5e3 runaway

[ho-mayed-stuff]

I've built a few 5e3's with good luck that have all followed the fender drawings pretty closely with the exceptions of center tapped transformers an the addition of a standby switch. The last build was similar in spec as all of the previous builds only this time I used sprague 715 series caps (orange drops) and the amp howls and runs away at high volume and tone. I've checked grounding and voltages. I start with a 5771 and split phase with a 12ax7, power with 6l6's and use a tube rectifier 5y3gt, same as all of the others. could orange drops be the culprit?

[Enzo]

Probably not.

Did you try reversing the plate wires from the output transformer? Or at least disconnecting the negative feedback line? That addresses the howling potentially.

When you say runaway, to me that means the power tubes sit there drawing more and more current and get red hot. Is that what you mean? If so, watch the control grid voltages to see if they climb tpwards positive. Also should try a different set of power tubes.

[Five_E35]

Did you use any bedding compound under the caps?Orange Drops are quite a bit bigger than Mallory 150's.

I used Orange Drop's in mine and had a problem with the .02 cap picking up microphonics from a bad tube in V1 and feeding them into the grid of V2 making the whole chassis microphonic. The .02 is right next to the very sensitive grid wire that dives under the board from one side to the other. Some additional bedding compound under the cap(and a new tube) fixed the problem.

[billyz]

As Enzo pointed out , reverse the OT plate leads. 5e3's don't normally have a NFB circuit. Could be a microphonic anything, Caps, tube. Get out your little rubber hammer , put one hand behind your back and tap around. Caps can be microphonic but it is rare in a 715 series. Did you use any little ceramics, they are prone to microphonics. Check your layout too. Grid and plate wires can interact , I like to cross them at 90 if possible , don't parralel them too close to each other or anything else. Especially the grid wires. A lot of builders resort to sheilded grids wires as they are prone to microphonics. I use teflon stranded wire and don't have any problems though.

[ho-mayed-stuff]

Thanks for all of your replies. Enzo, to your points. I don't use any ceramics and runaway may not be the correct term for this problem. What I meant is once it starts howling it gets louder and starts to feed on itself until I back down tone and volume. I don't see this problem at low volume. I'm not seeing any tube heating of self destruction. I will reverse the grid wires today. There's not much room in a 5e3 chassis to separate wires. Can grid and cathode wires be twisted together like I do with my heater wires? I use a Belden RG174 shielded coax to my grids and try to isolate them as much as possible. I've only tried an NFB one time and lost the gritty sound and I removed it. Once again thanks for your help.

[Enzo]

Plate wires, not grid wires. That is the blue and the brown wires that go to pin 3 of the power tube sockets. swap places. If there is no NFB, then this isn't the problem.

If it only does this at volume, next time it does, reach in there and grasp each tube firmly. Go down the row. If grabbing one of them makes it stop, you have a microphonic tube. If the tubes are too hot, use a rag or a glove.

[ho-mayed-stuff]

Enzo helped me out. I found a microphonic tube. Thanks to everyone who replied to help me out. Where are you getting bedding compound and who is the manufacturer?