair of them back in the day for our PA. I remember replacing a lot of output transistors (which were fortunately socketed). To be fair, unless you have a whole matched set of outputs, mismatched Betas etc., will cause more failures th...

The 'stiffness' of a supply is controlled by the size of the filter caps, and to a lesser extent, the impedance of the AC source. This is probably why the Marshall style uses a Pi filter (C/R/C) to compensate for the high source impedance. If you have a dedicated bias winding that can supply enough ...

I may be oscillation, these sometime pop up during note decay on multi-stage high gain preamps. Do you have a scope?

Wouldn't hurt to check the filter power supply caps. The amp is likely 30 years old at this point.

I remember that paper magazine article (from way back in the day). The writer was electrically competent, and this was way before the internet. This was a hobbyist magazine, though, not an industry journal. A couple of points. Most (all?) tubes were designed for AC heater power, so probably little t...

Something like that is easier to afford now that you can get $1-2 digital meters from AliExpress.