"Killer" Screen Driven Push-Pull Amp Initial Queries

[tubelab.com]

I've been collecting 35LR6s.....If anyone has had experience with the Edcor 5k, 100W push-pull transformers,

I have no experience with either of these exact items, but I offer this observation. I have been collecting 6, 26 and 36LW6's for years. They are a similar large sweep tube. They are found with several different plate constructions from big to immense. The wimpiest versions can dissipate 50 watts without glow, and the big guys smile at 90 watts. I have cranked these up to the 150 watts per pair level in both conventional G1 drive and screen drive. I find that these tubes work best with a much lower load impedance than 5K ohms. Most of my experiments were done using a 6600 ohm OPT with the 8 ohm load on the 16 ohm tap for a 3.3 K load. I have also used a 1250 ohm BIG 400 watt Plitron toroid OPT with good results. SY uses a 1250 ohm load in his 6LF6 screen drive amp. Both of these tubes are a bit bigger than the 6LR6.

I got 8 6LR6's off of Ebay for cheap a while back. Two are obviously toast (noted in the auction) so these will get "tested". The good ones will see some P-P amp testing as time permits.

[kenpeter]

Originally posted by wrenchone
I've bought from Edcor before - I wanted specific poop on the 5k, 100W transformer.

I own a pair. Recall measuring primary inductance at 20H per plate! Far too many Henries for my LCR meter plate to plate...
I'm guessing thats 80H P-P whilst both plates are in ClassA?
But only 20H for one plate in Class B. Or is that not the math
is done???

I forget now what I had measured for secondaries...
I can check that for you this weekend.

They are pretty and blue. But white Edcor logo is on a clear
peel-off sticker that tends to have ugly air bubbles under it,
so you will most likely wanna tear that sticker right off.

The mounting tabs on the bell housing don't quite seem as
substantial as a transformer of this mass deserves... Maybe
the bellhousing was stamped from too thin a sheet?

Edcor leads and insulation are substantial, much better than
the crummy leads on my Hammond choke of similar size. But
Hammond's ugly black bell seems to have the stronger tabs.

[smoking-amp]

A typical LCR meter uses a small test signal (like 1V or so) that does not get the magnetic field up enough to see large signal inductance. M6 (or most xfmr steels) have low Mu at tiny field strength, then improve considerably up around 1/2 Bmax. The usual inductance figures given for xfmrs are measured with large excitation (like from a Variac). But the initial Mu measured by the L meter is an important parameter for small signal performance. 70% Nickel alloys do much better at the low excitation level than the M6.

Don

[wrenchone]

If I wanted to do a "full George" and beat the living snot out of the tubes, I could use a lower impedance transformer than 5k P-P. However, I was keeping in mind the relatively high plate resistance of most screen driven sweep tubes and catering to that with a reasonable value of transformer impedance so as not to be so terribly dependent on feedback for low output impedance - correct me if this line of thought is faulty. A clean and honest 100W RMS would be more than enough for me - I'm 55, but I still seem to have relatively intact ears...

The 35LR6s were a very nice chance buy during the feeding frenzy at AES a while back - I couldn't resist hefty NOS sweep tubes at $5 each. AES has since recovered their senses, and the 35LR6s are now over 3X that price, though still a bargain compared to a lot of the big sweep tubes.

So far, I am seeing two types of 35LR6s - the GE type with a big plate with two simple side fins for heat dissipation, and the Sylvania type that has an equally big plate and a complex fin arrangement intended to spread out the radiating area some more. If I get around to it, I'll post a picture of the two types side by side. I can't recall if I also have an RCA version for comparison. The alt brand stuff I have so far seems to be relabeled GE or Sylvania.

[smoking-amp]

What tube are you planning to use for the HV driver?

Don

[wrenchone]

Well, I was planning to try a pair of matched JFETs cascoded by an 8CG7 as a differential gain stage/splitter, with another 8CG7 as voltage amp after that if necessary. If you look at the schematic for the old Berning sweep tube amp, they split up the gain stages in a similar fashion, though without the sand. I'll use source followers to drive the screens on the finals. I'm trying the JFET cascode first driving a pair of Russian 7189 clones as part of rebuilding an old Lafayette integrated amp to test the concept and see how much gain/drive voltage I get. Slow and easy does it, especially as I seem to be getting sidetracked into a gainclone project for a short while (curiosity killed the cat).

[wrenchone]

Late breaking stuff - I get a bunch of used nad NOS 35LR6s a couple of days back. I have an RCA in the bunch, and it looks exactly like the Sylvanias in terms of plate structure and marking, so we may only have two types - GE and Sylvania.

[wrenchone]

Some more back of the envelope stuff regarding the gain stages - in another project using PN4393 JFETS running at 2.5ma under a 12AU7, 250V B+, I'm getting a gain of about 125X. Taking this gain value as an attainable example, and assuming 80V drive will get me to the stated goal of 400ma peak plate current, and tracking through an output transformer with a 12.5:1 turns ratio on each side (5k), I get an open loop gain of 62.5. I would settle for a closed loop gain of 30-35X or so, so I will need some more open loop gain to lower the output impedance via feedback. I think I can get some more gain out of the input stage using the 8CG7 on top of the FETs by upping the drain current (more transconductance). I may look at using J310s or 2SK170s to pump up the gain. The higher available plate voltage will also allow me to use higher value load resistors. This may allow me to get all the gain I need (along with the phase splitter) in the first cascoded differential stage. If not, I have the option of using a modest common cathode stage like the Berning amp to eke out some more gain. I'd like to avoid this if I can.

One question - will the 8CG7s put up with 500V on their plates while they're warming up, or will I need to drop the B+ feeding them?

[smoking-amp]

Take a look at 6GF5, 700 V DC rating, 9 Watt, $1. Doesn't have the high Gm of the Fets though, so you will need to cascode still. Just run them with a fixed screen voltage. Could run local feedbacks to the g1's (Ummm... that's only working if there is a triode like device underneath though).

[wrenchone]

Actually, I was thinking of some other equally humble options. The 8CM7 and 8CS7 dissimilar double triodes both can withstand 500V on their plates, and triode 2 of either tube will allow sufficient voltage across the bottom JFETs at 5ma or so if used as a simple cascode with grid pinned to the JFET source. This leaves triode 1, which can be left idle or used for a second gain stage if necessary.

I would have to knock the plate supply down a bit from the main B+ to stay within the ratings, but I could tap the main output winding or just use a modest regulator, assuring a really clean supply for the input stage. I'd like to stay with the JFET front end, as it is then really easy to match devices for a differential stage, and you get a lot of gain with just one stage. Besides, I haven't seen anyone else trying it, and I like eccentric solutions...