Verellen - Meatsmoke Preamp  [traced]

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plush
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Post by plush »

vigilante397 wrote: 23 Sep 2021, 17:36
plush wrote: 23 Sep 2021, 16:48 Nice build.

But, I'd consider rerouting C2. Just as a precaution.
Any particular reason?
When dealing with high voltage (even though it's highly current-limited in this case), you still need to consider spacings between traces and HT sources.

In your case you've decoupled your signal line from HT with large C2 and then you routed your trace close to the pad which is tied to HT. This is potentially harmful practice.

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Post by vigilante397 »

plush wrote: 24 Sep 2021, 06:55 When dealing with high voltage (even though it's highly current-limited in this case), you still need to consider spacings between traces and HT sources.

In your case you've decoupled your signal line from HT with large C2 and then you routed your trace close to the pad which is tied to HT. This is potentially harmful practice.
At the voltages I'm running and the dielectric constant of the PCB, I need a minimum spacing of 16mil to eliminate the chance of arc from the high voltage lines. The entire board is set at minimum 20mil spacing.

This is not my first time doing a tube preamp PCB ;)

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Post by Reachahighernoon »

vigilante397 wrote: 24 Sep 2021, 13:17
plush wrote: 24 Sep 2021, 06:55 When dealing with high voltage (even though it's highly current-limited in this case), you still need to consider spacings between traces and HT sources.

In your case you've decoupled your signal line from HT with large C2 and then you routed your trace close to the pad which is tied to HT. This is potentially harmful practice.
At the voltages I'm running and the dielectric constant of the PCB, I need a minimum spacing of 16mil to eliminate the chance of arc from the high voltage lines. The entire board is set at minimum 20mil spacing.

This is not my first time doing a tube preamp PCB ;)
I designed several tube preamp pcbs and turns out my clearance spec is 6mil :shock:

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Post by vigilante397 »

Reachahighernoon wrote: 24 Sep 2021, 13:27 I designed several tube preamp pcbs and turns out my clearance spec is 6mil :shock:
And that still doesn't guarantee failure. In open air if you have a voltage of 250V between traces, you need 49mil spacing, but when you have a solder mask that's an additional layer of insulation, so you can get away with tighter spacing. I personally never go lower than 10mil spacing even for low voltage solid-state stuff, but depending on how thick the solder mask is, what it's made out of (different companies may do it differently), what traces are close together (don't put a HV trace by a ground trace or copper pour) you can still get away with tighter overall spacing.

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Post by Reachahighernoon »

vigilante397 wrote: 24 Sep 2021, 13:59
Reachahighernoon wrote: 24 Sep 2021, 13:27 I designed several tube preamp pcbs and turns out my clearance spec is 6mil :shock:
And that still doesn't guarantee failure. In open air if you have a voltage of 250V between traces, you need 49mil spacing, but when you have a solder mask that's an additional layer of insulation, so you can get away with tighter spacing. I personally never go lower than 10mil spacing even for solid-state stuff, but depending on how thick the solder mask is, what it's made out of (different companies may do it differently), what traces are close together (don't put a HV trace by a ground trace) you can still get away with tighter overall spacing.
Still it is better to keep this in mind the next time I design a high voltage pcb, even if the preamp runs on 180 volts its better to do 16mil

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Post by vigilante397 »

So I have an enclosure milled out and powdercoated for the preamp, but I haven't had a chance to box it up yet. Hopefully get that done tonight so I can get finished pictures and do an A/B test against the original. I went with "Brisket" as a working title, but after I finished it I renamed it to "Smoke Signal."

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Post by vigilante397 »

Kept getting distracted but finally got around to boxing this thing up. I haven't had a chance to A/B it with the original yet as it's late now, but hopefully I'll get to do that tomorrow.
IMG_20210926_003701.jpg
241855582_681285506171316_1779863490901497810_n.jpg

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Post by strato56 »

:applause: Very nice work : congrats.
The powdercoating looks great

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Post by Reachahighernoon »

I dont get something, what is the point of the line/instrument switch? It is basically a voltage divider to drop the volume of the unit so that it would work better with mixing consoles?

And how does the recording out work? Arent these supposed to be XLR outs?

And damn does the unit you built look fantastic

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Post by vigilante397 »

Reachahighernoon wrote: 26 Sep 2021, 18:35 I dont get something, what is the point of the line/instrument switch? It is basically a voltage divider to drop the volume of the unit so that it would work better with mixing consoles?

And how does the recording out work? Arent these supposed to be XLR outs?

And damn does the unit you built look fantastic
The line/instrument is precisely to drop the volume. If you're running it into the front of an amp or into additional pedals the output can get a little hot, so you may want to drop it to prevent clipping down the line. It's not that useful in my opinion as the output isn't uncontrollably hot, but people like switches so I left it in.

The recording out isn't a balanced DI output, just a speaker emulated output, so you could send the regular output to your amp and the speaker emulated output to FOH so you don't have to mic your amp. Or if you don't want to bring an amp you can run the speaker emulated output direct to PA or a power amp. It doesn't sound as good as an IR, but it sounds good enough for live use.

Also I finally got around to doing an A/B against the original unit, and they're pretty damn close. Mine is a little brighter sounding, but not harsh-bright, just clear-bright, I actually like it better. Gain levels and EQ seem to be the same, so I'm confident the trace is correct, I attribute the difference in tone to film caps vs MLCC, but obviously with the EQ you can compensate settings to get exactly the same tone.

And I forgot to mention, enclosure is UV print over raw enclosure then covered with sparkly clear powdercoat, my quick and easy finish of choice.

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Post by Reachahighernoon »

vigilante397 wrote: 27 Sep 2021, 13:22

The line/instrument is precisely to drop the volume. If you're running it into the front of an amp or into additional pedals the output can get a little hot, so you may want to drop it to prevent clipping down the line. It's not that useful in my opinion as the output isn't uncontrollably hot, but people like switches so I left it in.

The recording out isn't a balanced DI output, just a speaker emulated output, so you could send the regular output to your amp and the speaker emulated output to FOH so you don't have to mic your amp. Or if you don't want to bring an amp you can run the speaker emulated output direct to PA or a power amp. It doesn't sound as good as an IR, but it sounds good enough for live use.

Also I finally got around to doing an A/B against the original unit, and they're pretty damn close. Mine is a little brighter sounding, but not harsh-bright, just clear-bright, I actually like it better. Gain levels and EQ seem to be the same, so I'm confident the trace is correct, I attribute the difference in tone to film caps vs MLCC, but obviously with the EQ you can compensate settings to get exactly the same tone.

And I forgot to mention, enclosure is UV print over raw enclosure then covered with sparkly clear powdercoat, my quick and easy finish of choice.
Thanks for the info man, but whats an FOH?

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