I replaced my 2A3 driver stage (was 6SL7 SRPP, became 2-stage 6SN7 Audio Note circuit, then SRPP 6SN7) with a single stage 76. I fell in love with the tone of this tube. It makes Hendrix and The Cult sound even better.
I started with 2.5mA- 47K on the anode, 22K dropping from 310V B+, 3K3 on the cathode bypassed with 100uF Black Gate. The bass is soft.
Last night I made a 2.5mA CCS using the diyAudio CCS boards and made it the anode load. The cathode resistor is the same. The bass tightens up and have good control, but the low register, that creeping lows that my 45 makes me feel on the sofa is non-existent.
As a comparison, my 45 uses 5842, but I don't want to use another 5842 for the 2A3. Besides I really like the tone, and had the 76 the bass of the 5842, I'd pick it the winner now.
Is there something else I can do to improve the bass? Is it caused by the higher plate resistance of the 76? The 2A3 uses 470K as grid leak, should I change this?
I'd appreciate your experience on this tube guys! Thanks.
I started with 2.5mA- 47K on the anode, 22K dropping from 310V B+, 3K3 on the cathode bypassed with 100uF Black Gate. The bass is soft.
Last night I made a 2.5mA CCS using the diyAudio CCS boards and made it the anode load. The cathode resistor is the same. The bass tightens up and have good control, but the low register, that creeping lows that my 45 makes me feel on the sofa is non-existent.
As a comparison, my 45 uses 5842, but I don't want to use another 5842 for the 2A3. Besides I really like the tone, and had the 76 the bass of the 5842, I'd pick it the winner now.
Is there something else I can do to improve the bass? Is it caused by the higher plate resistance of the 76? The 2A3 uses 470K as grid leak, should I change this?
I'd appreciate your experience on this tube guys! Thanks.
Hi, the whole setup is Clearaudio Magnum turntable + Tangent arm + DL-103 into Clearaudio Symmetry phonostage into DIY WE407A preamp into the 2A3 (or 45)
I checked the loadlines again and changed the bias to -4V. The driver is now configured with 1K cathode resistor, 100uF BG bypass, DIYAudio CCS set for 4mA at the plate. Actual measurements revealed 4.5mA (4.5V on the cathode) and 121V at the plate of the 76.
This configuration gave respectful bass, but still short of that creeping lows the 5842-45 gives me. I was playing Erasure's A Little Respect and it is convincing but I still notice that lack of creeping bass to the sofa. It does sound good and I really like the tone of the 76-2A3 as I played Midnight Oil, Bad English and Eiji Ouie.
I hope there's still something I can do to improve further.
I checked the loadlines again and changed the bias to -4V. The driver is now configured with 1K cathode resistor, 100uF BG bypass, DIYAudio CCS set for 4mA at the plate. Actual measurements revealed 4.5mA (4.5V on the cathode) and 121V at the plate of the 76.
This configuration gave respectful bass, but still short of that creeping lows the 5842-45 gives me. I was playing Erasure's A Little Respect and it is convincing but I still notice that lack of creeping bass to the sofa. It does sound good and I really like the tone of the 76-2A3 as I played Midnight Oil, Bad English and Eiji Ouie.
I hope there's still something I can do to improve further.
This is George Rigney.
I build tube studio gear per custom order.
Here is a link to a photo-album with some of my work on
it.
http://photobucket.com/albums/b121/SalesBoy/
The Pics at the bottom of page one show a Vintage Mike
preamp using #37 and #76 tubes. I did a whole weekend
of testing out the new version of this circuit that
would be an 8-tube version of this that would have four
#76 triodes in parallel to drive the output
transformer.
So I have researched out these two triodes a fair
amount and they are great tubes.
This may sound bizarre, but I never use a cathode bypass cap.
The Kathode resistor is flat in frequency response and I have found best bass sound, dynamics etc. come from having no bypass caps anywhere. Small amounts of grid resistance and even some tiny amount of neg feedback from plate to grid or Kathode will compensate for miller effect etc.
3,3K is about where this tubes like it. You might try using two or three of these in parallel with a 33K series grid resistor. I have found that going against the past math, and throwing out all bypass caps works best and I have tested this and found flatter lower response without them..
I had a stereo preamp made from two mac C-4's that a freind bought. I had them back for a few mods and I tried adding a bypass cap to the line stage. The harshness and hiss came right up and went right away when I cut the leads.
