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This is a Fender Deluxe Reverb amp with new TungSol 6V6GT's.In the photo, the red spots look worse than they actually are because of the time exposure. It's a dull red glow.
The tubes are within their dissipation limits. Plate/screen voltage is around 450V, and in the photo they are pulling 24mA each. Negative
bias is -40V.I have to drop plate current down to 12 mA to make the hot spots disappear.
It looks to me like the electron beam is being concentrated on this one area of the plates. The tubes test normal.
Any ideas?
Edits: 06/08/17Follow Ups:
plate voltage is way too high (echo echo). 450 Volts is pushing the limit for a 6L6GC, let alone a 6V6. drop it by 100 volts. add an inductor and change the rectifier to one that drops more voltage or replace the power transformer
It should help by virtue of its higher voltage drop resulting in a little less voltage but i still think you nee to make changes to have the B+ below 400vdc. the GE tube manual for the 6V6GT shows recommended voltages of 285vdc in pp class ab. Of course the amp makers would "ignore" these recommendations and push the tubes to very limit. It is interesting to note that the characteristic curves in the GE manual show no points pass 400vdc!
This is not the case for 6V6GTA tubes for which the characteristic curves are plootted to 500vdc. So, you can try the GTA or 7408 tubes which have higher ratings and maybe ok in your unit but seriously, your B+ needs to be lowered with a dropping resistor or choke or a bucking transformer.
6V6GT is not designed for 450vdc plate voltages.
Edits: 06/08/17
I agree, 450V is too high, but that is how Fender built it. I didn't design it.
If I make changes, it will devalue the amp. A 65 Deluxe Reverb is worth upwards of $2,000, but less if modified, assuming the owner would approve it.
how much will the amp be worth when the power tubes go nuclear and take out the output trans and maybe the power trans.
Variations in the grid winding- Try different tubes?
Happy Listening
That seems really high for 6v6gt. I would think you would want to run it at around 350 / 300 or less
I don't have any choice. The power transformer is what it is.
The voltage is a little high, because the amp was designed to operate on 110VAC. I'm repairing the amp. I've seen a lot of them running high plate voltage, but don't usually see the red spots on one side of the plates.
If the tube is being stressed due to dissipation, it seems the entire plate should show color, not just one spot.
Alpha Al -Why do you say, "because the amp was designed to operate on 110VAC". The chassis label clearly states 117 VAC.
Alpha Al-Why do you say the amp is designed to operate on 110VAC, when the chassis label clearly states 117VAC?
What does your voltage look like if you run it through a Variac at 110v ?
Is the 24 ma. plate current or cathode current(which includes both plate and screen current)? And what's the screen voltage on the amp?
4 or 5 ma. of screen current might push the total tube dissipation up to or over 12 watts and may cause some discoloration in some cases.
That was plate current. I didn't measure screen current, but the screen voltage was just a little less than plate voltage, maybe 5 volts difference.
I was mostly curious why the hot area was so small, instead of the whole plate heating evenly.
"I was mostly curious why the hot area was so small, instead of the whole plate heating evenly."
I think it's always that way with beam power tubes. The beam deflector plates force the electrons to that area of the plate.
Is the bias voltage adjustable?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Yes, the bias is adjustable. If I reduce plate current down to 12 mA, the hot spots disappear completely, but the power and tone quality suffer.
12ma 450 vdc is only 5.4 watts
Something is wrong with those tubes.
The plate of the 6v6gt should be good for 12 watts without glowing.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Maybe a good idea to check all lines going to and from the 6V6GT. If over 5.4 watts per tube causes the tube anodes to glow red --- something is definitely amiss. I'd check any bypass cap and grid-to-ground resistors, along with other suspects.
Have you tried other know, good 6V6GT tubes?
Good luck!
8^)
Check the screen current and see what you find. That might be quite helpful.
I changed to a pair of JJ 6V6S, they don't have the hot spots.
Normally, I stay away from JJ octals, but in this case, they fixed it.
I think the TungSol was a defective pair.
The screens are drawing 2 mA each with either tube type.
That definitely could be the case. I've had ONE case of the 6V6GTs acting up in all the years I've had them. And many have been used at 425 -450 volts. But it can happen... Hopefully the vendor will stand behind yours!
Al, on many Fender amps... changing the screen resistors to wire wound or metal oxide, 5-watt 1k-ohm usually helps stabilize the output tubes.
My DR runs over 450VDC on the plates. I bias them to about 20mA per tube.
Photo is of my Super Reverb, I put 1k-ohm bricks onto the screen grids.
Good luck!
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