Using a 50 watt EL84 TX set for a 2x5881 design

[minim]

How doin?

I aquired a cheap Crate 4 x EL84 VC50 head which had blown up and melted various valve sockets, etc. I replaced the faulty PI coupling cap and now it hums along ok.
However, I've been wanting to build or modify a head to accommodate the Tung Sol 5881 because they sound so good. I'm not really clued up regarding matching output trannies to tube types. I'd be removing the board and just wiring everything on tagstrip and whatnot. I understand that it would be total rebuild, just mainly concerned whether the output transformer would be suitable. The PT puts out around 395vdc so it shouldn't cook the 5881s.

Any insights?

Regards

[soundguruman]

How doin?

I aquired a cheap Crate VC50 head which had blown up and melted various valve sockets, etc. I replaced the faulty PI coupling cap and now it hums along ok.
However, I've been wanting to build or modify a head to accommodate the Tung Sol 5881 because they sound so good. I'm not really clued up regarding matching output trannies to tube types. I'd be removing the board and just wiring everything on tagstrip and whatnot. I understand that it would be total rebuild, just mainly concerned whether the output transformer would be suitable. The PT puts out around 395vdc so it shouldn't cook the 5881s.

Any insights?

Regards

The output tranny will work.
The screen resistors are changed to 470 ohm for 5881 a lot of times.
Then the bias voltage is increased compared to EL34.
If EL 34 is running -32 bias 5881 might run -48 bias. (just an example, not necessarily in your amp, and depending on which tubes)
A resistor is changed in the bias circuit to increase the bias voltage.
I will use a scope, to see when the crossover distortion occurs, vs. bias voltage.
Another is to install resistors on the cathodes of the output tubes, measure across each resistor to see how much current each tube is drawing at idle.

You are basically going to dial in the current of the 5881, so it's not too high or low.
Other than screen grid resistors, and adjusting a resistor in the bias circuit,
there's not so much to doing it.
It's pretty basic and fairly simple, so go ahead and try it.

I mean it's a crate. What have you got to lose?

most amps can be changed from 6L6 to 5881 to EL34 to 6550 pretty easily.

[olddawg]

This is a 4 x EL84 amp producing 30 watts like a Vox AC 30 right? Running 2 x 5881 tubes would be quite an adventure. There are those much more experienced than me on this board, but I'm not sure the OT would work. There are lots of other considerations as well. You would have to use different tube sockets and basically gut the amp. You might consider selling it since it is working fine and start with something more compatible from scratch .

[minim]

The output tranny will work.
The screen resistors are changed to 470 ohm for 5881 a lot of times.
Then the bias voltage is increased compared to EL34.
If EL 34 is running -32 bias 5881 might run -48 bias. (just an example, not necessarily in your amp, and depending on which tubes)
A resistor is changed in the bias circuit to increase the bias voltage.
I will use a scope, to see when the crossover distortion occurs, vs. bias voltage.
Another is to install resistors on the cathodes of the output tubes, measure across each resistor to see how much current each tube is drawing at idle.

You are basically going to dial in the current of the 5881, so it's not too high or low.
Other than screen grid resistors, and adjusting a resistor in the bias circuit,
there's not so much to doing it.
It's pretty basic and fairly simple, so go ahead and try it.

I mean it's a crate. What have you got to lose?

most amps can be changed from 6L6 to 5881 to EL34 to 6550 pretty easily.

Cheers bro. Not sure if I made it clear though, this head has a 4 x EL84 power section. I'm clueless as to whether an OT designed for use with 4 x EL84s would at least work with two Tung Sol 5881s.

[minim]

This is a 4 x EL84 amp producing 30 watts like a Vox AC 30 right? Running 2 x 5881 tubes would be quite an adventure. There are those much more experienced than me on this board, but I'm not sure the OT would work. There are lots of other considerations as well. You would have to use different tube sockets and basically gut the amp. You might consider selling it since it is working fine and start with something more compatible from scratch .

Gotcha. I don't even know if this OT is even rated at fifty watts. I guess I assumed it's all fifty watt rated because the amp's a 'VC50'.

I understand that EL84s are noval and 5881s octal, not too concerned about major surgery. I'm a tireless tweaker.

[olddawg]

Gotcha. I don't even know if this OT is even rated at fifty watts. I guess I assumed it's all fifty watt rated because the amp's a 'VC50'.

I understand that EL84s are noval and 5881s octal, not too concerned about major surgery. I'm a tireless tweaker.

