Mark III noise question

for Dynaco Mark II/III/IV and DIY PP monoblocks

Mark III noise question

Postby Soundguy2856 » Sat Jun 29, 2013 7:24 pm

I've just finished the first of 2 Mark III restores using SDS cap board and Poseidon driver board. First test was fine using a fair set of tubes I had on hand. Power is good....freq response is good....distortion is good ...basically all is as expected but I have about 30mV of residual hum in the noise floor. Is that to be expected? The freq of the residual is 60Hz which makes me think it's source is the half wave bias supply.....but maybe, with this combination, 30mV is normal? Thanks for your input.
Soundguy2856
 
Posts: 4
Joined: Mon Jun 17, 2013 9:57 pm

Re: Mark III noise question

Postby Shannon Parks » Sun Jun 30, 2013 5:21 am

It should be under 1mV. The C- bias isn't the first place I'd look. Look at heater ground referencing and RCA input chassis isolation.

Shannon
User avatar
Shannon Parks
Site Admin
 
Posts: 3761
Joined: Tue Mar 18, 2003 5:40 pm
Location: Poulsbo, Washington

Re: Mark III noise question

Postby Soundguy2856 » Tue Jul 09, 2013 2:25 pm

Shannon/All - On your Poseidon board the ground point is configured as a star ground for the low level driver stuff. I took that point to my overall amp star ground point at a terminal strip on the chassis that was in reach of the transformer HV CT etc. I suppose I could have taken everything to the poseidon star ground. Was that the intention? I suppose I could have a circulating current form the low level board? I've definitely got the heater CT tied through the .02uf cap. Any thoughts?
Soundguy2856
 
Posts: 4
Joined: Mon Jun 17, 2013 9:57 pm

Re: Mark III noise question

Postby Shannon Parks » Sat Jul 13, 2013 6:00 am

All the low level input stuff should connect to the rest of the amp from 'C' (to the main original star ground). So you if disconnect 'C', your driver board and RCA ground should be high impedance when measured to the rest of the chassis. So try doing that and measure with a DMM - you should be isolated.

Shannon
User avatar
Shannon Parks
Site Admin
 
Posts: 3761
Joined: Tue Mar 18, 2003 5:40 pm
Location: Poulsbo, Washington

Re: Mark III noise question

Postby Soundguy2856 » Tue Jul 23, 2013 3:24 pm

I disconnected point C and I get 2k8 to chassis which is the feedback resistor through the output tranny gound reference. In my current build all the items that require a gnd ref are taken via there own 18Ga wire to a central point which is also where my HV ct and filament ref cap, power cord 3rd wire & chassis all come together. The hum I get is about 20mV and at 60Hz which tells me it's a common path through something with a common impedance. I've separated all the cap sections on the SDS board and taken them back to star gnd individually....no difference. If I pull the tubes out of the low level board the hum goes way down and becomes 120Hz which makes sense becasue that would be ripple related. Are there any items that shoud be reference together perhaps at point C rather than taken back to star gnd individually? Not sure how the haphazard scheme of the original Mark iii performs better than this? Any help is appreciated.
Soundguy2856
 
Posts: 4
Joined: Mon Jun 17, 2013 9:57 pm

Re: Mark III noise question

Postby Shannon Parks » Tue Jul 23, 2013 5:42 pm

Could you take some big pix and send them to me? Or post them? That noise level is ridiculous, so there's a big problem somewhere. Also, could you post your voltages per page 9? Resistances from page 10, too. That might give a clue.

Shannon
User avatar
Shannon Parks
Site Admin
 
Posts: 3761
Joined: Tue Mar 18, 2003 5:40 pm
Location: Poulsbo, Washington

Re: Mark III noise question

Postby Soundguy2856 » Wed Jul 24, 2013 1:07 pm

OK - Problem Solved -
Actually I believe that there were 2 problems...one is the common gound on the SDS board which ties all the cap stages together.....the charging current of the fisrt cap stage is imposed on the ground seen by all the others....but after separating and making that change I still had 60Hz hum. So lesson re-learned...start with the basics....Following Shannon's advice I started measuring voltages and pretty quickly realized that the first input stage of the low level board was drawing more than a normal share of current. Long story short the first stage 12AX7 was to blame......Bad old RCA tube! I changed it and went from this:
Image

to this:
Image

thanks and let the music play!
Soundguy2856
 
Posts: 4
Joined: Mon Jun 17, 2013 9:57 pm


Return to poseidon

Who is online

Users browsing this forum: No registered users and 1 guest