Tube Amp Archive

if the diodes in series don't have matching turn-off times, the faster of the two turns off before the other one does and is left holding off all the reverse voltage until its series buddy diode turns off too. So the faster turn-off diode can fail in reverse breakdown over time from the transients....

ACS712 based pcb seems small and with enough resolution.

The prospective fault current has to exceed 210mA +/- your sensing tolerances.

If you disconnect the anode then (depending on circuit config) the screen may get stressed - not a concern if the tube has had a non-recoverable fault, but a concern if the tube is not the cause of a glitch.

One issue with using the 'disconnect the cathode' method is that the cathode may float to a high voltage level. That is not a concern if the valve is already damaged, but some faults may not be due to the valve itself, and so a 'good' valve could then be damaged by excessive heater to cathode voltag...

I just finished a 2xEL34 garter cathode biased amp and used two MOVs between the plates.... The parts used were Eaton 625VAC/825VDC, 1.65kV, 230J, 280pF (Mouser 504-MOVHE14V625Y). Was that with one MOV from each anode to B+ ? The datasheet indicates a max capacitance of 200pF ? The 1mA Vdc is at le...

It can be a joy to eeek out info on such an amp. I have a prototype amp from a local music/service shop made 1964 when young aspiring band members couldn't afford a main label amp so pestered for something cheaper. The background story to that included hearing from a then young guy who recalled the ...

I had a quick look at how an LFO output changed with minor part alterations. There could well be an audible difference to using an LFO that has lower harmonic levels, as low frequency content getting through to the output stage and speaker could easily cause noticeable problems. https://www.dalmura....

Why only low frequency? A bypass cap won't boost only the lows. Yeh, arse-about comment - doh. Wrt the 10nF cathode coupling in the LFO stage, most Maggies just used 3M3 grid leak biasing, but a few models added 1k of cathode bias, and only the 460 (I think) also has a 10nF bypass. My only thought ...

The input stages don't use cathode bypassing (perhaps to constrain low frequency gain), but that then allows more hum to ingress from the heater, which they then try to mitigate with dc elevation of the heater. That also means they need to keep cathode resistance as low as practical, which they then...

Perhaps use a MT3608 to generate a regulated +12V or 15V, and then use a 12V to +/-15V converter.

Also many cheap ebay style dc-dc converters around to achieve whatever dc voltage you want, including step ups like MT3608 and XL6009 based modules, and even for B+ rails with perhaps 20W level grunt from a Baxandall style (Royer-like) small module. The step-ups can provide a regulated output level ...

Rearranging the preamp stages is likely the simplest effort - in a technical sense. You could even use a pre-out to check what any re-arrangements sound like. Changing the output stages is more technically demanding. You could revert to simple pentode mode, but you'd need to confirm the turns ratios...

The output stage is likely a unity coupled cathode-anode configuration with a form or UL screen drive, and a form of grid coupling. I don't recall seeing all those schemes in the one output stage before. You'd need to measure the turns ratios of the cathode and anode windings to see if they were uni...

Well done for having a go at a schematic. R8 and R12 are likely to have the same value, to make that phase splitter stage a cathodyne/concertina/split-load configuration. The output stage does appear to be a bit uncommon. There should be a main high voltage supply rail (B+) feeding into that circuit...

The amp looks to be very well made, and may well have few if any faults. Have you been able to locate a schematic for it? I'd recommend some caution and time delay before any knee jerk stripping of all wiring. You may well find that some of the circuitry is very similar to some classic guitar amp ci...