• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

PA1 Tube Microphone Preamp Build Issues

aheck

Member
2018-11-02 5:11 am
I found a circuit for a mic pre submitted as an "Instructional" on the interwebs and decided to build it. It wasn't problematic or complicated and I've checked my work several times, and it works, but the waveform degrades at about 1400 Hrtz. At about a 1000 Hrtz, it shows 2000 on the oscilloscope and as the frequency I feed it decreases, the waveform falls apart while the scopes reading increases. Above 1400 Hrtz it's a good solid sine wave, very linear.
So I need help with that...
Also, if anyone could tell me after seeing the schematic, if I were to couple the DC voltage at the 12AT7's Plate, could I use an audio transformer like a Sowter 8230f? I wouldn't think it would have been built to be able to supply plate voltages and current. The transformer in use now is a single ended output transformer I had laying around. The project calls for a reverb driver transformer. I did not have one, but the idea is the same-ish (or is this my problem?). The project page is here: https://www.instructables.com/The-PA1-DIY-Tube-Preamp-Efficiently-Built-With-Sal/
Thanks for any help or advice.
PA1 Mic Preamp Schematic with Voltages.jpg
 

20to20

Member
2010-06-23 9:25 pm
Well, I still have this issue, and I'm still looking for advice. I'm willing to fill in any blanks for anyone willing to help out. Thanks very mush.

A small signal driver tube can't push enough power into a common SE output transformer because the tube needs to see 20x the impedance those normally have. A reverb transformer is probably similar to an interstage transformer meant to pass and divide small signals modulated by a reverb unit to a high input impedance grid. You really can't expect a preamp to drive a speaker. That isn't even part of the project you are attempting to copy or extend.
 

aheck

Member
2018-11-02 5:11 am
A small signal driver tube can't push enough power into a common SE output transformer because the tube needs to see 20x the impedance those normally have. A reverb transformer is probably similar to an interstage transformer meant to pass and divide small signals modulated by a reverb unit to a high input impedance grid. You really can't expect a preamp to drive a speaker. That isn't even part of the project you are attempting to copy or extend.
Thanks. I'm not interested in driving a speaker. The project has this preamp driving a speaker using a reverb driver, but the author never specifies anything about which driver and I've seen quite a few different ones. I'm not trying to drive a reverb tank either. That's why I'm asking if I could modify it to capacitively couple the DC on the 12AT7's plate and run the signal through an audio transformer. What I'd like to do is to amplify the microphone's voltage to where it's usable with my audio interface. I want to use this as a microphone preamp plugged into the +4dB or -10dB line in on an Apogee Duet.
 
Last edited:

20to20

Member
2010-06-23 9:25 pm
I want to use this as a microphone preamp plugged into the +4dB or -10dB line in on an Apogee Duet.

OK, then you don't need the transformer, just the cap coupling to the output jack/line. I don't know the Apogee. If it is solid state and has a typical input impedance of 10K or less then you still have the same low impedance problem that a SE OPT has at 5K ohms. It takes a cathode follower type of driver to handle low impedance loads.
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
This was an uninspired "microphone preamp", there are better choices. But you built it, let's get something working OK.

And you have no specs for distortion or hiss?

I'm not proud of this hack. But at few-Volt output the over-worked 12AT7 will have tenths-percent THD, heavy 2nd-harmonic, won't stink. Zout is about 7k, so it will sag a bit into 10k-22k inputs, but it has way more gain than any sane recordist needs, so it will be fine if the knob is used with good taste.
aheck-mike.gif


You don't need to bypass 12AT7 cathode and you may be better off at the lower gain and increased NFB.

The "1Meg" pot on the original would, at half-gain, trim the upper octave or two even in guitar-amp service; Fender put a Brite cap around such pots which is a crude fix. Since gain is high and input levels should be low, a 250k gain pot does not hurt and will be more wide-band into the 12AX7 grid capacitance.
 
