heat and bias

[nuke]

It's really not going to change "headroom".

It's a class AB1 amplifier (I assume this is the Peavey Classic 30, 4x EL84). It will produce the cleanest tone when the bias is set around 65-80% of the output tube's plate dissipation. That's just baked into the circuit. Turning the bias "colder" (less plate current, more negative bias voltage) reduces the heat, and at some point along the way, an increase in crossover (aka notch) distortion, which kinda doesn't really sound that great.

Increasing the current increases heat, which reduces tube life, which leaves the "off" side of the push-pull on for an increasing part of the overall cycle. This wastes power, but up to a point reduces audible distortion in the power amp. Once the distortion minimum is reached, the only thing increasing plate current does is eat up tube life.

Peavey picked all the load lines, had the transformers wound to their specs, and calculated the bias they wanted that met their perceived tradeoff of warranty-problems, longevity and customer satisfaction. My estimation is they decided to err on the side of lower warranty problems and dialed it in on the cold side for longest tube life and fewest problems.

Diddling with it only goes so far, and it isn't really very far that it will go.

It just kinda is what it is. Unless you really get in there and change things up a good bit.

[Stevem]

wattage ( IE power ) as ohms law states is a product of voltage x amps.

If you monitor the amps output tube plate voltage as you adjust the bias you will clearly see that as you set the bias hotter the plate voltage will drop.

The main thing that always takes place as you bias a output stage colder is that less bottom end is produced for any given bass control setting on your amp.

The sooner in amps available output wattage you get to level of very punchy bottom end is made then the sooner the amp will distort.
Producing low end from a output stage eats up the most current the given PT has the fastest.

This is why on a amp such as yours where your on the edge of having enough clean wattage for your needs that the speaker you happen to be using is broken I well, regardless of how efficient it's rating is.

Here’s an automotive analogy.

Your cars engine has a red line rpm.
If you readjust your motor to idle at a higher rpm then it will reach its rpm limit sooner and make more heat in doing that and also waring out the engine faster, but it will be more fun to drive within the side effects above.

Also the ratio of preamp distortion to output stage distortion changes in regards to how hot the output stage is biased.
Preamp distortion and push pull ( class AB) output distortion sound and feel very different from each other .
For example metal players relie on all preamp stage distortion with a very tight and clean excess wattage output stage.

This does not sound to me like what you have posted about playing style needs .


It seems like you need anout a 50/50 contribution of distortion out of each amp section.

Also your new speaker you get needs to first be broken in for 8 to 10 hours so that more low end is made with less wattage input.
Only in this way will you have available the maximum amount of clean headroom once you get the bias set as you need it.

Here's a chart that tells you how much ac voltage to pump in according to your speakers wattage rating and impedance.

So as you can see from this chart a 30 watt 8 ohm speaker needs a ac input voltage of 8.94, and a 16 ohm driver needs 17.88 vac to be pumped in.
Since your pumping in 50 or 60 hz ac you will want to place the speaker in a small closed space like a closet and rapped up in a few blankets so it does not drive you nuts !

[iknowjohnny]

wattage ( IE power ) as ohms law states is a product of voltage x amps.

If you monitor the amps output tube plate voltage as you adjust the bias you will clearly see that as you set the bias hotter the plate voltage will drop.

The main thing that always takes place as you bias a output stage colder is that less bottom end is produced for any given bass control setting on your amp.

The sooner in amps available output wattage you get to level of very punchy bottom end is made then the sooner the amp will distort.
Producing low end from a output stage eats up the most current the given PT has the fastest.

This is why on a amp such as yours where your on the edge of having enough clean wattage for your needs that the speaker you happen to be using is broken I well, regardless of how efficient it's rating is.

Here’s an automotive analogy.

Your cars engine has a red line rpm.
If you readjust your motor to idle at a higher rpm then it will reach its rpm limit sooner and make more heat in doing that and also waring out the engine faster, but it will be more fun to drive within the side effects above.

Also the ratio of preamp distortion to output stage distortion changes in regards to how hot the output stage is biased.
Preamp distortion and push pull ( class AB) output distortion sound and feel very different from each other .
For example metal players relie on all preamp stage distortion with a very tight and clean excess wattage output stage.

This does not sound to me like what you have posted about playing style needs .


It seems like you need anout a 50/50 contribution of distortion out of each amp section.

Also your new speaker you get needs to first be broken in for 8 to 10 hours so that more low end is made with less wattage input.
Only in this way will you have available the maximum amount of clean headroom once you get the bias set as you need it.

