#26 pre amp

[Original Burnedfingers]

If I understand the VR circuit correctly you cannot parallel a large value cap to ground after the VR or you turn it into a pulse relaxation light. Cap value must be no more than .1 in value.

Do I stay with the one CCS before the VR tubes and then feed a choke and then the #26 plate?

[regal]

Originally Posted by Original Burnedfingers

If I understand the VR circuit correctly you cannot parallel a large value cap to ground after the VR or you turn it into a pulse relaxation light. Cap value must be no more than .1 in value.

Do I stay with the one CCS before the VR tubes and then feed a choke and then the #26 plate?

Check the mentioned ecp audio project, I am sure he had to use an electrolytic to kill the hiss from the vr tube shunt and he doesn't use electrolytics unless he has to. Much of what you are dealing with is in that write-up.

[Original Burnedfingers]

In the article he mentioned he used two (2K) 20 watt power resistors in series with a 100 mfd cap between them

[iko]

VR tubes cannot be used with large caps. 100nF max for most of them. I use a VR shunt reg in my 26 preamp currently and at very high volume there is a hiss, which I didn't have when I used my mosfet shunt regulator. But at the time the heat dissipated by the mosfet shunt reg bothered me.

Implement the Salas shunt reg if you want good performance and you're fine with the dissipated heat. As regal pointed out, there is/might be a GB going on for pcbs.

[iko]

But hey, you'll never know what we're saying if you don't try things. If you already got the VR tubes, it'll be an easy and cheap learning experience to plug them in your circuit. If you want to use a CCS before the VR tube, set it to about 40mA more than your load wants. If you will feed both channels from the same VR tube then you can set the CCS to about 55mA. You can then either load the 26 with a choke or with a CCS set to 5-7mA, depending on how you bias the 26.

[Original Burnedfingers]

right now I am gathering parts for two mono block power HV supplies and two mono block filament supplies.

I have Rod's boards finished and ready to use. I just ordered two cascode CCS boards.

I had plans to use a CCS before the VR tubes to eliminate the ripple. I haven't made my mind up yet if I want to go with a choke load on my 26 or go with a CCS there also.

Didn't plan on the Salas shunt regulator because I like the look of the VR tubes. I have plenty of the VR tubes and I have the chokes.

I also have the transformers for both the dual filament supplies and the two HV supplies. What I am missing and I wonder if I need them are the Hammond 157G chokes

[zigzagflux]

Originally Posted by Original Burnedfingers

Ok, so if I use a power supply with some ripple and I run it thru a cascoded DN2450 CCS and then a VR shunt regulator stage I should come out with a ripple of 1mv or so. At this point would I not be good to go for a choke loaded #26. Would I be better off to ditch the choke and go thru another CCS and then the #26's plate?

It says I could have 100 volts of ripple and as long as the stayed above the 220v the CCS would reject all the ripple. They are saying they want to maintain 200 volts on the output.

So if I used a CCS before my VR tubes and a CCS to feed the plate of the #26 I would have 0 ripple at the plate of the #26? Is this correct?

This is exactly correct. I have a total of three stages of gain fed with a cascoded DN2540 CCS into VR tubes, with very very little noise on the output. You can have your ear touching the tweeter and just barely hear some 'water flowing'. Not at all annoying, and you don't hear a thing from your sitting position. I have had such good success with it, I use it all the time. I can measure better ripple reduction with the cascoded version over the single device, so go cascode.

Noise floor of my 12B4 preamp attached. This tube has gained somewhat of a reputation for being noisy, yet I have a really low floor. What's not to like? 60 and 120 Hz is non-existent. The little spikes you see beyond 150 Hz are actually due to the tube used, not the power supply. Different tubes will show different little spikes (some are really quiet).

It's also nice to dissipate some heat above the chassis, instead of always having to deal with it under chassis.

[albertli]

Originally Posted by andyjevans

http://frank.yueksel.org/sheets/127/2/26.pdf

There are the curves for the 26. 145v and 11.5v bias looks like 4.5mA. That's fine - well within the comfort zone since the 26 can tolerate up to 6mA. As you see from the curves, increasing the bias voltage does decrease the current at the same plate voltage. 13v bias would take you down to around 3ma. Whether these theoretical values from the datasheet work in practice is another matter, and might be affected by the condition of the tube.

Andy

Andy,

I can't reach the site attached. Would you post the curves/graph so I could learn a bit more. My 1660 is only 10mA and originally aim at the battery bias. I have all the resistors with me and want to go with the filament bias instead.
My 1660 10mA is a SE, not a pp. Does it make a lot of different.

Albert

[vgeorge]

The only difference between ll1660 PP and 10mA is the core gap.
The PP has a little bit higher inductance and can tolerate about 5-6mA.
You should be fine.

[albertli]

Originally Posted by vgeorge

The only difference between ll1660 PP and 10mA is the core gap.
The PP has a little bit higher inductance and can tolerate about 5-6mA.
You should be fine.

What I worry is I don't know how to calculate the current/mA with filament bias. My plan is to run about 170v at anode and 18v dc to feed the filament. Would this be on the range??