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    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

#26 pre amp

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#1,103
Hi!
coolzero said:
I tried simple cathode bias and filament bias but end up with battery gird bias because I liked the sound. I initially tried passive LCL as well for PSU. But when used SALAS HVPSU, I felt a big improvement over sound and noise level so started using that.
Click to expand...

LCLC is not sufficient filtering for the linestage unless you use very high inductance chokes. The fact that you still had noise confirms that.

best regards

Thomas
 
#1,106
Vinylsavor said:
Hi!

Keep in mind that the sonic influence of the PSU is not the same with each circuit. A LC or RC coupled stage will react very different compared to a transformer coupled stage. It is important to keep the whole system in mind when you evaluate such things. What might work will in one context may not sound as good in another!

Best regards

Thomas
Click to expand...

Yes, that's a good point!

Andy
 
#1,108
Initial capacitor is a 0.1uF Russian teflon. L1 is 10H at 100mA, then 50uF cap, L2 is 15H at 30mA, then 47uf cap. Inside the preamp chassis I have a dropping resistor and then a 10uf cap. All caps are polypropylene motor runs. I have a 25K resistor to ground after L1 for critical inductance.

andy
 
Last edited:
#1,109
Hi!

I use LCLCLC filtering in my preamps. I use large inductances of 50-100Hy. Caps in the range 40-50uF. I want my preamps to be hum free.

The last LC section is separate for each channel.

Ultrapath requires much more ripple free B+ unless it is used in hum cancelling mode which I'm not doing. With conventional circuit you might get away with LCLC, but I'd use much bigger inductances. With the small currents needed by the 26, 100Hy is easily available

Thomas
 
#1,110
andyjevans said:
Initial capacitor is a 0.1uF Russian teflon. L1 is 10H at 100mA, then 50uF cap, L2 is 15H at 30mA, then 47uf cap. Inside the preamp chassis I have a dropping resistor and then a 10uf cap. All caps are polypropylene motor runs. I have a 25K resistor to ground after L1 for critical inductance.

andy
Click to expand...

I used almost similar configuration earlier (except different cap type) but I could hear slight noise. It almost disappeared with SALAS HV.
 
#1,111
Vinylsavor said:
Hi!

I use LCLCLC filtering in my preamps. I use large inductances of 50-100Hy. Caps in the range 40-50uF. I want my preamps to be hum free.

The last LC section is separate for each channel.

Ultrapath requires much more ripple free B+ unless it is used in hum cancelling mode which I'm not doing. With conventional circuit you might get away with LCLC, but I'd use much bigger inductances. With the small currents needed by the 26, 100Hy is easily available

Thomas
Click to expand...

This may be a dumb question but anyway I ask 🙂. I have two of 200H plate chokes. Is it possible to use those for this application?
 
#1,114
Hi!

Am new to this site and also a newbie to tube audio. 😱

After reading the posts from start to the latest (from coolzero) , I was inspired to resurrect my 26 pre amp, which I had ask a friend to build 2 years ago. Thanks to the wealth of info I got from this site, my 26 is now making music and is quiet. (It hummed and buzzed and sounded thin in my system before, that's why I abandoned it). Now I'm immensely satisfied with the sound of my system! 🙂

There's something that puzzles me with regards to the B+ reading I get. My friend told me the easiest way to get the B+ reading is to connect the probes of my multimeter to the cathode and plate of the tube. BTW my settings are Va= 138v; Vg=-9v; Ia=6mA. With the multimeter set at VDC, I did just as I was told to. With the RCA tubes in I got B+ readings of 144v & 147.9v. I only have 5 pairs of 26 tubes of different brands i.e., RCA, Silvertone, Philco, Sylvannia & NU and took B+ readings for each pair. The difference in B+ readings per pair were from a low of 2 volts (132.2 & 134.2) to a high of 13 volts (154.1 & 141.1). The pair with the lowest B+ reading, which was the Philco and which sounded poor in the original iteration of my 26 preamp sounded the best now!😕 ... Is this normal and did I do the correct procedure in getting the B+ reading?

Another thing that puzzles me is the filament reading I get. What I did was I took out the tubes from their sockets, turned on the preamp and stuck the probes of my digital meter to the pins assigned to the filaments. Both sockets gave me a reading of 4 VDC! ... When I placed back the tubes in the sockets, I get a reading of 1.4 VDC & 1.53 VDC. Just for curiosity sake, I added a 0.11R to the channel with the higher filament reading (hoping it would also bring down the B+ reading for that tube.) ... Turned on the preamp and took a reading of the filaments. I got 1.41 VDC & 1.45VDC, which I think is just fine. However, when I took the B+ reading for both tubes (same procedure as I did before), I got 144 VDC & 87 VDC! Before the addition of the 0.11R, the B+ readings were 144 & 147.9. I checked my solder connections and didn't find anything wrong with it. Turned on the preamp again and took a B+ reading ... same thing, 144 & 87! I took out the 0.11R and the reading is back to 1.40 & 1.53 (filaments) and 144 & 147.9 (B+). BTW these readings are for the RCA pair.

So sorry for the long post and newbie questions. Am still trying to learn more about tubes.

Please enlighten me on what's going on in my preamp. Why the difference in the voltages.

Humbly yours,
BenZene
 
#1,115
hi guys

i need help please. this is my 1st DHT preamp. i am using LOT for output and Rod's filamen

i just wired up my #26 and there is a loud hum. as loud as when we listen to music and maybe bit louder. i pass through the music tho. hence i dont wired it wrongly 🙂

my psu consist of 4 chokes. 2 for B+ and 2 for ground and 2 dropping resistor also one for ground and B+

the hum gets louder when i connect the inputs from the Dac which has output caps. while i dont have input caps for #26

when i float the ground from IEC earth, the hum is cut to half

im using 470K grid resistor to ground and 1K8 cathode resistor to ground

B+ is 170v
voltage at cathode is 13v
Plate voltage is 166v

any helps, pointers, suggestions are really appreciated
thanks in advance
erwin
 
#1,116
merlin el mago said:
For AZ1 tube hybrid rectification I only get 2 x 6VAC 12VA

MULTICOMP|MCF/N3506F|TRANSFORMADOR, 12VA, 2 X 6V | Farnell España

To drop 2V I will connect 1R 5W in each cathode, it's OK?
Click to expand...

andyjevans said:
Looks about right.
Click to expand...

Wich kind of resistors are the best suitables? both 1% tol.:

| Pasivos | Resistencias | Pasantes, fijas | No inductivo - Encapsulado por orificio pasante
TCR +-50ppm/ºC
Wire Wound Silicon Coated
5W

NS0051R000FB12 Vishay/Dale Wirewound Resistors - Through Hole
TCR 20 → 80ppm/°C
Thick film
15W

Others suggestions will welcome.
 
#1,120
 
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