Here's a chart that tells you how much ac voltage to pump in according to your speakers wattage rating and impedance.

So as you can see from this chart a 30 watt 8 ohm speaker needs a ac input voltage of 8.94, and a 16 ohm driver needs 17.88 vac to be pumped in.
Since your pumping in 50 or 60 hz ac you will want to place the speaker in a small closed space like a closet and rapped up in a few blankets so it does not drive you nuts !

The "new" speaker is new to me, but its used. HOW used i dunno.

[iknowjohnny]

It's really not going to change "headroom".

It's a class AB1 amplifier (I assume this is the Peavey Classic 30, 4x EL84). It will produce the cleanest tone when the bias is set around 65-80% of the output tube's plate dissipation. That's just baked into the circuit. Turning the bias "colder" (less plate current, more negative bias voltage) reduces the heat, and at some point along the way, an increase in crossover (aka notch) distortion, which kinda doesn't really sound that great.

Yeah. i found that out yesterday ! I'm setting the bias back up to 70%. Theres a nasty thing going on in the top end at a bit over 50%. It's hard to know who to listen to when you get opposing replies, but 50% is NOT good for tone just as i have always read.

[flemingmras]

Bias at 50% frees up more current which will give a tad more clean head room before the output stage starts to clip

Eh, not quite.

The operating class sets the margins in which the peak-peak signal can swing before the tube is driven to maximum forward conduction on the positive cycle (saturation), and zero forward conduction on the negative cycle (cutoff). Saturation is the point at which plate resistance is at its lowest, while cutoff is the point at which the plate resistance is infinite.

The control grid can only be driven to 0V (saturation) and some negative value, in most Class AB1 push-pull guitar amp designs, between -50 and -60V (cutoff). When the signal tries to drive the control grid above 0V, the control grid becomes positively charged. Opposite charges attract. Electrons being negatively charged, the positive grid sucks up electrons from the cathode. The negatively charged electrons collected by the positively charged grid reduces the control grid's voltage back to 0V. Some call this "grid blocking", or "blocking distortion".

The colder you bias the tube, the more positive headroom you have before you drive the grid to 0V.

If the control grid sits at -43VDC, you have a 43V swing margin.

If the control grid sits at -48VDC, you have a 48V swing margin.

You're always going to saturate at the same output current. The load impedance dictates what that will be. But cold bias will just take a larger control grid signal to drive it to that point.

If cold biasing limits the output of your amplifier, you have a very weak phase inverter.

[iknowjohnny]

If cold biasing limits the output of your amplifier, you have a very weak phase inverter.

It's a cathodyne by the way. Can the PI tube make a difference in clean headroom or volume from one model to the next? I tried a sovtek LPS long plate i had and i didn't try it in a band setting but at home it seems to give it a cleaner louder sound. But at home it's hard to tell w/o causing the neighbors to kill me. Should i test 12AX7's to find one that has the cleanest loudest tone? I have a ton of em.....mullard reissues, tung sol reissues, JJ long and short plates, old 80s chinese, EH, sovtek WA WB and the LPS and some more i can't think of off hand.

[flemingmras]

If cold biasing limits the output of your amplifier, you have a very weak phase inverter.

It's a cathodyne by the way. Can the PI tube make a difference in clean headroom or volume from one model to the next? I tried a sovtek LPS long plate i had and i didn't try it in a band setting but at home it seems to give it a cleaner louder sound. But at home it's hard to tell w/o causing the neighbors to kill me. Should i test 12AX7's to find one that has the cleanest loudest tone? I have a ton of em.....mullard reissues, tung sol reissues, JJ long and short plates, old 80s chinese, EH, sovtek WA WB and the LPS and some more i can't think of off hand.

If your amp has a cathodyne phase inverter, its output gain will be slightly less than 1 (around 0.95-ish). Gain factor of the tube in it would only come into play if the two sides weren't balanced perfectly and one output had a gain of something more than 1. You could certainly try different tubes, and you're not limited to just 12AX7. 12AT7 and 12AU7 can be interchanged as well.

[iknowjohnny]

If cold biasing limits the output of your amplifier, you have a very weak phase inverter.

It's a cathodyne by the way. Can the PI tube make a difference in clean headroom or volume from one model to the next? I tried a sovtek LPS long plate i had and i didn't try it in a band setting but at home it seems to give it a cleaner louder sound. But at home it's hard to tell w/o causing the neighbors to kill me. Should i test 12AX7's to find one that has the cleanest loudest tone? I have a ton of em.....mullard reissues, tung sol reissues, JJ long and short plates, old 80s chinese, EH, sovtek WA WB and the LPS and some more i can't think of off hand.

