26 Driver Tube for 45 - Operating Points

[eeyore]

Am currently spec'ing a interstage coupled 45 amp driven with a 26. I am aware of the hum and microphonic issues with the 26, however, I am keen in pursing a dht driving dht amp. Just wondering what are the some suggestions as to the operating point of the 26, and the method of bias that would be best. The 45 is operating at 50mA/250V (plate to filament, B+ of 300V)/-50V bias/5K load. Will be using a Tentlabs Autobias module along with the Tentlabs Filament supply.

The 26 will have a Tentlabs 1A low noise filament supply and coupled to the 45 via a 1:1 interstage. It looks like a 5K:5K unit, but might go for a custom impedance unit if alternative can be provided.

I'd assume that the IT will provide an almost equivalent choke loading of the 26.

Questions:

1. Can a 9V bias using a 9V rechargeable battery be good, or an alternate method of cathode bias?
2. What is a good plate voltage for the 26?

Thanks again!

[Miles Prower]

Originally posted by eeyore
Am currently spec'ing a interstage coupled 45 amp driven with a 26. I am aware of the hum and microphonic issues with the 26, however, I am keen in pursing a dht driving dht amp. Just wondering what are the some suggestions as to the operating point of the 26, and the method of bias that would be best. The 45 is operating at 50mA/250V (plate to filament, B+ of 300V)/-50V bias/5K load. Will be using a Tentlabs Autobias module along with the Tentlabs Filament supply.

Not too sure how good a #26 is in that application. The #26 has a rather high r(p) which means that you'll need more L in the primary for good low frequency operation. Best to run it hot. Here's a suggestion from the spec sheet:

Vpk= 180Vdc
Vgk= -14.5Vdc
Ipq= 6.2mA
r(p)= 7K3 (Problematic: more modern types can do way better with similar u-factors.)

A 12B4 would be a helluvalot better: r(p)= 1K03

I'd assume that the IT will provide an almost equivalent choke loading of the 26.

So long as you keep the 45 out of Class *2 operation, the secondary disappears for all practical purposes.

Questions:

1. Can a 9V bias using a 9V rechargeable battery be good, or an alternate method of cathode bias?
2. What is a good plate voltage for the 26?

1) No, you'll need more than 9.0Vdc. Your best bet is fixed bias for the #26. Since you're using an IT, cathode bias becomes problematic indeed. A cathode resistor bypass capacitor means capacitive currents in the cathode circuit, which means capacitive currents in the plate circuit, and that can cause resonances with the primary inductance. This will cause some nasty distortion. If you leave the cathode resistor unbypassed, that'll raise the effective r(p) and ruin your low frequency performance (gonna be difficult as it is, given the r(p)) besides, this puppy doesn't have the gain to spare.

Your best bet is fixed bias for the #26, with the cathode returned directly to ground.

[eeyore]

My original intent was to us a 417A/5842 to drive the 45, however, was wanting to try something different and use a DHT instead. Are there alternate DHT that are more suitable? From searching, most low Rp DHT are the power tubes like 300B, 2A3, etc.

I do have some 12B4A so not adverse to this tube, what do you think would be a good operating point.

Doing a search for other single triode low Rp tubes; 6CK4, PC25, VR40. Higher mu tubes come up as CV5112, 3A/167M, E288CC, 8233, 7044, 7370, 8456 and 6BX7.

The other ones I have also considered at D3a, C3g/C3m, 6C45pi, etc.

What alternatives exist?

[kmaier]

Recheck your operating points on the 45... 250V with -50V grid bias is about 34ma by the book. Also, don't exceed 36ma max on the cathode and/or 10-watts on the plate. Usable tube life will decrease quite a bit otherwise.

Regards, KM

[45]

If you want to work in Class A1 then the 26 is ok and, IMO, much better than the options you listed above..... the tube is microphonic only if you fix it directly to the chassis, otherwise it is fine.

I would go for 275V/33-34 mA with 5K load for the 45.

Having approx. 270V for the 26 you could bias with 1.8K cathode resistor, 22K anode resistor. In this way the tube should work at 135V/5.5mA with -10V bias.

Then if you want to improve and get also some more power output you could bias the 45 with a direct coupled 6SN7 cathode follower (or mosfet source follower).

Cheers,
45

[45]

P.S.
In case you decide to go for cathode follower driver I would use it also for the 26.... .... better, a JFET source follower.
This because you would be close to some grid current for the 26, too.
In this way you should also get few more volts for the anode supply.

[45]

Actually, if you self-bias the 45 you will need 275+56 = 326V and this would mean a better supply for the 26.
In that case, using a 2.2K cathode resistor and 22K anode resistor, the 26 would work at approx. 180V/6mA. Goal!!

In this case you should get approx. 2W, while with the previous solution 1.5W only.

Cheers,
45

[andyjevans]

I've used as drivers 12B4, 6CK4, 6AH4, 1626 etc etc and nothing is as good as a nice DHT like the 26. I've used that with a 1:1 interstage. The LL1660 is one possibility. As said, if you have the voltage, go for somewhere near 180v. Not over that.

Alternatives are 12A - similar operating points and less greedy on the filament, but 26 sounds better. 01A is a very nice sound but max plate voltage here is 135v and quoted as 90v for some of the older globes. 10Y is another very good choice. All these are excellent, and have about a mu of around 9. 71A and 31 sound nice but lower gain. 1J6g is mu of 15 and sounds good - that's a dual triode, common cathode. I use it in PP - never tried the sections in parallel, but could be good.

I see nothing wrong with going ahead with the 26. Others have used it with excellent results. It's a great tube. I used cathode bias with a polypropylene bypass (wouldn't use an electrolytic).

andy

[jimmyd53]

You could get really wild and build Gary Pimm's All C4S Loaded Parafeed 45 which uses the 26 as a driver. http://www.pacifier.com/~gpimm/45_all_active.gif
James

[eeyore]

Thanks for the input. Please see attached DRAFT schematic. I am pretty sure I got some of the numbers and values wrong, but would appreciate any feedback and comments.

The PSU now supplies around 300V at up to 150mA for both stereo channels.