Reeves Custom 50PS non-polarized electrolytic replacement?

[x-pro]

I'm not claiming that NP is smaller in size compared to a polarized capacitor. I was pointing out that it is smaller and more conveniently sized compared to film at the same voltage.

[Helmholtz]

I drew it myself, relying on photos of the area from the internet.

You probably have better eyes than me.
I agree that an NP cap would make sense in the NFB path.
But 10k//10µ would mean a corner frequency of 1.6Hz, so not much sense.

[x-pro]

In the Hiwatt 50, the FB resistor is 10k. This is the original circuit.
In Reeves Custom, the circuit is the same, but with a few changes: 100k resistor, and its shunting on the low-frequency band with a 10uF capacitor.

[Helmholtz]

In the Hiwatt 50, the FB resistor is 10k. This is the original circuit.
In Reeves Custom, the circuit is the same, but with a few changes: 100k resistor, and its shunting on the low-frequency band with a 10uF capacitor.

Shunting a 100k resistor with a 10µ cap means AC shorting the resistor down to low frequencies. Doesn't make sense to me.

[x-pro]

Your truth
I only assumed such a capacitor connection. I can't think of anything else. Let's wait for the author of the article to draw this part of the diagram.

[g1]

If it is indeed connected to the supply voltage, I was wondering about heater elevation. 330K and 100K in series off the supply node.

[g1]

As it seems the 100R is the voltage dropper for the screen node, perhaps it's just a place to sense the screen supply voltage for the power scaling.
(edit: that is assuming the voltage dropper mentioned in post #4 is the screen dropper, which may be incorrect)

[glebert]

Did Reeves respond to the schematic request? I would also ask if they can provide a replacement part since you are having trouble sourcing it. My patience for fixing boutique-ish amps has gotten much shorter the last couple years. I know I'm never going to get paid for the time I need to invest in researching the amp, so if it isn't something simple I suggest sending back to the manufacturer for repair. This is especially if they won't provide a schematic or if the schematics are wrong (looking at you Mesa).

[Pedro Vecino]

If it is indeed connected to the supply voltage, I was wondering about heater elevation. 330K and 100K in series off the supply node.

That's what I thought at first, although with that ratio the reference voltage would be higher than usual.
This would be easy to check if the junction point between the 100K and the 330K resistor is also connected to the junction point of a pair of resistors coming out of the filaments or from a central tap of the secondary that feeds them.

[Mick Bailey]

If for heater elevation, why connect it to the power scaling circuit? It may simply be a supply voltage for the power scaling, but the question remains, why use a NP cap? With one side tied to ground the cap could never see a reverse voltage. Maybe they just had them to hand.

My thought is that the cap/divider resistors sets the reference ratio for the power scaling. Owners of the TUT series of books may be able to provide a further insight.

[J M Fahey]

We NEED the schematic, reverse engineered if nothing else is available, and whatever it takes to draw it properly.

Flying blind is crazy, striking a mountain peak is easy and happens all the time.

That cap *exploded*, for God´s sake!

I have contacted Reeves for a schematic

GOOD!!!! Let´s wait for it.

EDIT:

Without a broader schematic that includes the power scaling module its difficult to determine what its doing, but as it's connected to the power scaling module I don't think it's part of a conventional tube circuit. Given that it's non-polarized, why would the amp builder specifically choose that component rather than a more readily available polarized type?

Maybe if the circuit was drawn out at least in part it would give more of a clue in the absence of any factory schematic.

Is the power scaling module encapsulated or otherwise obscured? If so, its likely that there wont be any detailed schematic for the module.

Exactly my points.
If it´s *labelled* "Power Scaling" then it must be the London Power version.
Circuits in general are not patentable but trade marks/brands are.

EDIT 2:

My patience for fixing boutique-ish amps has gotten much shorter the last couple years. I know I'm never going to get paid for the time I need to invest in researching the amp, so if it isn't something simple I suggest sending back to the manufacturer for repair. This is especially if they won't provide a schematic or if the schematics are wrong

THAT +1000

IF I have the physical chassis on my bench, probably can hand draw most any circuit in 1 - 2 - 3 - 4 hours, depending on complexity but more on how much it deviated from established practice.

BUT: is customer *paying* for that extra work?
Guess not, so forget it.

Back in the day, NO Internet anywhere , no access to Factory manuals, I spent hours and hours hand drawing schematics for anything interesting/modern/trendy that fell on my bench, there was no other source.

But now????

[x-pro]

IF I have the physical chassis on my bench, probably can hand draw most any circuit in 1 - 2 - 3 - 4 hours, depending on complexity but more on how much it deviated from established practice.
BUT: is customer *paying* for that extra work?
Guess not, so forget it.
Back in the day, NO Internet anywhere , no access to Factory manuals, I spent hours and hours hand drawing schematics for anything interesting/modern/trendy that fell on my bench, there was no other source.

I draw sometimes too, even in the days of the internet. Not everything is available and not all give schematics, but I have to do it.
Take for example Hypex modules (E/V ELX-series), Pascal modules (JBL PRX-7xx). I had to draw them. This helped many of my colleagues from different countries.
And in the future, with the joint help of colleagues and repair, the cost of such re-engineering pays off both in time and financially.

[Beyer160]

We NEED the schematic, reverse engineered if nothing else is available, and whatever it takes to draw it properly.

Agreed. I was hoping someone here had it, or maybe I'd get lucky and somebody knew what that cap was.

I talked to the owner at Reeves yesterday, he didn't want to send me a schematic (evidently they don't even have schematics for the power scaling boards, they just buy them from London Power) but asked for photos and said he'd pass it along to one of his techs and get back to me.

Failing that, I'll map out the circuit but I still won't know what voltages to look for.

It doesn't look like part of the bias circuit- it's not connected to the bias pot and the value looks wrong (and aren't bias caps always polarized?) but yes, we won't really know without a schematic.


Thanks to everyone who has responded!

[J M Fahey]

"In principle" Power Scaling or any of its variants is nothing more that a variable +V supply, from full original +V down to something so low that electrons don´t flow from cathode to plate any more.
What´s amazing is that amp works "properly" within most of that range.

In theory it could be fully passive, just a BIG rheostat, but since it would be horribly inefficient, next best s to use a normal potentiometer to control a MosFet gate; Drain connected to +V and Source provides regulated +V to power tubes (at OT center tap).

One problem is that it does need different bias along that range for proper functioning.

Easy if cathode biased, they sort of self adjust; now on fixed bias amps, this bias voltage must also track +V and relation is not exactly linear, so it becomes somewhat more complex.

London Power developed a properly tracking system so it´s a favorite, maybe others can too.

Most basic type, and it works fine for small-ish amps, think push pull 12-20W ,say 2 x 6V6/EL84 is this one.



Straight from:

https://www.emprizeamps.com/post_powerscaling.html

Now I bet the one fit to that Reeves amp is way more complex, but this is the basic idea.




Is it on a separate board or integrated on the main one?

Or is it built PTP?

[dmeek]

Many years ago I worked on a Marshall amp with a power control mod. It was called Claret Power Control. It was a variable power supply for the screen grids. The mystery for me is why an NP capacitor? Post a few photos of the area.

Claret Power Control US4286492.pdf