Can I use a PP interstage for SE?

[jev]

Hi all, I have an interstage (very nice from Tribute) that is designed for a PP 6h30 into a Hypex module (3+3:1+1). I had many problems with the amp and have decided to stop bugfixing it.
Can I use this for a SE amplifier by putting the primaries in parallel and the secondaries in series (so end up with 1.5:1)? Or is a PP interstage fundamentally differently build than a SE interstage? I read something about the air gap.... Thanks

 

[MrCurwen]

Yes, the air gap is very important and necessary if you have DC in the primary.

You can do whatever with the transformer if you go parafeed.

 

[jev]

Great. I understand parafeed is like the schema's (in the DHT Rob schema see the dots for capacitor positions as I understand).

Questions would be:
1. Is there a preferred position for the capacitor (green or red dot)?
2. What are the capacitor values that I need for interstage position in a end-amplifier?
3. What is the sonic advantage of an IT in parafeed above a capacitor?

 

[Shoog]

I prefer to put the parafeed cap in the green position because it keeps it biased up with maximum potential difference across it - which is good for linearity. I also suggest looking at taking the cold terminal to the valves cathode rather than earth. This takes the cathode bypass out of the signal path.
Many people undersize their parafeed cap going for something around 4-6uf. This introduces a resonance between the cap and the transformer which can give a bass boost. I would avoid trying this intentional sound shaping and go for at least 10-20uf to keep the resonance below 10hz.

Shoog

 

[MrCurwen]

Questions would be:
1. Is there a preferred position for the capacitor (green or red dot)?

You cannot have a capacitor in the green spot. It's at the secondary side and would leave the secondary floating and the primary would have DC flowing thru it.

If I understand you correctly, you are asking whether it's preferable to have the order

1) plate -> cap -> OT primary -> ground

2) plate -> OT primary -> cap -> ground

And the answer to that is: it doesn't really matter. I've done it both ways and absolutely no sonic difference.

It comes down to some marginal preferences. Even though transformers never ever break down, I like to think that if they were to break down, the cap in option #1 would protect the output. I only use parafeed in the output section.

2. What are the capacitor values that I need for interstage position in a end-amplifier?

5k primary? I'd start with 8 or 10 µF and measure the frequency response.

I use 4µF and 10k : 4R transformers, and it gives a small 1 to 2 dB bump centering at 10 Hz and completely level from 17 Hz up. Sometimes I use 8µF or 10µF. Makes no audible difference that I can hear with my headphones.

3. What is the sonic advantage of an IT in parafeed above a capacitor?

In an interstage coupling function? Well, none.

If you need gain, it could help with that. Many "only DHT" people use interstages because of that.

If you especially want to use an interstage for any reason, using it parafeed will make it perform to it's absolute best capability. So there's that.

 

[MrCurwen]

Also of course if you need phase splitting.

 

[Shoog]

Sorry I missed that it wont work as drawn.
You need a plate load CCS or choke to make it work in parafeed.
You should have just about enough voltage for a CCS if you use the 490V supply.

I also question if the use of such an IT in this application brings any benefits at all over a simple Cap, since the cap is still there whatever you do.

Shoog

 

[45]

Hi all, I have an interstage (very nice from Tribute) that is designed for a PP 6h30 into a Hypex module (3+3:1+1). I had many problems with the amp and have decided to stop bugfixing it.
Can I use this for a SE amplifier by putting the primaries in parallel and the secondaries in series (so end up with 1.5:1)? Or is a PP interstage fundamentally differently build than a SE interstage? I read something about the air gap.... Thanks

Unfortunately the air-gap is necessary for SE. You can ask Pieter if it is possible to put an air-gap.

You can use it in PP for driving a SE output stage, provided that the driver is not the splitter at the same time (you can use an input splitting transformer, for example). I have seen this done by Audio Tekne in the past to drive the 845 in class A1 with the typical small signal triodes. If I remember correctly the ratio was 2:1 (full primary to full secondary). It would be also useful in cases where the input impedance of power stage is low, like Schade-feedback output stage. If it works with 6H30, I suspect it will also work with tubes of similar or slightly higher plate impedance if you need different (higher) voltage swing.

Another way to use it is to drive 1/2 of primary with SE driver and the other 1/2 only DC (so with another active device) to compensate the DC induction but this would not be my first choice.

I don't like resonant parafeed....

 

[MrCurwen]

If it gets him to use a proper plate load, he'll come out miles ahead in terms of transparency.

I'd recommend a simple one FET gyrator instead of a CCS.

 

[Shoog]

Drive it with a LTP using the transformer as a PP > SE converter. Should work nicely with lower overall distortion.

Shoog

 

[45]

If it gets him to use a proper plate load, he'll come out miles ahead in terms of transparency.

I'd recommend a simple one FET gyrator instead of a CCS.

Even more transparent if there are no caps in the way...
Using a PP driver for SE output stage one can get high voltage swing with very low distortion.

 

[45]

Drive it with a LTP using the transformer as a PP > SE converter. Should work nicely.

Shoog

+1

Or with an input splitting xformer.

 

[MrCurwen]

Shoog gave excellent idea! Also, very rare so your amp will be different.

One benefit the IT could have also, is the low(ish?) impedance presented to the grid. Better grid drive conditions.

This could be solved with additional active circuitry, but I remember back when I used IT phase splitters the grid drive was much less of an issue.

 

[pieter t]

No, the airgap is not possible; the transformer is potted.
When using this interstage transformer in a single ended amplifier I would probably direct couple an input tube to a long tail pair driver stage, and use the IT to couple to the output stage. A 6BX7 might be a good candidate as driver with the 6H30 as input tube.

Ah I see more ideas along the same line

 

[45]

If used like PP interstage to drive SE output as always there is a trade-off. This is low distortion and especially 2nd harmonics of output stage cannot be (partially or mostly) cancelled anymore. So a very linear device or a device which gives low THD with local feedback is recommended as output tube.

 

[MrCurwen]

Whatever the transformer secondary, you could still use it to drive a balanced output stage. Use resistors to make a virtual center tap and ground that.

 

[pieter t]

Don't understand what you mean there; the IT is 3+3:1+1.

 

[MrCurwen]

Ah, right.

OP, do you have your mind set on a specific topology or are you just weighing your options?

 

[pieter t]

By the way, the IT has a minor air gap (as all PP transformers should have) so probably a driver tube with no more than some 10 mA of primary DC current could be tried as well. With the step down ratio there will be enough drive for the output stage.

 

[Shoog]

Surely you could still use the unused input to the LTP to inject feedback from the SE output if you got the phase right.
I would try to use the IT's step up potential and drive it with something like a 5687 in order to keep it a two stage.

Shoog