Vox AC50 OT

[soundmasterg]

Ok, I think I discovered my low power problem. The Heyboer output transformer that I used is about a 4600 ohm primary impedance, which is more suited to two EL34's than four I would gather. I wonder if they gave me a 50 watt output transformer instead of a 100 watt. It does sound good as it is, but I would guess maybe the higher load is working the tubes harder than needed?

Greg

Well I feel like an idiot but I screwed up when testing the transformer the other day. I re-tested it correctly this time and it is right around 1700 ohms for the primary impedance, so I must have a problem in my amp, probably in the phase inverter area I'm guessing, as far as why it is low on power. Hmmmmmm....where to start....

Greg

[Rockman5150]

I've got the exact same amp right here. Could you also post a pic of the 2 500V caps on the tube-side of the amp?

[soundmasterg]

Heck yeah!
A really good AC100, using fixed bias with the power tubes idling at about 15-16 watts each, will make about 105-115 watts output into a dummy load.
The cathode biased 50 watters ones are down in the 35-40 max watt range and run the power tubes way too hot at idle... but also sound glorious.

Hi Bruce,

I was just wondering if you measured the B+ on these AC100's that made 105w to 115w? The schematic shows around 470v B+, but thats on 110-115v AC...I wonder how much higher a real one is on 125v AC like we have today? The project one that I've been working on is 466v B+ on 125v AC, and is making around 80w or so last I checked......

Greg

[leitrim_lad]

Hey there, im wondering if ye could help me.. I have been given an old ac50 which isnt working.. When i plug in a guitar all i get through the cab is a harsh distortion noise when i play hard. If i play light nothing comes through. When i turn up the volume however i seem to get the same noise as when i play loud. It happens on both channels (normal and bril). Iv tried different came but to no avail.

Would anyone have any ideas what might be wrong? Iv checked all the soldering and everything seems to be ok but id be a fairly new at the DIY game!!!;-)

I followed the schematic and see that the R29 which seems to be a 750r 3w resistor seems to be bloted out. Im thinking it might be blown. Would this cause the sound im getting?

Thanks a million

Schematic:
http://backfromthesixties.co.uk/imag...%20Diagram.jpg

[Steve Conner]

Yes, if R29 were burnt out, it could explain the symptoms. But it may have burnt because of a bad EL34. If you replace the resistor and it just burns again, you'll probably want to get some new EL34s.

This amp is really a bit of a collector's item, worth a fair bit of money in working order. If you get stuck on it, and you're in the UK or Ireland, I'd suggest sending it to John Chambers in Nottingham, he'll fix it up for you.

Champ Electronics - Vintage Amp Repairs

[leitrim_lad]

Hey cheers for the reply, i really appreciate it. I replaces r29 and it just burns out straight away again. I have 2 sets of el34's and the same thing happens with both of them so i done think its the el34's.. Have you any other ideas on what it might be?

Thanks a million

[pdf64]

Are the EL34 tube sockets (and the wiring to them) good, no arcing tracks?
What's the max negative bias voltage (across C22), and what's the actual negative bias at pin5 of each EL34? Don't need tubes in for these checks, in fact best not to fit any more ouput tubes until the above and further basics are good.