#26 pre amp

[bequerel]

Originally Posted by andyjevans

Hi there! A 26 preamp is indeed the highest of high end. Hard to better!

I have one built which sounds pretty good - I use a glow tube to get 150v B+ and then a plate choke - the cheap Hammond 156C (150H). A resistor on the cathode right now, but will be replaced by something better.

However, a better solution is a cascode IXYS active plate load. I believe you have one already. Then I would try some red LEDs as cathode bias. Each is 1.7v, so say 130v plate, -8.5 (5 LEDs) for 6mA. I don't know what that would be as a sense resistor on the IXYS cascode - must try it and see.

For filaments I use a 22,000uf cap, a LM1084 chip as current source with a 1.25 sense resistor (min. 6w but better 12w). That's the "end bit" of a Ronan Reg. You can add a voltage reg before it but you may not need it. The current source is the important bit - better sound. You'll need a heat sink about 3" by 3" for the LM1084 - gets hot depending on the voltage going in.

andy

Andy,
What value is your coupling cap?

[kevinkr]

Originally Posted by bequerel

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.

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).

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? )

[andyjevans]

Hi,

I have a 0.1 FT-3 russian teflon cap. I'm going into a tube amp so not bothered with anything bigger.

andy

[andyjevans]

OK guys, in the interests of science I just did a blind test of a Holco cathode resistor bypassed with 47uF polypropylene and 4 x red LEDs. Changed one side but didn't know which until I checked after, then played a mono CD, listening to one side then the other.

The LEDs were much clearer, though ?? maybe a little more etched. The Holco combination was smoother, but a bit more muffled and not as exciting. So on the basis of this, I'm changing both channels to LEDs. I used four, for a bias voltage of 6.8v and I got 130v on the plate, corresponding to a current of 7.5mA. Should be just OK though if I use 5 LEDs the current should come down to 6mA which would be more comfortable. OK, so soldering iron out! Will report!

This is with a 156C Hammond plate choke coming off a 150v glow tube.

andy

[andyjevans]

Hmmmm. Loud hum from both channels with both sets of LEDs connected and no music! Back to the bench - what have I overlooked here.......

andy

[coldcathode]

Here is a 27 schematic I was playing around with.

[zigzagflux]

Looks like that last 150uF cap in the power supply is in parallel with the gas regulators. They don't really like that, maybe put a 100 ohm resistor in series with them; added bonus that you can measure current through them.

[coldcathode]

Originally Posted by zigzagflux

Looks like that last 150uF cap in the power supply is in parallel with the gas regulators. They don't really like that, maybe put a 100 ohm resistor in series with them; added bonus that you can measure current through them.

Zigzag,
Thanks I forgot about them not liking a cap at the input, I think I will put a 5 watt rheostat there then I can adjust the current taken up by the VR tubes to keep them happy.

I picked up an old Motorola radio chassis the other day in exchange for some work on a guys old Philco. Has LOTS of 5 pin sockets I think I might just bust out the soldering Iron and try this thing.

[coolzero]

Originally Posted by bequerel

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.

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).

Cap value was taken from the Jim D'Cort orginal design without a change. He had mentioned something in his site with regards to the maximum value of coupling cap. Pretty sure it was not that high as you mentioned. Didn't give much of an attention for that since I was hurried to listen to it

[coolzero]

Originally Posted by andyjevans

Hi there! A 26 preamp is indeed the highest of high end. Hard to better!

I have one built which sounds pretty good - I use a glow tube to get 150v B+ and then a plate choke - the cheap Hammond 156C (150H). A resistor on the cathode right now, but will be replaced by something better.

However, a better solution is a cascode IXYS active plate load. I believe you have one already. Then I would try some red LEDs as cathode bias. Each is 1.7v, so say 130v plate, -8.5 (5 LEDs) for 6mA. I don't know what that would be as a sense resistor on the IXYS cascode - must try it and see.

For filaments I use a 22,000uf cap, a LM1084 chip as current source with a 1.25 sense resistor (min. 6w but better 12w). That's the "end bit" of a Ronan Reg. You can add a voltage reg before it but you may not need it. The current source is the important bit - better sound. You'll need a heat sink about 3" by 3" for the LM1084 - gets hot depending on the voltage going in.

andy

Hi Andy,

How does the plate choke (H156C) sounds compared to tha normal plate resistor in place? Kevin recommended me to use MagneQuest but it is way above my budget. I have seen few articles about using 156C as plate chokes with good results.
I treid CCS as active plate loads but sound become uneven with unmatched tubes and didn't know how to fix it. So went back with plate resistors. I tried 3 stage filament regulator with LM317 (somethign similer to Ronan reg.) but the hum was too high. Best I had with the SMPS which I a musing now with no audible hum at all.
Alkiline battery bias worked for me very well and thinking of repalcing the batteries with small 9V PSU later.