E182CC, E88CC SRPP at low voltage

[yagoolar]

Hi,

I want to build an SRPP stage for a simple amplifier but ... I need a voltage drop on the upper triode to be 45 volts. Do you have experience with tubes, as in subject working in SRPP at, say, Ub=90 volts? I have seen a phono stage with 24 volts battery supply in the Net, so I guess the former might be possible.

Perhaps other noval tubes are more suitable for that? Any comments are welcome. Later on I will try to post the amp diagram.

Pawel

[fscarpa58]

hi

the Ecc86, for instance, has been designed for low voltage battery supply.

Federico

[Morgan L]

Yes, second that. The ECC86 is a fine little tube to use at Vak=15-25V, Ia=2-3mA. I use them in my CD player, running off the onboard +/-16V (only a heater transformer is added inside the box). Sounds great!
However - the question I would ask myself if I were restricted to Vb=90V: why do I want an SRPP stage? There are many more choices that may or may not work better when voltage is in short supply. E.g., if small to medium voltage swing is all you need, silicon constant current source loading can save you volts to use across the (non-stacked) tubes. (Otherwise, a voltage doubler is usually very easy to add to existing circuitry, at least if current demand is fairly low. That would get you some 160V or so, which is sufficient for stacked ECC88s.) But I think we need to know your intended circuit application to be able to help better.

Morgan

PS If you want to know more about the SRPP, Tube CAD.com has three excellent articles about it: SRPP At Last, SRPP Once Again, and SRPP Deconstructed. A Google search should bring you there.

[yagoolar]

Originally posted by Morgan L
Yes, second that. The ECC86 is a fine little tube to use at Vak=15-25V, Ia=2-3mA. I use them in my CD player, running off the onboard +/-16V (only a heater transformer is added inside the box). Sounds great!

Ok. Sounds great it sounds great. But I don't have it. I can choose from E88CC or E182CC at least as low voltage is concerned.

However - the question I would ask myself if I were restricted to Vb=90V: why do I want an SRPP stage? There are many more choices that may or may not work better when voltage is in short supply. E.g., if small to medium voltage swing is all you need, silicon constant current source loading can save you volts to use across the (non-stacked) tubes.

Look at the schematic. Initially 6c4c is to biased -45V. I know it is not true drd (direct reactance drive). It is just an exercise. I don't want to use a choke, but considered a CCS instead of one triode.

Some values:
Vb(6c4c)=315V
Vb(srpp)=90V
Vg(6c4c)=-45V
Ia(6c4c)=ca. 40 mA
Rk(srpp)=200 ohm
Comments?

PS If you want to know more about the SRPP, Tube CAD.com has three excellent articles about it: SRPP At Last, SRPP Once Again, and SRPP Deconstructed. A Google search should bring you there.

I read those too. Thanks

[Morgan L]

That won't work very well. Tubes need spare voltage across them to work, but in your circuit the SRPP tubes are required to swing all of their working voltage (+/- 45V). ECC88s can work at 45V with low idle current, but will only swing a few volts - not enough to get any substantial modulation of the output tube. Semiconductors can swing close to the rails, but not tubes.
The options include using a single triode with a choke for a plate load (chokes can "magically" swing past the 90V Vb), or one of the other example circuits in Tube CAD Journal May 2000. Anyway, the output tube needs to sit at a higher cathode voltage (ca 150V?) in order to give the input tube some working voltage for downward swings).
SRPP could be used if the upper tube connects to your output tube Vb, maybe via an RC combo to drop some voltage. The E182CC may be the better choice for both SRPP triodes, as it can swing quite low and also seems more suited for large swings. +/- 45V is quite a lot for a driver. Input sensitivity will be a bit low (which may be a good thing).
Anyway, the conclusion is: whether ECC88 or E182CC, you do need 100V or more across each triode to swing +/- 45V. So the choice is either to forgo the direct coupling or raise the output tube to provide more voltage for the driver. And either to connect the upper triode to 200V or more or skip it and use a choke connected to a voltage divider between the output tube cathode and ground instead. (Actually, just a lower triode with resistive loading to output cathode is also a possibility, though this won't be very linear near cutoff. Might compensate 2nd harmonic in the output tube though, so might be worth a try. The Broskie article discusses all these possibilities.)

Hope this was somehow helpful,
Morgan

[yagoolar]

Thanks Morgan. I will give it a try . Today in the evening...

[yagoolar]

Hi,

Finished assembling the amp. First listening impression promise quite much. It requires few components.
When I run measurement tests I will post results. It uses E88CC (Tesla) and russian 6C4C (NOS '73). PSU is Si-bridge based. Ua(6C4C)=315V and Ub(SRPP)=88V.
I will have to replace old xformers (polish TG 2,5-1-666) with new ones.
Do you any have suggestions for cheap 6C4C xformers?

Pawel
PS. I will post pictures later.