Plate or anode chokes

[CJ]

why? saves space

 

[joaquins]

why? saves space

I wasn't talking about that, I agree with you an active circuit could do the job, I just don't think a gyrator is the way to go, I'd rather go for a current source or a SRPP with lower complexity and better frequency response. The second one have the advantage of a much lower output impedance. If you don't want to expend another triode in it SS is possible as well. In fact the choke there is trying to emulate a current source, the advantage of the iron and wire is a higher voltage swing than the PS but we could get 50 or 100 extra volts and call it a day.

JS

 

[johnheath]

Thank you CJ and JS - I am settled with the "old" idea of SRPP last stage.

The idea with the plate choke  came up after looking at the V76 schematic and after questioning about it here I got the info that it could increase the voltage swing and therefore increase the overall gain but that was not something I wished for and all together with the noise problems with the chokes …. no, nothing for my budget right now to get shielded plate chokes just to test (I have already spent some money on the two I have) .

I did however build a clone of the V76's first stage with the EF86 and as you said CJ…it was a very nice input.

I am curious about the total gain though… I need som help here

Thanks

/John

 

[moamps]

I did however build a clone of the V76's first stage with the EF86
I am curious about the total gain though… I need som help here

Look here http://www.r-type.org/articles/art-010n.htm for pentode gain equations.

There are some errors in your design/schematic; chassis ground shorts HT heater bias, NFB RCR network doesn't have now any sense (except maybe partial PS hum canceling  )

 

[johnheath]

Thanks for the link

The NFB is actually a copy of the original except that instead of a rotary switch with the added total resistance of 30k I just replaced it with a 30k resistor. I tried to have a 25k pot to see what would happen in the first attempt and that was also without the gain pot... nothing really happened when turning the (rotary) pot so that is why I added a gain pot and replaced the rotary switch with a resistor.

Please tell me how to get the heater bias to the heater supply. It is all DC...12,6VDC and 6,3VDC

Regards

John

 

[moamps]

The NFB is actually a copy of the original except that instead of a rotary switch with the added total resistance of 30k I just replaced it with a 30k resistor. I tried to have a 25k pot to see what would happen in the first attempt and that was also without the gain pot... nothing really happened when turning the (rotary) pot ...

That happen because NFB network is now connected to +HT. Try it once again but connect it to the first anode of ECC82.

Please tell me how to get the heater bias to the heater supply. It is all DC...12,6VDC and 6,3VDC

You don't need it at all. ECC 82 will withstand Vkh of 130V. 

 

[johnheath]

Ok - thanks I will surely have a look at the NFB

And I also know about the 12AU7 and its rated peak heater to cathode voltage but the circuit is also a rest from a similar topolgy using a 5687 tube . So there is a big chance that I might use a similar tube in the future ... so if you like you could tell me about the heater elevation when using DC on the heaters?

Regards

/John

 

[emrr]

http://groupdiy.com/index.php?topic=25185.msg299760#msg299760

 

[vinyvamos]

Digging up an old thread, but in belief that my question fits best here . It is in relation to what PRR stated about calculating choke inductance...

2*Pi*Freq*H = choke impedance.

You need to pick an impedance. 12AX7 is often loaded with 100K resistor, so we will aim there.

You need to pick a frequency. Pick 20Hz.

There's a way to work the formula backward. But I'm lazy and just throw numbers at it.

Pick 100 Henries.

2*Pi*20Hz*100H = 12,566 Ohms impedance.

Not big enough by a long shot.

12AX7 internal plate resistance is 60K.

50Hz may be a useful low frequency.

2*Pi*50Hz*100H = 31,426 Ohms impedance, 100H at 50Hz

Say you can find 250H.

2*Pi*50Hz*250H = 78,540 Ohms impedance, 250H at 50Hz

Some further thumb-math leads to 60K impedance, 250H, at 38Hz.

Bass response will fall-off below that point.

So in a voltage-amplifier we sure could need hundreds of Henries.

Right now I am messing around with an output stage based on an RCA OP-6, and I am trying to calculate the various parameters around it. The thing is, when I am picking 20Hz as my roll-off frequency for the choke, I feel that this is too high. And even though the level might be sufficient (-3dB?) the phase error may be nasty for any kind of high fidelity recording situation. I have tried to find a rule of thumb for this, but I have so far only found the other end of the spectrum in Audiophile data related to SE parafeed power amps, where at Turner Audio he states a roll-off freq. of 3Hz...

I would also like to hear people's thoughts on my particular case, as I am working with a 6SJ7 running at a B+ of 300V. One datasheet has told me an internal anode impedance of about 8K ohm at 250V, but I really don't know if this is the value I should be taking into these calculations?? I will soon be trying a Hammond 156C (150H, 3K7 dcR) choke but would love to do the math on it to back up the measurements made

 

[PRR]

> I feel that this is too high.

Pick whatEVER numbers you like.

But with commodity chokes (there's no market for super-chokes) and wimpy tubes like 6J7, you can't get all that you can want.

If you want "low phase error", use transistors!! Why would you accept the ~~1%-5% THD of a tube, then choke on a 20 degree "phase error". Especially in bass...

 

[CJ]

here is a thread for that preamp>

https://groupdiy.com/index.php?topic=50018.20

that 1620 (6SJ7)  is wired with the screen to the pwr supply so you need to up your plate resistance. data sheet says Plate R 100 K.

200 Henries is mentioned, but at what level and frequency?

somebody used a Hammond choke, 150 H mentioned in a link in the above link but had rolloff at 10 K, probably from the choke having too much C, so you need a hi fi choke, 250 H to 400 H would be nice, if used for vocals you could go lower unless you are recording Billy Gibbons going " a how how how how"