Huge sweep of current on Express V5 under load?

[Cliff Schecht]

Measure the resistance of the primary between each plate winding and the center tap. I'll bet they aren't equal..

[guitarsnguns04]

ive noticed this as well. The ots primary are different from center tap to each leg..thats what I came up with as well on my build. Its never caused any issues but I spent a good bit of time trying to debug this..I guess its just the way it is with an express?

[rooster]

Gary - Ha! Yeah, now you are on my page. I will say that the amp sounds and feels great during all of this misbalancing. I suspect, too, that if the 10K tail was increased even to 15K, that much of the 'wobble' would be gone. But I am not going to do this.

I just have to assume this is all on the PI, and the fact that the preamp is capable of delivering such a large signal. And let's be clear on this, that when you reduce the gain (volume knob), the wobble is gone and the amp behaves typically. At least this is true for me.

OK, also understand I do not run my amp this way, full out like this. This was pointed out to me by a friend who DOES play this way. I have also loaned him the amp to play with for a few weeks - on another occassion, too - and he has failed to break it. No pedals, just a cranked Express.

ampgeek, if I understand you correctly, that is a definite problem. At idle, V5 and V4 should be the same if you have a matched set of tubes. In your case, you should measure the OT leads resistance, center to V4, center to V5, and then V4 to V5. These reading should all make sense, ie., X number, X number, X+X number. If what you say is true, your OT is FUBAR because the readings should be similar from center to V4 and V5. To me, a bad OT is the only explanation for what you report. And I pray you have a decent digital meter?

OK, have a great weekend all. Gary, yeah, ear plugs for the neighbors, too.

[ampgeek]

My imbalance only occurs with a signal at the input jack. Near perfect balance at idle with the 3 sets of EL34s that I have set aside for this Express.

Yep! I have two very fine Fluke DVMs. I installed external bias jacks on my build so I can set them both up at the same time and observe V4/V5 current draw simultaneously while banging it with a guitar signal.

Cheers,
Dave O.

[PlinytheWelder]

One of these days I'll put the amp back at the test bench and run it hard into a resistor and scope it, but it's more fun playing it....

[PlinytheWelder]

On another note, I have a LarMar PPIMV in mine and just hitting the PI hard and attenuating way down after it with the MV leaves the idle current steady....

[rooster]

Gary - Ditto on the results with the PPIMV.