Deluxe reverb 12AT7 reverb tube overheating

[macula56]

I have a silverface deluxe reverb that has a bit of an issue. the 1st 12AT7, by which i mean in the reverb circuit, gets way hotter than it should. and by hot i mean HOT. does anyone know what would cause it to overheat? would it have anything to do with the filament voltage? or possibly the plate voltage? maybe ever the cathode voltage? i haven't had a chance to open it up yet but i was wondering what to look for as i haven't had this problem in any other reverb amps that i own. any info greatly appreciated, JMac.

[tubeswell]

SF and BF reverb driver tubes are the most overstressed dual triodes in the pre-amp world. They will naturally run way hotter than other tubes, and the best thing to do is find a tube that can handle it. Or lower the B+ voltage

[eleventeen]

That tube "normally" gets really, really hot. It just does. It has big B+ on its plates, 410 volts, essentially the same as the output tubes! You could try moving the B+ feed from node "B" (415 volts) to node "C" (325 volts) which would be the junction of the 100K R and the 82K R that feed the phase inverter plates. This change *may* reduce the amount of reverb a little, but that should be no big deal...I myself can't use a Fender reverb control over let's say 3.5 > 4 without going overly surfy. So you have to turn it to 6 to get the same amount of 'verb. Big deal.

You could also try inserting a dropping resistor into the B+ feed to that tube BEFORE the reverb driver transformer..maybe in the 5K/1 watt range in an effort to knock the B+ down 50 volts or so. 

[macula56]

i have had a lot of Fender reverb amps and none of them have gotten this hot on that 12AT7. i'll check out the B+ and see what's going on there. what effect will lowering the B+ have on the reverb? thanks for the replies.

[Willabe]

what effect will lowering the B+ have on the reverb? thanks for the replies.

This change *may* reduce the amount of reverb a little, but that should be no big deal...I myself can't use a Fender reverb control over let's say 3.5 > 4 without going overly surfy. So you have to turn it to 6 to get the same amount of 'verb. Big deal.

             Brad     

[macula56]

thanks Brad. it's been a long day.

[floyd]

That 12AT7 has been EXTREMELY hot in every Fender amp I've owned.

[Ed_Chambley]

Seems common.  I have a 67 Deluxe Reverb measures close to yours.  Has the original JAN 12AT7 and it gets hot.  I have also lowered the voltage in Princeton Reverbs and really have not noticed any difference in the reverb depth.  My current Princeton has 334 on the plate and is a little cooler, but not enough where you want to pull it bare handed soon after shutdown.

The reverb circuit seems to have a little noise in most fenders.  Diddling with it I did get the noise where it seemed to be gone.  You can get away with a lower voltage if you want to use a 3 spring tank and dwell.  If it sounds good, let her ride.  12At7's are cheap.  Is your amp eating them?

[plexi50]

Does the reverb driver tube have to have a high plate voltage? If you lowered the driver plate voltage could this smooth out a over bearing ( too strong) reverb? You know how on some amps the reverb is too much too soon on the dial?

[phsyconoodler]

EXTREMELY HOT . 

Well not really.I pull them hot all the time.Hot yes,too hot,no. Some Silverface models have as much as 470v on the 12AT7 plate. If you look at the cathode resistor,it's 2.2k.I bet changing it to something like 4.7k or even bigger would help out immensely with both heat and reverb intensity.Worth a try I'd say.

[jim]

I use a 12AU7 reverb driver and a 12AT7 reverb recovery in my AB763 and still have ridiculous amounts of reverb.  Jim