Request for input and feedback on a circuit idea

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RhythMick
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#1 Request for input and feedback on a circuit idea

Post by RhythMick »

Having recently built SE amps for myself and my brother (5842/IT/45) which I'm pretty pleased with I'm next thinking about an all-DHT amp and quite likely Class A push-pull. I don't have a lot of experience, and NONE at all with push-pull, though I have built fully balanced differential amps and pre amps with balanced power supplies. Not a COMPLETE beginner, but green enough to need regular input from folks with a lot more experience.

Anyway, thinking about Class A push-pull. I love the balanced topology and a LTP with a CCS in the tail (another new area for me) feels like a good way to go. Lots of reading later (Morgan Jones, Merlin Blencowe and lots more) and I'm getting ready to get started. One question I have in my head is how to balance up the valves in the pair if they are not quite balanced, as is likely with some of the older valves. We want identical DC currents through the valves, right, to avoid output transformer core saturation ?

The latest SE amps I built were fixed bias, applied through the interstage transfomer secondary and I made trimmers accessible with safety measuring posts so we can keep the amps tuned to run 30mA through the valves, by adjusting the grid bias. That got me thinking that I could maybe use the same mechanism with the 1+1:1+1 interstages for the PP amp. I've crudely drawn it up and would appreciate comments - even if they are "don't be stupid" ... :|
Push Pull Fixed Bias.jpg
So the CCS in the tail will try to keep 60mA flowing while the + and - phases are applied to the grids. Tuning the valves would be a matter of adjusting the trimmers and therefore the grid bias of each valve while measuring the voltage drop across 10R sense resisters in the 45 cathodes (not on the diagram, sorry), to achieve 30mA through each valve.

Plenty missing from the diagram I know - the trimmers would be surrounded by fixed resistors to limit the adjustment range for example, plus caps to ground from the wipers.

Would this work in principle ?
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#2 Re: Request for input and feedback on a circuit idea

Post by Ant »

You can use interstages, they need to be specced so that they will take enough current not to saturate so you dont lose bass
I did a 45 class a pp, a 2a3 pp, and a 6s4s pp
My favourite way to get the phase split is to use an input transformer such as a sowter 3575 and run essentially 4 se channels. You could sit the power valves on a ccs each and tune the current sense resistor until each power stage valve pulls the same current
Loads of ways to do it
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RhythMick
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#3 Re: Request for input and feedback on a circuit idea

Post by RhythMick »

Thanks I should have added my input is already balanced.

I thought of CCS per output valve but I like the idea of the seesaw action of the LTP.

Anyone see issues with driving separate grid bias to reach valve via the interstage secondary in the way I've drawn it? The bias circuits each will draw about 2mA but the voltage tap to the grids should be very high impedance, so I wouldn't expect to see any DC through there?
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IslandPink
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#4 Re: Request for input and feedback on a circuit idea

Post by IslandPink »

What you have there looks very like the Amity and Raven pre-amp as designed by Lynn Olson and Gary Dahl.
http://www.nutshellhifi.com/triode1.html
Raven-MarkII.gif
There used to be a page on the Raven but for some reason it's missing.
So - you should be able to get something like that, as you've drawn working and sounding nice.
The Lundahl transformer used there -
http://www.lundahl.se/wp-content/upload ... s/1680.pdf
is designed for up to 5mA unbalanced current in the primary.
With that, and a choice of good twin-triodes like NOS 5687's , you should be fine.
If your outputs are not as tolerant of DC , you could test & match 5687's.
ps. I wouldn't use the LL1676 on the input - the 3575 Sowter is better. Also the ECC99's sound a bit hard and glassy.

Edit : Sorry you're looking at a power amp so scratch the bits about 5687 and 1680. See if there's anything in the 'Amity ' page that helps.
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Nick
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#5 Re: Request for input and feedback on a circuit idea

Post by Nick »

I would first decide on the circuit and transformers, then if needed you can use a balance pot to set the bias similar to what you have done. Depends on the transformer as to what DC it can stand. Most output transformers can take some.
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#6 Re: Request for input and feedback on a circuit idea

Post by IslandPink »

So, couple of questions -
what do you have in mind for these 30ma output valves ?
Have you got any output transformers in mind ?
I ran a PP 300B amp as per the Lynn Olson 'Aurora' for a while and it was very transparent. The outputs were various fancy 300B as 'matched' pairs, but the matching doesn't need to be perfect as the outputs can take a little bit of DC. I had LL1620's on there, then later O-Netics 6k PP's .
So the gain of the output valves in important because that dictates how big the input transformer will be. The 3575 for instance will handle a few volts but not eg. 50 to 100V .

edit - as Nick says ....
"Once you find out ... the Circumstances ; then you can go out"
RhythMick
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#7 Re: Request for input and feedback on a circuit idea

Post by RhythMick »

Lovely thanks for the feedback and inputs.

