#26 pre amp

[bequerel]

Originally Posted by kevinkr

Refer to my old 26 construction project on Positive Feedback Online for more details if interested in this approach. Providing that the transformers are well shielded from magnetic fields you get lower noise, and a much lower output impedance at the expense of gain.


Those Arcturus 27s are cool looking aren't they..

Edit: WRT to the 27 I recommend using fixed bias with the cathode grounded as this reduces the likelihood that cathode insulation leakage will result in excessive hum particularly if AC is employed for heating. Unlike later types it is not a good idea to elevate the filament supplies more than a few volts above ground as the cathode to filament insulation may fail. Unfortunately this seems to be one of the most common failure mechanisms in these tubes. (Well it was the first idht - so what do you expect? )

Thanks for all the information, Kevin.
Your construction project on Positive Feeedback online was the reason for my 26 interest. I also have some tipped 01 tubes, but from what I have heard these are even more prone to hum etc. than the 26.

[andyjevans]

Going back to the 156C plate choke, yes probably sounds better than resistors but not as good as IXYS cascode plate load. I have a friend with a 26 preamp and those were his findings. I think ultimately he used a LL1660 transformer. Those plate chokes pick up hum anywhere - can't have anything magnetic near them. You can use 2 in series - still cheap!

andy

[coolzero]

Originally Posted by andyjevans

Going back to the 156C plate choke, yes probably sounds better than resistors but not as good as IXYS cascode plate load. I have a friend with a 26 preamp and those were his findings. I think ultimately he used a LL1660 transformer. Those plate chokes pick up hum anywhere - can't have anything magnetic near them. You can use 2 in series - still cheap!

andy

Hi Andy,

I had similer experience with IXCY's where the sound was slightly better than resistors. But unbalanced channels were too unbareable and couldn't find a way to fix it than using a macthing pair of tubes (probebly a balance controller would be an easy solution but didn't want to add anything more). Only I missed to check with my pre was plate chokes becuase I didn't have something suitable.

[andyjevans]

Hello again,

Don't know what the glitch was - spooky! Anyway I now have the 26 preamp working with four LEDs and a 156C plate choke, fed by a 01A voltage reg. giving me 130v on the plate and 6.8v on the cathode for 7mA. Sound is pretty exciting! Very clear and great rhythm - nice boogie factor! I'm enjoying it! My my, a 26 preamp repays all the work you put into it!

andy

[andyjevans]

Sorry - glow tube is a 0A2 not a 01A. Really enjoying this preamp - it's great on rock music, really "fast".

andy

[Vargas]

Nice job and glad to hear it sounds nice. I have 4 #26's under the bench and you have inspired me to build one myself once I get moved and settled. I also have a bunch of 27's but after reading about your success I think I will go with the 26's! Cheers

[coolzero]

Originally Posted by kevinkr

While the 27 is cool the 26 seems to be much more reliable and somewhat less microphonic if you can believe that! (Frankly they are both horribly microphonic) I would go straight to the 26 although the 27 is a lot of fun. Sadly almost all of the 27 line stages I have encountered are now "retired" - mouldering in closets somewhere because as good as they sound the tubes are just unreliable. My prototype 26 dht pre just keeps soldiering on year after year with minor mods here and there. I'm only on my second pair of 26's in 8 years of relatively steady use.

On coupling caps choose the smallest that places the -3dB corner well below the LF cutoff of your amplifier/speaker combo.. Generally I think 5Hz is low enough in most instances.. A 1uF output capacitor into 100K has a -3dB corner in the vicinity of 2Hz which IMO is more than low enough, perhaps a bit too low if you are playing vinyl.

You should also consider plate to line transformers, Electra-Print can make these and old UTCs work well too. Refer to my old 26 construction project on Positive Feedback Online for more details if interested in this approach. Providing that the transformers are well shielded from magnetic fields you get lower noise, and a much lower output impedance at the expense of gain.


Those Arcturus 27s are cool looking aren't they..

Edit: WRT to the 27 I recommend using fixed bias with the cathode grounded as this reduces the likelihood that cathode insulation leakage will result in excessive hum particularly if AC is employed for heating. Unlike later types it is not a good idea to elevate the filament supplies more than a few volts above ground as the cathode to filament insulation may fail. Unfortunately this seems to be one of the most common failure mechanisms in these tubes. (Well it was the first idht - so what do you expect? )

Kevin,

I changed the output cap/resistor like you mentioned and the result was phenomenon. More control on the bass and nice extended treble than heard before. Thanks.

[JandG]

Andy,
Can you post a schemetic of your 26 pre..?
Joe

[andyjevans]

Hi Joe,

I don't have a circuit written out, but I could try and produce one!

andy

[JandG]

That would greatly be appreciated. I have built 26 pre before & know to keep PS far away from signal chassis, so I wouldn't be bugging you on WHY it hums...*s*
I am on the down hill portion of a PSEP amp build that will be switchable via flick of a switch between integrated mode & pure amp. It is a 2 stage 5687-IT-807 with seperate PS's for everything including screens. I really don't build other pre's as I have heard what a 26 can do so time to build my one & only last one & a good one. I have the life time stash of 26 allready.
Joe