The by-pass cap acts like a "Puddle of Electrons" that increases the instability of the tube and hiss and harshness.
Look at a Fisher 400 preamp schemo and other greats and you will see all sorts of cool ways to use triodes with out that basic caps all over thinking.....
If you use four #76 triodes in parallel;
The plate impedance will be 1/4, or 2375 ohms.
The uHmos or current gain will be 5800
I found playing that bringing the plate impedance down increased the ability to drive load more than current gain. But increasing both is major importance.
You can wire the fil in series for 24VDC. I use DC and I have a .25 cap from the + side to ground to "ground" the fil. I found this trick in a Dyna PAS preamp!
ANyway, sorry about blabbing on, I had a interesting time hooking up trioded to this 15K/600 ohm transformer to see what set of triodes like 6j5's, #37 or #76 had best head room etc...
Typical of tubes what happens is not what the math or theory says will haps...
gEo...
a tube like the #76 has a very low Uhmos or current gain.
I build tube studio gear per custom order.
Here is a link to a photo-album with some of my work on
it.
http://photobucket.com/albums/b121/SalesBoy/
The Pics at the bottom of page one show a Vintage Mike
preamp using #37 and #76 tubes. I did a whole weekend
of testing out the new version of this circuit that
would be an 8-tube version of this that would have four
#76 triodes in parallel to drive the output
transformer.
So I have researched out these two triodes a fair
amount and they are great tubes.
This may sound bizarre, but I never use a cathode bypass cap.
The Kathode resistor is flat in frequency response and I have found best bass sound, dynamics etc. come from having no bypass caps anywhere. Small amounts of grid resistance and even some tiny amount of neg feedback from plate to grid or Kathode will compensate for miller effect etc.
3,3K is about where this tubes like it. You might try using two or three of these in parallel with a 33K series grid resistor. I have found that going against the past math, and throwing out all bypass caps works best and I have tested this and found flatter lower response without them..
I had a stereo preamp made from two mac C-4's that a freind bought. I had them back for a few mods and I tried adding a bypass cap to the line stage. The harshness and hiss came right up and went right away when I cut the leads.
The by-pass cap acts like a "Puddle of Electrons" that increases the instability of the tube and hiss and harshness.
Look at a Fisher 400 preamp schemo and other greats and you will see all sorts of cool ways to use triodes with out that basic caps all over thinking.....
If you use four #76 triodes in parallel;
The plate impedance will be 1/4, or 2375 ohms.
The uHmos or current gain will be 5800
I found playing that bringing the plate impedance down increased the ability to drive load more than current gain. But increasing both is major importance.
You can wire the fil in series for 24VDC. I use DC and I have a .25 cap from the + side to ground to "ground" the fil. I found this trick in a Dyna PAS preamp!
ANyway, sorry about blabbing on, I had a interesting time hooking up trioded to this 15K/600 ohm transformer to see what set of triodes like 6j5's, #37 or #76 had best head room etc...
Typical of tubes what happens is not what the math or theory says will haps...
gEo...
a tube like the #76 has a very low Uhmos or current gain.
Hi Eli, that seems to be an easy mod for me to try. Thanks!
Hi George, without the bypass cap, my gain will go down further. I would have done that had I have more gain available. How did you know I have 8 pairs of 76? Just kidding. Though it's nice to that it's an option, my current chassis will prevent me from doing so. Thank you very much for the elaborate response.
Hi George, without the bypass cap, my gain will go down further. I would have done that had I have more gain available. How did you know I have 8 pairs of 76?
Just kidding. Though it's nice to that it's an option, my current chassis will prevent me from doing so. Thank you very much for the elaborate response. I'm really tempted to use LEDs on this even at 4.5mA
Now for battery bias I do know that non-rechargable is a no-no (due to trickle charge) on the cathode. I've seen some suggestions on the use of SLA, which is bigger than my amp
So that leaves me with Nickel Cadmium or Nickel Metal Hydride. On safety point of view, which is safer?
Battery experts, may I hear you?
Now for battery bias I do know that non-rechargable is a no-no (due to trickle charge) on the cathode. I've seen some suggestions on the use of SLA, which is bigger than my amp
So that leaves me with Nickel Cadmium or Nickel Metal Hydride. On safety point of view, which is safer?Battery experts, may I hear you?
