Dynavox VR70-E
 

Hi, first post! Im new to valve amps and decided to purchase one of
the afore mentioned as a cheap intro (ok, have a play with), Im
pleasantly surprised with what it offers. Ive made a decent input
selector and had a good snoop around inside. My only prob is how do I
bias the valves?? Ive no instructions on this. There are test points
at the side of the valve, I tried to get a DCV (350mV), and a
resistence (10ohms, I think) but TBH Im stuck, can someone help me
with this one???

Dynavox VR70-E
 

"StuH" wrote in message
om...
Hi, first post!

First off welcome!


You'll like it here - it's a regular 'home from home' for UK valvies!! :-)


Im new to valve amps and decided to purchase one of
the afore mentioned as a cheap intro (ok, have a play with), Im
pleasantly surprised with what it offers.


So far, so good then! ;-)


Ive made a decent input
selector and had a good snoop around inside.


Heh heh? What is it with valve amps that makes people want to get the lid
off as soon as they've got them??


My only prob is how do I
bias the valves?? Ive no instructions on this. There are test points
at the side of the valve, I tried to get a DCV (350mV), and a
resistence (10ohms, I think) but TBH Im stuck, can someone help me
with this one???


Right, I've been studiously avoiding facing up to one this ever since I got
into valves - due to having mostly cathode bias amps and a pair with a test
socket which clearly states 1.56VDC!! :-)

I don't yet fully understand the relationships between the voltage and
current readings myself yet. (I'm working on it, or will be in the v. near
future!) I have 2 of these amps and measured them as follows - both amps
have a resistance of 10R at the test point, power off and Amp No. 1 (with
valves marked '1a: 31 mA') seems to have a bias voltage of about 0.31V
(-0.31V with the leads the other way round I guess) and Amp No.2 (with
valves marked '1a: 27 mA') seems to be at the 0.33V mark - although this is
yet to be checked properly.

Someone here (Wally?) offered some 'correct settings', but I'm going to have
to do this all from scratch and try to understand completely that which I
only know in the broadest terms atm. So I'll be posting here as well as
worrying the arse off 'someone' offlist....!!! ;-)

Dynavox VR70-E
 

"Keith G" wrote in message
...

"StuH" wrote in message
om...
Hi, first post!

First off welcome!


You'll like it here - it's a regular 'home from home' for UK valvies!!
:-)


Im new to valve amps and decided to purchase one of
the afore mentioned as a cheap intro (ok, have a play with), Im
pleasantly surprised with what it offers.


So far, so good then! ;-)


Ive made a decent input
selector and had a good snoop around inside.


Heh heh? What is it with valve amps that makes people want to get the lid
off as soon as they've got them??


My only prob is how do I
bias the valves?? Ive no instructions on this. There are test points
at the side of the valve, I tried to get a DCV (350mV), and a
resistence (10ohms, I think) but TBH Im stuck, can someone help me
with this one???


Right, I've been studiously avoiding facing up to one this ever since I
got into valves - due to having mostly cathode bias amps and a pair with a
test socket which clearly states 1.56VDC!! :-)

I don't yet fully understand the relationships between the voltage and
current readings myself yet. (I'm working on it, or will be in the v. near
future!) I have 2 of these amps and measured them as follows - both amps
have a resistance of 10R at the test point, power off and Amp No. 1 (with
valves marked '1a: 31 mA') seems to have a bias voltage of about 0.31V
(-0.31V with the leads the other way round I guess) and Amp No.2 (with
valves marked '1a: 27 mA') seems to be at the 0.33V mark - although this
is yet to be checked properly.

Someone here (Wally?) offered some 'correct settings', but I'm going to
have to do this all from scratch and try to understand completely that
which I only know in the broadest terms atm. So I'll be posting here as
well as worrying the arse off 'someone' offlist....!!! ;-)






Hi Keith and Stu,

Fixed bias actually refers to the negative voltage applied at the grid of
the power valves to prevent them trying to pass infinite current and
destroying themselves. A typical value for an EL34 might be -35V.

When this bias voltage is established, we have a quiescent current through
the valve. Some amps have a built-in ammeter, to measure the current. When
using an external DVM it is a lot safer to measure a low voltage across a
fixed
resistor of known value between cathode and ground.

So, in your amp you have a resistor of 10 Ohms. That means, if you need a
quiescent current of 31mA you will see a voltage of 0.31V (i.e. 310mV
on your meter) when the bias is correctly set.. You probably have
four separate wirewound pots to adjust, one for each output valve.

I don't know what the plate (anode) voltage is on your amp, but 31mA
seems a little on the "lean" side to me. My own EL34 pp amp has a B+
of 425V, and is biased at 2V across a 39 Ohm resistor, and so has 51mA
idling current. You may have a higher anode (plate) voltage.

