Ghost notes on AC30/Rocket style amps

[Cliff Schecht]

I've built a few Rockets in different flavors now and in two of them, I've noticed the same "issue". In the first position on a few strings (namely the G and B strings), the amps have a ghost note that hits during hard transients and quickly dies off. To be more descriptive, it's a trash that rides on top of the signal when the volume control is set so that the amp is distorting. I could never fix this in the first amp (a 4x6AQ5 Rocket, now a great sounding 5F6A) but the other Rocket I built and sold to a friend didn't have this problem (or maybe it does, I need to listen to it again soon).

In the 6L6 Rocket I just finished up, I'm having the same issues. There's the same ghost note trash that shows up as the note is hit. I haven't chased it down yet and to be honest, I'm not sure I will. The amp does use quite a few old components but I work with this stuff often and know how to clean everything up so that it takes solder well. Chopsticking the shit every single solder joint in this amp (as with the 4x6AQ5 Rocket) showed that it isn't a problem there (although I did tame an oscillation issue ) and changing out all of the power supply caps for B+3 through B+5 didn't change a thing.

While I usually don't cite TGP as a source, this thread describes exactly what I've noticed in this circuit topology (i.e. cathode follower, no negative feedback, 220k plate load in the first stage, similar PSU, etc..): http://www.thegearpage.net/board/archiv ... 49895.html

After reading this, I'm thinking of upping the capacitance for the preamp stages (starting at doubling them) to see if I can squash this. Or maybe not. I need to play some other amplifiers with a similar topology to see if they exhibit the same problem. FWIW The 6L6 Rocket sounds great otherwise and much like every other Rocket I've built. The PPIMV is really nice to have as well and from what I can tell, the cut control is not so necessary with 6L6's vs. EL84's (it's out of the circuit now and I prefer it this way).

Has anybody else noticed a similar phenomena with this circuit topology?

[Roe]

ac30s are known to ghost with low filtering. solutions include:
- improving grounding scheme
- upping filtering
- adding a brian may style choke between rectifier and OT

[Reeltarded]

Aha the 63 A/TB stopped ghosting almost completely with last cap change. Hard to consider any stiffening. Love it low.

[xtian]

Cliff, does the ghosting exist with the PPIMV turned all the way up? This is the problem I'm trying to solve with my 1987--ghost notes, but only with master lowered.

[Cliff Schecht]

ac30s are known to ghost with low filtering. solutions include:
- improving grounding scheme
- upping filtering
- adding a brian may style choke between rectifier and OT

I'm pretty happy with the grounding scheme but since this is a recycled chassis (i.e. was a Bogen head a few days ago), the B+ filtering ended up being shoved off in a corner near the speaker output. I don't really like this and will probably change it before I put the bottom on, but I don't think this is what is causing the ghosting. Sorry no pics yet, I need to find my charger first!

I do think that upping the filtering would help. I might try adding some of the really small Nichicon e-caps right on the board for B+4 and B+5 to see if this helps any. It's worth a shot..

I don't specifically know what a Brian May style choke is. I already have a 16H choke in there in which I've used in many other amps successfully. Care to expand here?


Miles: My first B+ filtering is from a 30uF+30uF NOS can cap made by Sprague. I've used these in the past without issue (after reforming) and so I'm not sure it's this either, but I'm going to change out that particular cap anyways for sanity reasons (and to possibly clear up room for a vibrato circuit).

xtian: In my amp it's not related to where the MV is set. I can hear the trash at extreme attenuation and at full volume. I swapped out the OT transformer last night (which I was going to do anyways) to one more suited for 6L6's and this didn't fix anything. It did clean up the wiring near the output jack which was causing me other issues (parasitic squeal) too.

[Roe]

ac30s are known to ghost with low filtering. solutions include:
- improving grounding scheme
- upping filtering
- adding a brian may style choke between rectifier and OT

I'm pretty happy with the grounding scheme but since this is a recycled chassis (i.e. was a Bogen head a few days ago), the B+ filtering ended up being shoved off in a corner near the speaker output. I don't really like this and will probably change it before I put the bottom on, but I don't think this is what is causing the ghosting. Sorry no pics yet, I need to find my charger first!

I do think that upping the filtering would help. I might try adding some of the really small Nichicon e-caps right on the board for B+4 and B+5 to see if this helps any. It's worth a shot..

I don't specifically know what a Brian May style choke is. I already have a 16H choke in there in which I've used in many other amps successfully. Care to expand here?


Miles: My first B+ filtering is from a 30uF+30uF NOS can cap made by Sprague. I've used these in the past without issue (after reforming) and so I'm not sure it's this either, but I'm going to change out that particular cap anyways for sanity reasons (and to possibly clear up room for a vibrato circuit).

xtian: In my amp it's not related to where the MV is set. I can hear the trash at extreme attenuation and at full volume. I swapped out the OT transformer last night (which I was going to do anyways) to one more suited for 6L6's and this didn't fix anything. It did clean up the wiring near the output jack which was causing me other issues (parasitic squeal) too.

use a big choke after rectifier, but before the OT. may uses a 7h, 260ma. I tried a 5h, 300mA hammond 193 series choke that worked just fine. it will lower the voltages a tad and give a slightly smoother tone

[Cliff Schecht]

Alright... Interesting thought but I will probably hold off on that much modification for now. I removed the can cap and put in a 47uF e-cap right off the rectifier pin which coincidentally shortened the ground run for this cap considerably. The cut control is back in (much needed at higher volumes!!) and I think for now, I will button this up and play it for a while. Might roll some tubes before I put the top cage on but for the most part I am happy with this build for now. I'll bring it to a jam and see what she sounds like in a band. I'm digging the huge bottom end with the 6L6's and super punchy mids, definitely it's in the same league as my 5F6A. I can see why people dig the Rockster (although tbh my build has its voltages in the preamp damn near spot on what a real Rocket has which is part of the Rocket sound).