Shared screen resistor in push/pull amp

[Torquil]

Hi!

I have a Peavey Triumph 60 which is designed without screen resistors in its 2 x 6L6GC push/pull output section. I'm considering adding screen resistors, but it is not easy to find space for them in this amp, due to the way the output tube PCB is placed underneath another PCB, with little space in between. But I can quite easily add a single screen resistor which will be shared among the two output tubes.

What would be the disadvantages of using a single, shared screen resistor for the two tubes, compared to the traditional arrangement with individual resistors, given that its resistance and power dissipation capability is appropriately chosen?

[Roe]

You risk oscillation unless you use a small resistor at the socket

[pdf64]

A single shared resistor supplying the screen grids seems to have been a preferred arrangement for Phillips-Mullard with the EL34, see ‘operating characteristics’ p3 and 4 of https://tubedata.altanatubes.com.br/she ... e/EL34.pdf

Selmers tended to follow that https://el34world.com/charts/Schematics ... ii_50w.pdf

The ‘shared feeding individual’ arrangement of the JTM45 and Hiwatt DR103 seems a good option for retaining some grid stopper stability enhancement https://el34world.com/charts/Schematics ... ad_45w.pdf

https://el34world.com/charts/Schematics ... _dr103.pdf

But then the schematic for your shows screen grid resistors?
https://el34world.com/charts/Schematics ... 20_sch.pdf

It’s weird how both the above schematics have the same error, in that they show the OT CT being fed from the screen grid HT node.

[pompeiisneaks]

A single shared resistor supplying the screen grids seems to have been a preferred arrangement for Phillips-Mullard with the EL34, see ‘operating characteristics’ p3 and 4 of https://tubedata.altanatubes.com.br/she ... e/EL34.pdf

Selmers tended to follow that https://el34world.com/charts/Schematics ... ii_50w.pdf

The ‘shared feeding individual’ arrangement of the JTM45 and Hiwatt DR103 seems a good option for retaining some grid stopper stability enhancement https://el34world.com/charts/Schematics ... ad_45w.pdf

https://el34world.com/charts/Schematics ... _dr103.pdf

But then the schematic for your shows screen grid resistors?
https://el34world.com/charts/Schematics ... 20_sch.pdf

It’s weird how both the above schematics have the same error, in that they show the OT CT being fed from the screen grid HT node.

The schematic shows an * near the 100ohm screen resistors and when you look at the notes it says

R * Denotes a 120 amp only

Not sure why they omitted them.

I don't see an easy way w/o accessing that board, because you'll need to cut traces, as they've got solid PCB leading to the pin 4 on those tube sockets.

You could cut the traces to pin 4 on either side and then solder the resistors you want to use onto the pcb directly by scraping away some of the solder mask. A bit of a hack though, and it may be better at that point to either accept it as is, or maybe sell it and build a new amp to replace it? or decide to do something with the chassis and components where you gut it and rebuild... (the latter is more of a last ditch if your amp is just not working but you don't want/can't easily sell it)

~Phil

[pdf64]

...
The schematic shows an * near the 100ohm screen resistors and when you look at the notes it says

R * Denotes a 120 amp only

Not sure why they omitted them.
...

I assumed that was a drawing error, ie only 2 should have been marked up with the *?

[brewdude]

Are you certain they are not already on the PCB?

[pdf64]

Checking the pcb, there doesn’t seem to be provision for them on the version for the 60W version.
I can seen the attraction for doing a shared resistor, as it could just replace the link.

[Torquil]

Thanks for all the responses! I will take a look at the schematics provided above. As mentioned above, only the 120W version of the amp has screen resistors.

The output tube board is underneath the power supply board, and unfortunately they are layered upon each other so close that there is not even room for the ordinary white 5W "cement block" power resistors between those boards. And I cannot easily raise the power supply board, because there is not enough vertical space in the chassis. So it is difficult to get a 5W resistor close to the tube socket pins.

But there a 2 kOhm 5W resistor on the side of the board, and by rerouting some of the power supply traces it would be possible to put a shared screen resistor there, and relocate the 2 kOhm resistor. I have seen a picture somewhere of the Triumph 120 output tube PCB, and I think it has all the screen resistors located on the side of the board, i.e. not really close to the tube socket pins.

If I could use lower wattage screen resistors, it would be easier to get them closer to the tube pins. Would 2W or 3W power capacity be enough for individual screen resistors in this amp?

Would changing to a single shared screen resistor located around 5 cm of conductor length away from the tube pins increase the likelyhood of oscillation, compared to using no screen resistor at all?

Yeah, there are a few mistakes in the Peavey Triumph 60/120 schematic that is available on the web.

Changing to power tube sockets mounted on the chassis itself, and dropping the output tube PCB might solve the problem...

[Torquil]

A single shared resistor supplying the screen grids seems to have been a preferred arrangement for Phillips-Mullard with the EL34, see ‘operating characteristics’ p3 and 4 of https://tubedata.altanatubes.com.br/she ... e/EL34.pdf

Selmers tended to follow that https://el34world.com/charts/Schematics ... ii_50w.pdf

The ‘shared feeding individual’ arrangement of the JTM45 and Hiwatt DR103 seems a good option for retaining some grid stopper stability enhancement https://el34world.com/charts/Schematics ... ad_45w.pdf

https://el34world.com/charts/Schematics ... _dr103.pdf

Thanks for the links. Very interesting. The common/individual screen resistor splitting might be the best option if I do not decide to ditch the whole output tube board, since there will be space for the smaller, lower-wattage screen resistors.

[Andy Le Blanc]

There's an evolution of design with screen grids and circuit designs, if you look at old schematic for long enough.
Picture NO screen resistors and both screens of the P-P pair dressed directly to the center tap on the OPT.
They were originally run positive relative to the anode, very early 6l6 power pentodes were designed with this in mind.

you can measure the voltage difference between the screen and plate.