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Eimac 35T OTA experiment

I am experimenting with a 35T OTA (spud) amp. It's up and running and producing nice, clear music (subjective). I only need about 10-20W for my classical chamber music. So far so good. But, I am concerned the design is not optimized. Here is my concern and your input would be great to confirm/deny my thoughts:

1. My existing OPT is 5K:8ohm which will be replaced with a 14K:8ohm OPT. Rp ~ 8.5K. My theory is that this should help both the bottom end damping and top end extension.

2. Currently, I am using a 1:4 step-up transformer. I am thinking of going 1:8 (as shown in the schematic) to give my voltage drive. But what about current drive? I read somewhere, with little detail, that the 35T needs "current" to drive the tube. Is this only if A2 is being used? I am currently using only A1, from what I can tell (see curves with my operating points).

3. Operating point is 875V, -9V bias, 35mA. Thoughts on this for A1?

4. The 35T is not glowing red / orange as indicated by the user manual. But, I am getting nice music. Is glowing red / orange required and if so, what changes to the operating point should be considered?

I feel as thought this could be a very nice amp with some adjustments. Right now is sounds pretty darn good, but I suspect it could be better. As always, your input is appreciated.
 

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1. My existing OPT is 5K:8ohm which will be replaced with a 14K:8ohm OPT. Rp ~ 8.5K. My theory is that this should help both the bottom end damping and top end extension.

2. Currently, I am using a 1:4 step-up transformer. I am thinking of going 1:8 (as shown in the schematic) to give my voltage drive. But what about current drive? I read somewhere, with little detail, that the 35T needs "current" to drive the tube. Is this only if A2 is being used? I am currently using only A1, from what I can tell (see curves with my operating points).
1.) 14k versus 5k usually has more HF falling.
2.) 1:8 versus 1:4 usually has narrowed bandwidth (in both sides).

Most of transmitter tubes requiring "grid current" driving. Spud SUT unable to do so.
If you want more power, active stage needed.

3.) A1 versus A2.
Your operating point in A1 region (Ug<0).
If you draw loadline (5k or even 14k) in this op., until the line below 0V Ug, the tube working in A1.
The Ua swing would be few hundred Volt p-p, the Ia swing about few ten mA p-p, so estimated power not too much.

Unable to work (in this design) in A2 (Ug positive), because it's needed source for grid current.

"4. The 35T is not glowing red / orange as indicated by the user manual."
It's a problem. These types requires to activate getter to absorb gas inside the bulb. If it missed, the tube not working perfectly, would breaks down quickly.
 
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OldHector - the tube is negatively biased via "filament bias" using a 2ohm resistor.

Bela - Thanks for the great response. Just a few follow-up items:

1. You said "more FH Falling". Are you saying that the 14K OPT will have LESS high frequency extension?
2. OK, understood
3. OK, understood
4. hmmmm, I need to figure this out.

Right now, the amps sound really good. Really good. But, I am concerned about the lack of red / orange getter. The Eimac data sheet says that my operating points are good (possibly a change to -7 or -8 bias instead of existing -9V. Anyone with ideas on how to rectify the lack of red / orange getter glow?

Side note: I have 3 different types of 35T tubes: Eimac, Jan (Eimac make) with steel base and Heintz & Kaufmann. Anyone done any tube rolling with 35T tubes? Recommendations?
 
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1. You said "more FH Falling". Are you saying that the 14K OPT will have LESS high frequency extension?
Yes ... mainly.
High impedance OPT has much more turns, mostly thicker insulations between P-S layers (due to the higher voltages), better insulated wires ... etc.
Good 5k transformers achieves even 100kHz, 14-16k ones rarely exceeds 30..40kHz.

The OPT experts (for example 50AE) could say more about that.

p.s. Are you read Ale's blog?

https://www.bartola.co.uk/valves/?s=4-65a

4-65a also transmitter tube, so the basic behaviour of getter activating is similar.
 
Good evening Pat,

i've got hundreds of the 35T varieties (35T, 35TG, DET18 (from OSRAM), E.556 (from SFR), HK54, etc etc) to be used in my amplifier. See pictures: the 2 tube amplifiers on the right which powers the woofers in the full-fieldcoil-speakers. These tubes are BRIGHT even in daylight otherwise you're simply choking your precious tubes because the getter is not activated.

I've got 35T-varieties by the hundreds (really !) including all strange versions like the UH35, dET18 and the SFR e556.

Please stick to proper Eimac versions. The older (mostly with fins and bright anode [tantalum]) are nearly like the "newer" with zirconium-getter (no fins but a clearly visible application to the anode). To be avoided at all cost: Lewis-garbage......i don't know who accepted this sh&% then but this is proper cheapskate-stuff. Don't put money at Lewis-stuff; waste of money.

35T (and 35TG which is the same but the grid is externaly connected through the glass and not through the tube-base as this was better for HF-performance) first appeared as a simple cilinder (mu = 30).
Later a cilinder-cover was added in order to have a better gain (mu = 39) but the anode heat couldn't escape so fins ware attached.
Later-later the tantalum (expensive !) anode construction was replaced by a molybdeen version WITH zirconium-cladding (for gettering-purposed).

The first 35T(G) versions have a ceramic base. Later the cheaper metal-base was introduced.

Included some pictures from my stash. The current audio-set, the 35TG-amps (including bright shinging 35TG), a part of my 35T/G -collection and some exotics.
Please please please avoid Lewis-stuff. waist of time. Stick to Eimac (in ANY version) as this is the real deal. The older the better. Lewis is the B-version and simply not trustworthy.
Exotics like the SFR 3.556 and the Osram DET18 are exotic by nature. IF you have them: enjoy them. If not: be perfectrly happy with proper Eimac-stuff as this is made to last a lifetime (my 35TG's are running now for 20 years !)

Regards,
Reinout
 

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Yes ... mainly.
High impedance OPT has much more turns, mostly thicker insulations between P-S layers (due to the higher voltages), better insulated wires ... etc.
Good 5k transformers achieves even 100kHz, 14-16k ones rarely exceeds 30..40kHz.
One of the main culprit causing OPT mediocrity in transformers for high Rp tubes is the customer desire to use a higher Idc for triode stages in order to use the tube within a more linear operating point. With high Rp tubes, this dictates more DC flux density sacrificed to the thrash bin.
Although it is also problematic for low Rp tubes, but to a lower extent.
In an ideal world, if the Idc quiescent point would be the equivalent of the RMS current just enough equal for maximum power output, SE transformer would be more easier to build. Ironically, pentode CFB trannies are simpler to design, due to the feedback linearizing the right squeezed part of the curves, thus rendering the purpose of high quiescent current less meaningful.
High Rp transformers require less capacitance to leakage inductance ratio and vice-versa. Not necessarily hard to build, but one needs to master capacitance distribution and calculation well. Higher leakage transformers requiring more turns are prone to excite the dip resonance in a worse way.
 
Hi Marek,

on the venerable Frank's Tube database you will find 3 versions of the 35T datasheet (https://frank.pocnet.net/sheets/088/3/35T.pdf ), 3 versions of the 35TG (which is in essence the same except for the grid-location ) (https://frank.pocnet.net/sheets/140/3/35TG.pdf ) and if you don't like these charts you can always use the HK54 sheets which use more recognizable datasheets (https://frank.pocnet.net/sheets/114/h/HK54.pdf ). Remember that the HK54 uses 5V/5A at the filament instead of the 35T/TG's 5V/4A; the rest is the same.

Great tubes; enjoy !

Regards,
Reinout
(pictures added of my set and the 35TG-amps used for the lower register)
 

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