Fisher K10 Space Expander Spring Reverb- Dry leakage problem

Help Support GroupDIY:

Seabstudio

Active member
Joined
Jul 6, 2012
Messages
34
Good evening everyone,

After many days and evening tried to troubleshoot my problem, I come here to have the opinion of the pro in electronic :)
I am still relatively begineer to Tube but I am very careful to respect the basics.
I bought the Valve Wizard book 1 year ago  and learned a lot of things and tweak a lot of Preamp or DI with success
So I tried to optimize all the sensitive points of my spring reverb  and redo all my new unit (resistors 2W , Coupling caps with the same value) The reverb sounds really great, the noise was divided by two from original
But the problem (and from the beginning, no matter what the planof ground I use ( omnibuss / Star ground ...multistar I have a Dry leak to the WET signal from the preamp.
If I disconnect the RCA for Reverb Tanks I hear that Dry sound too ( not full bandwitch , a sort of hipass in dry with litlle distortion )
Here is the shematics I used for mod : (Dr. Zee)
I remove the dry channel tap , same leak too

https://drive.google.com/drive/folders/1Btv6iHbOhqUeYK9Pk1ORdRWz61VU_bAG?usp=sharing

Do you have any advice when looking at the schematic where the problem might come from please ?
I have been going around for months without being able to get rid of it  :-\ :-\
I noticed the reverb transformer also radiates a lot (12DW7 Cathode follower +  impedance matching transformer to drive a highZ tank ) , I noticed that it could radiate enormously (but I tried several positions without success.)
If you have any ideas , it will be really cool !
Thanks in advance
Seb from France :)




 

abbey road d enfer

Well-known member
GDIY Supporter
Moderator
Joined
Jan 22, 2008
Messages
14,334
Location
Marcelland
Sounds like the current from the drive stage is getting into the ground. Can you post pictures?
If possible showing the grounding arrangement of the tank, drive an drecovery stages.
 

Seabstudio

Active member
Joined
Jul 6, 2012
Messages
34
Many Thanks for reply
I will post pics in the GDrive folder OK
Grounding arrangement is this way :

- Tank is grounded at his input / Output insulated
- Inside the unit , the 2 ground leg of isolated RCA take place in the omnibus bar ( close to their respective capacitors )
The end of omnibus is the input jack , ground to chassis with an elevation ( 15R // 2 diodes and 0.1uF caps )
 

abbey road d enfer

Well-known member
GDIY Supporter
Moderator
Joined
Jan 22, 2008
Messages
14,334
Location
Marcelland
Seabstudio said:
- Inside the unit , the 2 ground leg of isolated RCA take place in the omnibus bar ( close to their respective capacitors )
I wouldn't do that;
-disconnect both phono sockets from the ground bus
-connect the input ground to the bottom of the drive xfmr secondary; no other grounding is necessary
-connect the ground of the output  to the ground junction of the cathode resistor of the recovery tube
See attachment.
Now the next thing is how ground is circulated between the drive stage and the recovery stage. I don't see the various decoupling caps and where they are referenced precisely.
 

Attachments

  • Fisher K10 gnd.jpg
    Fisher K10 gnd.jpg
    39.9 KB · Views: 21

Seabstudio

Active member
Joined
Jul 6, 2012
Messages
34
Thanks for time again .
I will follow your explanation and try this afternoon
But the ground drive xfmr primary is connect where ?
 

abbey road d enfer

Well-known member
GDIY Supporter
Moderator
Joined
Jan 22, 2008
Messages
14,334
Location
Marcelland
Seabstudio said:
Thanks for time again .
I will follow your explanation and try this afternoon
But the ground drive xfmr primary is connect where ?
Where I've drawn it, i.e. at the point where the primary bottom is grounded.
 

Seabstudio

Active member
Joined
Jul 6, 2012
Messages
34
I mean  where the "best "return point for this ground node  , because it s an omnibus , I use all isolated RCA and Jack plastic


 

Seabstudio

Active member
Joined
Jul 6, 2012
Messages
34
abbey road d enfer said:
Now the next thing is how ground is circulated between the drive stage and the recovery stage. I don't see the various decoupling caps and where they are referenced precisely.

Originally the decoupling caps are multican , solder to chassis in a corner
That the link of complete shematics mod
http://www.mzentertainment.com/pics/fisher_k_10/dr_zee_fisher_k10_spacexpander_spring_reverb_schematics.gif
 

abbey road d enfer

Well-known member
GDIY Supporter
Moderator
Joined
Jan 22, 2008
Messages
14,334
Location
Marcelland
Seabstudio said:
Originally the decoupling caps are multican , solder to chassis in a corner
It relies on a very strong ground bus for reducing the influence of currents circulating all over.
Apparently, since it's a new build, I would recommend placing individual caps where the current demand is, I mean the positive feeding the anode resistor and the negative at the ground connection of the cathode resistor (or load).
 

Seabstudio

Active member
Joined
Jul 6, 2012
Messages
34
Yes I understand , I Already try this but it s same dry appear
Another friend who have the Fisher with only the main mod ( preamp in front of Fisher ) have same problem ...

I just note this afternoon  that the first 1/2 12AX7 ( 12DW7/7247 )  in driver is powered by the same capacitor like output and preamp ....

I will try to use the same capacitor used by the Driver cathode follower , i thing if it s much than 235V it s cool for the Tube too and it s will split the driver with the rest ?

 

Seabstudio

Active member
Joined
Jul 6, 2012
Messages
34
abbey road d enfer said:
I wouldn't do that;
-disconnect both phono sockets from the ground bus
-connect the input ground to the bottom of the drive xfmr secondary; no other grounding is necessary
-connect the ground of the output  to the ground junction of the cathode resistor of the recovery tube
See attachment.
Now the next thing is how ground is circulated between the drive stage and the recovery stage. I don't see the various decoupling caps and where they are referenced precisely.

Hi , problem is same with that connection

A friend advice me to make a test , If I shunt the positive and ground in front of RCA ( for reverb recovery ) i still have a little dry , normally I think we should not hear it ...

Thanks
 

abbey road d enfer

Well-known member
GDIY Supporter
Moderator
Joined
Jan 22, 2008
Messages
14,334
Location
Marcelland
Seabstudio said:
Hi , problem is same with that connection

A friend advice me to make a test , If I shunt the positive and ground in front of RCA ( for reverb recovery ) i still have a little dry , normally I think we should not hear it ...

Thanks
Well, it indicates almost without doubt it's due to current circulating in the ground path. Very difficult to troubleshoot.
I suggest you use a piece of lare section copper wire (1.5mm²) and jumper grounds until you notice a significant change.
 

fede.tich

New member
Joined
Aug 21, 2018
Messages
2
Anyone has more ideas to test? I'm following this thread cause I actually have the same problem!!
Thank You
 
Top