Finished and working - 12 valve all DHT PP 26-71a-45

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RhythMick
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Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by RhythMick » Sun Dec 30, 2018 10:44 am

Lots of thinking overnight. Much of my thinking has been challenged and in some cases reversed. Things I thought were working well turn out to induce feedback in mine, but not on Geoffs. Think I need to let all that settle in.

If I can get rid of the buzz I induced by connecting a bias bypass cap to the right 71A then I'll leave it as is for a few days. Would be nice to actually listen to it.

Then I'm going to go as clean and simple as I can, ground the grids and use resistors to drop the b+ as Nick suggested. I suspect that will sound cleaner and tighter based on the last few days experience but I'd like to hear it for myself. I'll probably put those resistors in the PSU immediately before the umbilical.

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Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by Mike H » Sun Dec 30, 2018 12:24 pm

Nick wrote:
Sat Dec 29, 2018 2:15 pm
positive from cathode to grid drives the grid positive, makes cathode more positive,
I don't understand what you mean there.

The grid is connected to the cathode via the interstage and a resistor. Current through the resistor makes the grid -ve WRT the cathode. If the current through the cathode increases, the voltage across the resistor increases making the grid more -ve WRT to the cathode, so current decreases. That's negative feedback;
As I suggested earlier, if the tail choke is inductively coupling to the input transformer. I'm only guessing mind you. Alternatively, it could be acting like a reaction coil oscillator, which is two coils (one in anode or cathode path, other connected to grid) that are inductively coupled. Again I'm only guessing as to what may be causing these oscillations. Maybe it's niether, maybe the long-tailed pair is behaving like a multi-vibrator. (?)
 
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Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by Mike H » Sun Dec 30, 2018 12:34 pm

izzy wizzy wrote:
Sat Dec 29, 2018 5:53 pm
I'm going to have a play in ltspice to try and understand this. Just for fun.
Would be interesting.

Altho when I tried similar couple of years ago, I could get an o/p stage with p-p cente-tapped transformer, and a driver stage, with a CT choke as long-tailed pair balanced driver anode loads, to run perfectly, however when plugged together, spice always baulked and hung up on (apparently) very small time steps. Obviously some sort of high frequency instability / oscillation but couldn't find out what. (Because simulation would hang.)
 
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Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by IslandPink » Sun Dec 30, 2018 12:35 pm

izzy wizzy wrote:
Sun Dec 30, 2018 9:02 am
http://izzy-wizzy.com/audio/phono/Diffphono.gif


It's from a while ago I think.

Cheers,
Stephen
Ah yes, I remember that circuit - nearly 15 years ago I suspect.
So the DC conditions for the 8020's must be set by the choke DCR's ?
These days I'd be very sceptical of that circuit, too many transformers in which low-level signals can get lost.
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Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by izzy wizzy » Sun Dec 30, 2018 2:43 pm

IslandPink wrote:
Sun Dec 30, 2018 12:35 pm
Ah yes, I remember that circuit - nearly 15 years ago I suspect.
So the DC conditions for the 8020's must be set by the choke DCR's ?
These days I'd be very sceptical of that circuit, too many transformers in which low-level signals can get lost.
Yes, DCR sets bias.From what I've read, it can drift a bit using chokes this way.

As for signals getting lost, I've read some use chokes this way which is preferable to resistors. Caps lose signals. Just about anything does. I guess it's personal preference as to what is losing your signal and in which way. They're all distortion mechanisms of one sort or another. It's not a design for for me either. It's got to look good on paper first and that too is in the eye of the beholder.

There's bound to be a reason for that config but I reckon we'll never know why.

Cheers,
Stephen

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Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by RhythMick » Sun Dec 30, 2018 4:05 pm

I've converted the left 26 to a much simplified design. Grounded Grid, 1k cathode resistor with just over 10mA on the tail, no pots, dropper resistors where the umbilical comes in, before the 2nd LC. Voltages on the valve are about right and it plays music.

Proves the concept and approach. Took me about 3 hours.

There's a loud hum if I remove the bias cap from the 45. I think therefore that I will do the 45s next both channels.

There's also a distinct buzz only on the right, which I think is from the 71A, so actually I might do that one first.

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Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by Nick » Sun Dec 30, 2018 4:27 pm

Mike H wrote:
Sun Dec 30, 2018 12:24 pm
Nick wrote:
Sat Dec 29, 2018 2:15 pm
positive from cathode to grid drives the grid positive, makes cathode more positive,
I don't understand what you mean there.

The grid is connected to the cathode via the interstage and a resistor. Current through the resistor makes the grid -ve WRT the cathode. If the current through the cathode increases, the voltage across the resistor increases making the grid more -ve WRT to the cathode, so current decreases. That's negative feedback;
As I suggested earlier, if the tail choke is inductively coupling to the input transformer. I'm only guessing mind you. Alternatively, it could be acting like a reaction coil oscillator, which is two coils (one in anode or cathode path, other connected to grid) that are inductively coupled. Again I'm only guessing as to what may be causing these oscillations. Maybe it's niether, maybe the long-tailed pair is behaving like a multi-vibrator. (?)
Ok, yes, I agree, I think either interstage secondary to cathode choke, or interstage primary (in the anodes) to cathode choke coupling is the likely cause of the oscillation.
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Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by RhythMick » Mon Dec 31, 2018 4:32 pm

Long walk today, lots of time to think.

