external bias adjustment and test point terminals

[wafdaddy]

hi fellas, I was just wondering where I could find a 10 turn 2 watt trimmer that would mount on the back of my express and also what type of test points should I use? in other words I want to be able to change tubes and reset bias without having to take the chassis out of the cab. I checked mouser and some other places but I can't locate one, what do you guys suggest for a terminals to plug my dmm into? I was thinking banana plug terminals or is there a better choice to use? by the way is 2 watts enough or is that overkill.

[Structo]

The bias test jacks are called tip jacks.

[img:130:99]http://www.hoffmanamps.com/MyStore/cata ... pjacks.jpg[/img]

http://www.hoffmanamps.com/MyStore/perl ... =314116973

Don't know about a 10 turn trimmer.

I just use some single turn PEC pots I sourced locally.

[stink]

This is what I used in my build:

http://www.alliedelec.com/Search/Produc ... 22B-10-50K

http://www.alliedelec.com/Search/Produc ... 10&MPN=462

-Steve
________
HERB SCALES

[PlinytheWelder]

I've installed these surplus 25K Allen-Bradley pots from Surplus Sales of Nebraska in a couple of amps and I really like them....

http://www.surplussales.com/Potentiomet ... Adj-4.html
[img:102:103]http://www.surplussales.com/Images/Pote ... ysa253.jpg[/img]

[Structo]

That's the type I use.
Mine are made by PEC which are a sealed pot.

The locking nut is a great feature for a bias pot.

[Ron Worley]

Tom-

Can you share your PEC part number and where you get them??? Many thanks!

[wafdaddy]

allright!! you guys are great, thanks.

[Structo]

As I told Ron, I bought these PEC bias pots at a local surplus type electronics house.
They have new stuff too but I think a lot of it is liquidated overstock type stuff.

They have a small section that is all PEC pots. But they seem to be all linear. So I have bought two of the locking bias type pots and I think I have bought a couple 1M linears.
They charge like $10 each for the PEC pots.

It's always hit or miss at the at store. They have a pretty good assortment of resistors from 1/4 watt up through 25 watt. Flame proofs, etc.
The cap section is not as good.

I was looking for a 100K 1 watt a while back and they were out of it in the 1 watt, 2 watt, 3 watt.
I was in the other day and they were still out of those and it's been about 2 months. So their reordering system leaves much to be desired.
But, it's the only gig in town other than Rat Shack where you are lucky to find 1/4watt resistors let alone any other part related to a tube amp......

They have some electrolytic caps there that look 15-20 years old, new in the package.
When I buy small bypass caps or other small electrolytics, I always look the package over to try and guess it's age.
If the cardboard backing isn't yellowed with age I figure it was made this decade..........

[wallybob]

wafdaddy - By my calculations, a 2W pot is not necessary in this application. I certainly wouldn't hurt, but is not required. TIP jacks are what I used on my build - they are designed to be a mating connector for the meter probes.

[Fadedsunburst59]

I was wondering how to hook these tip jacks up they seem to only have one connection on them so where does that go?

[paulster]

I was wondering how to hook these tip jacks up they seem to only have one connection on them so where does that go?

Tie pins 1 and 8 together at each tube socket, and from there go to the red test point for each tube.

Tie both of the red test points to the black test point via a high-precision 1R resistor, and then tie the black one back to ground.

You'll then be measuring the voltage drop across a 1R resistor, which by Ohms Law makes it really easy to measure the cathode current.

[Fadedsunburst59]

So I would have two red's and two black's right? one of each for each tube? What would I tie them together with? Pic's?

[paulster]

So I would have two red's and two black's right? one of each for each tube?

No, you'll have two reds, one for each tube, and one black that's common.

What would I tie them together with?

Wire, ideally.

Pic's

This is an example that shows you how to do it. It has 4 power tubes, so each red test point is shared across two tubes, but it should still show you what you need.

https://tubeamparchive.com/download/file.php?id=7567

[Fadedsunburst59]

Ok, So in the pic I see two tubes are wired to the same red and because I have two el34's I would have two red's one to each and one black? and when I bias them what do I do? Sorry for all the question's but I am REALLY REALLY new to all of this and would like to learn what I could. Are there any Trainwreck wiring for dummies books that show you (If you change this resistor to this value it will sound more like this?)

[M Fowler]

Fadedsunburst59

Start here and read everything, then come back and ask questions so we all can help you understand better. Since you have an ill Ceriatone Express we can get you started by working on that amp and then graduate to you building a smaller simplier amp besides you will need a low volume amp anyway.


https://tubeamparchive.com/viewtopic.php?t=5691

Mark