#26 pre amp

[merlin el mago]

Hey guys I have inbalance problems: left channel sounds louder than right channel, also I use the TVCs alone & it seems my system sounds louder only with the TVCs and there isn't the inbalance problem...please help

 

[stajo]

sinus scope input-output on every device. could that be a start?

 

[merlin el mago]

Yes I guess is the way to catch the issue, but is a lot of strange that a pasive preamp sounds louder than an active preamp....

 

[merlin el mago]

Here the measurements:

Right channel
B+ 146V
Heaters 1,378V
Sinus: input 2Vpp output 4Vpp

Left channel
B+ 142,2V
Heaters 1,364V
Sinus: input 2Vpp output 4Vpp

Why have different B+ if both channels are supplied with a single 0D3? can be different heaters voltage? can be the reason I hear Left channel sounds louder than Right channel?

I'm sorry for a lot of questions.

TIA

 

[king4joy]

merlin....just a thought...just to be sure that the problem is with the line amp...please try at least once to switch left and right at the line amp output....to see if the problem still remains with the same channel....and go from there again...

 

[merlin el mago]

Thanks king4joy, switched the channels still same problem Left louder than Right without the lineamp only with TVCs both channels sound the same...what do you think of the measurements?

 

[merlin el mago]

I forgot to tell that I use 10 ohms as filament bias in place of 11R

 

[Noodle88]

Hi Merlin,
Please check line output wiring, if there's any dry joint or wrong connection.

Cheers,
Cheah

 

[merlin el mago]

That's a great idea, thanks. I will check but till tomorrow morning can listen with the the rest of the system.

 

[demetrios]

The problem depends on the type of tube 26 (year and way of construction , manufactor ...) regardless anode and heater supply.

 

[merlin el mago]

Done resoldered all input-output joints & checked the wiring connections.

 

[Vinylsavor]

Hi!

Here the measurements:

Right channel
B+ 146V
Heaters 1,378V
Sinus: input 2Vpp output 4Vpp

Left channel
B+ 142,2V
Heaters 1,364V
Sinus: input 2Vpp output 4Vpp

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.

Why have different B+ if both channels are supplied with a single 0D3?

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

 

[iko]

Yes, roll different tubes. I had the same problem with perfect measurements. Had to find two tubes which matched.

 

[merlin el mago]

Thanks guys I also suspected of the tubes, now I'm adjusting two Cunningham CX-326 globe

 

[king4joy]

Thanks king4joy, switched the channels still same problem Left louder than Right without the lineamp only with TVCs both channels sound the same...what do you think of the measurements?

That to me you have isolated the problem to within the line amp....only between the left and right within the line amp....then try switching left and right within the line amp (for example....tube...power to the tube....and filament....etc..etc...) ...and you will isolate the problem very soon...one can only hope....

 

[Vinylsavor]

Hi!

But the difference in tubes obviously is not the cause for your imbalance, as your measurement and test indicates.

When you reported some lack of bass with the TVC I already suggested that there seems to be some wiring problem. You should fix that

Thomas

 

[merlin el mago]

Hi,

I fixed: the OPT outputs where phase reversed & I taked off the siwtch between the OPT & GND (you can see in my latest uploaded schematic).

 

[Vinylsavor]

So problem is solved? Both channels equal now?

 

[king4joy]

 

[merlin el mago]

No, I fixed these when the TVCs.

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