Amps that have external bias points and adjust?

[MattyOvertone]

.... as oppeosed to internal biasing? I don't understand your comment.

 

For the most accurate tube biasing (for this method of biasing anyways), you want to find the static dissapation... basically how many watts the tube is thumping out. For this you need the plate voltage as well and you multiply them, because wattage=voltage x amperage. The blurb below is quoted from http://www.duncanamps.com/technical/lvbias.html.

 

"...many sources for biasing information just specify plate (or cathode) current settings; telling you to bias your 6L6s at "35 milliamps" is nonsense. Unless you take the plate voltage into consideration, a current specification is meaningless. For instance, 40 mA at 250 volts is 10 watts; the same 40 mA at 500 volts is 20 watts... TWICE as much. In both cases, the current is the same. Amps vary; two identical amps can have plate voltages which differ by as much as 20%. Just because you have a schematic that specifies the plate voltage in your amp as being at 450VDC, don't expect to see that voltage when you take a measurement. TAKE the reading, don't assume the voltage will be as specified. Trust your meter."

 

 

AND I'm not just a guy that wastes all his time reading {censored} online, haha. I bias my own amps and have for years now. Both externally and internally depending on the type. I quoted the above just to provide reference from a source that may be more reputable than me (a random forum guy, haha). Biasing externally just going by a current rating depending on the tube is the nonsense he's referring to.

 

Hands down, biasing externally is easier and faster and safer, but as I said it's less accurate.

[trey85stang]

 

For the most accurate tube biasing (for this method of biasing anyways), you want to find the static dissapation... basically how many watts the tube is thumping out. For this you need the plate voltage as well and you multiply them, because wattage=voltage x amperage. The blurb below is quoted from http://www.duncanamps.com/technical/lvbias.html . "...many sources for biasing information just specify plate (or cathode) current settings; telling you to bias your 6L6s at "35 milliamps" is nonsense. Unless you take the plate voltage into consideration, a current specification is meaningless. For instance, 40 mA at 250 volts is 10 watts; the same 40 mA at 500 volts is 20 watts... TWICE as much. In both cases, the current is the same. Amps vary; two identical amps can have plate voltages which differ by as much as 20%. Just because you have a schematic that specifies the plate voltage in your amp as being at 450VDC, don't expect to see that voltage when you take a measurement. TAKE the reading, don't assume the voltage will be as specified. Trust your meter." AND I'm not just a guy that wastes all his time reading {censored} online, haha. I bias my own amps and have for years now. Both externally and internally depending on the type. I quoted the above just to provide reference from a source that may be more reputable than me (a random forum guy, haha). Biasing externally just going by a current rating depending on the tube is the nonsense he's referring to. Hands down, biasing externally is easier and faster and safer, but as I said it's less accurate.

 

 

ahh ok, generally the ones biasing their amps like this would have a pretty good idea of what the plate voltages sits around anyways. But I see what you're saying the newb probably wouldnt have any idea. I know plate voltage drops some as bias is turned down (I may have that backwards) but it doesnt drop enough to make too much of a difference on teh dissipation calculation.

 

Plate voltage on my slo clone with my bias pot turned all the way down is like 499, and all the way up is like 504. 5 watts wont make or break that formula. (thats with a wall voltage at 115)

[MattyOvertone]

 

Don't need an expensive MM. Just an accurate one. Quality here doesn't mean expensive.

 

 

Exactly what i said, it's bad for people who don't have a quality one, i never said expensive. However as we all know, a lot of time quality does mean expensive, and in the realm of multi-meters, quality doesn't come cheap... that's for sure. Fluke all the way.

[MattyOvertone]

 

ahh ok, generally the ones biasing their amps like this would have a pretty good idea of what the plate voltages sits around anyways. But I see what you're saying the newb probably wouldnt have any idea. I know plate voltage drops some as bias is turned down (I may have that backwards) but it doesnt drop enough to make too much of a difference on teh dissipation calculation. Plate voltage on my slo clone with my bias pot turned all the way down is like 499, and all the way up is like 504. 5 watts wont make or break that formula. (thats with a wall voltage at 115)

 

 

Valid point, and yes plate voltage does vary as you adjust the trim pot but not a huge amount as you said. Where I notice a BIG difference in my plate voltages is when I switch around with tube brands. EH tubes always gave me higher plate voltage than Svetlanas. I would keep that in mind. In fact, years back when i got my first tube amp, the local music shop started selling EH tubes. I bought up 4 matched and slapped them in and they burnt up in no time, hahaha. that's when i started to read up about biasing. That ended up getting me into circuitry, and now I'm an engineer for GE... go figure.

