Lonestar bias mod
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dudleydawson
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attention Mr. Reeder and Splatter!
I have a bias-modded LSC on the way. so i've been reading up.
unless i'm missing something - one of you keeps talking in milli-volts, and one keeps talking in milli-amps.
now, i faked my way through 'circuits for non-electrical-engineering engineers' in college - but this doesn't make sense to me.
is one of you using improper units, or is this more complicated than i thought?
i really don't mean this as smart-*** - i'm legitimately confused and trying to get a handle on how to bias this thing when it shows up!
i'm just glad i've got you guys to learn from!!!
what am i missing? :?
I have a bias-modded LSC on the way. so i've been reading up.
unless i'm missing something - one of you keeps talking in milli-volts, and one keeps talking in milli-amps.
now, i faked my way through 'circuits for non-electrical-engineering engineers' in college - but this doesn't make sense to me.
is one of you using improper units, or is this more complicated than i thought?
i really don't mean this as smart-*** - i'm legitimately confused and trying to get a handle on how to bias this thing when it shows up!
i'm just glad i've got you guys to learn from!!!
what am i missing? :?
Charles Reeder
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dudleydawson:
Your confusion about MA or MV is understandable...but nothing to be concerned about.
First: Purchase a bias-probe. One of the cheaper, single tube units will work just fine. You put the bias-probe in one of your output tube sockets...leaving other three output tubes installed. Install the other output tube in the bias-probe's tube socket.
Second: You purchase a digital multi-meter ($20 at Radio Shack) any comparable unit will work as long as it has a setting for MA(milli-amps).
This is where the confusion originates: The bias probe has a 1-ohm resistor in it which converts a MA reading into MV. So with the multi-tester set on MA; you will get a MV (milli-volts) reading.
The bias probe you purchase should have adequate instructions to 'walk' you through the process (mine did); but just in case I'll add a few pointers.
Start your initial bias readings and adjustments in the 50-watt mode; silicon diodes rectification, with amp set on full power (or bold) setting (NOT on tweed). Use channel-1 with ALL tone, volume ,presense, reverb, etc.,controls set to ZERO.
Use one of the outer pair of output tubes for the 50-watt reading. JUST A WARNING! Make certain you don't install the bias probe in one of the tube rectifier sockets! On the Lonestar it's the big tube at FAR LEFT. On Stiletto Duece; they're the 2 big tubes at far right. On Ace combo it's the big tube at far left (as in the Lone Star). Ace head it's the big one far right. Stiletto Trident; 3 big ones at far right.
After setting the bias at 50-watts; you may want to check the 100-watt setting; just to be cautious. Install the bias probe in one of the inner pair of output tubes and set amp to 100-watts. Expect a slight difference. The 100-watt setting will usually read a few MA/MV lower than when run in the 50-watt mode.
Best of luck: Charles
Your confusion about MA or MV is understandable...but nothing to be concerned about.
First: Purchase a bias-probe. One of the cheaper, single tube units will work just fine. You put the bias-probe in one of your output tube sockets...leaving other three output tubes installed. Install the other output tube in the bias-probe's tube socket.
Second: You purchase a digital multi-meter ($20 at Radio Shack) any comparable unit will work as long as it has a setting for MA(milli-amps).
This is where the confusion originates: The bias probe has a 1-ohm resistor in it which converts a MA reading into MV. So with the multi-tester set on MA; you will get a MV (milli-volts) reading.
The bias probe you purchase should have adequate instructions to 'walk' you through the process (mine did); but just in case I'll add a few pointers.
Start your initial bias readings and adjustments in the 50-watt mode; silicon diodes rectification, with amp set on full power (or bold) setting (NOT on tweed). Use channel-1 with ALL tone, volume ,presense, reverb, etc.,controls set to ZERO.
Use one of the outer pair of output tubes for the 50-watt reading. JUST A WARNING! Make certain you don't install the bias probe in one of the tube rectifier sockets! On the Lonestar it's the big tube at FAR LEFT. On Stiletto Duece; they're the 2 big tubes at far right. On Ace combo it's the big tube at far left (as in the Lone Star). Ace head it's the big one far right. Stiletto Trident; 3 big ones at far right.
