abbey road d enfer

Re: Wilcox M57D1 tube limiter restoration
« Reply #20 on: April 01, 2021, 06:24:02 AM »
From RDH3: The 6SA7 is somewhat similar in construction to the 6L7-G but No. 3 grid has a super control characteristic whereas in the 6L7-G the first grid has a super control characteristic.
Weird. According to datasheet, G3 is the RF input; I wonder why signal should go to an input that has less linear transconductance...  ???
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.


Heikki

Re: Wilcox M57D1 tube limiter restoration
« Reply #21 on: April 01, 2021, 06:59:53 AM »
Weird. According to datasheet, G3 is the RF input; I wonder why signal should go to an input that has less linear transconductance...  ???
That seems to be the way it was usually done in compressors and expanders.

https://worldradiohistory.com/hd2/IDX-Site-Early-Radio/Archive-Radio-Engineering-IDX/IDX/35/Radio-Engineering-1935-12-OCR-Page-0026.pdf


abbey road d enfer

Re: Wilcox M57D1 tube limiter restoration
« Reply #22 on: April 01, 2021, 01:43:40 PM »
That seems to be the way it was usually done in compressors and expanders.

https://worldradiohistory.com/hd2/IDX-Site-Early-Radio/Archive-Radio-Engineering-IDX/IDX/35/Radio-Engineering-1935-12-OCR-Page-0026.pdf
Well, since there is no technical justification for this choice, I have to take it at face value.
I still feel it strange, particularly considering it's a feed-forward arrangement.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Re: Wilcox M57D1 tube limiter restoration
« Reply #23 on: April 10, 2021, 11:57:05 PM »
I've had a little more time to work on this...

200uA meter is working well strapped across a 220 resistor at the junction of R5 & R6/R7

I moved the sidechain signal tap to the 6V6 plates - each side through a 220K resistor & .047uf cap.  Otherwise the amp was getting too distorted before I could get the threshold into a useful range.

Next I will replace the carbon comps with metal film and then fine tune a bit more

It sounds pretty good

abbey road d enfer

Re: Wilcox M57D1 tube limiter restoration
« Reply #24 on: April 11, 2021, 12:15:49 AM »
I moved the sidechain signal tap to the 6V6 plates - each side through a 220K resistor & .047uf cap.  Otherwise the amp was getting too distorted before I could get the threshold into a useful range.
Not surprizing since you had one less stage in the side-chain after your mod.

Quote
Next I will replace the carbon comps with metal film
Don't throw them away. Put them on ebay. They're full of mojo.  8)
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Re: Wilcox M57D1 tube limiter restoration
« Reply #25 on: April 11, 2021, 11:33:48 AM »
Not surprizing since you had one less stage in the side-chain after your mod.

Not really of consequence, but I'm not sure this was modified before I got it - I could be wrong but the wiring all looked original.  Perhaps an earlier revision because the date codes pre-date the schematic I'm working from by about 8 years

Re: Wilcox M57D1 tube limiter restoration
« Reply #26 on: April 11, 2021, 12:04:01 PM »
When I inject sub 60hz signal into the unit I'm getting some motorboating that occurs only while limiting.  I didn't notice this when the sidechain was fed from the 6SN7.  Increasing C6 helps somewhat but it doesn't entirely disappear.  I haven't tried increasing C1 yet.

Before fine tuning I will wait to replace resistors and try to make sure the input tubes balance is OK - In particular, the anode resistor to V1 (R6) has really drifted in value.  I thought of also adding a balance trimpot on the cathodes of V1/V2.

abbey road d enfer

Re: Wilcox M57D1 tube limiter restoration
« Reply #27 on: April 11, 2021, 01:48:36 PM »
Not really of consequence, but I'm not sure this was modified before I got it - I could be wrong but the wiring all looked original.  Perhaps an earlier revision because the date codes pre-date the schematic I'm working from by about 8 years
  My understanding of your first posts, you took one 12BH7 and put a 6V6 instead...?
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Re: Wilcox M57D1 tube limiter restoration
« Reply #28 on: April 11, 2021, 02:06:28 PM »
  My understanding of your first posts, you took one 12BH7 and put a 6V6 instead...?

No, I redrew the circuit as it was when I got the unit.  The Army manual I have is from 1952 but the pot codes are 1946. It seems possible I have an earlier version with a pair of 6V6 on the output rather than a 6SN7 and minus the 6SN7 side chain gain stage

abbey road d enfer

Re: Wilcox M57D1 tube limiter restoration
« Reply #29 on: April 12, 2021, 11:54:19 AM »
OK, got it.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.


 

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