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Re: Dieselpunk Preamp of Doom and the Beast with Two Backs

Posted: Mon Nov 11, 2019 2:03 pm
by Mike H
Thermionic Idler wrote:
Mon Nov 11, 2019 12:39 pm
Thanks Mike - might be worth a try. This bit is on the PCB so adding it could be a touch fiddly.
I'd just bung it across on the track side. :D

Re: Dieselpunk Preamp of Doom and the Beast with Two Backs

Posted: Tue Nov 12, 2019 11:07 am
by jack
What frequency is the oscillation at?

If we're over a few 100 kHz, then just using a suitable ferrite bead close to the first grid may well be enough. Won't effect AF at all but will kill most RF.

I use these: https://uk.farnell.com/wurth-elektronik ... dp/1635667

Never notice any audible difference from the insertion, but they kill RF. I've given some to other group members who have used them with success.

Re: Dieselpunk Preamp of Doom and the Beast with Two Backs

Posted: Tue Nov 12, 2019 1:05 pm
by Nick
What frequency is the oscillation at?
Already asked that question.

Re: Dieselpunk Preamp of Doom and the Beast with Two Backs

Posted: Tue Nov 12, 2019 1:38 pm
by Ant
116.2khz calculated, says it in a post on the last page

Re: Dieselpunk Preamp of Doom and the Beast with Two Backs

Posted: Tue Nov 12, 2019 2:01 pm
by Mike H
Forgot about ferrite beads. Good thought.

Re: Dieselpunk Preamp of Doom and the Beast with Two Backs

Posted: Tue Nov 12, 2019 2:32 pm
by Thermionic Idler
Mike H wrote:
Mon Nov 11, 2019 2:03 pm
Thermionic Idler wrote:
Mon Nov 11, 2019 12:39 pm
Thanks Mike - might be worth a try. This bit is on the PCB so adding it could be a touch fiddly.
I'd just bung it across on the track side. :D
Sadly it's a double sided PCB with insulated tracks... :(

Re: Dieselpunk Preamp of Doom and the Beast with Two Backs

Posted: Tue Nov 12, 2019 2:39 pm
by Nick
Thermionic Idler wrote:
Tue Nov 12, 2019 2:32 pm
Mike H wrote:
Mon Nov 11, 2019 2:03 pm
Thermionic Idler wrote:
Mon Nov 11, 2019 12:39 pm
Thanks Mike - might be worth a try. This bit is on the PCB so adding it could be a touch fiddly.
I'd just bung it across on the track side. :D
Sadly it's a double sided PCB with insulated tracks... :(
I think Mike means the side without components. You can scrape the resist of the tracks if there isn't a convenient soldered point.

is there any frequency compensation in the feedback loop?

Re: Dieselpunk Preamp of Doom and the Beast with Two Backs

Posted: Tue Nov 12, 2019 2:48 pm
by Thermionic Idler
Nick wrote:
Tue Nov 12, 2019 2:39 pm
Thermionic Idler wrote:
Tue Nov 12, 2019 2:32 pm
Mike H wrote:
Mon Nov 11, 2019 2:03 pm


I'd just bung it across on the track side. :D
Sadly it's a double sided PCB with insulated tracks... :(
I think Mike means the side without components. You can scrape the resist of the tracks if there isn't a convenient soldered point.

is there any frequency compensation in the feedback loop?
Good point Nick.

I believe there is but I'm not sure I understand the circuit well enough to be sure. I'm sending you a PM.

Re: Dieselpunk Preamp of Doom and the Beast with Two Backs

Posted: Tue Nov 12, 2019 2:56 pm
by Mike H
LOL .. :lol:

Obviously it seems er, obvious, only to me :D

What I meant was:

Presumably there is a solder pad for V1A anode. Also presumably, there is a solder pad for C10 positive end wire. Solder the small cap between those two. If too far apart, add on a bit of wire. Seemples. :thumbleft:

Re: Dieselpunk Preamp of Doom and the Beast with Two Backs

Posted: Tue Nov 12, 2019 3:30 pm
by jack
Ant wrote:
Tue Nov 12, 2019 1:38 pm
116.2khz calculated, says it in a post on the last page
Thanks - missed that - bit late to this party...

Quite possible that there's something far higher going on and that 'scope can't see it. I'd definitely try the ferrite beads.

Re: Dieselpunk Preamp of Doom and the Beast with Two Backs

Posted: Tue Nov 12, 2019 3:55 pm
by Mike H
What is the bandwidth of the oscilloscope?

Re: Dieselpunk Preamp of Doom and the Beast with Two Backs

Posted: Tue Nov 12, 2019 3:59 pm
by jack
Mike H wrote:
Tue Nov 12, 2019 3:55 pm
What is the bandwidth of the oscilloscope?
My point entirely...

Re: Dieselpunk Preamp of Doom and the Beast with Two Backs

Posted: Tue Nov 12, 2019 4:44 pm
by Thermionic Idler
jack wrote:
Tue Nov 12, 2019 3:59 pm
Mike H wrote:
Tue Nov 12, 2019 3:55 pm
What is the bandwidth of the oscilloscope?
My point entirely...
It's a second-hand Tektronix 466 which apparently goes up to 100 MHz, or at least it did when it was new. I might need to get better probe connectors for it though, the one it came with disintegrated a few years ago and I bought rather cheap replacements...

Re: Dieselpunk Preamp of Doom and the Beast with Two Backs

Posted: Tue Nov 12, 2019 7:32 pm
by Mike H
OK fairy nuff!

Any higher RF stuff should be noticeable then.

Re: Dieselpunk Preamp of Doom and the Beast with Two Backs

Posted: Wed Nov 13, 2019 9:33 am
by jack
Mike H wrote:
Tue Nov 12, 2019 7:32 pm
OK fairy nuff!

Any higher RF stuff should be noticeable then.
Maybe. Maybe not!

I would try the beads for sure.