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Phase splitter choices on input of PP amp

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45

45

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Joined 2008
andyjevans said:
I'd like to know what are the choices for a phase splitter on the input of a PP amp so it could be driven by a preamp with gain using RCA inputs. Amp in question is an EL84 in PPP triode, so 8 output tubes in all. Choices I can think of include....

  • input transformer like 1:1+1 or 1:2+2
  • solid state splitter
  • valve concertina without the gain stage in front
  • ??? something else

I'd like something simple and transparent that could be driven with a 26 DHT preamp in this case, but this is a universal application so feel free to talk around choices as much as you would like.
Click to expand...
Build something like this below and forget 26, input transformers etc. etc.

https://diyaudioprojects.com/Schema...-6V6-6AQ5-Dynaco-A-410-Tube-Amp-Schematic.htm

I have done a similar one with the PCL82's and it just sounds glorious. Makes wonder why one should waste more money in 45's, other DHT's, transformers, preamps etc...
 
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andyjevans

andyjevans

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Joined 2003
artsandmedia.com
A conventional EL84 PP amp with feedback isn't in my plans, though I agree they can sound very nice. I'm very happy with my "Inverted DHT" amps - single ended 2 stage with 26 driving high gain output tubes like the EL12.

I just have a PP chassis which I'm wondering what to do with. I might use it to drive a pair of bass units with an active crossover, for instance. Just speculation at this stage. But my preference would be to use my 26 stage. I could build myself a new preamp stage with e.g. 4P1L into a LL1660 which would do the phase splitting. But I would prefer to put the phase splitter in the amp.
 
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ErikdeBest

Member
Joined 2003
45 said:
Build something like this below and forget 26, input transformers etc. etc.

https://diyaudioprojects.com/Schema...-6V6-6AQ5-Dynaco-A-410-Tube-Amp-Schematic.htm

I have done a similar one with the PCL82's and it just sounds glorious. Makes wonder why one should waste more money in 45's, other DHT's, transformers, preamps etc...
Click to expand...
Nice to hear this "simple" schematic sounds so well. Do you have an idea why it is so good? The current of the driver tubes is returned to ground through the OPT, first time I noticed that.
Sorry for the odd topic, OTOH I know Andy is open minded to accept that :)
 
45

45

Member
Joined 2008
ErikdeBest said:
Nice to hear this "simple" schematic sounds so well. Do you have an idea why it is so good? The current of the driver tubes is returned to ground through the OPT, first time I noticed that.
Sorry for the odd topic, OTOH I know Andy is open minded to accept that :)
Click to expand...
I have not used exactly that circuit but it's not much different except the "detail" you mention. Mine is a more conventional paraphase +UL and 6 dB feedback. IMHO, the paraphase makes it sound better and more SE-like, despite it is regarded less ideal than others. Overall distortion is still PP-like and very low. With 300V supply + LL1682 it makes 12W before you notice little flattening of the sinusoid on the scope.
 
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revintage

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Joined 2007
Played around with an old/odd idea of an iron concertina.

1710432764294.png
 
andyjevans

andyjevans

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Joined 2003
artsandmedia.com
How do you calculate the design? Do you leave the Va and B+ of the valve stage as they are and let the voltages in the Mosfet just find their natural level? Or is the Va of the valve stage critical? If you used a voltage divider in front what would determine the voltage of that, say in relation to 300V?

Presumably you choose the resistor value for the current you want - in your design 6mA.
 
R

revintage

Member
Joined 2007
DN2540 works well at 5-6mA. When you know the drivers Ua you can calculate Rk. Ra must be the same, ie you have to select B+ to get enough voltage over the depletion mosfet.

Wouldn´t go for a voltage divider with AC input, would do it like this instead:

1710443647409.png
 
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euro21

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Joined 2004
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FET phase splitter sample based on the above.
#26, FET splitter, EL84 PP_8k.jpg

p.s.
DN2540 almost unusable as next VAS (or in this case splitter) stage with high output impedance tubes as #26. It's HF behaviour is horrible, even over 10k the falling begins.
 
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revintage

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euro21 said:
DN2540 almost unusable as next VAS (or in this case splitter) stage with high output impedance tubes as #26. It's HF behaviour is horrible, even over 10k the falling begins.
Click to expand...
Remember the concertina has heavy NFB, like a source follower with two outputs. When adding your, in my view unnecessary voltage divider it might be, but with 8k directly from a 26 you reach -3dB at over 350kHz. With 50k you will only reach ca 70kHz.
Not familiar with the one you use, how much lower capacitance has it got?
 
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