6DJ8/ECC88-SRPP Tube Preamplifier

[merlin el mago]

I gorgot that did an anode follower with 88, sound energetic, very forceful but lacking of extreme highs, I will did with Salas HV version 1 & led instead resistor & capacitor in the cathode.

Cheers
Felipe

 

[revintage]

If lacking of highs, you did something wrong.

PM me with your E-mailaddress and I will send you a schematic. Waste of money with a fancy PSU to make it play well.

 

[merlin el mago]

Hi Lars,

You have pm with my email.

Brgds
Felipe

 

[merlin el mago]

I used B+ 280V & to elevate heaters I used:

R1 75K
R2 27K
C1 47uF

I used pin for B+ 280V

I have tube left channel:
between upper cathode pin 3 & 560R 147V
between 560R & 33K 144.9V
between 33k/15nF anode pin 6 142.7V

I have tube right channel:
between upper cathode pin 3 & 560R 136.5V
between 560R & 33K 132.2V
between 33k/15nF anode pin 6 121.7V

B+ connected 1st channel right & after in paralel channel left

Heaters connected 1st channel left & after in parallel channel right

Are the measurements OK?

I didn't see in both channels the filaments lit

 

[merlin el mago]

HV PSU have 200uF

 

[merlin el mago]

Attached pic

 

[merlin el mago]

For ECC81 it's necessary heaters at 12.6VDC when connected in parallel?

 

[revintage]

Hey Merlin,

Your voltages are totally wrong. Check what you used instead of 33k? 470ohm?

Series is 12,6V, parallell is 6,3V.

 

[merlin el mago]

Hi Lars,

The resistors values are OK:

R1 75K
R2 27K

See attached pic

Cheers
Felipe

 

[merlin el mago]

I disconnected B+, heaters don't lit, I measured heaters voltage pin 4 negative pin 5 positive both tubes 6.3VDC

 

[revintage]

R1 and R2 has nothing tio do with this.

If you have done your measuremants correct with ground as reference, your circuit is totally wrong.

And if you measure voltage over the resistors you should about 60 times as high voltage over the 33k than over the cathoderesistors.

Do you need the 34dB gain?

This circuit can also be made even better/equally good with a depletion fet instead of the upper tube. You can then dump most of the components.

 

[merlin el mago]

Yes all measurements referenced to ground, I measured with tubes connected, is OK?

 

[revintage]

If not connected you shouldn´t get any readings at all.

Still your voltages show you have done something totally wrong.

 

[merlin el mago]

Thanks Lars I will check again.

 

[merlin el mago]

Verified all components values & all components pins position and both are OK, it's possible the tubes are gone?

 

[revintage]

What is the voltage over only the 33k resistor?

What is the voltage over only the 560ohm resistor for each channel?

Note: Measure over the component not ref. ground.

Are you sure the tubes are ECC81/12AT7?

Is your voltmeter OK?

Have you ohm-measured the resistors?

 

[merlin el mago]

R2 is 27K my B+ is +280V

Voltage across R2 27K is 3.25V

Voltage across 560 ohms is 0.13V

I measured with two different voltmeters.

Yes I have ohm-measured each resistors and all are with correct values.

 

[merlin el mago]

I'm always speaking of the attached mu-follower schematic

 

[merlin el mago]

Yes both are ECC81 like the attached pic

 

[scott17]

I disconnected B+, heaters don't lit, I measured heaters voltage pin 4 negative pin 5 positive both tubes 6.3VDC

For 6.3V you connect one side of power to pins 4 & 5, the other side to pin 9.

I you have 6.3V on pins 4 & 5, then that's why your circuit does not work. The filaments are not running at the correct voltage.