After setting the bias at 50-watts; you may want to check the 100-watt setting; just to be cautious. Install the bias probe in one of the inner pair of output tubes and set amp to 100-watts. Expect a slight difference. The 100-watt setting will usually read a few MA/MV lower than when run in the 50-watt mode.
Best of luck: Charles
dudleydawson
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AHA! A 1 ohm resistor!
now it makes a little more sense.
thank you for the explanation...
cheers!
now it makes a little more sense.
thank you for the explanation...
cheers!
Hi everyone,
Ive had my mod kit laying arround for a while and finally got round to installing it (well nearly)
I got me a multimeter and bias prob and decided to take a reading before doing the mod.
All 4 valves are reading arround 27mA when in 100w mode but drop all the way down to arround 15mA when I switch to 50w!
I remember Charles saying they drop just a few mA when switched from 50w to 100w.
So im a bit confused why im getting such a big shift in readings, is it something to do with my amp being the 10/50/100w model?
Im getting 130mA in 10w mode along with noticable hum with the bias probe attached, I'm just putting that down to the valves operating differently in class A mode?
Anyway it seems when I install the mod I will be able to bias hot in 100w mode but 50w mode will still be cold.
Am I missing something or should I be concerned?
Ive had my mod kit laying arround for a while and finally got round to installing it (well nearly)
I got me a multimeter and bias prob and decided to take a reading before doing the mod.
All 4 valves are reading arround 27mA when in 100w mode but drop all the way down to arround 15mA when I switch to 50w!
I remember Charles saying they drop just a few mA when switched from 50w to 100w.
So im a bit confused why im getting such a big shift in readings, is it something to do with my amp being the 10/50/100w model?
Im getting 130mA in 10w mode along with noticable hum with the bias probe attached, I'm just putting that down to the valves operating differently in class A mode?
Anyway it seems when I install the mod I will be able to bias hot in 100w mode but 50w mode will still be cold.
Am I missing something or should I be concerned?
You need to measure the plate dissipation to get the full picture. The plate voltage is different in different mode. The bias probe should come with clear instruction on what to measure. Basically measure the plate voltage and multiply by the cathode current and bias the tube to about 70% of its max rated power. You can go more than 70% for hot, just use your ears at that point to fine tune.handyandy said:
Thanks ja22y,
Im still somewhat unsure about reading plate voltage and current etc.
I just worked out the huge drop was due to the rectifer comming on in 50w mode,
I switched to diodes and now the difference between modes is only a couple of mA.
Still though does this mean the valves will run cold when the rectifer is being used?
Or as you suggested do the voltages and currents change with the rectifer is engaged?
Thanks for any comments!
Im still somewhat unsure about reading plate voltage and current etc.
I just worked out the huge drop was due to the rectifer comming on in 50w mode,
I switched to diodes and now the difference between modes is only a couple of mA.
Still though does this mean the valves will run cold when the rectifer is being used?
Or as you suggested do the voltages and currents change with the rectifer is engaged?
Thanks for any comments!
I suggest that you do some research on the biasing topic so that it will make more sense, there are a few good ones:
http://www.aikenamps.com/Biasing.html
The bias probe gives you the cathode voltage across a 1 ohm resistor, since I=V/R, you can considered the multimeter reading is the cathode current. The cathode current alone doesn't tell you if your amp is properly biased or not. Biasing is basically set your tube while at idle to dissipate at a certain level based on its rated power. Rule of thumb for a starting point is 70% but really there is no rule. You'll see that the plate voltage varies as you adjust the cathode current, so the plate dissipation is the only way to tell if your tube is hot or cold. The simplified formula is P=IV, P is the plate dissipation, I is the cathode current, and V is the plate voltage. For example, with el34 in 100W mode:
EL34 rated pwr is 25W so 70% is 17.5W. This is the dissipation that you try to set the bias to.
