adjusting EL84 cathode bias?

18watt-specific Tech Talk - Building, Fixing, Parts, Mods...

Moderators: CurtissRobin, colossal, zaphod_phil, Daviedawg, Graydon

Post Reply
Walker
Unrated
Posts: 9
Joined: Sat 10/22/22 8:46 pm

adjusting EL84 cathode bias?

Post by Walker »

Hello all,

I've been tinkering with a 1974 circuit...long story short, a friend had a mojotone kit (basic 1974, not the TMB) and the thing sounds glorious, it's the only amp I ever want to play. So, I bought one for myself and have been trying in vain to recreate the magic of my friend's amp.

One of the biggest differences I've noticed, is that his amp has a much higher voltage drop across the output transformer and his output tubes wind up with a plate dissipation of about 90-110% (I know, I know, but listen I would sacrifice 2 EL84s a week for the tone of this amp...)

Output transformers are the same spec from mojotone, and resistance is within <5%, but he's getting a voltage drop of 12-14 volts, where I'm only getting a drop of about 8-9. My plate dissipation winds up being about 70%.

How can I adjust this? I'd like to at least try upping the plate dissipation. I've tried decreasing the cathode resistor from 130 to 115 with no significant change in dissipation...to be honest I'm not even sure if the cathode resistor is the place to start - how can I adjust the bias on my output tubes? Thanks for any tips!
0 x

User avatar
Bieworm
Verbose Moderator
Verbose Moderator
Posts: 2285
Joined: Mon 02/10/20 8:24 am
Location: Belgium

Re: adjusting EL84 cathode bias?

Post by Bieworm »

Could you give us the other numbers/Voltages? Here's a voltage chart, but watch out.. I think your V2 is the PI, and here V2 is the preamp of the tremolo channel.
You can notice that the Modern Classic's tube layout improves the lead dress, with less wires crossing.

files/JMPGuitars_18_Watt_Modern_Classic ... _Chart.pdf
1 x
"THIS should be played at high volume..preferably in a residential area"

Walker
Unrated
Posts: 9
Joined: Sat 10/22/22 8:46 pm

Re: adjusting EL84 cathode bias?

Post by Walker »

Bieworm wrote:
Thu 11/24/22 1:19 am
Could you give us the other numbers/Voltages?
Thanks for the reply! Yes, I'll need to get my friend's amp back and check those, but thanks for the chart I'll post all those voltages in a few days. There are definitely a lot of differences besides just the output tubes!

But generally, I'm still curious how does one go about adjusting the bias of the output tubes in this circuit? Is it primarily by changing the value of the cathode resistor or?

Cheers!
0 x

Daviedawg
Superior Amp Tech
Superior Amp Tech
Posts: 748
Joined: Fri 01/08/10 2:00 am
Location: Scotland

Re: adjusting EL84 cathode bias?

Post by Daviedawg »

My rule of thumb for the best 18w tone is to get as near 12v on the cathodes of the EL84s as possible. If you are within a couple of volts, set it with your cathode resistor.

Dd
1 x

User avatar
Bieworm
Verbose Moderator
Verbose Moderator
Posts: 2285
Joined: Mon 02/10/20 8:24 am
Location: Belgium

Re: adjusting EL84 cathode bias?

Post by Bieworm »

Walker wrote:
Thu 11/24/22 1:29 am
Bieworm wrote:
Thu 11/24/22 1:19 am
Could you give us the other numbers/Voltages?
Thanks for the reply! Yes, I'll need to get my friend's amp back and check those, but thanks for the chart I'll post all those voltages in a few days. There are definitely a lot of differences besides just the output tubes!

But generally, I'm still curious how does one go about adjusting the bias of the output tubes in this circuit? Is it primarily by changing the value of the cathode resistor or?

Cheers!
With cathode biased amps the most common way of setting the bias is adjusting the value of the cathode resistor. There are other factors too, such as the grid return resistor to ground... but I'd leave that one alone.

But don't chase your tail on obtaining the exact sound of your friend's amp. There are a lot of factors that determine the sound. I probably don't have to say this, but don't underestimate the substantial impact of the type of speaker. The individual voltages throughout the amp, starting with the right B+ voltage... all very important. With all things being equal you will closely approach the sound, but don't expect it to be a 100% the same.

