Finished and working - 12 valve all DHT PP 26-71a-45
#76 Re: Finished and working - 12 valve all DHT PP 26-71a-45
I reworked the diagram, changing the values to 01A values. I also reversed the current flow diagrams to show correct current flow direction.
#77 Re: Finished and working - 12 valve all DHT PP 26-71a-45
Thanks for that suggestion Steve. With the 71A and 45 in play there's no squeal. What's more by taking the input to the grids of the 71A I can get music. Doesn't sound too bad considering it's through crappy bookshelf speakers and there's a valve stage missing.
That's helped me pin it down. I think next I'll switch to the 26 valve which I know works well. That said my amp is a different physical build and layout compared to Geoffs, but I'm hoping I can at least get it playing for xmas.
That's helped me pin it down. I think next I'll switch to the 26 valve which I know works well. That said my amp is a different physical build and layout compared to Geoffs, but I'm hoping I can at least get it playing for xmas.
#78 Re: Finished and working - 12 valve all DHT PP 26-71a-45
Switched to the 26 valve (heaters set to 1.5v and chanegd the bottom resistor in the tail).RhythMick wrote: ↑Tue Dec 18, 2018 9:44 pm Thanks for that suggestion Steve. With the 71A and 45 in play there's no squeal. What's more by taking the input to the grids of the 71A I can get music. Doesn't sound too bad considering it's through crappy bookshelf speakers and there's a valve stage missing.
That's helped me pin it down. I think next I'll switch to the 26 valve which I know works well. That said my amp is a different physical build and layout compared to Geoffs, but I'm hoping I can at least get it playing for xmas.
No Banshee, but high pitched whistle so still feedback just a different form.
I'll measure everything tonight and check that we have no wires off.
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#79 Re: Finished and working - 12 valve all DHT PP 26-71a-45
Can you run just the 26 stage into another amp to rule out it interacting with the other stages or just oscillating on its own?
Cheers,
Stephen
Cheers,
Stephen
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#80 Re: Finished and working - 12 valve all DHT PP 26-71a-45
TBH I was wondering if all these transformers were coupling with each other? You only need a bit of positive feedback with a gain >1 and off it goes
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."
#81 Re: Finished and working - 12 valve all DHT PP 26-71a-45
Progress. The left channel makes music through the 26. The right whistles. Left has a buzz to track down but I'll come back to that.
I've swapped valves over including the rectifier. Run without right while measuring left and vice versa. Interestingly both channels measure very close to calculated design specs. It must be a layout thing.
I've taken valves out and measured resistances to each point in the tail and left matches right. No dodgy joints that I can see, though I can't rule out a dry joint somewhere.
I'll keep working my way through, after all progress has been made. All tips, tricks and magic touches welcome.
I've swapped valves over including the rectifier. Run without right while measuring left and vice versa. Interestingly both channels measure very close to calculated design specs. It must be a layout thing.
I've taken valves out and measured resistances to each point in the tail and left matches right. No dodgy joints that I can see, though I can't rule out a dry joint somewhere.
I'll keep working my way through, after all progress has been made. All tips, tricks and magic touches welcome.
#82 Re: Finished and working - 12 valve all DHT PP 26-71a-45
Yes interesting thought, however Geoff's channels both work fine and my left now does. I doubt it's the design and I doubt it's a connection (because the voltages all measure right, though it could be signal connection).
I'm left with either a signal connection difference or a layout difference I think. I might try sending the left 26 output to the right 71A and vice versa
#83 Re: Finished and working - 12 valve all DHT PP 26-71a-45
Actually the plate voltage is a bit higher than design, probably because my mains is high here at about 248v rms. I reckon the 300v secondary must be giving me about 320v - I'll measure back through the PSU tomorrow. However Vak isn't crazy high - it's about 154v (maximum for the 26 is 180). I don't believe it's that - especially as the left channel is working and is about the same.RhythMick wrote: ↑Wed Dec 19, 2018 10:09 pmYes interesting thought, however Geoff's channels both work fine and my left now does. I doubt it's the design and I doubt it's a connection (because the voltages all measure right, though it could be signal connection).
I'm left with either a signal connection difference or a layout difference I think. I might try sending the left 26 output to the right 71A and vice versa
#84 Re: Finished and working - 12 valve all DHT PP 26-71a-45
I when I have had similar issues I have swapped each stage on to the other channel ( one at a time). So the signal zig zags through the amp.. You will soon find the faulty stage.. then take it from there...
The tube manual is quite like a telephone book. The number of it perfect. It is useful to make it possible to speak with a girl. But we can't see her beautiful face from the telephone number
#86 Re: Finished and working - 12 valve all DHT PP 26-71a-45
I used the picoscope on the output of the right channel input transformer. Red trace is -ve phase, blue is +ve phase. CD playing (Mark Knopfler, you can tell), heaters are on (only because I wanted to eliminate them).
With no B+ applied, the red and blue traces balance each other nicely.
With B+ applied, the +ve trace goes haywire (and the amp throws out high-pitched squeal).
Cathode to grid voltage on both valves is correct (-9v). Bias is applied to the centre-tap of the output - since Vgk is correct the bias voltage must be getting through to the grids correctly. On that basis I think the wiring of the LL1544A is correct.
I'll keep digging.
With no B+ applied, the red and blue traces balance each other nicely.
With B+ applied, the +ve trace goes haywire (and the amp throws out high-pitched squeal).
Cathode to grid voltage on both valves is correct (-9v). Bias is applied to the centre-tap of the output - since Vgk is correct the bias voltage must be getting through to the grids correctly. On that basis I think the wiring of the LL1544A is correct.
I'll keep digging.
#87 Re: Finished and working - 12 valve all DHT PP 26-71a-45
ignore that - the scope connection was dislodgedRhythMick wrote: ↑Fri Dec 21, 2018 3:52 pm I used the picoscope on the output of the right channel input transformer. Red trace is -ve phase, blue is +ve phase. CD playing (Mark Knopfler, you can tell), heaters are on (only because I wanted to eliminate them).
With no B+ applied, the red and blue traces balance each other nicely.
With B+ applied, the +ve trace goes haywire (and the amp throws out high-pitched squeal).
Capture.JPG
Cathode to grid voltage on both valves is correct (-9v). Bias is applied to the centre-tap of the output - since Vgk is correct the bias voltage must be getting through to the grids correctly. On that basis I think the wiring of the LL1544A is correct.
I'll keep digging.
#88 Re: Finished and working - 12 valve all DHT PP 26-71a-45
Embarrassed ... I was measuring the input to the input tx, ie the XLR. Sigh. Still good to know that is wired ok, lol.
The output from the input tx tells a different story. The + and - are in phase with each other !
The output from the input tx tells a different story. The + and - are in phase with each other !
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#89 Re: Finished and working - 12 valve all DHT PP 26-71a-45
Have you got that shown as both traces as once coz the scope will trigger on leading edge and show both the same if you're doing one then the other but if you're showing both at once and triggering off one channel, then you can see phase.
Cheers,
Stephen
#90 Re: Finished and working - 12 valve all DHT PP 26-71a-45
Hmmm need to think about that one. Scope newbie hereizzy wizzy wrote: ↑Fri Dec 21, 2018 4:15 pmHave you got that shown as both traces as once coz the scope will trigger on leading edge and show both the same if you're doing one then the other but if you're showing both at once and triggering off one channel, then you can see phase.
Cheers,
Stephen