Epi Electar M 1260

[olddawg]

Did you replace those batteries?

[J M Fahey]

There is a 400v capacitor between Neutral and Chassis. So the chassis isn't really neutral... but definitely COULD be if that cap failed. I removed it thinking it's like a death cap, but there was a massive hum, so I put it back. I now have about 90vDC on the B+.

Iīm quite certain that your amp does NOT follow that schematic strictly ... which after all is not the factory one but drawn by some amateur who at certain points may draw what he *expects* to find andnot whatīs actually there, because in his mind "itīs a mistake" .
1) I bet you do not have 90V +V but are measuring wrong.

Please measure across the first main cap terminals and post here, not from "ground" to +V

Also measure from where first tube cathode resistor is grounded to the far end of its plate resistor.

I bet one of those voltages is the +90V you posted, the other hovering around 120 or 130V .

2) I bet you have TWO, countīem two grounds, separated by a high value "safety resistor" , usually between 22 and 68k , across which you also have that "400V cap" (wonder what happened about posting its *capacitance* )

Now onto checking voltages in the preamp... pretty sure I got at least one open or extremely large value-drifted resistor. I'll report back on my progress. I will definitely encourage the customer to pay for unity ratio power transformer and add a 3-prong cord.

Itīs more than encouraging, tell him that the amp must be upgraded to modern safety MANDATORY standards or that youīll give him the amp back as is, unrepaired, and just in case, he must sign some waiver (which you will keep) where he says he WANTS it as is, against your Tech advice, that he will never ever plug it (so if he does, as he will, it relieves you of responsibility) and that he specifically relieves you of any responsability, it being "implied" is arguable (against you) until the cows come home.
The moment you opened it and saw a deadly problem on it, YOU became responsible too.

About that neutral/chassis capacitor. Why/how does that cure the hum? Is curing hum its intended purpose?

Yes, of course, itīs audio joining 2 grounds separated by a large value resistor and around 30V DC.

[lowell]

Did you replace those batteries?

No, I changed to cathode bias, as stated in my previous post. (this is not a snide comment as it can be mistakenly read )

JM thanks I'll report back.

[lowell]

Please measure across the first main cap terminals and post here, not from "ground" to +V

Here's an updated PSU picture. All measured across the caps.

Also measure from where first tube cathode resistor is grounded to the far end of its plate resistor.

122v

2) I bet you have TWO, countīem two grounds, separated by a high value "safety resistor" , usually between 22 and 68k , across which you also have that "400V cap" (wonder what happened about posting its *capacitance* )

I do get 120k from chassis to PSU common. Although I've searched everywhere and do not see said safety resistor.

The capacitor is .05uf 400v.

The PSU is not written correctly so I've posted what it actually is.

The Filaments are not shown correctly either:
The 25L6 are all in series, not as the schematic shows. And there is a 45ohm resistor between mains HOT and the first 25L6.

The remaining tubes are: Mains HOT - rectifier plates - 70ohm resistor - rectifier filaments - 12SN7 - 6C6 - PSU Common (Neutral)
In that order.

The 6C6 is not drawn correct either. Here's what I have.

[J M Fahey]

I do get 120k from chassis to PSU common. Although I've searched everywhere and do not see said safety resistor.

I bet itīs "hidden in plain sight" meaning you see it but donīt recognize it as such because chassis and neutral are NOT directly connected but schematic shows they are.

Too sleepy now to suggest resistance tests but stay tuned.

[lowell]

Quick side note - any suggestions on a power transformer? About 100watts, simple 2 lead primary and secondary unity ratio? Like an iso, but mountable.

[J M Fahey]

What you describe *is* an iso transformer

Mountable how/where?
All transformers I know come with mounting ears or brackets, so you bolt them to chassis or, worst case, cabinet wall or floor.

[lowell]

Don't think there's enough room in or on the chassis. But, depends on how big we're talking'. May have to be cabinet wall.

[nickb]

Don't think there's enough room in or on the chassis. But, depends on how big we're talking'. May have to be cabinet wall.

I recommend that you add a full wave rectifier after the iso transformer. This will ensure the transformer does not saturate from the unbalanced load caused by the half wave rectifiers. It will also significantly reduce the power supply ripple.

[nickb]

Quick side note - any suggestions on a power transformer? About 100watts, simple 2 lead primary and secondary unity ratio? Like an iso, but mountable.

Mouser

[lowell]

Think this one should suffice: https://www.tubesandmore.com/product...solation-90-va

So rectify before the "rectifiers?" I seem to remember that rectifiers still operate as usual if fed DC. Correct? Or am I trippin'?

[nickb]

Think this one should suffice: https://www.tubesandmore.com/product...solation-90-va

The Mouser one is half the $$$

So rectify before the "rectifiers?" I seem to remember that rectifiers still operate as usual if fed DC. Correct? Or am I trippin'?

Correct. They'll be operating at 120Hz instead of 60Hz too.

[lowell]

Awesome thanks! Good call on the 120hz post rectified...so that'll reduce hum as well cause the filters are charged twice as fast. Got it.