Tube DI / Preamp missing gain

[saint gillis]

  Hello I'm working on The Brick from Groove Tubes, here's the schematic from GD's library : https://groupdiy.com/index.php?action=dlattach;topic=45261.0;attach=8677

  When I'm using the "instr input", with the gain pot CCW it works correctly, perfect unity gain, but when the gain pot is CW I only get +24dBs instead of the promised +30dBs... should I consider this as normal? (I checked the voltages, they seem normal)

 

[gyraf]

have you compared to a real one, or do you just trust manufacturer specs?

 

[saint gillis]

have you compared to a real one, or do you just trust manufacturer specs?

I'm checking a real one, and I trusted manufacturer specs...

 

[abbey road d enfer]

  Hello I'm working on The Brick from Groove Tubes, here's the schematic from GD's library : https://groupdiy.com/index.php?action=dlattach;topic=45261.0;attach=8677

  When I'm using the "instr input", with the gain pot CCW it works correctly, perfect unity gain, but when the gain pot is CW I only get +24dBs instead of the promised +30dBs... should I consider this as normal? (I checked the voltages, they seem normal)

If the schemo is correct, maximum gain is 24dB. 37 dB gain in the amp, 13dB loss in the xfmr. The culprit is the mktg dept.

 

[saint gillis]

If the schemo is correct, maximum gain is 24dB. 37 dB gain in the amp, 13dB loss in the xfmr. The culprit is the mktg dept.

Well thank you Abbey, I need to seriously learn how to compute tube's gain.

I have another problem, a HPF on the mic input, here are the frequency responses measured with RMAA :
https://image.ibb.co/kTCkap/Spectrum.png

I've changed the caps around V1 and demagnetized the input transformer, could it be the 6205 that got weak ?

 

[gyraf]

.. or a poor quality input transformer? Try making a plot of what appears right after transformer if possible?

 

[abbey road d enfer]

I've changed the caps around V1 and demagnetized the input transformer, could it be the 6205 that got weak ?

Tubes do not loose bass when they age, particularly there, where performance is constrained by significant NFB.
What is your source impedance for the measurement, and where do you measure it?
There are two stages, both have their LF response constrained by transformers. Try to measure their freq.response separately.
Actually, I've never felt comfortable with this preamp; its indefendibly low input impedance manages to smother most mics. Its used as a door stop now. I tried to mod it in order to make it more usable for me, but it would need a whole redesign, made very uneasy by its "particular" construction.

 

[abbey road d enfer]

.. or a poor quality input transformer? Try making a plot of what appears right after transformer if possible?

Well, the xfmr is supposed to be a Jensen 13K6, but I don't remember having actually seen a Jensen in mine... I'm too lazy to open it and check.

 

[saint gillis]

Ok thank you, I'll make those measures! The Input transformer doesn't look like Jensen : https://www.gearslutz.com/board/attachments/geekslutz-forum/415056d1408957272-groove-tubes-brick-schematic-brick_inside_funny.jpg

By the way what is VR1? only a gain trim?

 

[gyraf]

;D

..and look at that tube in there: marked 12AU7/ECC83 - From a reputable tube rebranding company, that's a serious error....

(I think I took the picture, actually?)

 

[saint gillis]

Haha, well done!

 

[abbey road d enfer]

The Input transformer doesn't look like Jensen :

Yes, now I remember; typical cheap Chinese stuff.

By the way what is VR1? only a gain trim?

It's the FET bias adjustment. It sets the operating point for both Q1and V1.

 

[saint gillis]

So I measured the input transformer with a 150ohm output impedance signal, back to the input of the soundcard with a very short cable.
Then I've measured the same signal throughout the whole circuit with the gain pot at minimum position, here are the graphs : https://image.ibb.co/mxrZ39/Spectrum2.png

 

[abbey road d enfer]

So I measured the input transformer with a 150ohm output impedance signal, back to the input of the soundcard with a very short cable.
Then I've measured the same signal throughout the whole circuit with the gain pot at minimum position, here are the graphs : https://image.ibb.co/mxrZ39/Spectrum2.png

So, the cause is both the circuit and the xfmr; they share about equally the LF roll-off. I would guess most of the roll-off in the rest of the circuit comes from the output xfmr. I don't know what to recommend. You may improve the circuit's response, but you would still have to do with the crappy xfmrs.
I never understood why Terry Manning uses them as his GP mic pre.
Now, as a DI box, it may be more acceptable.

 

[saint gillis]

But the DI response is quite flat... so I guess the roll off is in the Mic input stage.

Something else I don't understand :
  - I plug a -15dBU 1kHz sine in the DI input, I got +10dBU at the output = +25dBs amplification
  - I plug a -15dBU 1kHz sine in the MIC input, I got  +11dBU at the DI (point "OUT1" on the schematic) , and only +24 dBU at the output where I should supposed to get 11 + 25 = +36dBU 

  ... weird... I don't get it

 

[saint gillis]

Something else I don't understand :

Sorry I was saturating the circuit... the gain makes sense...

 

[abbey road d enfer]

But the DI response is quite flat... so I guess the roll off is in the Mic input stage.

Something else I don't understand :
  - I plug a -15dBU 1kHz sine in the DI input, I got +10dBU at the output = +25dBs amplification
  - I plug a -15dBU 1kHz sine in the MIC input, I got  +11dBU at the DI (point "OUT1" on the schematic) , and only +24 dBU at the output where I should supposed to get 11 + 25 = +36dBU 

  ... weird... I don't get it

Have you checked the output signal? This piece cannot output +36dBu. The signal must be seriously clipped. Try with a much lower input signal, like -40.

 

[saint gillis]

Exactly, sorry.. see my previous post...