Hi Simon, thanks for the information.
Desparate for a battery...
I adjusted the diyAudio CCS board for 4mA only to find out that it would be a pain looking for a 4V battery for the cathode. Most batteries are in the 3.6V range.
So I thought, heck why not set the current to 3.6mA which is still higher than the 2.5mA where I started. Darn I need a trim pot for my CCS board! I already hate soldering, de-soldering resistors into the board. I might break the traces too.
After the adjustment, I checked the voltage across the cathode resistor and it's close to 3.6V. I listened to some materials just to check if the character will change from 4.5mA to 3.6mA. Not much... maybe I'm deaf already. I played Sade Lover's Rock which has some heavy bass and I recall that track 2 had some nice lines from the bass guitar. Darn! I have to strain myself real hard just to try and figure out the bass guitar lines. In fact it's barely reprodued and mosty swamped by the kick drum. I scratch my head and said "that's it." I then ralized I don't have a battery holder for three AA batteries and I need two more for two channels!
Out of desparation and upon checking RS Components for the specs of water clear lens Blue LEDs, which mostly said 3.6V - 3.7V, what the heck... nothing's gonna be worse than this 1K Riken resistor and Black Gate coupling caps!
So into the cathode the water clear lens LED went. Played the same track and... geez, the bass guitar is there! With the detail and articulation I was hoping I would hear. It was just great! I noticed I'm getting more top end detail and air too, without compromising the mids. It's too early to make a call. So I turned off the amp and I checked the cathode voltage. I was surprised to find out that it's only 3V! Grrrr!
I need to make a string of LEDs now, maybe two red ones for 3.4V which is close enough to 3.6V...
I need a trim pot!!!
But I'm happy it's getting somewhere in the right direction.
Desparate for a battery...
I adjusted the diyAudio CCS board for 4mA only to find out that it would be a pain looking for a 4V battery for the cathode. Most batteries are in the 3.6V range.
So I thought, heck why not set the current to 3.6mA which is still higher than the 2.5mA where I started. Darn I need a trim pot for my CCS board! I already hate soldering, de-soldering resistors into the board. I might break the traces too.
After the adjustment, I checked the voltage across the cathode resistor and it's close to 3.6V. I listened to some materials just to check if the character will change from 4.5mA to 3.6mA. Not much... maybe I'm deaf already. I played Sade Lover's Rock which has some heavy bass and I recall that track 2 had some nice lines from the bass guitar. Darn! I have to strain myself real hard just to try and figure out the bass guitar lines. In fact it's barely reprodued and mosty swamped by the kick drum. I scratch my head and said "that's it." I then ralized I don't have a battery holder for three AA batteries and I need two more for two channels!
Out of desparation and upon checking RS Components for the specs of water clear lens Blue LEDs, which mostly said 3.6V - 3.7V, what the heck... nothing's gonna be worse than this 1K Riken resistor and Black Gate coupling caps!
So into the cathode the water clear lens LED went. Played the same track and... geez, the bass guitar is there! With the detail and articulation I was hoping I would hear. It was just great! I noticed I'm getting more top end detail and air too, without compromising the mids. It's too early to make a call. So I turned off the amp and I checked the cathode voltage. I was surprised to find out that it's only 3V! Grrrr!
I need to make a string of LEDs now, maybe two red ones for 3.4V which is close enough to 3.6V...
I need a trim pot!!!
But I'm happy it's getting somewhere in the right direction.
I was playing Christy Baron's Knocks Me Off My Feet early this morning comparing the sound of the 5842-45 and 76-2A3 and I can say that the 45 still has the edge but the 2A3 is now close as it can reproduce the deep bass of the tracks with the detail and articulation I am hoping for.
It's the deep bass I am seeking as the 76 do produce mid bass with enough balls.
It now occurs to me that even the worst LED can improve on the "best" (in this case a Riken and Black Gate) RC for the cathode, for this specific application.
I've got to buy myself some batteries and try them too! Although, price-wise the LEDs can't be beat!
It's the deep bass I am seeking as the 76 do produce mid bass with enough balls.
It now occurs to me that even the worst LED can improve on the "best" (in this case a Riken and Black Gate) RC for the cathode, for this specific application.
