Using a pot instead of 2 resistors for a heater center tap

[mooreamps]

I'm looking at the print, and it's actually laid out pretty good. The only thing I can think of ; at the moment ; to make the high voltage power supply really bullet-proof ; install a pair of 1n4007 diodes in front of V6, in series with pin 4 and pin 6. These will help keep the reverse peak voltage off the plates of the rectifier tube.


-g

[CitizenCain]

Yeah, the way I thought about this project was that since this amp was originally designed as a hi-fidelity amp, there's likely things in it that are not used in a typical guitar amp. I planned to keep most stuff intact, but some of the circuits I wondered about. This hum balance setup was one, the cap and resistor to ground from the PI grid was another. I had that one hashed through in another thread

I'm also adding a couple of mods to my friend's amp since I'm placing it in a different chassis and rewiring the whole thing anyway. I planned to add a gain control between V1a and V1b and play around with bypass caps on the triode cathodes. I also want to make V1a switchable.

I'm looking at the print, and it's actually laid out pretty good. The only thing I can think of ; at the moment ; to make the high voltage power supply really bullet-proof ; install a pair of 1n4007 diodes in front of V6, in series with pin 4 and pin 6. These will help keep the reverse peak voltage off the plates of the rectifier tube.


-g

Gary, not sure I follow you on this one. Wouldn't that sort of "pre-rectify" the voltage going into the rectifier? Would the rectifier tube be able to function properly then?

[Enzo]

The rectifier tube passes the part of the AC cycle going the right way and blocks that going the wrong way. If the wrong way part is already blocked, why should the tube stop passing the right way parts? All the tube knows is: pass the right polarity. It has no idea whether there was or was not any opposite polarity voltage knocking on its door.

A tune rectifier is still a rectifier, so having silicon diodes in series with the tube plates is no different from having two silicon diodes in series. That works, doesn't it?

[Enzo]

As to the hum balance pot, sure, keep it. Few people would bother to replace such a pot with the two resistors. What we would do it ADD two resistors where no center tap existed. The wiper of your pot was connected to the positive voltage from that voltage divider. One could just as easily connect the common point of two resistors to that same spot with the same benefit.

[Enzo]

What the hell does a filter cap do in an AC power circuit? It's not carrying signal and there's no way in hell you're going to filter the ripple out without rectifying it. Sounds like your amp builder was brain damaged.

.... what the fuck?

... WTF?
... and what the hell brain damage is this shit?

... See if the designers actually stumbled upon something or if this is as retarded as it looks.

Hey bluefoxicy. I genuinely hope any input I have provided has been helpful, and I think everyone else here has similar feelings. But I see in a number of your posts a trend to decide something is demented when you don't understand it.

For example you are all concerned that these heaters are at AC, and yet there are filter caps and DC involved. This is EXACTLY the same electrically as the 5VAC on the B+ rectifier tube cathode we discussed the other day. This is an AC voltage elevated by some DC amount.

A bit of advice: just because you don't understand something doesn't mean it is stupid, doesn't mean no one knew what they were doing. When you see some circuit that doesn't make sense, instead of thinking the professional engineers who designed it were idiots, try thinking about finding out why it was there.

I have been in electronics personally for 54 years now, and professionally 34 years. I have a substantial level of experience, I have learned a lot about electronics, and more about troubleshooting in particular. And yet I still learn something new EVERY GODDAM DAY as I go through this life. I learn things from the people here all the time. I see things in circuits and schematics I have not seen before. I learn from those opportunities.

You might be well advised to look at circuits you don't understand as learning opportunites yourself.

[CitizenCain]

The rectifier tube passes the part of the AC cycle going the right way and blocks that going the wrong way. If the wrong way part is already blocked, why should the tube stop passing the right way parts? All the tube knows is: pass the right polarity. It has no idea whether there was or was not any opposite polarity voltage knocking on its door.

A tune rectifier is still a rectifier, so having silicon diodes in series with the tube plates is no different from having two silicon diodes in series. That works, doesn't it?

Ah, I see now. Another wrinkle added to the gray matter

Thanks, Enzo!

[mooreamps]

Gary, not sure I follow you on this one. Wouldn't that sort of "pre-rectify" the voltage going into the rectifier? Would the rectifier tube be able to function properly then?

Without the series diodes, the plates of the rectifier will see the voltage potential of a positive pulse on one plate, and a negative pulse on the other plate. As the rectifier tube begins to age, it can produce an arc between the two plates.

With the series diodes, the plates of the rectifier will only see the alternating positive pulses on each plate. The voltage stress between plates is reduced to almost zero volts. Any rfi generated by the diodes are isolated by the tube plates themselves, so fast recovery diodes are not required.

-g

[CitizenCain]

Thanks, Gary. That's a neat idea, I've never seen it mentioned anywhere before. Tips and tricks! That's what it's all about