4in1 tube mic preamp SE/PP TRI/PEN

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solkatten

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Hi everyone....
I would like to share a tube mic preamp I design. I have not seen a mic preamp that can switch between SE/PP and TRI/PEN. It sounds nice och has a lot of diffrent flavours. The idea was also to design the preamp around cheap but good performing tubes. The russian/soviet 6J32P, 6N1P and 6N6P fit that bill.

I used the Carnhill WTB2291 as a affordable alt. output transfomer (even if it not design as a PP transfomer it does a nice job), I guess you could use a cheap pp transfomer from edcore if you live in US. As input transfomer anything from 1:4 to 1:10 should work fine depending on taste... headroom, s/n etc...

My build also has DI, HP, input and output PAD, phase reverse and 48V... The B+ is around 250VdC....

Now vers 2 of the schematics with gain switch

Audio sample post #24: 4in1 tube mic preamp SE/PP TRI/PEN
 

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solkatten

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HI I have not measured the preamp for optimum clean perforamance instead its build as a coloured preamp that goes from clean enough to light saturation. SE got more second harmonics. PP is crisper with cleaner bass. TRI is clean and pentode adds more gain and odd harmonics. I see it as a creative tool. I will post some more build pictures soon...
 

Winston OBoogie

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Looks like a lot of options there for different colours, cool :)

Ultra linear mode on the input pentode would be cool too but probably quite a bit more hassle to include and, maybe after all the effort it wouldn't add much more to the palette you have already anyway?
 
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solkatten

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Looks like a lot of options there for different colours, cool :)

Ultra linear mode on the input pentode would be cool too but probably quite a bit more hassle to include and, maybe after all the effort it wouldn't add much more to the palette you have already anyway?
Hi you actually get "UL" if the pot is in middle position.
 

ruffrecords

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I am surprised you got away with using the ungapped VTB2291 transformer - maybe that was intentional - instead of the VTB2290 gapped version.

Cheers

Ian
 

Winston OBoogie

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My understanding of UL is that we have a percentage of feedback happening. I forget what David Hafler used as the "magic" number, but 25% would be in the ballpark.
On your schematic, unless I'm mistaken, UL would take a signal from the anode, and present a certain proportion of it to the screen grid. Along with supplying the screen grid's current.

In a pentode, this'd probably require a buffer. If you made a pentode out of two triodes in cascode configuration, then you could do it without a buffer as the artificial screen grid would require no current.
 

solkatten

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Hi Ian a actually have both 2290 and 2291 at hand. I liked the higher primary inductance of the ungapped version, because the PP-stage can be well balanced with the pot it works fine... I connected it 8:1...
 

Winston OBoogie

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One thing I forgot to say is 'Welcome' 🙂

Your first post here was a schematic of your own design which is great.

Any suggestions here are part of what you might go through in a typical 'design review' at a company meeting.

Don't take them as criticisms.
 

solkatten

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Thanxs for the welcome. I think groupdiy is great forum and i have found many great projects here over the years. In all practical seens I think you could consider the stage as morphable from triode to UL and pentode depending, the amount of feedback between anode and screen is adjustble from 100-0 procent. The pot acts both as gain control and a harmonic content controll.

Do you mean that the pot only supply ac voltage and no dc current to the screen and that makes it no true UL?
 
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Winston OBoogie

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On thinking this through a bit more, I think for all practical purposes of your goals you're right.

I honestly don't know that there'd be any sonic colour differences doing it along the lines I suggested.
It might just end up as a whole lot more effort and parts, to end up with, basically, the same sonic options and flavours.

Cool.
 

ruffrecords

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Hi Ian a actually have both 2290 and 2291 at hand. I liked the higher primary inductance of the ungapped version, because the PP-stage can be well balanced with the pot it works fine... I connected it 8:1...
Yes you definitely get a lot more primary inductance with the un-gapped version. I understand Rupert's 'silk' enhancement is achieved by adding some dc to his output transformer so maybe there is another sonic effect ot be had by tweaking the balance a little bit off centre.

Cheers

Ian
 

solkatten

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Yes you definitely get a lot more primary inductance with the un-gapped version. I understand Rupert's 'silk' enhancement is achieved by adding some dc to his output transformer so maybe there is another sonic effect ot be had by tweaking the balance a little bit off centre.

Cheers

Ian
Yes that is definitely somthing you could explore. I’ve heard some people role their PP primary slightly off center for the second harmonic effect as well... My build has a switch between PP/SE but if you do it with a pot you get asymmertical PP in the middle position, but no DC saturation.
 
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abbey road d enfer

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SE got more second harmonics. PP is crisper with cleaner bass.
Have you made measurements/analysis of distortion in the different positions?
I would think the fact that the second triode is still connected in "SE" mode actually tends to produce less waveform assymetry than a true SE.
I understand you can't disconnect the 2nd half since it would result in very heavy DC unbalance in the xfmr pri.
 

solkatten

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HI I have not made any measurments maybe its time to get some software. To my knowlegde (just a audio hobbyist) the lower triodes grid is grounded it gets no ac-signal. but act as a constant current sink and balance the DC in the output transfomer in SE mode. The pot only effects the ac-signal. There should be no even order cancelation in SE but full even order cancelation in PP mode and somewhere inbetween in the asymmertic PP range...
 
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Sonic

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solkatten, hi! I assembled the triode-pentode according to this scheme.
There is a Hungarian transformer at the entrance. Output transformerless.
 

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solkatten

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solkatten, hi! I assembled the triode-pentode according to this scheme.
There is a Hungarian transformer at the entrance. Output transformerless.
Nice very similar to my build, it does not show in my schematic but its exactly as I have impIemented the DI, the input PAD and phantom power...
 

solkatten

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The bass guitar sounds delicious with this preamp. The degree and level of harmonics were measured using RMAA (RightMark Audio Analyzer 6.4.5).
Yeahs I like the sound very much. Very musical and its easy to dail in the right amount of harmonic enrichment. I do must of my testing with accoustic guitar and often end up in the middle position in the UL range.
 

abbey road d enfer

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HI I have not made any measurments maybe its time to get some software. To my knowlegde (just a audio hobbyist) the lower triodes grid is grounded it gets no ac-signal. but act as a constant current sink
No. A triode is not a constant-current sink. When the upper triode receives a positive on its grid, current increases, as well as Gm, and Rp decreases. At teh same time, the lower triode sees its plate voltage increase, as do current and Gm, for a decrease in Rp; this tends to linearize operation. Not as much as in a true push-pull, though.
 
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