#26 pre amp

[ncc]

Hi Rod,

I like your idea very much. Do you think it's OK if instead of using 3 x 30F5 which is difficult to obtain, we could use two 8CG7 (cheap on ebay) with heater in parallel (heater is 8.4V and 450ma) and a little resistor to make up for another 0.6V (or no resistor at all) and use one of the 8CG7 as cathode follower for each #26.

[mogliaa]

Hi Rod,
That's a brilliant idea. I remember now a similar circuit in a tiny headphone amplifier where the valve filament was fed from the output source follower.

Any particular reason why you chose the Mazda valve? Found a set of valves in eBay which I'm bidding for but they are not easy to get hold of. May need to look for some Russian pentodes instead!

I think this is a circuit variance which is very easy to implement, can try and listen to the results
I may adapt this one for my 01a preamp which I have currently breadboarded and will probably need only one valve per channel, so is dead easy to implement.

Would you run a cascoded MOSFET CCS (eg dn2540) or a simple power MOSFET (eg IRF840) instead?

Thanks
Ale

[Rod Coleman]

Originally Posted by ncc

Hi Rod,

I like your idea very much. Do you think it's OK if instead of using 3 x 30F5 which is difficult to obtain, we could use two 8CG7 (cheap on ebay) with heater in parallel (heater is 8.4V and 450ma) and a little resistor to make up for another 0.6V (or no resistor at all) and use one of the 8CG7 as cathode follower for each #26.

The 8CG7 could be used, but it is a little short of optimum. The gm is only 2.6mA/V on these, and as a 6SN7 type structure, they prefer 300V rather than the 150V we probably have in a 26 line stage.

Russian pentodes, triode-connected are a good bet. The 6Ж5П (6J5P) is rated for 150V operation and 9mA/V in pentode, which should mean over 10mA/V triode, and the heater is 6.3V 300mA.

you could also do the job with a single bottle:
The Russian 6H6П (6N6P) is a twin triode with 6.3V 900mA heater, and slope of 11mA/V.
Ebay seller boreca-75 and others have these cheaply. These should give excellent performance.

Please remember to use a grid-stopper though, starting at 10K, maybe going even higher.

[Rod Coleman]

Originally Posted by mogliaa

Hi Rod,
That's a brilliant idea. I remember now a similar circuit in a tiny headphone amplifier where the valve filament was fed from the output source follower.

Any particular reason why you chose the Mazda valve? Found a set of valves in eBay which I'm bidding for but they are not easy to get hold of. May need to look for some Russian pentodes instead!

I think this is a circuit variance which is very easy to implement, can try and listen to the results
I may adapt this one for my 01a preamp which I have currently breadboarded and will probably need only one valve per channel, so is dead easy to implement.

Would you run a cascoded MOSFET CCS (eg dn2540) or a simple power MOSFET (eg IRF840) instead?

Thanks
Ale

Hi Ale, The Russian 6H6П (6N6P) twin triode I mentioned above, is probably a better solution, and only one bottle is needed - now that I have taken a second look. Lots of valves will work in this position, but they will mostly be 1950s designed TV valves, intended for 150 to 180V operation, and with gm above 7mA/V

For the CCS you'll need 40-45mA for typical pentode (x3) or 60mA for both halves of a 6H6П. The cathode followers will operate with 130V to 160V (according to chosen type) across them. If we allow 30 to 50V across the CCS, then the dissipated power in the transistors will total 1.5W, which is no problem. I would start with the NPN bipolar CCS (with LEDs) that you drew earlier, and use a nice 250V NPN in a power package - TIP47, MJE340 etc. A tiny heatsink will suffice.

[mogliaa]

Hi Rod,
I have at hand the 6Ж51П (6J51P) which I can easily drop in my breadboarded 01a preamp. This has a 300mA heater so can easily replace the filament resistor for the 6Ж51П heater. Also dropping the bias to 6.3V should work nicely in my 01A setup.

I will look at the 26 preamp with the 6H6П (6N6P) as well!
Thanks
Ale

[mogliaa]

Hi Rod,
Quick update to my 01a preamp attached. I know the MOSFET gyrator is not optimal for a low anode current, should change it to Anatoly's when I get the chance as per my previous diagram. Anyway, this is a quick setup I can test easily.

Looked at SMD bipolars as was thinking to get a very small PCB board including SMD LEDs here.

I hope 15mA of idle current at the CCS should work fine in this case?

Thanks
Ale

[Vinylsavor]

Hi!

Since filament bias is used a lot lately, I decided to start a series of articles about his biasing scheme. The first one can be found here:

VinylSavor: Filament Bias, Part 1: Concept

More will follow. Hope you enjoy it. Keep those 26s glowing!

Best regards

Thomas

[andyjevans]

Hi,

For those not yet acquainted with Thomas' work on filament bias, we owe its modern usage to him! He started us all off on this, and very welcome it has been. So it's good to see these pages now - keep them coming!

We really owe you a thousand thanks!

Andy

[mogliaa]

Rod,
Looking for some help here. Attached is the simulation of the 26 DHT with mosfet source follower as discussed. Frequency response is better than a 26 with a 300H choke. However, distortion is slightly worse 0.07% against 0.05%. This is due to the gyrator which I can't set properly due to the HT level I guess. I think if I were able to increase the HT above 200V should be able to provide headroom to the PNP to enable bias at 170V and 6mA.

Cheers,
Ale

[mogliaa]

Andy,
Here is a quick sample of the impact of filament starvation on the DHT anode resistance. I have traced the 26 DHT with 1A and 700mA. You can see that the anode resistance increased from 8K to about 11K

A higher anode resistance is a clear drawback if you want to starve filaments with a choke / gyrator load to drive the cables and amp's input capacitance directly

cheers,
Ale