I've got to buy myself some batteries and try them too! Although, price-wise the LEDs can't be beat!
arnold,
Try the 76 operating points used by Asano in his 300B kit:
http://www7a.biglobe.ne.jp/~asano/300bamp(english).pdf
Use a 9V batt + 1.2V in series at the grid for fixed bias and bypass with 0.01-.022uF high quality cap as option. OR run the 76 with cathodes unbypassed and use a wide bandwidth line level input high ratio step-up transformer like the ones from Lundahl, etc. You would need a very low output Z preamp or a TVC to drive this though, and maybe load the secondary with a the proper R value for the 76.
Try the 76 operating points used by Asano in his 300B kit:
http://www7a.biglobe.ne.jp/~asano/300bamp(english).pdf
Use a 9V batt + 1.2V in series at the grid for fixed bias and bypass with 0.01-.022uF high quality cap as option. OR run the 76 with cathodes unbypassed and use a wide bandwidth line level input high ratio step-up transformer like the ones from Lundahl, etc. You would need a very low output Z preamp or a TVC to drive this though, and maybe load the secondary with a the proper R value for the 76.
If the UhMos or current gai of the triode is increased, then the triode will "pull" more current through the load resistor and then more gain.
So if two tubes in parallel were used then the voltage gain might increase about the amount the bypass cap gives to one triode.
I went crazy trying to get more gain out of a 1933 Wurlitzer juke-box amp that had #56 triodes at the same voltage gain as the #76 at 13.8.
They are a 2.5VDC version of the #76 a modern AC filiment tube.
In my how well can I drive a 15K/600 ohm tranny, I tried all sorts of triodes and combos...
So I studied tube swaps and found four #76 just under 6j5 with #37 on the bottom....
The newer version of the #76 is a 6j5 metal tube. This triode has a voltage gain of 20, which is more than the cap gives you on the #76 at 13.8. I play with everything and I found small gain increase with a bypass cap. Lowering the Kath impedance helps.
I would try a 6j5 and there are glass versions..
These tubes are really nice sounding.
Good Sound!
So if two tubes in parallel were used then the voltage gain might increase about the amount the bypass cap gives to one triode.
I went crazy trying to get more gain out of a 1933 Wurlitzer juke-box amp that had #56 triodes at the same voltage gain as the #76 at 13.8.
They are a 2.5VDC version of the #76 a modern AC filiment tube.
In my how well can I drive a 15K/600 ohm tranny, I tried all sorts of triodes and combos...
So I studied tube swaps and found four #76 just under 6j5 with #37 on the bottom....
The newer version of the #76 is a 6j5 metal tube. This triode has a voltage gain of 20, which is more than the cap gives you on the #76 at 13.8. I play with everything and I found small gain increase with a bypass cap. Lowering the Kath impedance helps.
I would try a 6j5 and there are glass versions..
These tubes are really nice sounding.
Good Sound!
I have a few glass 6j5 tubes.
I have some VT versions that are mint.
I would try a tube before taking anyon's word.
It seems to easy to say the metal versions are the worst. He might be right but a lot of gear was made with 6sj7, 6sk7, 6j5s and others. I have restored big CapeHart recievers that have awsome fidelity using same tubes.
The trickx is around the cathode. I would try a 33K resistor in series with the grid and see what haps. The triode likes a leetle neg feedback.
I was using a 5.6K after the plate to the coup cap, then a 33K from cap to grid. the two resistors push against the cap and help control overshoot.
If the Kath has some dry resistance, say 56 to 220 ohms between the cap and the Kath, this gives neg to the KAth which lessens the grid drive. A "schock Asorber" for the tube.
Increases the speed of the stage and clarity. Then you can bypass the est of the cathode.
Loads of fun...
I have some VT versions that are mint.
I would try a tube before taking anyon's word.
It seems to easy to say the metal versions are the worst. He might be right but a lot of gear was made with 6sj7, 6sk7, 6j5s and others. I have restored big CapeHart recievers that have awsome fidelity using same tubes.
The trickx is around the cathode. I would try a 33K resistor in series with the grid and see what haps. The triode likes a leetle neg feedback.
I was using a 5.6K after the plate to the coup cap, then a 33K from cap to grid. the two resistors push against the cap and help control overshoot.
If the Kath has some dry resistance, say 56 to 220 ohms between the cap and the Kath, this gives neg to the KAth which lessens the grid drive. A "schock Asorber" for the tube.
