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cyclecamper 12th July 2012 08:34 PM

PAiA stack-in-a-box guitar preamp kit
 
http://www.paia.com/prodimages/siabsch.pdf
This looked interesting until I saw the schematic and all those op-amps. Probably an interesting hybrid, but I'm just into tubes for guitar preamp.

I'd like to build their theramin and add some kind of conversion to MIDI perhaps.

cyclecamper 12th July 2012 08:51 PM

I might like to modify the envelope follower guitar kit and build it into a tube amp. Tap into the preamp just before after the first tube stage where the envelope hasn't been compressed, convert to voltage control, then a nice voltage controlled filter placed after the preamp, for when the preamp adds its own tonalities and distortion.

Nigel Goodwin 13th July 2012 12:02 PM

Quote:

Originally Posted by cyclecamper (Post 3090227)
http://www.paia.com/prodimages/siabsch.pdf
This looked interesting until I saw the schematic and all those op-amps. Probably an interesting hybrid, but I'm just into tubes for guitar preamp.

I don't see your problem?, it's got a valve in there to give the distorted valve sound, the opamps are mostly filtering and buffering (pretty impractical to do anywhere near as well with valves).

benb 13th July 2012 05:36 PM

The big problem here is the plate supply voltage of 30V (see the inverters driving the diode-capacitor voltage multiplier circuit). I've read about this type of "low-voltage" tube preamp circuit before (also in the "Art Tube Pre"), it doesn't give the same type of distortion as with a 150 to 250V plate supply voltage.

Printer2 13th July 2012 10:34 PM

Quote:

Originally Posted by benb (Post 3091151)
The big problem here is the plate supply voltage of 30V (see the inverters driving the diode-capacitor voltage multiplier circuit). I've read about this type of "low-voltage" tube preamp circuit before (also in the "Art Tube Pre"), it doesn't give the same type of distortion as with a 150 to 250V plate supply voltage.

Do you really think the plate supply is 30V? What is the multiplier for then?

Loudthud 14th July 2012 12:17 AM

Quote:

Originally Posted by Printer2 (Post 3091409)
Do you really think the plate supply is 30V? What is the multiplier for then?

There is that note on the schematic near the output of the multiplier, 30V; 10% tolerance. Doesn't make any sense to me either. I saw an article somewhere once that suggested using the flash supply from an old disposable camera. Something around 200-300V @ 2mA.

djgibson51 14th July 2012 12:25 AM

42v out of the multiplier (allowing for losses).
You could;
Add some more diode/cap stages to the multiplier,
Reduce the values of the plate resistors,
Try other 12A*7 tubes,
Replace cathode resistors with LEDs;

Ie. Hours of fun. Go for it :)

JimG

Printer2 14th July 2012 01:56 AM

The 30V 10% from I can figure is the multiplier capacitor ratting. The supply for the chip from the note says 15V so then we are looking at 75V not counting losses. A little low but not horrid.

Loudthud 14th July 2012 02:59 AM

@ Printer2, check this thread at MEF: Charge pump for bias/switching supply?

I know you're a member there. Non-members won't see some of the attachments. A voltage octupler is used to make 56V @ ~1mA from 6.3VAC 60Hz.

benb 14th July 2012 03:08 AM

Okay, looking again, that 30V 10% surely IS the cap rating. I found a similar circuit (the "inverter" plate supply circuit is the same) on page 25 that says 42V, though the text on page 24 says 45V:
http://www.paia.com/manuals/docs/930...ead-manual.pdf
The open-circuit voltage may well be 75V, but with a CMOS inverter chip driving the voltage multiplier, it's easy to see how it can drop a lot with a few milliamps of loading.

There's more on "starved plate" tube circuit design here:
PAiA: Tube Sound?
Here's a thread that discusses some differences between the lower voltage "starved plate" design and "full" voltage design:
What Should Be The Cut Off For Calling a Circuit "Starved Plate" - Gearslutz.com


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