If your amp has a cathodyne phase inverter, its output gain will be slightly less than 1 (around 0.95-ish). Gain factor of the tube in it would only come into play if the two sides weren't balanced perfectly and one output had a gain of something more than 1. You could certainly try different tubes, and you're not limited to just 12AX7. 12AT7 and 12AU7 can be interchanged as well.

I do have a few AT7's (mainly old 80s chinese) but won't the lower gain be opposite of what i wanna achieve? I have some UA7's too but those thing drop the volume in a huge way.

[flemingmras]

It's a cathodyne by the way. Can the PI tube make a difference in clean headroom or volume from one model to the next? I tried a sovtek LPS long plate i had and i didn't try it in a band setting but at home it seems to give it a cleaner louder sound. But at home it's hard to tell w/o causing the neighbors to kill me. Should i test 12AX7's to find one that has the cleanest loudest tone? I have a ton of em.....mullard reissues, tung sol reissues, JJ long and short plates, old 80s chinese, EH, sovtek WA WB and the LPS and some more i can't think of off hand.

If your amp has a cathodyne phase inverter, its output gain will be slightly less than 1 (around 0.95-ish). Gain factor of the tube in it would only come into play if the two sides weren't balanced perfectly and one output had a gain of something more than 1. You could certainly try different tubes, and you're not limited to just 12AX7. 12AT7 and 12AU7 can be interchanged as well.

I do have a few AT7's (mainly old 80s chinese) but won't the lower gain be opposite of what i wanna achieve? I have some UA7's too but those thing drop the volume in a huge way.

Not in a cathodyne phase inverter, as the gain of a cathodyne PI is slightly less than 1 regardless of the tube's gain factor.

However, if more clean headroom is what you're after, a lower gain tube in any other circuit that incorporates gain would be what you would want.

[iknowjohnny]

This can't be right, but it may be my ears. I tried a chinese 12AT7 in the drive channels tube and it seemed way louder. How can that be? Wouldn't a 12AX7 be just as loud or louder just by adjusting the gain pot for less drive? I mentioned my ears because i have trouble with my ears needing to pop quite often and at ties when i'm playing they'll pop and it;s like turning the master u a number or even more. But i don't think thats the case right now so i gotta wonder how a lower gain tube would make the amp a lot louder.

[Stevem]

Do you have a tube tester that test for Gm of a tube ?

A normally good AT7 can work better then a very weak AX7 .

A easy way to prove out which tube is working better is to just set your meter for ac volts and you can then read the audio drive signal you have coming into the input of the PI .

You can read the audio signal level anywhere in the amp down stream of any of the plate decoupling caps

Of course it’s best to drive the amp with a test tone so you input level is steady.

I find that when any amp is in good working condition and with all good tubes that a audio input signal level of 150 mv is more then sufficient to drive the amp to maximum peak output.

[iknowjohnny]

Do you have a tube tester that test for Gm of a tube ?

A normally good AT7 can work better then a very weak AX7 .

A easy way to prove out which tube is working better is to just set your meter for ac volts and you can then read the audio drive signal you have coming into the input of the PI .

You can read the audio signal level anywhere in the amp down stream of any of the plate decoupling caps

Of course it’s best to drive the amp with a test tone so you input level is steady.

I find that when any amp is in good working condition and with all good tubes that a audio input signal level of 150 mv is more then sufficient to drive the amp to maximum peak output.

No, but i just got done trying a S ton of AX7s in it and the AT7 is always way louder. But, what i found is i *don't think* it is when turned up to band volume. It's only louder at home volume. Once you crank the master it seems they are close or the same when real loud. The asterisks above are there because it;s hard to be sure when i can't play loud here. Just quick jab at a chord to get an idea. But thats what i seemed to notice, plus it actually seems at the loud end of the knob the AX7 pulls ahead and is louder. So the T seems to be louder at low master settings and the X is louder overall when turned up high.

Anyways, been trying to figure out whether this amp will be loud enough since i got it, and the speaker is the final decider. On the 4th i'll be doing a gig with the redback and get a definitive answer. If it doesn't work out i'll be amp shopping. probably go back to my old standby, a 50w masrshall. Didn't wanna resort to that kinda weight at my age but it seems like thats the only way to get my sound w/o issues like this. Small amps have their place but they tend to cause me a lot of issues in getting everything i need w/o issues.