[O'Rourke]

My Laney TT50 combo has the trim pot accessible through a little hole in the chassis with a rubber grommet but there aren't any external test points. Gave me an excuse to buy a Weber Bias-Rite and I love that thing.

 

The DSL and TSL JCM 2000 heads have external trim pots and test points. The two trim pots can be a pain in the ass to balance though as turning one affects the other a lot. It's much easier if you have a Bias Rite so you can just flip the knob from A to B rather than moving the multimeter probe back and forth every time you turn one of the pots.

[MattyOvertone]

 

It's much easier if you have a Bias Rite so you can just flip the knob from A to B rather than moving the multimeter probe back and forth every time you turn one of the pots.

 

 

I know this may be asking a lot from someone who isn't a electrical nut like myself, but ideally you would want two meters setup so you can see in real time exactly what is happening with the bias system as you make adjustments.

[solarburn]

 

My Laney TT50 combo has the trim pot accessible through a little hole in the chassis with a rubber grommet but there aren't any external test points. Gave me an excuse to buy a Weber Bias-Rite and I love that thing. The DSL and TSL JCM 2000 heads have external trim pots and test points. The two trim pots can be a pain in the ass to balance though as turning one affects the other a lot. It's much easier if you have a Bias Rite so you can just flip the knob from A to B rather than moving the multimeter probe back and forth every time you turn one of the pots.

 

 

My bias rite will give you plate voltage as well with a flick of a switch. I only use it when I want to determine PV cause I have to take the amp out of the chassy to use it. Mostly use a MM due to pv staying in the same neighborhood from EL34 to EL34 in my DSL. Bias numbers vary in small increments per pv and I run mine warm not hot. If I want exact numbers I'll get pv.

[solarburn]

 

I know this may be asking a lot from someone who isn't a electrical nut like myself, but ideally you would want two meters setup so you can see in real time exactly what is happening with the bias system as you make adjustments.

 

 

Yep.

[MattyOvertone]

...If I want exact numbers I'll get pv.

 

I agree with that.

[solarburn]

I disagree with that. I agree with that.

 

Have you seen much of a variance in pv from EL34 to EL34?

 

Edit: I'm talking in my DSL.

[solarburn]

I know they vary but my PV is around the 470 give or take 10 and what El's I'm using. Anyways not a whole lot of change with bias numbers per PV in the same neighborhood.

 

Jim Jones table:

EL34 - 25W Hot (70%) Avg (60%) Cool (50%)

300V 58mA 50mA 42mA

325V 54mA 46mA 38mA

350V 50mA 43mA 36mA

375V 47mA 40mA 33mA

400V 44mA 38mA 31mA

425V 41mA 35mA 29mA

450V 39mA 33mA 28mA

475V 37mA 32mA 26mA

500V 35mA 30mA 25mA

[MattyOvertone]

Yeah, I have. Though thinking back on it, I also saw increased bias current. So I suppose adjusting the bias current to the suggested xx mA would probably drop it into the acceptable range but definitely a lot hotter than the previous brand. Basically I've had instances where i switched tube brands and noticed the bias was pretty outta' wack. In the case of switching svetlanas with electro harmonic el34s, the plate voltages were much higher as were the currents with the EHs. Maybe it was just that set, i don't know. I've seen differences switching between other brands too, but pretty large differences going to EH tubes. Now in that instance, if I set the current to the suggested xx mA, the tube would be running much hotter than if I found the correct plate voltage and adjust the current in accordance to the static dissappation and how much i wanted to push my tubes. You may find that you can get away with the xx mA setting, but it may be running them really hot and kill 'em quite, or it could be running them really cold and make the amp sound dull, or they could be ok. You never know until you bias it correctly.

 

All that being said, when i stick with the same brand, say replacing svetlana el34s with svetlana el34s i never saw a huge variation in bias to really worry about, but i always adjust them to where I want.

 

I have in the past attached a multi-meter to read the plate voltage and a multi-meter to read the voltage across a one ohm resistor soldered on the cathode (that voltage reading equals you bias current) and have plugged the amp in and played it and adjusted the bias by ear and figured out the static dissappation as I adjusted just to make sure I was in the acceptable range. I found that, for me and my amps anyways, having them biased hot sounds worse than having them biased normal or a little warm. they just get too harsh and non-musical for me when they're really hot.