Look at some schematics for amps using a PP 5881 arrangement. The PI will be different. I'm pretty sure you will need a bias tap on the PT for 5881s. You need to look at the B+ and the current available. Lots of considerations. It's usually a good idea to start with a known good design and layout too. I hear those VC 50s are the poor man's AC 30 and sound pretty good with decent tubes through decent speakers. A shame to make a mess of it IMHO.

[minim]

Look at some schematics for amps using a PP 5881 arrangement. The PI will be different. I'm pretty sure you will need a bias tap on the PT for 5881s. You need to look at the B+ and the current available. Lots of considerations. It's usually a good idea to start with a known good design and layout too. I hear those VC 50s are the poor man's AC 30 and sound pretty good with decent tubes through decent speakers. A shame to make a mess of it IMHO.

Mate I don't make any messes.

I'm not partial to the EL84 sound.

Just forget it. All I wanted was advice on tube/transformer matching.

[pdf64]

I can't see that it would be a problem, see schematic The Fillmore Pedal blog: Crate Vintage Club 50 guitar amplifier schematic
The OT primary impedance and current handling is almost certain to be fine.
The only circuit mods required are for the bias supply; the JTM45 arrangement could be used http://www.raw-sewage.net/images/jtm45ri-schm1.jpg though it may need tweaking for 5881 and the screen grid resistors, R48-51 increased to 470ohms @ 1W, though that would be beneficial for EL84 also.
Pete

[soundguruman]

Cheers bro. Not sure if I made it clear though, this head has a 4 x EL84 power section. I'm clueless as to whether an OT designed for use with 4 x EL84s would at least work with two Tung Sol 5881s.

Yes it will work.
I could do it in about 45 minutes.

The OT is compatible.
You do not need to make any major change.

The bias supply can be adjusted for the right voltage, as is.

[g1]

The bias supply can not be adjusted for enough voltage as is.
Like pdf mentioned, no problem with the OT. For example/reference Marshall JTM30 (2x5881) uses same OT as DSL401 (4xEL84).
You will need an alternate bias circuit for more range, the one pdf linked is a good option.
If anyone tells you they can punch the chassis for 4 new tube sockets, rebuild the bias circuit, and have it up and running and tested and reliable in 45 minutes... run away as fast as humanly possible.

Edit: Oh, I forgot, some self proclaimed supertechs don't know there is a difference between EL34 and EL84.

[minim]

Thanks guys for all your replies. I didn't even think to refer to the DSL40/JTM30 schems. As the amp stands, I've fitted a bias transformer to try and cool the thing down, it ran so hot with the max available negative bias voltage. I'll likely use this for the 5881s.

Mitch

[Tooboob]

It's doable if you're willing to wire up the sockets. Some things you have to know that you may know already:

1. The plate and screen of the power tubes are at high voltage. Use wire with a sufficient insulation rating.
2. Most replacement bias circuit solutions tap off the high voltage secondary. This point runs maybe 300V AC and can deliver a couple hundred milliamps. It's the most dangerous point in the amp. It shuts up right away if the amp is unplugged, but don't forget, and check your work carefully before power up to avoid catastrophic failure. There will probably be a cap close to this HV connection in series with it, maybe through a resistor, diode, etc. It needs a good voltage rating. 500V is a minimum. There will be another cap that goes to ground, maybe a part or two away from a diode. it should be at least 60V, and 400V is better. It will probably be large value and polarized. The + end goes to ground, since the other end is a negative voltage.
3. You can test your bias circuit with power tubes uninstalled, but most amps discharge their high voltage much more slowly without tubes (and the tubes don't help much once their heaters are cool, so there will probably be some residual voltage around). Putting the amp on standby for a few seconds before you turn it off helps by keeping the tube heaters running as the voltage discharges. Check the big high voltage supply caps with a suitable volt meter before messing with the amp, even if it's unplugged. A clip-on jumper wire between a preamp tube plate and ground usually get things discharged pretty quick. If you have schematics, you can find the resistor going to the plate and connect there (tube end).
4. One hand in your pocket, and don't rest your chassis on a bowling ball.
5. Don't get nervous or you'll kill yourself.
6. Don't drink and mod.
7. Google "tube amp safety" and "amp repair safety" and study up. I haven't given you a complete list. I haven't met anybody who'd admit to killing themselves with a tube amp, but it will at least happen someday, and folks around here could probably tell you stories ...