Untitled Document
From:
Date:
Subject:
Jason
Page 1 of 9
(j1c1a1@aol.com)
12/6/2001 10:00 PM
Fixed Bias EL-84's--Woo Hoo!
I think I like 'em best this way.
After becoming quite bored(so sorry) with my Komet clone using 6V6's, I decided to tear the beast
apart and check out some Ideas I had regarding fixed bias EL-84's. I decided to go with my
"standard" preamp(which I think blows away anything I have tried up to this point), a pair of fixed
bias EL-84's(EMC Iron--nice stuff used in the 18 Watt club), a Hammond PT, and my trusy and
beat up BF Deluxe Reverb sized cabinet loaded with 2 X10's(Celestion). BTW, these are kind of
yucky, IMO, and I immediately noticed that these 10's sounded a whol lot better after the
"rebuild". Also, this amp rocks even better through my Thiele. I also decided to refinish the BF
cloned chassis in Rustoleum hammered black. Nice choice.
Preamp (12AX7 EH): First Stage--100K plate, 1 Meg grid, 2.7K/.68uF cathode, .0022uF coupling
cap, 1 Meg pot(directly into second stage).
Second stage (12AX7 EH)--100K plate, 2.7K/.68uF cathode(I usually use a 1.5K/4.7uF here for EL34 amps), direct coupled to cathode follower.
Cathode follower (12AX7 EH): plate to raw B+, 100K cathode load(to tone stack)
Tone Stack[check this out--it was kind of an accident because I am running low on .022uF caps]-220pF treble, .047uF mid and bass caps, 100K slope, 250K treble pot, 25K mid pot, 1 Meg bass
pot.
PI (12AX7 EH): .01uF input, 100K grid loads, 1.2K cathode, 27K tail, 100K/82K plate loads, .01uF
coupling to EL-84 grids.
Power amp (Sovtek EL-84's): 220K grid loads(to bias -VDC), 5.6K grid stoppers, 820 Ohm screens.
Bias supply: 100K dropping/blocking resistor to 1N4007(reverse bias), 10uF/250VDC, 10K
decoupling resistor to next 10uF/250VDC to 10K/10 turn pot and lastly 10K "set" resistor to
ground. AC voltage is fed from either high VAC leg before the rectifier.
Power supply: first filter--55uF/700VDC(110uF in series with 220 K bleeders), to 1K/5Watt(instead
of choke), to 110uF/700VDC(220uF/350V in series with 220K bleeders), to 20uF/500V through a
1.2K/1 Watt, to another 20uF/500V through a 10K/1 Watt, to the last 20uF/500VDC fed through
another 10K/1 Watt resistor. Please note that I use a .047uF/630V to hot side of standby from GZ34 or sand state rectifier. PT is 300-0-300VAC @ 250 ma. 6.3VAC and 5.0VAC windings for
filaments and rectifier.
Pots are PEC and Clarostat 2 Watt RV4N series.
E-Caps are Mallory TC, Sprague Atoms
Poly Caps are Mallory 150.
Teflon/Silver wire.
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Resistors are 1 Watt carbon film and 2 Watt metal film for first and second stage plate loads.
Power resistors are Dale RS2B and RS5 series.
Blah Blah Blah.
Have fun!
Jason C. Arthur
From:
Date:
Subject:
Bruce /Mission Amps
(missionamps@aol.com)
12/7/2001 12:14 AM
Re: Fixed Bias EL-84's--Woo Hoo!
Glad to see you come around!
Fixed bias El84 amps kick ass.
Your amp description is similar to mine but I am using 500K tone pots, a MV to the PI/driver,
slightly larger value driver coupling caps and 2700 screen resistors.
Ripin' little bugger.
Bruce
From:
Date:
Subject:
Steve Ahola
(steve_ahola@yahoo.com)
12/7/2001 1:09 AM
Re: Fixed Bias EL-84's--Woo Hoo!
Jason:
So are you using NFB, and if so what value resistor, etc.?
I have a fixed/cathode bias switch on my Blue Express, but haven't used the cathode bias mode
for ages... So what is your idle bias current? I think I have mine set for about 28mA per tube (with
the bias voltage around -12.5vdc).
