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Old 24th June 2012, 03:15 AM   #1891
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Can anyone tell me how hot the 26 runs? By hot, I mean the radiant heat from the tube itself (NOT the bias) ;^)

Doing a chassis layout plan and have stuff on top I need to place fairly close...

THANKS

JD
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Old 24th June 2012, 07:37 AM   #1892
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26 runs barely warm - at least at 3mA. I don't ever remember it being any more than that.
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Old 24th June 2012, 07:42 AM   #1893
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Originally Posted by harrylime6 View Post
Hi, Andy
Did you ever manage to listen to Mills MRA12's in the 26 preamp? If yes, what was your impression of them? Many thanks.
No - for the filament bias resistor I listened to alu clad (OK-ish), thick film (awful), ceramic (OK-ish), and then various wirewounds which were clearly the best. Best of those were the black ones with the two ears, discontinued Dale stock on ebay. In filament bias this does matter!!

andy
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Old 24th June 2012, 08:18 AM   #1894
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Quote:
Originally Posted by Jeffrey Davison View Post
Can anyone tell me how hot the 26 runs? By hot, I mean the radiant heat from the tube itself (NOT the bias) ;^)

Doing a chassis layout plan and have stuff on top I need to place fairly close...

THANKS

JD
As Andy said, heat is not a problem. Picking hum it could well be. So layout is more critical when you are thinking on having power supplies and pre on same chassis, something I'd strongly not recommend. 26 are prone to pick hum from chokes, etc.

Something I noticed looking at the spectrum of the valve whilst testing THD of the 40 one samples is that on some old ones, you could see the 50/100 Hz byproducts that when you shielded the tube disappeared. I remember that someone did this with aluminium foil. I take it's a workaround for those 26 which are defective (for whatever reason that would be great for anyone to explain here) they are very sensitive to picking hum? Testing setup and conditions didn't change. Is important to clarify that I only found this on a couple of samples which remind me clearly to the workaround suggested here by someone...
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Old 24th June 2012, 08:24 AM   #1895
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Originally Posted by albertli View Post
As I remember you did lots of gyrator loaded amps lately. I thought you like gyrator more. I have some DN2540 would love to try. Do you make them with PCB or ??
Yes, and I prefer the gyrator with mu-follower output. It may be a question of personal taste, but also found the CX301a (which has thoriated filaments) sounds better in my opinion to the 26. I did a blind test with my wife (amateur choral singer) and she also liked the cx301a. Subtle differences...
As a said my blind test was done 26 OT vs. 01a gyrator. Not a like for like comparison.
Thomas Mayer wrote recently an interesting article about what influences sound. Is very hard to say "26 sounds better than x" as it's not just the valve what defines the sound in your pre (or amplifier). It's a complex system with many variables which can affect sound in many ways, and the valve is just one of them.
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Old 24th June 2012, 08:27 AM   #1896
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Re PCBs, yes I make my own. Not the best looking, but works fine.
I have also built CCS using point to point wiring, but you better first drill the holes to mount the mosfets in the heatsink as they will provide support to other components. A bit tricky, but works...
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Old 24th June 2012, 08:33 AM   #1897
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Originally Posted by mogliaa View Post
As Andy said, heat is not a problem. Picking hum it could well be. So layout is more critical when you are thinking on having power supplies and pre on same chassis, something I'd strongly not recommend. 26 are prone to pick hum from chokes, etc.

Something I noticed looking at the spectrum of the valve whilst testing THD of the 40 one samples is that on some old ones, you could see the 50/100 Hz byproducts that when you shielded the tube disappeared. I remember that someone did this with aluminium foil. I take it's a workaround for those 26 which are defective ..
If you put your hand around some of these DHTs - 26, 01A etc - you get a big hum with some of them!!! Funny - some hum a lot others not at all.

Last edited by andyjevans; 24th June 2012 at 08:36 AM.
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Old 24th June 2012, 01:56 PM   #1898
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That's precisely what I looked at the test. I always selected the best valves for the amps so never had experienced it before. I think is a clear characteristic of a valve. Not sure if you would classify it as defective, but certainly it'd be a pain in the back to use one of those in the preamp. You will have to wrap it in aluminium foil and ground it as someone did here before. I prefer getting hold of a good one....
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Old 24th June 2012, 03:34 PM   #1899
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re: aluminum foil...

One could get creative and design and build an open mesh Faraday cage/cover to fit over to tube to make it look somewhat more presentable, it could also be wired to circuit ground or chassis earth. Better than a kludge fix that looks like a bandaid fix. Just a thought worth looking into. imo

JD
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Old 24th June 2012, 03:39 PM   #1900
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CeCo globes are excellent if you can find any..
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