The Amp Garage

A quick PSUD2 assessment of a JTM45 with 350-0-350V secondary (effective 80R winding per side), with GZ34 into a 32uF filter with 5k simple load for 88mA continuous supply indicates a 250mA T IEC fuse in the CT link should have some margin to cope with a hot-start, and is operating at about 75% of c...

As RG indicates, adding a fuse is not a straight-forward task, and I'd suggest is quite a technical effort to appreciate what faults can be protected against and how the added fuse would respond. If you've had to replace PT's, then there may be incentive to do such a technical assessment when adding...

This is delving into hazardous testing, which you may not have done before, in which case imho you should work with a competent person to raise your experience. The commonly used technique is to insert a 1kΩ resistor in series with the filter cap and clipping a handheld multimeter across the resisto...

Measure the leakage at rated DCV. Measure the ESR. Measure the capacitance. Look for deformities. Fairly simple, but it takes time. It can be done by lifting one leg with care, and can be done using the amp's power supply when you have a variac and use a series resistor for current sensing.

If you notice some hum/ripple getting through the SE amp to your speakers then one way to suppress B+ ripple is to parallel a 'tuned' capacitor across the choke to form a ripple trap. The parallel LC forms a brick wall for 2nd harmonic to mains frequency, which is the major ripple component.

Yes that is a reasonably well known power supply 'technique'. The other advantage of that input capacitor is that it alleviates solid-state diode switching noise by lowering dV/dt at that node. Perhaps better to use UF4007, instead of 1N4007, for that application too. Not such an issue with valve re...

I'd also suggest the polarity of the 10uF 50V ecap in series with the coring diodes is showing the wrong polarity.

I find it strange that some happily measure and set output stage tube currents etc to better than 5-10% accuracy, and spend many $$ on tubes, but don't take the time to measure mains ac current at idle, and when cranked, for their nominal mains AC voltage, and then buy and use an appropriately value...

Your modified schematic (without varistors) may well work ok, but you would need to confirm the stages are being adequately biased, and the frequency response is acceptable. The 18k allows the 820pF cap to pass inverse phase signal to that being passed through the varistor network in the cathode leg...

Ta - I missed seeing those parts, doh! Possible that they are 3.5V or 35V. Another aspect of restricting the peak signal voltage being applied to the disk could be related to time 'smear', for example where the applied voltage is sufficient to leak across to nearby stripe of disk that has, or is, ab...

I don't have a Standel schematic that include any zener diodes - can you provide a link please. The back-to-back diodes in the two schematics in the first post could plausibly have been two back to back zeners, or a diode and a zener, as I'd anticipate their function would be to avoid any high volta...

I'd suggest it would be an 'ear' adjustment to suit your playing style. A can has a fairly restrictive frequency and dynamic range response, which are aspects of what the screen adjustment would modify. The input stage is grid leak biased, so is already going to be dependent on the playing style you...

Depending on the actual amp, the anode voltage can 'fly' around quite wildly under certain playing conditions, as only stray capacitance both inside and outside the output transformer limits the voltage excursions (simply described by the transient energy equation I = C . dV/dt). Adding extra capaci...

Franco, I'm not too sure - will have a look.

The schematic is not imho showing the jfet correctly for the tremelo function. Afaik, the jfet input would be driven by the low frequency oscillator, and would be shunting V1A or B's cathode to ground, or not. Also the Depth setting introduces feedback to the oscillator, which is interesting.