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 » Fri Dec 28, 2018 4:50 pm

Sorry Nick, didn't mean to give offense there. I realise I'm a newbie at this and truly appreciate the advice. If I could wave a magic wand and instantly soak up all the experience you guys have I'd do it, but in the absence of that I need to work my way through step by step and try to get clear in my head what works and what doesn't, and try to comprehend why.

For now my foundation is that the circuit works when built in Geoffs amp. If I replicate that and still have a problem then it's likely to be layout.

I'd like to get it working first. From there it's easy to remove the chokes and switch to simple resistance to check how it sounds. I'll then start looking into CCS design.

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

Post by RhythMick » Fri Dec 28, 2018 5:05 pm

Nick wrote:
Fri Dec 28, 2018 4:36 pm
RhythMick wrote:
Fri Dec 28, 2018 4:30 pm
Yes I don't understand it either. And by definition what is positive to one Grid will be negative to the other.
That's only true for a signal that is induced in the transformer core, DC shifts of the common point will raise both grids at the same time in the same direction. Current has to flow through the coil to generate flux, the grid ends are in effect disconnected so no current can flow in the secondary.
Of course, you're right. Thanks.

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

Post by izzy wizzy » Fri Dec 28, 2018 5:26 pm

Thomas Mayer does a similar thing with choke in the tail of a diff pair. Some use a CCS. Lynn Olson uses a resistor as he reckons balance is restored at each It. I've tried a CCS over resistor and it wasn't for me but hey, it wasn't a great CCS.

I was only postulating an idea re positive feedback. It kinda works in my head. But I am displaced at present only with a phone so would be too hard to expand further on from the link I posted.

Cheers,
Stephen

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

Post by Nick » Fri Dec 28, 2018 6:49 pm

Thomas Mayer does a similar thing with choke in the tail of a diff pair.
With a inductive anode load? I just fail to see how the choke has any advantage in that position, especially in this case when it is not seen by the valve grids. All I see it doing is letting the grid circuit float at AC and increasing the impedance of the supply.
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Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by izzy wizzy » Sat Dec 29, 2018 12:26 pm

Nick wrote:
Fri Dec 28, 2018 6:49 pm
Thomas Mayer does a similar thing with choke in the tail of a diff pair.
With a inductive anode load? I just fail to see how the choke has any advantage in that position, especially in this case when it is not seen by the valve grids. All I see it doing is letting the grid circuit float at AC and increasing the impedance of the supply.
He floats the grid center tap on a choke to ground too.

I've not done much study on this but I've seen some use a choke instead off CCS in the tail of a cathode follower. If this allows greater swing, not sure it does, like it does in the anode, then we have feedback off the top of this to the grid, that seems to me to be positive feedback ie it's in sync with the input. Isn't that the basis of an oscillator?

PP isn't a perfect cancelling circuit and the imbalance appears across the tail. A large impedance here forces balance. Here we have a large cathode impedance, being fed back to the the grids. The 26 is a highish Z valve so strays in the circuit become relevant and possibly relevant to causing phase shifts to set it off or creating the imbalance that sets it off. It's wishful thinking that the topology will cancel stuff out as none of the devices, active or passive are perfectly symmetrical.

Hence the need in my head to decouple the feedback for AC leaving just the DC for bias. Hence that link I posted for adjustable cathode bias.

All this might be rubbish but it makes some sense to me. It would also explain why one of 813 pp amps did the exact same thing with a very similar biasing strategy.

I realise the other amp works but that doesn't mean it will work every time as was experienced earlier when a bias change set off the 71a. The balancing pots maybe the contributor to having great dc balance but creating an ac offset.

Or it could be something else entirely.

Cheers,
Stephen

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

Post by Mike H » Sat Dec 29, 2018 1:01 pm

Nick wrote:
Fri Dec 28, 2018 4:23 pm
Just remove the choke in the cathode, its not doing what you think it is.
I tend to agree, I was going to say short them all out, and see what happens.
 
