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Building a Langevin 5116B preamp - bad bass response
Old 3rd May 2020
  #1
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Building a Langevin 5116B preamp - bad bass response

Hello !
I was given nice big old irons with the goal to build 2 langevin 5116B preamps.
The irons are Peerless / Altec Lansing corp. Transformers, with references : T-101, T-102
Nothing about thoose irons online, but with some mesurements and help Help finding data on vintage Peerless Transformers we decided we could build a nice tube preamp
So it's gonna be a langevin 5116B (schemo included)

I wired the thing on a prototype chassis. I get about 300V B+.
My tubes are 12AX7 for V1 and 12AU7 for V2 as i don't have any 12BH7 right now.

After some time to troubleshoot the thing (mostly transformer induced hum and ground issues) i got it working. It's souding great !
Time for REW measurements now...
It's behaving great above 100hz but i would like to have the bass cut rather at 20hZ that 100hz....
I made measurements along the circuit : bass response is great at V1 grids, just after the input transformer, and then it begins to get funky at V2 grids (must be impedance issues with my meter connected?)
Output is as i said flat above 100hz but i'd like it to be flat to 20hz
then i tried to remove the NFB (disconnect R5 and R6, last measure) but it has no effect on bass response.

Any idea of what i should do to recover bass response ?

For my test i use a scarlett 2i2 sound card, so maybe my tests are bad.

Output transformer wiring is also questionnable, it's a speaker out transformer, not line out...

Thanks !
Attached Thumbnails
Building a Langevin 5116B preamp - bad bass response-mesure4.jpg  
Attached Files
File Type: pdf Langevin_5116B.pdf (812.3 KB, 9 views)
Old 3rd May 2020
  #2
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nosebleedaudio's Avatar
 

Do you have a speaker connected to output?
If so, no wonder..
Old 3rd May 2020
  #3
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Quote:
Originally Posted by nosebleedaudio View Post
Do you have a speaker connected to output?
If so, no wonder..
no load !
so i made some tests with some loads (easy with resistors as the power il very low)

the output transformer has many taps
power tube is a 12AU7 so wants to see about 20k
The most i can get from the OT is 1:25 turn ratio so with a 8R load it's 5k on the primary.
With that wiring, hooking a load has no effect at all on bass response... tried 8R to 100k

So i tried the lowest ratio of the OT : 1:1.8 !
with a 8R load that's 26R load on the power tubes??
Now the load on the output has an effect on the curve, it's not bad with a 8R load, see measurements
BUT :
- i'm not sure my 12AU7 like to see such an impedance ???
- output is VERY low : about 0.2V pp ... not so great...

Any idea ?
Attached Thumbnails
Building a Langevin 5116B preamp - bad bass response-load-no-load.jpg  
Old 3rd May 2020
  #4
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nosebleedaudio's Avatar
 

Do you have a scope?
If so just start at the input and go from there..
Old 3rd May 2020
  #5
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Quote:
Originally Posted by nosebleedaudio View Post
Do you have a scope?
If so just start at the input and go from there..
yeah i do, and a function generator, but how can i use it to tweak bass response ?
Old 3rd May 2020
  #6
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nosebleedaudio's Avatar
 

Quote:
Originally Posted by pi_amp View Post
yeah i do, and a function generator, but how can i use it to tweak bass response ?
You need to see WHERE it's dropping..troubleshooting...
Old 6th May 2020
  #7
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emrr's Avatar
T-101 and T-102 are references to part #s on a schematic, not the transformers themselves. No telling what response they are capable of.

In a balanced amp like this, current balance in the output transformer tends to affect low frequency response. A cathode balance pot may or may not help, probably not a lot.

The feedback path capacitors are or can be tailored to compensate low frequency. Think of them as EQ elements, plan from there. With the response you are getting, you may not be able to EQ that far, even with multi-section EQ networks in the feedback.

Correct output load will probably affect low frequency response also. Can be found with load experiments.
Old 6th May 2020
  #8
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Radardoug's Avatar
 

If the circuit called for a 12BH7, why are you not using a 12BH7?
Old 6th May 2020
  #9
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Quote:
Originally Posted by emrr View Post
T-101 and T-102 are references to part #s on a schematic, not the transformers themselves. No telling what response they are capable of.

