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Power Amp Cathode Bias

10/6/1997 11:14 AM
Bob Hunter	Power Amp Cathode Bias I'm in the process of building my first DYI amp. I have read that cathode bias of the power amp will result in a richer harmonic flavor (with reduced power). Could someone share their opinion(s) on cathode biased vs. fixed bias power amps. The amp that I'm currently designing will use a pair of 6V6s, and a standard Fender front end. I'm looking for a rich overdriven sound at a relatively low volume. Thanks All Bob Hunter

10/6/1997 11:14 AM

Bob Hunter

Power Amp Cathode Bias

I'm in the process of building my first DYI amp. I have read that cathode bias of the power amp will result in a richer harmonic flavor (with reduced power). Could someone share their opinion(s) on cathode biased vs. fixed bias power amps. The amp that I'm currently designing will use a pair of 6V6s, and a standard Fender front end. I'm looking for a rich overdriven sound at a relatively low volume.

Thanks All
Bob Hunter

10/6/1997 2:44 PM
David Axt	Bob, Check this out! http://www.mojotone.com/techtip2.html Dave

10/6/1997 11:52 PM
Bruce	Hi Bob, I just finished up a project using a pair of 6V6GTAs in push pull with cathode bias. Don't let the tiny bit of extra circuitry of a adjustable fixed bias amp stop you from trying both! I found that with about 370VDC on the plates and 365VDC on the screens....... A single 10 watt 800 ohm cathode resistor (conencted to the parallel cathodes), measures about 32.5 volts... 32volts/800 ohms = 40ma. And with a 25V/25uF bypass cap, it sounded pretty good. I actually measured about 17.5ma and 19ma across each plate through an old Deluxe OT transformer. Pretty close to the estimate using the math... (not taking the G2 currents into consideration). I could probably drop the resistor value down a little more if I wanted to. I figue about 12 watts ouput now into an 8 ohm load at the point where I can really tell the amp is starting to get fuzzy. At that point the amp is LOUD when connected to a 94dB/watt 10" speaker. It is plenty loud enough for a small practice amp and breaks up a little earlier then I expected. I used 2 12AX7s for the preamp and PI/driver. 250 volts on those 2 tubes with about 1.0 ma each tube half. I found it sounded alittle better if I used a 3k5 resistor on the cathode of the first preamp triode and bypassed it with a .5uF cap. This is similar to what you are talking about but I like the Marshall tone stack values instead of the Fender ones. I am using 330pF on the treble cap, and .022uF caps for the bass and mid section. Also I use a 68K slope resistor and a 25K pot for the mid control, 500K bass and 250K treble. IT has plenty of bass and gets pretty snarly. I have mixed feelings about the self biased power tube amps. I like it but then when I want it to be alot louder and real punchy ... it isn't. But they sound very bluesy and funky when you play a little too loud in to them. Sounds cool! A character all it's own. Bruce

10/6/1997 11:52 PM

Bruce

Hi Bob,
I just finished up a project using a pair of
6V6GTAs in push pull with cathode bias.
Don't let the tiny bit of extra circuitry
of a adjustable fixed bias amp stop you from
trying both!
I found that with about 370VDC on the plates and
365VDC on the screens.......
A single 10 watt 800 ohm cathode resistor
(conencted to the parallel cathodes),
measures about 32.5 volts...
32volts/800 ohms = 40ma. And with a 25V/25uF
bypass cap, it sounded pretty good.
I actually measured about 17.5ma and 19ma across
each plate through an old Deluxe OT transformer.
Pretty close to the estimate using the math...
(not taking the G2 currents into consideration).
I could probably drop the resistor value
down a little more if I wanted to.
I figue about 12 watts ouput now into an
8 ohm load at the point where I can really
tell the amp is starting to get fuzzy.
At that point the amp is LOUD when connected to
a 94dB/watt 10" speaker.
It is plenty loud enough for a small practice amp
and breaks up a little earlier then I expected.
I used 2 12AX7s for the preamp and PI/driver.
250 volts on those 2 tubes with about
1.0 ma each tube half.
I found it sounded alittle better if I used a
3k5 resistor on the cathode of the first preamp
triode and bypassed it with a .5uF cap.
This is similar to what you are talking about
but I like the Marshall tone stack values instead
of the Fender ones.
I am using 330pF on the treble cap, and .022uF
caps for the bass and mid section.
Also I use a 68K slope resistor and a 25K pot
for the mid control, 500K bass and 250K treble.
IT has plenty of bass and gets pretty snarly.
I have mixed feelings about the self biased
power tube amps.
I like it but then when I want it to be alot
louder and real punchy ... it isn't.
But they sound very bluesy and funky when you
play a little too loud in to them. Sounds cool!
A character all it's own.
Bruce

