Compression in a Rocket? (Clean vol almost = dirty vol)

[strat_addict]

I really love the AC30 circuit but would really love to get the volume of my clean tone almost equal the volume of my dirty tone when I use my vol knob. What could I change in the Rocket to allow it to have the same effect as in the Express? My volume knob is hardly never being touched when I play, the clean and dirty volume compression sounds like the best thing in an amp ever. What kind of things could I change in the circuit to get this (if something like this would even be possible)?

[dave g]

It's not going to be as dramatic of an effect compared to the Express. Even the liverpool can't do clean to mean as well as the Express.

You can put some extra series resistance in line with the rectifier tube - try a 100 or 200 ohm 25 watt resistor. This works best if you plan to do this from the getgo and use a higher voltage PT. You may want to use a smaller resistor if you're already running low voltages. Also, replacing the choke with a 25 watt ceramic resistor (or adding one in series with the choke) can help. Try something between 250 ohms and 1k.

Be careful, because you may run into some muddiness if you add too much resistance, but it should improve the clean-to-mean quality of the amp.

[M Fowler]

Sounds like you need the Liverpool not the Rocket.

You could add a 25uf/25v cap across R5 (1k5) coming from pin three (bias) v1A to ground for more gain. Use a switch to bring this in and out of the circuit.

[strat_addict]

Sounds like you need the Liverpool not the Rocket.

You could add a 25uf/25v cap across R5 (1k5) coming from pin three (bias) v1A to ground for more gain. Use a switch to bring this in and out of the circuit.

maybe, but the Liverpool just has way too much gain, I'm the clean to light/medium gain type of person.

Maybe I could put 12AY7s in it or something

[paulster]

You'd still struggle to get the clean to mean thing going on in a Liverpool with 12AY7s I'd think, because key to this effect is getting the power supply to bog down under load, exactly in the kind of way that Dave has suggested.

That's the reason for the seemingly high 1K resistor instead of the choke.

To get this effect you need the power stage to work really hard though, since it's the only part of the amp that will be able to affect the voltages since the preamp is all class A and therefore constant current.

If you don't get your power tubes cooking you won't get them pulling large amounts of current and therefore creating a large sag in the power supply.

I'd probably stick with the Rocket topology and use a high voltage transformer (or secondary tap if available) and a sag resistor to get the voltages into a normal range, which will then compress more as the gain goes up.

You will probably also want to lose the choke since, by definition, a choke will attempt to oppose changes in voltage, which is kind of the opposite of what you want!

[passfan]

It's not going to be as dramatic of an effect compared to the Express. Even the liverpool can't do clean to mean as well as the Express.

You can put some extra series resistance in line with the rectifier tube - try a 100 or 200 ohm 25 watt resistor. This works best if you plan to do this from the getgo and use a higher voltage PT. You may want to use a smaller resistor if you're already running low voltages. Also, replacing the choke with a 25 watt ceramic resistor (or adding one in series with the choke) can help. Try something between 250 ohms and 1k.

Be careful, because you may run into some muddiness if you add too much resistance, but it should improve the clean-to-mean quality of the amp.

Just for my own clarity , this would be after rectification in the main B+ line coming off the rectifier ?

[passfan]

You'd still struggle to get the clean to mean thing going on in a Liverpool with 12AY7s I'd think, because key to this effect is getting the power supply to bog down under load, exactly in the kind of way that Dave has suggested.

That's the reason for the seemingly high 1K resistor instead of the choke.

To get this effect you need the power stage to work really hard though, since it's the only part of the amp that will be able to affect the voltages since the preamp is all class A and therefore constant current.

If you don't get your power tubes cooking you won't get them pulling large amounts of current and therefore creating a large sag in the power supply.

I'd probably stick with the Rocket topology and use a high voltage transformer (or secondary tap if available) and a sag resistor to get the voltages into a normal range, which will then compress more as the gain goes up.

You will probably also want to lose the choke since, by definition, a choke will attempt to oppose changes in voltage, which is kind of the opposite of what you want!

So we want them biased on the edge and hot ? Above the standard 70% point, right ? I'm likin' this already.

[paulster]

Actually, too hot bias won't help you as it'll tend towards class A rather than AB. Once you get into true class A the power drawn will be constant (of course this implies prior to power tube distortion), so you won't get much sag and therefore any power supply compression.

What you want is the power supply to start bogging down as soon as you get into distortion, so that the volume will effectively drop, bringing it down to closer to the clean volume.

What you probably want to do is to get the preamp on the verge of clipping, so you're in the 'clean' zone. Then adjust the input into the power tubes to get as high an output as you can without the current drawn suddenly beginning to increase rapidly. If you can get the gain staged like this then as soon as you get into overdrive you'll force the output tubes to draw more current and pull the voltages down.

You could very easily experiment with this by adding a PPIMV (post-phase inverter master volume) temporarily and the sag resistor and then tweaking to taste. It would probably be quicker than setting up a SPICE simulation or taking measurements and experimenting with resistor values to get the gain staged just right.

[fishy]

Why not build a Liverpool and tune R7 and C8 for less gain. This can help some and even a low vale of R7. Around 33k can still produce a significant amount of gain.

[Gaz]

You could try increasing the screen resistors to 1k, which would add a more compression, and sound/feel louder at all volumes, which might get you the effect you're looking for. I never noticed how low the screens resistors were on the Rocket (100ohms). Some folks prefer as high as 2.2k on the screens of EL84s.

