[adamus]

I want to make a simple preamp, so a common cathode triode should fit the bill.

In the parts bag i have: 210v tranformer (With 6.3 tap). 6n6p's, good signal caps. Ez81, a 40uf, 220uf caps.

Question is what operating point shall i run at?

reading around its suggested to go hot with 6n6p, 18ma looks reasonable.

how does this sound?

240V B+ ( i may go solid state to get this, the EZ81 will probably drop too many volts).

110 on the plate
Ra = 7.2k
cathod bias at -3v so looking at Rk of 170ohms. I culd even use a couple of 1.6v leds here.

The powersupply will need to be failry clean.

Any advice? This is my first from scratch circuit design, so spot the errors!

 

[nikongod]

output impedance with a common cathode is going to be pretty high. certainly high enough that long cable runs are out of the question. perhaps you could drive a transformer, but then your adding that, and from the sound of this project thus far a transformer worth putting in a preamp will be a budget buster.

If your system needs gain: I would look at SRPP, or hybrid tube/ss type stuff. you can probably afford to throw a little gain away to get lower output impedance...

If you dont need gain, but a buffered volume control: white cathode followers or simple cathode followers love you like a fat kid loves cake.

 

[dsavitsk]

Just to be disagreeable

Quote:

Originally Posted by nikongod /img/forum/go_quote.gif output impedance with a common cathode is going to be pretty high. certainly high enough that long cable runs are out of the question.

Not really. Rp is about 2K, which means that as long as the cables are kept reasonable (3m or less) there won't be an issue.

Quote:

Originally Posted by nikongod /img/forum/go_quote.gif If your system needs gain: I would look at SRPP

I wouldn't, though a cathode follower of mosfet follower is probably OK

Quote:

Originally Posted by nikongod /img/forum/go_quote.gif If you dont need gain, but a buffered volume control: white cathode followers or simple cathode followers love you like a fat kid loves cake.

Look at this (SYclotron Audio » The Heretical Preamp) if you want to do a cathode follower -- 6n6p is good for the design.

Now, for the original questions, a single LED should be sufficient for bias -- find one about 2V. I think I'd use the EZ81, get ~200V or so B+, and use a 8K load. Should run around 15mA. If you do use solid state rectification, move to a 10K load.

 

[adamus]

thanks dsavitsk. I'll give the link a run through.

as said, i dont need the gain, so a cathode follower could be an option. Reading around many people seem to think waste of time though.

my other options is just to use jisbos.

First look at the 'heretical linestage' is that its probably over budget. sowter transformers arent cheap, but are one of the few uk sources.

I could probably get away with a simple cathode follower, without the 10k input transformer. Maybe i could voltage double the 6.3 to get the -12 for the CSS (I dont see why its bipolar.... maybe it would be clear if i read on). It would bias the 6.3v but not by much, something i doubt would be noticed. I could also make things super simple by using a 'ring of two'. Morgan jones does warn of oscilliation, but i could try and scope it.

I'll also have to keep an eye on the old tranformer, it was out of a leak amplifier, and doesnt show any problems, but i dont know what the 6.3v rating is, it was powering about 6 tubes, but all small signal or radio + an ez80. I gues 800 + 300*5 = 2.5 amps. two 6n6ps and an EZ81 would give 2.6amps, so i should be able to get away with it.

 

[Siemens]

Hello ,
i will run it at Ub=300V Ua=100V Ia=10mA Ug1=2V for a Phase-Inverter (Kathodyn).
It sounds very good .
 
Greetings
Bernard

 

[Ant1Xr1st]

My friend's 6-chanell version of this buffer works good.