BluegrassDan

Concertone - one more question - 4:1 output TX
« on: May 31, 2020, 12:27:49 AM »
One more question about the Concertone.

I installed a 4:1 output transformer and now the output headroom is way too low. Without the transformer, it just barely starts to get a bit of harmonic distortion right before clipping my AD converter input.

But with the 4:1 transformer, it's totally crunched at around -3dB or -2dB before it even sees the interface.

Any "easy" fixes to squeeze a few more dB of output headroom out of this circuit? (Attached)

P.S. Another odd situation is the 12AU7 meter buffer circuit decreases the output headroom even more when engaged. I mean, it's like a fuzz box.


ruffrecords

Re: Concertone - one more question - 4:1 output TX
« Reply #1 on: May 31, 2020, 03:46:58 AM »
How about re-purposing the 12BH7 bias osc as a White Follower and fit the 4:1 to that?

But I just realised you really need 12dB more gain to compensate for the 12dB loss in the transformer. SO you could re-purpose the 12\BH7 as an SRPP then, in conjunction with the half tube used as the cathode follower you could make a version of my twin line amp design.

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'

rackmonkey

Re: Concertone - one more question - 4:1 output TX
« Reply #2 on: June 04, 2020, 06:10:54 PM »
How about re-purposing the 12BH7 bias osc as a White Follower and fit the 4:1 to that?

But I just realised you really need 12dB more gain to compensate for the 12dB loss in the transformer. SO you could re-purpose the 12\BH7 as an SRPP then, in conjunction with the half tube used as the cathode follower you could make a version of my twin line amp design.

Cheers

Ian

This is exactly what I did with a Sony SRA-2 a while back. Had a 12BH7 bias oscillator that I rewired as an SRPP output. Worked pretty well.
Whether you think you can or you think you can't, you're probably right.

BluegrassDan

Re: Concertone - one more question - 4:1 output TX
« Reply #3 on: June 04, 2020, 08:34:35 PM »
Do you have a schematic for that?

rackmonkey

Re: Concertone - one more question - 4:1 output TX
« Reply #4 on: June 07, 2020, 12:34:47 AM »
I’m still hunting down my notes from that job, Dan. Found the SRA-2 starting schematic, but having trouble finding my hand-drawn SRPP circuit for the 12BH7. Will update when I locate it.
Whether you think you can or you think you can't, you're probably right.

BluegrassDan

Re: Concertone - one more question - 4:1 output TX
« Reply #5 on: June 07, 2020, 04:53:47 PM »
I’m still hunting down my notes from that job, Dan. Found the SRA-2 starting schematic, but having trouble finding my hand-drawn SRPP circuit for the 12BH7. Will update when I locate it.

Thanks so much!

BluegrassDan

Re: Concertone - one more question - 4:1 output TX
« Reply #6 on: June 08, 2020, 01:24:55 AM »
Any ideas for converting this 12AU7 into a better VU buffer/driver without so much distortion?

rackmonkey

Re: Concertone - one more question - 4:1 output TX
« Reply #7 on: June 12, 2020, 12:33:36 AM »
Found the annotated schematic, Dan. This copy just has the 12BH7 SRPP circuit I used. I recall making a small change or two in testing, but this was my starting point and it sounded good wiith a 2:1 step down transformer. Wish I could recall all the details, but this was several years ago. (I made many more changes to this thing to get it halfway usable. Changed grid leak input to common cathode, removed several early 60s packaged filter parts, lowered overall gain etc. The guy really wanted it for the magic eye tubes.)

Whether you think you can or you think you can't, you're probably right.

vitopower

Re: Concertone - one more question - 4:1 output TX
« Reply #8 on: July 08, 2020, 01:13:53 PM »
Thanks so much!
Hey BluegrassDan, would it be possible to know what type of caps are in the mic/line preamp signal path (v101 c102/103 and v104 c116)?
I have only found one very blurry gut shot and its hard to tell....  Looks like it might have been black beauties on 12ax7 but can't tell  about either stage.

BluegrassDan

Re: Concertone - one more question - 4:1 output TX
« Reply #9 on: July 09, 2020, 07:46:15 AM »
It’s already gone back to the client and I didn’t take detailed photos. Seems like there were a few old black beauty caps in there.

(Although, I don’t understand the appeal of them.)


vitopower

Re: Concertone - one more question - 4:1 output TX
« Reply #10 on: July 09, 2020, 03:04:48 PM »
Thanks, I found some more pics, just wanted to see if they were using ceramic coupling.


 

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