tube rect/ss rect circuit

[Dave H]

Vacuum rectifier max specs on first filter caps amount to an educated guess about what the peak currents would be in a "typical" rectifier setup, and what peaks will give a long-enough life in such setups.

It's a fairly well educated guess For a capacitor input filter as well as the max capacitor value they usually also specify the min value of the "limiting resistor" (or transformer winding Rt) to be used per plate for different transformer voltages. For example EZ81 data sheet says Rt should be 2 x 150R for a Vt of 2 x 250V and Rt should be 2 x 310R for a Vt of 2 x 450V.

[R.G.]

Those old guys back then used any possible cheat! Imagine, using math and stuff to figure things out!!

[Chuck H]

Indeed! But it's fair to say that they were also experienced engineers that had to KNOW how to cheat. That is, they weren't ordering transformers with specific DCR. They noticed it, knew the effects and took advantage of it in situ. Not that you inferred that they cut corners. I'm just backing your sentiment.

[J M Fahey]

No, both things are objectives, best performance possible at a still competitive price

That said, DCR is THE big stumbling block in transformer design.

Severely limited window area means you are severely limited on wire area/diameter which directly affects DCR.

Only way to increase window area is to go to next higher lamination size, might easily double transformer cost.

[pdf64]

If intended for tube rectifiers, a fair bit of dcr in the HT winding is beneficial, as it saves on adding limiting resistors.
The PT design / winding insulation then needs to accommodate the heat generated (and reservoir caps the surge voltage).
I suspect that 'cool running' transformers (and the absence of limiting resistors) is one reason why modern rectifiers (in modern amps) are reported to fail frequently.

[trobbins]

I'd also be recommending making the JTM45+ more bullet proof by adding another 1N4007 in series - and in series with both the valve and existing 1N4007 diodes, for the double benefit of also protecting against a gassy GZ34 in to the future.

I'd recommend using PSUD2 to cross-check what valve diode peak current levels occur in your amp if you are really worried. You can measure the transformer winding resistances, and filter capacitance if you have a meter, and feed them in to the simulation. This kind of check is similar to how you would choose a secondary side fuse, as described in https://www.dalmura.com.au/static/Va...p%20fusing.pdf.

[tpaairman]

I do have a fear of toasting tubes, especially the rectifier. It sounds like the switch should be thrown with the amp off?

The "proper procedure" here would be to have it set for tubes, or as they are calling it standby, then turn on the amp, and after the amp has warmed up, you can switch to diode. Here's the thing. Typically a standby switch will simply turn off the B+. And it's really not a truly needed feature in those cases. You hear people go on about damaging the tubes from the rush of current, however there isn't a rush of current since the rectifier tube has to heat up as well before it starts conducting. Also, keep in mind that guitar amps were the only devices with standby switches. None on TV's, Radios, etc.

In this case, you could make the argument that the switch is needed, so you still have the tube rectification to get started with, then when the tubes are heated up, you can switch over to SS. But unless the tubes are running closer to 1Kv, it's well debated back and forth if damage would occur to the tubes from a sudden rush of current before they warm up.