LTSpice model for a commercial toroidal output transformer

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Preliminary LTSpice results

If I run this transformer in UL mode with no cathode feedback, gain at 1W is 36.5db. Distortion at 80W is nearly 4%.

If I connect the CFB gain at 1W is reduced to 27dB. So, CFB is reducing gain by nearly 10dB.

Output power is drastically reduced. I can get 30W with 1.6% THD at which point the drive is providing 250V pp! 6550 dissipation is 30W.

Menno van der Veen was reporting 50W with this configuration albeit with his much more expensive transformer.

Looks like I need less CFB which would require a custom transformer.
 
My experience is with the VDV-2100-CFB/H
they have 33% UL taps and 10% Cathode Feeback taps.

For those they are definitely talking voltage ratio.

As the UL feedback is screen to cathode voltage then when you connect the cathode feedback in the output tube cathodes then the tubes actually operate at 33 + 10 = 43% Ultralinear Mode. That is the tubes operate at the "standard" 43% Ultralinear with the added benefit of cathode feedback.

Have you looked at the Norman Koren stuff. He had some simple spice models for this sort of tranny.
The Emperor's New Amplifier

Cheers,
Ian
 
Have you looked at the Norman Koren stuff. He had some simple spice models for this sort of tranny.
The Emperor's New Amplifier

Ian

Thanks Ian. I skimmed the article a few weeks ago but it's time I had a more thorough look.

The price of the VDV-4070-CFB is prohibitive but I'll create a model for that and give it a try.

Still looking for something a bit different to do with my 2 tubelab Universal Driver Boards.
 
Note that the VDV OPTs cathode feedback is voltage ratio.

Without seeing any circuit it's hard to say anything that can make sense.

Only thing is that for 80W into that Polish transformer with 24% CFb (voltage ratio) means that the driver should deliver the power tube bias + 127V peak voltage. So it's even more than 250V grid-to-grid.

Lower output power also results from increased primary load that becomes about 7K instead of 4K. If the amp is optimised for 4K will hardly be for 7K.

Regarding distortion I simply don't trust simulators unless they use the ACTUAL curves of the tubes in hand.

P.S.
If you want to run a comparison with 4K no CFb amplifier then when using CFb it would be better to use the 2K Polish transformer with 24% CFB which would result in 3.5K total primary impedance, IMHO.

Also 24% CFb + 33% additional UL = 57% standard UL so even less power....
 
Disabled Account
Joined 2013
When Lcfb is increased, Nfb is increased, the input drive is increased, but 30W is all you can get (with UL 33%), how do you get 80W from a pair of 6550 or EL34 or is it 2 pairs (PPP)?
1 pairs EL34 sim I got:

Lcfb 14.5 THD 0.38% In=120V-p out 22V-p
Lcfb 1.45 THD 0.8% In=60V-p out 22V-p
Lcfb 0 THD 1.8% in 36V-p out 22V-p
 
30W is all you can get (with UL 33%),
No, it becomes 57% UL (it's 33+24) when CFb is connected.

Without connecting UL tap it still is 24% UL unless the G2 is AC referred directly to the cathode to become real pentode.

I suggest a good reading of the N. Crowhurst 1956 article "Triode vs Pentodes: which?". I cannot attach it here because my .pdf version is too big but you can easily download it from the net.

Note how the circuit in Fig.16 where G2 has separate supply still is UL (the so called modified ultra-linear).

Note that for all UL and pentode PP connections the TOTAL primary load is the same at 6.6K for the 6L6 tubes while the turns are distributed in a different way from one circuit to another. You need to compare apples to apples as much as possible. :)
 
Thanks everyone for all the input.

@45

"Without seeing any circuit it's hard to say anything that can make sense." - I've attached the schematic with the Toroidy OPT. The 2 LTPs and MOSFET drivers are tubelab's Universal Driver Board.

"Lower output power also results from increased primary load that becomes about 7K instead of 4K. If the amp is optimised for 4K will hardly be for 7K."

