Studio Tube Preamp

[morebass]

Built this Instrument Preamp from plans by Fred Nachbaur. Amp works except I can not get the tone controls to work. Being a newbie ( other than a simple repair) I'm not sure where to start. I've checked and rechecked my wiring and I really can't find anything that looks out of place. One mod was I eliminated the bypass and ran directly to the tone controls. Any suggestions?

Freds Web Link http://www.dogstar.dantimax.dk/tubestuf/instpre.htm

MoreBass

[Geek]

Are the controls connected correctly as per pinout and in the right place on the board?

[dhaen]

Ensure that the phasing of Sw1 sections is correct.
Did you fit a component in the position of Ctwk? If so, remove it for now.

[morebass]

Pinouts of my pots look OK and I eliminated SW1ab completely (bypassing the flat response switch. Just ran signal directly to the tone stack). No cap in the tweek before C3 either. Being my first post and new to this I appreciate your responses.
Perhaps a better question about my nonfuntioning tone controls is should this schematic work as designed? From Freds web site on this project he said he did not construct this project but felt it is correct in design. I haven't found anyone else who may have built this preamp to compare notes, and I know there are a lot of knowedgeable people on this forum, so if others can say this design is solid, I'll assume the problem is on my end and troubleshoot my construction.

Thanks,
MoreBass

Freds Web Link http://www.dogstar.dantimax.dk/tubestuf/instpre.htm

[Geek]

I was a personal friend of Fred (RIP) and can indeed attest to it's working good. Even got PCB's here for it

[morebass]

Geek, I appreciate your response and from what I've read, Fred was a wealth of knowledge and most willing to share it, that's what prompted me to build his design. By eliminating the possibility of a flaw in the schematic, I know the problem is on my end. Was not aware boards were available but building it point to point and constructing an enclosure was part of the fun and education to understand tube design.
When time permits, I'll go over my build and post any progress.

MoreBass

[Geek]

No problem! Good luck

IIRC, DigitalJunkie has the majority of those PCB's now. Maybe he'll shoot you a couple for a buck or two.

[dhaen]

I too think it should work, and have confidence in Fred's work.
For the tone controls to work, there must be gain - lots of it as they are in the feedback path.
Try removing the feedback take out C15, and see if the gain jumps right up (it may go unstable)..
Also, are the regulator tubes glowing? If not, measure some voltages and report.

[morebass]

Busy Holidays, but found time to trouble shoot.
In my first post I mentioned I deviated from the orig. design by eliminating the bypass switch, so thinking that may be the problem, I installed the switch for tone/bypass with no better results. Switching to tone gives me good sound but no control of tone; bypass produces more noise and ocsillation.
Bypassed C15 resulting in distorted low volume(neon bulbs did not light), so I probed some voltages which are represented on the left. On the right are recommended voltages from Freds Phono Preamp(similar design, not sure if applicable).

B+ 366V 360-390V
Plate V1A( bottom of R6) 204V 190-210V
Cathodes V1(pin 3&8) 31.5V 26-32V
Grid Supply V1A(+ side C2) 29.5V 1.5V less than Cathodes
Grid Supply V1B(+ side C4) 29.9V 2V less than Cathodes
Grid V2(pin 2) 207V Same as V1A(190-210V)
Grid V2(pin 7) 288V N/A
Cathode V2(pin 3) 202V 1.5V higher than Grid V2
Plate V2 (pin 1) 288V 290-310V
Filaments +- 6.5V +- 6.3V
R9 and L1 191V N/A
C15 (- side) about 30V N/A

Thanks Again for the Suggestions
MoreBass

[Geek]

Leaky caps? I saw the RA-100 this was designed for and Fred used low-leakage sweep/SMPS caps.

Also, 12AT7's are kinda finicky unless you're used to them.