Techtalk

Well fired #1 up !!

So I am getting unequal clipping (only on bottom of wave) on channel 2 at about 3V p-p while the channel 1 will gain up until the transformer clips and whole thing breaks apart way above 3V p-p

I measured the heaters to be all at 6VAC. So my transformer is a little weak but will provide enough I am thinking ???

Or is that the cause of my problem?


Opinions?

-ChuckD

since channel one is working I don't think the problem are the heaters.

remove the T4B and sidechain tubes - still the same ?

I'll have to wait until tommorow it's 2am and sleepy + 250VAC = danger

Ask me about how I almost killed myself with the Vari Mu !! I was literally knocked on the floor when the chassis became hot....

Thanks that is a good tip to try I'll get back to you tomorrow.


-Chuck

if it is still the same with the sidechain removed, swap the audio tubes between left and right.

ChuckD hat geschrieben:2am and sleepy + 250VAC = danger

Smart decision!

OK I'm back

Hey Volker did you mean pull everything except leave in the Voltage Amp 12AX7 (V201) and the Cathode Follower 12BH7A (V202) ???

Is that correct?

Will this show my gain structure in both channels only for debugging and possibly matching the tubes better... or finding a bad one.

-Chuck

Ok the voltages do seem off

Node at :
10K (R117) input to the 12BH7A and Solen 100n

Working one = 250 VDC
Broken one = 340 VDC

CJ's Voltage chart = 210 VDC

I expect little higher since the Sidechain is pulled out but 340VDC is too high here for sure

What could be the cause?

-Chuck

ahhh

Fixed ! The 1K resistor 118 was not soldered on one side!



That was easy once I saw way more voltage drop on one side then the other.
Got lucky that time but will have to see how it sounds now.

Volker,
What side do I put the 3.9K on the Vu. I read it should be on the negative side? Is that correct? oh and did you check my labels on the previous question / Photos ? Did I get it right ?

-Chuck

Huuuuuuummmmmmmm

damnit! Here's a snapshot



Centered around 70Hz and some nasty high end stuff
Followed the correct star grounding scheme and made sure to ground the XLR and shielded the gain and peak.

Tubes :

JJ ECC803
Electro Harm 12BH7
GE NOS 68qs


any ideas??

-Chuck

hi chuck,

maybe change your tubes, read on page 5 . i had also a hum problem.

Hum increases with more gain as well.
At max gain with no input hum is peak at -40dBu

Did you end up getting some telefunkens from ebay to solve your problem after all?

Also did you only have to replace v101 and v201?

-Chuck

ChuckD hat geschrieben:What side do I put the 3.9K on the Vu. I read it should be on the negative side? Is that correct? oh and did you check my labels on the previous question / Photos ? Did I get it right ?

I always put it on the positive side, but I don't think it matters.
Yes, your drawings are right.

I think v101 and 201 are the most important tubes for humming in my unit.

yes, i put the same telefunken tubes in.

ChuckD hat geschrieben:Hum increases with more gain as well.
At max gain with no input hum is peak at -40dBu

shouldn't be the tubes then

How do I connect the Edcors to ground?
Also should my input transformers be connected to ground as well?

Might as well shield everything imaginable first right.
I will also grab some copper tape from work for shielding the power lines.

What else can cause humming.... seems like a grounding problem to me.

-Chuck