[strat_addict]

Ok, Here's what I've got so far:
-put some extra series resistance in line with the rectifier tube - 100/200 ohm 25w resistor
-higher voltage PT
-No choke (1K resistor instead)
-sag resistor
-Increase the screen resistors to 1k

Tell me if some combination of this is bad and what all of this would do tone wise. I'm very to new to idea of building a tube amp, so not much of this makes sense to me. I'll going to be getting an apprenticeship later this year with a guy in town, so I want to be able tell him all the stuff I want changed in Rocket circuit before I go and begin to buy parts.

[strat_addict]

Back to the Liverpool idea, I just can not make up my mind for anything haha.
https://tubeamparchive.com/files/giveaway_507.mp3
Would something like this (the singing character) be possible with a Rocket (especially at 37s to the end) or is it just too much gain?

https://tubeamparchive.com/files/rocket_clip_799.mp3
This is a Rocket recording and it sounds good but it doesn't have the same singing character that the Liverpool clip had.

I guess my question here is: Is the Rocket capable of the tonal characteristic in the 1st clip?

EDIT: I know that some of the singing quality from the 1st was is the guy's fingers and playing techniques (if that makes sense to anyone)
EDIT: sorry about the double post
EDIT (again ha): no offense to either to the clip's creators, both of them were very good and enjoyable

[paulster]

Ok, Here's what I've got so far:
-put some extra series resistance in line with the rectifier tube - 100/200 ohm 25w resistor
-higher voltage PT
-No choke (1K resistor instead)
-sag resistor
-Increase the screen resistors to 1k

Tell me if some combination of this is bad and what all of this would do tone wise.

The 100/200ohm "sag" resistor will drop some voltage to the rectifier at idle (that's why the suggestion of using a higher voltage tap on the power transformer to set the idle voltage back to a normal level) and will increase the voltage drop as the current increases (Ohm's Law: V=IR, so voltage dropped is proportional to current drawn across the resistor).

That will increase power supply compression at high output, lowering your volume to a degree.

Changing the choke for a 1K resistor will have a similar effect. In this case the voltage drop will only be at the screens and down the chain to the preamp, since the plate supply is before this point, so you might find that sticking with a choke will keep the feel closer to a regular Rocket.

Increasing the screen resistors from 100ohms to 1K will have a similar effect, and will also contribute to longer tube life as 100ohms is pretty low for new tubes and the Rocket/AC30 run them pretty hard.

I use 1K screens in mine after changing from 100ohms and the difference is very subtle but I like the slight added compression and the fact that my expensive tubes will be happier for longer.

If I remember correctly the Rocket transformers that most people use are dual tap, and you usually use the lower voltage tap. In that case what I'd do is build my Rocket basically stock, including a choke, and measure voltages using the low voltage tap. Then I'd switch to the high voltage tap and choose a sag resistor to get the voltages at idle down to about where they were before.

I'd also add a post-phase inverter master volume (PPIMV) which is completely out of circuit when it's on 10, so you can tweak the drive to the power tubes to maximise your clean-scream window. Then leave it set or pull it out and replace it with fixed resistors.

I'd also change the screen resistors to 1Ks for longer tube life and a tiny extra bit of compression.

You'd still have all the essentials of a Rocket and the ability to use it stock if you wanted to.

[passfan]

Ok, Here's what I've got so far:
-put some extra series resistance in line with the rectifier tube - 100/200 ohm 25w resistor
-higher voltage PT
-No choke (1K resistor instead)
-sag resistor
-Increase the screen resistors to 1k

Tell me if some combination of this is bad and what all of this would do tone wise.

The 100/200ohm "sag" resistor will drop some voltage to the rectifier at idle (that's why the suggestion of using a higher voltage tap on the power transformer to set the idle voltage back to a normal level) and will increase the voltage drop as the current increases (Ohm's Law: V=IR, so voltage dropped is proportional to current drawn across the resistor).

That will increase power supply compression at high output, lowering your volume to a degree.

Changing the choke for a 1K resistor will have a similar effect. In this case the voltage drop will only be at the screens and down the chain to the preamp, since the plate supply is before this point, so you might find that sticking with a choke will keep the feel closer to a regular Rocket.

Increasing the screen resistors from 100ohms to 1K will have a similar effect, and will also contribute to longer tube life as 100ohms is pretty low for new tubes and the Rocket/AC30 run them pretty hard.

I use 1K screens in mine after changing from 100ohms and the difference is very subtle but I like the slight added compression and the fact that my expensive tubes will be happier for longer.

If I remember correctly the Rocket transformers that most people use are dual tap, and you usually use the lower voltage tap. In that case what I'd do is build my Rocket basically stock, including a choke, and measure voltages using the low voltage tap. Then I'd switch to the high voltage tap and choose a sag resistor to get the voltages at idle down to about where they were before.

I'd also add a post-phase inverter master volume (PPIMV) which is completely out of circuit when it's on 10, so you can tweak the drive to the power tubes to maximise your clean-scream window. Then leave it set or pull it out and replace it with fixed resistors.

I'd also change the screen resistors to 1Ks for longer tube life and a tiny extra bit of compression.

You'd still have all the essentials of a Rocket and the ability to use it stock if you wanted to.

Does this resistor go on the centertap to ground of the HT winding ? I'm not sure where to place this.

[paulster]

I'd usually place it just beyond the rectifier, in the single B+ line coming out of the rectifier tube.