Rocket tone control values

[harryk]

In Rocket schematic tone control has one 0,022 uF and one 0,0022 uF cap and a tail resitor with value of 1 kohm. When I compared this schematic to original Vox and also Matcless schems I noticed that there is significant difference between these values. Vox and Matchless values are 0,022 uF, 0,022 uF and 10 kohm. Now I am a little confused if this Rocket schematic is right. Can somebody explain? Thanks Harry

[nickt]

Yes I'm confused as well - Mark Abbott's schemo shows some different values to his layout and another schemo just has the Vox values? There are also some pictures of Paul Ruby's Rockets that show some further different values?

[tubedogsmith]

If you use the .002 mid cap then use a 500pf for the treble cap, there was at least one real rocket with these values. The PI cathode resistor is 1.2k, the tail is 47k. This tone stack combo gives you a real strong mid tone with more apparent gain. It also sounds cool in a liverpool.

[harryk]

I mean with tail resistor that one with value of 1 kohm from bass pot to ground. In Vox and Matchless this value is 10 kohm. Maybe this odd tone control combination is correct despite I have never seen such a one. Regards Harry

[nickt]

If you use the .002 mid cap then use a 500pf for the treble cap, there was at least one real rocket with these values. The PI cathode resistor is 1.2k, the tail is 47k. This tone stack combo gives you a real strong mid tone with more apparent gain. It also sounds cool in a liverpool.

Sounds interesting, I've just removed a 500p from my Lightning because it was somewhat under-whelming. I'll give the 500p/0.02u/0.002u a try. Thanks for posting!

[Doug H]

Here are some things to think about. Download a copy of Duncan's Tone Stack Calculator, if you haven't already. Mark mentions 2 different Rocket tone control configurations. Reading his notes, the values on the schematic are from a 2-Rock clone (?!?!).

Okay, plug the 2-Rock values into the Vox tab on the calculator: it's a very middy response. It looks kind of strange for a tone stack but okay.

On Mark's schematic, he also mentions another tone stack configuration in the notes. This is just an AC30 top boost tone stack with a 470p subbed for the 47p treble resistor. Plug this into tone stack calculator. Now plug the Express "Kelly" values (470p/100k/22n/22n/1M/1M/25k) into the calculator on the Fender tab. Switch between the Fender and Vox tabs.

The frequency response of the 2nd Rocket tone stack configuration that Mark mentioned is almost exactly the same as the Twreck Kelly values. The Kelly stack is a little less lossy but the curves are nearly identical.

I don't really know what all this means, I just thought it was interesting...

[jelle]

What do you think?!

[Doug H]

I'm not sure what I think but I'm going to find out. I've played with subbing a 470p for the 47p in the Vox tone stack in amp prototypes and pedal circuits. It adds a nice upper midrange kick that I liked. I'm getting ready to mod my Firefly into a "baby Rocket" and I'm going to try both the tone stacks and see which one I like better. That's what got me looking into this.

[nickt]

Downloaded the calculator and had a play. Lines on a graph very musical However my peanut brain finally caught up with "Hey Nick! why don't you see what the response looks like for the stack settings youve been using on your AC30 for the last 30 years?"

The result is the "normal" vox settings give a flat frequency response but attenuated by about 25dB. Interesting...

Now if you plug in the Rocket values suddenly you are getting the same flat response but attenuated 42dB. The main driver of the attenuation is the 10K/1K mid resistor.

[Doug H]

I don't understand your plots. All you changed was the 10k and the output load, which IMO should be the same between the Rocket and AC30. I think the PI's of the two amps are roughly the same circuit. If you use the 1k in the Rocket circuit, C1 should be a 2.2n and C2 should be 470p.

This is what I found with the tone stack calculator:

[harryk]

I think that I start with AC30 tonestack values and then Rocket values. The one which sounds better will stay in this amp. It is only a matter of changing a few components to test it in real life. Harry

[nickt]

I don't understand your plots. All you changed was the 10k and the output load, which IMO should be the same between the Rocket and AC30. I think the PI's of the two amps are roughly the same circuit. If you use the 1k in the Rocket circuit, C1 should be a 2.2n and C2 should be 470p.

This is what I found with the tone stack calculator:

What you didn't do was set the tone controls to where they are actually used by (IME over last 30 years ) most AC30 TB players - that is, treble full off bass full off. This also lines up with folks like Brian May who use the normal channel only (ie no tonestack so flat response).

In this configuration you get -25db attenuation for the Vox and -42db for the Rocket with the flat response shown. Changing the 56k cathode-follower resistor to 68k makes +/- 1dB difference so it's probably inaudible. Note that we are assuming this calculator is actually correct in its output.

I changed the output load because the Rocket has 220k into 1M. BTW this made pretty much no difference to the response curve.

