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#26 pre amp

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mach1

mach1

Banned
Joined 2004
Andy,

The main reason I asked about your choke orientation was to find out whether you had tried twystd's recommendation (see below). I have mine orientated in the same manner as you and have never got round to trying it myself.

I wrote the recent post you were looking for. I bread boarded a parafeed preamp, first I tried a single choke and got some hum. Then I mounting 2 of them in series base to base on standoffs like Gary suggested, and the hum went away. Then I started thinking that with the 2 cores in direct contact, that I may be losing some inductance due to cancelling.

I next mounted them in pairs, but wired out of phase, and placed the individual series wired chokes in close proximity, and parallel to each other. The parallel pairs (left and right channels) were mounted at right angles to each other. I could hear a definate improvement in bass response, and no hum after this, which would indicate that I was getting more inductance, and still having some hum cancellation. The results were so obvious, that I didn't really feel the need to measure it. Here's the original post that started the discussion: Tube DIY Asylum: Cheap iron parafeed preamp by twystd
and the link below gives the explanation of why it works.

twystd
Click to expand...

I believe Magnequest recommend that their parafeed chokes be mounted on insulating standoffs.
 
mogliaa

mogliaa

Member
Joined 2009
bartola.co.uk/valves/
If you enjoy the sound of a 26 (like me), you may be interested in looking at how this valve performs from a purely linearity perspective (not how it sounds between various brand types, etc.). I have a stock of about 40 valves of 12 brands and mainly used and ST, albeit some NOS and globe as well. So statistical results here must be taken with a pinch of salt:

26 THD analysis | Bartola Valves

Cheers,
Ale
 
mogliaa

mogliaa

Member
Joined 2009
bartola.co.uk/valves/
Can't remember any significant difference. I do like RCA and Sylvania. Globe provide a warmer tone as they have more H2 presence. Difficult to find good CX326/226 that are in good shape at a reasonable price these days. I found an audible difference between OT and gyrator loaded. Want to do a proper listening test but that would imply building another 26 pre to do a quick switch between the two :)
 
andyjevans

andyjevans

Member
Joined 2003
artsandmedia.com
harrylime6 said:
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.
Click to expand...

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
 
mogliaa

mogliaa

Member
Joined 2009
bartola.co.uk/valves/
Jeffrey Davison said:
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
Click to expand...

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...
 
mogliaa

mogliaa

Member
Joined 2009
bartola.co.uk/valves/
albertli said:
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 ??
Click to expand...

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.
 
andyjevans

andyjevans

Member
Joined 2003
artsandmedia.com
mogliaa said:
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 ..
Click to expand...

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:
mogliaa

mogliaa

Member
Joined 2009
bartola.co.uk/valves/
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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