ampage Tube Amps / Music Electronics	For current discussions, please visit Music Electronics Forum.

ampage archive

Vintage threads from the first ten years

High Bias- will it damage the amp?!

12/24/1999 10:32 PM
rb	High Bias- will it damage the amp?! I checked the bias on my blackface Princeton Reverb and noted the meter is showing about 36-37 mv; I understand the reasonable range for 6V6s is about 25mv. The amp is fixed bias (no bias pot), so is hard for me to adjust the bias...I know too high a bias will shorten the tube's life, but will it otherwise harm the amp? Harm the internal components (caps, resistors, etc), transformers, etc? Does it affect the sound (harshness, tone, dyamics, etc). Thanks for your comments

12/24/1999 10:32 PM

High Bias- will it damage the amp?!

I checked the bias on my blackface Princeton Reverb and noted the meter is showing about 36-37 mv; I understand the reasonable range for 6V6s is about 25mv. The amp is fixed bias (no bias pot), so is hard for me to adjust the bias...I know too high a bias will shorten the tube's life, but will it otherwise harm the amp? Harm the internal components (caps, resistors, etc), transformers, etc? Does it affect the sound (harshness, tone, dyamics, etc). Thanks for your comments

12/24/1999 10:50 PM
John Stokes	Wow, that is a bit on the hot side. You can either mod the bias supply to install a trimpot, or you have to dink with fixed resistors. If I were you, I would fix this situation.

12/24/1999 11:21 PM
tboy	What method did you use to measure the bias? Was that the reading for both tubes, or just one?

12/26/1999 8:27 PM
Bruce	Both John and Steve are alluding to this but I'll go a little further. No, it's pretty unlikly the 37ma idle current level will hurt the caps, resistors or the output transformer. Yes, too high of current can overheat the PT a wee bit and wear out the power tubes faster. I think, too much idle current can ruin the sound of the amp by allowing the power tubes to be current saturated before there is much signal delivered to them at all. The dynamics will be poor, after a certain drive point, and the amp will sound distorted at high volumes when the tubes are handling too much current, but, too low of current will sound distorted too. The trick is to get the power tubes idling in a nice sounding but dependable current level. Primer: If by 36mv to 37mv "bias" can we assume you mean you are measuring the current with a 1 ohm resistor from cathode to ground on each individual tube? This is not really a good way to refer to idle or bias current IMO. If you were using just one resistor and holding both the tubes above ground with it, measuring 37mv, then the power tubes are over biased and will be idling at a lower current level, each. Not bad but not optimum. BTW, bias, is a term that loosely describes the relative voltage between the grid and cathode when compared to the anode voltage and the current flow of the tube. Over biased would mean too much negative bias voltage (low idle current and cold) and under biased means not enough negative voltage (high idle current and hot). Sometimes both terms are used incorrectly here, other sites and even in print by famous amp "gurus". In tube speak, bias or bias current, generally is the term describing the amount of current that is flowing at idle from the cathode through the plate, when there is NO signal applied. That same current has to flow through the 1 to 10 ohm resistor, that you might have on the cathode to ground, thus dropping a very tiny measurable amount of voltage (milivolts to hundreds of milivolts) across that same resistor. The more current , the more voltage drop. This is often referred to as a current shunt. The idle current, of course, depends on what the negative bias voltage and positive plate voltage is. Now, in your Princeton, since you aren't likely to be changing the plate voltage, the best thing you can do is change the bias voltage. Look at the way the bias supply is made and note that the negative bias voltage is predetermined by a pair of resistors, set up as a voltage divider, with the actual applied bias voltage coming from between the two. The two resistors are the 100K and the grounded 22K. Yes there is a diode rectifier in between but the main thing to note is the 22K resistor is sending a large portion of the derived negative voltage to ground. If you leave the 100K alone, that 22K is what sets the max negative voltage. You should be able to see that if that 22K resistance was a variable 22K resistance, you could always send more voltage to ground and lessen the available negative bias volatge amount, making it more positive with respect to the grounded cathode. That would mean more idle current, but what you really need in your amp is to raise the negative bias volatge. Here's a $2.00 trick I do to EVERY amp like this I get in for repairs. Remove the 22K resistor and install a small 1/4 watt 25K trim pot, wired as a variable resistor, with one leg grounded and the other soldered to a 1/2 watt 10K resistor and then back to the eyelet that feeds the bias voltage to the amp. Now you can vary the "bias voltage set resistance" from 10K to 35K and consequently, the bias voltage is adjustable. With the total circuit resistance set at 35K, there will be more negative bias voltage available to lower the idle current and with the pot turned the other way, you can decrease the negative bias voltage and get more idle current. Depending on the player, the tubes, and the speaker, I like the way a PP, class AB 6V6 amp, like your Princeton, sounds when the power tubes are idling at around the +9 to 11 watts area. Others, yourself included, might like a different idle current and you can experiment with that when you get the bias circuit modified to be adjustable. Be carefull you don't end up setting the idle current for what sounds the best at low volume in the short run, and is damaging to the power tubes in the long run. Keep in mind although a 6V6GT can run safely at 14 watts for many many hours, the power transformer in these amps are not huge and shouldn't be run this high for extended periods. Also, the OT is not a brute and it's dynamic range can suffer a bit when there is large amounts of current flowing through it too. So keep that in mind too, if you like the way the amp sounds with higher idle current flowing through each tube. I've only had one or two bad Princeton OTs in the many I've worked on so I don't think it's a weak point but, these amps also sound good if you go to a larger, but different lower impedance, Deluxe Reverb OT if you like a slightly bigger sound. Bruce

12/26/1999 8:27 PM

Bruce

Both John and Steve are alluding to this but I'll go a little further.
No, it's pretty unlikly the 37ma idle current level will hurt the caps, resistors or the output transformer.
Yes, too high of current can overheat the PT a wee bit and wear out the power tubes faster.
I think, too much idle current can ruin the sound of the amp by allowing the power tubes to be current saturated before there is much signal delivered to them at all.
The dynamics will be poor, after a certain drive point, and the amp will sound distorted at high volumes when the tubes are handling too much current, but, too low of current will sound distorted too.
The trick is to get the power tubes idling in a nice sounding but dependable current level.

