Maximising headroom

[rjgtr]

Thanks for the explanation.

I know it's blasphemy, but I already have an 18 watt for getting down and dirty with, and I can always mod the amp in the future. (If history is anything to by it'll get modded to death.) I treat every build as a learning curve.

Cheers

I was just kidding - it's all about getting the amp the way you want it.

[dynosoar1]

The first (and favorite) was simply to change the grid leaks on the power tubes from 470k to 220k. This will allow the power section breath and bounce a little better.

T

Why is this??? Wouldn't this significantly drop the the output/volume on the amp?? Aren't you in effect bleeding more to ground by reducing this resistor?

[Merlinb]

The first (and favorite) was simply to change the grid leaks on the power tubes from 470k to 220k. This will allow the power section breath and bounce a little better.

T

Why is this??? Wouldn't this significantly drop the the output/volume on the amp?? Aren't you in effect bleeding more to ground by reducing this resistor?

It doesn't affect the power section at all. It loads down the phase inverter more, reducing it's gain and output swing, so it can't drive the output stage as hard or as soon.

[sub]

I think dynosoar1 has right in volume drop...
I have a PPIMV in my amp (1Mohm resistors, 1Mohm dual pot), and when i set the pot about 2 o'clock (i measured it was about 220Kohm) the amp volume significantly dropped.

[Merlinb]

I think dynosoar1 has right in volume drop...
I have a PPIMV in my amp (1Mohm resistors, 1Mohm dual pot), and when i set the pot about 2 o'clock (i measured it was about 220Kohm) the amp volume significantly dropped.

Well, yeah, if you turn down the volume, the volume goes down...
The power stage hasn't changed though. It's still capable of 18 watts if you drive it fully.

[zaphod_phil]

The first (and favorite) was simply to change the grid leaks on the power tubes from 470k to 220k. This will allow the power section breath and bounce a little better.

Why is this??? Wouldn't this significantly drop the the output/volume on the amp?? Aren't you in effect bleeding more to ground by reducing this resistor?

It doesn't affect the power section at all. It loads down the phase inverter more, reducing it's gain and output swing, so it can't drive the output stage as hard or as soon.

Well, yeah, if you turn down the volume, the volume goes down... The power stage hasn't changed though. It's still capable of 18 watts if you drive it fully.

Actually, all of you are right. In terms of where the amp breaks up in pure loudness terms from the speaker, nothing really changes. So if you need an amp that's louder when clean, the 36W power stage helps buy you that.

However, another frequent observation made about Marshall 18W style amps is that they start to break up at around 3 or 4 on the volume knob and after that point, don't get a lot louder, just get dirtier and more compressed. Changing the power stage grid reference resistors from 470k to a Vox-like 220k, scales back some of the drive to the EL84s, so that you get more clean range (ie later breakup) on the amp's volume knob. At max volume, you still get plenty of distortion and grind, although not quite as dirty and compressed as with the regular 470ks. This also means that the EL84s operate a bit more in their more linear zone, so that like dynasoar1 says, they "breathe and bounce a little better".

So to some people, reduced value grid reference resistors (220k or compromise values of 330k or 270k) on the power valves make more sense. I once tried the 220ks, and then out the 470ks back - as many folks know, I personally think too much clean tone is a waste of a good tube amp!

HTH

[Merlinb]

This also means that the EL84s operate a bit more in their more linear zone, so that like dynasoar1 says, they "breathe and bounce a little better".

But isn't that just the same as if you had kept the 470k's and just turned down the master vol a bit? I mean, the EL84's don't actually operate any different with 220k...

[zaphod_phil]

Yes, it's exactly the same as partly turning down a post-PI MV (which most 18W type amps don't actually have). So I don't really see any problem there either. In fact some people will say that one of these controls will make the amp a lot more flexible in use.

This raises another interesting point, which I've been re-iterating quite a lot recently. Since a lot of the distortion in typical Marshall 18W/36W type amps is produced in the power valves, a post-PI MV control doesn't behave quite the same as an MV in typical MV amps that have a lot more preamp/PI distortion. So I am now advocating that it be called a "Drive" control instead of MV.. Electrically, it does the same thing in any case, but hopefully it helps the average player understand it better.

[Brewmaster]

Ysame as an MV in typical MV amps that have a lot more preamp/PI distortion. So I am now advocating that it be called a "Drive" control instead of MV.. Electrically, it does the same thing in any case, but hopefully it helps the average player understand it better.

