#26 pre amp

[merlin el mago]

Attached pic right channel, scope set 5mV/cm 20uS

 

[merlin el mago]

Attached pic left channel, scope set 5mV/cm 20uS

 

[merlin el mago]

I guess DN2540 is oscillating?

 

[Rod Coleman]

A detailed photo of the wiring will be helpful, to debug any oscillation problems.

But 10mV of ultrasonics should not be enough to give such a bad reading for anode current.

 

[Rod Coleman]

I swaped the tubes (right for left) & the left heater reg the max voltage can reach is 1.365V, voltage at the input heater reg is 15.12V

With a 10Ω resistor for filament bias, you need 16,5-17,5V on the input, as shown on the diagram you posted.

 

[merlin el mago]

Wiring pic

 

[Rod Coleman]

Grid Wiring is much too long.

It would be better to mount the input sockets very near to the 26.
But, if you don't want to do that, please change the grid wire to screened cable (high quality). Quality Microphone cable can be used.

 

[merlin el mago]

With a 10Ω resistor for filament bias, you need 16,5-17,5V on the input, as shown on the diagram you posted.

Without load I measure 18.82V, isn't enough?

 

[Rod Coleman]

How long are the wires from the Filament Regulator to the raw dc capacitors?

 

[Rod Coleman]

Without load I measure 18.82V, isn't enough?

Off-load voltage does not matter. On-load, it must be 16V minimum, 17V normal.

 

[merlin el mago]

2.5 meters

 

[Rod Coleman]

Please check the PDF manual.

Above ~400mm wiring, You need to add Cap 1000uF 35V at the input to the Regulator, right at the PCB.

Panasonic FC, FM etc. is preferred.

 

[iko]

But 10mV of ultrasonics should not be enough to give such a bad reading for anode current.

I agree.

 

[merlin el mago]

Shortened the cable to 1 meter and added the capacitor 100uF 63V Pana FC at the regs input. Attached measurements right channel new tube.

 

[Rod Coleman]

OK, it should be working and sounding OK now?

The cap should be 1000uF, or at least 470uF.

When the grid wiring is shortened, or shielded, it should sound very good!

 

[merlin el mago]

Typo cap is 1000uF 63V Pana FC

 

[iko]

Shortened the cable to 1 meter and added the capacitor 100uF 63V Pana FC at the regs input. Attached measurements right channel new tube.

This looks much more plausible.

 

[merlin el mago]

Rod, left channel max reach 1.36V with shortened wiring and 1000uF cap.

 

[Rod Coleman]

Felipe, Have you corrected the low voltage at the Regulator input? for 10 ohm you need 16,5V min.

If that does not fix it, please post a photo of the regulator, so that I can see all the parts.

 

[merlin el mago]

Rod, both regs hace two equal and separate PSU, following your advice I check both PSU to ensure your regs have proper DC input.