sound city 50R problem

[chief mushroom cloud]

ok, all you brainiacs....I have a challenge:

I have a SC 50R on the bench.
Everything works dandy...EXCEPT...
there is 6Vp-p 120Hz ripple on the HT caps when standby is switched on. Textbook ripple. Real purty. This translates to over 4p-p on the OT secondaries. Loud.
Amp puts out healthy 50W clean (just before clipping).
Amp draws 105mA at idle.
I disengaged the preamp HT from power and screen HT. Ripple is still there. Unsoldered all the OT connections and measured DCR w/ DMM. Looks good, but I also know that doesn't tell the whole story.
Checked windings ratio (per Jelle's suggestion) and looks good...nice 3.4k OT.
I've isolated the EL34s from incoming coupling caps. Ripple still there.
I've moved wires around (and chopsticked)...absolutely no change in ripple.
When I shut off standby (disengages all circuitry from rectified HT)...HT goes flat, very nice ripple free 495V).

Now, I can only surmise that for whatever reason, there is a massive DC loading issue with the OT...and it's not able to cancel it out. But the idle draw doesn't raise a red flag here. But now I'm suspecting PT.

EL34s biased at 70% plate dissipation

Before I go out and buy another PT or OT (at my expense) and tack in for test...I need to know if I've missed anything.

Is there any good way to test PT in situ?

TIA

'as built' schematic attached

[martin manning]

What about the 2x 200uF reservoir caps? With no load the plate supply node might look ok, but not so much when the standby is on and there is current draw.

[JazzGuitarGimp]

My guess would be one of two things, or perhaps a bit of both:

- undersized PT
- under filtered

Another possibility might be somehing pulling heavy curent in the heater circuit, robbing power from the HT winding.

[chief mushroom cloud]

What about the 2x 200uF reservoir caps? With no load the plate supply node might look ok, but not so much when the standby is on and there is current draw.

well, I thought of that...BUT..these are brand spankin new F&Ts, and I tried brand spankin new ARS cap, too. Same prob.
I have a 100W Burman that uses the same power supply topology w/ two 200uF caps in same config....that amp is dead quiet.
but yea, no load...it's flat...w/ load it's overkill ripple. The 100W Burman has about .25V ripple w/ same caps.

[chief mushroom cloud]

My guess would be one of two things, or perhaps a bit of both:

- undersized PT
- under filtered

Another possibility might be somehing pulling heavy curent in the heater circuit, robbing power from the HT winding.

Stock Partidge trannies and same filter as 100W Burman.
I will check the heaters now.....

[M Fowler]

What about the bias cap?

[chief mushroom cloud]

What about the bias cap?

entire bias circuit new and rock steady...besides, it's a half wave rectifier, so it generates 60Hz ripple.....this is 120Hz

but keep the ideas coming...I appreciate ANY response at this point

[overtone]

Morning all,
with the EL34s disconnected from the coupling caps - then surely any AC still lurking there could be from the filaments / heater circuit?
But why then does it measure no ripple with standby off, circuit disconnected from HT?

Got me running in circles, I need a huge coffee. I was asking myself if you have heavy rectifier pulse getting into the heater CT. Maybe elevate the heaters as a quick test. Saw something similar with a GA40 where the filament winding was funky. Separate filament iron was a simple fix and the original PT carries on without the heavy lifting.

Curious, is the PI filter grounded to the chassis as drawn?

Best, tony

[martin manning]

I'm leaning toward an OT problem. 6V p-p is not much ripple on 500V... 1.2%, and in theory the OT should be canceling it. But 4V p-p shoes up on the secondary? That doesn't sound right at all

[billyz]

Maybe it needs a choke .

[tubeswell]

Rectifier diode?

[tubeswell]

Is there any good way to test PT in situ

measure the secondaries with dummy loads connected across each winding?

[martin manning]

Rectifier diode?

Nope. No blown fusees and 120Hz ripple says they are working.

Chief, you could sub in any working OT (like a 4k Zpri Fender-type) to see if that is the problem.

[jjman]

Have you disconnected the NFB to rule that route out?

[chief mushroom cloud]

Rectifier diode?

Nope. No blown fusees and 120Hz ripple says they are working.

Chief, you could sub in any working OT (like a 4k Zpri Fender-type) to see if that is the problem.

I think you're right. I've been in contact w/ Mark Huss, and he suggested the same thing. I have an A470 OT I'm going to tack in...if the ripple get's cancelled...the Partridge OT get's a rewind.

I suspected this at first, but had to eliminate everything else and the OT behaves and measures normally except for this. I forgot I had the A470 (and I actually have another 3.4k Partridge that I forgot about, but it's the size of a 100watter!)

stay tuned