In a balanced amp like this, current balance in the output transformer tends to affect low frequency response. A cathode balance pot may or may not help, probably not a lot.

The feedback path capacitors are or can be tailored to compensate low frequency. Think of them as EQ elements, plan from there. With the response you are getting, you may not be able to EQ that far, even with multi-section EQ networks in the feedback.

Correct output load will probably affect low frequency response also. Can be found with load experiments.
Yeah i had the same idea, i got good result with 22uF feedback path capacitors (had to use electrolytic) and also reduced feedback resistors (tweakable with a stero pot) At the price of a low gain... about 16db

About load, i got the best results with a 33R load, wich equals to a 20k load on the valve side

Here i am now, several curves with several resistor feedback values


About the valve : i used what i have on hand, so it's a 12AU7 but i will order some 12BH7s ! Could the give a better bass response ?
Attached Thumbnails
Building a Langevin 5116B preamp - bad bass response-33r.jpg  
Old 9th May 2020
  #10
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Chassis almost wired for stereo operation :D

Any idea of why such a small value (0.005uF) for the coupling cap between the 12AX7 and the 12BH7 ? Maybe a bigger cap would reduce bass cutoff ?
Attached Thumbnails
Building a Langevin 5116B preamp - bad bass response-img_20200508_180542.jpg   Building a Langevin 5116B preamp - bad bass response-img_20200508_175826.jpg  
Old 9th May 2020
  #11
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Langevin 5116b

Quote:
Originally Posted by pi_amp View Post
Any idea of why such a small value (0.005uF) for the coupling cap between the 12AX7 and the 12BH7 ? Maybe a bigger cap would reduce bass cutoff ?
it does seem to be very small, even taken into account the plate resistance of the 12AX7a and feedback around the stage.
for comparison, the McCurdy AU300 has similar circuit resistances yet the coupling capacitor is .05 uF.
Attached Thumbnails
Building a Langevin 5116B preamp - bad bass response-au300.jpg  
Old 9th May 2020
  #12
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Quote:
Originally Posted by cathode View Post
it does seem to be very small, even taken into account the plate resistance of the 12AX7a and feedback around the stage.
for comparison, the McCurdy AU300 has similar circuit resistances yet the coupling capacitor is .05 uF.
Thanks,
interesting circuit !
Well i changed the coupling cap to 0.1uF as a test
success... bass slope is now about -3db/octave (it was very steep, about -15db/octave with the 5nF caps!)
(gains are different, different volume)
Attached Thumbnails
Building a Langevin 5116B preamp - bad bass response-coupling-cap.jpg  
Old 10th May 2020
  #13
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emrr's Avatar
The stock coupling cap in a real 5116B yields proper response to 20Hz. Certainly situation dependent, related to transformer bandwidth AND feedback.
Old 26th May 2020
  #14
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Quote:
Originally Posted by emrr View Post
The stock coupling cap in a real 5116B yields proper response to 20Hz. Certainly situation dependent, related to transformer bandwidth AND feedback.
Well i got the 12BH7A,
Gives me a little more gain, i'm around 22/24db, still far from the 40db from the schematics, but maybe i'm wrong with my measurements ? With a 400hz sine, 0.5V AC signal i get about 8V AC RMS output (fluke 87V readings)

Is there any disadvantage of what i changed to the schematics :
- using 0.1uF coupling caps instead of the 0.005 that the schem calls for ?
- using 22uF (polarised electrolytics) for feedback caps
Old 26th May 2020
  #15
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emrr's Avatar
Quote:
Originally Posted by pi_amp View Post
Well i got the 12BH7A,
Gives me a little more gain, i'm around 22/24db, still far from the 40db from the schematics, but maybe i'm wrong with my measurements ? With a 400hz sine, 0.5V AC signal i get about 8V AC RMS output (fluke 87V readings)

Is there any disadvantage of what i changed to the schematics :
- using 0.1uF coupling caps instead of the 0.005 that the schem calls for ?
Doubtful, though 0.025 would be plenty.

Quote:
Originally Posted by pi_amp View Post
- using 22uF (polarised electrolytics) for feedback caps
Stick with the original 0.22 or smaller, not larger. Smaller makes more bass, if it's there to make. Make them smaller until you see too much bass, if possible, then dial back up until optimal.
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