Modded Peavey Butcher blows Heater Fuses

[Gr8Scott]

I converted this amp for use with EL34 tubes. It puts a little more strain on the heater circuit, but it's equipped with an 8 amp fast blow fuse. Initially this amp worked fine with the EL34 tubes, but now it will blow the fuse as soon as you turn the amp on. I thought maybe it was the hum balance feature as the pot was looking burned and the wire running to ground from the pot was showing signs of being really hot as well. I removed it from the circuit entirely and that did nothing. The heater fuse continues to heat up to a cherry red the millisecond I flip the amp on. It takes about a full second for the fuse to burn out. I've changed all the tubes just in case and no improvement whatsoever. It will run on two output tubes, but not on all four at the same time. Whatever increased the current draw, it did it by about 3 amps apparently and it did so very suddenly.

Any suggestions would be greatly appreciated.

I've checked the AC voltage across the fuse and it's a touch over 6v. When I try to measure amperage, it shows a very low figure .13 amps.

[demonufo]

I've checked the AC voltage across the fuse and it's a touch over 6v. When I try to measure amperage, it shows a very low figure .13 amps.

That doesn't make sense. How are you measuring amps?
You should be measuring in series (using the meter as a part of the circuit) and not measuring across the fuse. Not saying you don't know what you're doing, just curious as to how 130mA could blow an 8A fuse. Ideally, you should be using slo-blow fuses though. Even healthy amplifiers can trip fast blows at the correct rating from time to time.

[Gr8Scott]

Demonufo - I truly don't know what I'm doing. I'm very new at this, but I'm learning along the way. I'm being safe about it and I'm taking no chances.

When I remove all tubes and remove the fuse and then put the meter on each side of the contact points for the fuse and turn the amp on, I'm getting 2.4 amps of current draw with no tubes in the amp whatsoever. That is definitely enough to cause the fuse to blow with all the tubes in the amp.

I can only imagine the 2.4 amps being pulled from either a socket with some semi conductive debris in it or from the power board having some leaky component that is causing the amps to be drawn into it.

Edit - After thinking about it, it makes complete sense to test amperage in series for the circuit. It makes things somewhat difficult because I'll have to desolder some wires here and there to check the individual boards and components, but it gives me the tools I need to test and diagnose the problem.

Thank you for your post. I think I have what I need to figure it out. Thanks for pointing out what should have been obvious to me, but wasn't because I'm very very new at all this.

[Gr8Scott]

I feel like a complete dummy. I thought my DMM would automatically know if the signal was AC or DC and it doesn't. You have to press the "mode" button to get it to read AC. This explains all the weird readings I've been getting because I was trying to measure DC amperage on an AC circuit. My DMM is fused at 400ma and 10A and I blew the 10 amp fuse very quickly once I switched over to that input. So now my DMM is useless until I buy more fuses and I'm no closer to figuring out what is going on with the amp. Live and learn I guess...

Now that I know how to use the DMM a bit better , I can hopefully do a bit better figuring out what is going on with the amp.