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#26 pre amp

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#3,221
I have a lot of hum today...)
I measured the Rods reg Raw AC ripple is around 20-40mV,
the filament has less then 0.01mV AC ripple. I assume this is not the source of hum.
When I put my hand on one of the caps for raw filament filtering the hum changes but its not less, also when I touch the heatsinks the hum changes a bit..
Now to my B+, I am using the RGN504 followed 0.5uF + 50H choke + 5uF + 50H choke + 10uF. The AC ripple at the end of B+ is 20mV. I guess this could be the source of hum.
The psu togeter whit raw filament is on one board and the amp is separate board,
around 30-50cm from it...
Had some one similar problems?

The 26 is also very sensitive to touch, very microphonic!!!
 
#3,223
DIYspaceW said:
I have a lot of hum today...)
I measured the Rods reg Raw AC ripple is around 20-40mV,
the filament has less then 0.01mV AC ripple. I assume this is not the source of hum.
When I put my hand on one of the caps for raw filament filtering the hum changes but its not less, also when I touch the heatsinks the hum changes a bit..
Now to my B+, I am using the RGN504 followed 0.5uF + 50H choke + 5uF + 50H choke + 10uF. The AC ripple at the end of B+ is 20mV. I guess this could be the source of hum.
The psu togeter whit raw filament is on one board and the amp is separate board,
around 30-50cm from it...
Had some one similar problems?

The 26 is also very sensitive to touch, very microphonic!!!
Click to expand...

no the 26 is very silent with good DC regulator like the coleman ( i had a 102DB speaker) but you probably need more cap filter i also had a separate CLCLC filter on mine but the last two C are 30uf and 60uf MKP .
i'm sure it will helps 🙂
 
#3,225
Rod Coleman said:
Hi - Please use the circuit shown in the PDF document ANdht01.

The problem here: the rectifier and capacitors must not be grounded at any point - this will cause the cathode-bias to be (near) zero.

The correct circuit is shown in the PDF.

If you like, you can add extra chokes (CMC and differential), but please check that the total dc resistance adds up to about the same as the examples in the PDFs.

You can add extra high frequency caps, to try them out.
Click to expand...

Hi Rod,

I have few filament boards for previous plan of pre-tube phono many years ago. This is Salas' design. I intend to use them for DHT and connect 2 tubes 26 in series like DHTrob's way.

D17 will be non using.

How do you think about this ?
 

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#3,226
There are all kinds of filament regs that work, that's not a problem. But Rod's is a step up because it's been carefully designed through several iterations to be a level higher than "just working". It depends how important you think the filaments are. If you're using filament bias, you would probably take the view that they are critically important. If not, it's up to you really. Rod has put down the marker.
 
#3,227
I have few filament boards for previous plan of pre-tube phono many years ago. This is Salas' design. I intend to use them for DHT and connect 2 tubes 26 in series like DHTrob's way.
Click to expand...

Not being Rod, I would like to mention that if you are using one filament voltage for both '26's in series, center connected to ground, this design will be quiet enough.. Using Rod's filament supply will result in better performance though..
If using filament bias, this design will be noisy (hissing noise from speakers).
 
#3,228
tcqanh said:
Hi Rod,

I have few filament boards for previous plan of pre-tube phono many years ago. This is Salas' design. I intend to use them for DHT and connect 2 tubes 26 in series like DHTrob's way.

D17 will be non using.

How do you think about this ?
Click to expand...

Hi, I think the earlier one you posted with current-control - will be easier to make point-to-point, and current control will be better,.

But watch out for noise [and degraded sound from intermodulated noise, etc]
as Andy and Rob mention.
 
#3,237
6em7 better than 27

coldcathode said:
The hum is ripple in your filament supply. What you did "by mistake" was twofold. You "star grounded" and added a "C" filter.

You can actually incorporate this into your filament supply circuit.
Maybe increase the value of that capacitor also. 1,000uF??

Make sure the B+ capacitors and chokes are of good quality, but I doubt the hum is coming from there.

By the looks of it the "bias" of that tube is actually more around -15V (~6.2ma thru a 2K2 cathode resistor.

It appears he took the operating point for the 26 straight out of the RCA manual. Vp 180 Grid Bias -14.5V Current 6.2mA

What you might consider is placing the 2K2 Cathode resistor and the 470uf on the center tap of a 50 Ohm 1W potentiometer. The two outer tabs of the potentiometer then go to the filament pins.

This is a "hum balance" pot.

Check your voltages proper operation should be at idle (short the input)

~194 - 195V at plate connection to ground
and pretty close to -14.5V grid to ground.

He is running the 26 kind of "hot" the other designs I have seen show it running more at Vp135 current 5.5mA
Find a copy of RCA RC-10 and look at specs.

Good luck, I am working on a 27 preamp right now.
Click to expand...

I like my 26. However, sometimes, I prefer my 6em7 preamp.

using ultra path parafeed, that amp has best highs I heard and solid base.
 
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