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#26 pre amp

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.. :D

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? :p )

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.
 
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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
 
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.
 
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
 
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.. :D

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? :p )

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.
 
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
 
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

Hi Joe,

I built mine (image/schematic at the beg. of the thread) in single chassis (signal+psu) with no hum issue what so ever.

Kanishka
 
Looking at the 220nF coupling cap at the output; isn’t that generally too small?
In normal (not DHT) linestages this is usually 2-4uF. Are there special considerations with DHTs?

A very nice build btw., coolzero.:cool:

I am confident that a well constructed 26 linestage is one of the best linestages money can buy... regardless of price.
I have not jumped on the DHT linestage wagon myself yet, but I have a lot of 26 tubes.
Think I will try a 27 linestage first. I have collected a small truckload of Arcturus Blue 27 (tipped).:cool:

I changed the output cap/resistor to 680nf/220K and best sound I heard out of this pre so far.
 
I have built your original posted signal schemetic exactly w/a glow tube regulated 150v. I like the 135v 5.5ma fixed version you show also. i am heavily intrested in active loading of the plate, at least to try it. easy enough to go back to tranny loading or resistor which still sounds excellent. I see you got away with PS in same chassis, this is something i have never been able to do, your layout is most likely much better than i ever did with single chassis try's. I will play it safe & go with 2 chassis. battery bias also works well with 26 w/ NiMH bieng what i used.
Joe
 
I have built your original posted signal schemetic exactly w/a glow tube regulated 150v. I like the 135v 5.5ma fixed version you show also. i am heavily intrested in active loading of the plate, at least to try it. easy enough to go back to tranny loading or resistor which still sounds excellent. I see you got away with PS in same chassis, this is something i have never been able to do, your layout is most likely much better than i ever did with single chassis try's. I will play it safe & go with 2 chassis. battery bias also works well with 26 w/ NiMH bieng what i used.
Joe

I tried active load with CCS as well. Sound was perfect but channel balance was an issue. Kevin advised me not to use NIMH claiming those sound worst. I used alkaline he had recommended which sounds great btw.
 
I'm getting interested in these 26 preamp designs! I'm just building another to try out some things. Two chassis - seems the most predictable way of eliminating hum. Instead of the 156C choke I'm going to try an IXYS active load. Need to do some calculations - need a bit more volts out of the glow tube than 150v, so maybe two 105v glow tubes to give me 210v. I'll start with four red LEDs for 6.8v on the cathode and see how that works out - shoot for 130v on the anode and 7mA as before.

andy
 
Next experiment - mixing LED bias and cathode resistor. Replaced 4 LEDs with one LED and a 1k cathode resistor. Doesn't seem to sound any worse. I'm trying to get rid of the glare of the LEDs and figured one might be better than 4. Will have to listen some more.

I also built a another preamp where I used IXYS active loads (cascode). Right now it has a pair of VT-67 (30 special) at 3mA, with four LEDs for cathode bias. 6.8v on cathode, 114 volts on the anode and 150v supply from the glow tube. Sounds pretty similar overall to the 26 - but slightly flatter and not quite so nuanced, so the 26 wins still, though it's more hungry on filaments! Will go on to try 26s in this new one and then 10Y.

Since I'm now using a separate power supply it's much quicker to build just the preamp chassis.

andy
 
Well - some conclusions. took out the red LEDs and went back to cathode bias with wirewound resistors. Best sound so far. Nice and clean, and the glare of the LEDs is gone. With a wirewound cathode resistor the VT-67 preamp sounds really good with its IXYS 10M45 cascode active load. Very nice! 114v on the plate, 2.4k cathode bias for 3mA. IXYS sense resistor is 1k. Fed by a 150v glow tube.

26 preamp now has 1.2k wirewound resistor for 7mA and still the 156C choke load, for 130v on plate, again fed by a 150v glow tube.

andy