Plate voltage measured to be 450V so the bias current that you want to set is 17.5/450= 39mA
Do the above calculation for 50 and 10 modes. Even if the current reads lower, its plate voltage might be higher so effectively you're still within the range. You should only see may be a .5W or 1W different between the modes.
Hope this helps.
BTW be very careful as you are dealing with high voltage source. Don't do this if you're not sure what you're doing.
http://www.aikenamps.com/Biasing.html
The bias probe gives you the cathode voltage across a 1 ohm resistor, since I=V/R, you can considered the multimeter reading is the cathode current. The cathode current alone doesn't tell you if your amp is properly biased or not. Biasing is basically set your tube while at idle to dissipate at a certain level based on its rated power. Rule of thumb for a starting point is 70% but really there is no rule. You'll see that the plate voltage varies as you adjust the cathode current, so the plate dissipation is the only way to tell if your tube is hot or cold. The simplified formula is P=IV, P is the plate dissipation, I is the cathode current, and V is the plate voltage. For example, with el34 in 100W mode:
EL34 rated pwr is 25W so 70% is 17.5W. This is the dissipation that you try to set the bias to.
Plate voltage measured to be 450V so the bias current that you want to set is 17.5/450= 39mA
Do the above calculation for 50 and 10 modes. Even if the current reads lower, its plate voltage might be higher so effectively you're still within the range. You should only see may be a .5W or 1W different between the modes.
Hope this helps.
BTW be very careful as you are dealing with high voltage source. Don't do this if you're not sure what you're doing.
Thanks ja22y!
Done the mod, took the voltage, did the maths.
50 & 100 watt modes there is little difference, I'm reading 419 volts and biased to 45mA which has got me round the 65% or 19.5w mark.
It turnes out I'm very cold with the valve rectifer on, 376 volts and 24mA! I should be getting about 51mA at that voltage to stay arround 65%!
However the amp has improved on all modes so I'm very happy about the mod, Its nearly like lifting a blanket that was covering the speakers, thats my best description anyway.
I'm glad I did it, the lonestar keeps getting better for me and I have now learned how to properly bias an amp into the bargin so couldn't be happier!!!
Oh I found a very convinent way of mounting the circut board inside the amp with the pot still adjustable from the outside, no drilling needed!
I used a block of wood with a cut out to accept the circut, glued it in with hot glue gun and stuck it behind the vents at the back of the amp with some industrial tape, really strong stuff, I used it to stick effects to a pedal board once and it took a layar of plywood off when i tried to remove it!!
Done the mod, took the voltage, did the maths.
50 & 100 watt modes there is little difference, I'm reading 419 volts and biased to 45mA which has got me round the 65% or 19.5w mark.
It turnes out I'm very cold with the valve rectifer on, 376 volts and 24mA! I should be getting about 51mA at that voltage to stay arround 65%!
However the amp has improved on all modes so I'm very happy about the mod, Its nearly like lifting a blanket that was covering the speakers, thats my best description anyway.
I'm glad I did it, the lonestar keeps getting better for me and I have now learned how to properly bias an amp into the bargin so couldn't be happier!!!
Oh I found a very convinent way of mounting the circut board inside the amp with the pot still adjustable from the outside, no drilling needed!
I used a block of wood with a cut out to accept the circut, glued it in with hot glue gun and stuck it behind the vents at the back of the amp with some industrial tape, really strong stuff, I used it to stick effects to a pedal board once and it took a layar of plywood off when i tried to remove it!!
indifferent
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Just completed the bias mod and ZOWIE! The amp never, ever, sounded so good. I having been thinking about building another vintage Fender clone to get the tone I desire but no need now for sure. I ended up drilling a hole slightly bigger than the adjust screw and simply glued the top of the pot to the inside of the chassis on the left near the fuse holder. It is very accessible and you wouldn't see it unless you were searching because it is within a bunch of printed text. I biased it up to 70% using the Eurotubes bias probe. Piece of cake. I am using the Black Sable 6l6 tubes, just on the outside cause I never go to 100w and a mix of new Mullards and Tunsol preamp tubes .... just can't spend the cash to mess with the high dollar NOS stuff, and honestly, it blows me away as is now.