What all the above indicates (to me at least) is that the Vox frequency response (as used by Vox players) is more to do with coupling caps than the tone stack. If you replaced the tone stack with a pad you'd (well I'd ) probably get the same sound (unless twisting the phase response makes some sort of tonal difference).


BTW it is interesting that you are so definitive regarding what values should or shouldn't be used in a Rocket tone stack. I'll read back through the various threads but I haven't seen anything "definitive" about the rocket - perhaps I missed it

[Doug H]

We must be looking at 2 different schematics. I'm referring to Mark's Rocket schematic in the "Trainwreck Files" section of this board https://tubeamparchive.com/files/Mark_R ... fo_cct.jpg . There's no "220k into 1M" and the cathode follower resistor is not 68k. I'm going by his notes which outline 2 tone stacks that were seen. I plugged both of them into the TSC and made an observation. The only tone stack change you made was the 1k/10k swap (compared to the AC30) when there were other changes Mark outlined in his notes. Again, maybe we're looking at 2 different schematics?

I never claimed anything was "definitive". In fact I did say "I don't really know what all this means" and "I'm not sure what I think but I'm going to find out" and "I'm going to try both the tone stacks and see which one I like better". Maybe you haven't really read my responses.

I agree that if you are running bass and treble on zero, it makes the tone stack pretty useless. Why not just use the normal channel? Maybe the top boost channel has tighter bass.

[nickt]

We must be looking at 2 different schematics. I'm referring to Mark's Rocket schematic in the "Trainwreck Files" section of this board https://tubeamparchive.com/files/Mark_R ... fo_cct.jpg . There's no "220k into 1M" and the cathode follower resistor is not 68k. I'm going by his notes which outline 2 tone stacks that were seen. I plugged both of them into the TSC and made an observation. The only tone stack change you made was the 1k/10k swap (compared to the AC30) when there were other changes Mark outlined in his notes. Again, maybe we're looking at 2 different schematics?

Yes I think we may be. Trouble is that when you look on Mark's rocket layout he has the 68K and 220k resistors. Plus the 10k Mid resistor and 50pF and 0.022u x 2 tone caps. Theres' yet another schematic somewhere which is different again and, as I say in a post above, Paul Ruby's rocket clones seem to have further differences.

I never claimed anything was "definitive". In fact I did say "I don't really know what all this means" and "I'm not sure what I think but I'm going to find out" and "I'm going to try both the tone stacks and see which one I like better". Maybe you haven't really read my responses.

Possibly not but you did say "If you use the 1k in the Rocket circuit, C1 should be a 2.2n and C2 should be 470p." which sounds pretty definite to me Don't be offended

I agree that if you are running bass and treble on zero, it makes the tone stack pretty useless. Why not just use the normal channel? Maybe the top boost channel has tighter bass.

Also an extra gain stage, cathode follower, lower coupling cap and bright cap on the vol pot. None of those are actually reasons of course. The reason is "hey that sounds more gooder!"

The really interesting thing for me is that I've found out something about my rig that I didn't know before that almost makes sense. So thanks for posting the pointer to the Duncan site.

[Doug H]

Trouble is that when you look on Mark's rocket layout he has the 68K and 220k resistors. Plus the 10k Mid resistor and 50pF and 0.022u x 2 tone caps. Theres' yet another schematic somewhere which is different again and, as I say in a post above, Paul Ruby's rocket clones seem to have further differences.

Yeah, I see what you mean. There are a lot of differences between component values in Mark's schematic and layout for the Rocket. I had not looked at the layout. My guess is that you probably don't need the 220k. IIRC, the AC30's used 220k mixer resistors when connecting the normal and top boost channels to the PI.

Possibly not but you did say "If you use the 1k in the Rocket circuit, C1 should be a 2.2n and C2 should be 470p." which sounds pretty definite to me

Yeah well, I was just referencing Mark's schematic and notes. I didn't realize his layout differed. Maybe that was the source of the misunderstanding.

Also an extra gain stage, cathode follower, lower coupling cap and bright cap on the vol pot.

I thought about that too. I wonder if the extra stage and follower make that much difference if the tone stack is that lossy.

The reason is "hey that sounds more gooder!" Very Happy

That's the bottom line. None of the other stuff really matters.

I've played around with the top boost tone stack quite a bit. I can say that just subbing a 470p for the 47p and leaving the rest of it stock sounds pretty "Ken-y" to me-Big gain boost in the upper mids. I'm not saying that's what he did in the Rocket. It's always worth it to try a lot of different ideas.

Also, if you want more gain try increasing or removing the 10k. Did you know the original top boost tone stack was a mistake? There's a nice article about it out there somewhere. The idea came from a Gibson amp. Try moving the 10k from the bass pot wiper to the end lug that is grounded. Insert the 10k between the pot lug and ground. This gives you a basic TMB tone stack like Marshall/Fender.

Lots of fun to be had...