Vox AC30C2

[matt mincer]

Hey All,

I'm looking for some help repairing a Vox AC30. What is happening is that all 4 of the 3.3K grid resistors for the EL84 tubes are getting smoking hot. Also, my DC voltage on those resistors is up around 400V with respect to ground. My guess is that one of the resistors coming from the preamp is bad but none of them look burnt. I am planing to replace them but I figured I would ask here first. Any thoughts?

Thansk
Matt

[PRR]

Genuine vintage, re-issue, clone?

What watt-size are the present resistors?

Can you measure the power tube's cathode current? Can you see if they are re-plating? What voltage on the cathode resistor(?)?

400V sounds like a LOT for any EL84(?) amp. Was the PT or rectifier replaced? (or ill-bought?)

> resistors coming from the preamp is bad

Does not make sense. Preamp does not shove power to the final, and does not use enough power to bother the final.

[sluckey]

Pull all four power tubes. Does the 400V on those grid resistors go away?

[matt mincer]

I'm at my office right now but this is what I remember. I'll post any corrections later.

This is a reissue with diodes rather than a rectifier tube. The owner told me he was using it and it blew a fuse. He replaced it and it blew again immediately. He had the ELs replaced but it did the same thing.

When I got it I noticed the 3.3Ks were burnt though they all ohmed out okay. I replaced them anyway. They are probably 1/4 or 1/2 watt (check later but I think 1/2). They were the stock ones.

The 400VDC I'm talking about is on the control grid but the screen grid is reading 400VDC too, both to chassis ground. The voltage remains when I remove the tubes.

I meant capacitors from the preamp. If they were shorted, they would bleed B+ to the grid of power tubes. There are a few problems with that theory. 1, They are newer style poly (I think) caps that look okay but I'm going to test them when I get home. 2, There are 2 of them so if one of them shorted, I would expect high voltage to one set of the power tubes but not both. I said preamp resistors because when I was testing on my variac, I noticed a waft of smoke from the area where they are located. I didn't see any evidence of burning and I couldn't get it to reoccur.

Thanks
Matt

[kagliostro]

How many volts do you measure on heaters ?

Franco

[pompeiisneaks]

I'm at my office right now but this is what I remember. I'll post any corrections later.

This is a reissue with diodes rather than a rectifier tube. The owner told me he was using it and it blew a fuse. He replaced it and it blew again immediately. He had the ELs replaced but it did the same thing.

When I got it I noticed the 3.3Ks were burnt though they all ohmed out okay. I replaced them anyway. They are probably 1/4 or 1/2 watt (check later but I think 1/2). They were the stock ones.

The 400VDC I'm talking about is on the control grid but the screen grid is reading 400VDC too, both to chassis ground. The voltage remains when I remove the tubes.

I meant capacitors from the preamp. If they were shorted, they would bleed B+ to the grid of power tubes. There are a few problems with that theory. 1, They are newer style poly (I think) caps that look okay but I'm going to test them when I get home. 2, There are 2 of them so if one of them shorted, I would expect high voltage to one set of the power tubes but not both. I said preamp resistors because when I was testing on my variac, I noticed a waft of smoke from the area where they are located. I didn't see any evidence of burning and I couldn't get it to reoccur.

Thanks
Matt

Just calling it a 'reissue' is still not quite specific enough, is it an AC30C2, CC2, HW, etc?  The schematics I've seen so far all have 1.5k screen resistors, so I can't yet find one with 3.3k.  As sluckey mentioned, try removing the power tubes, see what happens to the voltage then.   

[sluckey]

There should be zero volts on those 3.3K resistors. Are you sure there is 400V on all four tubes? I only see a few ways to get 400v on them...

1. Tubes shorted between control grid and screen grid (or plate). Pulling tubes would answer that.

2. Grid coupling caps (C50 and C51) shorted or leaking badly. I doubt the voltage would be 400V. Probably more like 200 or 250. Disconnect caps to answer this.

3. J31 plugged in wrong. Notice that two control grids are on pin A and screen grids are on pin B. Adjacent pins!

4. J31 cable shorted.

[sluckey]

The schematics I've seen so far all have 1.5k screen resistors, so I can't yet find one with 3.3k.

The AC30C2 schematic in Hoffman's schematic library shows 3.3K.

[pompeiisneaks]

Oh ok, that's an abomination of a schematic :P But yes that's SS rectified and has the 3.3k so that's likely the beast.

~PHil

[PRR]

> 400VDC ...is on the control grid

That is, as said, just wrong. And would be instant burn-up, so something changed. (Remember that clients do not always know, or tell, the whole story....)

It may not be wired like you think. You may be looking at it wrong. It may be a surge zapped all the grid caps (unlikely). A ground may have come loose, floating everything up to B+.

After a good look and re-check of assumptions, I guess I would start taking one leg of these grid caps out.

[matt mincer]

I figured it out. When I removed all of the connectors from the tube board, I was reading 45k between the input of the transformer and grid resistor. Turns out there was a arc between the J29 and the trace above (see photo in link). It was letting B+ bleed back into the grid. I scraped it off and it's working fine. The picture is post scrape so it's not a great visual aid but I think you'll get the idea. Thanks for all the help.

Matt

https://flic.kr/p/Hkcg9H

[sluckey]

Good find. That would explain 400v on two of the 3.3Ks. But you said all four 3.3Ks had 400v on them. Was that just confusion?

I want to clean up the soldering on those tube sockets with denatured alcohol. 

[matt mincer]

It's actually a good question. There was 400 on all 4 resistors and all 4 had some signs of burning. After cleaning off the carbon, the resistors were all at well below 1V and I can't explain why. Best I can figure is that it was somehow feeding back through the resistors on the preamp though I don't know how. It's also possible I'm remembering wrong. I do know all 4 were burnt because I saved them to show the customer. Senior problems!

I'll clean thee flux, thanks for the tip.

Matt

[boogieWoogie]

I just fixed the same Vox AC30C2 failure: arcing from OT transformer spade lug to the grid.  My symptoms were no audio, buzz, and finally ~240VDC on the output tube grids.  I removed all coupling caps (suspecting that they'd died & were leaking the plate's B+ DC into the grid), removed cabling.  Finally I was down to bare bones of a circuit & there was 56k ohms between the grid & plate.

There is another thread on this board of someone who's fixed the exact same problem (i've also posted this there).  That makes 3 of us.  Vox shoulda done a recall.

My fix was to grind out the arc'd pile of carbon & metal using a dremel wheel, and pack the ditch w/ hand-molded epoxy.

Arc, pre-fix
--------------


Post-fix (grinding + epoxy)
------------------------------

[shooter]

The connector in your pic has failed on 2 that I worked on along with arcs n burns
I didn't have fun so I passed on any more