#26 pre amp

[kevinkr]

Originally Posted by CFT

Great, thanks for the info Kevin : )

Can I eliminate the cathode by-pass e-cap anyway since it's a constant current that flows throught the cathode resistor? and can I connect the parafeed cap at the cathode resistor rather than the ground?

And, just one more question, I noticed some filament filter circuits that employ a common mode choke between cathode and regulator, is this something helpful for further removing hum? Any recommendation there?

Thanks so much!

CFT

Unfortunately although you are operating in constant current mode removing that bypass capacitor will result in a much higher rp which will result in rather poor HF performance.

I would recommend trying battery bias as previously discussed using RC coupling in the grid circuit to inject the bias voltage - this would allow you to remove both the cathode bias resistor and the bypass capacitor.

The parafeed cap can be connected to the cathode connection in any of the situations described above, however the cathode bypass cap is required for the reasons mentioned.

Incidentally the cathode bias wants to set your operating point as well as your CCS, in this case you should allow the CCS to do it. If you do not want to use the battery bias mentioned something like 5 red leds in series in place of the cathode bias resistor & cap should make for an audible improvement. You can try connecting the parafeed cap between the leds and the filament. (Try both there and normal circuit ground - you may or may not prefer one over the other.

Hope this all helps.

[coolzero]

Originally Posted by Curly Woods

Some tubes do sound their best when run at their "upper edge", but given the cost of these tubes anymore, it raises the cost as they will surely not make it as long. If you are willing to pay the cost of more frequent tube changes, go with what moves you. I try to find a happy medium.

Hi,

How long the tube will survive if I continue the same op points? At least for an hour daily, I am using this Pre nowadays. I don't mind of changing tubes every 2 years and having the same sound which I like.

[kevinkr]

Originally Posted by coolzero

Hi,

How long the tube will survive if I continue the same op points? At least for an hour daily, I am using this Pre nowadays. I don't mind of changing tubes every 2 years and having the same sound which I like.

Probably OK if you have a target life span of a couple of thousand hours or less.

[CFT]

Hi Kevin,

Just saw your post on Tubes to transistors : )

Could you share your thoughts or experience on using something like a FW B1 buffer placed directly at the coupling output cap? Given that the buffer is said to be quite neutral, would you think that the output transformer can be replaced?

Thanks!

[coolzero]

Originally Posted by kevinkr

Hi Kanishka,
Try it at 150V and 6 - 7mA and let us know what you think sonically. I think this is an ok compromise between sound quality and service life.

Hi Kevin,

Tried 150v/6ma and sound is pretty impressive. I think I can settle down with this as well. Bass response is slightly inferior but overall sound is perfectly fine. Thanks.

[kevinkr]

Originally Posted by CFT

Hi Kevin,

Just saw your post on Tubes to transistors : )

Could you share your thoughts or experience on using something like a FW B1 buffer placed directly at the coupling output cap? Given that the buffer is said to be quite neutral, would you think that the output transformer can be replaced?

Thanks!

I've never heard a B1, but the word on the street is that it is quite neutral - if you have one on hand it would be a worthwhile experiment. I am not sure what the input impedance of a B1 might be, but the higher the better from the perspective of cap size, and load on the 26..

[andyjevans]

Kevin refers to red LEDs, and I just tried those in place of a cathode resistor. Initially I was excited - very clear and dynamic. But there was an edge to the sound I couldn't live with, and I took them out and put in a wirewound cathode resistor. I found that fine - the smoothness returned and sound was only marginally less clear and dynamic.

I'm using cap out, and I didn't find any loss of treble without a bypass cap so I left it out, so with cap coupling that's a good option. With transformer OPT, just as Kevin says, you want to keep the Rp low.

As for operatint point, I've been using a 150v glow tube and that's really too low - I need two to give 210v or 265v depending on the tubes. So with 3mA through the 26, the sound really is a bit thin. So will need to up the HT and the current as said in previuous posts. Looks like the 26 likes to be run hard - or at least quite hard!

andy

[coolzero]

Originally Posted by andyjevans

Kevin refers to red LEDs, and I just tried those in place of a cathode resistor. Initially I was excited - very clear and dynamic. But there was an edge to the sound I couldn't live with, and I took them out and put in a wirewound cathode resistor. I found that fine - the smoothness returned and sound was only marginally less clear and dynamic.

I'm using cap out, and I didn't find any loss of treble without a bypass cap so I left it out, so with cap coupling that's a good option. With transformer OPT, just as Kevin says, you want to keep the Rp low.

As for operatint point, I've been using a 150v glow tube and that's really too low - I need two to give 210v or 265v depending on the tubes. So with 3mA through the 26, the sound really is a bit thin. So will need to up the HT and the current as said in previuous posts. Looks like the 26 likes to be run hard - or at least quite hard!

andy

Hi Andy,

Actually I tried all these biasing methods and settle down with battery bias as Kevin suggested because it gave the most natural sound out of all. It reduced the hum considerably too. About the op points; if you playing Rock music, run those 26 hard as you can. It will bring the band to your house . Also it will give life to vocal jazz too. This is what I observed so far.

[kevinkr]

Originally Posted by coolzero

Hi Kevin,

Tried 150v/6ma and sound is pretty impressive. I think I can settle down with this as well. Bass response is slightly inferior but overall sound is perfectly fine. Thanks.

This is where I run mine, and is the maximum plate current I can run without danger of saturating the UTC HA-133 plate to line transformers. It's pretty authoritative, particularly when I am not overtaxing my 8Wpc 300B SE amplifier on loud, low pitch music. The new GM70 amp should resolve that issue if I ever get it done.

[andyjevans]

Hi Kevin,

Seems like you have some nice amplification there! What speakers are you using, and how did you make that choice? Interested to know!

andy