[Leigh Orf]

Hi,

Does anyone know what the current rating is for the 15.6 ohm bias
resistor in the Dynaco ST-70 (from pin 8 of the EL34's to ground)?

Among my upgrades was to replace these resistors, which had drifted,
with 1/4 watt 10 ohm resistors, which I keep frying at loud volumes. I
suspect 1/2 watt would be OK but am not taking my chances.

Leigh

--
Leigh Orf http://orf.cx
SDF Public Access UNIX System - http://sdf.lonestar.org

[Leigh Orf]

Leigh Orf wrote:
Hi,

Does anyone know what the current rating is for the 15.6 ohm bias
resistor in the Dynaco ST-70 (from pin 8 of the EL34's to ground)?

Among my upgrades was to replace these resistors, which had drifted,
with 1/4 watt 10 ohm resistors, which I keep frying at loud volumes. I
suspect 1/2 watt would be OK but am not taking my chances.

Leigh


Sorry for the repost - ignore. My question has been answered.


Leigh




--
Leigh Orf http://orf.cx
SDF Public Access UNIX System - http://sdf.lonestar.org

[Phil Allison]

"Leigh Orf"

Does anyone know what the current rating is for the 15.6 ohm bias
resistor in the Dynaco ST-70 (from pin 8 of the EL34's to ground)?

Among my upgrades was to replace these resistors, which had drifted,
with 1/4 watt 10 ohm resistors, which I keep frying at loud volumes. I
suspect 1/2 watt would be OK but am not taking my chances.



** Can't you see the THREE replies here to this same dumb question you
have already ?

Don't you believe them ?



Leigh Orf http://orf.cx


** OK - a ham radio guy & an " egghead" .

Explains it all.




........ Phil

[Leigh Orf]

Phil Allison wrote:

** Can't you see the THREE replies here to this same dumb question you
have already ?

Don't you believe them ?

I was having newsreader problems and hence did not see them (I tried to
cancel the second article once I realized this). The articles weren't
showing up.

** OK - a ham radio guy & an " egghead" .

Explains it all.

Yeah well it could be worse - I could be Australian.

Leigh

--
Leigh Orf http://orf.cx
SDF Public Access UNIX System - http://sdf.lonestar.org

[Phil Allison]

"Leigh Orf"
Phil Allison

** Can't you see the THREE replies here to this same dumb question you
have already ?

Don't you believe them ?

I was having newsreader problems and hence did not see them (I tried to
cancel the second article once I realized this). The articles weren't
showing up.

** OK - a ham radio guy & an " egghead" .

Explains it all.

Yeah well it could be worse - I could be Australian.


** Geez - *another* total ****wit, Septic Tank.

This clown thinks changing a clearly overloaded 1/4 watt rated resistor 1/2
watt will do some unspeakable damage.

Ham radio licence = certificate of stupidity.

PhD = Pin Headed Dope.

( Abbott & Costello were never more accurate)


**** Orf - Leigh.




........ Phil

[Patrick Turner]

Leigh Orf wrote:

Hi,

Does anyone know what the current rating is for the 15.6 ohm bias
resistor in the Dynaco ST-70 (from pin 8 of the EL34's to ground)?

Among my upgrades was to replace these resistors, which had drifted,
with 1/4 watt 10 ohm resistors, which I keep frying at loud volumes. I
suspect 1/2 watt would be OK but am not taking my chances.

Leigh

--
Leigh Orf http://orf.cx
SDF Public Access UNIX System - http://sdf.lonestar.org

I would ONLY use 5 watt wire wound resistors, 15 ohms will be close
enough,
or 18 ohms 5W plus a trimmer of 120 ohms 1 watt to bring R close to the
non standard
value of 15.6 ohms.

But I also would not use the existing bias adjust methods, and
would have 4 adjust pots, one for each output tube, and
a 10ohm x 5W ww from each cathode to 0V. Then 0.5V = 50mA of cathode
bias current.

A 1/2 watt 15.6 ohm resistor needs only 179mA for the R to reach 1/2 a
watt dissipation,
and depending on speaker load the resistor sure will smoke because
average class AB current
exceeds 179mA. I have seen 1 watt 10 ohm R blow when used with EL34.
But that was partly due to the guy using bad speakers.

