Mic/DI Tube Channel Project

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genghiscohen

Member
Joined
Dec 28, 2012
Messages
6
I'd appreciate any and all feedback on the attached schematic.

The first pentode stage was inspired by a discussion between PRR and blandman74 here: http://www.groupdiy.com/index.php?topic=24442.5

My main uncertainty concerns that first stage, as I have no prior hands on experience with pentodes. I.e. calculating bypass and screen cap sizes, intuition for how it will behave with different input signal levels.

Also, on the output. For direct coupling the cathode follower, will setting the preceding triodes plate and thus the cathode follower grid at B+/2 be sufficient or are there additional issues I should be keeping in mind?

I realize the topography is about as primitive as it gets, but I am after something that has some (hopefully interesting or gritty) color to it. On that note, will the given placement of the two volume controls give some control over the distortion? I'm starting to suspect that replacing the pentode with two triodes with a pot in between them will give me more flexibility towards that end. As I understand the current iteration the only stage that would clip with any pleasant effect would be the one directly preceding the output, which doesn't seem very flexible.


 

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It gets a lot more primitive than that  :)

Nice circuit - should do very well with lots of scope for this-and-that. I'm doing something reasonably similar in my current instrument tube amp build.

Good luck with it.
 
Thanks Alex,
I'd like to see what you end up with. I say primitive because its pretty much a guitar preamp...

Here's two other options.
As mentioned previously, two triodes at the input.

upload foto

or here's one ignoring the overwhelming advice against loading down a common cathode gain stage with a tonestack.

upload foto
 
Yep - all of the above are commonly done.

My pref generally is #2 but these days more like #1. I started out with #3.

The interesting thing that is not common but much liked by some is the final tube as cath follower driving the traffo. Pultec did this sort of thing I think. I've always wanted to try that.

My chain is more or less : i/p -> constant current stage (grounded cath triode amp with dc cath follower)

followed by tone stack (low z ip) to a pentode stage with volume divider at output

followed by grounded cath triode amp with another volume divider at output

followed by the line amp stage. My current choice is a push-pull output stage with traffo.


But - it's all good. Just goto try a few. Generally they can remain pretty clean. Especially if you have a negative feed back loop in there somewhere. The dirt comes from some very obvious overdriving input stages. The challenge is to keep the noise down as you stage gain up-and-down-and-up-again :)

Eventually you find a config you like for this period in time until your taste's change.

That's the beauty of point-point wiring in as spacious an enclosure you can manage.

Cheers
 
There are a number of things you need to watch out for:

1. Input stage overload. Pentodes can give as much as 40dB gain and this combined with 20dB gain in the transformer can easily lead to input stage overload even with a 20dB pad switched in.
2. You probably need to load the mic transformer with something less than the 1M you have at present. You should also hatch out for interference problems with the instrument input.
3. I am not sure your input pad will work as intended. I am not sure if it is meant to be a line in switch or just a pad. Either way, with a 200:50K transformer, the 470 ohms is a little on the high side.
4. You PSU may need some values tweaking. I would certainly be tempted to increase the HT smoothing capacitor values to around 220uF.

Good Luck.

Cheers

Ian
 

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