LOL! I finished my 36 watter a few weeks ago and labeled the Volume "Drive" and what most call the MV, "Volume". Bass akwards I guess!

[wiseowl]

I think the last few posts have convinced me that I've taken the correct route in building an amp with more power to get more volume while remaining clean.

It did cross my mind that maybe tweaking the preamp would help, but it would seem that while it may make the amp have more clean range on the volume knob, the overall volume at which the amp broke up would remain the same, or at least very close to it.

Thanks all,

Martin

[zaphod_phil]

You done right. Apart from the fact that, as Shwartzneger would say, "clean is for girly men"

LOL! I finished my 36 watter a few weeks ago and labeled the Volume "Drive" and what most call the MV, "Volume". Bass akwards I guess!

As long as you don't mind the sound coming out of your guitar pickups.

[Phil_S]

I realize I'm a latecomer on this thread. Lowering the grid leak R's to 220K is the classic way to clean up. I did this on my 18W and feel it was a big improvement. As they say, there's no accounting for taste. Maybe if I played real loud into a 4-12, I would have been happy with the stock 470K. But I don't.

The other classic way of cleaning up is the put more volts on the power tube plates. In the 18W, you are already beyond design center specs, so doing this with regular EL84's is probably not recommended. Get yourself a pair of NOS 7189's and you are good to go!

This brings me to the 36W, which I don't know first hand, but it seems that the added power is a way to finesse the plate voltage question. You might also consider building one with a pair of JJ 6V6, which I understand will take quite a bit of plate voltage without flinching.

One more possibility is the see if 400 plate volts will actually wake up a pair of Sovtek EL84M, which, in my limited experience sound like poo, at least at lower voltages. Is there anyone out there with experience on these EL84M?

[rjgtr]

In a TMB, the PPIMV does allow you to get some distortion from the preamp. Of course, the 18watt Plexi or other cascaded designs get even more distortion in the preamp!

Unfortunately, even though Drive is an accurate description electronically, there's been a lot of amps that have used Drive for preamp drive, among those a few Marshalls. Maybe if you had a PPIMV that went between about 100k and 500k you could call it 'Kick', 'Crunch' or something similar.

That's the problem - we're running out of cute names to call these controls.

So maybe the standard should be -

Gain - preamp gain or volume after the first triode
Volume - the control that governs the volume of the channel
(like the master on a TMB)
Drive - PPIMV

[BackwoodsJoe]

How much headroom would you gain if you switched the PI 12ax7 to a 5751?

[zaphod_phil]

In a TMB, the PPIMV does allow you to get some distortion from the preamp.

Yes, I agree. So the MV can indeed act a little more like a real MV.

Unfortunately, even though Drive is an accurate description electronically, there's been a lot of amps that have used Drive for preamp drive, among those a few Marshalls. ...That's the problem - we're running out of cute names to call these controls.

I'm not letting that deter me. The commonly used Marshall amp terms just don't quite fit an 18W/36W IMO. So for a typical TMB amp, with a post-PI MV, I might label the TMB channel controls Gain, Volume and Drive. I think those terms still help a non-EE guitarist understand how to use his/her amp better.

How much headroom would you gain if you switched the PI 12ax7 to a 5751?

In real terms - none. In terms of volume control range, like I explained before, it would be roughly similar to changing the grid leaks to 220k.

Anyway, what's all this sudden unhealthy obsession with clean tones everyone seems to have got? IMO someone who wants clean should be playing an SF Twin, not an 18W/36W...

[sub]

I think dynosoar1 has right in volume drop...
I have a PPIMV in my amp (1Mohm resistors, 1Mohm dual pot), and when i set the pot about 2 o'clock (i measured it was about 220Kohm) the amp volume significantly dropped.

Well, yeah, if you turn down the volume, the volume goes down...
The power stage hasn't changed though. It's still capable of 18 watts if you drive it fully.

I didn't touch the volume knob...

[leslie]

However, another frequent observation made about Marshall 18W style amps is that they start to break up at around 3 or 4 on the volume knob and after that point, don't get a lot louder, just get dirtier and more compressed.

This might be true if you playing humbucker equiped guitar.
But with single coils you get significant difference in loudness beetween 4-8.
Also with single coil you get more headroom,so if somebody wants more headroom there you go...