2-EL84 Rocket Tribute with reverb underway

[gktamps]

The addiction grows...

With the kind and generous help of Richie, who provided me with his latest layout and parts values, as well many helpful tips, and using the iron and chassis from a Marsh 18watt clone that sounded awful, I built an 18watt TMB that just blows my mind. It took me more time than anticipated, and I abandoned the VVR implementation due to excessive hum and insufficient space, but all the time and effort was well worth it. Each channel has its own character, it has plenty of grit and grind, as well as clean, and the TMB circuit is warm and complex. Even the wife likes it, and she ain't nuts about the noise, uh, loud guitar!

So, I've started my 2-EL84 Rocket Reverb build, using helpful guidance from Mark F. and Richie, a chassis, circuit parts, and kind guidance from RJ, Heyboer iron from Dartanion, PEC pots, and my own cherry cabinet, waiting for good weather for the second coat of varnish. I'm persisting with the VVR, and using Richie's suggestion to use can caps to open up space for the VVR circuitry. I've got to make a few changes and additions to the board - this is the first go around.

I've said it before - this forum is stacked with helpful, generous, talented builders - thanks to all who have helped me out.

Waiting now for more parts, so progress reports will follow later.

Cheers,

Greg T.

[Reeltarded]

Yes! Talented builders, no doubt.. and it looks like you might be one of them or at least becoming one of them.

That looks fantastic!

[gktamps]

Wow - thanks for that compliment! The builders here set the bar high, so lots of good examples to learn from.
Cheers

[gktamps]

Hello,

Busy work week, and waiting on more parts from RJ when he returns from overseas, so slow progress, but here's another pic.

Question: am I overfiltering the power section for my 2-EL84 build if I stay with the standard cap values for a Rocket? I like a tight bottom end (who doesn't? ), but don't want to lose the feel of the Rocket. I'm using Heyboer 260/290V iron.

Also, I'd like to use both taps (260 and 290) on the PT; anyone have a switch recommendation that's safe?

Thank you,
Greg T.

[gktamps]

Howdy!

Making progress, albeit a little slow due to work and waiting on parts. Waiting on the front cherry panel varnish to dry; taking forever in this cold, wet weather, so I can't finish front control installation.

I've made a few minor changes. Now incorporated a switch for 260v and 290v taps, though since adding, was advised by Richie to use only 260v for this amp. Added a CT fuse for the PT, diode protection for the tube rectifier, upped the cathode resistor to 150ohm. The added perf board is to hold the reverb driver cap and resistor, fuse, and for potential later addition of parts as the amp evolves.

A little unsure what values to use for the screen resistors; the original value is 100r, but I've seen 470r and 1k being used as well - any input on this appreciated.

Another stumbling point is the best place to apply VVR. The reverb driver is supplied at B2, so applying voltage control to the power tubes and PI will also affect this circuit. My instinct is to leave this node out of the VVR circuit so the voltages stay where they need to be to operate properly, but I'm not sure how I would do this and still control voltages at B3 (PI). Is that even possible, or should I include the reverb in the VVR circuit?

Thanks for youir comments!

Greg T.

[RJ Guitars]

That is just a way cool build... lots of innovative ideas there. Looking forward to the liftoff of this amp. rj

[kwijabo]

well the schematic has the 100R listed, and that's what i used on the first try of it so maybe try it as written first. but since then, i've done two other amps with 1.5k 5W. went back and changed my rocket.

i've only done EL84 amps, and someone can/should comment if i'm wrong, but anecdotally i feel like with the 1.5K's vs. 100R or 470R the output gets to that nice tube feel at a lower volume setting. so the feel/sound at a volume setting of 6/10 with the 100R happens at about 4/10 for the 1.5K.

there's an exchange about screen resistors regarding tube life over here, and higher Rscreen will translate into a longer (but how much so?) than a lower Rscreen:
https://tubeamparchive.com/viewtopic.ph ... 03a34ec1fb

[gktamps]

Thank you, Kwijabo,

That link was very interesting - Novosibir's rheostat was a pretty cool idea. I actually installed higher screen resistors on my recent 18 watt TMB build, and liked the tone, and the idea I was conserving my tubes. Looks like the consensus is that 1k on my Rocket tribute would be just fine, though I might start with 470r then see how that sounds.

Now to choose a direction to go with the VVR...

Cheers,

Greg T.

