seeking: tube for a SEP parallel feed like an EL84, but with a lower Ra

[aletheian]

I think the title says it all. I want to tinker with parafeed, and I like EL84's, but the 40K-ish Ra is murder! I'd need a 300H or so choke to get 20Hz out of it.

I don't need much power, and the B+ can be whatever it needs to be, but I'd like to keep the bias current around 50mA, so larger pentodes are out of the question... I mean, I guess I could run an el34 at 400v and 50mA, and then have the lower 15K Ra and a similar sound...

Anyway, any suggestions?

[mach1]

If you use local feedback from the plate of the EL84 to the plate of the driver tube (for an example see RH84 posts) the effective plate resistance drops in proportion to the EL84 transconductance (s) and feedback ratio (n).

if s = 10,000 and n=0.15, then Rp (effective) = 100/0.15 = 666R.

In Mr Kitic's circuit n is approx 0.1, so Rp(effective) = 1000R

Nice figures for a parafeed choke n'est ce pas?

regards

[aletheian]

that's a great idea... I'll have to look into it. Thanx.

[Tubes4e4]

Hi aletheian,

if you want to stay with noval socketed tubes, EL86/6CW5 or their 300mA heater variants PL84 (no typo) / 15CW5 may be of interest.

Philips made a commercial SEPP OTL amp with them, the circuit can be found in the Philips EL86 data sheet at frank.pocnet.net, IIRC.

Tom

[Shoog]

"If you use local feedback from the plate of the EL84 to the plate of the driver tube (for an example see RH84 posts) the effective plate resistance drops in proportion to the EL84 transconductance (s) and feedback ratio (n).

if s = 10,000 and n=0.15, then Rp (effective) = 100/0.15 = 666R.

In Mr Kitic's circuit n is approx 0.1, so Rp(effective) = 1000R

Nice figures for a parafeed choke n'est ce pas?

regards"

You need to be careful using this approach. By lowering the resistance of the Pentode you up the requirement for the parafeed cap. 4uf caps of good quality are relatively easy to find, but good quality caps at 100uf and 1000V are not.
If you ignore this issue you can expect a peak in the low to mid frequency response, and a fall of in responsew below the peak - not good.

Shoog

[rdf]

Depending on how you feel about sand an IXYS chip might work well here too. If nothing else it permits playing with the concept for little money.

[tubelab.com]

I have done the chokeless parafeed thing using the IXYS IC's and it works well. There are however a few drawbacks. Since there is no choke the power supply voltage must be raised to supply the headroom. The B+ voltage must be higher than the highest expected positive excursion at the plate of the output tube. For a 2 watt amplifier using a 45 tube I needed 500 volts. This is about the most that you can put through the IXYS chip. The chip will dissipate about 10 watts to do this and it will require a heat sink. The tab on the chip is connected to B+, this makes it almost impossible to insulate it from the heat sink, and you will use a lot of bad language when you touch it!

If you get past all of these drawbacks, it does sound sweet, even with a cheap OPT.

There has been a discussion of using an active load, including some novel CCS concepts proposed in another thread. I am going to experiment with some of these ideas as soon as I have my lab back up.

Active load output stage

[aletheian]

Those are all cool ideas, but before I get all 'fancy,' I think I'll try the 'ol choke plate load first... just to start at the beginning. I may try both the NFB/EL84 thing, and the EL34 thing. I think I have an EL86 around somewhere too, so maybe I'll kick that around to see how it sounds. I have a sinking feeling that I'll be hearing more of the choke quality in the output than the tube's tonality, but I guess I'll see.

[aletheian]

Are there any other feedback techniques to lower the output tube's effective internal resistance other than feeding it to the driver? i suppose that would work well, but I was planning on using a 12ax7 or 12at7 there, and that FB worries me. I could easliy use a different driver triode... i just usually experiment with the ones that I can pick up down the street since many meet their untimely demises during the tweaking stage.

[kevinkr]

How much power do you actually need? If it's really only a couple of watts or less you could run the EL84 in triode connection and Rp under those conditions would be around 2K or perhaps less. Triode connected EL84's sound pretty good imho..

Kevin