No clean headroom

[hebaton]

I have a JTM45 clone bult by Stephen of Trinity amps.
I read everywhere those are a good clean platfrom at low volume, but with PAF style Humbuckers, mine gets realy loud and dirty at 2 o either channel.
Where should I look ?

[maxkracht]

Maybe try a 12at7 for the PI and/or v1?

[pdf64]

...
Where should I look ?

Volume pot taper?
Power amp gain?

[bepone]

add master volume, without amp is useless on low volume

[hebaton]

how does a Master volume help with clean headroom ?

[cdemike]

Master volume could actually help by limiting the amount signal reaching the power section and/or phase inverter, but it'd depend on where the breakup is happening. I'm wondering how loud the amp is when it breaks up. If it's loud when it's breaking up, I agree that the pot taper is too linear for what you're looking for. If not, there could be a number of things happening, in which case it'd be helpful to know more about your setup. Off the bat the things that come to mind are turning down the bass control, using only the bright channel (as long as it doesn't have a gigantic bright cap), using higher wattage or higher efficiency speakers, and biasing it cooler. It's totally possible you tried those things, but I'd want to know how the amp is behaving under those conditions before modifying for headroom since it'd help narrow where you could get the most headroom.

[hebaton]

Thanks, I did cool down bias a touch and installed a 12at7 at V3. That of course softened it up some.
I went looking foo deviations in value on the components and noted a couple of rssitors that on the JTM45 schematic call for 270K in reality measure 330K each. ( 165K incircuit reading )
See attached for reference. What would be the effect of this change ?

[Stevem]

To best answer your question get a reprint of the RC-30 ( RCA tube manual ) or look up and print off from that manual on line pages 641-642-643-644-645 and 647.

These pages are massively helpful to messin with tube circuits!

Any time I am building Or especially repairing an amp I always test the new components I am installing to confirm their value.
This 4 seconds long step can save you a lot of later trouble shooting time!

[hebaton]

Thank you Stevem, I will try to educate myself. I did not build this amp though, I am a player, not a builder. Just trying to understand why this particular unit, although it seems to have been built with period-correct components ( mostly ) does not behave like others I have played before.

[hebaton]

Can someone tel me what those 2 resistors do and how their value will affect the output ?

[sluckey]

Can someone tel me what those 2 resistors do and how their value will affect the output ?

They are the mixing resistors for the two channels. Larger values may reduce the output slightly but it may not be noticeable.

[Stevem]

Here’s a voltage chart that should help you.
How do your voltages on pins 1 and 6 of the preamp tubes compare to this chart?

[cdemike]

Can someone tel me what those 2 resistors do and how their value will affect the output ?

In addition to what sluckey said, changing the value will also change the EQ response by changing the peaking network formed by those resistors and the 500pf cap across the bright channel's mixing resistor. I second Stevem's request for voltage readings, and I'd still be interested to know how loud the amp is when it's breaking up. A JTM45 should easily be able to stay clean while keeping up with a decently loud drummer, especially with a chilly bias and a 12AT7 phase inverter. In other words, short of playing a guitar with hot humbuckers through the bass channel with the bass all the way up, a stock JTM45 (which this appears to be based on the board) should have all the headroom you'd need for any modern practice or gigging scenario I can think of short of playing in a large outdoors venue.

[R.G.]

I hate to use the "A" or in polite company, bu there is an architecture to clipping and distortion.

Clipping happens when a stage is driven with an input signal bigger than its output can follow cleanly. Yes, that's a well, duuuh fact, but hang on. A single coil pickup puts out about 100mV peak, more or less. A 12AX7 stage with a 100K plate resistor has a voltage gain of about 160-ish if it's set up with a cathode decoupling cap, and it output can swing 75-100V peak if it's biased just... right. So a 12AX7 stage can only stand an input of about 100V/160 = 625mV. It will for certain clip if you feed it more than about 1-2V on the grid, because it's usually biased at about -1 to -2V on the grid relative to the cathode. (These are VERY approximate numbers, just to illustrate the points.)

A 100mV peak input signal to a 12AX7 stage can make a 0.1V X 160 = 16V peak signal at the plate. So a single 12AX7 stage driven by a single coil pickup can drive a second 12AX7 stage to massive grid clipping if there is no signal loss between them.

So: you need some signal loss between gain stages or the thing will have almost zero headroom, even with single coils. Also, you get to pick which stage is clipping and by how much by choosing how much attenuation you put between the triode stages. This can all happen before the output stage ever gets a chance to clip as controlled by a master volume.

I think that the OP's problem is that the triode stages are clipping before the output stage, and it might be controlled by introducing some more loss between the first few stages - a gain control as opposed to a volume control.

The more experienced folks already know this, of course, so sorry for belaboring the points, but for someone new, this might help understanding the gain/distortion path.

[hebaton]

Can someone tel me what those 2 resistors do and how their value will affect the output ?

In addition to what sluckey said, changing the value will also change the EQ response by changing the peaking network formed by those resistors and the 500pf cap across the bright channel's mixing resistor. I second Stevem's request for voltage readings, and I'd still be interested to know how loud the amp is when it's breaking up. A JTM45 should easily be able to stay clean while keeping up with a decently loud drummer, especially with a chilly bias and a 12AT7 phase inverter. In other words, short of playing a guitar with hot humbuckers through the bass channel with the bass all the way up, a stock JTM45 (which this appears to be based on the board) should have all the headroom you'd need for any modern practice or gigging scenario I can think of short of playing in a large outdoors venue.

How significant is that EQ change ? 330K instead of 270K tht'S what 22% over ?