4 tube push pull question

[tpaairman]

This is more out of curiosity for down the road......

On amps that use more than two tubes for a push-pull output, it looks like you just put the tubes and associated parts in parallel with each other. Do you have to change the supply voltage to the sets of tubes, as opposed to just two tubes?

[Jazz P Bass]

Have to?
No.

[tpaairman]

Well, OK. But would you want to?

[Enzo]

I guess that depends what you want to do.

[pdf64]

In real world tube guitar amps, supply voltages vary according to the mains and signal conditions.
Consider that more that more tubes will draw more current, hence (all else being equal), supply voltages will sag.

[Helmholtz]

In real world tube guitar amps, supply voltages vary according to the mains and signal conditions.
Consider that more that more tubes will draw more current, hence (all else being equal), supply voltages will sag.

All above comments are justified.

So I suggest to reformulate the OPs question into "Why do 4 powertube 100W amps often have higher B+ than their smaller 50W brothers using just 2 powertubes of the same type?"

If the OP agrees to this, I might do some speculation.

[g1]

I'd say the current capability of the supply (power transformer size) is a bigger factor than the voltage.

[tpaairman]

If the OP agrees to this, I might do some speculation.

Works for me. I appreciate the info.

[Justin Thomas]

[Post deleted for misinformed speculation & other such nincompoopery]

Justin

[g1]

One exception to this is the larger Ampegs, thel best example I've seen myaelf being the VT-40/VT22. The 2-tube amp was running an easy 590V on the plates & had been retrofitted to Sovtek 5881WXTs due to unavailability of 7027s. I checked the schematics, and the VT-40 was shown as having a higher B+ than the VT-22.

V2(VT40) and V4(VT22) use different PT's (and OT's). No idea why they ran the lower power version at such a high B+. (590V vs 545V of V4)

[tubeswell]

On amps that run high plate voltages (500+) like a Plexi Superlead 100 with 4 x EL34s, its fairly important to get all the tubes in the quad matched as closely as possible. EL34s are finicky because the signal grid is wound close to the cathode for maximum gain, which makes them prone to grid current, and when the amp is pumping out maximum power, a tube with more grid current will lose bias first (which in a fixed-bias amp, spells doom for the tube)

[Justin Thomas]

Thanks for that. I'ma delete my erroneous post.

Justin

[Old Tele man]

Two principle ways to make more power: increase voltage swing (▲Vp) and/or increase current swing (▲Ip); however, (A) BOTH are constrained by the equivalent plate load slope and (B) tubes can only handle so much current flow. So, typically, it's easier to boost voltage (if the tubes can handle it, such as EL34, etc.) and "stretch" the load line over a longer distance, creating a larger voltage change for the same current change, yielding more output power.

[g1]

Thanks for that. I'ma delete my erroneous post.

Justin

Well, I've always considered it a noteworthy exception, so I'm leaving that part.

[Chuck H]

Ok...

Jazz said "Have to? No." A little curt Accurate though. But it should be understood that you can't just stick two more tubes into a two tube amp. I mention it because it's hard to interpret what your question means.?. Additional sag to the supply voltage with more current was mentioned. Not mentioned was that for most guitar amp purposes the available current from a two tube amp will be insufficient. Same goes for the OT rating.

Helmholtz mentioned that most four tube amps do indeed have more voltage than their two tube counterparts. If this the reason for the question then Jazz's answer sums it up. So why do most four tube amps have more higher plate voltage? We'll get more speculation than my own, but I think it's because an amp that uses two tubes is just an amp, maybe even targeting a certain output wattage below max. When guitar amp makers get to building with four tubes there's only one reason. Maximum power and headroom. And you get that with higher voltages. JM2C