200mA slo fuses are a good idea in cathode to ground of each output tube
where you don't have active excess output current protection circuits
fitted.

If you blow such fuses, your speaker load is too low, and should be
moved from 8 ohm to 4 ohm outlet.
If they still blow, speakers could be faulty. Or you need to turn the
volume down, you must be well into clipping.

Patrick Turner.

[Patrick Turner]

Leigh Orf wrote:

Phil Allison wrote:

** Can't you see the THREE replies here to this same dumb question you
have already ?

Don't you believe them ?

I was having newsreader problems and hence did not see them (I tried to
cancel the second article once I realized this). The articles weren't
showing up.

** OK - a ham radio guy & an " egghead" .

Explains it all.

Yeah well it could be worse - I could be Australian.

Leigh

I'm Australian, and it doesn't seem to affect me.

Its not a big deal that you may have posted twice.

Good luck with smoke prevention measures.

Patrick Turner.



--
Leigh Orf http://orf.cx
SDF Public Access UNIX System - http://sdf.lonestar.org

[Peter Wieck]

On Sep 13, 8:54 am, Patrick Turner wrote:
Leigh Orf wrote:

Hi,

Does anyone know what the current rating is for the 15.6 ohm bias
resistor in the Dynaco ST-70 (from pin 8 of the EL34's to ground)?

Among my upgrades was to replace these resistors, which had drifted,
with 1/4 watt 10 ohm resistors, which I keep frying at loud volumes. I
suspect 1/2 watt would be OK but am not taking my chances.

Leigh

--
Leigh Orfhttp://orf.cx
SDF Public Access UNIX System -http://sdf.lonestar.org

I would ONLY use 5 watt wire wound resistors, 15 ohms will be close
enough,
or 18 ohms 5W plus a trimmer of 120 ohms 1 watt to bring R close to the
non standard
value of 15.6 ohms.

But I also would not use the existing bias adjust methods, and
would have 4 adjust pots, one for each output tube, and
a 10ohm x 5W ww from each cathode to 0V. Then 0.5V = 50mA of cathode
bias current.

A 1/2 watt 15.6 ohm resistor needs only 179mA for the R to reach 1/2 a
watt dissipation,
and depending on speaker load the resistor sure will smoke because
average class AB current
exceeds 179mA. I have seen 1 watt 10 ohm R blow when used with EL34.
But that was partly due to the guy using bad speakers.

200mA slo fuses are a good idea in cathode to ground of each output tube
where you don't have active excess output current protection circuits
fitted.

If you blow such fuses, your speaker load is too low, and should be
moved from 8 ohm to 4 ohm outlet.
If they still blow, speakers could be faulty. Or you need to turn the
volume down, you must be well into clipping.

Patrick Turner.- Hide quoted text -

- Show quoted text -

Thank you, Patrick, for a rational response to the basic question.
Leigh's OP on the ST-70 wondered why a 1/4 watt resistor smoked after
a mod that he did. The similar mods with which I am directly familiar
call for a 10-ohm, 3-watt resistor (despite Bret's blathering). I also
agree with your suggestion of a fuse as a protective measure.

I will be mildly hunting for a 70 at Kutztown in a few days, there are
usually half-a-dozen in evidence. If I find one, I will do the
separate-resistor mod together with the separate pots and such... I
might just make it a "let's try this" test-bed. I write "mildly" as I
kinda-sorta would like to get along with my own home-brew, and have no
need for additional diversions.

Peter Wieck
Wyncote, PA
Kutztown Space 338

[Peter Wieck]

On Sep 13, 4:07 pm, Bret Ludwig wrote:

The resistor is SUPPOSED TO BLOW. Yes you can use a fuse (or even
certain values of small light bulbs) but why? Resistors are cheap,
cheaper than fuses. You can even buy engineered "fusistors" but why
bother?

