NYD one bottle mic preamp questions

[Toxophilite]

Thinking of building a couple of these preamps inside my current project.




I'm curious as to the role of 220K 2 watt resistor to ground on the B+.
 I know the 22uf electrolytic is likely a filter, what does the 220K 2watt resistor do?


Also could I get away with using a higher  value cap on the  plate of v1b /output ?
I can't seem to find one with high enough voltage as the plate could have pretty high voltage on it
Or could I use a 3.3 450 volt electrolyic ?

[WimWalther]

The 220K serves two potential purposes. 1) It's a safety bleeder for the HV cap(s) and 2) It preloads the power supply, slightly improving stability / regulation.

It's specified as 2W as it runs continuously at 400mW and they don't want it getting too hot.

[tdvt]

I can't seem to find one with high enough voltage as the plate could have pretty high voltage on it

AES has a couple high voltage non-polarized 3.3uF

https://www.tubesandmore.com/products/capacitors?filters=3109a3149c112a3109

[Toxophilite]

Found some solens that cost an arm and a leg.


Which begs two questions


Why is the output cap so large (3.3uf) when the decoupling cap between the stages is comparatively small (.02uf)


- I happen to have some high voltage .22 orange drops needing a new home. Could they be used in the output application?


- And is there any advantage in increasing the value of the decoupling cap between the stages?


Thanks!

[sluckey]

Found some solens that cost an arm and a leg.

The one tdvt linked only cost $1.29.

[Toxophilite]

True enough but they are in another country. International shipping hasn't gone down in price recently.
I wasn't ignoring the link and I appreciated the information. I was just picking up something locally to try this out.


Thanks very much


Which begs two questionsWhy is the output cap so large (3.3uf) when the decoupling cap between the stages is comparatively small (.02uf)- I happen to have some high voltage .22 orange drops needing a new home. Could they be used in the output application?- And is there any advantage in increasing the value of the decoupling cap between the stagesThanks!

[PRR]

> Why is the output cap so large (3.3uf) when the decoupling cap between the stages is comparatively small (.02uf)

The cap must suit the lowest frequency and the LOAD.

The interstage load is 1Meg, 1,000,000 Ohms.

The box-output load is not defined (and you have not discussed it), but will rarely exceed 100k and these days is often the 10k (10,000 Ohms) of an "interface". It is rational for the output cap to be much larger than the interstage.

A minor point: the interstage is INside a NFB loop which will extend the response even more.

[sluckey]

True enough but they are in another country. International shipping hasn't gone down in price recently.

You're what? 50 miles from the border. Drive down. Surely there are ways.

[shooter]

 

[Toxophilite]

True enough but they are in another country. International shipping hasn't gone down in price recently.

You're what? 50 miles from the border. Drive down. Surely there are ways.

S. Luckey and Shooter let me just check the price of fuel and time and get back to you on the cost effectiveness of this great new idea. 

Thanks PRR for answering my questions. I appreciate it and it helps me understand more!

[shooter]

Tox, my laugh was aimed WAY higher than you. you, like the rest are a "casualty" of why I laughed at "the bigger picture"

the border. Drive down. Surely there are ways.

[WimWalther]

@Toxophile

That 3.3uF cap anticipates driving a load as low as 5-10K or so.. (it's -3dB at 10Hz into 5K). It's the same value output cap you see in many tube linestages.

If the load you need to drive is another tube stage,  with a Zin of 100K or so, you can cut the value to 0.47uF (470nF) or 1uF. Either should be easier to source than a decent audio quality 3.3uF.

Also, you can up the value of that 220K across the output to 1M without any issues, making the load even easier (i.e. deeper into the bass) for a 0.47uF cap.