#26 pre amp

[merlin el mago]

Merlin,
Have you tried LL2745 for your 26 preamp?
With 5.6:1 step down, lower output impedence will better match with your low impedence AVC.

Cheah

No, I haven't tried. I used the TVCs with SS mono amps (Erno Borbely "Millenium" 75W RMS Class A) so I didn't used tube amps (can match better, but I don't know).

 

[castellu]

Hi Magz,
beautiful work !

There are pretty 26 bijoux here...

B r u n o

 

[Rod Coleman]

hi Felipe,

1. The scope pictures are input and output?

2. If the gain is not correct, maybe the transformer is not wired the way you intended. If you need help with the wiring, you must give us a link to the exact model of transformer you have, and draw exactly how the transformer has been connected. With Lundahl, it's more complicated than with other maker's parts.

 

[merlin el mago]

Yes the scope are input & output.

Attached Lundahl PDF.

 

[merlin el mago]

Interstage transformer pics, cotton wire are input/output to anode, red wire are signal output-

 

[Rod Coleman]

If the vertical (Voltage) scale on the scope pics is the same for In & Out, Then it looks nearly 2x gain.

Does it sound good?

 

[merlin el mago]

New pics of LL1660 wiring B+ & anode

 

[merlin el mago]

New pics of LL1660 wiring signal output & output ground.

 

[merlin el mago]

If the vertical (Voltage) scale on the scope pics is the same for In & Out, Then it looks nearly 2x gain.

Does it sound good?

Yes is the same vertical scale voltage, yes looks near 2x gain but when I no connect the 26 and only use the TVC sounds louder....checked the TVC and there is no gain 1x😕

 

[merlin el mago]

Rod discovered the issue, right channel doesn't amplifies the signal. Any idea where to start to fix?

 

[Rod Coleman]

Debug in same way as any amplifier.

Measure all dc voltages.

Measure Anode current. Measure filament current.

Use scope to see ac waveform on grid, anode, and output.

 

[merlin el mago]

Thank you.

 

[Rod Coleman]

Felipe, you will get more help with amplifiers if you install a program to draw schematics.

You can use LTspice, if you don't want anything better. It runs on Windows (any version), and even better, it is designed for WINE, so any Linux PC, or even Mac can use it.

It takes only a few minutes to learn the schematic interface. And you can use triode, pentode and DHT symbols.

You can take a Screenshot of the schematic, and post it here.

With no schematic, it is very hard to give you definite help.

other ways of drawing schematic:

DIPTRACE (excellent) or Eagle (widely used) PCB design packages.
They have free versions for non-profit use.

 

[stajo]

If you have a tube with 6dB amplification and connect an opt 4.5:1 you will devalve the voltage of the signal with more then half. Do we agree on that?

 

[Magz]

If you have a tube with 6dB amplification and connect an opt 4.5:1 you will devalve the voltage of the signal with more then half. Do we agree on that?

mu of the 26 is 8

 

[stajo]

ok then the signal looks fairly good to me. No?

 

[merlin el mago]

Debug in same way as any amplifier.

Measure all dc voltages.

Measure Anode current. Measure filament current.

Use scope to see ac waveform on grid, anode, and output.

1st scope pic AC waveform on grid, 2nd scope pic AC waveform on anode & 3rd pic AC waveform on output. Also pic with schematic with measurements. Of course all pics and measurements are of the right channel where is no gain.

 

[stajo]

first pic, input, is around 1,5 units. second on anode is around 7,8 if you havent changed scale. That is a gain of 5,2 or 14,3 dB. Third is about 1,9 units which is about 1:4.5 from second (with some load loss)

 

[merlin el mago]

I haven't changed scale.

 

[merlin el mago]

Pics of left channel, 1st grid, 2nd anode & 3rd output.