In the context of the ST-70, the precision resistors were not supposed
to blow. Again as you appear to have been frightened by a 70 in-utero
and so have an irrational dislike of them, this may have escaped you.
True, there was little protection on the unit other than blowing the
tubes...

Now, consider the fuse and the fuse-holder. More expensive than a
resistor for sure. But the work of a few seconds of dry work to
change, no solder required. Which would you choose?

I will be mildly hunting for a 70 at Kutztown in a few days, there are
usually half-a-dozen in evidence. If I find one, I will do the
separate-resistor mod together with the separate pots and such... I
might just make it a "let's try this" test-bed. I write "mildly" as I
kinda-sorta would like to get along with my own home-brew, and have no
need for additional diversions.

Then why don't you just build it? Rig up a test bed, quick and dirty,
on a board or a scrap chassis. Surely you can get a piece of off-fall
aluminum extrusion or as I have said many times the olde automatic
transmission oil pan. Various dead computer items work fine as well.
Don't think too much, just start cutting and building.

Several circuits have been breadboarded and tested and found stable.
Time is the difficulty now as I have decided on the 6L6/5881 PP
configuration driven by a single 12AX7/5751 per side. A mighty simple
little amp which is quite appealing. I have a sketch to bend the
metal and a couple of places to send it when I get a few moments. The
tube sockets (ceramic) are in hand, and any bits, pieces and pots I
might need are on the list for Kutztown. Before I send out the bending
sketch, I want to have all the drillings laid out as well. And as to
the bending, one guy will actually back-cut for me at no extra charge
so I can get true "square" corners.

Here's a thought: get a good BENCH SUPPLY for testing circuits. One
with heater, bias and HV outputs.

Yeah. So?

You should get out more and perhaps see a large group of individuals
who actually enjoy themselves at the hobby...

Peter Wieck
Wyncote, PA

[Patrick Turner]

Peter Wieck wrote:

On Sep 13, 8:54 am, Patrick Turner wrote:
Leigh Orf wrote:

Hi,

Does anyone know what the current rating is for the 15.6 ohm bias
resistor in the Dynaco ST-70 (from pin 8 of the EL34's to ground)?

Among my upgrades was to replace these resistors, which had drifted,
with 1/4 watt 10 ohm resistors, which I keep frying at loud volumes. I
suspect 1/2 watt would be OK but am not taking my chances.

Leigh

--
Leigh Orfhttp://orf.cx
SDF Public Access UNIX System -http://sdf.lonestar.org

I would ONLY use 5 watt wire wound resistors, 15 ohms will be close
enough,
or 18 ohms 5W plus a trimmer of 120 ohms 1 watt to bring R close to the
non standard
value of 15.6 ohms.

But I also would not use the existing bias adjust methods, and
would have 4 adjust pots, one for each output tube, and
a 10ohm x 5W ww from each cathode to 0V. Then 0.5V = 50mA of cathode
bias current.

A 1/2 watt 15.6 ohm resistor needs only 179mA for the R to reach 1/2 a
watt dissipation,
and depending on speaker load the resistor sure will smoke because
average class AB current
exceeds 179mA. I have seen 1 watt 10 ohm R blow when used with EL34.
But that was partly due to the guy using bad speakers.

200mA slo fuses are a good idea in cathode to ground of each output tube
where you don't have active excess output current protection circuits
fitted.

If you blow such fuses, your speaker load is too low, and should be
moved from 8 ohm to 4 ohm outlet.
If they still blow, speakers could be faulty. Or you need to turn the
volume down, you must be well into clipping.

Patrick Turner.- Hide quoted text -

- Show quoted text -

Thank you, Patrick, for a rational response to the basic question.
Leigh's OP on the ST-70 wondered why a 1/4 watt resistor smoked after
a mod that he did. The similar mods with which I am directly familiar
call for a 10-ohm, 3-watt resistor (despite Bret's blathering). I also
agree with your suggestion of a fuse as a protective measure.

I will be mildly hunting for a 70 at Kutztown in a few days, there are
usually half-a-dozen in evidence. If I find one, I will do the
separate-resistor mod together with the separate pots and such... I
might just make it a "let's try this" test-bed. I write "mildly" as I
kinda-sorta would like to get along with my own home-brew, and have no
need for additional diversions.

