Rocket versus ZWreck/Matchless etc

[Mark]

I should clarify, I'm making reference the the 22uF cap that goes across the 1.5K cathode resistor (preceding the cathode follower) and the 220K voltage divider prior to the P.I. stage.

I noticed that this circuit had been put into a Trainwreck Rocket after discussion with Ken Fischer. As I'm up for a different voicing option I thought I'd implement this into my amp. (This circuit has made an appearance in many other amps besides the Rocket and the ZWreck.)

http://ceriatone.com/images/layoutPic/m ... iatone.jpg

I did some preliminary work where I put the 22uF cap across the 1.5K resistor and I heard the signal being boosted and a definite change in tone. However upon putting the entire circuit into the amp, I'm struggling to hear a distinct difference. I can hear a difference when I strum a chord and switch between the two settings, but it is quite subtle.

If I play the Stratocaster (enamel wire pickups = CBS era pickups) and switch the circuit in and out I don't hear any change in tone when the amp is set to 10 o'clock and less. In order to hear a difference I turned up the bass and treble controls fully up and then switched the circuit in and out. There was a distinct difference this time. The 22uF and voltage divider offered reduced bass which was in line with what I thought I was hearing.

I haven't done any testing of the amp at considerable volume, but I suspect the mod MAY come into it's own at higher volume levels, where the extra gain provided by the bypass cap causes extra compression via the cathode follower. This hasn't been verified yet though.

Another note completely aside, I did try Merlin Blenecowe's mod to save the cathode follower, he recommends a diode and a 10K cap. I had this circuit on a switch whilst putting a signal into the amp and looking for changes to the waveform with a CRO. I saw no difference to the waveform with this mod installed in the amp. I've heard no difference in tone to the amp with this mod in the amp. This one sounds like just the trick to save those NOS tubes from an early death.

[Blackburn]

I've done it and to me it's a vast difference from a stock Rocket. There's a boost in overall gain. I like the richness and that it distorts earlier with the mod, but I prefer less grit and more clean and slightly overdriven tones with picking dynamics as a stock Rocket, but that's just me. I do think the cleans are better stock. If you're looking for a dirtier Rocket, I think it's a great mod. Seems to be more of a classic AC30 type of overdrive with it.

[Mark]

Thanks for the reply Blackburn. It's sounds like you put the cap across the 1.5K resistor but didn't put the 220K resistor in the circuit (prior to the P.I. stage) which reduces gain by the same magin .

Without the resistor there is an increase in gain and volume (which is quite dramatic) however when the voltage divider prior to the P.I. is implemented the volume level is the same and I didn't find it particularly dramatic.

[Blackburn]

Mark, I did employ both mods, the bypass cap and the divider, and my post is about both used simultaneously. I never split them up.

[Mark]

Here is the thread I was referring to:

https://tubeamparchive.com/viewtopic.ph ... b&start=30

Joe Con says this:

"...adding that extra 220k to ground at the PI entrance cuts the signal going into the power section by -6dB, giving the 2nd preamp stage much more of a chance to clip before the power tubes go into hard clipping. Giving that stage a chance to clip more not only prevents the power section from clipping too hard, but also means that the signal going in to the power stage is now asymmetrically clipped, so a lot more even order harmonics are preserved."

Martin Manning said:

And switching in the cathode R bypass cap on the previous stage will boost its gain by about 6dB, so it all evens out.

I installed the resistor first and it did cut down the volume of the signal and then I added the cap which brought the signal level to where it was before.

[pdf64]

Maybe Blackburn was using hotter pickups than those of a v1ntage strat?
Compared to my Strat RI, my LP R8 RI with Burstbucker PAFs pushes pre-amps into overdrive very easily, such that it's difficult to set a clean level on many amps.

Yes, under linear conditions, bypassing such a cathode would provide about a 6dB signal increase.

However, when the stage is overdriven, (to me) the change seems more significant, seeming to allow a more intense degree of distortion than an unbypassed cathode.

[matt h]

(deleted)

[Mark]

I should point out that I'm not refuting Blackburn's findings but legitimising my own findings. To my way of thinking I am getting the expected results, however I'm happy to be proven wrong.

My thoughts on the circuit are that it is there to push the cathode follower harder and get more compression/distortion from it. When I use my 50 watt Weber Mass, the distortion sounds pretty ordinary but it does sound different when I switch the circuit in and out.

When I simply have the amp hooked up to speakers the amp has much better sounding distortion and the amp sounds rounder when the circuit is out.

I should point out that I do need further listening tests (preferably with ear plugs).

[Blackburn]

Maybe Blackburn was using hotter pickups than those of a v1ntage strat? Compared to my Strat RI, my LP R8 RI with Burstbucker PAFs pushes pre-amps into overdrive very easily, such that it's difficult to set a clean level on many amps.

pdf64, that's exactly what I'm using!

Mark, no worries brother! I'm just going by what I remember from when I ran that mod for a week or so and there was a notable distinction from the stock circuit. I think pdf64 may be hitting the nail on the head by mentioning the different guitars used and thus, different pickups and output.

[Mark]

Okay, I had a bit more of a play with the switch today.

I found either the treble on the amp or the bass has to be cut a bit for the effect to be more pronounced. I was amazed that the amp is actually louder without the switch in.

I'd be inclined to alter the board layout so this mod is easily incorporated. For me it was an after thought and I wasn't prepared to pull the amp apart to tap a few more turrets into the garolite board.

Another worthy mod comes from Richie and that being switching between 500pF and 50pF. The man knows his amps!