Heathkit Mono Amp (Heathkit W-5M) Filtering and Max Voltage Questions

[misterc57]

Bunch of questions on filtering and voltages.

I want to replace all the original filter caps on an old mono audio tube amp. The amp has caps in series with balancing resistors. Voltage rating doubles in series, I assume this is why they did this.

I wonder why not use a single capacitor instead without the balancing resistors?

Example

replace (2) 40 uF @ 450V in series with (1) 22 uF @ 550V
replace (2) 20 uF @ 350V in series with (1) 10 uF @ 550V

Currently there is 500V at the first filter cap. The amp calls for a pair of KT-66 but has a pair of 6L6GC tubes and there is 470V at the plates.

I am wondering if it is a good idea to lower the high V with a dropping resistor, and if so what value and where to place it?

Schematic attached. Thank you

[Helmholtz]

6L6GC plate voltage limit is 500V and screen voltage limit with UL operation is the same. So no problem here.

550V cap rating will not be enough.
Pull both power tubes and measure B+, then add 10% safety margin to find safe cap ratings.

[Jon Snell]

I would imagine that as there is a possibility of 643 volts across the first pair, (455 x 1.414), even now that would be almost impossible to purchase due to the breakdown voltages of electrolyte and the ripple current required must be high enough to avoid overheating, so small can types would be no use in this employment.
Dual electrolytics are still commonly used as the ESR is low and the ripple current handling is high.

Resistors do not lower the voltage, they lower the current available and in turn the voltage will be less whilst the valves are drawing current but return to the high value when the current reduces.

[misterc57]

Thank you all. Great info!

I did pull the output tubes and watched the V rise at the first pair of caps. Once it hit 600V I shut it down.

I did replace those first two pairs with new 47uF @ 450V and new 100K 2W resistors.

[misterc57]

I have refurbished this amp. All filter caps replaced. Almost all coupling caps. Amp sounds great and has a lot of punch.

I have read many times to not install 3 prong cords on these old audio amps as it can introduce noise.

I kept the 2 prong cord installed. I am however seeing 12 VAC leaking on the chassis, if I change the AC plug orientation I read 3 VAC. I do not see any safety caps in this amp or in the schematic.

Any thoughts on this?

I think the original design this amp would be connected to a pre-amp, perhaps the pre-amp had the safety cap(s).

Thank you, MC

[Pixel]

Do you see any voltage at the wall socket between ground and neutral?

[misterc57]

Do you see any voltage at the wall socket between ground and neutral?

Yes, when the amp is turned on. 12 VAC and 7 VAC depending on orientation of the amp AC plug.

[Pixel]

Yes, when the amp is turned on. 12 VAC and 7 VAC depending on orientation of the amp AC plug.

Nothing between ground and neutral when the amp is not plugged in?

[Helmholtz]

There are no safety caps. No cap can improve safety.

As the amp most probably doesn't comply with class II requirements, it needs to be safety grounded.
Otherwise it doesn't comply with safety regulation and is a potential risk.
Look up your UL safety standards.

I'm sure there's a solution to any noise problem.

[misterc57]

Nothing between ground and neutral when the amp is not plugged in?

About 0.2 VAC

[The Dude]

There are no safety caps. No cap can improve safety.

As the amp most probably doesn't comply with class II requirements, it needs to be safety grounded.
Otherwise it doesn't comply with safety regulation and is a potential risk.
Look up your UL safety standards.

I'm sure there's a solution to any noise problem.

^^^^^^ THAT. Please make the amp safe.

[Tom Phillips]

... I have read many times to not install 3 prong cords on these old audio amps as it can introduce noise...

That warning was most likely about the possibility of introducing a ground loop between the power amp an the connected pre-amp that causes a low level background hum. If that happens there are solutions that allow you to keep the earth safety ground connected. It's not always a problem. You could do a test by grounding the chassis to the earth ground at the wall socket where you have the power amp plugged in. You can access the ground easiest by connecting the jumper wire between to the screw on the outside of the wall socket cover plate. Feel free to ask further questions if this isn't clear to you.
Cheers,
Tom

[loudthud]

In just about any transformer connected to the Mains, there is capacitive coupling between primary and secondary. To avoid this, you need a grounded shield between primary and secondary, or some kind of split bobbin arrangement so the secondary is not wound over the primary. In the USA, one side of the 120VAC Mains is grounded so by reversing the 2 wire Mains plug, you change the Voltage behind that capacitance between windings.

This capacitive coupling is why you need to ground the center tap of the heater winding. If you disconnect the center tap, you can measure AC Voltage to ground that can be over 100VAC because the heater winding is usually wound over one end of the high Voltage secondary. That big AC Voltage is easily picked up by high impedance preamp circuitry. Even a resistive virtual center tap is enough to conduct this unwanted Voltage to ground.

The same capacitive coupling is why early guitar amps in the USA had "Death Caps". The idea was that a capacitor connected between the chassis and the grounded side of the Mains would conduct the unwanted Voltage to ground, but it only worked if the Ground Switch (which selected which side of the Mains the cap was connected to) was in the correct position. If the switch was in the wrong position, the chassis and the strings of your guitar were connected to the HOT side of the mains through a capacitor. If the cap shorted ... just ask Keith Richards.

[The Dude]

Yeah, but Keith is still alive (I think- it's hard to tell for sure ). The guy you'd wanna ask is Les Harvey, except you can't anymore.

[loudthud]

I think this article is about the time I was thinking of: https://www.sacbee.com/news/local/hi...161670488.html
Technical details may not be totally correct.

Cool picture showing Brian Jones playing through a blackface Dual Showman.