The problem with having ONE 15.6 ohm resistance for two common cathodes
is that you are monitoring the sum of the cathode currents, and the EL34
tubes could
have become quite unmatched for biasing, although otherwise quite
serviceable.

So maybe there is 70mA in one tube and 30mA in the other, with the same
applied grid voltage.

So having separate bias R of say 10 ohms each is good practice,
and you'll soon see why individual bias is a must when you measure the
cathode currents.

Quad-II has a common 180 ohm resistance large enough to act as a bias
regulating R,
but I have often seen 90mA in one KT66, and 30mA in the other, and as
one tube
tries to conduct more I it tends to cut off the other, exacerbating the
imbalance.

Its common to find maybe a 4 Vdc difference in grid bias voltage to get
the same bias current.
When the bias currents are balanced within 5%, there is good fidelity.
With 10%, muddiness sets in, and with 30% the OPT is beginning to
saturate
with the dc flux; there is no gap in the core, and the magnetic length
is short,
and it does not take many amps x turns / ML to give magnetic saturation.

I like cathode bias because even with quite un-matched tubes, you will
find the
voltages at the cathodes nearly equal, and hence cathode currents are
equal.
One can gild the lilly by adding a bias balance pot anyway, and once set
the balance then remains
good for a long time.

The transconductance may not be equal though, but while in class A the
tubes
lack of match mildly increases 2H, not a big deal.

You can measure each tube's gm quite easily in a fixed bias amp with 4
pots.
Set the bias for each tube equally. The begin by recording grid voltage
for No1,
and its Ik, which BTW is the sum of screen and anode currents.
The adjust the gid bias down 1V and quickly measure the Ek, and hence
Ik.
if you wait a minute, maybe you find the bias has sagged a little more,
because the tube will have cooled a bit,
and gm sags with lower T.
Anyway, be precise and quick to get the change of Ik with Eg1.

Move the bias back to where you began, and do the same with each tube,
and measure
the current change for -0.5 Eg1 change.

Divide the change in Ik by change in Eg1, and its mA change/voltage
change, and gm = mA/V.

With only 50mA of Ik, you won't get the data figure of 10mA/V for EL34,
maybe only 1/2 that
and its because the gm changes with Ia, and at high Ia, say 100mA, gm
may indeed be 10mA/V.

Maybe you get 5, 7, 4, & 6 mA/V This would show two tubes are a "bit
down" in emissions.

After a the amp has been on for 20 minutes, keep an eye open, because
the tubes could be
getting gassy, and you will read a large vdc across the 270k Rg for each
EL34.
A Vdc reading where the grids are slightly negative with respect to the
bias voltage at the pot
indicates a fine healthy new tube. But maybe you read a grid or two is
at say 0.1Vdc positive
with respect to applied bias from the pot.
The tube is going slightly positive at grid, and its not new.
But say you find a tube with say a volt or more across the 270k.
Wait another 20 minutes, has the voltage increased? if it has, the Ik
will have risen,
temperature increased, and grid current also will have increased.
Old tubes slowly run away thermally; the grid gets too positive, and
fails to hold down Ia,
and the getter doesn't seem to want to absorb any more gas molecules.
The tube is stuffed.

Its very common for ppl to buy an old amp, and tubes are said to be good
by the seller,
but have actually been teetering on the edge
for some time.
Old tubes tend to drift after biasing.

Patrick Turner










Peter Wieck
Wyncote, PA
Kutztown Space 338

[tubegarden]

Hi RATs!

http://www.tubecad.com/2005/May/blog0046.htm

Scroll down to Blumlein auto bias.

It is wonderful

Happy Ears!

Al

[robert casey]

Phil Allison wrote:


This clown thinks changing a clearly overloaded 1/4 watt rated resistor 1/2
watt will do some unspeakable damage.

Well, it turns out the resistor in question was designed partly as a
fuse, to burn out open circuit to protect the tube and output
transformer. Using a higher wattage resistor loses you that protection.

Ham radio licence = certificate of stupidity.

