It is currently Mon Mar 16, 2020 8:56 pm

All times are UTC - 7 hours



Post new topic Reply to topic  [ 9 posts ] 
Author Message
Post subject: Bias probe that reads plate voltage and current?
Posted: Tue Jul 29, 2014 6:50 am
Offline
Hobbyist
Hobbyist

Joined: Thu Jun 12, 2014 9:28 pm
Posts: 55
I am looking for a bias probe. I see some measure current only but some like the Eurotube bias probe and Compu-bias measure voltage and current. Is that necessary or is one that measures just the current ok?


Top
Profile
Fender Play Winter Sale 2020
Post subject: Re: Bias probe that reads plate voltage and current?
Posted: Tue Jul 29, 2014 7:28 am
Offline
Amateur
Amateur

Joined: Sat Jul 07, 2012 3:21 pm
Posts: 164
In order to determine the correct bias current to adjust your amp to, you need to know the voltage at the plate. Also, bias current will be different depending on the tube type, i.e. 6V6, 6L6, EL34, etc. You can measure the plate voltage with a digital multimeter at the correct tube socket pin, or you can buy a bias probe that has the plate voltage measurement built into it.

Once you know the tube type and the real plate voltage, you can find the correct starting point, typically the 70% point, with this online bias current calculator:

http://www.ax84.com/biascalc.html


Top
Profile
Post subject: Re: Bias probe that reads plate voltage and current?
Posted: Tue Jul 29, 2014 2:58 pm
Offline
Rock Icon
Rock Icon
User avatar

Joined: Tue Jun 29, 2010 1:20 pm
Posts: 9640
Location: Indiana
It is fairly easy to measure the Plate voltage with a multimeter, not so with the current. While a probe that measures the voltage is not necessary, it makes things more convenient.

My choice of probes, offered with either simple current probes, or probes that measure current and voltage.

http://www.asharpfretworks.com/5543.html

Image

_________________
---> "The amp should be SWITCHED OFF AND UNPLUGGED before you do this!" <---

Por favor, disculpe mi español, no se llega a la práctica con mucha frecuencia.


Top
Profile
Post subject: Re: Bias probe that reads plate voltage and current?
Posted: Tue Jul 29, 2014 4:20 pm
Offline
Aspiring Musician
Aspiring Musician
User avatar

Joined: Sun Apr 14, 2013 4:41 pm
Posts: 764
Location: Sydney. Australia
Mowgli77 wrote:
I am looking for a bias probe. I see some measure current only but some like the Eurotube bias probe and Compu-bias measure voltage and current. Is that necessary or is one that measures just the current ok?



Go for the eurotubes probe, its an easy to operate tool & accurate.

The downside is that you can only measure one tube at the time, but this doesn't bother me.

Eurotubes are known for top notch customer service too.

:)


Top
Profile
Post subject: Re: Bias probe that reads plate voltage and current?
Posted: Tue Jul 29, 2014 4:55 pm
Offline
Amateur
Amateur

Joined: Sat Jul 07, 2012 3:21 pm
Posts: 164
Unless you have individual bias pots on each tube, you don't need to measure the bias current on more than one tube...assuming you have a matched pair or quad in your amp. In all amps that I'm aware of, there is one bias pot that adjusts bias for both or all four tubes. Wherever you set the bias for one tube, all the others will be similarly affected. Therefore, all you need is a one-socket bias probe. It's nice and also convenient to have a probe that reads both plate voltage and bias current, but not essential. It's also convenient to read voltage and current on both or all four tubes, but not essential. It's really up to you and how much you want to spend.


Top
Profile
Post subject: Re: Bias probe that reads plate voltage and current?
Posted: Tue Jul 29, 2014 5:19 pm
Offline
Aspiring Musician
Aspiring Musician
User avatar

Joined: Sun Apr 14, 2013 4:41 pm
Posts: 764
Location: Sydney. Australia
Lynn Nicholson wrote:
Unless you have individual bias pots on each tube, you don't need to measure the bias current on more than one tube...assuming you have a matched pair or quad in your amp.


Assuming .. 'matched pairs' are not always matched as sellers claim.
You need to measure each tube to ascertain where the bias and VDC are.
Do not assume anything, better measure and bias all power tubes .. :mrgreen:


Top
Profile
Post subject: Re: Bias probe that reads plate voltage and current?
Posted: Tue Jul 29, 2014 5:24 pm
Offline
Hobbyist
Hobbyist

Joined: Fri Nov 01, 2013 5:33 pm
Posts: 35
This one will read Plate voltage as well as current, you just have to swap one lead around during the process. You'll need a DMM that will read Milivolts, a cheap one works fine.
ThePros:
Works great and is very in-expensive.
Cons:
Its a kit so some assembly and a soldering iron required.

https://www.tubedepot.com/products/tubedepot-bias-scout-kit


Top
Profile
Post subject: Re: Bias probe that reads plate voltage and current?
Posted: Fri Aug 01, 2014 7:34 am
Offline
Amateur
Amateur

Joined: Sat Jul 07, 2012 3:21 pm
Posts: 164
I have a confession to make, Mowgli77. Just a couple of days ago I put a new pair of JJ power tubes in my DRRI. Since my bias probe only reads current and it is essential to measure plate voltage with the new tubes installed and operating, I had to remove the chassis from the amp cabinet, install the tubes and bias probe, use two meters, one to measure plate volts on pin 3 and one to measure bias current out of the probe. After I got it right, I had to reassemble the whole thing. It is only the second time I've been through the procedure, which took about an hour.

Watching the "How to" videos on the Eurotubes website where Bob Pletka uses a probe with both meters built in, the whole procedure takes about 15 minutes including warm-up time for the amp.

Now, I'm almost 67 years old and I hope that I've done my last retube. You shouldn't have to do this very often unless you are an experimenter or pro tech. But if you DO have to retube frequently, then the probes with the meters built in are real time savers and worth the money!


Top
Profile
Post subject: Re: Bias probe that reads plate voltage and current?
Posted: Fri Aug 01, 2014 7:52 am
Offline
Hobbyist
Hobbyist

Joined: Thu Jun 12, 2014 9:28 pm
Posts: 55
Hi, I cancelled the Allessandro (sp?) probe order and instead I ordered a Compu bias Probe that gives me the additional readings. Thanks!


Top
Profile
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 9 posts ] 

All times are UTC - 7 hours

Fender Play Winter Sale 2020

Who is online

Users browsing this forum: Google [Bot] and 2 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum

Search for:
Jump to: