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AC 50/2 Head driving me MENTAL!!!

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10/5/2005 5:55 AM

AC 50/2 Head driving me MENTAL!!!

Hi there.

I've got a 64 ac50 head with the ez34 rec valve that's realy taking its toll on my brain. The first three gigs I did with it were just amazing and it sounded wonderful, and then...disaster!

Problem - If i plug an overdrive pedal into it, it seems that there are two notes, one clean and one distorted. I switch the overdrive off and it's clean, but when I turn the pedal back on again, the overdive is great for about 1/2 a second, then it sounds like it gets sucked back into some black hole and the tone is back to some 1/2 and 1/2 thing again.

I have checked everything I can possibly think of. New valves, filter caps and all electrolytics replaced. Balanced driver section as well. I have even put in a set of THD yellow jackets to see if that fixes the problem, but no joy. Can anyone offer me some suggestions, please, I'll try anything.

10/5/2005 1:43 PM
Daniel	And i've checked the pedal with other amps.. it works fine. The amps tone is very lack luster and harsh. Could a busted output transformer be the prob? I can still get plenty of volume out of it, so I don't think that's it.

10/5/2005 3:01 PM
Bruce /Mission Amps	Of all the amps I've worked on with odd tonal nuttiness that drove me crazy, it has been AC50s and AC100s with an OT that has "pin holed" insulation or shorted windings in the primary. They always biased up right and made decent power but sounded bad when driven harder. If everything else checks out... I'd be swapping in a replacement OT for a sound check. Bruce

10/6/2005 12:30 AM
Daniel	I was afraid of that. Oh bugger! I'll give it a try. Thanks Bruce

10/6/2005 1:38 AM

It's as you said Bruce. I checked the primary. One side measured .49 Ohms and the other .47. No probs there, but when ichecked the secondary, my 16 Ohm tap was 1.6 Ohms and the 8 Ohm tap was 1.7 Ohms. So I would say it's buggerd. I understand that the ratio is wrong, but could you enlighten me as to what causes this to be wrong?
Also, what OT did you replace yours with. I know FNS do a direct replacement but I've no idea if it's any good.

Thanks again for your help.

Daniel

10/6/2005 8:48 AM

You might be able to tell whether it's an internal pinhole with the inductive transformer checker schematic I have at GEO (http://www.geofex.com) in the tube amp technology section. This uses a 6V battery and an NE2 neon bulb hooked to one winding. You load up the winding with DC from the battery then open the switch. If there's a short, the short eats the inductive kickback. If there's no short, the inductive kickback blinks the neon light. All the windings have to be open circuit for this to work, of course.

In your case, I'd try hooking the 6V/neon to the output winding. That would guarantee enough voltage to arc the pinhole in the primary.

I'm not sure that the resistance ratio on the output windings is a good measure of the condition of the windings other than open/not open. The windings may be internally of different gauge wires and may be series/paralleled in unexpected ways. Even truly centertapped windings (like a primary is supposed to be) will vary in resistance with the length of the wires, which varies with where in the window the wire is wound.

10/6/2005 12:29 PM
Daniel	MR Keen, you are a genius. I will do this and get back to you. Thanks

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