New Reverb Rocket build

[M Fowler]

I am finishing up on another Rocket should be done today.

Uses rjguitar blank 19.5 x 8 x 2.5 chassis very good quality by the way. Can't wait for his #124 chassis and board kit.

The PT and OT are on the schematic.

Gabkit Dumble ODS cabinet 112

Faceplate was in my head and Colossal made it happen with his magic, thanks Dave.

I will post the final pictures later.

Mark

[Structo]

Looks real pro Mark.

How was the faceplate done?

[M Fowler]

Thanks Tom finishing up the wiring today.

Colossal made a scaled drawing using his Mac software and then I get a pdf. which I convert to coral draw for bnp lazers onto silver alumamark.

[Colossal]

Hi Mark,

That amp came out great. Nice work! RJ's chassis' look really solid. Can't wait to hear how it sounds.

Regards,
Dave

[M Fowler]

Thanks Dave, your help was very appreciated.

rjguitars chassis is made of nice material and has welded corners, I highly recommend it.

Mark

[Bob S]

Nice work Mark - looks very professional. Well done !
Interested to hear how it sounds when you finish it.
I was going to build a kt66 rocket reverb, but seeing this is giving me second thoughts

[M Fowler]

Bob,

Thanks I play my KT66 version and my EL34 versions more if that helps. Also did a 80w version I really like with Edcor iron nice deep bass response.

Wouldn't you know it I was down to the last wire and ran out of solder

Mark

[Structo]

I see you have a master volume on your front panel.
How did you implement that in the circuit?

[M Fowler]

It is a 25kL/2w PEC pot in their as of yesterday wired with a 33k/3w resistor across the pot pins. This pot is wired between the PI tube pins 8/3 and the 1k5 PI resistor on the board.

This is a watts control rather than a Master but I labelled it as master.

Mark

[Structo]

Not familiar with that type of master.

You are saying it is wired between the cathodes of the PI?

I've done a boot strap master on the grids before but not on the cathodes.

How do you like it?

[Colossal]

Not familiar with that type of master.

You are saying it is wired between the cathodes of the PI?

I've done a boot strap master on the grids before but not on the cathodes.

How do you like it?

Is this how it is wired? (excuse my chicken scratch drawing)

Tom, the method cold-biases the PI and works quite well as a means to reduce volume while preserving tone. Caveat emptor: it doesn't seem to be a good fit for everyone or every amp. I ran this by the 18watt crowd and they poo-poo'd it saying a bunch of them had tried it and it messed too much with the 18watt magic. The 18watt Marshall (1974) may be a specialized case since it is all about driving the output stage into severe distortion so anything that reduces the signal to the power amp "upstream" messes with that magic.

I've enclosed a drawing of how I implement it. I've modeled various combinations of pots and paralleled resistors and the 25kB||33k is a very good one because it gives good sensitivity (which is likely the reason Bruce Egnater chose this combination of values for his "Watts" control). I have used this technique in Marshally amps and I really like it. It's very effective for volume reduction while retaining the distortion characteristic of the amp. Like any method (VVR, attenuator, PPIMV, etc) there is a point where it tends to sound a bit off. It does add a good bit of compression but with added warmth at low volume levels. This does create some mud but it's doable for home practice and you still get that punchy, chunky tone. I was quite surprised and pleasantly so.

I've added the mod to some client amps as well. Case and point was a retrofit onto a '72 JMP 50W. This amp was loud as hell and tended to get a bit edgy and a little too much "over-the-top" when dimed. Backing off on the "scale" pot (or whatever you want to call it) took the instability out of the amp and just cleaned it up just slightly. This is NOT to be interpreted as "clean" meaning less distorted, but I mean took the ragged edge off of a dimed amp and brought back a measure of definition at very high volume.

I have also added it to a recent new 50W build for the same client. He loves it. I told him before the amp was built that it was a good trick for reducing the overall sound pressure level of the amp and he said "ok, let's call it Pressure then". So that's how the knob got labeled. I used it in conjunction with a LarMar PPIMV and the combination of the two produces some incredibly usable tones (see enclosed photo - last two knobs on the left).

Hope this helps.

[M Fowler]

Tom,

I tried this on my Express and did not like it at all.

However, I tried this on a bunch of my various Rockets 6L6, EL34s, and quad EL34s and I like this feature a lot. I can dial down my amp for home jam but still get a great distortion retaining the tone I want. I can also crank my volume pot up and dial in the watts control and use the cut control to my desired tone.

I don't find that it affects the tone in my Rockets. To take Dave's (colossal) point and add this control along with a MV would really add a lot and I plan on doing just that in another upcoming K60 project.

Love your new amp Dave.

Mark

[Structo]

Hey thanks a lot guys, I'll have to give it a try.
Thanks for the schematic Dave, I had it all wrong.

So this changes the bias of the PI tube, interesting.

I could see where this may be a good tool to dial in an amp.

Thanks again.
Tom

[Colossal]

Thanks for the schematic Dave, I had it all wrong.

No worries!

So this changes the bias of the PI tube, interesting.

Yes. It basically makes the amplitude of the outgoing signals to the PA tubes smaller! It does add some compression since you are changing the bias point of the tube (colder) which causes some muddiness at very low settings but the added benefit is that it seems to add warmth as well, so an interesting tradeoff there.

I could see where this may be a good tool to dial in an amp

I don't think it would be good for every amp and I think a study of how a particular candidate amp is set up to provide overdrive would be needed. In Mark's case, he tried it in his Express and didn't care for it (which I was surprised to hear). The Express is set up in a very specific way as is the venerable Marshall 18W 1974 so changing the PI in those amps has a more dramatic effect on the behavior of the power amp.

Mark and I learned of this when we dissecting the Egnater Rebel amp but it was also featured in Merlin's book. There are variations on this as well such as what is used in the Carlsbro T60 amp. I have tried it in mostly Marshall plexi and JCM type amps and I think it is very useful particularly where those amps are getting all of the killer tone at high volume levels. The beauty of the mod is its simplicity: one pot and a resistor. Even backing the knob off slightly takes some edge out of a cranked amp. It does not really start to greatly affect the tone until you get down a little, but the plot of cathode current vs. resistance on that schematic shows that. The tone is still quite doable, even at low levels, and the volume is cut way down. You could add a clean boost to the front end to recover some of the aggression from the amp while keeping the cops out of your neighborhood

[M Fowler]

Speaking of the Rebel, I still want to get a four gang pot built to try that mix function along with the watts control on a higher powered amp.

I'm considering wiring the tone stack of a Reverb Rocket differently more like the Komet 60.