[MattyOvertone]

 

I know they vary but my PV is around the 470 give or take 10 and what El's I'm using. Anyways not a whole lot of change with bias numbers per PV in the same neighborhood. Jim Jones table: EL34 - 25W Hot (70%) Avg (60%) Cool (50%) 300V 58mA 50mA 42mA 325V 54mA 46mA 38mA 350V 50mA 43mA 36mA 375V 47mA 40mA 33mA 400V 44mA 38mA 31mA 425V 41mA 35mA 29mA 450V 39mA 33mA 28mA 475V 37mA 32mA 26mA 500V 35mA 30mA 25mA

 

 

 

Perhaps that's true for your amp/your tubes of choice, i'm just saying what i've seen and through out there the fact that setting the current by a # on a sheet isn't the most accurate. Most of the time it'll get you in the acceptable zone but sometimes it won't and for someone who gigs a lot, reliability is nice. Also, it's always nice to know exactly where you're at, not just what ball park your in.

 

I realize some people prob don't give a {censored} as long as the amp sounds ok and doesn't crap out. however without biasing correctly you run the rick of it not sounding ok and possibly crapping out. It could end up being costly and have very bad timing.

[solarburn]

 

Yeah, I have. Though thinking back on it, I also saw increased bias current. So I suppose adjusting the bias current to the suggested xx mA would probably drop it into the acceptable range but definitely a lot hotter than the previous brand. Basically I've had instances where i switched tube brands and noticed the bias was pretty outta' wack. In the case of switching svetlanas with electro harmonic el34s, the plate voltages were much higher as were the currents with the EHs. Maybe it was just that set, i don't know. I've seen differences switching between other brands too, but pretty large differences going to EH tubes. Now in that instance, if I set the current to the suggested xx mA, the tube would be running much hotter than if I found the correct plate voltage and adjust the current in accordance to the static dissappation and how much i wanted to push my tubes. You may find that you can get away with the xx mA setting, but it may be running them really hot and kill 'em quite, or it could be running them really cold and make the amp sound dull, or they could be ok. You never know until you bias it correctly. All that being said, when i stick with the same brand, say replacing svetlana el34s with svetlana el34s i never saw a huge variation in bias to really worry about, but i always adjust them to where I want. I have in the past attached a multi-meter to read the plate voltage and a multi-meter to read the voltage across a one ohm resistor soldered on the cathode (that voltage reading equals you bias current) and have plugged the amp in and played it and adjusted the bias by ear and figured out the static dissappation as I adjusted just to make sure I was in the acceptable range. I found that, for me and my amps anyways, having them biased hot sounds worse than having them biased normal or a little warm. they just get too harsh and non-musical for me when they're really hot.

 

 

When I have done mine hot I thought the amps tone was a bit flubby, muddied and not desirable. The cooler settings have proven better and I still have great overtones and harmonics.

 

I'm going to take more PV readings and make sure of the differences between tubes cause I do like to experiment with tubes. Marshall's recommendations are pretty damn hot settings compared to where I bias mine. I'm usually at a 34 to 38 at the most with Tung Sol, Ruby and Svets. Marsh's recommends 40/45 for EL34's on a DSL50.

[solarburn]

 

Perhaps that's true for your amp/your tubes of choice, i'm just saying what i've seen and through out there the fact that setting the current by a # on a sheet isn't the most accurate. Most of the time it'll get you in the acceptable zone but sometimes it won't and for someone who gigs a lot, reliability is nice. Also, it's always nice to know exactly where you're at, not just what ball park your in. I realize some people prob don't give a {censored} as long as the amp sounds ok and doesn't crap out. however without biasing correctly you run the rick of it not sounding ok and possibly crapping out. It could end up being costly and have very bad timing.

 

 

We need them to last for sure. I agree its better to know the numbers for best biasing. For tone and longevity.

[Speed_Racer71]

JSX

Soultone and many more booteek amps

[MattyOvertone]

 

When I have done mine hot I thought the amps tone was a bit flubby, muddied and not desirable. The cooler settings have proven better and I still have great overtones and harmonics. I'm going to take more PV readings and make sure of the differences between tubes cause I do like to experiment with tubes. Marshall's recommendations are pretty damn hot settings compared to where I bias mine. I'm usually at a 34 to 38 at the most with Tung Sol, Ruby and Svets. Marsh's recommends 40/45 for EL34's on a DSL50.

 

 

 

Yeah Solar, i mean i'm not say who's wrong or right on this, just saying I feel that if you're capable of getting plate voltages... get them and do the quick calculation. you'll be able to bias better and have a healthier running amp. Plus tubes are expensive and you don't wanna' run the risk of killing them much sooner than need be, especially for those grabbing up the high dollar NOS stuff.

