#26 pre amp

[merlin el mago]

Both

 

[dhtrob]

just to give some more info.. If on one 26:
It maybe that heat on the plate is causing some tension/vibration on the inner structure of the 26. If it starts to "whistle" try putting your finger on it, gently pressing.. If the whistling diminishes, you know that it is a faulty tube that is susceptible to mechanical stress on the inner structure probably caused by heat..
Swap the 26 with the other one, if the problem moves with the 26.. well, you know..

 

[dhtrob]

Ah, posted at the same time..
Do they share the heater supply?

 

[merlin el mago]

No, only share B+

 

[merlin el mago]

I changed the AZ1 & both 26's and still the same issue....

 

[dhtrob]

Since it is common to both channels, it is likely that the problem is in something that the 26 share.. Can you be more specific what is shared and what not?
To rule out my earlier post: Do you have spare 26's so that you can try these before seeking the pin in the haystack (or B+)?

 

[merlin el mago]

Can be the OD3?

I have a resistance of 330 ohms between the paralleled cap of 0,1uF and ground.

 

[dhtrob]

Are you using one for both channels?
In my experience: No. It either ignites or not, or is igniting continuously which sounds like a *thumb* on the speakers each time is does.
Maybe the CCS is getting too hot?

 

[merlin el mago]

Yes, I'm using only one OD3 for both channels, I guess is enough?
No, I don't have the "thumb" when power ON.

 

[dhtrob]

Well, to rule out the obvious: Do you have spare? Swap everything that is easily swappable..

 

[merlin el mago]

OK I will do tomorrow starting with the OD3, I share my listening room.

 

[merlin el mago]

Rob YHPM

 

[merlin el mago]

Isn't the OD3, now I have "bufff" sound like having a lot fr air pressure at loudspeakers, I changed to the old OD3 but Buff still continue, changed AZ1 but still the same issue, checked CCS and is OK 32,25mA for both channels

Checked all B+ and heaters filaments and all voltages are ok, B+ left channel 143V & right channel 147V, also filaments are Ok right channel 1,381V left channel 1,368V, voltage across cathode resistor right channel 8,82V left channel 9,26V

 

[H.Michi]

I have a strange issue, after 2 or 3 hours of playback the #26 sounds but start to add to the song other not desired sound (exactly the kind of sound is like the end of a vinyl) once added this strange sound after 30 minutes or one hour start to sound a loud sound like 2 or 3kHz constant so I have to disconnect the preamp, any ideas to fix?

I had similer trouble on my 71A pre amp bfore.
That was direct coupling between two tubes.
Please try covered 26 with metal cans.

The purpose is to seal electrostatic noise as well as shielding direct coupling between two tubes. Additionally suppressing air-borne vibration is achievable.
Mechanical osillation (around 2kHZ) like a tuning fork occured on my naked experimental chassis.

Yes it is sensitive to the microphonics, so I built the amplifier in a stiff enclosure for shutting off air-born vibration and mounted 71A`s sub-chassis on to the main chassis via the rubber suspensions

 

[merlin el mago]

I never had microphonic noise with 26's, also I have the OD3 just between the two 26's...

 

[dhtrob]

Now that the issue is gone with the whislin', do you have the noise (air pressure sound as you call it) also with the other pair 26's?
I have some 26's that are really noisy, just like you described it (TungSols)

 

[merlin el mago]

Do you suspect of tubes? Ok I will change and rapport.

Rob YHPM

 

[merlin el mago]

Rob what's the "wishlin'"?

 

[dhtrob]

I mean the 2˜3khz tone, like a whistle..

 

[merlin el mago]

Thanks now I understand, the old sound was 1-1,5kHz not like a whistle.