PSE with different tubes (6V6 & 6L6GC) - Doubt about OT ..........

[kagliostro]

A friend is planning to build a PSE with a 6V6 tube and a 6L6GC tube as power tubes

the 6V6 will be always connected and the 6L6GC also, but a MV will be used to increase and lower (till zero) the power of the 6L6GC

B+ will be around 360V, the question is about the OT that will be wrap by my friend

on datasheet 6V6 in SE at 315V B+ requires a load of 8.5K (5K at 250V B+)

and the 6L6GC in SE at 350V B+ requires a load of 4.2K (2.5K at 250V B+)

My friend can wrap the OT in many way, a separated primary with the correct impedance for each tube, a single primary with a primary impedance equal to the higher impedance requested by the tube and a tap to feed the tube that requires a less reflected impedance, or a primary winding with an impedance that is a compromise between the two requested impedances

There is no problem to wrap the OT in the various way because my friend wrap it by himself

I would like to know wich will be a good primary impedance for the 6V6 tube with B+ at 360V

and, more, which will be your choice for the arrangement of the OT (?)

Thanks

Franco

[PRR]

What is the purpose? Different tone? Or different power?

I'd run 4K or 3K load. The 6V6 alone will make more like 3 Watts (not 5-6W), a lower power. With the 6L6 opened up, you get for-sure the ~~10W of the 6L6 and maybe another couple Watts from the 6V6, a higher power.

If it will be self-wrapped, bring out taps at 70% and 140% of computed turns, and try each way. 

[kagliostro]

Thanks for the answer PRR

Franco

[shooter]

bring out taps at 70% and 140% of computed turns

You've dually confused me, is each tube gonna have a different tap instead of both plates tied together?
my AC theory has rusted, is there any phase/signal cancelling issues by "inserting" inductance between the otherwise parallel plates

[PRR]

> is each tube gonna have a different tap instead of both plates tied together?

I doubt it, unless we go crazy with switching.

In general, if both tubes have the same supply voltage, they will give nearly equal swings, and ought to have about the same turns-ratio to the load. But we do NOT know what turns-ratio is "best" for this odd combination. With 70%/140% options, he can try 2:1 impedance up or down to see if it makes a difference. (Or I suppose he could wind 2/4/8/16/32 output and move the speaker around.)

[DummyLoad]

why not bond two output transformers at the secondary? that way each valve gets the proper load for the applied potential? there will be some difference required to drive to clipping in drive signal. k, how are you planning to compensate for that? not that it matters if the smaller output valves clips before the larger... 


--pete

[PRR]

> bond two output transformers at the secondary?

#2: "my friend wrap it by himself". While the total kilos of metal will be similar, the labor will essentially *double*. (More than double, because smaller cores need more turns.)

> that way each valve gets the proper load

It won't be "proper load".

I assume he has just the one speaker, isn't swapping speaker impedances while he turns the 6L6 drive pot. Assume 8 Ohms for example.

What is proper load?

Assume 250V supply.

6V6 will want just shy of 250V 50mA thus a 5K load.

For simplicity, assume two 6V6, one constant and one variable drive.

One tube working wants a 5K:8r OT.

Two tubes working wants two 5K:16r OTs. So that 16||16 makes the 8 Ohms we will load with. Alternatively: one 2.5K:8, so each of two tubes sees 5K load.

6L6 will pull 250V 100mA and so a 2.5K load. Both 6V6 and 6L6 together should be a 1.66K load.

So what is the intention? MAX power in either condition? Probably not. What happens with a single 1.6K load? The 6V6 alone wants 5K, so with 1.6K the Watts will be down and the THD will be up. Probably 1.5W-2.5W max depending how much THD you tolerate. Now crank the 6L6 up and you may get 13 Watts from both tubes. That *may* be the intent. A ~~2W small bedroom amp plus a ~~12W small stage amp at the turn of a dial.

While 6V6 and 6L6 sound a little different, in this combination I think the "sound" will be more about the compromised loading than the basic tubes. Which is why I suggested some way to change the loading in the try-out stage of the build.