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My current amp project is poised to use an EF86 driving a 6C4 cathodyne (1/2 12AU7). I've been comparing the 6C4 in this mode to a 6J6 as a long tail pair, and the results in SPICE have been surprising. The 6J6 is pretty clean regarding distortion, even when the outputs are pushed to 60V p-p (the approximate level needed to achieve full output from the 5881s). The spectrum looks pretty typical, with 2nd and 3rd harmonics fairly similar and higher frequency energy decaying quickly. The 6C4 cathodyne, on the other hand, doesn't fare so well. The 2nd and 3rd are pretty well controlled, but there's a lot of energy popping up at the 4th, 5th and 6th. Even the 7th looks as though it might be audible. I don't rely on SPICE as a primary tool for distortion analysis, but I hadn't expected to see anything like this. I'm just wondering if anyone here has experience with the 6J6 as a LTP, and what your impressions (or measurements) have been. It would be easy to change the topology at this point, and the additional gain afforded by the LTP compared to the cathodyne would come in handy.
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Buy Chinese. Bury freedom.
Follow Ups:
It takes a bit of sorting to get 6J6's that match well enough( IMO ) for LTP use. There are Three types, and one is allegedly selected for section-section matching. 6099 I think is one, and 6101 is the other.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Thanks Douglas, I'll look for those. It's becoming evident from additional SPICE work and calculations that I'll be short of gain in this design (EF86 - 6C4 cathodyne - 5881 push-pull). Even with the EF86 operating at highest gain, 20 dB of feedback will create a slight shortfall.
The chassis is already cut for a 7-pin in the splitter position, so the choices are limited. The easiest option would be to incorporate positive feedback from the 6C4 back to the EF86, similar to the RCA 30W design. I believe this technique will approximately double the total driver gain. The other possibility is to replace the cathodyne with a 6J6 LTP like I mentioned. That would provide better than a X10 increase. SPICE is telling me the 6J6 is a cleaner approach, but I'm not convinced yet. It would be great if I could find an example of this somewhere. Thanks again for your suggestion!
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Buy Chinese. Bury freedom.
You could consider a 6BN4 in place of the 6C4. It's also a 7 pin and they have higher transconductance and gain while the other specs are similar to the 6C4.
nada aqui
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Buy Chinese. Bury freedom.
6AB4 (1/2 a 'T7) μ = 60 and 6GK5 μ =78.
Eli D.
Those appear to be significantly less linear than the 6BN4 - probably OK in a cathode follower or cathodyne splitter as long as the quiescent current is high enough.
The rating for "heater negative with respect to cathode" is a problem for the majority of possible subs in cathodyne service. With +400V B+, voltage on the cathode of the splitter will be around +100V DC in addition to 60V AC pk-pk signal. It's not clear to me yet whether I'll elevate the heaters in this driver. In any event, I don't see much reason at this point to look for a different tube type. I have a decent stock of 6C4, and this tube has proven it's capabilities in this service in many commercial amplifiers. I have bigger problems at hand right now, like aligning 3D dimensions in the various chassis pieces using 2D software. :)
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Buy Chinese. Bury freedom.
those are single elements, yes?
Go totally nuts; 6HV5A. mu of 300, gm of 65 mA/V...not a 7-pin though...LOL
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
JMO, use 6J6 specimens whose sections are reasonably well balanced and force symmetry between the sides with a tail CCS.
Given its μ of 38, approx. 20 dB. of stage gain should be realized from a 6J6 LTP.
Eli D.
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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Buy Chinese. Bury freedom.
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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