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In Reply to: RE: Cathodyne VS LTP posted by Eli Duttman on May 01, 2021 at 16:28:47
Which tube type would you suggest for the CCS? :) I'm thinking an AC balance pot might be beneficial, ala Citation V.
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I'd be curious of how well an LTP sims with a choke loaded tail.
I've used the setup recently and know it works - but don't have distortion measurements.
I'm really good at frying solid state devices!
I have a small stock of 70H audio chokes, so that's what I use in SPICE for scenarios like this. The sims indicate that, while current balance is improved, distortion at each anode doesn't benefit much. Specifically, the improvement appears to be 3dB or so with the 6J6. That's not insignificant, but it may not be worth the cost of good chokes if the overall design includes NFB that pushes harmonics below audibility.
The issue of the contribution to distortion at the output of the amplifier is a different matter, because LTP balance plays a role. It's tempting to assume even-order harmonics will be improved with the choke (or a SS CCS) in the tail. However, best balance isn't necessarily achieved simply by equalizing current through the tubes. The exact value of the anode resistors also comes into play, as does the output loading on each section. All things considered, I believe any LTP should have a balance pot and should be tested and realigned periodically to compensate for aging of the various components. In my mind, this points to the most significant benefit of the CCS or choke in the tail, which is not that it balances the sections (that can be accomplished with a pot), but that it compensates for aging in the tube. That should minimize the need for periodic maintenance and significantly lengthen the alignment intervals.
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Buy Chinese. Bury freedom.
thanks for the info!
I'll try this first with a design similar to the 30W RCA circuit in RC-30. This circuit creates positive feedback between the pentode and cathodyne, significantly increasing the gain. Max Robinson discusses this circuit on his site and (thank you Max!) provides data regarding gain. HK used this technique also, although it was a triode-triode front end. In fact, I was involved in discussions right here a few years back regarding this technique in the HK A500.I don't know why my SPICE simulation indicates issues with the 'AU7 cathodyne, but I think there must be an error in the model. With that in mind, moving forward with the cathodyne rather than the LTP seems to be the simplest approach. At this power level (35W), the 5881s need about 60V pk-pk at each grid for full output. This drive level is well within the capability of the pentode-cathodyne with the voltages I'm using, so long as there's enough gain.
Here's the RCA circuit redrawn by Max. It's unusual in that it provides negative *DC* feedback to the screen of the pentode, while simultaneously providing positive *AC* feedback to the cathode. The former helps to stabilize the operating conditions of the cathodyne, which was Max's focus in the article. He measured the gain at about 450 (!), an increase of more than 6dB.
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Buy Chinese. Bury freedom.
Edits: 05/01/21
"Sand" could easily be best for the constant current sink (CCS), say the 10M45S integrated circuit. If 100% "hollow state" is desired, a multi-grid type whose current handling capability is appropriate gets the nod. Ground g2 and employ a negative rail that's "tall" enough to ensure correct operation. A member over at DIY Audio suggested regulating the negative rail, to emphasize multi-grid constant current behavior. The negative rail is g2 B+.
The previously linked 6J6 data sheet suggested 8.5 mA./triode as being the "typical" "idle" current. A type that can comfortably handle a 17+ mA. plate current is needed for the CCS. The 6CL6 is quite adequate for the role.
Eli D.
If you need the tube CCS, the type 6AC7 should do. There is also a tube cascode, triode on the bottom, pentode on top. This gets remarkably expensive voltage-wise though.
I prefer the SS version. A DN3545N3 under a 10M45 does quite well also. The 10M45 has double the bias voltage of a DN2540N5 and leaves the 3545 more voltage D-S.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
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