Increases the speed of the stage and clarity. Then you can bypass the est of the cathode.
Loads of fun...
Hi Franstar, how do you make out the values for the Rs and C that you mentioned?
Here's the bad news- I changed the cathode bias to two green LEDs which gave me a perfect 4V on the cathode. Perfect as the voltage is, it sounds BAD on the midrange. It's like carrying tons of grain only on that range
So I changed to two red LEDs which gave me around 3.1V on the cathode. But there's aggression on Neil Young's Reactor album (track 4, side 1)
And so far the best tone was produced by the 1K + 100uF cap
Maybe I'd try LED(s) on the cathode and R or L on the plate...
Here's the bad news- I changed the cathode bias to two green LEDs which gave me a perfect 4V on the cathode. Perfect as the voltage is, it sounds BAD on the midrange. It's like carrying tons of grain only on that range
So I changed to two red LEDs which gave me around 3.1V on the cathode. But there's aggression
on Neil Young's Reactor album (track 4, side 1)And so far the best tone was produced by the 1K + 100uF cap
Maybe I'd try LED(s) on the cathode and R or L on the plate...
Hi. I am super busy and I have little time to
talk. Great stuff on the forums and I am glad
youse guys are doing tube thangs.
If you look at early tube circuits, the resistors
and cap values were odd numbered. There is a lot
of 33K, 3.3K, 5.6K etc. and I found these values
to give best performance when using a scope and
testing for max out level, freq response etc..
Around the 1960's is when even values came in.
I use a 33k series grid R and a 330K bias.
If a high current tube like 12au7, then a 12K
series grid and 560K grid.
I took a standard two 6v6 push-pull amp with 6sl7
driver and ran around the block with it. I found
that if the tubes are "too close" the performance
goes down.
I ran 12au7's with 560 ohm Kath, and 5.6K to 8.2K
plate, maybe up to 12K at 150 volts for almost 40
volts swing down to around 2.2K load. No bypass
caps. I always test cap effect by scope and ear
test before use.
I also found that a small amount of negative
feedback straightens up a 10KHz sqaure wave going
through. If you put a small amount, 2.2Meg from
out cap to in the volume control or tone controls,
the square wave response at high freq straightens
up. This is the effect where when using a old
Hi-Fi preamp, you always want to have the Treble
control on notch up so it sounds "flat"..
Around 1970, there was almost no parts to build
anything with. I have Audio Amatuer magazines from
the 1970's and they were frantic for coupling caps
and decent resistors.
WHen you put a resistor in series with current
flow, the voltage that builds across thr resistor
is in oppisition to the current flow.
A 33K resistor in series with the grid of a low
current gain tube like a 12ax7 of 76 or 37 tube
will give a small amount of negative feedback and
help the triode be more stable, quieter and less
hard response on highs.
I found when building amps, if I put a 5.6K
resistor in front (from the plate of the previous
to the capand a 15K resistor behind, (from the cap
to the grid of the output tube) then the two
resistors "push" against the cap and if they are
carbon, will slow down sharp response, overshoot
and improve clarity.
You can use resistors in series in circuits to add
small amounts of Neg feedback and the character of
the resistor will change the tone. I would like to
make a power resistor block that you could put in
series with a tweet, that would soften harshness
because the resistive elements used would be slow.
The #76 tube has very low current gain at
1450uhmos, close to a 12axy at 1250. So when 1
volt of grid voltage changes, a #76 triode will
pull a lot less current than a 12au7 at 2200, or a
6sn7 at 2600, or a 6j5 at 2600.
What this means is that if you take a 12ax7, 37,
76 and drop the plate resistor to say 22k, the
voltage gain goes down. If you do the same with a
12au7, 6fq7, 6sn7, no change in voltage gain
happens.
Then the impedance of the tube, and how much
voltage swing it can produce is from both plate
impedance and current gain.
If you have a scope, you can run a steady signal
and look at when the sign wave streches.
I have a Heath switcher and I found switching
between sign and square at about 100 Hz shows
power response, overshoot etc. and then adjust
values for best response or tone.
Carbons are soft and noisy
Carbon film are quieter and faster response
Metal Oxide are fast and hard. I do not use them.