Set the bias after the amp has been switched on for about an hour, and
check it again after another couple of hours.

I have a pal who reckons he can hear the sweet-spot and set the bias by
ear. His amp sounds good, but get gets through a lot of valves:-)
Not recommended:-)

Iain

Dynavox VR70-E
 

"Iain M Churches" wrote


Hi Keith and Stu,

Fixed bias actually refers to the negative voltage applied at the grid of
the power valves to prevent them trying to pass infinite current and
destroying themselves. A typical value for an EL34 might be -35V.

When this bias voltage is established, we have a quiescent current through
the valve. Some amps have a built-in ammeter, to measure the current.
When
using an external DVM it is a lot safer to measure a low voltage across a
fixed
resistor of known value between cathode and ground.

So, in your amp you have a resistor of 10 Ohms. That means, if you need a
quiescent current of 31mA you will see a voltage of 0.31V (i.e. 310mV
on your meter) when the bias is correctly set.. You probably have
four separate wirewound pots to adjust, one for each output valve.

I don't know what the plate (anode) voltage is on your amp, but 31mA
seems a little on the "lean" side to me. My own EL34 pp amp has a B+
of 425V, and is biased at 2V across a 39 Ohm resistor, and so has 51mA
idling current. You may have a higher anode (plate) voltage.

Set the bias after the amp has been switched on for about an hour, and
check it again after another couple of hours.

I have a pal who reckons he can hear the sweet-spot and set the bias by
ear. His amp sounds good, but get gets through a lot of valves:-)
Not recommended:-)



OK Iain, that's good stuff. Have a look at this pic:

http://www.apah69.dsl.pipex.com/keit.../valvebase.jpg

and could you explain what the figures represent and how they relate to your
explanation above.

What does the following stand for:

Ua

Vg2

Vg1

Ia

S

and what do the Voltages tell us? Do they give us the plate voltage. Is that
an amp design thing or a valve design thing?

My guess would be that the bias can be set at 36mA (?), but what does that
handwritten figure mean? It's obviously taken from measuring the valves. All
four valves have the same figure and are presumably 'matched'???

On the other amp the figure on each of the valves is 27mA. Does this mean
there is a different bias figure for each amp?

Other pix on:

http://www.apah69.dsl.pipex.com/keit...chineseamp.htm


Also, what is the best way to set the bias - mA or Volts?

I guess what we (me and Stu) are looking for is the *correct* figure to set
the bias at and the best way to do it? (You are correct about the adjustment
pots, btw...)

Apologies for my iggerance - I'm new to this game, know bugger-all (never
claimed any different) and only learning as I go along! ;-)

Many thanks.

Dynavox VR70-E
 

Hi,

In message , Keith G
writes

"Iain M Churches" wrote

[Snip of good stuff from Iain]

OK Iain, that's good stuff. Have a look at this pic:

http://www.apah69.dsl.pipex.com/keit.../valvebase.jpg

and could you explain what the figures represent and how they relate to your
explanation above.

What does the following stand for:

Ua

Vg2

Vg1

Ia

S

Keith, have a butcher's at
http://www.hamradioindia.com/HRI-THE...es/valves.html

for a quick overview of what all those abbreviations mean. Bear in mind
that I did my EE degree at a time when it wasn't trendy to talk about
bottles, so I know exactly nothing about valve amps, but it seems to be
a decent introduction. Apologies if I'm teaching granny how to suck
eggs.


Apologies for my iggerance - I'm new to this game, know bugger-all (never
claimed any different) and only learning as I go along! ;-)

You and me both!


--
Regards,
Glenn Booth

Dynavox VR70-E
 

"Glenn Booth" wrote


Keith, have a butcher's at
http://www.hamradioindia.com/HRI-THE...es/valves.html


Jeez Glenn, that's right on the money!! Many thanks!

(Could cause even more questions, I suspect though....)



for a quick overview of what all those abbreviations mean. Bear in mind
that I did my EE degree at a time when it wasn't trendy to talk about
bottles, so I know exactly nothing about valve amps, but it seems to be a
decent introduction.


Yes it does - better than anything I've seen yet. I'll give it a good going
over!


Apologies if I'm teaching granny how to suck eggs.


What's an 'egg'....??? ;-)



Apologies for my iggerance - I'm new to this game, know bugger-all (never
claimed any different) and only learning as I go along! ;-)

You and me both!


Hmmm, except in my case I *mean* it!! :-)

Dynavox VR70-E
 

Keith G wrote:



OK Iain, that's good stuff. Have a look at this pic:

http://www.apah69.dsl.pipex.com/keit.../valvebase.jpg

and could you explain what the figures represent and how they relate to your
explanation above.

