AC50 bias supply causing super high B+ voltage

[randalp3000]

updated post below, thanks for looking

title says it all, amp has a strange bias supply and if I disconnect it from the power transformer voltages are all good.

I received a 65? AC50 today and went to check the bias and had a plate voltage of around 650 volts, ouch. Looks like it might be using the wrong taps on the PT. The voltage selector had gone bad at one point and it has been wired straight to the power switch(brown and black) with the remaining taps taped off .

There are no markings on the trans but it has grey covers and I think it is an Albion. Hoping someone here might know the color code to one of these and could save me some time. The leads available are brown, black, green, red, yellow, and blue. Brown and black are being used at the moment.

thanks for your help
rp

[Roe]

Here's a pic: https://www.facebook.com/photo.php?fbid ... =3&theater

See amparchives for more.

Correct voltage is 460-70v

[Dingleberry]

If you have a variac it's an easy and fast job to check which primary lead is which. If you don't have a variac you could measure the dc resistance of the primary windings with an ohm meter. It takes a bit more time but it's totally plausible. Been there, done that. Now when you already know two taps, brown and black, we can probably assume that one of those is 0 (common) and other is ???V. Rest is just measuring leads towards both of them and determing which really is the common. When you are sure about that just arrange the leads that the lowest resistance between common and thd lead is lowest voltage and so on. Google: identifying power transformer leads for more info. Are you in 115v or 230v area?
Never serviced an AC50, but about dozen AC30's.
Every vintage AC30 I've serviced have had an overwound power transformer. Always had to tap the primary to "next higher" tap to get the voltages to reasonable level (230v into 245 tap).

-T

[Midnight59lp]

Did you ever get the problem with the ac50 plate voltage worked out?
I have one that is reading 500 volts on the plates and i got the plate voltage down to 450 by putting a transformer in line with the wall to reduce it..but i dont like using that method.
Thanks,mike

[Gaz]

I think 500vdc is normal considering tolerances and higher wall voltage today. I personally would not sweat it, however I would change the 100R screen resistors to 1K since modern EL34s are crap. Look in the back of the amp at the power tubes. Through the holes in the plates you should see the spiral screen grid. Hit a hard chord at high volume and watch it light up. This is the equivalent of "red-plating," only on the screen instead.

[randalp3000]

Did you ever get the problem with the ac50 plate voltage worked out?
I have one that is reading 500 volts on the plates and i got the plate voltage down to 450 by putting a transformer in line with the wall to reduce it..but i dont like using that method.
Thanks,mike

Not yet, going to measure all the primaries with a meter as described above. I think the 0 tap is black so it should be pretty easy to figure out the sequence. After sleeping on it, being in the us (120v) the secondary voltages should only be lower and not higher with the wrong tap. Is this correct?

I'm trying to bum a variac from the great Jon Cusack of Cusack pedals. I think I need to bring it up slowly and check some other voltages like the filaments to see if it's not some other issue. Good thing someone put some nice F&T's in there but they're only rated at 500 volts.

Surprising that the amp is functioning but it was not very loud. Good thing I didn't run it very long and checked the bias. Hopefully there is not damage.

Thanks for the ideas everyone.
rp

[randalp3000]

I think 500vdc is normal considering tolerances and higher wall voltage today. I personally would not sweat it, however I would change the 100R screen resistors to 1K since modern EL34s are crap. Look in the back of the amp at the power tubes. Through the holes in the plates you should see the spiral screen grid. Hit a hard chord at high volume and watch it light up. This is the equivalent of "red-plating," only on the screen instead.

I think the screen resistors have been replaced but I'll double check the values.

If these do run at 500 volts should I worry about 500v caps???

It's great that you mentioned the screens going red because I had a build do that which was based on a AC30 power section and I think I brought those up as well but I'll double check. The AC50 came with some RCA 6ca7's but they tested pretty week on the Amplitrex which is unfortunate.

thanks for the info
rp

[Gaz]

Yeah, I think those 6CA7s could take it, but if you said they were weak...

That's a good point about the caps. They will probably be okay, but their life will be shorter, which is probably a moot point if they are original since they are probably on their way out anyway.

[randalp3000]

Yeah, I think those 6CA7s could take it, but if you said they were weak...

That's a good point about the caps. They will probably be okay, but their life will be shorter, which is probably a moot point if they are original since they are probably on their way out anyway.

Someone already put all new F&T's in it.

[Gaz]

You know the caps aren't just gonna blow at 500vdc, and I think that rating is the nominal not the max. Electros have a big 20% tolerance, so the ratings are not too tight I would think. I think in theory they will have a shorter life, but personally I would just rock them since they're already in there. JM2C.

[Kagliostro]

Here a large documentation about the Vox AC50

I hope you can find there info that can help you

http://www.backfromthesixties.co.uk/Pages/Vox_AC50.aspx

K

[randalp3000]

Here a large documentation about the Vox AC50

I hope you can find there info that can help you

http://www.backfromthesixties.co.uk/Pages/Vox_AC50.aspx

K

That's great stuff, thanks. I did find all those layouts and that nice clean version of the schematic last week. According to that layout I should be using the Black and Brown wires but I do have a different PT without the exposed lugs.

thanks for the help
rp

[randalp3000]

Finally had time to dig in with new variable power supply.
Turns out the bias supply is causing my B+ to slowly rise to ridiculous voltages, takes about a minute to settle. Replaced diode and all caps in bias supply. With 50v AC powering amp I'm getting 450v on my B+. Disconnect bias supply and at 120v AC I get a good 460v on B+. I also decoupled the power amp section from the PI so it's all isolated. There was some DC coming off the .1uf coupling caps so I need to replace those as well.

Any help would be greatly appreciated.
thanks
rp

[VacuumVoodoo]

Please measure secondary (HV) AC voltage with both 50VAC and 115VAC from Variac, should be around 165VAC and 380VA respectively. If you measure much higher, like over 200VAC and 450VAC respectively it might indicate internal short between primary & secondary HV winding.