Cathode bias on that amp is kinda like wrapping a pillow around your head after dosing yourself
up with Nyquil... ;)
BTW I've heard from a few people who have been using IRF820 MOSFET's for cathode followers
in their amp circuits. Mike Donovan made some changes to his Electar 30 rebuild (this amp uses
the MOSFET's for reverb and tremelo) so this is the latest downloadable:
http://www.blueguitar.org/mikede30.pdf
--Thanks for your post!
Steve Ahola
From:
Date:
Craig D
12/7/2001 2:34 AM
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Page 3 of 9
Re: Fixed Bias EL-84's--Woo Hoo!
Hey Steve,
The link didn't work for me...Is it correct??
Craig
From:
Date:
Subject:
Mark Lavelle
(logimark@pacbell.net)
12/7/2001 3:53 AM
More on fixed vs. cathode bias EL84s?
Subject says it all, I guess. I'm planning to do a push-pull EL84 combo next year but haven't
decided on any real details (other than the fact that it'll have reverb).
It seems to me all the Voxes are cathode biased, and they have a pretty clean sound, so what
more do you get out of the fixed bias?
Signed,
Curious in California
From:
Date:
Subject:
Steve Ahola
(steve_ahola@yahoo.com)
12/7/2001 8:25 AM
Re: More on fixed vs. cathode bias EL84s?
Mark:
It seems to me all the Voxes are cathode biased, and they have a pretty clean sound...
... until you crank them up. I just call it the "warm fuzzies" because it smooths things out and
can cover up all sorts of flaws in an amp design (or in your playing :( ). I was sold on cathode
bias when I built my Torres SuperTexan because that seemed to do the trick in smoothing out the
OD channel (which many people here thought had a certain "fizziness" to it). So I've added
cathode/fixed bias switches to many of my amps, but usually only use cathode bias if I have the
MV set for a very low playing volume... it gives it a compressed sound that makes it seem like you
are really cranking up your amp. And if there is excessive brightness in an amp, cathode bias might
might be able to smooth that out a bit. But other than that I think that fixed bias gives more
definition and dynamics to the notes you play... they seem to jump out at you more.
Randall was telling us about this blindfold test he had done with his picker friends to see if they
preferred cathode or fixed bias. These guys are killer guitarists and they all preferred the fixed bias
since it seemed to articulate their playing better.
Bruce Collins had mentioned a trick he'd used to make fixed/cathode switching more appealing
to his customers: he'd set the idle bias current for the cathode mode a few mA higher than the
fixed mode so you'd get a little boost when you'd switch over to cathode bias.
--Good luck with your amp next year! Gee whiz that is only a few weeks away...
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Steve Ahola
From:
Date:
Subject:
Mark Lavelle
(logimark@pacbell.net)
12/7/2001 5:16 PM
Re: More on fixed vs. cathode bias EL84s?
I guess that's one more switch for the Verberator.
I like cathode bias too much to give it up completely (I don't often crank my amps, and my playing
can use some cover!), but it sounds like I'm gonna want the fixed bias for when I venture into surf
territory (if you'll excuse the pun). Probably better for those rare & feeble attempts at jazz, too...
From:
Date:
Subject:
Jason
(j1c1a1@aol.com)
12/7/2001 6:06 PM
Thanks for the responses!
What a response!
Yeah Bruce, fixed bias absolutely ROCKS the house with EL-84's. It's interesting that I never tried
this before(maybe stuck on the Vox vibe?). Damn, this may be the best EL-84 sound I have ever
experienced. Much tighter and rounder than anything I built or played to this point. To be honest,
this was basically a boredom project, and I think I'll stick with it because the amp is almost
"perfect", IMO. Don't tell anyone[haha], but I may have found a design that I like equally or maybe
even more than the EL-34 amp I toiled on for years!
Steve, I forgot to go into greater detail about -FB loop. I use a 47K/1W from the 8 Ohm tap(4-816) on the OT. I still use the almighty 5K/.1uF presence control(I can't seem to get away from
using this simple circuit--I like to use it on everything I build). For some reason, I HAVE to have a
little bit of -FB in all my stuff. I just can't get into the loosey goosey no -FB sound.
