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Dynaco VTA-inspired KT88 PP AMP project
Dynaco VTA-inspired KT88 PP AMP project
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Old 5th February 2019, 11:30 AM   #1
AnatolyAA is offline AnatolyAA  Russian Federation
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Default Dynaco VTA-inspired KT88 PP AMP project

Dear Friends,

Inspired by the discussions around the possible schematics to use my father came up with the idea to combine the driver stage from the VTA ST70 AMP using 6SN7 tubes with the output of Mark III based on KT88 for more power. The draft of the schematics is attached.

Power transformed will be of toroid type giving 250 Wt output. B+ voltage before the choke will be 504 V.
I have some questions regarding the schematics:
1. Is this in general fine or have I missed something?
2. I want a OT with wide frequency response. The original schematics shows a 4.3K transformer. I found the LL1679-PP 9k7, 4k5, 2k6, giving 45, 105, 188 Watt transformer. Will it suite the needs? Or can you recommend something else?
3. There will be 2 chokes (1 per channel power supply). The Mk3 has this choke: 1.75 Henries, 200ma. DC current, 62 ohms, 400 vdc (max) 10% Tolerance. are these parameters enough? I saw in other schematics that chokes of 4-8 Henries are used.
4. Various schematics advice different capacity of C13 and C15 - from 22 to 47 uF. The more the better?
5. Did I correctly draw up the NFB? It is taken from the VTA 70 schematics.
6. Any suggestions on the capacitor types? I can take made in China or some European brands?
7. Is there any software to imitate and check how would this project actually work and whether some changes are needed?

Thank you in beforehand for your help
Attached Images
File Type: jpg Dynaco style 6SN7 KT88 PP with CCS.jpg (247.5 KB, 648 views)

Last edited by AnatolyAA; 5th February 2019 at 11:39 AM.
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Old 6th February 2019, 12:43 AM   #2
Alllensoncanon is offline Alllensoncanon  United States
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Dynaco VTA-inspired KT88 PP AMP project
Your common 11.2R output tube cathode resistor seems odd. I would thinks you want an even resistor value=(ie: 10R) for easy calculation. Also if you separated it into two 10R (or 1R), you can read bias for each tube.

Unless you want to match the tubes yourself, adding an AC balance resistor may be an good idea (add an trim pot in series with R27.).

I would use 100uf+ for C13 and C15.

GNFB seems fine to me.
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Old 6th February 2019, 01:20 AM   #3
rayma is offline rayma  United States
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Quote:
Originally Posted by Alllensoncanon View Post
Your common 11.2R output tube cathode resistor seems odd.
Chosen by Dyna for 1.56VDC when biased, so a fresh battery can be used for comparison, on a lousy meter. http://www.curcioaudio.com/mk3_mnl.pdf

Last edited by rayma; 6th February 2019 at 01:22 AM.
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Old 6th February 2019, 02:38 AM   #4
Francois G is offline Francois G  United States
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With a 6sn7 voltage amp I expect that you will not swing enough voltage to drive your power tubes sufficiently. Consider a 6SL7 or 12ax7 in stead.

For reference look at diytube.com Poseidon and Eiclone designs. The Eiclone uses a 12ax7 voltage amp and 6sn7 phase splitter in a classic design with modern implementation. See: http://www.diytube.com/ike/IKEREVB.pdf
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Old 6th February 2019, 02:38 AM   #5
Alllensoncanon is offline Alllensoncanon  United States
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Dynaco VTA-inspired KT88 PP AMP project
You mean one of these
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File Type: jpg tempthumbnail.jpg (28.8 KB, 612 views)
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Old 6th February 2019, 05:25 AM   #6
kward is offline kward  United States
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Considering that the output stage will need approx -45V bias, then in rough terms, driver voltage swing per side will need to be 45V/1.4 = 32V in order to drive the output stage to full power. With a 4.3K primary, I expect full power output at the speaker terminals will be around 55 to 60 watts, if using a stout 490V B+.

