Rocket build probs

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jph118
Posts: 53
Joined: Wed Apr 19, 2017 1:35 pm

Rocket build probs

Post by jph118 »

Hey everyone, daily reader, rare poster... i'm working on a Rocket build from Mark Abbott's well known schematic, with a few slight deviations, listed below:

1. Only using one 40uF as reservoir, with a 2nd following choke, then 3x22uFs.
2. Choke is a Weber rated at 9H/120mA, which seems like a good fit here.
3. dropping resistors (all 2w m.ox) are 15k-10k-10k, and B+ voltages are right on spec per chart found on this forum.
(I've also added a 3-way bright switch, 3-way cathode bypass on input, 500pf/4.7nf coupling cap switch, and tone stack bypass, but ignoring all of that for now...)

Only problem: getting 290V on plates, but about 293-294V on screen grids. No bueno. Everything else on the amp is functioning fine, and the tone is nice, except for a bit of hum and intermittent loud popping. Any thoughts? FYI I've got 1K 5Ws on all four screen grids.

Thanks in advance guys!
jph118
Posts: 53
Joined: Wed Apr 19, 2017 1:35 pm

Re: Rocket build probs

Post by jph118 »

Update: just flipped it on again, and now i have a loud hum and a doubled/ring modulated-type sound when i play. Weird.
jph118
Posts: 53
Joined: Wed Apr 19, 2017 1:35 pm

Re: Rocket build probs

Post by jph118 »

Problem solved! Was swapping out the choke (Weber W022699) for a 470R 5W, just to rule that out of the equation, and tested my capcan grounds; they were inconsistent, intermittent. So i hardwired one of them to a wirewasher, and the big problem above was solved. Screen grids dropped 3-4V lower than plates as well. Still a little noisy, but sounding great overall.

Thanks for letting me talk it out here, i'll just leave it up for anyone with similar future problems.
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romberg
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Re: Rocket build probs

Post by romberg »

Thanks for sharing this! Information like this is great to keep in the back of ones mind for the next time one might encounter something similar. The old saying is that experts have done more things wrong than beginners have even tried. It is great to share some of these wrong moments and let everyone become a bit more of an expert along with you. :)

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RJ Guitars
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Re: Rocket build probs

Post by RJ Guitars »

Thanks JPH, we all learn a little form these kind of posts. I suspect the DC resistance of the choke was less than your 470R resistor... FWIW, the typical Rocket choke is around 400 ohms. Also, one interesting note is that occasionally that screen voltage and plate voltage can be essentially the same and the amps still sound great... the HiFi guys don't generally like this but that's a different sort of ball game in itself. If you have a couple photos that would also be useful to us. thanks again, rj
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jph118
Posts: 53
Joined: Wed Apr 19, 2017 1:35 pm

Re: Rocket build probs

Post by jph118 »

Thanks guys! I'll get some pics up soon, appreciate your support. Ended up going a little leaner on the PI coupling caps, .01uFs, as well as a 250kA bass control (also added a 25kL for mids); the low end was just out of control with original values and a Weber Silver Bell 12". I built this for a friend of mine in Nashville who's been using an AC30TB for years but is tired of lugging it around, i'll deliver it to him this weekend and hope it really knocks him out. I couldn't be happier with it!
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