Finished and working - 12 valve all DHT PP 26-71a-45
#16 Re: Finished and working - 12 valve all DHT PP 26-71a-45
Nice build, you must be chuffed.
Sorry, I couldn't resist!
#17 Re: Finished and working - 12 valve all DHT PP 26-71a-45
Thanks - lots of time and effort, but well worth it.izzy wizzy wrote: ↑Fri Aug 03, 2018 12:49 am Impressive work. Love every aspect from schematic to build. Well done!
Cheers,
Stephen
#18 Re: Finished and working - 12 valve all DHT PP 26-71a-45
When I powered on I had a sequence of tests to go through. 5 of the 6 power circuits worked perfectly but the left 26 was odd. The 10r resistors in the anode reported current through both valves but the tail resistors reported next to nothing. Eventually I realised - I'd missed a mica spacer on one of the metal tabbed transistors for the DC boards so the anode current was going to ground.
I didn't have spares with me so instead I isolated the heatsink from the copper chassis with duct tape!
After that it worked and I confess a tear or two may have leaked out...
#19 Re: Finished and working - 12 valve all DHT PP 26-71a-45
A grand job, look forward to hearing it at the next Owston.
#20 Re: Finished and working - 12 valve all DHT PP 26-71a-45
If you can get that cap away from the DHT heater, it will sound even better. The cap is a voltage source and will be attempting to "regulate" the music signal on the DHT's heater.
Will the boards you bought not do the job?
Great job, by the way!
Andrew
Will the boards you bought not do the job?
Great job, by the way!
Andrew
Analogue, the lost world that lies between 0 and 1.
#21 Re: Finished and working - 12 valve all DHT PP 26-71a-45
Thanks Andrew. There are 8 DC boards in the amp, feeding the 26 and 71a valves.
The 45s are good old fashioned choke-filtered DC. I was originally planning to use DC boards for the 45s too, but decided on this a) to reduce space requirements in the amp and b) because having never heard DC boards I wanted to also try the choke approach. I may well try DC boards on the 45s at some point, but it means finding room for 2 more toroidals and 4 more DC boards, with heatsinks etc.
When you say "get that cap away from the DHT heater" do you mean physically away (eg move it into the PSU) or electrically away (for example put a choke after the cap, so LCRCL) ?
#22 Re: Finished and working - 12 valve all DHT PP 26-71a-45
Electrically away.
Analogue, the lost world that lies between 0 and 1.
#24 Re: Finished and working - 12 valve all DHT PP 26-71a-45
I would use the phrase "shorting out" instead of regulating in this case.The cap is a voltage source and will be attempting to "regulate" the music signal on the DHT's heater.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
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#25 Re: Finished and working - 12 valve all DHT PP 26-71a-45
I keep looking at the schematics above and can't see either the DHT heaters shown or a cap that might be across the output - can you put me out of my misery ?
"Once you find out ... the Circumstances ; then you can go out"
#26 Re: Finished and working - 12 valve all DHT PP 26-71a-45
It's this diagram
#27 Re: Finished and working - 12 valve all DHT PP 26-71a-45
Yep, agreed, Nick, "shorting out" is a better description.
Analogue, the lost world that lies between 0 and 1.
#28 Re: Finished and working - 12 valve all DHT PP 26-71a-45
I have a question, more about my learning but it might be something to try, see if there's any audible difference.
For this discussion, lets ignore questions about safety earth. I know I'll come back to it, but it may simplify the question.
As can be seen from the PSU and Signal circuit diagrams in post 1, there are 5 sections to the amp which are transformer coupled ...
Balanced input
(transformer coupled using LL1544A to)
26 pair
(transformer coupled using LL1621PP Interstage to)
71A pair
(transformer coupled using LL1621PP Interstage to)
45 pair
(transformer coupled using LL1620PP OPT to)
Lowther DX4 Speaker & Active Sub
The valve sections are totally galvanically isolated from each other except in one respect - they are all referenced to 0V at the bottom of their respective LTP.
So here's the question : What would happen if I did not reference each section to 0V?
Take the 45 section - secondary winding of the LL1665 goes to the tube rectifier to the 1st choke, return to the centre tap of the winding. Cap and bleed complete the PSU section, returning to the choke and thence the CT. B+ pair (send & return as I always call them) go to the amp. B+ goes through another LC filter, then to the CT of the OPT, through both anodes, down through the valves and virtual cathodes where they join, through the LTP and at the bottom of the lowest tail resistor return back to the 2nd choke and back to the PSU. At present I reference to 0v at the bottom of the LTP, but if I didn't - does it matter ? The grids of the 45s are fed by the outputs of the 2nd LL1621PP interstage, with a tap from the 45 LTP feeding bias voltage to the CT of the interstage output.
The only problem I can see (and I may be dreaming this up) is if the transformer (either interstage or OPT) were to build up capacitative charge which would have nowhere to drain ? Is that just nonsense ?
Anyone see a reason why I shouldn't try fully isolating each stage by removing it's reference to 0v ?
For this discussion, lets ignore questions about safety earth. I know I'll come back to it, but it may simplify the question.
As can be seen from the PSU and Signal circuit diagrams in post 1, there are 5 sections to the amp which are transformer coupled ...
Balanced input
(transformer coupled using LL1544A to)
26 pair
(transformer coupled using LL1621PP Interstage to)
71A pair
(transformer coupled using LL1621PP Interstage to)
45 pair
(transformer coupled using LL1620PP OPT to)
Lowther DX4 Speaker & Active Sub
The valve sections are totally galvanically isolated from each other except in one respect - they are all referenced to 0V at the bottom of their respective LTP.
So here's the question : What would happen if I did not reference each section to 0V?
Take the 45 section - secondary winding of the LL1665 goes to the tube rectifier to the 1st choke, return to the centre tap of the winding. Cap and bleed complete the PSU section, returning to the choke and thence the CT. B+ pair (send & return as I always call them) go to the amp. B+ goes through another LC filter, then to the CT of the OPT, through both anodes, down through the valves and virtual cathodes where they join, through the LTP and at the bottom of the lowest tail resistor return back to the 2nd choke and back to the PSU. At present I reference to 0v at the bottom of the LTP, but if I didn't - does it matter ? The grids of the 45s are fed by the outputs of the 2nd LL1621PP interstage, with a tap from the 45 LTP feeding bias voltage to the CT of the interstage output.
The only problem I can see (and I may be dreaming this up) is if the transformer (either interstage or OPT) were to build up capacitative charge which would have nowhere to drain ? Is that just nonsense ?
Anyone see a reason why I shouldn't try fully isolating each stage by removing it's reference to 0v ?
#29 Re: Finished and working - 12 valve all DHT PP 26-71a-45
It could and probably would do that.The only problem I can see (and I may be dreaming this up) is if the transformer (either interstage or OPT) were to build up capacitative charge which would have nowhere to drain ? Is that just nonsense ?
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
#30 Re: Finished and working - 12 valve all DHT PP 26-71a-45
Ok thanks. Aside from that, is there any other reason this wouldn't work?