The phase inverter/driver stage will have gain I expect of somewhere between 6 and 7 (let's say 6.5) per side (meaning between each inverter output and ground). Voltage amp will have gain before feedback of say 12. So if you apply say 14 dB feedback (which is a voltage gain reduction in first stage of about 5x), then input sensitivity of the amp will be approximately 32V/(6.5 * 12/5) = 1.64V. Which is just about right, IMO.

Therefore a 6SN7 voltage amp will offer plenty of gain IF you keep feedback to a reasonable amount (around 14 to 15 dB).

Negative feedback topology looks correct. Final FB resistor size is easiest determined on test. Final FB capacitor size is output transformer dependent, and must be determined on test.

===

Things I'd add or consider changing:
  • 100Ω resistor between each screen pin and its respective UL tap lead.
  • Consider direct coupling the voltage gain stage to the inverter. That will remove the time constant (high pass filter) of that R/C coupling between the gain and inverter stages, and make it easier to apply feedback while keeping the amp stable. However DC coupling those stages will require raising the cathode voltage on the inverter stage to something like 100V to 120V, so the biasing circuit/cathode circuit of the inverter will need to change to conform.
  • Add current sense resistors in the cathode circuits of each output tube so you can test for DC balance of each tube independently.
  • Add a way to bias/balance each output tube separately (maybe already accounted for, we don't know for sure since you haven't shown the bias circuit). It can take the form of individual bias adjustments for each tube as you've eluded to, or it can be a bias/balance scheme for the pair of tubes.
  • By the book, the 150K grid return resistor on the output stage is too large for fixed bias. I'd follow the spec sheet max value listed: 100K. But if you want to be able to sub in a 6550 or KT120, you should make it no larger than 51K.
  • Going with a 51K grid return resistor on the output stage solves another lurking problem: If you keep the R/C coupling between first and second stage, the time constants between the two stages' R/C filters are too close together. If you move them farther apart you will have an easier time obtaining good LF stability when feedback is applied. Using a 51K grid return resistor with a 0.33 uF coupling cap as shown will give you a ~9.5 Hz 'cutoff' frequency, which will separate it from the cutoff frequency of the first stage R/C filter by about 2.7 octaves, which is much better separation than currently shown on your schematic, which is only one octave.
  • You may need some additional HF compensation after feedback is applied. Look for example at the Dynaco Mk. III schematic at the small feedback cap from one of the UL primary taps back to the junction in the cathode circuit of the first stage. You may also need/want to add an HF step circuit. These tuning circuits and values chosen are best determined on test. HF tuning in a feedback amp that includes the output transformer in the loop is output transformer specific.

Last edited by kward; 6th February 2019 at 05:33 AM.
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Old 6th February 2019, 10:48 AM   #7
John Ess is offline John Ess
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Default I'm looking at a similar project

But using a little less B+...probably end up between 450-460vdc using Edcor for power and LL1682 OPT...5.5K/5. Will also be dual mono, so I've got pairs of both. The first 6SN7 duties will be split into 2 and handled by a pair of 6J5's.The schematic shown is the one given with VTA's 6SN7 replacement board for the ST70, and should be able to accommodate several different output tubes, given proper B+. The pic I'm posting should help with bias scheme.

Screen-2019-02-06_06-43-05.jpg
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Old 6th February 2019, 10:59 AM   #8
AnatolyAA is offline AnatolyAA  Russian Federation
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kward thank you for this vast input, which I will definitely take into consideration. Would you recommend the LL1679-PP 4,5k OT for this schematics?

John Ess, thank you.
Yes, that is one of the versions for the bias scheme I have available, which also comes with one of the VTA boars versions for the ST 70. I hope and as kward calculated that it will be able to work with KT88. The LL1682 does not seem to fir for me as I am targeting 60 Wt per channel.

Francois G, ok, I will once again recheck the schematics to be sure that everything fits.

Thank you all for your input!
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Old 7th February 2019, 01:45 AM   #9
kward is offline kward  United States
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I've not ever used Lundahl transformers so I have no opinion of them. I have used Dynaco clone transformers for example from Dynakit Parts. These are excellent.
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Old 7th February 2019, 01:50 AM   #10
rayma is offline rayma  United States
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Quote:
Originally Posted by Alllensoncanon View Post
You mean one of these
Exactly.
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