  • Like
Reactions: 1 user

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
a reverb driver, but the author never specifies anything about which driver
The unspecified reverb transformer is usually the small one from the Fender integrated-reverb amps. (That whole 12AT7 stage IS the reverb send from a Fender Twin Reverb, tho the bias is changed??) Depending who you believe, the impedance is 15k or 23k. This will give less bass-suck from a hi-Z source than your 5k part, but is still NOT a deep-bass solution. (Heavy bass in spring reverb is just mud-slide.) At $17 it could be worth having one for play. It would reduce the excess gain (by about 70X!!).
 
  • Like
Reactions: 1 user

aheck

Member
2018-11-02 5:11 am
OK, then you don't need the transformer, just the cap coupling to the output jack/line. I don't know the Apogee. If it is solid state and has a typical input impedance of 10K or less then you still have the same low impedance problem that a SE OPT has at 5K ohms. It takes a cathode follower type of driver to handle low impedance loads.
Okay, I was hoping using a 5:1 Sowter transformer would help lower impedance by a factor of 25:1 and better drive the interface. What would the output impedance of the preamp be without the transformer or how would I calculate that?
 

Attachments

  • Tube Mic Preamp Output.pdf
    15.1 KB · Views: 12

aheck

Member
2018-11-02 5:11 am
The unspecified reverb transformer is usually the small one from the Fender integrated-reverb amps. (That whole 12AT7 stage IS the reverb send from a Fender Twin Reverb, tho the bias is changed??) Depending who you believe, the impedance is 15k or 23k. This will give less bass-suck from a hi-Z source than your 5k part, but is still NOT a deep-bass solution. (Heavy bass in spring reverb is just mud-slide.) At $17 it could be worth having one for play. It would reduce the excess gain (by about 70X!!).
Right, Thank you, I'm really planning on using this for a ribbon mic to record electric guitar so under 80 Hrtz is pretty much moot anyway.
This was an uninspired "microphone preamp", there are better choices. But you built it, let's get something working OK.

And you have no specs for distortion or hiss?

I'm not proud of this hack. But at few-Volt output the over-worked 12AT7 will have tenths-percent THD, heavy 2nd-harmonic, won't stink. Zout is about 7k, so it will sag a bit into 10k-22k inputs, but it has way more gain than any sane recordist needs, so it will be fine if the knob is used with good taste.
View attachment 1078717

You don't need to bypass 12AT7 cathode and you may be better off at the lower gain and increased NFB.

The "1Meg" pot on the original would, at half-gain, trim the upper octave or two even in guitar-amp service; Fender put a Brite cap around such pots which is a crude fix. Since gain is high and input levels should be low, a 250k gain pot does not hurt and will be more wide-band into the 12AX7 grid capacitance.
I'll make these changes, I can also post pics of my build. I very much appreciate the guidance.
 

aheck

Member
2018-11-02 5:11 am
This was an uninspired "microphone preamp", there are better choices. But you built it, let's get something working OK.

And you have no specs for distortion or hiss?

I'm not proud of this hack. But at few-Volt output the over-worked 12AT7 will have tenths-percent THD, heavy 2nd-harmonic, won't stink. Zout is about 7k, so it will sag a bit into 10k-22k inputs, but it has way more gain than any sane recordist needs, so it will be fine if the knob is used with good taste.
View attachment 1078717

You don't need to bypass 12AT7 cathode and you may be better off at the lower gain and increased NFB.

The "1Meg" pot on the original would, at half-gain, trim the upper octave or two even in guitar-amp service; Fender put a Brite cap around such pots which is a crude fix. Since gain is high and input levels should be low, a 250k gain pot does not hurt and will be more wide-band into the 12AX7 grid capacitance.
After making these changes and testing functionality, gotta say it sounds a crap-ton better. Thanks for the advice and lessons. There are some noise issues and I may have to bypass the pot because there is a muddiness to the sound and it's motorboating when pushed, but it does work and I get to try to correct these issues. It's the best way for me to learn.