Primer:
If by 36mv to 37mv "bias" can we assume you mean you are measuring the current with a 1 ohm resistor from cathode to ground on each individual tube?
This is not really a good way to refer to idle or bias current IMO.
If you were using just one resistor and holding both the tubes above ground with it, measuring 37mv, then the power tubes are over biased and will be idling at a lower current level, each. Not bad but not optimum.
BTW, bias, is a term that loosely describes the relative voltage between the grid and cathode when compared to the anode voltage and the current flow of the tube.
Over biased would mean too much negative bias voltage (low idle current and cold) and under biased means not enough negative voltage (high idle current and hot).
Sometimes both terms are used incorrectly here, other sites and even in print by famous amp "gurus".
In tube speak, bias or bias current, generally is the term describing the amount of current that is flowing at idle from the cathode through the plate, when there is NO signal applied.
That same current has to flow through the 1 to 10 ohm resistor, that you might have on the cathode to ground, thus dropping a very tiny measurable amount of voltage (milivolts to hundreds of milivolts) across that same resistor.
The more current , the more voltage drop.
This is often referred to as a current shunt.
The idle current, of course, depends on what the negative bias voltage and positive plate voltage is.
Now, in your Princeton, since you aren't likely to be changing the plate voltage, the best thing you can do is change the bias voltage.
Look at the way the bias supply is made and note that the negative bias voltage is predetermined by a pair of resistors, set up as a voltage divider, with the actual applied bias voltage coming from between the two.
The two resistors are the 100K and the grounded 22K.
Yes there is a diode rectifier in between but the main thing to note is the 22K resistor is sending a large portion of the derived negative voltage to ground.
If you leave the 100K alone, that 22K is what sets the max negative voltage.
You should be able to see that if that 22K resistance was a variable 22K resistance, you could always send more voltage to ground and lessen the available negative bias volatge amount, making it more positive with respect to the grounded cathode.
That would mean more idle current, but what you really need in your amp is to raise the negative bias volatge.

Here's a $2.00 trick I do to EVERY amp like this I get in for repairs.
Remove the 22K resistor and install a small 1/4 watt 25K trim pot, wired as a variable resistor, with one leg grounded and the other soldered to a 1/2 watt 10K resistor and then back to the eyelet that feeds the bias voltage to the amp.
Now you can vary the "bias voltage set resistance" from 10K to 35K and consequently, the bias voltage is adjustable.
With the total circuit resistance set at 35K, there will be more negative bias voltage available to lower the idle current and with the pot turned the other way, you can decrease the negative bias voltage and get more idle current.
Depending on the player, the tubes, and the speaker, I like the way a PP, class AB 6V6 amp, like your Princeton, sounds when the power tubes are idling at around the +9 to 11 watts area.
Others, yourself included, might like a different idle current and you can experiment with that when you get the bias circuit modified to be adjustable.
Be carefull you don't end up setting the idle current for what sounds the best at low volume in the short run, and is damaging to the power tubes in the long run.
Keep in mind although a 6V6GT can run safely at 14 watts for many many hours, the power transformer in these amps are not huge and shouldn't be run this high for extended periods.
Also, the OT is not a brute and it's dynamic range can suffer a bit when there is large amounts of current flowing through it too.
So keep that in mind too, if you like the way the amp sounds with higher idle current flowing through each tube.
I've only had one or two bad Princeton OTs in the many I've worked on so I don't think it's a weak point but, these amps also sound good if you go to a larger, but different lower impedance, Deluxe Reverb OT if you like a slightly bigger sound.

Bruce

12/28/1999 4:39 AM
Curtis	Bruce,how many milliamps are we talking about in the 9-11 watt range?

12/28/1999 5:13 AM
Bruce	It depends on the amp, the type of tubes and the condition of the tubes, including the rectifier. Roughly though, if the normally biased amp is showing about 400vdc to 420vdc on the plates, then the bias current can be in the 24ma to 28ma range pretty easily and still sound really good. I have a few guys that like them up to 32ma. That's pretty hard on the tubes though and can get the power transformer heated up. I also like to install a seperate 1K 2W resistor from the circuit board to the screen connection point. All I do is remove the single screen supply wire that goes from the eyelet board to the #2 power tube and use a single 1K 2W resistor instead. This will drop the screen voltage a very tiny amount, but it also adds a little insurance to the screen supply and the 6V6GT tubes themselves. Bruce

12/28/1999 5:13 AM

Bruce

It depends on the amp, the type of tubes and the condition of the tubes, including the rectifier.
Roughly though, if the normally biased amp is showing about 400vdc to 420vdc on the plates, then the bias current can be in the 24ma to 28ma range pretty easily and still sound really good.
I have a few guys that like them up to 32ma.
That's pretty hard on the tubes though and can get the power transformer heated up.
I also like to install a seperate 1K 2W resistor from the circuit board to the screen connection point.
All I do is remove the single screen supply wire that goes from the eyelet board to the #2 power tube and use a single 1K 2W resistor instead.
This will drop the screen voltage a very tiny amount, but it also adds a little insurance to the screen supply and the 6V6GT tubes themselves.

Bruce

12/28/1999 6:14 AM
Curtis	Thanks Bruce...............Do you ever sleep?Curtis

Page 1 of 2

Next>

Last Page>>