AMP-HEAD Dual Bias Tester - is true?
mig50
could someone tell me something about this?
AMP-HEAD Dual Bias Tester - Guitar Amp Tube Probe
http://amp-head.com/product_info.php?products_id=31
is it effective or easy? im far from a good technician ,and it would
be great for me doing the bias myself on my mig 50.
Thanks anyway
PMG
I wouldn't buy one from them. You can get them from other people.
There aren't many amps that you can bias properly without removing the
chassis from the cabinet, and I didn't notice any indication that
theirs gives you a plate voltage reading, so what they're showing you
in their picture, I'm not sure.
Aside from how I wouldn't buy a bias probe from those guys, read this:
http://duncanamps.com/technical/lvbias.html
Which may seem slightly intimidating, but pull out a notebook, and
write down notes pertaining to you, and be careful not to leave out
anything important, and be very careful.
I use a bias probe from Weber fwiw.
Pete
--
Now you know why I used the qualifier "practically" --Bender
Dave Curtis
I use a dual bias probe from Fluke; One Fluke for the voltage and the
other Flike for the current.
:o)
-Dave
PMG
wrote:
Fluke sells a bias probe? I didn't know that.
Who's Drivin?
"PMG" <avo...@comcast.net> wrote in message
news:428g40pr5nrpp98fn...@4ax.com...
He's kidding Pete, one fluke to read plate volts another to read current.
same way I do it. no probe, just two meters.
Dave
PMG
I use two DMMs, but not Flukes, and I DO use a probe.
Zorrro_2k
a) Does your Mig 50 amp HAVE an adjustable bias ? If not, then this
item will be of no use to you.
b) You will probably have to remove your amp chassis from the cabinet
and put it on a bench/table to get at the bias adjustment. Do you feel
comfortable about this ?
c) Do you know the bias point of your amp is so that you would know,
from the readings taken from the tester, IF it needs to be re-biased
and where to set the re-bias at ?
Zorrro_2k
Another EXCELLENT disertation on biasing is on the AIKEN Amplifier web-site.
Go to www.aikenamps.com
Click on TECH INFO on the left side
Click on ADVANCED (AIKEN)
Scroll down and select 'The Last Word On Biasing'
Read........
You might also want to read " Idle Current Biasing- Why 70%" , and
"The Last Word On Class A" while you are there.
PMG
wrote:
Is it really the last word on Class A? I'd love to see that happen.
There seems to be an obsession with that subject around here.
Zorro_2K
I like a single ended Class A amp for it's clean tone, but for 'nads I'll
take the old AB1 amp, thank you.
Dave Curtis
What he said. (Thanks, other Dave)
>
>I use two DMMs, but not Flukes, and I DO use a probe.
>
>Pete
I guess I learned to bias without one and never bought (or made) one.
I would guess it's easier, and I know it's safer. How do you like your
probe, Pete? Did you get it from Weber? (I think it was you that I
recall saying that)
-Dave
PMG
wrote:
Yep. I just got the probe, instead of the setups with the meters.
It works just fine. My only real complaint about it is that it's body
is a piece of copper tubing (around 1" dia.) so it can get kind of
hot, so sometimes I have to wait a while for it to cool down. Makes
me wish I'd bought two sometimes, because my SC120 has separate bias
adjustments for each side.
mig50
> a) Does your Mig 50 amp HAVE an adjustable bias ? If not, then this
> item will be of no use to you.
Yes it have
> b) You will probably have to remove your amp chassis from the cabinet
> and put it on a bench/table to get at the bias adjustment. Do you feel
> comfortable about this ?
there愀 no problem about that
> c) Do you know the bias point of your amp is so that you would know,
> from the readings taken from the tester, IF it needs to be re-biased
> and where to set the re-bias at ?
I have write to amp-head and they told me it comes with instruction
about readings but I supposed it would be the same as any other amp
with two 6L6 and three 12ax7, or not?
I havent changed those for two years and i will like to change my
power tubes once a year or two (more or less)
Bias is needed when you change power tubes, no?
Any information would be great.
Thanks again
WakyAmps
news:nu8i40d7uuvqp7t18...@4ax.com:
[snip]
> I guess I learned to bias without one and never bought (or
> made) one. I would guess it's easier, and I know it's safer.