Radio Shack of all people, in their Pro catalog,
had 2Watt Metal Oxide Film that looked like the
bands were painted on. They were great tony big
dynamic sound.
I used 12au7, 6fq7 or 6sn7 with no bypass caps
ever for real big tone with very solid clear,
round tone bass. The bypass cap has it's own
response time that is not in sync with the circuit
and muddies the bass.
I also use Metal-Oxide 5watt in the cathodes of
out tubes with no bypass or have about a 33-51 ohm
in series with the big value and bypass the big
value so a small amount of dry resistance is at
the kath. Raises grid impedance, less harsh,
quiet, (no puddle effect) more clarity.
The #76 tube does not have enough current gain to
be that good a driver except for tone.
I think there is a desperate need for new versions
of these triodes. With say four versions at diff
current/voltage gains so you could build with all
seperate triodes with big plates and big tone.
If you can set you CD player to repeat on sound,
then you could adjust values by killing b+ and
clip leading in another value.
Good Tone!!!
arnoldc []
diyAudio Member
Registered: Apr 2003
Location: Makati
reply to the below
Hi Franstar, how do you make out the values for
the Rs and C that you mentioned?
--------------------------------------------------
Here's the bad news- I changed the cathode bias to
two green LEDs which gave me a perfect 4V on the
cathode. Perfect as the voltage is, it sounds BAD
on the midrange. It's like carrying tons of grain
only on that range
So I changed to two red LEDs which gave me around
3.1V on the cathode. But there's aggression on
Neil Young's Reactor album (track 4, side 1)
And so far the best tone was produced by the 1K +
100uF cap
Maybe I'd try LED(s) on the cathode and R or L on
the plate...
I have a few glass 6j5 tubes.
I have some VT versions that are mint.
I would try a tube before taking anyon's word.
It seems to easy to say the metal versions are the
worst. He might be right but a lot of gear was
made with 6sj7, 6sk7, 6j5s and others. I have
restored big CapeHart recievers that have awsome
fidelity using same tubes.
The trickx is around the cathode. I would try a
33K resistor in series with the grid and see what
haps. The triode likes a leetle neg feedback.
I was using a 5.6K after the plate to the coup
cap, then a 33K from cap to grid. the two
resistors push against the cap and help control
overshoot.
If the Kath has some dry resistance, say 56 to 220
ohms between the cap and the Kath, this gives neg
to the KAth which lessens the grid drive. A
"schock Asorber" for the tube.
Increases the speed of the stage and clarity. Then
you can bypass the est of the cathode.
Loads of fun...
__________________
The ToneMaster
hi franstar, I have read good things about the 6J5
and Morgan Jones' book says the metal 6J5 is one
of the worst the Pinnacle 6J5 being the best in
terms of his measurements. While they seem to be a
good option, I don't have those tubes too
I was playing Christy Baron's Knocks Me Off My
Feet early this morning comparing the sound of the
5842-45 and 76-2A3 and I can say that the 45 still
has the edge but the 2A3 is now close as it can
reproduce the deep bass of the tracks with the
detail and articulation I am hoping for.
It's the deep bass I am seeking as the 76 do
produce mid bass with enough balls.
It now occurs to me that even the worst LED can
improve on the "best" (in this case a Riken and
Black Gate) RC for the cathode, for this specific
application.
I've got to buy myself some batteries and try them
too! Although, price-wise the LEDs can't be beat!
arnold,
Try the 76 operating points used by Asano in his
300B kit:
http://www7a.biglobe.ne.jp/~asano/300bamp(english)
.pdf
Use a 9V batt + 1.2V in series at the grid for
fixed bias and bypass with 0.01-.022uF high
quality cap as option. OR run the 76 with cathodes
unbypassed and use a wide bandwidth line level
input high ratio step-up transformer like the ones
from Lundahl, etc. You would need a very low
output Z preamp or a TVC to drive this though, and
maybe load the secondary with a the proper R value
for the 76.
By the way, you should be able to use a CSS to
furnish the "missing" 6 mA of current to "properly
bias your LED's
This should be a voltage divider for AC of 8 ohms(
2 RED LED) to 100k (CSS), and should not
negatively affect either AC or DC operation.
Doug
The tone you are looking for would come with more current gain in the 76. I found because of the low uhmos, the Kath R wants to be up around 3.3K to 5.6 k for max swing and load drive.
