what's the deal with parallel triodes?

somebody help me out here. what happens when you have two triodes in parallel sharing the same plate and cathode resistors? what are the benefits? how about drawbacks?

does dynamic plate resistance half as if it were two resistors in parallel? how about capacitance between all those extra electrodes? How does this interact with the miller effect? Is noise performance better or worse for low-level gain stages?

I see it some for transformerless outputs in headphone amps and such. presumably for lower output Z and more power.

any notable designs I should check out that use this topology? just curious.

joe

for a moment I thought you were doing a jerry sienfield bit.

ha! i thought about that, but my jokes are usually terrible...somewhat like sienfelds...

I would be interested in this too. This is done in the LA2 EL pre-driver amp as well as in the Fender Reverb driver amp. I can see twice the plate current available but not twice the voltage gain happening. So maybe twice the power gain, but the same volts gain?

I don't get the Seinfield thing.

:?:

[quote author="funkydiplomat"]somebody help me out here. what happens when you have two triodes in parallel sharing the same plate and cathode resistors? what are the benefits? how about drawbacks?

does dynamic plate resistance half as if it were two resistors in parallel? how about capacitance between all those extra electrodes? How does this interact with the miller effect? Is noise performance better or worse for low-level gain stages?

I see it some for transformerless outputs in headphone amps and such. presumably for lower output Z and more power.

any notable designs I should check out that use this topology? just curious.

joe[/quote]

Assuming the operating point for each is the same as a single tube, capacitances and transconductance for a pair in parallel doubles. Plate resistance halves. Equivalent input noise goes down by 3dB. Mu of the composite stays the same.

If the tubes are very poorly matched it's maybe a good idea to have individual cathode resistors to enforce shared currents. But tubes are much more forgiving of this than bipolar transistors.

I don't get the Seinfield thing.

Click to expand...

Jerry sienfield usually starts off his comody segments with what is the deal with fill in the blank

so based on the title of the thread. oh well never mind...

Gm doubles as well, natch.