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#1 More SEPs

Posted: Sat Mar 11, 2023 7:20 pm
by Cressy Snr
This one is different from the recent ones, so I thought I’d better start a new thread. There’s nothing new about it as such but I’ve moved back into Gary Pimm ‘Tabor’ territory, although it is not a push-pull design. Also it is very much a work in progress, so suggestions are welcomed.

Anyway this is what I have so far.

A veritable, Manhattan skyline of tubes. :lol:
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#2 Re: More SEPs

Posted: Sat Mar 11, 2023 7:21 pm
by Cressy Snr
This amp uses D3a as the input/driver:
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#3 Re: More SEPs

Posted: Sat Mar 11, 2023 7:30 pm
by Cressy Snr
As the audio circuit has become simpler, the power supply has become more complex, but that’s fine as the circuit sounds great. The D3a plate resistor is also the feedback resistor and the whole thing is a bit of a back-of-a-fag-packet job.

Ideally I would like to use 150V on the screens, but at the moment, there is not really enough HT off the plate of the output tube to enable that option, as it puts the driver plate volts below the screen volts

Also the gain is a bit low, as it would be by using the D3a plate resistor as the RFb. To get the gain up, I might have to go to separate plate and RFb, or use an active preamp, but the first priority I think, is to get the driver screen volts to 150V with the plate volts higher or the same.

#4 Re: More SEPs

Posted: Sat Mar 11, 2023 7:39 pm
by Nick
Would it not be better to use a lower GM pentode as the driver? Say e180f. Would give you more options with anode resistor.

#5 Re: More SEPs

Posted: Sat Mar 11, 2023 7:43 pm
by Cressy Snr
Thanks Nick,
I’ll look into that option.

#6 Re: More SEPs

Posted: Sat Mar 11, 2023 8:41 pm
by Paul Barker
Great to see another project.

A lot easier when heaters seperate from cathode.

I’m thinking of a cooking version all heater valves.

#7 Re: More SEPs

Posted: Sun Mar 12, 2023 7:38 am
by izzy wizzy
Have you tried a low impedance device in the d3a cathode to get the gain higher? Diode, led, RC combo would all sound different.

#8 Re: More SEPs

Posted: Sun Mar 12, 2023 1:04 pm
by Cressy Snr
Thanks for the input guys.

This isn’t about gain per se, but I wonder what would happen if I did this:
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Taking the feedback from the UL tap but keeping the fixed screen volts on the output stage. I haven't a clue without trying it.

#9 Re: More SEPs

Posted: Sun Mar 12, 2023 2:09 pm
by Nick
IMHO, like the other circuit, all you are doing is applying feedback to the output valve in just the same way as UL, but unlike real UL, you are also making the driving valve have to work harder. What would be interesting is doing something like I did on the ref phono, but take the feedback from the secondary side of the transformer level shift it and use that to drive the g2 feedback. You may need to add some extra gain to compensate for the transformer step down.

#10 Re: More SEPs

Posted: Sun Mar 12, 2023 3:06 pm
by Cressy Snr
Just tried it and it was a mess of oscillation, dependent on the position of the volume control. It was interesting as the further around the track of the pot I got, the higher went the frequency. Apparently I built myself a signal generator, er... not quite what I want :shock:. Fun nonetheless.
I'm ordering some E180Fs tomorrow. Cheap as chips.

#11 Re: More SEPs

Posted: Sun Mar 12, 2023 3:15 pm
by Nick
Not sure what you tried, maybe my explanation was at fault. This is the sort of thing I had in mind. Works in spice.

#12 Re: More SEPs

Posted: Sun Mar 12, 2023 4:05 pm
by Cressy Snr
Ah!

See what you mean there. I’d never have thought that. Thanks Nick.

At the moment I’m in the process of setting the amp up with a separate feedback resistor. With the first try, I have too much gain, which in addition to being bloody loud, also causes the D3a to become microphonic. However, on a positive note, I now have sufficient HT to get the driver screen to 150V whilst keeping the driver plate 27V higher. Just need to increase the feedback level to reduce the gain a tad. I’m therefore in a better position than I was yesterday.

#13 Re: More SEPs

Posted: Sun Mar 12, 2023 4:07 pm
by Cressy Snr
Nick wrote: Sun Mar 12, 2023 3:15 pm Not sure what you tried…
I used the UL tap to feed back to the driver. Didn’t work.

#14 Re: More SEPs

Posted: Sun Mar 12, 2023 4:10 pm
by Nick
It probably would, but you would need to invert the feedback.

What you could do was connect the b+ to the ul tap, then take the unused tap that used to be b+ and use that to feed g2. Would invert the feedback.

#15 Re: More SEPs

Posted: Sun Mar 12, 2023 5:01 pm
by Cressy Snr
Nick wrote: Sun Mar 12, 2023 4:10 pm It probably would, but you would need to invert the feedback.

What you could do was connect the b+ to the ul tap, then take the unused tap that used to be b+ and use that to feed g2. Would invert the feedback.
Thick as pigsh*t that's me. Never mind. :)