Mine is WA2ISE, what's your callsign? :-)

[Arny Krueger]

"Bret Ludwig" wrote in message
oups.com...
On Sep 14, 7:17 am, tubegarden wrote:
Hi RATs!

http://www.tubecad.com/2005/May/blog0046.htm

Scroll down to Blumlein auto bias.

It is wonderful

Not really. Traditional fixed bias IS BEST.

Traditional fixed bias for tubes is an oxymoron, because the
chararacteristics of the tube are changing.

[Patrick Turner]

Robert Casey wrote:

Phil Allison wrote:


This clown thinks changing a clearly overloaded 1/4 watt rated resistor 1/2
watt will do some unspeakable damage.

Well, it turns out the resistor in question was designed partly as a
fuse, to burn out open circuit to protect the tube and output
transformer. Using a higher wattage resistor loses you that protection.

You are right of course, but maybe you'd get "nuisance blows" of a low
wattage R value.

I have used 10r 1 watt carbon film for cathode current monitoring in
EL34 circuits
and seen them flame up when they blew. But it was due to the owner not
having
a high enough load for the PA amp, a Lennard from the 1960s
with 900V supply, and 8 x EL34. Ik could easily reach saturation levels
of .3A, giving 0.9Wpd, and you'd think that's OK, but the R still blew,
after the tubes began to glow red hot with too low a load, so I assumed
the
combination of high dc plus probable RF oscillations during the red
anode trauma
well exceeded the Pd rating for the Rk.

I later changed all R to 10r x 5W ww, and no more blowings. His speakers
were 2 x 6driver coffins
but he'd wired them all in parallel, so load was less than 1 ohm instead
of 8 ohms needed,
and after sorting all that out the amp didn't keep smoking.

The better way to protect the circuit is with active cathode current
detection,
and a relay that turns the whole amp off, but which can be reset.
With hi-fi, it will rarely ever work unless a tube goes gassy and
conducts too much dc,
and that's just when you need to catch it before it wrecks an OPT.

The ac operation of tubed hi-fi amps never causes excessive strain on
any tubes
or OPT unless you have a speaker load way too low, or have no load at
all
and you turn up the volume and the back emf from stored energy in the
OPT leakage inductance
causes anode swings about 3 times normal maximums, thus maybe starting
arcs at tube sockets ot in OPT.
low cathode resistance wattages are no protectiection against the
latter.

Its mostly always the dc operation that can traumatise the OPT, PT,
choke,
or all 3, plus the reservoir cap and rectifiers.



Ham radio licence = certificate of stupidity.

Mine is WA2ISE, what's your callsign? :-)

Hello!, Hello, that's my call signal....

Patrick Turner.

[Patrick Turner]

tubegarden wrote:

Hi RATs!

http://www.tubecad.com/2005/May/blog0046.htm

Scroll down to Blumlein auto bias.

It is wonderful

Happy Ears!

Al

In output tube circuits, separate Rk are definately the best for high
class A content output
unless the use of dynamic bias stabilizarion is used as shown at
http://www.turneraudio.com.au/schem-...tabilizer.html

Otherwise Blumlien's garter is good, but also needs high class A content
to stop
bias drift with rising Ek from class AB high positive current peaks.

With the Garter, if one tubes decides to conduct a lotta dc current, it
tries to make the other do the same,
so its Ek rises, and this raises bias Ik of the tube which started to
rise in the first instance. Not good.

Common Rk are always the worst, as in Quad-II, unless the amp is in
class A and tubes are matched.
Willimanson also had common Rk, unbypassed, to all easy full
cancelation of 2H currents in the cathode circuits. But W gave a pot for
dc balance adjustment.
But W amps also were for class A, and distortion is horrid when they
lurch into class AB.

Fixed bias is king for class AB, unless you use my above dynamic bias
stabilization method.

Patrick Turner.

[Leigh Orf]

Patrick Turner wrote:

I would ONLY use 5 watt wire wound resistors, 15 ohms will be
close enough, or 18 ohms 5W plus a trimmer of 120 ohms 1 watt
to bring R close to the non standard value of 15.6 ohms.