Here's my story to support this... I made the prototype Tremolo TMB a few years ago. I have built a bunch of Tremolo TMB's since then, but none of them sounded as good ast the prototype. Earlier this year I made an exact copy of that amp.. with all the imperfections of the original. Same brand of parts, tubes,transformers, voltages.. the lot! Well.. when I plugged that amp into the speaker, which still sat in the prototype combo.. it sounded really great, but not even close to the sound of the prototype. And that is as close a copy as one can ever make, by -mind you- the same builder.. and still a very clear difference in tone.
1 x
"THIS should be played at high volume..preferably in a residential area"

Daviedawg
Superior Amp Tech
Superior Amp Tech
Posts: 748
Joined: Fri 01/08/10 2:00 am
Location: Scotland

Re: adjusting EL84 cathode bias?

Post by Daviedawg »

That happens B. Guitars you can explain by no two pieces of wood being the same. But why do manufactured items create varying outcomes? Amps do break in over time but that does not explain your experience which mirrors other peoples' experiences.

Dd
1 x

User avatar
Bieworm
Verbose Moderator
Verbose Moderator
Posts: 2285
Joined: Mon 02/10/20 8:24 am
Location: Belgium

Re: adjusting EL84 cathode bias?

Post by Bieworm »

Daviedawg wrote:
Thu 11/24/22 5:09 am
That happens B. Guitars you can explain by no two pieces of wood being the same. But why do manufactured items create varying outcomes? Amps do break in over time but that does not explain your experience which mirrors other peoples' experiences.

Dd
I don't mind though.. you gotta have your favorite amp. I think it's something that takes place on a quantum level. The Higgs boson of my Tremolo TMB prototype
If even I can't copy it, nobody can either. So I'm the proud owner of the IMHO best specimen!!
:D :D :D
0 x
"THIS should be played at high volume..preferably in a residential area"

Walker
Unrated
Posts: 9
Joined: Sat 10/22/22 8:46 pm

Re: adjusting EL84 cathode bias?

Post by Walker »

I'm afraid I'll chase my tail on this one forever, and I'm ok with that - when I nail it that's like the day my life begins. They're both heads so I have used them with the same speaker - honestly I've gotten mine pretty close at this point, but I don't feel it's as loud and it's definitely not as smooth...his sags and compresses instantly a very soft but rich and cutting attack. Mine still has kind of a cold, barky attack and not as much sustain. But the tonal spectrum is quite similar so I feel like heating up those output tubes could help with the volume and overall squish. I'm also replacing my plate resistors with carbon comp (as they are in his) haven't done that bit yet. Anyway appreciate all the advice as I'm quite new to this, and I will update with the voltages in a few days. Cheers yall!
1 x

User avatar
JMPGuitars
Super Duper Admin
Super Duper Admin
Posts: 3951
Joined: Tue 09/18/12 8:00 pm
Location: South Central, MA
Contact:

Re: adjusting EL84 cathode bias?

Post by JMPGuitars »

The 12V rule is okay, but it doesn't cover everybody accurately. Some wall voltages, or transformers, will make that 12V a poor bias point. The best bet is to shoot for 85% dissipation. This calculator is handy: https://robrobinette.com/Tube_Bias_Calculator.htm

Bieworm, on your amps, did you match the voltages and output dissipation?
1 x
'I installed a skylight in my apartment yesterday... The people who live above me are furious.' - Steven Wright

Modern Ground Schemes
Soldering Technique
B+ Voltage Reduction
Amplifier Tools & Parts Info


Web Design: DolceVittoria.com
Guitars / Amps / Effects: JMPGuitars.com
(anti)Social: Facebook · Instagram
Items for Sale

User avatar
Bieworm
Verbose Moderator
Verbose Moderator
Posts: 2285
Joined: Mon 02/10/20 8:24 am
Location: Belgium

Re: adjusting EL84 cathode bias?

Post by Bieworm »

JMPGuitars wrote:
Thu 11/24/22 3:42 pm
The 12V rule is okay, but it doesn't cover everybody accurately. Some wall voltages, or transformers, will make that 12V a poor bias point. The best bet is to shoot for 85% dissipation. This calculator is handy: https://robrobinette.com/Tube_Bias_Calculator.htm

Bieworm, on your amps, did you match the voltages and output dissipation?
I did in fact.. within 5% approx..
I stichting with the higgs boston theory 😉
0 x
"THIS should be played at high volume..preferably in a residential area"

User avatar
JMPGuitars
Super Duper Admin
Super Duper Admin
Posts: 3951
Joined: Tue 09/18/12 8:00 pm
Location: South Central, MA
Contact:

Re: adjusting EL84 cathode bias?

