building a Vox AC30 from the Tino Zottola book!!!

[Curranproducer]

OK! so I did the MV mod from this thread and I love it! I also changed out the .1uf caps off of the PI to .022uf/400v for even tighter bass than the .047uf trainwreck mod. I also did the power section mods of upping the wattage for the grid and screen resistors from the 1watt to 100r/5watt, and 1k5/2 watt on the amp.and I beefed up the resistor/capacitor set coming off of the power section cathodes to the higher wattage pieces as well. so now I am enjoying the amp and the tone BUT the el84's are RED HOT the plates(grey metal sheets in thetubes) are orange!!! AND I saw a little blue spark jump in the recifier tube. I shut thethe amp off after I saw that and left the studio for the day. What is going on? the amp is not noisey, and there is no crackling going on. the transformers are all cool, and there is no smoke. those el84's are DAMN hot though. I grounded the MV to one of the ground lugs on my multisection cap 40/20/20, and everything looks right from the mod links on this site BUT I dont think the tubes should look like little red/orange fire demons?!?!?!?

[martin manning]

I'm thinking you have an ultrasonic oscillation, possibly related to the lead dress in the MV circuit mod. Are the leads to and from the pots and the power tubes shielded or twisted?

[Curranproducer]

no they are absolutely NOT!!! I used cloth covered cable for each lead, So I should replace all that lead dress with shielded cable then? So he cabling in there can make the tubes burn crazy hot like that? I did not even shield going into the 1st stages of the amp from the input jacks? the amp is not excessively noisy by any means?!?!?! on my little ax84 I did shield at many points in the circuit but the zitolo book did not give me those instructions so I never thought twice about it. besides the MV section, and the input should I shield anything else in the circuit?

[Curranproducer]

I have some shielded cable from an 1176 compressor I built, what should my minimum voltages be for the wire? I am thinking 300v for sure, will this be OK?

[martin manning]

If you have some two-conductor shielded cable you can use that as shown in the diagram posted earlier in this thread, and you can use the shield to carry the bias voltage (0V, in this case since the power tubes are cathode biased) to the dual pot instead of a separate wire. If you don't have any cable like that, just twist the pairs of wires going from the PI coupling caps to the pots, back to the board, etc. This wire only has bias voltage on it, so it doesn't need to be high voltage rated. I wouldn't worry about shielding other runs that aren't causing a problem.

[Curranproducer]

and that is going to make the tubes NOT crazy red/orange hot? very weird and interedting. This is fun learning all this about amps vintage PTP wiring and stuff very interesting. You don't think the .022uf caps instead of the .047uf PI caps are stressing the tubes do you? I am a wonderful novice so when things like this happen I need to learn what is happening FNG!!!!

[martin manning]

If you put a scope on it you could confirm my suspicion of an ultrasonic feedback oscillation, but I'm betting that it will settle down if you shield or twist the wires. The size of the coupling caps will not affect the stress on the tubes, and since they are mainly affecting the roll-off of bass frequencies they are probably not enabling the oscillation either.

[Curranproducer]

well that is just darn neat-o!!! I am going to need to get my grubbies on a scope, which kind should i try and find? Man this is the best, i really like this site!

[martin manning]

Good, older analog scopes often go pretty cheap ($50-$400) on Craigslist or e-Bay. You don't need anything fancy for audio, but I'd look for something that is at least two-channel, 20MHz or higher, includes probes, and is <20 years old. New digital scopes are getting cheap, ~$400, so that is another option.

[Curranproducer]

Sooooooooooooooooo.............. I am pretty sure I figured out what the red/orange EL84 tube plate stuff is all about!!!! I did nor remove the 220k resistors from the PI circuit to the power section after putting in the MV!!!! So I am creating a MONSTER voltage divider going into the power tubes!!!! am I right?

[Curranproducer]

plates are still crazy red/orange on the power tube plates, I twisted the cloth wire coming off of the PI and the MV pot.

[martin manning]

Leaving the 220k's across the pots wouldn't cause this. Have you got the bottom (CCW lug) of the pots grounded?

[Curranproducer]

yeah I did a master ground from BOTH of the CCW lugs to the ground on one of my multicap canisters, if it is a cold joint could this be causing the issue?

[Curranproducer]

I also removed the 220k's from the circuit

[martin manning]

If you turn the MV all the way down does the redplating stop? That will ground the grids, which should kill any oscillation.