One other thing I did differently to Geoffs was the routing of the b+ and b- wires. I try to think less of voltages and more of current in a circuit, send and return wires. I also try to minimise the area in that circuit, to minimise the possibility of picking up hum or inducing hum into other circuits.

Power comes in from the umbilical in send/return (b+/b-) pairs for each valve stage. Those go through a second LC filter before being presented to the appropriate transformer, so for the 26 anodes the b+ goes to the centre tap of the 1st interstage. I use a solder post there to anchor the b- and keep it close to the b+, those wires are twisted.

From that interstage there are 2 send wires which feed the anodes, but following the same concept there is also a return wire (b-) which returns current from the bottom of the cathode tail back to the solder post, from there back to the last smoothing capacitor.


I twisted those 3 wires : 2 b+ and 1 b-. It occurs to me those connect to 2 anodes and 1 cathode. If we're looking for ways in which cathode or Grid is coupling with anode, is this a candidate? In trying to minimise current loop area have I inadvertently created oscillation?

For completeness, a separate wire connects the bottom of the cathode tail with mains earth to give a 0v reference. There is no other 0v reference for that valve stage. The pattern repeats for all valve stages.

Should be easy to test. I'll disconnect the b- wire from the cathode and use a clip lead to provide a separate return path. Thoughts would be welcome.

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Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by Nick » Mon Dec 31, 2018 6:24 pm

I would have thought using your terms, the two b+ being out of phase would cancel out, and the b- is a low impedance supply, so should be ok as it stands, but wont do any harm to try. Generally wire to wire coupling will be low capacitance, so unless on very high impedance supplies will generally introduce RF and above instability. Of course you could have one of more RF signals mixing to give a audible signal, but I would expect that to behave like a thermion, and to be all over the place frequency wise.
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Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by RhythMick » Mon Dec 31, 2018 6:28 pm

Nick wrote:
Mon Dec 31, 2018 6:24 pm
I would have thought using your terms, the two b+ being out of phase would cancel out, and the b- is a low impedance supply, so should be ok as it stands, but wont do any harm to try. Generally wire to wire coupling will be low capacitance, so unless on very high impedance supplies will generally introduce RF and above instability. Of course you could have one of more RF signals mixing to give a audible signal, but I would expect that to behave like a thermion, and to be all over the place frequency wise.
Ok thanks Nick.

Generally speaking am I right to try to minimise the circuit area, eg by twisting wires?

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Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by Mike H » Tue Jan 01, 2019 1:35 pm

Very interesting. Experience leads me to think about putting capacitors close to any actual stage, especially if supply wires are long. I have used long and twisted + & — wire pairs but tended to have a capacitor across the supply at the end, even if it's only 1uF, 100nF, or 10nF
 
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Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by Nick » Tue Jan 01, 2019 2:12 pm

RhythMick wrote:
Mon Dec 31, 2018 6:28 pm
Generally speaking am I right to try to minimise the circuit area, eg by twisting wires?
Generally, yes, but I agree with Mike, bipass caps as close as possible to load point, at least a small bipass, say 100uf electro in the main supply, and 1uf film as close as possible.
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Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by RhythMick » Tue Jan 01, 2019 2:30 pm

Nick wrote:
Tue Jan 01, 2019 2:12 pm
RhythMick wrote:
Mon Dec 31, 2018 6:28 pm
Generally speaking am I right to try to minimise the circuit area, eg by twisting wires?
Generally, yes, but I agree with Mike, bipass caps as close as possible to load point, at least a small bipass, say 100uf electro in the main supply, and 1uf film as close as possible.
Ok thanks. I'm rethinking the layout along with changing the design to ground the grids.

Generally speaking I've been taking the b+/b- wires from the output pins of the last smoothing chokes, with a wire from those same pins to the caps. The distance from choke to cap is fairly small, wires are probably 100mm ish.

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Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by Nick » Tue Jan 01, 2019 2:46 pm

Should be fine, they are guidelines not hard and fast rules, unlikely the world will end if you don’t rigorously follow. But generally if the choice is between neatness and better layout, I would go for the better layout every time.
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Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by RhythMick » Tue Jan 01, 2019 2:53 pm

Nick wrote:
Tue Jan 01, 2019 2:46 pm
Should be fine, they are guidelines not hard and fast rules, unlikely the world will end if you don’t rigorously follow. But generally if the choice is between neatness and better layout, I would go for the better layout every time.
The suggestion of a smaller film cap right next to the load makes sense. The 2nd filter smoothing caps are 200uF (45) and 100uF (26 & 71A), all Mundorf Tubecap. I'll continue to take the power from the chokes, with feeds to the caps, but I'll also mount local 10/20/30 uF Tubecaps next to the centre taps. I have plenty to spare.

I'm also going to lose the front boards, which just results in unnecessarily long wires.

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