> How do you like your probe, Pete? Did you get it from Weber?
> (I think it was you that I recall saying that)
>
> -Dave
I'm using one of Ted Weber's probes. For me, it is just simpler
than using a pair of DMMs (or one DMM and range flipping).
Ted sells them as plain adapters with leads hanging off for
your DMM, and also with a small digital meter head. I am using
the latter (else, kinda, what's the point?)
He makes 'em with up to four socket adapters and is willing to
do odd combinations. The one I'm using has two octals and two
nine-pins. Having the extra two sockets flopping on the bench
is a bit of a nuisance, but it's nice to be able to do octals
and EL-84s (or any other 9 pin) with the same device.
The version I have has three switches:
- on-off
- a rotary selector to choose which adapter head the meter is
monitoring
- a small toggle to select VDC or mA display.
To use, just throw it in between tube(s) and chassis, let
things warm up, flip switches and look at numbers. Keep a
calculator (or an open excel sheet) handy for figuring
dissipation. Ted claims that there is a version coming that
will display watts directly.
The good things:
- It is safe and simple. You don't have to solder in a resistor
on the client's amp, you don't have to hold a DMM on the OT.
- I've compared readings against my DMMs and found the device
to be accurate (or at least as accurate as my DMM)
The neither good nor bad things:
- You have to decide whether it is worth the $100 (or whatever
it works out to these days). For me, doing bias jobs all the
time, it's paid for itself easily in saved time.
- As Pete points out, the copper sleeves get pretty hot.
Particularly in something with inverted tubes. Again, for the
casual user this can be a pain, but I keep a heat-proof tray
and a kitchen mitt near the bench anyhow for messing with hot
tubes.
The bad things:
I'm actually on my second one. The first one had switches in
the octal adapters for selecting cathode on pin 8 or pin 5 (so
you could do 7591A biasing). One of the switches went up in
smoke.. not sure if it was just a bum switch, couldn't take the
cathode current, couldn't take the heat, or a combo of all
three. Ted replaced the unit (sans switches) promptly.
In summary, if you're doing bias jobs all the time, I'd argue
that having one or two of these is well worth it. If you're an
occasional user the value may not be there for you.
WakyAmps
news:c365eab7.0403...@posting.google.com:
As I wrote earlier, I would not recommend the amp-head unit
that you originally referenced. It only tells you half of the
story. Unless you know the plate voltage on the output tubes,
knowing the plate current[1] doesn't do you any good. Ted Weber
, Bias-King, and others make a similar unit that show both
volts and amps. Multiply the two together to get watts of
dissipation.
You won't need to use it on the 12AX7s in your amp. These
probably don't have adjustable bias anyhow.
For the 6L6s, assuming that your amp is biased normally for
class AB1 operation, a good rule of thumb is to adjust the
power section at idle to 70% of rated dissipation _or_less_
A 6L6GC is rated at 30w, so your maximum should be around 21w
at idle. Please read:
http://aga.rru.com/TechTips/disp.cgi?file=LV/BiasAdj.tt
before proceeding.
[1] Small caveat, these types of units actually show plate and
screen current combined, so the calculated dissipation is
actually a tad higher then what's really going on.
PMG
I like the idea of how you're doing it.
What I'm doing, to make a long explanation short is clipping a second
DMM to pin 3, and the chassis, with alligator clips, and that just
stays hooked up the whole time.
One interesting thing that I've noticed from doing this a few times,
is that the plate voltage drops to the specs listed on the amp's
schematic and layout, right about the time I get the amp biased to a
range that sounds good to my ears. Which isn't something that I can
explain, but it does seem to happen.
mig50
thanks,
do you know if he is still selling those bias meters? outside USA?
i´m from Spain and maybe one of those could be usefull
Best regards
PMG
wrote:
>thanks,
>do you know if he is still selling those bias meters? outside USA?
>i´m from Spain and maybe one of those could be usefull
>Best regards
I don't know what's involved in buying one if you're outside of the
USA. I think they can ship international, but you might want to study
what's on their webpage, and maybe email them.
There's always the possibility that someone over in Europe sells bias
probes.
Some guys make their own bias probe, but I didn't have a spare tube
socket, and didn't feel like doing the work. I felt that it was worth
the $25 I paid for it, just to have someone else make it for me.