Rocket Reverb - Less Headroom than Expected

[jhaas]

I've just completed a 4xEL84 Reverb Rocket build. My only previous Rocket experience was a 4x6V6 Reverb Rocket. Unfortunately I no longer have possession of the 6V6 version to do a side-by-side comparison, but the EL84 version sesms to have a lot less headroom. Is it normal for an EL84 version to break up sooner than a 6V6 version?

All voltages appear normal. I don't have them in front of me but from memory:
EL84 Plates/Screens/Cathode - 293V/291V/9.0V
and all the preamp voltages seem within acceptable limits.

I'm using an Edcor tranny set from RJ.

This amp is for a country picker who will need more headroom, so I'm considering two options:
Changing V1A plate resistor from 220K to 120K as Mark Fowler has done on a recent build
Adding a pre-PI resistor to ground to bleed some signal, as RJ did with his "PIG" pot posted about a couple years back. I would install a temporary pot to get the desired amount of attenuation and then hard-wire a resistor.

But first I want to me sure the amp working normally. As it is now, volume pot at 3-4 gives quite a bit of crunch. If I switch in the V2 Ck, even more! (as expected) Is this normal?

[fishy]

Headroom with what. If its a HB guitar, headroom issues are not unexpected in my experience. With single coils it should have the headroom you are looking for.

[jhaas]

Most of my testing has been with my strat with "vintage noiseless" single coils. A buddy of mine played his Tele, and Trussart (bridge-HB) thru it as well, but mostly singles.

[telentubes]

I also just finished a 4 X EL84 Rocket Reverb. The voltages I get match the voltage chart very very closely. My amp is quite clean, well into higher volumes, simply getting thicker, meatier, and more harmonically rich when the volume is turned up. There is no break-up in the 3-4 range, and I wouldn't describe it as having "crunch" along the way. It will start to "drive" in the 7ish zone (out of 10), and it's pretty loud at that point. I play in a modern country style (amongst others) and find this to be a great amp for that. This is with a tele with Harmonic Design Vintage Plus single coils.

[jhaas]

telentubes, what you have described is pretty much what I *expected* but not what I'm getting, so I must have something going on with the amp. Thanks for sharing your results.

[Zippy]

This amp is for a country picker who will need more headroom, so I'm considering two options:
Changing V1A plate resistor from 220K to 120K as Mark Fowler has done on a recent build
Adding a pre-PI resistor to ground to bleed some signal, as RJ did with his "PIG" pot posted about a couple years back. I would install a temporary pot to get the desired amount of attenuation and then hard-wire a resistor.

What are your voltages in the preamp?

The PIG will give you what you want only if the distortion if coming from the PI or power amp. If your lack of headroom is from the preamp, the PIG won't help.

Do you have a 12AY7 handy? You could also remove the bypass cap on the first stage. If V1 voltages are low, you might adjust the dropping string to bump them up.

How are you running the tone stack? Have you tried adjusting the midrange?

The Rocket is very friendly to this kind of tweaking. Of course, for ultimate Rocket headroom, go with the KT66 Rockster.

[KellyBass]

I had similar results as yours with my edcor PT/OT rocket build. I ended up making some changes around V1 that opened up the sound quite a bit.

Going from memory, I think I switched V1a's cathode bypass cap from 22uf to 10uf and left the 1.5k resistor alone...but switched V1b's cathode resistor from 1.5k to 1k. I made some notes on this, so I'll double check when I get home and post the results.

[jhaas]

I made some notes on this, so I'll double check when I get home and post the results.

Thanks KB, please do, I'll be very interested to know what you did.

Zippy, Thanks, I'll post complete voltages tonight when I get home.

[M Fowler]

I think the problem is with this circuit is the cathode follower (CF) adding a lot of gain.

I tried to clean the Rocket up by reducing the plate resistor to 120K rather then the 220K , which 120K reduces the gain of V1a. Or you can simply keep the 220K and use different 12A_7 type of tubes or 5751 in V1. Use a 12AT7 in the PI.

I like to keep the voltage high on the plates as well so use the 290-0-290 rather then 240 or 250 or 260 volt HT as some use.

I keep changing up the circuit on each build.

Mark

[Colossal]

I think the problem is with this circuit is the cathode follower (CF) adding a lot of gain.

The cathode follower is stealing current from the previous stage, adding some gain to it. To reduce compression a little, you could try increasing the 56k cathode resistor on the cathode follower up to 100k.

I am a compression freak and love the lower values for cathode followers and the warmth they add. I have little doubt that Ken chose the value he did deliberately.

[mlp-mx6]

I think the problem is with this circuit is the cathode follower (CF) adding a lot of gain.

Don't cathode followers by definition have a less-than-unity gain?

[jaysg]

What el-84s are you using? Do you have a scope? If so, you need to figure out how much clean power it's outputting. When I brought up my Liverpool, it was only getting ~10W clean. It was the el-84 set. I couldn't tell how loud it was until I A/B'd it with some other amps, at which point it was obvious.

[Colossal]

I think the problem is with this circuit is the cathode follower (CF) adding a lot of gain.

Don't cathode followers by definition have a less-than-unity gain?

Yes, that's correct.

[M Fowler]

The CF is the only place in the amp that I can see where a problem could exist. But I am all ears for more knowledgeable to offer some advice on the Rocket circuit to get more head room.

[CapnCrunch]

What el-84s are you using? Do you have a scope? If so, you need to figure out how much clean power it's outputting. When I brought up my Liverpool, it was only getting ~10W clean. It was the el-84 set. I couldn't tell how loud it was until I A/B'd it with some other amps, at which point it was obvious.

+1

My Rocket is stock per the documents here on TAG. I haven't AB'd other tubes but I suspect your tubes play a huge roll in the amount of available headroom. I read many posts from the early days of Rocket builds here. It seems that some have lots of headroom, i.e. Allyn's first Rocket clone, Rawnster, etc. Others don't. There was speculation early on that the transformers were the major player in this department, however, I think Rawnster's Rocket used Edcors.

My amp doesn't develop significant "hair" with single coils until the volume is at 70% or so (vintage single coils, not Texas Specials). At that point, it is extremely loud. I have it tubed with Saratov el84's and a mish mash of vintage 12ax7's- GE shortplate in v1, Sylvania short plate in v2, and a GE longplate in v3. It sounded so good with this set up, that I've never got around to swapping and AB'ing tubes. I'm here to say that you can get a lot of clean volume from a stock Rocket. I never play in settings where I can push this amp into its own overdrive, it's just WAY too loud. I have to use pedals. I'd swap some tubes and see what that does for you.