10/9/1997 7:55 AM
Mike T	I've converted 2 amps of mine to cathode bias, and they both work real well. I use 250 ohm 5 watt resisters on both bypassed by 50uF@50V caps, one uses (2) 6V6s, or 6L6s, or EL34s and I get anywhere from around 20 to 30 watts from it depending on the power tubes and rectifier. It was a Princeton, but I cut the baffle for a 12 and rewired it as a 1987 Marshall. The other amp has bigger transformers and can use 6L6s or EL34s and gets from 35 to 45 watts. It was a Bassman and is also now a 1987 Marshall. If they were fixed bias, I'd probably get a little more from them. The 250 ohm cathode resister makes them run hotter and gives them a more headroom than an 800 ohm cathode resister. I can't tell you the readings I got now because I havn't got them with me, but I can tell you that nothing was exceeded, current or dissipation. I've been using them both for a couple of years now, especially the Princeton, and they both work fine and sound great... Cathode bias is a much more harmonically rich sound than adjustable bias, IMO. The breakup and rhythm crunch is smoother and less abrasive, the overdriven tone is without a doubt more violin like, especially with a 5Y3GT or a 5U4GB (as opposed to a 5AR4/GZ34 or 5V4 or SS) rectifier that sag pretty well and add compression. Mike

10/9/1997 7:55 AM

Mike T

I've converted 2 amps of mine to cathode bias, and they both work real well. I use 250 ohm 5 watt resisters on both bypassed by 50uF@50V caps, one uses (2) 6V6s, or 6L6s, or EL34s and I get anywhere from around 20 to 30 watts from it depending on the power tubes and rectifier. It was a Princeton, but I cut the baffle for a 12 and rewired it as a 1987 Marshall. The other amp has bigger transformers and can use 6L6s or EL34s and gets from 35 to 45 watts. It was a Bassman and is also now a 1987 Marshall. If they were fixed bias, I'd probably get a little more from them. The 250 ohm cathode resister makes them run hotter and gives them a more headroom than an 800 ohm cathode resister. I can't tell you the readings I got now because I havn't got them with me, but I can tell you that nothing was exceeded, current or dissipation. I've been using them both for a couple of years now, especially the Princeton, and they both work fine and sound great...
Cathode bias is a much more harmonically rich sound than adjustable bias, IMO. The breakup and rhythm crunch is smoother and less abrasive, the overdriven tone is without a doubt more violin like, especially with a 5Y3GT or a 5U4GB (as opposed to a 5AR4/GZ34 or 5V4 or SS) rectifier that sag pretty well and add compression.

Mike

10/9/1997 10:09 PM
Bruce	Hi Mike. What plate voltage are you running that will allow you to use a 250 ohm cathode resistor for a push pull 6V6 amp? Are you biasing the finals for class A? I have been fooling around with similar projects all summer and now into fall. I have built 4 different configurations. Mine last one is a PP class AB1 with 2 12AX7 into a single 12AT7 PI/driver. SS rectifier with a 150ohm series resistor and diode to the preamp section for isolation. I'd like to build this one again only use my 300 VAC transformer with the 5v winding and stick the 5Y3 back in. I got such a great deal on a bunch of these 590V/80ma CT transformers that I have just been going crazy throwing little 10-20 watt amps together. But only the 590V CT and a single 6.3 V 3 amp secondary. OH well ... no tube recitifer on these. My best compromise sound and volume comes from 350 volts on the plate (Anode to cathode) and a cathode resistance of 800 ohms with a 68uf cap across it. I am close to 20ma through each tube for bias. Of course the cathodes are tied together in parallel. I am guessing at about 12-15 watts of LOUD output. It is real easy to drive into a very bluesy distortion and sound GREAT playing slide guitar. I built a small combo box up for it and for fun I put a single 10" Eminence speaker in it. Combo cab and amp is 18"x16"x8". It sounds ok and nice and small but this amp sounds REAL BIG with my Celstion V-30 2 x 12" bottom. It is really a shock to hear what 10-15 watts sounds like when driving some good speakers. Lots of fun.... and it doesn't hurt my ears too much either. ..... I hope. Bruce

