VHT cathodyne PI

[Speedypancake]

Still experimenting with my current high gain build. Started out as a JCM800 but I'm now rebuilding the first preamp along the lines of the VHT Deliverance.

Even though I'm only using the 2 preamp valves from the 2205, the amp already sounds much "tighter" and fuller than the JCM800.

I've now reached the phase splitter / power amp section, and learning that the VHT uses something called a "Cathodyne" circuit here, which is something new to me, so I'm interested to see how it compares to the standard Marshall PI.

One thing I've noticed is the differing values of the resistors going into the output valves highlighted below. In every other circuit I've seen, these resistors are the same value, but here we have a 2.2K & a 10K... Assuming this isn't a mistake, does this have something to do with balancing (or even unbalancing) the cathodyne driver?

Thanks!

[Stevem]

Go to the Aiken amplification site and read the technical papers he has on all the different types of PI circuits.

[roberto]

I'd add the cathode followers after the cathodyne phase splitter, as they do in many of their amps.
I'd actually add source followers instead of cathode followers, using CCS loaded mosfets.

I'm proposing to develop a similar pcb on another forum, I'll report here the sketch I'm preparing.
It's a pcb to be connected between the original circuit and the grids of the power amp, with independent biasing for each tube and drivers dc coupled to the grids of the power tubes.

[Speedypancake]

Thanks for the link Stevem.

Roberto! Hope you will keep us up to date with your current project.
And will try the alt CF you suggested one day. Were you thinking the orignal circuit would be underpowered to drive a couple of el34s? I'm not too bothered about vol as this is a bedroom / workshop amp, and I've even scheduled in a power / B+ reduction once I get the tone I'm after.

Did you have any thoughts on the original question about the mismatched el34 grid resistors??

My own project seems to be creeping into something more like the Synergy system, which allows me to plug and play various preamp boards.. even run them in parallel... or chained one into the other!......

Cheers!

[roberto]

I've noticed something similar on Marshalls too.
It will cause some second harmonic when pushed hard (close and into AB2).
There could be other reasons as well.

[Speedypancake]

It will cause some second harmonic when pushed hard (close and into AB2).

Had been hoping that might be the reason - looking forward to trying it out this weekend. Unfortunately I can't A/B this new PI so a lot will depend on my memory, how I'm feeling, how much JD we have left

[Phil_S]

Did you have any thoughts on the original question about the mismatched el34 grid resistors??

I may have a theory. The cathodyne (aka split load or concertina) PI has much lower voltage coming off the cathode side of the PI compared to the plate side. This requires the use of different grid stoppers. BTW, I can't see any of the component values. What are the 2 resistors?

[sluckey]

Signal voltage is the same on the plate and cathode. DC voltage doesn't matter because of the coupling caps.

[stephenl]

Pete Millett sells these buffer boards on his ebay store. I haven't used this but have used some of his other stuff - well designed...

http://www.pmillett.com/A2_buffer.html

[Speedypancake]

Well I've just road tested the new cathodyne layout..


Think this was the missing secret sauce, for me anyway. Tight as a (insert fav similie here), and bursting with rich harmonic distortion. So touch responsive as well.

You know I hadn't played through a real amp for @ 10years, making do with the convenience of a desktop pc sim (guitar rig) when the fancy took me. But this is totally different ball game. The better the amp sounds, the more I want to play. It makes playing exciting again

Also loving the new presence and depth controls around the cathodyne driver. Will have to seriously work on my sloppy layout though as I just lashed everything together and it's exposing quite a bit of high freq instability with pres anything above 3 or 4. Will also take a look at adding a 470K resistor going into the cathodyne grid as recommended elsewhere.

I stuck pretty faithfully to the Deliverance schematic (but with EL34s intead of KT88s) and used the 2K2 /10K grid resistors for now. Thinking it might not be the exact values so much as the ratio between the 2 sides...

So much more to explore and try.. including the challenge of coaxing a clean(ish) tone out of this beast!

Thanks all for your help & interest so far.

[nworbetan]

One thing that may help tame the amp and help with the clean tone is reducing the negative feedback (either increasing R33 or moving the nfb connection to a lower impedance tap on the OT). Since EL34s are a higher gain tube, they'll need less input signal into V4A to get the maximum output (and maximum nfb signal) compared to the KT88s.

Where did you get the schematic, and do you have the preamp also? Thanks for sharing the power amp!

[Speedypancake]

Where did you get the schematic, and do you have the preamp also?

https://music-electronics-forum.com/for ... power-amps

[gingertube]

Merlin covers the Cathodyne Splitter quite well.
http://www.valvewizard.co.uk/cathodyne.html
It is reputed to have quite nasty behaviour when overdriving the output tubes as the loads on the anode and cathode of the splitter become unbalanced (with output tube grid current).

The fix for this is to push the output tube grid stop resistors way up, say 33K or 47K.

If you are a bit of a techhead and don't mind maths then see my reply #27 toward bottom of page here:
https://www.guitargear.net.au/discussio ... c=38629.15


Cheers,
Ian

[Speedypancake]

Thanks GT - yes I had seen that excellent write up by Merlin. To be honest I've just stuck with the original schem as is and very pleased with the results. Not sure if I'm going to be able to hear differences between upping the el34 grid resistors, or adding a 470K to the cathodyne input, but we'll see..

.. And no, definitely no head for maths here.. Ohm's Law is about my limit I'm afraid!

Plan now is to tidy everything up a bit, and start experimenting with dropping the B+ with a variable mosfet reg circuit

Cheers

[pdf64]

...It will cause some second harmonic when pushed hard (close and into AB2)...

Just to note that to achieve AB2, the preceding stage must be able to maintain signal linearity with the power valve’s Vg1>Vk, the positive grid / grid current area of operation.
Almost by definition, AB1 amps can’t do that. Although the clipped signal peak may rise a little above Vk, that shouldn’t be taken as meaning that they’ve entered AB2.