Fender Twin Tube Shields

[redir]

You know what I am talking about? The little metal shields that fit over the preamp tubes and lock into place. This is a question for the amp builder and repair folks. A while back the face plate of my Twin was getting insanlly hot after about an hour playing. So hot that you could not even touch it. I actually noticed that their is a warp in the faceplate and I think it's due to the heat.

Anyway the amp had been retubed a few years ago and then sat barely used for a good two years before I decided to start using it again. That's when I noticed it. So I looked back and all the tubes seemed to have a normal glow but then I noticed in the back of the amp a little ziplock bag with the old spare tubes and those little shield cans.

So I put those on and now I swear the thing never gets so hot anymore.

I looked around online and there really are not a whole lot of conversations about them and most are arguments that say your amp sounds better without them vs with them and that they are really just some sort of electronic oscillation shielding and not heat shields per se.

[Paultergeist]

From everything I have read, the primary purpose of those shields is to block RF noise from reaching the amplification circuitry inside the pre-amp tubes. Should un-intentional noise reach the first stage of the pre-amp, for example, it will get re-amplified several times as it progresses through the amplifier circuit. Such shields are generally not used on the power tubes, however, as the power tubes are the last step in the amplification circuit, and are thus less sensitive to RF noise.

I do suppose, however, that there might also be a heat-shielding effect. I am surprised, however, that the pre-amp tubes themselves could generate that much heat. In my experience, it is usually the power tubes that really gfive off a lot of heat. Interesting......

[clintj]

Which end of the face plate? Looking from the front, right side has the power transformer, rectifier, power tubes, and the center tap for the heater wiring along with the pilot light. Lot of potential heat sources, IOW. Center of chassis has the output tranny (potential heat source), reverb driver, and choke. Neither of the latter two should get hot. Left has the doghouse with the filter caps, which if they're questionable may generate heat.

I would have someone look it over closely, as nothing in there should generate as much heat as you're describing. Toasty warm, yes. Warping things, no. With the amp running at idle, standby switch on, the power tranny should be a little warm and the output tranny should be cool.

[BTF]

I believe the thing about sounding better on/off applied more to higher gain amps. There used to be this theory that the cans reflected back electrons which escaped the tubes and affected the tone (not my theory, mind you). Some Marshall owners swear heads sound better with the old fabric web holders than the newer metal ones on their power tubes. I honestly can't hear a difference. I built a VERY high gain preamp for my rack, and I use tube shields on it and feel no audible detriment.

If I may suggest, you may want to have your amp's bias checked. If the power tubes are under-biased, they'll run very hot, distort sooner and have a shorter usable life. Under-biased tubes will heat a Fender chassis (especially Twins) to uncomfortably warm temperatures. Just a guess.

If the tube shields help, I say leave them on. Nothing's worse than getting to a gig and having a radio channel come through your amp... unless it's Mexican Radio (sorry, Wall of Voodoo fan here!)

Regards, Bill.

[redir]

It was real hot all the way over to the right mostly so I guess that was the power tubes. It's odd that it doesn't do it anymore. Here is a pic that shows the warped face plate:

[MikeBmusic]

How old's the amp? My Bandmaster used to get really hot, but never warped the faceplate - that would take some extreme heat, I would guess.

[clintj]

Odd indeed. Fender added extra grid stoppers and snubber caps in some of the silverface to fight against parasitic oscillations, that were partly brought on by sloppier construction and lead dress. Maybe pulling the shields was just enough to put it over the edge and start one?

Bias is a likely culprit, too. There's a filter cap in the bias circuit that could cause the tubes to be underbiased if it starts leaking DC through to ground. Retubing usually involves rebiasing, so if it quit doing it after that that may be it. If it does it again, it'd be a fun challenge for a good repairman.

I just had another thought before I hit "submit". Some of those silverfaces don't have a proper bias circuit, they have a bias balance adjustment instead. Basically, they aimed for a safe bias point on one pair and left the other adjustable so they could be matched. I'd have to look up the schematic for that particular amp to find out.

[redir]

Thanks for all the details. Yeah I'll have to see if it ever happens again. Fortunately I've got two guys local that are really good at amp repair.

I was told this amp was a 1972 when I bought it 20 years ago but it's not easy to date the thing.

[clintj]

There may be a sticker in the cab with the circuit number, and maybe even a production date if it hasn't been lost to time. If you're really burning to know, the transformers also have a date code on them.

[redir]

I'll take a look thanks. I think I remember seeing a tube chart sticker in there.