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

Posted: Sun Jan 13, 2019 8:18 pm
by steve s
I would have loved to hear it but I'm busy most weekends this month
Well done on getting it sorted.. roll on the next build !!!
If your still up for bringing it across to mine in February your welcome..

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

Posted: Sun Jan 13, 2019 8:32 pm
by IslandPink
So pleased it's worked out. I remember how it was when I first got the PP 300B working - eye-opener !

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

Posted: Sun Jan 13, 2019 9:04 pm
by RhythMick
We've designed the PSU and amp to mount on industrial rollers into the cupboard. M8 rod connects the layers. The top dome nuts can be removed, connecting nuts and rods then allow the amp to be turned upside down protecting the valves, so I can get to the insides. Works well. Here it is in maintenance mode.
1547413483666282.jpg

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

Posted: Sun Jan 13, 2019 10:43 pm
by RhythMick
steve s wrote:
Sun Jan 13, 2019 8:18 pm
I would have loved to hear it but I'm busy most weekends this month
Well done on getting it sorted.. roll on the next build !!!
If your still up for bringing it across to mine in February your welcome..
The hum has been tamed. If I kill the buzz by then I'd love to bring it over. Or you bring your speakers here.

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

Posted: Mon Jan 14, 2019 3:52 pm
by RhythMick
Interesting.

- removed rectifiers for 26 & 71A valves
- removed 26 & 71A valves (so no heater noise)
- grounded the 45 grids with clip leads

Design reminder following the simplifications ...
PSU.PNG
Left channel was silent 1 foot away from speaker (Lowther DX4), small buzz with ear next to it. 30uF cap bypassing cathode stopped that buzz dead. Unclip it and the slight buzz returns, clip on and it goes, very clear.

Right channel was silent.

So I measured the valves.

Left : Vgk = 47v, Vak = 245v, Valve current measured across 10R were 31.6 & 26.2 mA
Right : Vgk = 47v, Vak = 250v, Current = 32.4 & 25.7mA

So I swapped the outside valves figuring to make two closer matched pairs. The mA stayed mismatched, the voltages changed slightly, but most interestingly unclipping the bypass cap now makes zero difference on the left channel. Both channels completely silent - in fact I went back to check they were still on, even though I'd just measured voltages. Measurements after swapping the valves below.

Left : Vgk = 48v, Vak = 249v, Valve current measured across 10R were 31.1 & 27.1 mA
Right : Vgk = 48v, Vak = 253v, Current = 33.2 & 25.6mA

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

Posted: Mon Jan 14, 2019 6:00 pm
by RhythMick
99% of the buzz had been killed. 30uF caps across the 26 cathode resistors killed the hum, the buzz turned out to be valve mismatches.

From the sofa I can BARELY tell the system is on.

Listening to the same album I heard at lunchtime, on same volume, everything is blacker and more atmospheric.

I'm so happy I might cook tea.

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

Posted: Mon Jan 14, 2019 7:33 pm
by izzy wizzy
RhythMick wrote:
Mon Jan 14, 2019 6:00 pm
99% of the buzz had been killed. 30uF caps across the 26 cathode resistors killed the hum, the buzz turned out to be valve mismatches.

From the sofa I can BARELY tell the system is on.

Listening to the same album I heard at lunchtime, on same volume, everything is blacker and more atmospheric.

I'm so happy I might cook tea.
Happy it's worked out well for you. I know only too well how frustrating these "simple" amps can be.

Cheers,
Stephen

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

Posted: Mon Jan 14, 2019 8:13 pm
by RhythMick
Everyone who has helped and supported, welcome to beer food and accommodation here any time you need or want it

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

Posted: Mon Jan 14, 2019 10:40 pm
by Ray P
I see you use Lowthers; maybe from the sublime to the ridiculous for your next project with a 6C33C or 13E1 SE-OTL build?

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

Posted: Mon Jan 14, 2019 10:49 pm
by RhythMick
Ray P wrote:
Mon Jan 14, 2019 10:40 pm
I see you use Lowthers; maybe from the sublime to the ridiculous for your next project with a 6C33C or 13E1 SE-OTL build?
Yeah I've been watching those threads with interest.

I definitely want to bring this system to Owston. I want someone else to have a listen and tell me is not just my biased imagination.

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

Posted: Mon Jan 14, 2019 11:24 pm
by IslandPink
Which Lowthers do you use and how are they employed ?

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

Posted: Tue Jan 15, 2019 12:15 am
by RhythMick
We bought Icon Audio cabs (folded horn in a tower with a rear exit for the bass) with DX2 units originally, since upgraded to DX4.

Geoff just recently bought and built 15" subs which replaced the B&W 610 units we previously used. We have 2 each and they integrate really smoothly. I'll get the details from Geoff.

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

Posted: Tue Jan 15, 2019 12:49 am
by RhythMick
We bought Icon Audio cabs (folded horn in a tower with a rear exit for the bass) with DX2 units originally, since upgraded to DX4.

Geoff just recently bought and built 15" subs which replaced the B&W 610 units we previously used. We have 2 each and they integrate really smoothly. I'll get the details from Geoff.

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

Posted: Tue Jan 15, 2019 6:26 am
by Ray P
RhythMick wrote:
Tue Jan 15, 2019 12:15 am
We bought Icon Audio cabs (folded horn in a tower with a rear exit for the bass) with DX2 units originally, since upgraded to DX4.

Geoff just recently bought and built 15" subs which replaced the B&W 610 units we previously used. We have 2 each and they integrate really smoothly. I'll get the details from Geoff.
Do you have the 15ohm DX4s?

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

Posted: Tue Jan 15, 2019 7:11 am
by RhythMick
No 8ohm, wish we'd opted for 15