It is such an improvement I can't image Mesa not offering the adjustable bias as an option for those that care more about tone than the cost of replacing the tubes more frequently or the inconvenience of carrying spares in case they give out on a gig. This mod is well worth the effort and expense. The Mansfield Guitar mod is well made, as simple as it is, but it does take about 2 weeks to get it once ordered. This mod is a definite no brainer!!!!!!
It is such an improvement I can't image Mesa not offering the adjustable bias as an option for those that care more about tone than the cost of replacing the tubes more frequently or the inconvenience of carrying spares in case they give out on a gig. This mod is well worth the effort and expense. The Mansfield Guitar mod is well made, as simple as it is, but it does take about 2 weeks to get it once ordered. This mod is a definite no brainer!!!!!!
Ok, I've read the LSC pot mod thread and this one as well. I must say that I'm quite pleased at the lack of opinion bashing, so much so that I've decided to "ask away". Imagine, a forum that's really helpful!
Perhaps it's my new-ness to the forum or my concentration of the two threads mentioned above. What I haven't seen, please forgive me if it's there and I haven't found it, is what tubes are you all using when you've been adding these two mods?
The reason I'm asking is that I have a slight hummmmmm when using CH 2. I do have the gain up fairly high, 2-3 o'clock. I do expect some but not as much as I'm getting. Its not enough hum to make you think "wow, something must be wrong", it's just enough to be annoying. This has lead me down the road of researching diff preamp tubes.
I've not tried any diffent tubes yet so I'm all ears.
Thanks in advance.
Rog
Perhaps it's my new-ness to the forum or my concentration of the two threads mentioned above. What I haven't seen, please forgive me if it's there and I haven't found it, is what tubes are you all using when you've been adding these two mods?
The reason I'm asking is that I have a slight hummmmmm when using CH 2. I do have the gain up fairly high, 2-3 o'clock. I do expect some but not as much as I'm getting. Its not enough hum to make you think "wow, something must be wrong", it's just enough to be annoying. This has lead me down the road of researching diff preamp tubes.
I've not tried any diffent tubes yet so I'm all ears.
Thanks in advance.
Rog
plan-x
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You can use the stock mesa tubes for the mods. I use JJ's and get a smoother respone. It's not dramatic though. As for the hum, maybe the guitar? Running the gain at 3:00 might contribute to that. I run my gain much lower and let the drive do the work.
Lonestar Buff
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I have a Lonestar Classic with an adjustable bias control in it, but the bias jumps up from the 40's (mv's) to the high 50's, low 60's when I switch from 50 to 100 watt mode which seems contrary to what Charles mentioned above that his drops a couple mv. Anybody know why or how this can be managed so that I can go from 50 to 100 w and not have to re-bias? Thanks.
thanks for the info Charles . I had no Idea that mesa offered different tubes like that .
here are some pics of the amp after the mod .
as you can see from the photos I opted to mount the bias pot on the outside so that when I change tubes I won't have to open the amp up . If you don't like the way it looks you can always mount it on the inside .\
Here is a link to ebay where you can find the pot .http://cgi.ebay.com/Bias-Mod-Mesa-Boogie-Marshall-Fender-Peavey-Vox-/320510207372?cmd=ViewItem&pt=LH_DefaultDomain_0&hash=item4a9fe5a58c
This is where I got mine. It comes with a schematic with the load resister circled . You can also send your amp to them and have them preform the mod . but then theres shipping and bench fees . If you don't feel comfortable preforming the mod then order the part and have a local tech do it . Remember there are VERY HIGH VOLTAGES PRESENT EVEN WITH THE AMP OFF AND UNPLUGGED .
I hope this helps ,this mod made all the difference between me keeping and selling the amp.
