"Test Rocket" - 10 lbs. of candy in a 5 lb. bag

[gktamps]

I've repaired 3 amps, partially re-built 4, and this is my fourth scratch build (last was a 6V6 KF60-inspired, which turned out great). Not an electrical engineer or nearly as experienced as many here, but I have built guitars, effects and have worked on many, many electronics projects for 45 years, tube amps only for last 3 years. My TAG reading to posting ratio is about 400:1, which is as it should be unless you're one of the Maestros here!

While waiting for parts for my Rocket tribute build to arrive, I picked up a small, but usable chassis, a Heyboer HTS 4831 260/300v TT and a pair of Dynaco A470 OTs for next to nothing, so thought I'd do a no-holds barred version with bells and whistles to serve as my advanced learning tool, and have been slowly working on for about 6 weeks whenever I get an hour or two of time.

It's got a PPIMV, VVR3, RJ's Rocket Booster circuit, and footswitchable second gain stage cathode bypass boost. I used can caps and played with their location as well as that for the choke. Incorporated the Hiwatt speaker jack wiring just for kicks, and diodes across the GZ34 for the belt and braces approach.

First lesson learned; doing this with a smaller chassis was probably not the best move, as there was very little room to shift components around. Makes for some less than optimal wire runs, but oh well, this is my advanced learning tool after all.

Fired it up the other day without tubes, checked voltages, and found some were off, but nothing too wild, so stuck in some tubes and bias probe, found the EL84s dissipating 12.5 watts, so changed out the cathode resistor, got it down to 9.5 watts for testing, then plugged in a Paul.

And was met by strangled tone, very low volume, so immediately I checked voltages again, and got the readings that follow (voltage chart coming up soon).

My first steps were to up the B+5 dropping string resistor to drop some voltage, but plate 1 was still high, and plate 2 was still low. I then dropped the V1 plate 2 resistor which helped, but voltage on that plate is still low. I also upped the B+3 dropping resistor value, but might have made a mistake since plate 1 was low to start with. Now I think I'd better up the V1 cathode resistor, because the reading is way off.

Next thing will be to temporarily clip out the RJ Rocket Booster mod, since that's pulling current from B+5, and try to start with a cleaner circuit.

Photos being posted next, then my voltage chart.

Cheers,

Greg

[gktamps]

Photos.

Wires hanging out are second stage bypass cap boost circuit, not yet wired to stereo jack.

[gktamps]

Here's my voltage chart, prior to disconnecting the Rocket Booster circuit, which hooks up from C9 to B5, essentially.

[romberg]

Nice! What is that coiled wire running off the picture at the bottom in your first image?

Mike

[gktamps]

Thanks, Mike. That's the second gain stage bypass cap wiring. I went with two different value caps, then connected those wires with the shield coil to a stereo jack after that photo. The circuit will be foot switch controlled. The shield coil is an approach used on the Komet, a variation of which was my previous build.
Cheers,
Greg

[Reeltarded]

Greg, it looks great! Nice to see an amp around here!

It looks like it was a little tight to work on and you did a great job on that count as well!



Miles!

[katopan]

Squeezy! Test amp builds are so great to experiment and learn from. Keep it comin'.

[gktamps]

Thanks, Gents! I'll have more to be proud of when I get it sounding like it should. Next update after I make some adjustments; if I'm not good enough to figure it out, I'll be asking for some critical eyes on my voltage chart, at least!

Oh yeah - I'm thinking of a metal cage cab for this, or some variant - try to pull in the Dynaco heritage, maybe? Suggestions are welcome, even willing to be told no.

Cheers,

Greg

[M Fowler]

Greg, nice project you have going.

My Rockets run higher voltage then the expected values too.

[romberg]

Followup question (if I get one . What is a WR3? Is that an autowah?
Or something completely different?

Mike

[gktamps]

Hi Mike,
That's V V R 3 - Variable Voltage Regulator from Dana Hall.
Cheers,
Greg

[romberg]

Ah, that makes more sense! Once you get things dialed in (all the candy in the jars , I'd be very interested in your opinion of the VVR3. I've never played around with power scaling. It has some appeal over using an attenuator.

Mike

[overtone]

Greg, good to see a tight space build. Tricky devils those.
Do you still have a strangled sound / low volume?

I had look at your voltages and the first thing that strikes me is the cathode voltage on V1a.

On the output section: what cathode and screen resistor values are you using right now? From here it looks like you have around 14W dissipation on each EL84 if it is a standard Rocket set-up. I don't follow the part in the OP where you said you changed out the cathode cap to get down to 9.5W.

Ahoy, tony

[Smokebreak]

Oh yeah - I'm thinking of a metal cage cab for this, or some variant - try to pull in the Dynaco heritage, maybe? Suggestions are welcome, even willing to be told no.

Cheers,

Greg

That's a great looking build! I've been thinking about the metal cage cab lately, too. Where ar you looking for manufacturers? I know Hammond makes them, and of course :http://www.lead.co.jp/rackcases/pdf/case/MK.pdf
But I havent sen any others. I think a birdcage would look cool, too...the Right birdcage

[gktamps]

Ahoy Tony! How did you know I was a sailor? Lived aboard a Tayana 37 with my wife for 10 years.

Still have the strangled tone/low volume, but I suspected the high cathode voltage when I first took readings (you caught that, too), since it is so far over range, and haven't yet had time to correct that. I've got to get the voltages more in line, then evaluate again.

Screen resistor values are standard, but I had a brain glitch when I first wrote "cathode cap", meaning of course, "resistor". I changed that to 68 Ohms, which dropped the dissipation to where I can keep troubleshooting without toasting marshmallows.

Will make changes and report tonight.

Thanks, Tony,

Greg

Greg, good to see a tight space build. Tricky devils those.
Do you still have a strangled sound / low volume?

I had look at your voltages and the first thing that strikes me is the cathode voltage on V1a.

On the output section: what cathode and screen resistor values are you using right now? From here it looks like you have around 14W dissipation on each EL84 if it is a standard Rocket set-up. I don't follow the part in the OP where you said you changed out the cathode cap to get down to 9.5W.

Ahoy, tony