KT-77 /big bottle 6CA7/ EL-34's + Biasing plate voltage

All about tube choices for the JVM series. See workbench for biasing and tube problems you may have.

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oldstringman
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KT-77 /big bottle 6CA7/ EL-34's + Biasing plate voltage

Post by oldstringman » Wed May 11, 2011 3:31 pm

Hi all,

I am fairly new here....got a JVM410H....want to retube it...need to retube it..
Anyway, I understand the whole bias thing....done that before...65-70mV for a stock EL-34....but if you are going to try KT-77's or Bib Bottle 6CA7's do you still bias to that same setting? Or is there a change to get each type of tube working it's optimal.
Ordering a brace of 4 of each KT-77 Gold Lions, Elec Harm 6CA7-eh, and Mullard EL-34's. So, can anyone give me a tip on what bias settings I should be shooting for on the KT's and the 6CA's?


Any tips will be greatly appreciated.

John

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big dooley
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Re: KT-77's or big bottle 6CA7's or EL-34's????

Post by big dooley » Wed May 11, 2011 6:33 pm

EL34's E34L's KT77's and 6CA7's are all biased on the same calculation

which means in order to know where to set it at, you need to know what the plate voltage is
guitars:
93 gibson les paul, seymour duncun triple shots, P-rails pickups, (rail coil, P90, series humbucker or parallel humbucker)
elixer baritone strings: .012 .016 .022 .038 .052 .068 in standard tune
amplifiers:
marshall JVM410H: stock components, main voltage boosted with a variac at 240Vac, bias set at 68% of max plate dissipation
JJ KT77 powertubes, V1 and V2 JJ ecc83s, V3 JJ ecc803, V4 stock marshall (shuguang), V5 JJ ecc83s
marshall AFD100: completely stock, bias set at 47mA
marshall VS100r combo: celestion G12T75 speaker, general electric 5751 tube

cabinets:
marshall JCM900 1936 cabs each with a celestion G12H30 anniversary and vintage 30
JVM drives the V30's, AFD drives the H30's

effects:
dunlop cry baby, boss NS2, DD3 CE3, custom A/B looper box

oldstringman
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Re: KT-77's or big bottle 6CA7's or EL-34's????

Post by oldstringman » Thu May 12, 2011 1:37 am

big dooley wrote:EL34's E34L's KT77's and 6CA7's are all biased on the same calculation

which means in order to know where to set it at, you need to know what the plate voltage is
And how do I determine this plate voltage? Is this a part of the spec sheet on a tube set? Or is this dynamically measured with a special fixture/DMM setup? If the latter, where do I get one of these fixtures? I assumed that tube types/models had a voltage rating, etc....and that the bias was set to what the tube wants to work with comfortably. New to this part of the equation/information. I am no dummy, know how to measure, and read meter, have a fluke 87 that I use alot for my job....I have some entry skill at electronics...but tube stuff is brand new to me...at least this aspect of it.

John

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big dooley
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Re: KT-77's or big bottle 6CA7's or EL-34's????

Post by big dooley » Thu May 12, 2011 11:00 am

this is a little tutorial i wrote about a year ago... first a little disclaimer:
WARNING!!!
inside the amp there are lethal voltages, make sure you've set the multimeter to DC voltage and be extremely careful making measurements... if you're not having a steady hand, DON'T DO IT!!!



ok let's go trough this from the top...

an EL34, E34L, KT77 or 6CA7 has a maximum of 25watt's of plate dissipation
bias is needed to set the idle where the tube is running... just like setting up a carburator on a car
with bias, you add a negative voltage to the inside of a tube to regulate the amount of current flowing trough the tube... with the amp at idle (no signal, volume's at 0 and the amp hooked up to a cab) you have to set the bias, so that you won't exceed 70% of the EL34's maximum plate dissipation (17.5 watts)... this is not a setting where you have to set the amp to, this is an absolute limit, you don't want to exceed... everything under that is safe and you can adjust it to your likings... if the amp sounds better to you on 58% instead of 68% for example, it wouldn't make sence to run the amp at 68%

plate dissipation is plate voltage times plate current
let's take a look at the attachment

plate voltage is measured with the black lead of your multimeter on ground (somewhere on the chassis) and the red lead on pin3 of one of the powertubes (encircled in RED)

the JVM comes with testpoints (encircled in GREEN) to make readings of current in milliamps.. since these currents go trough a resistor of 1 ohm, you can read the points at millivolts, while these nuimbers are the same as milliamps... the middle prong is for the black lead the left prong for the left 2 tubes, the right prong for the right 2 tubes, where you touch off the red lead

so... let's say you measured 450Vdc on pin 3 and 70mV on both testpoints, what does that mean?
well, do the calculation: 70mV is 70mA per pair, so that's 35mA per tube or 0.035A per tube
450*0.035=15.75 watt or 63%
you adjust each pair with the biaspots (encircled in YELLOW) until the sides are in balance and at the preferred level
when you bias the amp this way you're good to go