One more note: Shelly came upstairs as I was finishing this amp up and plugging it in. As I started
to play, the most noticable thing was the lack of noise and hum. She immediately noticed this. I
guess I did something right. Actually, I must have done something a little bit better in this rebuild
because it uses a steel chassis with slight rust in places(hence the rustoleum), a BF Fender style
interior, and some Radio Shack PERFBOARD(I NEVER use this stuff--just for prototyping!!!) for the
filters. This amp is actually a bit quieter than my finished EL-34 amp(which is very quiet in it's own
right--no DC filaments--just good layout). This setup used to be a bit on the noisy side. Must be
that paint or something. BTW, I use a typical star ground technique in all my stuff. Plate and
screen filters to same ground point as entire power amp(OT, cathodes, speaker ground, etc.). PI
filter grounded to a seperate terminal with just bottom of presence pot tied to it along with bias
ground point. Cathode follower, second stage cathode, and bottom of mid pot to same point as the
filter for this stage. The first gain stage gets it's own ground point as well tied to it's filter ground.
Lastly, the input jack(1 Meg and shield) gets it's very own ground point. All jacks are isolated
Switchcraft using fiber and nylon shoulder washers. All ground points are in a "stream" on the
chassis heading towards the AC mains ground point. The PT CTaps also get their own ground point.
The only thing I would change is the PT. IMO, it works very well so far, but I may lean down the
voltages a bit and go with a 250-0-250 VAC PT secondary if I were to make more of these. Still,
the Mullard GZ-34 rocks(I had one previously and my buddy Matt @ Acacia, his neighbor was
cleaning his house and gave Matt a Stromberg/Carlson radio which contained a bunch of funky
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tubes including a pristine Mullard GZ-34!! Too cool!).
Sorry for no sound samples. Give a ring anytime and I'll be happy to let anybody listen through the
lo-fi phone line!
Jason
From:
Date:
Subject:
jaysg
(jayfbv@yahoo.com)
12/7/2001 1:13 AM
Re: Fixed Bias EL-84's--Woo Hoo!
My Marshall 18 Watt clone is definitely mo' betta fixed bias.
From:
Date:
Subject:
Chuck
12/7/2001 7:09 AM
Re: Fixed Bias EL-84's--Woo Hoo!
I have a Mesa Subway that I bastardized from the PI back and in the power amp I added a cathode
bias switch. This amp is running all of 400v on the plates in fixed bias (386-7 cathode biased) so it
could be a factor, but my amp sounds better in cathode bias mode. I wasn't getting enough bottom
end through with a 25uf bypass cap so I changed it to 47uf and bingo. At 400v it sounded real stiff
fixed. I have it biased a bit hotter in cathode bias mode and since el84s don't take much bias
voltage my amp is just as loud in cathode bias as it is fixed.
Any opinions on the voltage issue as it relates to the type of bias chosen?
From:
Date:
Subject:
Steve Ahola
(steve_ahola@yahoo.com)
12/7/2001 8:39 AM
Re: Fixed Bias EL-84's--Woo Hoo!
Chuck:
Any opinions on the voltage issue as it relates to the type of bias chosen?
Well, I think that one critical rating for a power tube is the voltage measured between the plates
and the cathodes. Assuming that the plate voltage remains the same (not always a given) the plate
to cathode voltage will be lower using cathode bias because of the voltage drop across the cathode
resistor. Your own measurements indicated that the plate voltage was around 15v lower with
cathode bias so that would reduce the plate-to-cathode voltage even more.
I think that you can run more risks adding a fixed bias mode to a cathode biased amp than the
other way around. Like with a 5E3 tweed deluxe with the plate voltage approaching the maximum
ratings; adding in a fixed bias mode might possibly push the 6V6's over their limits.
Except for cases like that it doesn't really hurt to add a fixed/cathode bias switch so that you can
check out both options...
--Thanks!
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Steve Ahola
From:
Anthony Noel
Date:
12/7/2001 2:40 PM
Subject:
(anoel@gte.net)
Re: Fixed Bias EL-84's--Woo Hoo!
