Night of the living AC50 MK iii

[Mr_bibbles]

Hey y'all,

Have an vintage AC50 MKiii on the bench in very bad shape, it's been unused for 20+ years.

http://www.voxac50.org.uk/images/sch...ic_mark_03.jpg

After repairing some basics like an amature cord replacement, bypassing the Brimistor, replacing 33/33uf Caps, new tubes, new rectifier, it was still pulling a lot of current on the limiter. After disconnecting one end of the choke (measured 190 ohms across) it settled down, and I could see 500 on the plates WITH the limiter. The bulb was now pretty dim, but no sound / hiss / hum. I know the output is making it to the speaker because I can hear a -pop- when I drain the B+ at the OT tranny.


For test purposes, I tried installing a resistor in place of the choke; after putting about 4k on it I have 290 plate volts, still no sound, and it's pulling down the AC so that the preamp tubes are not visibly lit. Without the choke, the filaments are fine across the board.

My main questions:

I know you can replace the choke with a resistor, but in this case, can I just bypass it until we have signal again? It's possible something like a failed electrolytics in the preamp are cutting the sound, and since it seems like it's job is just a filter, is this an option? If not, how would I calculate the value of a temporary replacement resistor for testing purposes?

2. I've never run into a bad choke before. Resistance was under 200 ohms, OL to chassis while disconnected, so according to GEOfex it's 'likely ok'. Could I be missing something here? Could it be functioning well, but allowing a problem upstream when connected??

Thanks!

[The Dude]

Choke is probably fine. I'd reconnect it and temporarily pull the output tubes. Does it draw excessive current with the tubes removed? While they're out, check the bias voltage. Is it there?

[Mr_bibbles]

Sorry, should have stated at the outset, WITH the choke installed, no tubes, the plate voltage drops to 35 Volts. If I install the tubes it goes down to 25. Pretty pertinent information im guessing....

Grid voltage shows up no problem, have about -10 at the lowest settings.

[The Dude]

Bias voltage should be in the neighborhood of -50V. It will definitely draw lots of current at -10V.

Edit: Did you maybe replace the bias filter cap and install it backwards?

[Mr_bibbles]

-10 at the lowest settings, WITHOUT the choke installed i could crank it up to -50 I'm sure, but,

WITH the choke installed it's about ...-2.

[Mr_bibbles]

I guess to clarify further, it seems like all the voltages are spot on as they should be without the choke, but I have no signal whatsoever.

With the Choke, all the voltages are pulled down SIGNIFICANTLY, still no signal, Lightbulb limiter fairly bright.

[The Dude]

Well, the choke feeds output tube screens and those tubes are removed, so the problem is not there. It also feeds other nodes for preamp tubes. Try pulling out the preamp tubes and see if your still drawing excessive current. If it's not a shorted tube, I'd suspect a leaky filter cap after the choke. Also, make sure the voltage selector is set correctly for your line voltage.

[drewl]

Also measure the resistance of each B+ node to ground with tubes in, then out.

[Mr_bibbles]

Thanks for the responses-

Drewl- The resistance from Plate to chassis is the same whether or not the tubes are installed- Roughly 270 ohms.

Dude- Pulled the preamp tubes, no change. Replaced the preamp filters (16/16/16/16 --> 22/22/22/22), no change in the amount of current being pulled.

I can see what you mean when you say the Choke is feeding the Screens, but It seems like they would still be fed when it's completely removed from the circuit... the + of each filter cap goes to the same place regardless of whether or not the choke is there. Pretty much the only thing I know about Chokes in this application is that they're used to smooth out AC, so wouldn't the amp still be able to theoretically make noise with more hum than usual?

Thanks for your patience and replies!

[The Dude]

...... The resistance from Plate to chassis is the same whether or not the tubes are installed- Roughly 270 ohms......

Which plate?

[The Dude]

......I can see what you mean when you say the Choke is feeding the Screens, but It seems like they would still be fed when it's completely removed from the circuit... the + of each filter cap goes to the same place regardless of whether or not the choke is there......

Yes the 2nd cap is still connected to the screens, but there is no voltage on the 2nd cap with the coil removed. With the coil removed, you only have voltage on the plates.

[Mr_bibbles]

Yes the 2nd cap is still connected to the screens, but there is no voltage on the 2nd cap with the coil removed. With the coil removed, you only have voltage on the plates.

AHH! Of course. Because the second cap is not connected to the rectifier, DUH! To reply to your edit, the caps are not backwards, but I just installed them in parallel to the old caps for a quick test. Just removed the old ones completely, now I'm getting 36V off the positive of C27 (before choke) and 1.6vdc off C28 (after Choke).

What Plate?

Pin 3 of both power tubes.

[The Dude]

Is your 36V at C27 with the light bulb limiter? You shouldn't have only 270 ohms from output tube plates to ground. Is that with or without the coil installed?

[Mr_bibbles]

Is your 36V at C27 with the light bulb limiter? You shouldn't have only 270 ohms from output tube plates to ground. Is that with or without the coil installed?

That is the measurements WITH the coil installed. WITHOUT the coil, OT plate --> ground shows OL. I've never run into a faulty choke before, but the more I work on this the more I suspect it. Plate and grid voltage at the OT is right on the nose when it's NOT installed....

Is there a way I can temporarily install a resistor in it's place? If that solves the issue I would order a suitable replacement choke.

[drewl]

Just bypass the choke with like a 470-1K, 5-10w power resistor for now.