Difference between 2 x EL84 and 4 x EL84 board architecture?

[olddawg]

So I picked up a couple of these THD chassis. One was a 2 x EL84 model complete except it had an Australian PT. I swapped in another PT (actually one from a 30 watt/ 4 x EL84) and it's working fine. The other is a 30 watt/ 4 x EL84 has everything but the board. They look virtually the same layout wise.... all the controls, the front panel is exactly the same, wiring dress, etc... except one has two EL84s and one has four EL84s. And the transformers of course are slightly larger. My question is... since these amps seem to be identical except for the number of output tubes, what would be different in the board. Are the other two tubes just paralleled off of the phase inverter? I don't have a schematic.

[nickb]

They may well be driven in parallel off the PI. The output transformer will have about half the impedance for the 4 x EL84 version.

[bob p]

they are typically driven in parallel, though the grid load resistors might differ in value by 2x. at least they should.

[Malcolm Irving]

Also the dropper resistance (if there is one) to the screen supply node ought to be halved.

[olddawg]

Thanks everyone. Makes sense. I also found a schematic! Well a layout anyway....

[Chuck H]

...though the grid load resistors might differ in value by 2x. at least they should.

Yet, in guitar amps they usually don't. YMMV. But this commonality, I think, is definitely one of the reasons players tout the tone of four tube amps or two tube amps, respectively, as being notably different and/or preferred with models that come in both incarnations. Marshalls being the most recognizable example.

[vintagekiki]

They may well be driven in parallel off the PI. The output transformer will have about half the impedance for the 4 x EL84 version.

And main transformer is dimensioned for twice much secondary current.

[olddawg]

And main transformer is dimensioned for twice much secondary current.

Understood. The iron is for the 36 watt amp. The whole chassis is complete except for the board. It wasn't pulled.... just never installed. It's all new stuff. I used a PT for a 36 watt amp to replace the Australian one on the 18 watt chassis because I got it for $5. Sounds fine to my ears. Guess it's overkill.

[bob p]

Yet, in guitar amps they usually don't. YMMV. But this commonality, I think, is definitely one of the reasons players tout the tone of four tube amps or two tube amps, respectively, as being notably different and/or preferred with models that come in both incarnations. Marshalls being the most recognizable example.

Looking at the Fenders, I think they didn't bother in the Tweed era but they did bother in the SF era. So depending on time frame, Fender was pretty good about adjusting grid loads. I can't say why Marshall wasn't ... maybe they were just dummies who copied old Fenders and got lucky, lol.

Amp               Tube Compliment     Grid Load
Tweed
5E8-A             1 pair 6L6          220k
5F8               2 pair 6L6          220k

SF
AA371 Bassman     1 pair 6L6          100k
Bassman 135       2 pair 6L6          68k
AA769 Twin Reverb 2 pair 6L6          68k
Super Twin Reverb 3 pair 6L6          33k

[Chuck H]

Looking at the Fenders, I think they didn't bother in the Tweed era but they did bother in the SF era. So depending on time frame, Fender was pretty good about adjusting grid loads. I can't say why Marshall wasn't ... maybe they were just dummies who copied old Fenders and got lucky, lol.

Amp               Tube Compliment     Grid Load
Tweed
5E8-A             1 pair 6L6          220k
5F8               2 pair 6L6          220k

SF
AA371 Bassman     1 pair 6L6          100k
Bassman 135       2 pair 6L6          68k
AA769 Twin Reverb 2 pair 6L6          68k
Super Twin Reverb 3 pair 6L6          33k

Yep. That really illustrates my point. Thank you It's worth noting that the SF amps were poorly received. Albeit for other reasons. But laymen don't differentiate WRT details most often. I won't argue that the grid load should be adjusted for the driven load. I completely agree. I've even said as much already. I was only noting that there are some coveted designs that failed to recognize this design element and the resulting manifestation of "improper" design has been well received tonally by players. Not sure how much more discussion that deserves, but it is something that could go unnoticed (or at least unmentioned otherwise).

[cjenrick]

Note that in the Peavey Classic 30 they run two EL84 tubes with screen resistors and two without.

[Malcolm Irving]

Note that in the Peavey Classic 30 they run two EL84 tubes with screen resistors and two without.

Yes, that's a strange thing. Could it be just a mistake? Screen resistors protect screens from overheating and act as 'screen stoppers' to improve stability.

Doesn't make sense to only protect one of the pair from overheating.

I guess if you stabilize one tube of a parallel pair it would help to stabilize overall - but just to save the cost of two resistors?

It would also have some effect on the tone by making the tube with the screen resistor reach grid limited clipping before its companion.

[bob p]

that sure is odd. it makes me wonder whether they were trying to circumvent the simul-class patent when they thought of that.

[nickb]

Yes, that's a strange thing. Could it be just a mistake? Screen resistors protect screens from overheating and act as 'screen stoppers' to improve stability.

Doesn't make sense to only protect one of the pair from overheating.

I guess if you stabilize one tube of a parallel pair it would help to stabilize overall - but just to save the cost of two resistors?

It would also have some effect on the tone by making the tube with the screen resistor reach grid limited clipping before its companion.

I've seen the same thing in a few different amps and wondered why too. Marshall JVM205 springs to mind. If it was for tonality and you wanted safety wouldn't you just use sensibly sized but different valued resistors?

[g1]

I've seen amps with no screen resistors, screen R's on some but not all the power tubes, and screen R's on all power tubes.
I thought it was basically an evolution but I could be wrong.