#26 pre amp

Vinylsavor · 2011-07-04 5:45 pm

merlin el mago said:
So if I use LL1660 & TVC can I avoid the output cap?

Yes, but the last cap in your PSU or reg if you have goes fully into the signal path and will influence the sound. Only the best should be used there

Thomas

merlin el mago · 2011-07-04 6:05 pm

Ok thanks for advice Thomas.

Felipe

Vinylsavor · 2011-07-04 6:15 pm

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.

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

andyjevans · 2011-07-05 10:00 am

I have to admit that my LCLC filtering is exactly the same as Rod Coleman's. I don't have an issue myself.

What do you guys think of glow tubes? I've used them in the past, but not sure they are any better than caps and chokes.

Andy

Vinylsavor · 2011-07-05 10:30 am

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

andyjevans · 2011-07-05 1:04 pm

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

Yes, that's a good point!

Andy

coolzero · 2011-07-05 4:33 pm

andyjevans said:
I have to admit that my LCLC filtering is exactly the same as Rod Coleman's. I don't have an issue myself.

What do you guys think of glow tubes? I've used them in the past, but not sure they are any better than caps and chokes.

Andy

What values you use for L1, L2 and C4?

andyjevans · 2011-07-05 5:08 pm

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

Vinylsavor · 2011-07-05 5:19 pm

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

coolzero · 2011-07-05 6:29 pm

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

I used almost similar configuration earlier (except different cap type) but I could hear slight noise. It almost disappeared with SALAS HV.

coolzero · 2011-07-05 6:33 pm

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

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?

Vinylsavor · 2011-07-05 6:39 pm

Hi coolzero,

if their current rating is sufficient, yes.

Thomas

coolzero · 2011-07-06 5:41 pm

Vinylsavor said:
Hi coolzero,

if their current rating is sufficient, yes.

Thomas

It is rated 10ma.

BenZene · 2011-07-10 4:33 pm

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

milen007 · 2011-07-17 6:37 pm

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

merlin el mago · 2011-07-17 7:01 pm

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?

andyjevans said:
Looks about right.

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.

andyjevans · 2011-07-17 7:15 pm

merlin el mago · 2011-07-17 7:19 pm

I used Welwyn too, are great resistors, thanks Andy.

Vinylsavor · 2011-07-17 7:22 pm

Hi!

milen007 said:
my psu consist of 4 chokes. 2 for B+ and 2 for ground and 2 dropping resistor also one for ground and B+

In order to help, some more information is required. A schematic maybe? Did you measure the ripple present on B+ and filament?

Is the signal ground connected to chassis?

also a photo of the wiring would be helpful

Thomas

milen007 · 2011-07-18 9:52 am

hi Thomas

i sketch up a schematic. hope they good enough
B+ is vary around 2v

hope the link i paste the pictures works

https://picasaweb.google.com/lh/photo/ujJxq5OzLZvEgEAbXpUVNni7EE8OttjnbsitFa40ZWg?feat=directlink

https://picasaweb.google.com/lh/photo/0ZHRzPHD6FUC9MLmJoN8Yni7EE8OttjnbsitFa40ZWg?feat=directlink

schematic
https://picasaweb.google.com/lh/photo/boFi7EXH6syn0P5fv9ZKD3i7EE8OttjnbsitFa40ZWg?feat=directlink

ground wiring
https://picasaweb.google.com/lh/photo/ORsPdRnbHmgGyqO1-r-kpHi7EE8OttjnbsitFa40ZWg?feat=directlink

#26 pre amp

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