You could bias the grid with a little positive volts, ALA MArantz style, and get a higher idle current and a bigger tone.
Leave the carbon Kath resistor in at about 3.3K, bias up and watch the Dissapation rating, and you would get more swing and tone. Marantz did this in his preamps and X-overs etc. with 12ax7 tubes to get bigger tone from low current tubes..
Geo...
talk. Great stuff on the forums and I am glad
youse guys are doing tube thangs.
If you look at early tube circuits, the resistors
and cap values were odd numbered. There is a lot
of 33K, 3.3K, 5.6K etc. and I found these values
to give best performance when using a scope and
testing for max out level, freq response etc..
Around the 1960's is when even values came in.
I use a 33k series grid R and a 330K bias.
If a high current tube like 12au7, then a 12K
series grid and 560K grid.
I took a standard two 6v6 push-pull amp with 6sl7
driver and ran around the block with it. I found
that if the tubes are "too close" the performance
goes down.
I ran 12au7's with 560 ohm Kath, and 5.6K to 8.2K
plate, maybe up to 12K at 150 volts for almost 40
volts swing down to around 2.2K load. No bypass
caps. I always test cap effect by scope and ear
test before use.
I also found that a small amount of negative
feedback straightens up a 10KHz sqaure wave going
through. If you put a small amount, 2.2Meg from
out cap to in the volume control or tone controls,
the square wave response at high freq straightens
up. This is the effect where when using a old
Hi-Fi preamp, you always want to have the Treble
control on notch up so it sounds "flat"..
Around 1970, there was almost no parts to build
anything with. I have Audio Amatuer magazines from
the 1970's and they were frantic for coupling caps
and decent resistors.
WHen you put a resistor in series with current
flow, the voltage that builds across thr resistor
is in oppisition to the current flow.
A 33K resistor in series with the grid of a low
current gain tube like a 12ax7 of 76 or 37 tube
will give a small amount of negative feedback and
help the triode be more stable, quieter and less
hard response on highs.
I found when building amps, if I put a 5.6K
resistor in front (from the plate of the previous
to the capand a 15K resistor behind, (from the cap
to the grid of the output tube) then the two
resistors "push" against the cap and if they are
carbon, will slow down sharp response, overshoot
and improve clarity.
You can use resistors in series in circuits to add
small amounts of Neg feedback and the character of
the resistor will change the tone. I would like to
make a power resistor block that you could put in
series with a tweet, that would soften harshness
because the resistive elements used would be slow.
The #76 tube has very low current gain at
1450uhmos, close to a 12axy at 1250. So when 1
volt of grid voltage changes, a #76 triode will
pull a lot less current than a 12au7 at 2200, or a
6sn7 at 2600, or a 6j5 at 2600.
What this means is that if you take a 12ax7, 37,
76 and drop the plate resistor to say 22k, the
voltage gain goes down. If you do the same with a
12au7, 6fq7, 6sn7, no change in voltage gain
happens.
Then the impedance of the tube, and how much
voltage swing it can produce is from both plate
impedance and current gain.
If you have a scope, you can run a steady signal
and look at when the sign wave streches.
I have a Heath switcher and I found switching
between sign and square at about 100 Hz shows
power response, overshoot etc. and then adjust
values for best response or tone.
Carbons are soft and noisy
Carbon film are quieter and faster response
Metal Oxide are fast and hard. I do not use them.
Radio Shack of all people, in their Pro catalog,
had 2Watt Metal Oxide Film that looked like the
bands were painted on. They were great tony big
dynamic sound.
I used 12au7, 6fq7 or 6sn7 with no bypass caps
ever for real big tone with very solid clear,
round tone bass. The bypass cap has it's own
response time that is not in sync with the circuit
and muddies the bass.
I also use Metal-Oxide 5watt in the cathodes of
out tubes with no bypass or have about a 33-51 ohm
in series with the big value and bypass the big
value so a small amount of dry resistance is at
the kath. Raises grid impedance, less harsh,
quiet, (no puddle effect) more clarity.
The #76 tube does not have enough current gain to
be that good a driver except for tone.
I think there is a desperate need for new versions
of these triodes. With say four versions at diff
current/voltage gains so you could build with all
seperate triodes with big plates and big tone.
If you can set you CD player to repeat on sound,
then you could adjust values by killing b+ and
clip leading in another value.
