Heathkit - power supply question (replacing diodes)

[Guitarist]

I'm about to swap out the olde diodes for some 1n4937 Freds and I was wondering if I can omit the two first caps. If so, should I convert to a full wave bridge?
If not, can I run BOTH secondary wires into the diodes (forward) and NOT ground the lower diode, kind of like a Marshall? I'm no wiz on PSs.
I will be adding a 3-prong power cable.
thanks.

[Jazz P Bass]

Curious what you are about here.

What are the Freds going to do that a plane jane diode wouldn't?

And why are we reinventing the circuit? (what is the gain goal?)

[Enzo]

That is my reaction too. This is a simple circuit that works - Heath engineered things well in general - so why change it?

[vintagekiki]

Detail sematics you have shown is a classic power dupler who must work exactly as it was designed by experts in Heathkit.

If you paid attention to the power supply transformer secondary voltage 117 VAC got that after rectification due to a projected detail schematics (silicon rectifiers, C18, C19) is approximately twice the value of the dc voltage (295 VDC)

If you convert this schematics to classic full wave bridge, after rectification you will have to lower the voltage on the first cap (117VAC * 1:41 = 165VDC), which is insufficient for any serious work amplifiers.

Engineering advice:
If you are not skillful to work on amplifiers or devices that use high voltage (> 50VDC), better not to experiment.
High voltage gives only one chance, without another chance in case of failure.

This is a high voltage which often can be done with the ultimate fatal outcome.



knowledge of the language: https://translate.google.com
sorry

vk

Useful links:
Voltage Doubler,Voltage Tripler,Voltage Quadrupler circuit
Voltage doubler - Wikipedia, the free encyclopedia
Voltage multipliers : Diodes And Rectifiers - Electronics Textbook

[Guitarist]

Thanks for the replies gentlemen. I did some reading prior to the post and just looked over kiki's doubler links. I now realize the 'net-sourced schematic is arranged different than what's in the manual here, and that's because there looked to be an odd connection there that wasn't. Now att. is a photo of what I have in hand. I keep humble but am not a total noob, still, looking at diodes is a harder aspect of tube amps for me anyways.

I am recapping the unit, and you've convinced me to leave the rectifier design alone And I do have some F&T 30uF @ 500V caps for the doubler caps. Originals were 200V. Should I up the uFs? I'll be increasing the uF downstream with 100 (or 200) at the diodes and after the first dropping resistor.

My 70 year old blacksmith friend always had issues with the power supply. He built the amp when he was 11 and his wiring looks amazing actually. The schematic is saying 290V at 'A' but he suspected cap issues.

The ec86 look to have a max plate voltage of only 250V.

[vintagekiki]

Schematics that attach also is voltage doubler only different is drawn.
Substituting 2 diodes with capacitors classic full wave rectifier schematics, becomes full wave voltage doubler.

The value of the dc voltage full wave voltage doubler, does not depend from the values (in uF) capacitors C100 and C101.
From their values (in uF) depends on how much current can give voltage doubler, by principle more uF more mA.

An unwritten rule for good rectifier is 1000uF (and more uF) to 1 A of current or 100uF to 100mA. Operating voltage C100 (C101) is minimum half of the total output voltage


knowledge of the language: https://translate.google.com
sorry

vk

Useful links:
Voltage Multipliers - Doubler, Tripler, Quadrupler

[Guitarist]

Hey that's helpful. So those double caps are now 2 of 100uF @250V so that approaches the plates drawing in mA. Thanks so much!

[R.G.]

I completely agree that the circuit topology should not change. Replacing caps - Good!

Replacing diodes is optional. Yes, the originals were good choices from what was available then. However, we have half a century of semiconductor advances now, and also half a century of heat cycling on the diodes in there. They are probably fine, but replacing them **if it's done as a correct replacement, no mistakes** is also OK. There are only marginal reasons to change the diodes. One of them is the generic "put in new ones with no wear" idea. The idea of replacing with FREDs is OK, but really only an advantage if it also gets rid of hum/buzz that is caused by the slam-off and RF ringing of the original diodes. That may or may not be the case in this amp. If the amp is not experiencing RF-squark buzz from the rectifiers, fixing something that's not broken won't help.

[vintagekiki]

When rebuild (audio) devices generally replace only defective components and components subject to wear or aging (electron tubes, electrolytic capacitors). Semiconductors (diodes, transistors) are replaced very rare, unless defective.

There is no reason why would be replacing the correct components.
When servicing the unit should if possible as much as possible to remain in its original factory condition.

We should not forget that the factory in design to those of professional engineers, which is in the design for a given performance projected schematics that are performed by the (audio) device. Any deviation from the original concept of losing original idea of the unit.



knowledge of the language: https://translate.google.com



sorry

vk