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Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by Mike H » Sat Dec 29, 2018 1:07 pm

Nick wrote:
Fri Dec 28, 2018 4:27 pm
will stop the cathode(s) feeding positive feedback into the input transformer.
May be missing something, but I don’t see how you can get positive feedback from the cathode to the grid without something adding 180 degrees phase shift.
No need for phase inversion, as they are already in phase. E.g., positive from cathode to grid drives the grid positive, makes cathode more positive, etcetera... the thing saturates, goes negative, ditto... inter-winding capacitance in the transformer = tuned circuit, = audible whistling. (As it happens.)

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

Post by Nick » Sat Dec 29, 2018 2:11 pm

He floats the grid center tap on a choke to ground too.
Ok, any link I would be interested in seeing.
PP isn't a perfect cancelling circuit and the imbalance appears across the tail.
Ok, ignoring the fact that the coupled inexpediences in the loads will do much of the balancing, and the valves are already being fed a balanced signal from the interstage.
Here we have a large cathode impedance, being fed back to the the grids.
But how is it being fed into the grid? The grid is on top of the choke. How does the valve see the choke at all?
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Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by Nick » 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;
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Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by Nick » Sat Dec 29, 2018 2:19 pm

How does the valve see the choke at all?
Just an addition, if you decouple the interstage to ground with the cap you added, then the grid does see the choke, but not as originally drawn.
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Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by IslandPink » Sat Dec 29, 2018 3:18 pm

Re. positive feedback from cathode - rang a bell. Just went looking for Thorsten's WE91B amp variant ( which I built ) . Thankfully someone saved the original thread with all the pictures ( which have disappeared on AA original thread ) as a PDF :
https://pearl-hifi.com/06_Lit_Archive/1 ... s_WE91.pdf
Some way down you'll see some comment on the circuit
"The odd looking circuit between the cathode of the 300B and the 300B gridleak resistor is also original WE, it is a form of positive
feedback that overcomes the limited LF frequency response cause by using fairly small value (but high quality) cathode bypass
capacitors. In the old days large value capacitors were too large and expensive, today the same holds true for high quality types, hence
this circuit still has use. Note it only works with pentode drivers or in specific configurations with interstage transformers, no point
adding it to a triode driver amplifier"
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Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by Nick » Sat Dec 29, 2018 4:05 pm

cthode of the 300B and the 300B gridleak resistor is also original WE, it is a form of positive
feedback that overcomes the limited LF frequency response causec
Looks to me to still not be positive feedback, it looks more like a negative feedback circuit that reduces the amount of feedback below the RC point set by the 1.5u and 10k resistor. So the gain is increased at LF by reducing the negative feedback.
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Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by RhythMick » Sat Dec 29, 2018 4:21 pm

Well, although I haven't reported anything I have been working hard.

1. All 6 valve pairs have been changed to reflect Geoffs design - so there is no balancing pot, the VCs are joined and connected to top resistor (which sets minimum bias), then a 2W pot with bias from the wiper, then tail choke, then bottom resistor to ground. Still whistling.

(side note - because of the current going through the tails the bias pot for the 71A is 1K 2W, with a 1K 2W resistor in parallel, turning this into 500R 4W. Similarly the 45s have 2 x 1K 2W in parallel, making 333R 6W)

2. Added back in the 30uF caps on bias using clip leads. Whistle reduced but god awful hum. Experimentation lead me to conclude the hum is from the bias cap on the 71A - removing this loses the hum. So with caps on 26 & 45, but none on 71A, we have music with a lower volume high pitched whistle.

3. Removed bias caps. Replaced 71A tail choke (LL1667, 2400DCR) with resistor (2k8 made from 2 x 5k6 5W - best I could do from what I have to hand). Whistle gone - and I'm sure I have more gain.

Nick I thank you for that suggestion. Clearly I have not yet "grokked" this. I will have to replay your earlier posts many times over I think. Meanwhile I'm removing the tail chokes for the 26 & 45 too.

Beers on me at Owston.

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

Post by RhythMick » Sat Dec 29, 2018 5:27 pm

All tail chokes out of circuit.

I found that the 45s need that bias cap, then the hum drops. Plays nicely, and I'm sure I have more gain.

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

Post by RhythMick » Sat Dec 29, 2018 5:34 pm

Ha. Spoke too soon - tried the cap across the right 71A and now there's a buzz where it was silent. Perhaps an imperfect solder joint I nudged. Still, I'm sure I'll track it down.

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