now... there is a catch.. 70mA is NOT plate current, it's cathode current
cathode current is plate current and screen current combined
and if you are like me, you like to know the exact numbers

to know the actual plate current you have to know how much current you're drawing on the screens, so you can subtract that number from your testpoint readings
there is a method of measuring plate current directly, but this is way too dangerous for noobs like us to perform, so i won't explain this
to know how much current the screens are drwaing is pretty simple, you have to measure the voltage drop between the screen resistors (encircled in blue) the way i do it, is measuring voltage where i touch off the leads under those resistors (encircled in black)

those resistors are 1000 ohm so if you measure 3 volts, thats 3mA of screen current (per tube)
pick the lowest value of each side, double that and subtract it from your tespoint number and you have calculated the plate current for that side...
do the same on the other side

Image
guitars:
93 gibson les paul, seymour duncun triple shots, P-rails pickups, (rail coil, P90, series humbucker or parallel humbucker)
elixer baritone strings: .012 .016 .022 .038 .052 .068 in standard tune
amplifiers:
marshall JVM410H: stock components, main voltage boosted with a variac at 240Vac, bias set at 68% of max plate dissipation
JJ KT77 powertubes, V1 and V2 JJ ecc83s, V3 JJ ecc803, V4 stock marshall (shuguang), V5 JJ ecc83s
marshall AFD100: completely stock, bias set at 47mA
marshall VS100r combo: celestion G12T75 speaker, general electric 5751 tube

cabinets:
marshall JCM900 1936 cabs each with a celestion G12H30 anniversary and vintage 30
JVM drives the V30's, AFD drives the H30's

effects:
dunlop cry baby, boss NS2, DD3 CE3, custom A/B looper box

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Bam Bam
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Re: KT-77's or big bottle 6CA7's or EL-34's????

Post by Bam Bam » Fri May 13, 2011 8:00 pm

I have yet to try anything but EL34s in my JVM, but I did put a quad of EH 6CA7s in my DSL and biased it exactly the same as the EL34s (85mV, that's for the DSL, not the JVM). They made the amp sound HUGE compared to the EL34s. I'd imagine they would do a a similar thing in a JVM.

Bam
Guitars: 2012 Ibanez 25th Anniversary RG3 w/Dimarzio D-Activator/Air Norton S, 2005 Charvel USA Custom Shop Pointy Head Reissue w/80's Jackson J-95/Duncan '59, 2010 Charvel Pro Mod San Dimas Style 1-2H w/Custom Custom/Jazz (was JB/'59 stock), 1987 Kramer Aerostar ZX30 w/Dimarzio Super Distortion/EMG Selects (passive single-coils)
Amps: JVM 410H (2007), EVH 5150III 50w head (2011), Peavey Series 2600 Mark III
Cabs: Avatar Contemporary 4x12 loaded with Hellatone 60s (Vintage 30s), Early 80's Peavey 2x12 loaded with Eminence Governors
Pedals: ISP Decimator, Morley Mark Tremonti Wah, Fulltone OCD (4th gen), Digitech Whammy (5th gen), MXR 10-Band EQ, Boss DD-3 Delay, Boss BF-3 Flanger, Korg Pitch Black Tuner, others

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big dooley
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Re: KT-77's or big bottle 6CA7's or EL-34's????

Post by big dooley » Fri May 13, 2011 8:54 pm

there is only one way to find out, bam bam [smilie=icon_wink.gif]
guitars:
93 gibson les paul, seymour duncun triple shots, P-rails pickups, (rail coil, P90, series humbucker or parallel humbucker)
elixer baritone strings: .012 .016 .022 .038 .052 .068 in standard tune
amplifiers:
marshall JVM410H: stock components, main voltage boosted with a variac at 240Vac, bias set at 68% of max plate dissipation
JJ KT77 powertubes, V1 and V2 JJ ecc83s, V3 JJ ecc803, V4 stock marshall (shuguang), V5 JJ ecc83s
marshall AFD100: completely stock, bias set at 47mA
marshall VS100r combo: celestion G12T75 speaker, general electric 5751 tube

cabinets:
marshall JCM900 1936 cabs each with a celestion G12H30 anniversary and vintage 30
JVM drives the V30's, AFD drives the H30's

effects:
dunlop cry baby, boss NS2, DD3 CE3, custom A/B looper box

User avatar
Bam Bam
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Posts: 3936
Joined: Sat May 29, 2010 9:06 pm
Location: Mesa, Arizona

Re: KT-77's or big bottle 6CA7's or EL-34's????

Post by Bam Bam » Sat May 14, 2011 3:52 pm

big dooley wrote:there is only one way to find out, bam bam [smilie=icon_wink.gif]
Which is what? Let you do it and tell me how it works out? [smilie=icon_mrgreen.gif]

Yeah, maybe I'll order another set of them for the JVM next!