I'd looked a long time at the Raven and Karna amps and a lot of what I'm aiming at comes from there.

I don't need a lot of gain. Our Lowther DX4s are plenty loud enough driven by 5842-IT-45. I use an autoformer for passive attenuation while my brother uses a balanced pre amp (12AU7 based), so I've put a step down transformer on his input to increase the input impedance and let his gain stages breathe a bit. I'm motivated to do this because a) I like to learn and I'd like to build a push pull amp, b) I'm intrigued by the idea of carrying the balanced signal right through to the OPTs.

I'm aiming for an ALL-DHT power amp. Most likely 26s into 71As into 45s. I have the LL1620-PP ready to go for OPT and the LL1621-PP for interstage duties. I shouldn't need an input transformer as the XLR input will go direct to the grids of the 26. DHT long tailed pairs are I think going to need CCS in the tail to make a solid fulcrum for the seesaw (or whichever metaphor you prefer...). I'll probably go with Rod Coleman regulators for the heaters, at least the 26.

I've never used CCS before so I'll be back for advice I'm sure, but nice to know I'm at least starting out on vaguely the right tracks. Thanks all.
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#8 Re: Request for input and feedback on a circuit idea

Post by Ant »

Iirc, i think that tx complement is the same as Mr i's pp 45 amp
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#9 Re: Request for input and feedback on a circuit idea

Post by IslandPink »

All sounds very good.
RhythMick wrote: Sat Mar 03, 2018 12:40 pm I'll probably go with Rod Coleman regulators for the heaters, at least the 26.
Have a read of the first few pages here, because the Coleman ones are not great in the treble, we did better with this project :
http://www.audio-talk.co.uk/phpBB3/view ... t=filament
The Coleman ones were used as 'reference' for bass and midrange.

I'm sure you'll be OK to use AC on the 45's as per an 'Amity' , the 300B PP I built was very quiet.
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pre65
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#10 Re: Request for input and feedback on a circuit idea

Post by pre65 »

IslandPink wrote: Sat Mar 03, 2018 1:05 pm
I'm sure you'll be OK to use AC on the 45's as per an 'Amity' , the 300B PP I built was very quiet.
I have some brand new (unopened box) Hammond 266M5 transformers if you are going 2.5v AC.

PM me if interested.
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RhythMick
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#11 Re: Request for input and feedback on a circuit idea

Post by RhythMick »

Thanks Il but I just bought some 266M6 and also 266GB6 for the 5842. However I may not need them - I just discovered that listening for the Humbucker sweet spot didn't work as well as measuring it across the speaker terminals (at least for me). I've gone from 10mV ripple down to < 1mV and can barely hear it. Much improved. Might still try them later.

That linked thread is 57 pages, but I will read it later thanks. I'm definitely in for at least 4. Given that I'm pairing DHT in LTP what's the recommended way of driving the heaters?

- separate regulators for each valve
- single regulator, series (2x V)
- single regulator, parallel (2x A)
- something else...
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Nick
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#12 Re: Request for input and feedback on a circuit idea

Post by Nick »

I would use one heater supply per valve. You are making what is going to be a costly amp with all the iron, I would not bother skimping on the heater supply’s. I would also try with and without the CCS in the tails, I think a simple resistor will do a good job, the fact that you have a balanced input and a transformer to couple the anodes at AC will mean that you will be getting very little AC imbalance anyway.
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#13 Re: Request for input and feedback on a circuit idea

Post by Cressy Snr »

pre65 wrote: Sat Mar 03, 2018 1:33 pm
I have some brand new (unopened box) Hammond 266M5 transformers if you are going 2.5v AC.

PM me if interested.
I'll have 'em if Mick doesn't want 'em Phil.
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#14 Re: Request for input and feedback on a circuit idea

Post by pre65 »

Cressy Snr wrote: Sat Mar 03, 2018 2:27 pm
pre65 wrote: Sat Mar 03, 2018 1:33 pm
I have some brand new (unopened box) Hammond 266M5 transformers if you are going 2.5v AC.

PM me if interested.
I'll have 'em if Mick doesn't want 'em Phil.
I'll PM you later Steve.
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#15 Re: Request for input and feedback on a circuit idea

Post by IslandPink »

RhythMick wrote: Sat Mar 03, 2018 1:44 pmThat linked thread is 57 pages, but I will read it later thanks. I'm definitely in for at least 4.
I did say 'Have a read of the first few pages here' , but then given how closely I read your first posting before replying, I shouldn't talk :!:
The first 10 pages has all of the meat of what we found and how the circuit ended up the way it is.

Why couldn't you use one heater supply per pair if the valves share a cathode connection , like the Amity/Aurora, Nick ?
The 300B's on mine were driven by one AC winding.
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