Post by JMPGuitars »

Bieworm wrote:
Thu 11/24/22 5:15 pm
JMPGuitars wrote:
Thu 11/24/22 3:42 pm
The 12V rule is okay, but it doesn't cover everybody accurately. Some wall voltages, or transformers, will make that 12V a poor bias point. The best bet is to shoot for 85% dissipation. This calculator is handy: https://robrobinette.com/Tube_Bias_Calculator.htm

Bieworm, on your amps, did you match the voltages and output dissipation?
I did in fact.. within 5% approx..
I stichting with the higgs boston theory 😉
Well, there are other factors. Break in period, emotional attachment, etc.

I wonder if you could tell the amps apart in a recording. Play something into a looper pedal, then have somebody else play it through 2 or 3 of your amp builds and record it with the same speaker cabinet and mic setup. Then see if you can tell them apart.
1 x
'I installed a skylight in my apartment yesterday... The people who live above me are furious.' - Steven Wright

Modern Ground Schemes
Soldering Technique
B+ Voltage Reduction
Amplifier Tools & Parts Info


Web Design: DolceVittoria.com
Guitars / Amps / Effects: JMPGuitars.com
(anti)Social: Facebook · Instagram
Items for Sale

Walker
Unrated
Posts: 9
Joined: Sat 10/22/22 8:46 pm

Re: adjusting EL84 cathode bias?

Post by Walker »

Added an 820 in parallel with the 130 cathode resistor for 111 ohms, and this amp is already sounding muuuuch better, and louder!

I'd like to go lower on the cathode, like perhaps replace the parallel duo with a single 100 ohm...but I'm not sure how to do the math to see where 100 would put my plate dissipation. I understand V=IR but not sure how I apply that to the cathode R value? Any tips? Thanks!
0 x

User avatar
JMPGuitars
Super Duper Admin
Super Duper Admin
Posts: 3951
Joined: Tue 09/18/12 8:00 pm
Location: South Central, MA
Contact:

Re: adjusting EL84 cathode bias?

Post by JMPGuitars »

Walker wrote:
Mon 11/28/22 5:47 pm
Added an 820 in parallel with the 130 cathode resistor for 111 ohms, and this amp is already sounding muuuuch better, and louder!

I'd like to go lower on the cathode, like perhaps replace the parallel duo with a single 100 ohm...but I'm not sure how to do the math to see where 100 would put my plate dissipation. I understand V=IR but not sure how I apply that to the cathode R value? Any tips? Thanks!
Don't guess, use the calculator: https://robrobinette.com/Tube_Bias_Calculator.htm
1 x
'I installed a skylight in my apartment yesterday... The people who live above me are furious.' - Steven Wright

Modern Ground Schemes
Soldering Technique
B+ Voltage Reduction
Amplifier Tools & Parts Info


Web Design: DolceVittoria.com
Guitars / Amps / Effects: JMPGuitars.com
(anti)Social: Facebook · Instagram
Items for Sale

Walker
Unrated
Posts: 9
Joined: Sat 10/22/22 8:46 pm

Re: adjusting EL84 cathode bias?

Post by Walker »

JMPGuitars wrote:
Mon 11/28/22 8:22 pm

Don't guess, use the calculator: https://robrobinette.com/Tube_Bias_Calculator.htm
But that’s where I get confused. To calculate using cathode resistance I have to enter the voltage drop across the cathode resistor. But I don’t know the voltage drop until I change the resistor and measure the voltage drop so how do I start with the calculator? I don’t think I’m using the right variables, maybe if I plug in some other numbers I can calculate what the voltage drop would be? I want to know what my plate dissipation would be with a 100ohm cathode resistor just not quite sure how to get there. Thanks for any advice!
0 x

User avatar
Bieworm
Verbose Moderator
Verbose Moderator
Posts: 2285
Joined: Mon 02/10/20 8:24 am
Location: Belgium

Re: adjusting EL84 cathode bias?

