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

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

Post by steve s » Tue Jan 15, 2019 9:33 am

A Lowther supported by a large efficient bass driver is a good way to go in my view.. a few years ago I used the same and got some good results
Matching the Lowther mid range peak with suitable efficient wideband bass driver(s) can give a flattish response, I was surprised how my similar my tpl150s sound to them but a blacker background if that makes sense.
I think my best lowther speaker (going by Owston comments ?) Was a twin 12" set up with my ex4s off axis... I went down hill with the next few set ups after, a vitavox horn period then the tpl150 took some time to get a good sound out of them.
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

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

Post by RhythMick » Tue Jan 15, 2019 10:42 pm

Here is the PSU and amp installed into the cupboard along with the DAC/CD/TV boxes. Geoff did all the woodwork, using an old piece of local Cedar found in a farmer's shed. Came up nicely and set off by the walnut dowels and fronts.
1547591972745419.jpg

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

Post by IslandPink » Tue Jan 15, 2019 10:44 pm

Looks like the sort of cupboard where you could dry off your wet walking boots quite nicely !
"The bass is the king of the instruments - it has no known natural predator" (Wobble)

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

Post by RhythMick » Tue Jan 15, 2019 10:47 pm

:lol:

Toasty and warm now too with all those valves and heatsinks in there...
IslandPink wrote:
Tue Jan 15, 2019 10:44 pm
Looks like the sort of cupboard where you could dry off your wet walking boots quite nicely !

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

Post by izzy wizzy » Tue Jan 15, 2019 10:49 pm

IslandPink wrote:
Tue Jan 15, 2019 10:44 pm
Looks like the sort of cupboard where you could dry off your wet walking boots quite nicely !
Bet he was wondering what to do with that spare space on the left.

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

Post by RhythMick » Tue Jan 15, 2019 10:51 pm

izzy wizzy wrote:
Tue Jan 15, 2019 10:49 pm
IslandPink wrote:
Tue Jan 15, 2019 10:44 pm
Looks like the sort of cupboard where you could dry off your wet walking boots quite nicely !
Bet he was wondering what to do with that spare space on the left.
Reckon I could fit a beer fridge in there, disguised as an amp? Mount a few valves on top...

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

Post by RhythMick » Wed Jan 16, 2019 10:24 pm

steve s wrote:
Tue Jan 15, 2019 9:33 am
A Lowther supported by a large efficient bass driver is a good way to go in my view.. a few years ago I used the same and got some good results
Matching the Lowther mid range peak with suitable efficient wideband bass driver(s) can give a flattish response, I was surprised how my similar my tpl150s sound to them but a blacker background if that makes sense.
I think my best lowther speaker (going by Owston comments ?) Was a twin 12" set up with my ex4s off axis... I went down hill with the next few set ups after, a vitavox horn period then the tpl150 took some time to get a good sound out of them.
I was really quite surprised at how the introduction of the dual 15" subs (replacing dual 10") improved the sound throughout the frequency range. Better bass I expected, but suddenly everything else seemed so much more realistic. Live performances in particular, films have much more atmosphere, but also both male and female voices much better. And the better recordings stand out much more.

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

Post by steve s » Thu Jan 17, 2019 8:44 am

RhythMick wrote:
Wed Jan 16, 2019 10:24 pm
steve s wrote:
Tue Jan 15, 2019 9:33 am
A Lowther supported by a large efficient bass driver is a good way to go in my view.. a few years ago I used the same and got some good results
Matching the Lowther mid range peak with suitable efficient wideband bass driver(s) can give a flattish response, I was surprised how my similar my tpl150s sound to them but a blacker background if that makes sense.
I think my best lowther speaker (going by Owston comments ?) Was a twin 12" set up with my ex4s off axis... I went down hill with the next few set ups after, a vitavox horn period then the tpl150 took some time to get a good sound out of them.
I was really quite surprised at how the introduction of the dual 15" subs (replacing dual 10") improved the sound throughout the frequency range. Better bass I expected, but suddenly everything else seemed so much more realistic. Live performances in particular, films have much more atmosphere, but also both male and female voices much better. And the better recordings stand out much more.
That's great to hear Mick my view is large speakers add so much realisum in the lower octaves
Which 15" drivers are you using ?
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