What does the following stand for:

Ua

Vg2

Vg1

Ia

S

and what do the Voltages tell us? Do they give us the plate voltage. Is that
an amp design thing or a valve design thing?

May heva been in the link, but my nameserver can't find it.

Neither, they are just the conditions that the valve was measdured at.
They would have set the Ua (voltage on anode) to 430v, Vg2 (voltage on
second or creen grid) to 440v, Vg2 (the first control grid) to -36v and
measured the current that flows through Ia (anode current), in this case
31ma. They then would have sorted each valve according to Ia, and those
with the same valye would be caled a matched set. I am guessing the S
could be for measuring transconductance.

If you look at the curve

http://www.mclink.it/com/audiomatica/tubes/el34.htm

And draw a line up from the x axis (Anode voltage, and see where it
crosses the grid curve at the grid voltage, that will give the expected
current (look at the pentode mode one, as g2 is in use).

--
Nick

Dynavox VR70-E
 

"Keith G" wrote in message
...

"Iain M Churches" wrote


Hi Keith and Stu,

Fixed bias actually refers to the negative voltage applied at the grid of
the power valves to prevent them trying to pass infinite current and
destroying themselves. A typical value for an EL34 might be -35V.

When this bias voltage is established, we have a quiescent current
through
the valve. Some amps have a built-in ammeter, to measure the current.
When
using an external DVM it is a lot safer to measure a low voltage across a
fixed
resistor of known value between cathode and ground.

So, in your amp you have a resistor of 10 Ohms. That means, if you need
a
quiescent current of 31mA you will see a voltage of 0.31V (i.e. 310mV
on your meter) when the bias is correctly set.. You probably have
four separate wirewound pots to adjust, one for each output valve.

I don't know what the plate (anode) voltage is on your amp, but 31mA
seems a little on the "lean" side to me. My own EL34 pp amp has a B+
of 425V, and is biased at 2V across a 39 Ohm resistor, and so has 51mA
idling current. You may have a higher anode (plate) voltage.

Set the bias after the amp has been switched on for about an hour, and
check it again after another couple of hours.

I have a pal who reckons he can hear the sweet-spot and set the bias by
ear. His amp sounds good, but get gets through a lot of valves:-)
Not recommended:-)



OK Iain, that's good stuff. Have a look at this pic:

http://www.apah69.dsl.pipex.com/keit.../valvebase.jpg

and could you explain what the figures represent and how they relate to
your explanation above.




Greetings Gents!

The pic above confirms our thoughts so far:

What does the following stand for:

Va
Plate (anode) voltage - measured anode to cathode.
which, in your case it is purely academic as the cathode is less than a
volt above ground. If it were cathode bias, then the ground to cathode
potential would be considerably more, and need to be taken into
consideration. So to all intents and purposes the anode is 430V
above ground. (just for reference, my amp is 425V)


Vg2
Screen grid voltage


Vg1
Input grid DC voltage, i.e. the bias. At -36V this is mighty close to
the -35V I thought you might find:-)

There are actually three grids, but g3 (pin 1) is usually tied to
the cathode (pin 8) Take a peep:-)


Ia
Anode current. i.e. the idling current flowing through the valve with
no audio present. This is the figure we are trying to achieve when we
set the bias.

and what do the Voltages tell us? Do they give us the plate voltage. Is
that an amp design thing or a valve design thing?

Yes. They tell us everything we need to know:-)
To borrow a quote from a member of another group
"Ratings are for transistors....tubes have guidelines"

There are many sets of operating conditions for valves in push pull,
depending on the circuitry, fixed bias, cathode bias, triode connected,
UL etc etc,


My guess would be that the bias can be set at 36mA (?), but what does that
handwritten figure mean? It's obviously taken from measuring the valves.
All four valves have the same figure and are presumably 'matched'???

Yes. Manufacturers/builders usually match power valves in pairs, but one
does not always see a label attached:-)
That's good attention to detail.

On the other amp the figure on each of the valves is 27mA. Does this mean
there is a different bias figure for each amp?

Yes presumably.

Also, what is the best way to set the bias - mA or Volts?

Do you have a set of test points to measure the voltage across the 10 Ohms
cathode resistor, cathode to ground? If you have, then that is the way to do
it.
So set the bias to give you 310mV on your meter then you will have 31mA of
current flowing.

I guess what we (me and Stu) are looking for is the *correct* figure to
set the bias at and the best way to do it? (You are correct about the
adjustment pots, btw...)

The manufacturer has been kind enough to supply that information precisely
for you on a label attached to each valve.

Apologies for my iggerance - I'm new to this game, know bugger-all (never
claimed any different) and only learning as I go along! ;-)

That's one of the fascinations about valve audio, it's a "hands on" hobby
(though hopefully not hands on 'lytics or anodes:-)


Cheers
Iain

Dynavox VR70-E
 

"Iain M Churches" wrote in message
...

