fender twin reverb

[mellowamoya]

hi all,
have a fender twin reverb that i have been asked to repair when i look inside i find that it a has two 6l6gc tubes in v7 and v10 and two 5881 tubes in v8 and v9 and i ask how can this work properly as i understand they have different bias setting and different power outputs, the grid resistor 1.5k on v9 is a piece of carbon now and v10 screen grid resistor 470ohm is open, the amp went dead by the look of things when brown wires to the output transformer rubbed through on the 470ohm resistor the resultant flash bang killed the amp for now.
the twins that i have worked on before had elther a set of 6l6gc or a set of 5881, so my question has anyone seen an amp with this setup before and if so how does it work?
i have been trying to date the amp but no tube layout card in the cab the number stamped on the chassis f115345 which i think puts it 1981 the eia numbers on the output tr is 606 125 and on the mains tr is 606 044, if that helps.
the owner bought this as a fully serviced vintage twin, it has none matching speakers fitted as well but i have seen that before
thanks any help will be appreciated.

[Enzo]

It isn't Kosher, but really, a 5881 is just a variant of 6L6. Don't overthink this, just consider it a mismatched set of 6L6s. If you consider any differences between the two types on paper, you will probably find greater differences between two 6L6s or two 5881s individually.

I am of Scottish descent, and if you compare my DNA to that of a Chinaman, you may find some differences. But you just as easily can find a greater difference in DNA between me and some other Scot. it is like that.

The tube mix has nothing to do with resistors burning. That is caused by other things, like tubes failing. It is hard for a 6L6 to burn up the 1.5k grid stopper. Do any of your power tubes have a broken off center peg, allowing the tube to be installed facing the wrong way? That is a common reason for odd resistor burns.

Check all four 470 ohm resistors, replace any that are burnt or damaged, and also any that are now way off value. Not unusual to find one sitting at 750 ohms instead of 470.

Anything is possible, but in my experience, your brown wire didn't do anything but sit next to a resistor that burnt. Burnt screen resistors are pretty much only caused by tube failure. The tube screen is the only current path available to it.

personally I never worry about the date, I car about condition much more, but the pots all ought to have date codes on them.

[Justin Thomas]

Couplea questions: which specific model TR is it? It could be as 71 or 81. Big difference. Pics, or a list of features? BF or SF?

Also, with all respect to Enzo, if those are Sovtek 5881s, I wouldn't worry, they're as stout as any other 6L6GC. If vintage real 5881s or reissue Tung Sol 5881, then they could easily cook in a Twin if not set up for them. The original and reissue TS 5881s are 23W dissipation; the Sovteks can be 30+W and at e frequently used as "torture test" tubes. My point being, we don't know what the operating conditions for the specific amp are. If it IS an 81, it could be running 500+V on the plates, and maybe someone biased the amp hot, and it could kill a 5881...

Just so we know which schematic to use and which specific tubes are in it, then we can go.

And, the mix can work - so long as you have one of each type per side, which you do.

Justin

[mellowamoya]

Ok bit more info I couldn't find the correct schematic but finally did a little while ago,so he unit ties up with the twin reverb 135 watt and I think that that is correct as it is a very heavy chassis and it has a 2.7k 10 w resistor in the ht power line as well as a 30 k 20w from the ht line this is open.the owner has another set of tubes which can make up a quad set of either 6L6gc or a set
Quad set of 5881 they are all jjs,
Will progressing with it when I get the now resistors on Monday
This now concerns me about the speakers as they I think from a standard twin.

[mellowamoya]

the mains transformer is marked A012405, EIA606 044, UL1411, its a BF

[mellowamoya]

i installed a new set of 6L6GC and replaced the plate and grid resistors on the power tubes,also the 30k 20w, the amp worked but not as nice as it should of been went on to check all the plate resistors and ended up changing them all, checked as many resisters as i could. setup the bias on the output hey presto now the amp is cooking with gas just about blows the door off,it is so loud, the owner has a smile on his face.
thanks for the help