Good Tone!!!
arnoldc []
diyAudio Member
Registered: Apr 2003
Location: Makati
reply to the below
Hi Franstar, how do you make out the values for
the Rs and C that you mentioned?
--------------------------------------------------
Here's the bad news- I changed the cathode bias to
two green LEDs which gave me a perfect 4V on the
cathode. Perfect as the voltage is, it sounds BAD
on the midrange. It's like carrying tons of grain
only on that range
So I changed to two red LEDs which gave me around
3.1V on the cathode. But there's aggression on
Neil Young's Reactor album (track 4, side 1)
And so far the best tone was produced by the 1K +
100uF cap
Maybe I'd try LED(s) on the cathode and R or L on
the plate...
I have a few glass 6j5 tubes.
I have some VT versions that are mint.
I would try a tube before taking anyon's word.
It seems to easy to say the metal versions are the
worst. He might be right but a lot of gear was
made with 6sj7, 6sk7, 6j5s and others. I have
restored big CapeHart recievers that have awsome
fidelity using same tubes.
The trickx is around the cathode. I would try a
33K resistor in series with the grid and see what
haps. The triode likes a leetle neg feedback.
I was using a 5.6K after the plate to the coup
cap, then a 33K from cap to grid. the two
resistors push against the cap and help control
overshoot.
If the Kath has some dry resistance, say 56 to 220
ohms between the cap and the Kath, this gives neg
to the KAth which lessens the grid drive. A
"schock Asorber" for the tube.
Increases the speed of the stage and clarity. Then
you can bypass the est of the cathode.
Loads of fun...
__________________
The ToneMaster
hi franstar, I have read good things about the 6J5
and Morgan Jones' book says the metal 6J5 is one
of the worst the Pinnacle 6J5 being the best in
terms of his measurements. While they seem to be a
good option, I don't have those tubes too
I was playing Christy Baron's Knocks Me Off My
Feet early this morning comparing the sound of the
5842-45 and 76-2A3 and I can say that the 45 still
has the edge but the 2A3 is now close as it can
reproduce the deep bass of the tracks with the
detail and articulation I am hoping for.
It's the deep bass I am seeking as the 76 do
produce mid bass with enough balls.
It now occurs to me that even the worst LED can
improve on the "best" (in this case a Riken and
Black Gate) RC for the cathode, for this specific
application.
I've got to buy myself some batteries and try them
too! Although, price-wise the LEDs can't be beat!
arnold,
Try the 76 operating points used by Asano in his
300B kit:
http://www7a.biglobe.ne.jp/~asano/300bamp(english)
Use a 9V batt + 1.2V in series at the grid for
fixed bias and bypass with 0.01-.022uF high
quality cap as option. OR run the 76 with cathodes
unbypassed and use a wide bandwidth line level
input high ratio step-up transformer like the ones
from Lundahl, etc. You would need a very low
output Z preamp or a TVC to drive this though, and
maybe load the secondary with a the proper R value
for the 76.
By the way, you should be able to use a CSS to
furnish the "missing" 6 mA of current to "properly
bias your LED's
This should be a voltage divider for AC of 8 ohms(
2 RED LED) to 100k (CSS), and should not
negatively affect either AC or DC operation.
Doug
The tone you are looking for would come with more current gain in the 76. I found because of the low uhmos, the Kath R wants to be up around 3.3K to 5.6 k for max swing and load drive.
You could bias the grid with a little positive volts, ALA MArantz style, and get a higher idle current and a bigger tone.
Leave the carbon Kath resistor in at about 3.3K, bias up and watch the Dissapation rating, and you would get more swing and tone. Marantz did this in his preamps and X-overs etc. with 12ax7 tubes to get bigger tone from low current tubes..
Geo...
Morgan Jones' book says the metal 6J5 is one of the worst the Pinnacle 6J5 being the best in terms of his measurements. While they seem to be a good option, I don't have those tubes too >>
this was posted a while back. I have a box of the very same Pinnacle 6J5 that Morgan tested - they are from the same batch. I have too many for my own use and am happy to sell them cheaply - just email me if you want some. Andy
this was posted a while back. I have a box of the very same Pinnacle 6J5 that Morgan tested - they are from the same batch. I have too many for my own use and am happy to sell them cheaply - just email me if you want some. Andy
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