Post by Bieworm »

Walker wrote:
Fri 12/02/22 1:46 pm
JMPGuitars wrote:
Mon 11/28/22 8:22 pm

Don't guess, use the calculator: https://robrobinette.com/Tube_Bias_Calculator.htm
But that’s where I get confused. To calculate using cathode resistance I have to enter the voltage drop across the cathode resistor. But I don’t know the voltage drop until I change the resistor and measure the voltage drop so how do I start with the calculator? I don’t think I’m using the right variables, maybe if I plug in some other numbers I can calculate what the voltage drop would be? I want to know what my plate dissipation would be with a 100ohm cathode resistor just not quite sure how to get there. Thanks for any advice!
Easy.. you take the voltages on pin 7, which are the plate voltages. Then you take the voltage on pin 3, which are the cathode voltages. You then subtract the cathode voltage from the plate voltage and voila! That's the plate to cathode voltage you need for the calculator.
Example: pin 7= 345V minus pin 3 = 11.5V --> platevto cathode voltage = 333.5V

That would make 84.2% dissipation
Screenshot_20221202_205541_Samsung Internet.jpg

Screenshot_20221202_205553_Samsung Internet.jpg
You do not have the required permissions to view the files attached to this post.
1 x
"THIS should be played at high volume..preferably in a residential area"

Walker
Unrated
Posts: 9
Joined: Sat 10/22/22 8:46 pm

Re: adjusting EL84 cathode bias?

Post by Walker »

Bieworm wrote:
Thu 11/24/22 1:19 am
Could you give us the other numbers/Voltages? Here's a voltage chart, but watch out.. I think your V2 is the PI, and here V2 is the preamp of the tremolo channel.
You can notice that the Modern Classic's tube layout improves the lead dress, with less wires crossing.

files/JMPGuitars_18_Watt_Modern_Classic ... _Chart.pdf
Hey @Bieworm do you know where I can find the schematic for this amp? Filling out the voltage test sheet now but I'm not sure what "A" "B" "C" and HV1 and HV2 are? Thanks!
0 x

Walker
Unrated
Posts: 9
Joined: Sat 10/22/22 8:46 pm

Re: adjusting EL84 cathode bias?

Post by Walker »

Haha ok nevermind, all I had to do was replace "voltage test sheet" with "schematic" in the url and lo & behold :D
0 x

User avatar
JMPGuitars
Super Duper Admin
Super Duper Admin
Posts: 3951
Joined: Tue 09/18/12 8:00 pm
Location: South Central, MA
Contact:

Re: adjusting EL84 cathode bias?

Post by JMPGuitars »

Walker wrote:
Sat 12/03/22 8:38 pm
Haha ok nevermind, all I had to do was replace "voltage test sheet" with "schematic" in the url and lo & behold :D
You're welcome. 😉😉
2 x
'I installed a skylight in my apartment yesterday... The people who live above me are furious.' - Steven Wright

Modern Ground Schemes
Soldering Technique
B+ Voltage Reduction
Amplifier Tools & Parts Info


Web Design: DolceVittoria.com
Guitars / Amps / Effects: JMPGuitars.com
(anti)Social: Facebook · Instagram
Items for Sale

Walker
Unrated
Posts: 9
Joined: Sat 10/22/22 8:46 pm

Re: adjusting EL84 cathode bias?

Post by Walker »

Thanks everyone for your help. All of my voltages lined up - but there was one peculiarity....whenever I would touch pin 7 on the PI I heard a crackling/crunching noise (not through the speaker, just coming out of some component :? ). Turns out the 0.01uF cap on the PI was not connected to ground and now that I've connected it, this amp is sounding exactly as I wanted it to, it's screaming!

To understand how the heck I got here, this amp was originally a TMB when purchased, I wanted to gut the TMB channel and convert it to an 18W lite. In doing so, I removed the TMB master volume and inadvertently left that capacitor hanging in the breeze! Without the master volume, one side of the cap was totally disconnected...duh. Rebiasing the output tubes was helping the tone and volume, but apparently was not the real culprit.

Now this amp is just ruling and I'm super stoked, thanks again!
3 x

Jon_Kayak
Unrated
Posts: 2
Joined: Sat 03/18/23 7:21 am

Re: adjusting EL84 cathode bias?

Post by Jon_Kayak »

Thx a lot for the explanations! Very very useful!
0 x

Post Reply