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

If they glow, this is the place to be
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Ray P
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#16 Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by Ray P »

Nice build, you must be chuffed.
Sorry, I couldn't resist!
RhythMick
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#17 Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by RhythMick »

izzy wizzy wrote: Fri Aug 03, 2018 12:49 am Impressive work. Love every aspect from schematic to build. Well done!

Cheers,
Stephen
Thanks - lots of time and effort, but well worth it.
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#18 Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by RhythMick »

Ray P wrote: Fri Aug 03, 2018 6:54 am Nice build, you must be chuffed.
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...
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Ali Tait
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#19 Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by Ali Tait »

A grand job, look forward to hearing it at the next Owston. :D
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#20 Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by Andrew »

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

Post by RhythMick »

Andrew wrote: Fri Aug 03, 2018 9:42 am 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
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) ?
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#22 Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by Andrew »

Electrically away.
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#23 Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by RhythMick »

Andrew wrote: Fri Aug 03, 2018 10:25 am Electrically away.
OK Thanks - I may well try that. I could move the cap into the PSU and mount a small choke into the back of the amp. Interesting experiment and well worth trying, cheers.
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Nick
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#24 Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by Nick »

The cap is a voltage source and will be attempting to "regulate" the music signal on the DHT's heater.
I would use the phrase "shorting out" instead of regulating in this case.
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IslandPink
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#25 Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by IslandPink »

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 ? :!:
Andrew wrote: Fri Aug 03, 2018 9:42 am 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?
"Once you find out ... the Circumstances ; then you can go out"
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#26 Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by RhythMick »

It's this diagram
PP DHT Final-45 Heater (1).jpg
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#27 Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by Andrew »

Yep, agreed, Nick, "shorting out" is a better description.
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#28 Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by RhythMick »

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 ?
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Nick
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#29 Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by Nick »

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 ?
It could and probably would do that.
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#30 Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by RhythMick »

Nick wrote: Mon Aug 06, 2018 11:08 am
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 ?
It could and probably would do that.
Ok thanks. Aside from that, is there any other reason this wouldn't work?
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