We've been discussing the 18watter as a Tone Brewery module over at the AX84 forum. (actually
it's been prototyped and supposedly rules.) Paul Ruby over there pointed out that the Bypass cap
on the cathode resistor is way too small and should be at least a 470uF to get optimal bypass with
no frequency shelving (he ran the numbers and they are posted over there if you want to verify).
That should bring the performance of the cathode bias on the 18watter closer to the performance
of a fixed bias version.
From:
Date:
Subject:
Bruce /Mission Amps
(missionamps@aol.com)
12/7/2001 4:06 PM
Re: Fixed Bias EL-84's--Woo Hoo!
For what it's worth, in my amps, I use 47uF to 68uF caps for cathode bypass if the cathode resistor
is less then 270 ohms. The rest of the preamp would be tweaked to lessen some of the bass
response.
Although these frequency numbers are not dead accurate because of the tubes themselves, a 22uf
cap across a 120 ohm resistor has a -3dB point of around 53Hz.
From that, I'd venture a guess that the bypass would be flat (or fully boosted?) from 100Hz on up
and -3dB at +50Hz.
47uF cap = -3dB @ 28Hz, 68uF cap = -3dB @ 19.5Hz
A 470uF cap across a 120 ohm resistor has it's -3dB point at less then 3Hz and maybe flat from
around 6Hz on up.
The point? Not much...
I'd be surpirsed if he could hear much, if any, difference using a 68uF across a 120 ohnm resistor
compared to a 470uF... regardless of the math.
No big deal, but a 470uF cap across a cathode resistor could charge up like a leaky/weak 9-12v
battery! ha ha.
Bruce
From:
Anthony Noel
Date:
12/7/2001 5:33 PM
Subject:
(anoel@gte.net)
Re: Fixed Bias EL-84's--Woo Hoo!
I'm most certainly not the mega engineer like Paul is. Here's the text from the explaination.
Audibly or not it still makes good sense when trying to achieve, using cathode bias, the equivilent
performance level of the same circuit running in fixed bias.
-----------------------------------------The AC response of any node is a function of the RC time constant. The cathode resistor and cap
form a high pass filter. The time constant is the resistance times capacitance. In this case R is 130
ohm and C is 150e-6 farads. Multiply those together and you get 0.0195 seconds. That means the
cathode can charge 63% of the way to some new value in ~20ms. If you fed the grids with a
square wave that was high for 20ms and low for 20ms, the cathode would have a sawtooth wave
moving 63% of the way to the new bias point every 20ms. That square wave has a period of 40ms,
frequency of 1/40ms=25hz. The point is that the cathode is able to move quite a bit (63% of what
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it would move without the bypass cap) for a 25hz square wave. It'll move ~30% for a 50hz. 15%
for 100hz, etc. So, the cathode is still able to move around by a noticable amount for the low E
frequency of ~80hz. I used 470uf in mine. So my RC time constant is 60ms and my 63%
movement square wave frequency is 1/120ms or 8 hz. So, my cathode is not budging at the
guitar's low E frequency.
The above is my attempt at an intuitive description of what's going on. I always strive to gain an
intuitive grasp of stuff so I can make quick assessments and decisions. But, you could also do the
math of the -3db frequency, too. The formula is 1/(2*PI*R*C). For each case above:
1/(2*3.14*130*150e-6) = 8 hz
1/(2*3.14*130*470e-6) = 2.6 hz
This is the frequency at which the signal is attenuated to half it's value. From this, you can
conclude that either cap will pass the low guitar frequency well. But, from my analysis, prior, I
showed that the cathode moves quite a bit, ~20% of the no-cap condition, for the 150uf cap given
an 80hz square wave. So, maybe I'm erring on the conservative side, but I've chosen a cap value
that holds the cathode rock solid for the guitar's low E. Here's a link to a good explanation of RC
filters...
http://www.qrp.pops.net/filter1.htm
Also, I just did a sim of a 12ax7 input stage with a cathode bypass cap to enusre my intuition is
properly honed. Using a 1K bias resistor and 1uf bypass cap, my intution says:
1/(2*1k*1e-6) = 500hz is where there will be noticable diference in the signal. Indeed the sim
shows a measureable (but pretty insignificant) roll off at 1khz and clear difference in signal at
500hz. But, the -3db calculation would say:
1/(2*PI*1k*1e-6) = 160hz for the 50% cut point and again the sim shows the signal rolled off
50% at 160hz.