[Casey4s]

 

For the most accurate tube biasing (for this method of biasing anyways), you want to find the static dissapation... basically how many watts the tube is thumping out. For this you need the plate voltage as well and you multiply them, because wattage=voltage x amperage. The blurb below is quoted from http://www.duncanamps.com/technical/lvbias.html . "...many sources for biasing information just specify plate (or cathode) current settings; telling you to bias your 6L6s at "35 milliamps" is nonsense. Unless you take the plate voltage into consideration, a current specification is meaningless. For instance, 40 mA at 250 volts is 10 watts; the same 40 mA at 500 volts is 20 watts... TWICE as much. In both cases, the current is the same. Amps vary; two identical amps can have plate voltages which differ by as much as 20%. Just because you have a schematic that specifies the plate voltage in your amp as being at 450VDC, don't expect to see that voltage when you take a measurement. TAKE the reading, don't assume the voltage will be as specified. Trust your meter." AND I'm not just a guy that wastes all his time reading {censored} online, haha. I bias my own amps and have for years now. Both externally and internally depending on the type. I quoted the above just to provide reference from a source that may be more reputable than me (a random forum guy, haha). Biasing externally just going by a current rating depending on the tube is the nonsense he's referring to. Hands down, biasing externally is easier and faster and safer, but as I said it's less accurate .

 

 

Finding the correct idle current bias point by reading the plate voltage is a given. But it is your terminology that has me confused. I have never in all my years heard anyone refer to "internal" vs "external" biasing methods. If your adjustment pot is located inside or outside the amp the adjutment is just as accurate, because no matter where the pot is located you will calculate your bias the same way. If you have external test points for the idle current and plate voltage it is still adjusted exactly the same, the access is just more simplified.

 

So again, I don't understand why you think that having an internal adjustment is more accurate than having an external one. You are achieving the same calculated idle current either way.

[MattyOvertone]

I've never come across an amp with external plate voltage test points. From what's been said I guess you can use a bias rite to find the plate voltage without testing inside, idk i never used a biasing tool like that. But any amp I've seen with external bias pots and probes (mainly only Marshalls) only have probes to test idle currents. Plate voltage probes would be crazy as you would typically see a range of 300-500VDC on those probes when the amp is power which could definitely make some sparks and cause some damage if grounded.

[Casey4s]

Some bias probes like the Weber have a plate voltage feature but if you are somewhat careful you can read the PV from pin 3 of a bias probe without the feature which is what I do on amps without a PV test port.

 

 

Here's a plate voltage test point on the bottom of the chassis....

No sparks so far...

 

 

 

Here's a bias adjustment recessed in the bottom of the chassis....

 

 

 

Idle current test points & ground located on the back pannel.....

 

[MattyOvertone]

Haha you win with that one... I have never seen any like that. Guess maybe because the only amps I've tinkered with were mid 90's and older marshalls... and some fender and peavey stuff too.

 

What kind of amp is that? Is that a stock feature?

[MattyOvertone]

And when I said plate voltage probes would be crazy I was picturing the typical marshall metal pins with 400VDC on them, haha, sparks and denger would come with that. I never gave banana plugs a thought.

[Casey4s]

 

Haha you win with that one... I have never seen any like that. Guess maybe because the only amps I've tinkered with were mid 90's and older marshalls... and some fender and peavey stuff too. What kind of amp is that? Is that a stock feature?

 

 

You can add those features to virtually any amp if you don't mind drilling a few holes in the chassis. I wouldn't add these test points to a vintage or collectable amp for obvious reasons. But it's pretty easy to do on any DIY amp or stock factory tube amp.

 

This is a DIY variation on the BF Princeton Reverb >>>---> http://s238.photobucket.com/albums/ff243/Casey4s/DIY%20BF%20Princeton%20Reverb%20Variation%20Walnut/?start=all&paginator=top

[MattyOvertone]

ok, yeah I can't imagine it would be too much of a task... i was mainly just curious if you did that or if that is factory for your amp. What kind of amp is it?

 

I've never seen that on a production amp and I was wondering if companies are building them that way now or if that is a DIY setup??

[Casey4s]

ok, yeah I can't imagine it would be too much of a task... i was mainly just curious if you did that or if that is factory for your amp. What kind of amp is it? I've never seen that on a production amp and I was wondering if companies are building them that way now or if that is a DIY setup??

 

Yes, it's a DIY amp, look at the link.