Bam
Guitars: 2012 Ibanez 25th Anniversary RG3 w/Dimarzio D-Activator/Air Norton S, 2005 Charvel USA Custom Shop Pointy Head Reissue w/80's Jackson J-95/Duncan '59, 2010 Charvel Pro Mod San Dimas Style 1-2H w/Custom Custom/Jazz (was JB/'59 stock), 1987 Kramer Aerostar ZX30 w/Dimarzio Super Distortion/EMG Selects (passive single-coils)
Amps: JVM 410H (2007), EVH 5150III 50w head (2011), Peavey Series 2600 Mark III
Cabs: Avatar Contemporary 4x12 loaded with Hellatone 60s (Vintage 30s), Early 80's Peavey 2x12 loaded with Eminence Governors
Pedals: ISP Decimator, Morley Mark Tremonti Wah, Fulltone OCD (4th gen), Digitech Whammy (5th gen), MXR 10-Band EQ, Boss DD-3 Delay, Boss BF-3 Flanger, Korg Pitch Black Tuner, others

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big dooley
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Re: KT-77's or big bottle 6CA7's or EL-34's????

Post by big dooley » Sat May 14, 2011 5:28 pm

Bam Bam wrote:
big dooley wrote:there is only one way to find out, bam bam [smilie=icon_wink.gif]
Which is what? Let you do it and tell me how it works out? [smilie=icon_mrgreen.gif]

Yeah, maybe I'll order another set of them for the JVM next!

Bam
no experience with the 6CA7's, but i do use KT77's for quite a while now... the JJ 6CA7's will be next... they can handle the same amount of screen voltage as the winged C EL34, which is why i choose them instead of the EH 6CA7's... the KT77's can have even more screen voltage by the way

as for the KT77's, they have more headroom, extended low end, and a bit more highs...
guitars:
93 gibson les paul, seymour duncun triple shots, P-rails pickups, (rail coil, P90, series humbucker or parallel humbucker)
elixer baritone strings: .012 .016 .022 .038 .052 .068 in standard tune
amplifiers:
marshall JVM410H: stock components, main voltage boosted with a variac at 240Vac, bias set at 68% of max plate dissipation
JJ KT77 powertubes, V1 and V2 JJ ecc83s, V3 JJ ecc803, V4 stock marshall (shuguang), V5 JJ ecc83s
marshall AFD100: completely stock, bias set at 47mA
marshall VS100r combo: celestion G12T75 speaker, general electric 5751 tube

cabinets:
marshall JCM900 1936 cabs each with a celestion G12H30 anniversary and vintage 30
JVM drives the V30's, AFD drives the H30's

effects:
dunlop cry baby, boss NS2, DD3 CE3, custom A/B looper box

User avatar
Bam Bam
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Posts: 3936
Joined: Sat May 29, 2010 9:06 pm
Location: Mesa, Arizona

Re: KT-77's or big bottle 6CA7's or EL-34's????

Post by Bam Bam » Sat May 14, 2011 5:34 pm

big dooley wrote:
as for the KT77's, they have more headroom, extended low end, and a bit more highs...
Expect the same from the 6CA7s! [smilie=icon_wink.gif]

And thanks for the JJ vs. EH screen voltage info- good to know!

Bam
Guitars: 2012 Ibanez 25th Anniversary RG3 w/Dimarzio D-Activator/Air Norton S, 2005 Charvel USA Custom Shop Pointy Head Reissue w/80's Jackson J-95/Duncan '59, 2010 Charvel Pro Mod San Dimas Style 1-2H w/Custom Custom/Jazz (was JB/'59 stock), 1987 Kramer Aerostar ZX30 w/Dimarzio Super Distortion/EMG Selects (passive single-coils)
Amps: JVM 410H (2007), EVH 5150III 50w head (2011), Peavey Series 2600 Mark III
Cabs: Avatar Contemporary 4x12 loaded with Hellatone 60s (Vintage 30s), Early 80's Peavey 2x12 loaded with Eminence Governors
Pedals: ISP Decimator, Morley Mark Tremonti Wah, Fulltone OCD (4th gen), Digitech Whammy (5th gen), MXR 10-Band EQ, Boss DD-3 Delay, Boss BF-3 Flanger, Korg Pitch Black Tuner, others

monnkey2
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Re: KT-77 /big bottle 6CA7/ EL-34's + Biasing plate voltage

Post by monnkey2 » Mon Nov 05, 2018 3:06 pm

Hey all,

I just did the negative feedback and plexi mods, so I’m rebiasing while I’ve got it out. I’m a little concerned though. I’m reading 495V off of pin 3. That is higher than what I’ve seen posted in this forum (most people are in the 450-460 range). Is 495V safe, or is my amp about to explode?

Also, I’m a little confused about how to get the screen current. When I measure the screen resisters I get 480V and 476V. Do I subtract these numbers to get 4V and then double it to get 8V? If so, do I subtract that 8 from 495, so my plate voltage is 487?

Thanks for the help!

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