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

Post by RhythMick » Thu Jan 17, 2019 10:38 am

steve s wrote:
Thu Jan 17, 2019 8:44 am
That's great to hear Mick my view is large speakers add so much realisum in the lower octaves
Which 15" drivers are you using ?
https://iplacoustics.co.uk/SW6%20subwoofer.htm

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

Post by steve s » Thu Jan 17, 2019 1:13 pm

RhythMick wrote:
Thu Jan 17, 2019 10:38 am
steve s wrote:
Thu Jan 17, 2019 8:44 am
That's great to hear Mick my view is large speakers add so much realisum in the lower octaves
Which 15" drivers are you using ?
https://iplacoustics.co.uk/SW6%20subwoofer.htm
They look great... are your lowthers in open baffle?
Where roughly in Sheffield are you... in case I get chance to bring my speakers across..
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

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

Post by RhythMick » Thu Jan 17, 2019 1:18 pm

No they are in Icon Audio cabs. Seen here in front of the subs.
1547731046904838.jpeg

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

Post by RhythMick » Thu Jan 17, 2019 1:24 pm

Much tidying needed I realise having posted that. However anyone fancy popping into Penistone for a listen would be welcome.

I'm away next week (son is looking after house for us) but then will make a determined effort to track down and eliminate the buzz. It's now inconsistent. Monday it was there for a few minutes then receded. Yesterday it receded after an hour. Today it's still there after a few hours. Odd. I'll work through the stages from the output, tying grids to ground. The system is the best I've heard by some margin, in fact it kind of shocked me into realising what I'd been missing. Of course I haven't heard the otl amps yet. I need to get rid of that buzz though.

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

Post by RhythMick » Thu Jan 17, 2019 1:32 pm

Is this a reasonable tactic for isolating the source of the buzz?

I figure that if I replace each valve in a pair with a DC equivalent resistor I should be able to measure and scope the b+ to see if the noise is there, or if it's the valve (or interaction of valve and PSU) which is producing it.

Example the 2 45s have 250v across them and draw 28mA each so a 9k resistor would substitute. It would need to dissipate 7W, but would be temporary in nature so don't need to oversize as I normally would. Could probably mount on an old valve base even. Anyone know where to source valve bases (not sockets, but the bases of the valves) otherwise I'd have to find a couple of duff old valves and break the glass.

I don't have a proper scope, but a picoscope. Works ok for signal but is limited to 20v, so I'd need to use a 100x probe and I'd lose resolution. Must look into a proper scope soon.

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

Post by RhythMick » Thu Jan 17, 2019 1:52 pm

The bases I use have pins, so they would actually stack. Expensive solution though.
1547733107333469.jpg
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RhythMick
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Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by RhythMick » Mon Jan 28, 2019 9:01 pm

Hmmm.

Working through the amp tonight I find that one of the 45 heaters had failed. In that pair one valve was conducting no current, so the shared cathode resistor was causing almost double the current through the other valve. Yikes.
PSU.PNG
PSU.PNG (19.25 KiB) Viewed 1618 times
Reattached the simplified design. Simplified but still with a shared cathode.

One option would be to change this so that each valve has it's own cathode resistor. Reluctant to go down that road because i lose the balanced differential nature of the amp by doing so.

Another option would be to use one DC board to drive both heaters in series. Then if one fails they both fail. That would involve losing the virtual cathode resistors and the 10R resistors which I measure across would need to go to the anodes. Also the valve cathodes would be at different voltages and therefore the bias on the valves would be different (since the grids are grounded via the transformer centre tap).
Screenshot_2019-01-28-22-14-19-1.png
Screenshot_2019-01-28-22-14-24-1.png
Alternatively I could live with the risk that losing a valve heater will cause the other valve in the pair to run at very high plate current.

Views?

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