[rooster]

Hey there. That choke is 347 ohms and remember that it's in series with your screen voltage. So with that and a 100 ohm resistor you have probably got yourself a 4-5 volt drop on the screens vs. the plates. Which should be pretty typical vintage sparkly tone. When the higher value screen resistors are mentioned they don't come with an explanation of the choke value? So here you just don't know if using a 470 ohm value is giving you typical tone. And it might if the choke is 100 ohms. But clearly 1K or 1.5K is not in the park even if there is no choke.

Just sayin. Tube life is overrated, life is short when tone is good, enjoy it!

[M Fowler]

The amp is looking good.

Mark

[gktamps]

Thank you, Mark, and to all who have commented,

I've taken a break the past couple of weeks because work got busy, and because of indecision about several details of the build. In that time, I did a lot of reading of past postings on this forum, as well as others, which helped me come to some conclusions, including the following:

1) We relative beginners ask a lot of redundant questions about stuff that's been done before (sorry).
2) We're not just talking theory here, but practice; what works in a build for one person may not be right for another.
3) The theoretical and applied effects of various changes are interpreted differently by different people.
4) There's no silver bullet.

So, as of now, I'm going to finish this build but make incremental changes to see how I like them, even though it's a PITA to keep yanking it from the cabinet. Based on all of the comments I've received, the off-line suggestions from Jelle, and lots of reading, this is the plan:

No VVR for now. No MV for now. 1K screen resistors. 150R cathode resistor (for 2-EL84s, current production). Boost switch/cap. Bright switch ala Liverpool. Button it up and play. If I don't, this amp will sit unfinished as I wallow in self-doubt and confusion.

Later;

1) VVR PA and PI, not whole amp (don't want to drop voltages on reverb driver), and not screens only as I had considered.
2) Add 40uF reservoir cap before VVR.
3) Adjust power dropping resistors for proper voltages on unscaled sections.
4) Play with screen resistor values, down to 100r, up to 1.5k. 130R cathode resistor. May go stock if I use my NOS Russian tubes.
5) LARMAR PPIMV to keep from overdriving the power tubes when scaled.

Thanks to all who have provided input and suggestions, and all who have done this stuff before and posted your experiences.

Greg

[M Fowler]

Greg,

I think what you have decided is a good plan. Enjoy this 18watter for a while stock or as is and then plan your next move.

The VVR alone maybe the best next mod without the PPIMV and go from there.

Mark

[gktamps]

This build took me longer than I hoped, but it's finally in the cabinet and sounding fantastic! Last thing needed is to mount the reverb tank and make some short rca cables to connect the tank to the chassis.

Here's how it ended up:
2 x EL84 Rocket tribute with reverb circuit, per RJ's Rockster Reverb circuit. No changes to that circuit, and it sounds lush, full, and quiet.
Lar/Mar PPIMV
Footswitchable gain control, ala Mark Fowler
Multi can-caps for main PS, aux. board for reverb filter and PT center tap.

After I've enjoyed the amp for awhile, I'll be connecting up the VVR, which is already installed, scaling the entire amp. The caps are installed on the board, and the 1M resistors will be installed on the sockets when I'm ready to make that change.

I'm going to play around with a bright switch, not yet wired up.

The amp is quiet, and sounds really tasty and well balanced. I had to change the screen resistors and cathode resistor for the power tubes to get rid of some crossover distortion caused by screen voltages being too high, and too close to plate voltages, but that was easy to identify thanks to the great posts on this forum and taking careful measurements.

I owe many thanks to several folks who gave me extra help beyond the comments on the forum; Mark Fowler for help with the overall strategy, and details about the boost switch; Jelle for talking with me on the phone about several details and helping me decide on a direction to go with the VVR, though I chickened out and didn't experiment with scaling screens only as he was hoping; RJ for providing some great parts, and a schematic to branch off from for my own creation; and in particular, Richie Hall, who spent a great deal of time helping me through my confusion about the effects of VVR on the reverb circuit, proper implementation for scaling the whole amp, checking out my build pictures, and providing much appreciated, detailed advice and suggestions. THANK YOU!

Now it's time to play through this thing, while I think about what to build with the Classic Tone 18watt iron I just bought. How pitiful is that?

Greg T.

[dartanion]

Looks great Greg. One option on the VVR that eliminates the need for the 0.1uF caps and resistors on V1 is to change the limiting resistor on the VVR to change the lower limit to around 30% instead of 10%. IIRC, the original limit resistor is a 100K, so swapping that with say a 330K will do the trick. The reason I do this is that the layout of the caps and resistors at the socket is critical and can be a real PITA when it all looks right otherwise.

Anyway, it's just a suggestion to eliminate some frustration.

Enjoy your new amp brother Greg!

[rmroza]

Looks great man! Need some clips!