Phase inverter

[Sherman]

Yes the trafo is key in this type of phase inverter. That's why in the end I had a couple made by Jack at Electra-Print. He has a couple specifically designed for phase splitting. They are autoformers rather than transformers. On the downside they are a more expensive solution at about $55 each.

In the tests I did the best results didn't come from the interstages I tried but from PP output trafos! The inexpensive Edcor PP trafos at $20 did a great job. I connected them without using the secondaries. I took the signal from the input tube into one of the primaries, connected the center tap (normally used for B+) to ground and then connected each primary to the grid of an output tube. Pretty easy and very good sound. The trafos are a bit big though so mounting can be an issue.

[FlaCharlie]

Sounds interesting. What model of Edcor? What are the specs? I'm sure I've got some orphaned OTs around.

Will this work for PP using other output tubes, specifically 6B4Gs?

Thanks . . . Charlie

[Sherman]

The Edcors I tried were XPP15-8-8K. I also tried the TF110-48-UL from Triode Electronics which also worked well. Finally I tried a potted toroidal power transformer 120v:120:12V:12V. With the primary connected it worked great with the secondary connected, not so well. Finally I tried an Edcor SE trafo with UL taps but it didn't sound very good either. (It sounds great as a regular OPT though, especially for $20.)

I suspect any PP OPT might work. Keep in mind that I was only going on sound and don't pretend to have 'golden ears'. My scope went south on me just before I started testing these trafos so I couldn't do a real frequency response test. I did compare the sound against a couple other amps I have both SS and valve.

I also tried a standard concertina type phase splitter with the same output stages. I tried these experiments with both EL84 and KT88 output stages. With the EL84 setup I used a 6N1P as the voltage amp and with the KT88s I used a 6SL7 as the voltage amp. B+ in the EL84 setup ranged from 325V to 360V and with the KT88 I tried from 350V to 400V.

[zobsky]

Originally posted by Sherman
Yes the trafo is key in this type of phase inverter. That's why in the end I had a couple made by Jack at Electra-Print. He has a couple specifically designed for phase splitting. They are autoformers rather than transformers. On the downside they are a more expensive solution at about $55 each.

In the tests I did the best results didn't come from the interstages I tried but from PP output trafos! The inexpensive Edcor PP trafos at $20 did a great job. I connected them without using the secondaries. I took the signal from the input tube into one of the primaries, connected the center tap (normally used for B+) to ground and then connected each primary to the grid of an output tube. Pretty easy and very good sound. The trafos are a bit big though so mounting can be an issue.

Hi Sherman,

1. Are you saying that, for phase splitting applications, the edcor OPTs (used as autoformers) worked better than the Electra-Print PSA-2 autoformer-splitters ?

2. Which interstage did you compare with?

3. What difference did you hear comparing the "iron" splitters to the tube-based splitters.

Thanks

[Sherman]

Originally posted by zobsky


Hi Sherman,

1. Are you saying that, for phase splitting applications, the edcor OPTs (used as autoformers) worked better than the Electra-Print PSA-2 autoformer-splitters ?

2. Which interstage did you compare with?

3. What difference did you hear comparing the "iron" splitters to the tube-based splitters.

Thanks

1- No, I'm not saying that at all. I tested the OPTs then ordered the purpose built Electra-Print splitters. I just haven't built the amp with the Electra-Prints yet. I tore down my bread-boarded amps before I received the PSA-2N autoformers so I haven't even tested them yet. I'll actually start building the amp next week and I'll report back when it is done (I work pretty slowly though so it could be a while.)

2- I tried three different interstages, one from Edcor- XSM10K-10K, one with no identifiable markings I got on eBay and an old Triad pulled from a console (though I don't think it was original to the amp)

3- For the EL84 setup I tried both 12AU7 and 6922 concertina splitters. They both worked fine though I might give a small edge to the 12AU7. It seemed that the bass was more defined but it was definitely not a big difference. For the KT88 setup I only used a 6SN7 for the splitter. Again it worked fine.

Bottom line, to my ears the iron splitter with the Edcor OPT was a match for the tubes. I just with I could have done some FR tests for more objective info.

[zobsky]

Originally posted by Sherman



1- No, I'm not saying that at all. I tested the OPTs then ordered the purpose built Electra-Print splitters. I just haven't built the amp with the Electra-Prints yet. I tore down my bread-boarded amps before I received the PSA-2N autoformers so I haven't even tested them yet. I'll actually start building the amp next week and I'll report back when it is done (I work pretty slowly though so it could be a while.)

2- I tried three different interstages, one from Edcor- XSM10K-10K, one with no identifiable markings I got on eBay and an old Triad pulled from a console (though I don't think it was original to the amp)

3- For the EL84 setup I tried both 12AU7 and 6922 concertina splitters. They both worked fine though I might give a small edge to the 12AU7. It seemed that the bass was more defined but it was definitely not a big difference. For the KT88 setup I only used a 6SN7 for the splitter. Again it worked fine.

Bottom line, to my ears the iron splitter with the Edcor OPT was a match for the tubes. I just with I could have done some FR tests for more objective info.

Thanks for clarifying. Please report back when you finish with the PSA-2N. It should be an easy "plug and play" replacement for the edcor "autoformer" connected OPT, assuming you load the secondaries as described by Jack.

FWIW, I have a pair each of PSA-2N and conventional electra-print interstages sitting around, I hope to get started on my PP 2A3 when I can find some time.

[fred76]

Hi,

Looking at the electraprint schemas, it seems that Jack prefers to put a R across the PSA-2N to damp LF Resonance and 'get away' with using a low value coupling cap. I'm just don't know how one calculates for the resonance point (~5-8Hz?) using a damping R across the choke especially if one would also use a plate choke for the driver tube. Or maybe it entails one of those spice programs to get ballpark values for an initial breadboard amp.

[zobsky]

Originally posted by fred76
Hi,

Looking at the electraprint schemas, it seems that Jack prefers to put a R across the PSA-2N to damp LF Resonance and 'get away' with using a low value coupling cap. I'm just don't know how one calculates for the resonance point (~5-8Hz?) using a damping R across the choke especially if one would also use a plate choke for the driver tube. Or maybe it entails one of those spice programs to get ballpark values for an initial breadboard amp.

When I spoke with him some time ago, he suggested not bothering with a driver plate choke, and just go with a plate resistor on the driver.

[ThSpeakerDude88]

So is there really anything wrong with just plain using the cathodyne in the ST35? I understand its obviously not the BEST amplifier in the world, but still darned good sounding, and I am dieing to have a decent sounding tube rig running again that will at least sound better than my solid state amp.

[SY]

No, nothing wrong with it at all. You could drop in the input stage from Morgan Jones's Bevois Valley design or my Red Light District and you'll have a very fine-sounding amp.