#26 pre amp

[merlin el mago]

Hi!



Seems your linestage is not out of balance.
Also your test with reversing the channels shows that it is not the line stage.
Most likely a bad wiring as Cheah already suggested.



This is the big disadvantage of filament bias, if the tubes differ in emission they draw different currents. This causes a different voltage drop across the primary. In your case it means the difference between the two tubes is about 3mA which is a lot.

When using filament bias the tubes should be tested and the current should be verified in circuit since there is no self adjustment mechanism any more as in cathode bias

Best regards

Thomas

How do you calculate the 3mA, TIA for teaching me.

 

[king4joy]

Ooooops...forgot...you may want to try to switch the in-put into the line amp too....to see if the problem stays with the same channel....

 

[merlin el mago]

My lineamp only have one stereo input & one stereo output: at the moment no selector

 

[king4joy]

yes....switch the left input to the right....and the right to the left.....and hear....

 

[merlin el mago]

Now I can't do it, I will do tomorrow morning.

 

[Vinylsavor]

Hi!

No, I fixed these when the TVCs.

So you still have the channel imbalance?

Now I resoldered again & checked all input-outputs at Right channel also I'm rolling new tubes to see what's happen because the measurements are OK

Instead of just rolling tubes, maybe try a more structured approach.

Your measurements indicated identical gain. Try at different frequencies also to make sure

When you switched the left and right inputs at the TVC, it was still the same channel which appeared louder?

Again this would indicate that the problem is not in the line stage.

Best approach:

With everything connected, measure the signals at both channels:

At linestage input
linestage output
TVC input (inside your TVC box right at the TVCs)
TVC output

That will tell you where exactly the problem occurs

It would also be helpful if you report back more clearly what you have done and where you measured to help further

best regards

Thomas

 

[Vinylsavor]

How do you calculate the 3mA, TIA for teaching me.

Ohms law

 

[merlin el mago]

N.B. When I do the measurements I connected the probes: input direct to 100R-100K junction in place of the input RCA, output where connected to output RCA so can be a cold solder at the input RCA so I resoldered & tomorrow I will listen.

 

[merlin el mago]

Hi!



So you still have the channel imbalance?



Instead of just rolling tubes, maybe try a more structured approach.

Your measurements indicated identical gain. Try at different frequencies also to make sure

When you switched the left and right inputs at the TVC, it was still the same channel which appeared louder?

Again this would indicate that the problem is not in the line stage.

Best approach:

With everything connected, measure the signals at both channels:

At linestage input
linestage output
TVC input (inside your TVC box right at the TVCs)
TVC output

That will tell you where exactly the problem occurs

It would also be helpful if you report back more clearly what you have done and where you measured to help further

best regards

Thomas

Yes I have channel imbalance.

I have the TVCs in a separate box, I used without the lineamp and NO difference of gain between channels

So last measurements done without TVCs

Thanks for help

 

[merlin el mago]

Ohms law

I guess so but how do you calculate the resistance?

 

[Vinylsavor]

Hi!

So please do the measurement steps which I listed to isolate the problem.
Report back the results exactly.
Otherwise it gets confusing

Best regards

Thomas

 

[Vinylsavor]

Hi!

I guess so but how do you calculate the resistance?

Nothing to calculate look it up in the data sheet of the transformer. There is even a resistance mentioned in your schematic but the actual resistance is 1250 Ohms.

But don't get side tracked, you should hunt down your imbalance issue. When that is solved, you can use matching tubes

Thomas

 

[merlin el mago]

Here the new measurements with others tubes:

Right channel
B+ 141,6V
Heaters 1,35V

Left channel
B+ 142,3V
Heaters 1,35V

 

[merlin el mago]

Now it's only 0,7V of difference!!!!

 

[Noodle88]

So left and right channel balance?

 

[merlin el mago]

I can't listen today, tomorrow morning I will listen and let us know.

 

[king4joy]

It could be something very simple....and I hope so.....cheers...good night...

 

[merlin el mago]

Thanks to everybody.

 

[Vinylsavor]

Hi!

I can't listen today, tomorrow morning I will listen and let us know.

Why don't you do the measurements as suggested instead of blindly poking around?

Thomas

 

[merlin el mago]

Because I guess is a cold joint & the measurements were OK except the difference of B+ that's the reason I rolled the tubes & resoldered the connections.