4in1 tube mic preamp SE/PP TRI/PEN

[solkatten]

The clear winner in the THD department is triode input plus PP output. The PP does a good job on canceling the H2 which dominated the distortion in SE mode...

The pentode plus SE has lower THD than triode plus SE, which I thought was a suprise. But in combination with PP the pentode gave higher THD... is there some cancelation mechanism? What do you guys make out of this?

THD at 1K 0dBu

TRI PP: 0,3%
PEN SE: 0,6%
TRI SE: 1,4%
PEN PP: 1,7%

 

[solkatten]

The lowest THD is actually "UL" 3/4 towards pentode mode, plus PP at 0,15%. The plot thickens...

 

[solkatten]

You live and you learn...

Increasing the output towards +15db didnt increase THD much, but a hotter input signal did, so it was the input stage that generated the distortion. The CF was at half the supply voltage a 5ma so it should be in a good place, the 6j32p anode was anemic at 0,9 ma and a supply voltage of 160v. Now supply voltage 200v bias at 3ma. I set it up as Morgon Jones says it should be done... I also realized I hooked up the screens pin 2 and 7 to the grid instead of the cathode... And pin 8 is now connected to cathode instead of pin 1. And now the THD looks better... and the measurments makes more sense...

The slightly hotter PP is 0.18% at 1K
SE is at 0,35%

 

[solkatten]

Because this is a coloured pre, how much THD can it crank? Pentode/SE/no feedback/+15db its 2,7% of delicious valve harmonics

 

[solkatten]

Maybe we dont need the gain switch. Triode mode is clean enough and got a lot of headroom. And you have good controll over gain by the PP/SE and TRI/PEN controll. PP has 11db more gain than SE and PEN having 10db more gain than TRI... And you have no longer got interaction between gain switch and tri/pen controll...

..and I really like the openess of the sound in this preamp, whitout any global feedback..

 

[solkatten]

Now I think I am satisfied with the design. It sounds and measures good. I will remove the gain switch... I will record a couple of samples, then the preamp will do a studio tour and do some real life work... Thanxs for the feedback

 

[solkatten]

Updated schematics input stage

 

[solkatten]

Freq response with the new home brewed mic trafo. Sound card is -1db at 20K so my preamp is down 1db at 20K. The carnhill is doing a good job...

I feel satisfied with the result, now Im going to focus on my planar v72 mic pre...

 

[rock soderstrom]

Thank you for the project presentation and the detailed measurements!

Final question. Is all the flexibility worth it in practical use or has one setting proven to be best?

 

[solkatten]

Hi Rock I like the different mode texture of the pre. I like the soft character of SE triode. And the complex/balanced harmonics of PP/triode and SE/pentode

I guess you could get by with SE/PP or a triode/pentode switch. Though the PP is by far the cleanest.

 

[solkatten]

Hi long time no see....
This weekend the 4in1 preamp is going to a local studio to record Harp, violin och vocal. I will update the thread with some audio samples....

 

[solkatten]

If there is any interest, I could build a couple of eyelet cards for the preamp section of 4in1 mic pre. Like this: The planar v72 from scratch

 

[janos.hardi]

If there is any interest, I could build a couple of eyelet cards for the preamp section of 4in1 mic pre. Like this: The planar v72 from scratch

I might be interested at some point when I find some time to finish all the guitar amps and the redd style pres. Looking forward to hear the samples Did you eventually dropped the DI idea btw? It would be really cool to have a DI input (maybe with attenuation) especially with all the color you packed into this pre.

 

[solkatten]

I might be interested at some point when I find some time to finish all the guitar amps and the redd style pres. Looking forward to hear the samples Did you eventually dropped the DI idea btw? It would be really cool to have a DI input (maybe with attenuation) especially with all the color you packed into this pre.

Hi my pre got a DI like in post 25. Without gain switch. Yep if you separate the DI and mic input you could add a volume control to the DI input

This pre definitly got some Reddish flavour if the input stage is at pentode/UL and the line stage is at SE

what kind of guitar amps are you building?

 

[janos.hardi]

I'm in the middle of a Hiwatt DR103 style amp. Next patient is a Selmer 100 watter from the late 60s, 99% original state, alas not the most desirable piece but still have that distinct Selmer flavor. I have a few other small heads to fix but they can wait

Good intel on the DI part! I'll keep in mind your project.

 

[solkatten]

You could do the DI like this

 

[solkatten]

Updated schematics phase splitter and line stage

 

[solkatten]

Output PAD + Load

 

[solkatten]

Now I have updated post 1 with new schematics and measurments and all that juicy stuff...

 

[solkatten]

Im gonna try the UTM3580 push pull trafo. 15K CT: 600.