Request for input and feedback on a circuit idea

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andrew Ivimey
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#16 Re: Request for input and feedback on a circuit idea

Post by andrew Ivimey »

Ditto my Amity (2a3s) no probs at all.
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#17 Re: Request for input and feedback on a circuit idea

Post by RhythMick »

Nick wrote: Sat Mar 03, 2018 2:08 pm 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.
Interesting thanks Nick. I'd considered building out +ve & - ve rails to give a bigger cathode resistor but with the 45 it would be only 4k ish (250v drop 60mA). Given the low mu I'd assumed that would be a bit weedy but I can give it a try.

Assuming I then go with CCS, anyone fancy helping me with CCS designs for 12mA, 36mA & 60mA? I have zero experience of SS designs and the sheer number of possible components seems impossible to contemplate to me.
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#18 Re: Request for input and feedback on a circuit idea

Post by Nick »

You could use one supply. If we believe that the heater supply matters, then that implies the voltage and current in the heater affects the sound then I would worry joining both ends with a high impedance supply may do something. I said what I would do, someone else may do something different.
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#19 Re: Request for input and feedback on a circuit idea

Post by RhythMick »

IslandPink wrote: Sat Mar 03, 2018 12:08 pm 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.
Having read that DC heater thread I know know you as Mark, hope you don't mind my using that.

Looking again at that Raven circuit, the valves are cathode biased, 36mA through a shared 87R.

I'm looking at applying fixed bias to each valve independently through the secondaries of the interstage. So the secondaries will not be directly joined in the middle in that design, but each will be fed by its own bias voltage trimmer. The coils would be joined indirectly and I wanted to make sure nobody saw a real problem with that.
20180304_083514.jpg
20180304_083514.jpg (7.84 KiB) Viewed 7046 times
Also having not used CCS before I don't know enough about exactly how they control the current and in particular what will the voltage be at the top of the CCS? I guess I'm going to wind up with a mixture of cathode and fixed bias and I need to get to grips with how to design a CCS. Any good articles recommended?
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#20 Re: Request for input and feedback on a circuit idea

Post by Nick »

Thinking a bit more about this. I would (if I was worried, and the transformers needed it) balance the two valves with a 47R pot between the cathodes. You could take the CCS to a -ve point (or the shared resistor) of you could stand the grids up a bit, either would work, =ve grids would probably be simpler

CCS-Triodes.jpg
.
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#21 Re: Request for input and feedback on a circuit idea

Post by RhythMick »

Nick wrote: Sat Mar 03, 2018 11:55 pm You could use one supply. If we believe that the heater supply matters, then that implies the voltage and current in the heater affects the sound then I would worry joining both ends with a high impedance supply may do something. I said what I would do, someone else may do something different.
My question wasn't prompted by cost savings. Given everything I put into my amps that would be daft.

I was more thinking that that any residual hum could be applied the each amp identically and in phase, therefore cancelling in the OPT.

I was also thinking that if the valve heaters were applied in series then the failure of one heater would cause the other to turn off as well, avoiding large DC going through the OPT. Probably the last failure scenario to worry about I guess.

Does this change the advice? Would you still use separate supplies?
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#22 Re: Request for input and feedback on a circuit idea

Post by pre65 »

Some many years ago I did a KT66 PP amp.

I did a CCS on each KT66 (using LM317HV ) and joined them as per the third diagram from the bottom on this link.

http://www.tubecad.com/2011/08/blog0210.htm
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#23 Re: Request for input and feedback on a circuit idea

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RhythMick wrote: Sun Mar 04, 2018 11:03 am
Nick wrote: Sat Mar 03, 2018 11:55 pm You could use one supply. If we believe that the heater supply matters, then that implies the voltage and current in the heater affects the sound then I would worry joining both ends with a high impedance supply may do something. I said what I would do, someone else may do something different.
My question wasn't prompted by cost savings. Given everything I put into my amps that would be daft.

I was more thinking that that any residual hum could be applied the each amp identically and in phase, therefore cancelling in the OPT.

I was also thinking that if the valve heaters were applied in series then the failure of one heater would cause the other to turn off as well, avoiding large DC going through the OPT. Probably the last failure scenario to worry about I guess.

Does this change the advice? Would you still use separate supplies?
Yep, my reply wasn't directed at you in this case. I personally dont like to use cancellation, just because the hum cancels its no reason to assume the intermod harmonics do as well, and IMHO they are the problem, not the 100Hz. It may work, try it and let us know. The problem with the valves in series is there is nothing to make the two heaters dissipate equal power, they may be, they might not. It wont be large DC unless the cathode is broken, and if that is a worry use a fuse.