10/9/1997 10:09 PM

Bruce

Hi Mike.
What plate voltage are you running that will allow
you to use a 250 ohm cathode resistor for a push
pull 6V6 amp?
Are you biasing the finals for class A?
I have been fooling around with similar projects
all summer and now into fall.
I have built 4 different configurations.
Mine last one is a PP class AB1 with 2 12AX7 into
a single 12AT7 PI/driver. SS rectifier with a
150ohm series resistor and diode to the preamp
section for isolation.
I'd like to build this one again only use my 300
VAC transformer with the 5v winding and stick the
5Y3 back in.
I got such a great deal on a bunch of these
590V/80ma CT transformers that I have just been
going crazy throwing little 10-20 watt amps
together. But only the 590V CT and a single
6.3 V 3 amp secondary.
OH well ... no tube recitifer on these.

My best compromise sound and volume comes
from 350 volts on the plate (Anode to cathode)
and a cathode resistance of 800 ohms with a
68uf cap across it. I am close to 20ma through
each tube for bias.
Of course the cathodes are tied together in
parallel.
I am guessing at about 12-15 watts of LOUD output.
It is real easy to drive into a very bluesy
distortion and sound GREAT playing slide guitar.
I built a small combo box up for it and for fun I
put a single 10" Eminence speaker in it.
Combo cab and amp is 18"x16"x8".
It sounds ok and nice and small but this amp
sounds REAL BIG with my Celstion V-30 2 x 12"
bottom.
It is really a shock to hear what 10-15 watts
sounds like when driving some good speakers.
Lots of fun.... and it doesn't hurt my ears too
much either. ..... I hope.
Bruce

10/10/1997 1:46 AM
M. Helin	I built a small combo box up for it and for fun I put a single 10" Eminence speaker in it. Combo cab and amp is 18"x16"x8". It sounds ok and nice and small but this amp sounds REAL BIG with my Celstion V-30 2 x 12" bottom. It is really a shock to hear what 10-15 watts sounds like when driving some good speakers. It's the efficiency of speakers that's forgot quite often. As you know, doubling the output power of amp adds only 3 dB output level. This same effect can be get with a speaker system that's 3 dB more efficient. Compared to 50 W amp with 90 dB/w/m speakers, a 12.5 W amp with 96 dB/w/m speakers is as loud. So it may be worth checking those 15" speakers, as efficiency is a function of size, too. Guitar has not much anything over 6.5 kHz, so a good high-frequency response is not so critical anyway. Good bass response at area 150-200 Hz is important, maybe a high-q element gives more power at that area. Mikko

10/10/1997 1:46 AM

M. Helin

I built a small combo box up for it and for fun I
put a single 10" Eminence speaker in it.
Combo cab and amp is 18"x16"x8".
It sounds ok and nice and small but this amp
sounds REAL BIG with my Celstion V-30 2 x 12"
bottom.
It is really a shock to hear what 10-15 watts
sounds like when driving some good speakers.

It's the efficiency of speakers that's forgot quite often. As you know, doubling the output power of amp adds only 3 dB output level. This same effect can be get with a speaker system that's 3 dB more efficient. Compared to 50 W amp with 90 dB/w/m speakers, a 12.5 W amp with 96 dB/w/m speakers is as loud. So it may be worth checking those 15" speakers, as efficiency is a function of size, too. Guitar has not much anything over 6.5 kHz, so a good high-frequency response is not so critical anyway. Good bass response at area 150-200 Hz is important, maybe a high-q element gives more power at that area.

Mikko

10/10/1997 6:02 PM
Mike B	Bruce, Just a thought...You can use a 6CA4/EZ81 tube rectifier - it runs off a 6.3V filament supply with no separate filament winding necessary. It draws 1 amp of filament current, so you should be able to run 2 6V6's,3 12AX7's and the rectifier from your 3A filament winding (2.8A total). This tube was used in the original Vox AC15 and an early Marshall 18W combo, I believe. It's a fairly efficient tube within it's operating range, so it won't sag as much as a 5Y3. But, it still beats using SS rectifiers.... Mike

10/10/1997 6:02 PM

Mike B

Bruce,
Just a thought...You can use a
6CA4/EZ81 tube rectifier - it
runs off a 6.3V filament supply
with no separate filament winding
necessary. It draws 1 amp of
filament current, so you should
be able to run 2 6V6's,3 12AX7's
and the rectifier from your 3A
filament winding (2.8A total).

This tube was used in the original
Vox AC15 and an early Marshall 18W
combo, I believe. It's a fairly
efficient tube within it's operating
range, so it won't sag as much as
a 5Y3. But, it still beats using
SS rectifiers....
Mike

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