  • Can you show me the logic and arithmetic behind that please? I assume the 9.5dB of cathode feedback is increasing the effective load resistance.

  • In my sim if I were to divide the RMS anode voltage by the RMS anode current should I get 7K? If I do that I get 1.43K.
"If you want to run a comparison with 4K no CFb amplifier then when using CFb it would be better to use the 2K Polish transformer with 24% CFB which would result in 3.5K total primary impedance, IMHO." - I'll give that a try.

Given the UDB I'm limited to 2 output tubes rather than 4 although I suppose I could replicate the MOSFET drivers off the PCB.

"I suggest a good reading of "Triode vs. Pentode - Witch - Norman H. Crowhurst (1956)"" - trying to find a source that doesn't require me to sign-up for some sort of trial.

Meanwhile, I've applied what I've learned here to modelling the Van der Veen 4070-CFB-PP transformer.
 

Attachments

  • UDB 6550 6CG7 Toroidy OPT with CFB.png
    UDB 6550 6CG7 Toroidy OPT with CFB.png
    61.7 KB · Views: 224
Meanwhile, I've applied what I've learned here to modelling the Van der Veen 4070-CFB-PP transformer.

When you simmed this transformer.....:

http://www.trafco.rs/download/4070-CFB-PP WEB.pdf

....I am afraid you still have to learn :confused:

For the specs of the 4070CFB-PP transformer:
L1 = 118.7 H
L2 = 28.8 H
L3 = 28.8 H
L4 = 118.7 H
L5 = 1.32 H
L6 = 1.33 H
L7 = 1.33 H
 
I use almost 500V for the plate -60V bias as your last attachment.

I'm guessing the difference between our results is your perfect signal sources.

I don't have pentode3.asy. I replaced your symbols with mine and adjusted the links but I must be missing something. -220.76KV on the grids. Where did that come from? (Rhetorical question - I'll work it out!)



6550a pp-2 cfb.png
 
Last edited:
Disabled Account
Joined 2013
Pin orders in asy must be the same as model, this Pentode3.asy or use Pentode.asy and change pin order in model, either way will be ok.

Code:
Version 4
SymbolType CELL
LINE Normal 48 -32 48 0
LINE Normal -48 -32 -48 0
LINE Normal 0 -80 0 -48
LINE Normal 20 -48 -20 -48
LINE Normal 20 -44 -20 -44
LINE Normal 20 -48 20 -44
LINE Normal -20 -48 -20 -44
LINE Normal -20 -32 -12 -32
LINE Normal -4 -32 4 -32
LINE Normal 12 -32 20 -32
LINE Normal 27 -32 39 -32
LINE Normal 48 -16 28 -16
LINE Normal -20 -16 -12 -16
LINE Normal -4 -16 4 -16
LINE Normal 12 -16 20 -16
LINE Normal -48 0 -28 0
LINE Normal 20 0 12 0
LINE Normal 4 0 -4 0
LINE Normal -12 0 -20 0
LINE Normal 24 12 -24 12
LINE Normal 32 48 32 20
LINE Normal 24 12 32 20
LINE Normal -24 12 -32 20
LINE Normal 28 16 -28 16
LINE Normal 39 15 39 -32
LINE Normal 32 20 39 15
ARC Normal 48 -80 -48 16 48 -32 -48 -32
ARC Normal 48 -48 -48 48 -48 0 48 0
WINDOW 0 -16 -96 Right 2
WINDOW 3 15 64 Right 2
SYMATTR Value Pentode
SYMATTR Prefix X
SYMATTR Description symbol for a subcircuit macromodel , P G1 C G2
PIN 0 -80 NONE 0
PINATTR PinName Anode
PINATTR SpiceOrder 1
PIN -48 0 NONE 0
PINATTR PinName G1
PINATTR SpiceOrder 3
PIN 32 48 NONE 0
PINATTR PinName Cathode
PINATTR SpiceOrder 4
PIN 48 -16 NONE 0
PINATTR PinName G2
PINATTR SpiceOrder 2
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.