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

If they glow, this is the place to be
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RhythMick
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Finished and working - 12 valve all DHT PP 26-71a-45

Post by RhythMick » Thu Aug 02, 2018 2:05 pm

Well my pride and joy, after lots and lots of hard work, is now living and breathing and sounding beautiful to my ears. I intend to bring her to the next meet whenever that is and I'd LOVE to hear her on other speaker systems (Steve - hint).
WhatsApp Image 2018-07-26 at 19.16.00.jpeg
The PSU is separate (for now it's a very rough build, but we intend to build it looking like the amp itself).

Each channel consists of 3 pairs of DHT valves in PP configuration : 26s driving 71As driving 45s, into Lowther DX4 drive units in simple cabs with parallel 12" active subwoofers. Each PP pair has a long-tail including a high-inductance choke to maximise impedance for the AC signal. Within each tail is a trimmer allowing bias to be adjusted).

Input to the amp is via Tribute Autoformer, into Lundahl input transformer. Between the stages are Lundahl PP Interstage Transformers and the OPT is Lundahl 1665.

Each pair of valves has fully independent power supply right back to separate transformer secondary windings. PSU circuits are tube-rectified and LCLC smoothed using Lundahl chokes and Mundorf Tubecaps.

Heaters are DC. The 26 and 71a use Andrew's boards (thanks Andrew), while the 45s use LCRC smoothing with Lundahl 2733 chokes and 2 x 22,000uF caps.

Circuit diagrams below.
PP DHT Final-PP PSU with LL.jpg
PP DHT Signal.jpg
When is the next meet ?

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

Post by RhythMick » Thu Aug 02, 2018 2:18 pm

PP DHT Final-45 Heater (1).jpg

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

Post by RhythMick » Thu Aug 02, 2018 2:29 pm

Oh and the PSU includes a Lundahl LL1662 mains isolation transformer, output from which is 230v "balanced" (ie +115v, -115v) and everything (both channels) drives off this.

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

Post by Andrew » Thu Aug 02, 2018 5:08 pm

Where are the caps on the 45 heaters? Is is CLC?

cheers,

Andrew
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Cressy Snr
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Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by Cressy Snr » Thu Aug 02, 2018 5:18 pm

Fantastic stuff Mick.
Nice to see someone building something. :wink:

You're welcome to put them through my Fanes sometime in the Autumn.
“ What a load of cobblers! ”.....Said Dougal.

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

Post by RhythMick » Thu Aug 02, 2018 6:24 pm

Cressy Snr wrote:
Thu Aug 02, 2018 5:18 pm
Fantastic stuff Mick.
Nice to see someone building something. :wink:

You're welcome to put them through my Fanes sometime in the Autumn.
That would be really good thanks.

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

Post by RhythMick » Thu Aug 02, 2018 6:25 pm

Andrew wrote:
Thu Aug 02, 2018 5:08 pm
Where are the caps on the 45 heaters? Is is CLC?

cheers,

Andrew
The 45 heaters are LCRC. The choke and 1st cap are in the PSU, the 2nd cap and resistor in the back of the amp.

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

Post by simon » Thu Aug 02, 2018 7:05 pm

Lots of respect for completing amp like this.

So the PS. Having come this far with an amp like this are you going to try separate galvanically isolated supplies for each stage? Even though it's PP and not SE I wouldn't be surprised if you could hear the difference :-).

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

Post by RhythMick » Thu Aug 02, 2018 8:37 pm

simon wrote:
Thu Aug 02, 2018 7:05 pm
Lots of respect for completing amp like this.

So the PS. Having come this far with an amp like this are you going to try separate galvanically isolated supplies for each stage? Even though it's PP and not SE I wouldn't be surprised if you could hear the difference :-).
Hi Simon and thanks. Just checking I understand you. What's meant by "galvanically isolated"?

The power supplies are already completely independent right back to the mains transformers. The 45 supply is from the LL1665. The 26 & 71A are separate secondary windings on the same mains transformer (a custom made by Dagnall from a previous amp).

Are you suggesting it would be worth putting the 26 & 71A on separate mains transformers? I did consider it. I think my brother would cry if I tried to fit anything else in, but I can try it.

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

Post by simon » Thu Aug 02, 2018 9:16 pm

Yes, and apologies for not reading carefully enough as you're pretty much where is reasonable to go I'd reckon!

The ultimate would be a separate supply for each stage and channel, but it's perhaps questionable whether you'd hear an improvement over what you have now :-).

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

Post by Dave the bass » Thu Aug 02, 2018 9:51 pm

simon wrote:
Thu Aug 02, 2018 7:05 pm
Lots of respect for completing amp like this.
Agreed, awethumne work, well done MrRythMick.
Dirt eating, low-voltage boy but "'Occasionally phenomenal'"

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

Post by IslandPink » Thu Aug 02, 2018 10:16 pm

Good effort !
Very 'Direct Heating' style.
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RhythMick
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Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by RhythMick » Thu Aug 02, 2018 11:05 pm

simon wrote:
Thu Aug 02, 2018 9:16 pm
Yes, and apologies for not reading carefully enough as you're pretty much where is reasonable to go I'd reckon!

The ultimate would be a separate supply for each stage and channel, but it's perhaps questionable whether you'd hear an improvement over what you have now :-).
No apology needed, thanks for the input. This forum helped me a lot along the way. In particular Nick helping me get my head around the heater current through the virtual cathode resistors. Awesome group.

I should have been clearer, the supplies are totally separate for each stage and channel. The only sharing is that the 26 and 71a secondaries are on the same mains transformer. Also the heaters for the rectifier tubes are secondary windings on the LL1665, which I would really have preferred to keep separate.

The amp is stereo on a single chassis. The PSU is also stereo on a single chassis, with separate umbilicals carrying b+ pairs x 3 and 45 heater pairs x 2, per channel. There's a single mains cable which carries balanced mains (+115,-115 and Earth) to the amp, which powers the 4 toroidals for the 8 DC heater boards. Safety earthing is kept separate but joined at the earth inlet to avoid loops.

The amp tops fold back like wings for maintenance. The woodwork and physical build was all Geoff (brother).

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

Post by RhythMick » Thu Aug 02, 2018 11:36 pm

This is the amp with the wings flat...
1533249361430824.jpg

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

Post by izzy wizzy » Fri Aug 03, 2018 12:49 am

Impressive work. Love every aspect from schematic to build. Well done!

Cheers,
Stephen

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