[nuke]

If you want to work out the details on a graph to understand electrically what's going on:

Grab and print this data sheet.

https://www.tube-data.com/sheets/019/e/EL84.pdf

Use the graph on the top of page EL84-3 that says, "Vg2 = 300v". That's very close to the operating point voltages of your amp.

Get yourself a pack of colored pencils and a ruler.

https://robrobinette.com/Drawing_Tube_L ... e_vignette

And skip down to the part where power tubes are discussed, and do the tiniest bit of math and draw the lines on the EL84 graph.

That's exactly how engineers would have done this in 1960.


I think the OP is just trying to solve the problem of not being loud enough. The bias control is never going to fix that. Bias needs to get set to the point the power amplifier is working correctly with the minimal amount of idle current necessary to achieve that.

You could try a more efficient speaker, speakers having more influence on tone than most of the electronics.

If you need more clean volume, you're probably going to find that by changing the amplifier. This one is very hard to mod in meaningful ways. Like I said, it is what it is. If you want a little more power and a little more sparkle, a Vibrolux might be a great choice. If you need a lot more then you need something with more oomph. A 60-watt 2x 6L6GC type amp, like a Blues DeVille or a late 70's silverface Super Reverb, will sparkle you right up. On the hand, you might find a Peavey Transtube series amplifier, from the made-in-USA days to be a good trick. They're solid state and they sound really good.

More power gonna be more heavy, unfortunately.

Most of the venues I frequent or have played at, are really frowning on loud amps on stage. I regularly drag out an 18-watt Blues Junior, since it is light and easy to carry. It is plenty loud to play over drummers and everything else in a decent size bar/restaurant. I usually just everyone play it all night on jam nights if I'm hanging out.

If you're gigging in places with PA, you might want to look at a modeler as a more practical solution. You can just aim a powered speaker at yourself to hear what you're doing, and feed the signal to the board so the sound man can keep the house volume where they like it.

Many venues are demanding that these days.

[iknowjohnny]

If you want to work out the details on a graph to understand electrically what's going on:

Grab and print this data sheet.

https://www.tube-data.com/sheets/019/e/EL84.pdf

Use the graph on the top of page EL84-3 that says, "Vg2 = 300v". That's very close to the operating point voltages of your amp.

Get yourself a pack of colored pencils and a ruler.

https://robrobinette.com/Drawing_Tube_L ... e_vignette

And skip down to the part where power tubes are discussed, and do the tiniest bit of math and draw the lines on the EL84 graph.

That's exactly how engineers would have done this in 1960.


I think the OP is just trying to solve the problem of not being loud enough. The bias control is never going to fix that. Bias needs to get set to the point the power amplifier is working correctly with the minimal amount of idle current necessary to achieve that.

You could try a more efficient speaker, speakers having more influence on tone than most of the electronics.

If you need more clean volume, you're probably going to find that by changing the amplifier. This one is very hard to mod in meaningful ways. Like I said, it is what it is. If you want a little more power and a little more sparkle, a Vibrolux might be a great choice. If you need a lot more then you need something with more oomph. A 60-watt 2x 6L6GC type amp, like a Blues DeVille or a late 70's silverface Super Reverb, will sparkle you right up. On the hand, you might find a Peavey Transtube series amplifier, from the made-in-USA days to be a good trick. They're solid state and they sound really good.

More power gonna be more heavy, unfortunately.

Most of the venues I frequent or have played at, are really frowning on loud amps on stage. I regularly drag out an 18-watt Blues Junior, since it is light and easy to carry. It is plenty loud to play over drummers and everything else in a decent size bar/restaurant. I usually just everyone play it all night on jam nights if I'm hanging out.

If you're gigging in places with PA, you might want to look at a modeler as a more practical solution. You can just aim a powered speaker at yourself to hear what you're doing, and feed the signal to the board so the sound man can keep the house volume where they like it.

Many venues are demanding that these days.

As far as speakers, as i said in a previous post i'm picking up a celestion redback i'll be installing for a band event on the 4th. The current stock is a celestion midnight 60 thats very inefficient. (60w/96 SPL) The replacement is a celestion redback, 150w/100 SPL. Like i said above, if tat doesn't work i'll be looking to buy a marshall combo. As to modelers, thats what i've been using for the years now and this amp i bought to get away from modeling and back to tubes. I was just checking the volume and i turned my modeler (100w combo) up to the same volume i use in the band then matched it with the peavey. It seemed as loud but it doesn't have much headroom left at that point. But the fact it matched my modeler at the volume i use it at in the band tells me a 150w speaker with much less (read probably NONE) compression/distortion and 4 dB louder should get me that bit of headroom i need. If l=not like i said, back to amp shopping.