#26 pre amp

[king4joy]

if you want to test your caps, solder cap into preamp output and bias with 1m to output.
then measure the dc voltage accross the resistor till it stabilizes. compare results with 200 , 400 or 1000v caps.
AC test would be to run 10khz into a 1k ohm load and measure voltage drop accross capacitor.


Appreciate if you could draw a simple diagram for each the above tests please ??

 

[euro21]

what is the lowest voltage rating that I could use please ??

The (theoretically) maximum output swing is V(B+) peek to peek, so output capacitor voltage range must exceed this value (about 1.5 multiplier).

So if you have 220V as B+, 400V capacitor is enough.
If you have 400V, 630V cap required.

 

[king4joy]

Thankyou.

Just for example, if I have 400V B+, can I use 400V main cap, and by pass it with a 1600V small cap ??

 

[merlin el mago]

Thankyou.

Just for example, if I have 400V B+, can I use 400V main cap, and by pass it with a 1600V small cap ??

No, better use 450V or more.

 

[sumotan]

Slightly off topic guys, without having to take Az1 route, whats a
nice sounding rectifier that you would recommend ?

Thks

 

[Vbenonisen]

5y3g or 80 is very nice if you plan to go directly heated.

 

[awkwardbydesign]

I tried the 80, but went back to the meshplate AZ1. In my set up it was a distinct improvement.

 

[sumotan]

Isn't the 80 over rated for this application ?
May I ask how does the AZ 1 sound like
& must it be only mesh plate

Thks guys

 

[Vbenonisen]

AZ 1 mesh sounds more open and natural. Yes, it has to be mesh for best sound.

I'll guess those electrons just loves that vacuum that's sipping trough the mesh everywhere

 

[sumotan]

Now that's an ouch for me . It's getting very expensive
the AZ1 especially the mesh plates. Isn't there any other
varieties that sound just as open ?

Thks

 

[awkwardbydesign]

I added an octal socket to my PSU, so I could quickly compare the AZ1 meshplate with Mullard AZ31s, supposedly the same as an ST AZ1 but with an octal base. The AZ1 meshplate sounded distinctly smoother and more refined, just the way it had against an ST AZ1. I use the AZ31 or ST AZ1 for testing now, rather than risk the meshplate!
This is the PSU chassis before I changed the second socket from UX4 to octal.

 

[Vbenonisen]

Now that's an ouch for me . It's getting very expensive
the AZ1 especially the mesh plates. Isn't there any other
varieties that sound just as open ?

It's more to sound and musicality than rectifiers. I'm using the 5y3g in my 26 preamp and it sounds just fine. Just build the amp with some flexibility (such as a removable board for the rectifier and some space for a couple of big wirewound resistors), and then you know you can add some expensive mesh'es later on. Anyway, the AZ1 is hardly the last word in mesh rectifiers. There are even better and more expensive ones out there.

By the way - Happy Easter fellow 26 and 01A preamp constructors!

 

[euro21]

went back to the meshplate AZ1.

I'm not surprised.
Mesh AZ1 (for preamp) is the best choice.

 

[awkwardbydesign]

I'm not surprised.
Mesh AZ1 (for preamp) is the best choice.

I'm playing with the AZ31 now, and it's lively, but "splashy", if that makes any sense! Some people might prefer it, as some prefer pop over classical (for example, not a judgmental opinion), but not so much to my taste. Still, I'll leave it in for a bit and see if I appreciate it more when I'm sober!
I love valves, the way I can mess around with the sound. I just wish I was a bit cleverer and understood them more.

 

[sumotan]

Well looks like I've no choice & I hate splashiness. lol
Think I read somewhere that the 6x series rectifiers
sounds good but can't recall

Thanks again guys

 

[Vbenonisen]

I'm in the prosess of finishing up my 2 chassis 26 preamp. Is it ok to use only one connector for the HT and filament wiring? Do I need some kind of separate screening of the wiring inside the umbilical?

 

[Rod Coleman]

Assuming that the Filament Regulator is near to the valve-socket, and all of the Raw DC caps are in the power-chassis,

AND

The B+ supply capacitors and chokes are in the Power chassis (a small cap in the signal chassis is OK).....

If you build it like this, the 'umbilical cable' should have only static dc voltages, and steady dc current.

Like this, running the B+ and filament raw dc in the cable is OK.

But if there is any pulsing-current in the cable, caused by capacitors in the signal-chassis - there will be some noise-coupling.

 

[Vbenonisen]

I do indeed have B+ capacitors and chokes in the signal chassis. Sigh! Ok then, two connectors with separate screening it is. Thank you Rod for responding so quickly.

 

[awkwardbydesign]

I use this circuit, with a choke followed by a cap in the signal chassis, with a single screened umbilical, and haven't noticed noise coupling.

Having said that, I do have a hum problem with the 01A, using the same layout, but a gyrator instead of a transformer.

 

[sumotan]

Hi Rob
Re regs how large do they need to be for testing purpose. ?
Oh on 01A's

Thanks