EmRR

Re: How to add gain adjustment of first triode
« Reply #40 on: June 22, 2020, 11:15:29 AM »
I ran your first stage through LTspice. Stage gain is 29dB as is. With the 20dB from the transformer that's a total of 49dB before the first pot. Fortunately tubes can take a lot of level before they fall apart. I simulated yours with 0dBu on the grid and its +29dBu output has about 1% distortion at 1KHz which is a bit of tube tone but not noticeably nasty. This means you should be able to input -20dBu before things start to get into serious distortion.

Acknowledging the later comment about the 10K pot dragging it down, it's useful to point out that this sort of -20 max in is the usual ballpark for vintage tube preamps with 20+ dB gain in the transformer.   49 dB before a gain control is always a problem in modern times with anything hot in output. 

Back to the pot, tons of vintage tube interstage gain controls in the  100K-250K range with no treble loss, none I know of with a CF as you are doing. 
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde


BluegrassDan

Re: How to add gain adjustment of first triode
« Reply #41 on: June 22, 2020, 12:58:53 PM »
It's amazing what a couple resistor values can do. Here's the quick report.

Paralleled the 56k and 2.2k to make 27k and 1.5k respectively. Clipped in and out whilst running a hot test signal showed thing cleaning up significantly.

Attached is a photo of waveforms from me singing a somewhat strong passage through a U87Ai. The top (original bias) shows the asymmetrical junk that was happening. The bottom does exhibit some gentle distortion, but it is largly symmetrical and sounds more like compression than the fuzz I was getting.

I thought I was losing my mind with this thing. Thank, Ian, Winston, et al for helping me figure this out. How can I repay?

Gonna run some sweeps now and see what they reveal.

Re: How to add gain adjustment of first triode
« Reply #42 on: June 22, 2020, 01:04:17 PM »
Hey, hey, great  :)
I think Ian &  Doug did the heavy lifting, I was just a cheer-leader. 
Good to hear it's on its way there anyway, have fun.
Note to self:  Don't let c**ts wind you up

BluegrassDan

Re: How to add gain adjustment of first triode
« Reply #43 on: June 22, 2020, 01:11:23 PM »
Running more test tones. Hands down, I have never gotten distortion figures this good from anything I've ever built before. I might even stick with the 1:10 now instead of 1:5, or perhaps make it switchable.

TWO DANG RESISTORS!

This is excellent!

EmRR

Re: How to add gain adjustment of first triode
« Reply #44 on: June 22, 2020, 01:18:14 PM »
cool
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

BluegrassDan

Re: How to add gain adjustment of first triode
« Reply #45 on: June 22, 2020, 01:21:06 PM »
And it is now spitting out exactly 70dB of gain. That is a nice round number to work with.

ruffrecords

Re: How to add gain adjustment of first triode
« Reply #46 on: June 22, 2020, 03:48:18 PM »
Team groupDIY scores again.  Very pleased you got it working so you like it. There's nothing like a bit of basic engineering coming right to make you feel good.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

Re: How to add gain adjustment of first triode
« Reply #47 on: June 23, 2020, 11:41:43 AM »
I have enjoyed reading along in this thread. I know I don't often post here but I do love reading and learning all the time.

Are you able to post the updated schematic so that I would be able to print and study it a bit?

Thanks so much.

Gus.

BluegrassDan

Re: How to add gain adjustment of first triode
« Reply #48 on: June 23, 2020, 04:00:01 PM »
Here is the update.

I do still wonder if updating the second CF would be beneficial in some way.

ruffrecords

Re: How to add gain adjustment of first triode
« Reply #49 on: June 23, 2020, 04:28:56 PM »
Here is the update.

I do still wonder if updating the second CF would be beneficial in some way.

Unless we hit some magic sweet spot with the change of operating point and bearing in mind the CF has 100% NFB anyway, it is unlikely the second CF will itself benefit from the change. However, the actual voltage at its cathode itself will probably change which in turn will mess with the bias arrangement of the White follower since its bias conditions are controlled directly by the CF cathode voltage.

In any case, the impedance this CF drive is near infinite being the bare grid of the top triode of the White follower so its load will be negligible so it should not clip due to excess loat as was the case with the pot.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'


BluegrassDan

Re: How to add gain adjustment of first triode
« Reply #50 on: June 23, 2020, 10:24:57 PM »
Unless we hit some magic sweet spot with the change of operating point and bearing in mind the CF has 100% NFB anyway, it is unlikely the second CF will itself benefit from the change. However, the actual voltage at its cathode itself will probably change which in turn will mess with the bias arrangement of the White follower since its bias conditions are controlled directly by the CF cathode voltage.

In any case, the impedance this CF drive is near infinite being the bare grid of the top triode of the White follower so its load will be negligible so it should not clip due to excess loat as was the case with the pot.

Cheers

Ian

I'm not sensing any clipping, Ian, so I'll leave it as is. I'm just blown away how those small changes in resistive values totally saved the day.

abbey road d enfer

Re: How to add gain adjustment of first triode
« Reply #51 on: June 26, 2020, 12:19:17 AM »
Is it possible...?
Yes, somewhat.
As others have already mentioned, the bias of the input tube is probably way too low, with about half the drive capability it would have if optimally biased.
Then the cathode resistor could be left un-decoupled, that would reduce gain by about 10dB.
I see that you use a low ratio input xfmr, which can accomodate a 100k potentiometer (or switched attenuator) between secondary and grid, without risks of treble loss.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.


 

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