"Keith G" wrote in message
...

"Iain M Churches" wrote


Hi Keith and Stu,

Fixed bias actually refers to the negative voltage applied at the grid
of
the power valves to prevent them trying to pass infinite current and
destroying themselves. A typical value for an EL34 might be -35V.

When this bias voltage is established, we have a quiescent current
through
the valve. Some amps have a built-in ammeter, to measure the current.
When
using an external DVM it is a lot safer to measure a low voltage across
a
fixed
resistor of known value between cathode and ground.

So, in your amp you have a resistor of 10 Ohms. That means, if you need
a
quiescent current of 31mA you will see a voltage of 0.31V (i.e. 310mV
on your meter) when the bias is correctly set.. You probably have
four separate wirewound pots to adjust, one for each output valve.

I don't know what the plate (anode) voltage is on your amp, but 31mA
seems a little on the "lean" side to me. My own EL34 pp amp has a B+
of 425V, and is biased at 2V across a 39 Ohm resistor, and so has 51mA
idling current. You may have a higher anode (plate) voltage.


Iain, I would add here to ensure all bias pots are fully down before
applying B+ !!!
I also find adding a bit of anode current helps the warmup time prior to
setting the bias.



Set the bias after the amp has been switched on for about an hour, and
check it again after another couple of hours.


....and ensure the readings are steady... as you could have a suspect valve.


I have a pal who reckons he can hear the sweet-spot and set the bias by
ear. His amp sounds good, but get gets through a lot of valves:-)
Not recommended:-)


Guitarist seem to like doing this..but hey they can generally afford the
replacements :-)




OK Iain, that's good stuff. Have a look at this pic:

http://www.apah69.dsl.pipex.com/keit.../valvebase.jpg

and could you explain what the figures represent and how they relate to
your explanation above.




Greetings Gents!

The pic above confirms our thoughts so far:

What does the following stand for:

Va
Plate (anode) voltage - measured anode to cathode.
which, in your case it is purely academic as the cathode is less than a
volt above ground. If it were cathode bias, then the ground to cathode
potential would be considerably more, and need to be taken into
consideration. So to all intents and purposes the anode is 430V
above ground. (just for reference, my amp is 425V)


Vg2
Screen grid voltage


Vg1
Input grid DC voltage, i.e. the bias. At -36V this is mighty close to
the -35V I thought you might find:-)

There are actually three grids, but g3 (pin 1) is usually tied to
the cathode (pin 8) Take a peep:-)


Ia
Anode current. i.e. the idling current flowing through the valve with
no audio present. This is the figure we are trying to achieve when we
set the bias.

and what do the Voltages tell us? Do they give us the plate voltage. Is
that an amp design thing or a valve design thing?

Yes. They tell us everything we need to know:-)
To borrow a quote from a member of another group
"Ratings are for transistors....tubes have guidelines"

There are many sets of operating conditions for valves in push pull,
depending on the circuitry, fixed bias, cathode bias, triode connected,
UL etc etc,


My guess would be that the bias can be set at 36mA (?), but what does
that
handwritten figure mean? It's obviously taken from measuring the valves.
All four valves have the same figure and are presumably 'matched'???

Yes. Manufacturers/builders usually match power valves in pairs, but one
does not always see a label attached:-)
That's good attention to detail.

On the other amp the figure on each of the valves is 27mA. Does this mean
there is a different bias figure for each amp?

Yes presumably.

Also, what is the best way to set the bias - mA or Volts?

Do you have a set of test points to measure the voltage across the 10 Ohms
cathode resistor, cathode to ground? If you have, then that is the way to
do it.
So set the bias to give you 310mV on your meter then you will have 31mA of
current flowing.

I guess what we (me and Stu) are looking for is the *correct* figure to
set the bias at and the best way to do it? (You are correct about the
adjustment pots, btw...)

The manufacturer has been kind enough to supply that information precisely
for you on a label attached to each valve.

Apologies for my iggerance - I'm new to this game, know bugger-all (never
claimed any different) and only learning as I go along! ;-)

That's one of the fascinations about valve audio, it's a "hands on" hobby
(though hopefully not hands on 'lytics or anodes:-)


Cheers
Iain

Dynavox VR70-E
 

"Mike Gilmour" wrote in message
...


Iain, I would add here to ensure all bias pots are fully down before
applying B+ !!!
I also find adding a bit of anode current helps the warmup time prior to
setting the bias.

I mentioned this because a friend of mine (not too clued up about valves)
actually put a new set of valves in with the bias set to provide him with
glowing anodes!!
its good to have some current through the valves during warm up as it
provides some load for B+




Set the bias after the amp has been switched on for about an hour, and
check it again after another couple of hours.

Exacamento


Cheers
Iain