But I also would not use the existing bias adjust methods, and
would have 4 adjust pots, one for each output tube, and a 10ohm
x 5W ww from each cathode to 0V. Then 0.5V = 50mA of cathode
bias current.

A 1/2 watt 15.6 ohm resistor needs only 179mA for the R to
reach 1/2 a watt dissipation, and depending on speaker load
the resistor sure will smoke because average class AB current
exceeds 179mA. I have seen 1 watt 10 ohm R blow when used with
EL34. But that was partly due to the guy using bad speakers.

200mA slo fuses are a good idea in cathode to ground of each
output tube where you don't have active excess output current
protection circuits fitted.

If you blow such fuses, your speaker load is too low, and
should be moved from 8 ohm to 4 ohm outlet. If they still blow,
speakers could be faulty. Or you need to turn the volume down,
you must be well into clipping.

Patrick Turner.

Patrick,

Thanks a lot - excellent information. I am running off the 4 ohm outlet.
The speakers are ProAc Super Tablettes. They are small monitors that
roll off at about 60 Hz (I have a SS subwoofer running off the preamp).
I believe they are rated at about 6 ohms, can't remember offhand. The
resistors I've fried twice are cheapo radio shack carbon film resistors.
They were all I had in the shack. I might try carbon composite or
wirewound. Toying with this amp is a hobby for me, although I enjoy just
listening to the music that goes through it. I'm surprised how good it
sounds, actually. At some point I might go back and put carbon composite
resistors back in (some claim the sonic characteristics are superior for
certain applications).

I would love to mod the amp so that I can bias each tube separately.
That just makes sense.

Someday - when things like obtaining tenure are no longer a concern - I
want to take the time learn more about amplifier design etc. so I can
answer these types of questions myself just by looking at a schematic.
The math is really quite easy, I know. I've always found this group to
be very helpful for answering my questions throughout the years, ever
since I accidentally discovered hollow-state amplification. Thanks.

Leigh


--
Leigh Orf http://orf.cx
SDF Public Access UNIX System - http://sdf.lonestar.org

[Phread]

"Arny Krueger" wrote in message . ..

"Bret Ludwig" wrote in message oups.com...
On Sep 14, 7:17 am, tubegarden wrote:
Hi RATs!

http://www.tubecad.com/2005/May/blog0046.htm

Scroll down to Blumlein auto bias.

It is wonderful

Not really. Traditional fixed bias IS BEST.

Traditional fixed bias for tubes is an oxymoron, because the chararacteristics of the tube are changing.

Hey Arny,

What part of being a Born Again Christian requires you to
insult as many others as you possibly can, as often as you
possibly can? I'm speaking specifically of the tube lovers
on RAT.

Has being born again taught you that your intellect is so
vastly superior to all others' that you should spend your life
raining pain and humiliation on all who fail to recognize or
acknowledge your greatness?

Born again, my ass.

Maybe you could help us all out by spending a little more
time praying for guidance and a little less time attempting to
destroy all who fail to grasp your wonderfulness.

Or you could maybe do a little penance and go away forever
from RAT, which as you know has no wish to deal with your
vindictive Born Again Christian disgust for all who fail
to worship your education and knowledge and greatness
and wonderfulness.

Born again, my ass.

Fred

[West]

"Phil Allison" wrote in message
...

"Leigh Orf"
Phil Allison

** Can't you see the THREE replies here to this same dumb question
you
have already ?

Don't you believe them ?

I was having newsreader problems and hence did not see them (I tried to
cancel the second article once I realized this). The articles weren't
showing up.

** OK - a ham radio guy & an " egghead" .

Explains it all.

Yeah well it could be worse - I could be Australian.


** Geez - *another* total ****wit, Septic Tank.

This clown thinks changing a clearly overloaded 1/4 watt rated resistor
1/2
watt will do some unspeakable damage.

Ham radio licence = certificate of stupidity.

PhD = Pin Headed Dope.

( Abbott & Costello were never more accurate)


**** Orf - Leigh.

What's wrong with Ham Radio guys? They merely do it at a higher frequency.

west



....... Phil