I would still use separate supplies, but you can do whatever you want.
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#24 Re: Request for input and feedback on a circuit idea

Post by RhythMick »

Nick wrote: Sun Mar 04, 2018 10:51 am Thinking a bit more about this. I would (if I was worried, and the transformers needed it) balance the two valves with a 47R pot between the cathodes. You could take the CCS to a -ve point (or the shared resistor) of you could stand the grids up a bit, either would work, =ve grids would probably be simpler


CCS-Triodes.jpg.
Interesting and thanks - food for thought. Pot in the cathode vs separate bias pots. I need to get to grips with CCS design so I know what voltage the cathodes will be at, then I'll come back to this.

I might well take your advice though and build it with a simple shared resistor to begin with. That way I can get an initial prototype fairly quickly and see whether the CCS improves it.

I did also consider a big choke in the tail. LL2743-70mA is 64H which would give 4k at 10hz, so as good as the resistor without the - ve supply. DCR is 400R which gives 24v at 60mA. Maybe worth trying - I have the 90mA spare actually which is 50H.

3k at 10hz
30k at 100hz

Just got your other post and thanks. Yes I'm inclined to separate supplies, just chewing the cud.
Last edited by RhythMick on Sun Mar 04, 2018 12:30 pm, edited 1 time in total.
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#25 Re: Request for input and feedback on a circuit idea

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Nick wrote: Sat Mar 03, 2018 11:55 pm You could use one supply. If we believe that the heater supply matters, then that implies the voltage and current in the heater affects the sound then I would worry joining both ends with a high impedance supply may do something. I said what I would do, someone else may do something different.
Well I certainly believe a heater supply matters !
Are we talking about the case of the cathodes on a CCS ( not a resistor ) where you see a problem ?
For the Amity-style circuit I can't see a problem, you would replace the centre-tapped AC winding with a DC DHT supply to both valves and have either the supply Neg or the supply Pos attached to the shared cathode resistor - so one end would not be high impedance.
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#26 Re: Request for input and feedback on a circuit idea

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IslandPink wrote: Sun Mar 04, 2018 11:49 am
Nick wrote: Sat Mar 03, 2018 11:55 pm You could use one supply. If we believe that the heater supply matters, then that implies the voltage and current in the heater affects the sound then I would worry joining both ends with a high impedance supply may do something. I said what I would do, someone else may do something different.
Well I certainly believe a heater supply matters !
Are we talking about the case of the cathodes on a CCS ( not a resistor ) where you see a problem ?
For the Amity-style circuit I can't see a problem, you would replace the centre-tapped AC winding with a DC DHT supply to both valves and have either the supply Neg or the supply Pos attached to the shared cathode resistor - so one end would not be high impedance.
As far as the valve is concerned if one end of its cathode is a high impedance, then both ends will have to be.

The problem I see (and we are in a world where small changes to the heater supply is heard), then if one end of each filament is connected together, then that point will be equal voltage. The anodes are out of phase, so they have different signals on them. If the anodes have different signals, and one end of each cathode is at the same voltage, then the other end of each filament will also have different signals on them. Connecting them together will mean those signals are forced to be the same. The result may be better or worst, I cant say without trying, but I would expect it to be different.
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#27 Re: Request for input and feedback on a circuit idea

Post by IslandPink »

OK, I get your drift.
I see Lynn's Karna version with the DC supplies has what you're suggesting, though that may be just a function of the Tent supplies not doing more than about 2A.
http://www.nutshellhifi.com/Karna-alternate-a.gif
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#28 Re: Request for input and feedback on a circuit idea

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I did also consider a big choke in the tail. LL2743-70mA is 64H which would give 4k at 10hz, so as good as the resistor without the - ve supply. DCR is 400R which gives 24v at 60mA. Maybe worth trying - I have the 90mA spare actually which is 50H.
I do think you are putting too much on the need for a long tail. If the anodes were resistively loaded I would be with you all the way, but the transformer coupling will do all you need to ensure balance.
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#29 Re: Request for input and feedback on a circuit idea

Post by RhythMick »

IslandPink wrote: Sun Mar 04, 2018 1:17 pm OK, I get your drift.
I see Lynn's Karna version with the DC supplies has what you're suggesting, though that may be just a function of the Tent supplies not doing more than about 2A.
http://www.nutshellhifi.com/Karna-alternate-a.gif
@Mark, thanks for that diagram - I hadn't seen that version of the Karna.

@Nick, you may well be right. I'll pare it down and start simple. I contacted Jac (Lundahl dealer) and he said the Lundahl 1660 had 15% of the total magnetism reserved for DC imbalance and that the worst pair of 45s I could find wouldn't trouble it.
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#30 Re: Request for input and feedback on a circuit idea

Post by Nick »

Jac (Music) knows of what he speaks.
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