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#61 Re: Another go at PP -> IT -> PP

Posted: Sun Nov 06, 2022 6:59 pm
by Max N
Still thinking about trying to shrink this down. When I originally tried the followers, I was using the sowter step up transformers for gain, so without the followers I was driving the output stage with quite a high impedance. I preferred the sound with the followers. I repeated the experiment with the valve gain stage and again preferred the followers, but at that time I wasn’t using the mu output of the CCSs, so again the driving impedance without the followers was quite high.
Today, using the mu output, I again tried without the followers and again didn’t like the sound. I was trying to figure out why. The followers weren’t the only variable…
With the followers I’m able to use a 1Mohm resistor after the coupling cap, which means I can use a small cap. Without the followers I’m limited to a 50kohm resister so I need to use a larger cap.
For the small cap I’m using some ERO KP1832 film and foils, my favourite caps, but I don’t have any larger than 0.075uF, and they don’t make them anymore. For the large caps I’m using some ‘decent’ film caps that I used in the past for speaker crossovers.
Still without the followers, I tried bypassing the large caps with the EROs, and the magic was mostly back.
Losing the followers will allow me to eliminate a toroidal transformer, a positive and negative supply, and quite a lot of heat inside the chassis. But I’ll need 4 caps of at least 1uF, that are as good as the ERO film and foils. I’m considering the Mundorf M Cap ZN, tin foil and film. Any other suggestions?

#62 Re: Another go at PP -> IT -> PP

Posted: Sun Nov 06, 2022 8:12 pm
by IslandPink
Max N wrote: Sun Nov 06, 2022 6:59 pm With the followers I’m able to use a 1Mohm resistor after the coupling cap, which means I can use a small cap. Without the followers I’m limited to a 50kohm resister so I need to use a larger cap.
Can you explain this a bit more ( or remind me if I missed it ) ?
I am familiar with the trade-offs on grid leak values when going from auto-bias to fixed bias, sounds similar...

#63 Re: Another go at PP -> IT -> PP

Posted: Sun Nov 06, 2022 10:24 pm
by Max N
I’ll try :D
Both scenarios are fixed bias. Without the buffers, the data sheet for 6b4g says 50kOhm. My understanding is that as grid current starts to flow, it develops a voltage across the grid 50kOhm which reduces the negative bias, so more current flows, and we get run away.
With the buffers (see circuit in post 59), the follower is doing a really good job of holding the DC voltage at the grid of the output valve, steady at the fixed bias voltage. Actually it’s a few volts below the bias voltage because of the gate to source voltage of Q1. But anyway, the follower soaks up any grid current without allowing the grid voltage to change, and we can pretty much go as high as we want for R4.

#64 Re: Another go at PP -> IT -> PP

Posted: Sun Nov 06, 2022 11:07 pm
by IslandPink
OK thanks, I see what you mean.

#65 Re: Another go at PP -> IT -> PP

Posted: Sun Nov 20, 2022 9:43 pm
by Max N
I worry about what would happen if the bias supply failed. I probably should put some fuses in somewhere. In the cathode returns to earth? Just normal fast-blow glass fuses?

#66 Re: Another go at PP -> IT -> PP

Posted: Sun Nov 20, 2022 10:37 pm
by Nick
I would think the only safe place is in the anode circuit. In the cathode it would not help with a grid to anode failure. And I would look at 32mm fuses to get close to the voltage rating that would actually do any good in the case if a failure.

#67 Re: Another go at PP -> IT -> PP

Posted: Sun Nov 20, 2022 10:44 pm
by Max N
Thanks Nick I’ll take a look

#68 Re: Another go at PP -> IT -> PP

Posted: Mon Nov 21, 2022 4:42 am
by izzy wizzy
Max N wrote: Sun Nov 20, 2022 9:43 pm I worry about what would happen if the bias supply failed. I probably should put some fuses in somewhere. In the cathode returns to earth? Just normal fast-blow glass fuses?
I had that happen. Took out the HT tx and the choke. Somehow the OPT survived. I put fuses in the HT supply for the Tx rating; being full wave, 1 in each leg.

Cheers,
Stephen

#69 Re: Another go at PP -> IT -> PP

Posted: Mon Nov 21, 2022 6:24 pm
by Max N
Hi Stephen, I remember reading about that and wincing. I’ve been meaning to do something about it ever since, it’s only taken me 6 months to get my finger out!

#70 Re: Another go at PP -> IT -> PP

Posted: Mon Nov 21, 2022 9:07 pm
by izzy wizzy
Max N wrote: Mon Nov 21, 2022 6:24 pm Hi Stephen, I remember reading about that and wincing. I’ve been meaning to do something about it ever since, it’s only taken me 6 months to get my finger out!
Don't delay. I even had the parts but....