So, it behaves as expected. The point is, you need to know what you want for the node in question.
question. In my case, I want rock solid cathode voltage down to 80hz. That takes a 470uf bypass
cap on the 130ohm cathode resistor. 150uf gets you close but the signal has started to roll off at
the guitar frequency of 80hz.
- Paul
From:
Date:
Subject:
kg
12/7/2001 7:31 PM
little caveat w/paul's analysis
this is not entirely correct either:
The cathode resistor and cap form a high pass filter. The time constant is the
resistance times capacitance. In this case R is 130 ohm and C is 150e-6 farads.
Multiply those together and you get 0.0195 seconds.
it doesn't tell the whole story--there's something else hanging off that circuit node, namely the
cathode itself. the cathode is pretty low impedance, approximately 1/gm, so it MUST be taken into
account when figuring the effective R in the RC time constant. once that is done, you will find that
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the effective R is lower (as it always will be when paralleling resistances) and therefore the TC will
be smaller as well.
this effect (as well as any other concerning the cathode bypass capacitance) will be minimized in a
push-pull stage, and will be altogether absent in push pull class a.
as for the math, or intuitive analysis, everything looked fine.
ken
From:
Date:
Subject:
MBSetzer
(QualityAna@aol.com)
12/7/2001 3:26 PM
Re: Fixed Bias EL-84's--Woo Hoo!
Well, I'm having fun but it sounds like you are having lots of fun too :)
Am I mistaken or do I recognize the continuous improvement process at work on your bench?
Anyway, each time you detail your progress I get more inclined to want to hear some sound
clips ;)
Best Regards,
Mike
From:
Date:
Subject:
KB
(ktbktb@eatel.net)
12/7/2001 4:27 PM
Re: Fixed Bias EL-84's--Woo Hoo!
Yeh me too Bruce dish em out :) Hey just curious but wouldn't that amp get real flabby at those
low HZ rolloffs like around 6 to 20 hz ?,
KB
From:
Date:
Subject:
Bruce /Mission Amps
(missionamps@aol.com)
12/8/2001 5:43 PM
Re: Fixed Bias EL-84's--Woo Hoo!
Flabby? Well, no not really Kerry, if the rest of the circuit is up to the task.
Keep in mind, I'm not a big fan of guitar amps that are built so they can be run down into the sub
bass harmonic frequency areas. As a matter of fact, to me, it seems to be a waste of energy,
especially if you really play electric guitar music with others and have a bass player who is the
bottom end of it all anyhow.
If you play for fun at home with no one else... OK. Especially if you like that rumble bass
accompanyment... IMHO, a guitar amp should be built so the -3dB point should be around 30Hz to
50Hz or so and that's about it. Even that might be too bass heavy, but that's why most good amps
have tone controls!!
Again not considering the effects of the paralleled cathode R with the cathode RC components,
most power tubes, that I can think of, a 470uF cap across a 130 ohm cathode biasing resistor have
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a -3dB point frequency of under 2.6Hz and if you want to call that 50% voltage output of the same
tubes, then twice that is stil under 6Hz.
So yes, I'd say that is about flat (in terms of max output) since most of us can't here below 15Hz
to 20Hz anyhow. If you say you need to feel it down there, then your in the wrong biz or playing
the wrong axe! Ha ha.
with a 130ohm cathode resistor:
150uF is under 9Hz or 18Hz if you double it again for that 0dB shelving.
Is it really times 4 for a +3dB boost?
68uF is 18Hz or 36Hz doubled/0dB shelving and 72Hz if X's 4 for +3dB.
47uF is 26Hz or 52Hz doubled, 104Hz if X's 4..
22uf is around 56Hz and 112Hz if doubled.
Hence why I use 47uF to 68uF.
Bruce
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... 12/8/2001