Monkeymatic Rocket 64

Express, Liverpool, Rocket, Dirty Little Monster, etc.

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xtian
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Re: Monkeymatic Rocket 64

Post by xtian »

didit wrote: Wed Oct 11, 2017 11:40 pm Unbolt power transformer, and rotate to the limit wires allow on all three axis, pitch yaw and roll. If hum changes notably you know what's required.
Thx, but i did placement very carefully by ear. Also, you can observe the level of noise from the iron by powering up with no tubes. Mine is acceptable.
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sluckey
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Re: Monkeymatic Rocket 64

Post by sluckey »

You isolated the hum to the output stage when you pulled the PI tube. Hum in a push/pull amp can be caused by an imbalance of current through the OT. The push/pull amp uses high ripple B+ (10VAC is not uncommon) but a perfectly balanced current through each half of a perfectly balanced OT will theoretically totally cancel the ripple (hum) due to the common mode rejection characteristic of the push/pull circuit.

But if each side of the push/pull amp are not equal some of the ripple will remain. This can happen when the tubes are not matched ***OR*** if each side of the primary windings are not matched.

If you are confident that the close proximity of your OT and PT are not causing the hum, here are some checks I would do...

1. Tubes first. Pull two of the output tubes so you only have one tube on each side of the OT primary. This will narrow down the number of possible culprits. If you had two separate cathode resistors you could quickly determine if the current is equal on both sides of the OT.

2. Now the OT. Check the resistance of each primary winding plate lead to the center tap. Should be approx. the same and should be approx. half of the plate to plate resistance. A better check on the balance of the OT primary would be to use your variac to inject about 3 or 4 VAC into the speaker jack (amp turned off and no speaker connected). Now measure the voltage from each primary plate lead to the center tap. Voltages should be equal (not important what the exact voltage is). If the voltages are not equal then the OT will hum.
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xtian
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Re: Monkeymatic Rocket 64

Post by xtian »

I measured 69R1 vs 77R on the OT primary.
Dissipation at 60.8mA vs 51mA.

Then I swapped the output tubes right to left, and got
Dissipation at 52.1mA vs 59.7mA.

They're mismatched, but not much. Should be fine, in my experience. Yours?

THE GOOD NEWS is that the amp sounds stellar with my Weber Mass attenuator. The tone stack is very nice, with a behavior that I didn't notice on the first two Rockets I built. When turned clockwise past noon, the mids drop away, giving a very Fendery scoop, without being too bass heavy.
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sluckey
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Re: Monkeymatic Rocket 64

Post by sluckey »

That mismatch should be acceptable. So, is your hum gone?
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RJ Guitars
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Re: Monkeymatic Rocket 64

Post by RJ Guitars »

xtian wrote: Fri Oct 13, 2017 2:23 am I measured 69R1 vs 77R on the OT primary.
Dissipation at 60.8mA vs 51mA.

Then I swapped the output tubes right to left, and got
Dissipation at 52.1mA vs 59.7mA.

They're mismatched, but not much. Should be fine, in my experience. Yours?

THE GOOD NEWS is that the amp sounds stellar with my Weber Mass attenuator. The tone stack is very nice, with a behavior that I didn't notice on the first two Rockets I built. When turned clockwise past noon, the mids drop away, giving a very Fendery scoop, without being too bass heavy.
Congrats Aaron! Sounds like you are almost there.

I am curious if it's possible to balance an unbalanced OT using power tube pairs that are similarly but oppositely unbalanced and have a good tone and reliability from the amp? I'm not saying this is a strategy to aim for in your build plans but it might just create some magic in the event you find yourself with an unbalanced OT. I know this presumes that the tubes won't drift over time but it gives you a tool to work with. FWIW There are some who like the flavor of a little tube mismatch anyway as long as it's not enough to hum.

The Rocket (Vox) TS does have a huge mid scoop, especially if you dime the tone knobs. I read in Rob Robinette article on Fender tonestacks that the mid scoop on most designs was intentional to balance the tendencies for most pickups to be mid focused. I had worked hard to get that out of my tonestack using the Duncan amps TS Calculator... however once I succeeded I didn't like how it sounded and eventually went with something closer to the original Fender design.
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xtian
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Re: Monkeymatic Rocket 64

Post by xtian »

Sorry to disappoint, but my report meant to say, even after swapping power tubes around, it made NO difference to the level of hum.

Using the attenuator made the hum far less noticeable. The overall signal-to-noise ratio is OK—the amp is LOUD. But when you turn it on in a quiet room, the hum is quite obvious, and this is not something I normally allow in my builds!

I don’t have another suitable PT, but I can swap the OT for one of my 60w Schumacher 4K OTs. I may do that this weekend.
I build and repair tube amps. http://amps.monkeymatic.com
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M Fowler
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Re: Monkeymatic Rocket 64

Post by M Fowler »

I built a Princeton Reverb that started humming so I changed filter caps and replaced coupling caps, checked wiring repeatedly.

Naturally changed tubes but in the end I replaced the PT and all is well again.

This amp was running for 3-4 years?

Hope you resolved your amp woes.

Mark
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xtian
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Re: Monkeymatic Rocket 64

Post by xtian »

Already changed filter caps and coupling caps, thanks Mark. Just ordering a new PT to try. Also have another OT to try.
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didit
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Re: Monkeymatic Rocket 64

Post by didit »

xtian wrote: Thu Oct 12, 2017 12:49 am
didit wrote: Wed Oct 11, 2017 11:40 pm Unbolt power transformer, and rotate to the limit wires allow on all three axis, pitch yaw and roll. If hum changes notably you know what's required.
Thx, but i did placement very carefully by ear. Also, you can observe the level of noise from the iron by powering up with no tubes. Mine is acceptable.
Thanks Aaron. Curious though. Did you try face down, as it was designed?

Best .. Ian
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xtian
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Re: Monkeymatic Rocket 64

Post by xtian »

No, I did not. But during fitting stage, I did use headphone trick to listen to various orientations, including lay down.
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didit
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Re: Monkeymatic Rocket 64

Post by didit »

Thanks sir.

More than idle curiosity. Half dozen larger Baldwin multichannel chassis from the same era here in my junk shelf. Each about 80W, with basically same output components as this. Static resistance off one sample output transformer - 144 ohms across the primary, definitely imbalanced with 76 & 68 ohms from centre. Mismatch of about 10%. A common characteristic of simple wound iron. Fits with sluckey's hypothesis. Lacking ambition at the moment to isolate and wire up a dynamic test.

Hard at this late stage of design to resolve inductive noise coupling. However, it is more than just the two transformers at play. There is magnetic material in tubes. Cathode rod is nearly always nickel-based.

Keep at it. There's solution there somehow.

Best .. Ian
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xtian
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Re: Monkeymatic Rocket 64

Post by xtian »

Thanks, Ian. I'll be trying out new iron near the end of the week.
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xtian
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Re: Monkeymatic Rocket 64

Post by xtian »

I'm throwing in the towel on this one. I tried a couple more drastic measures: tacked in a new 60w OT, and changed tube recto for solid state, both resulted in no change to base hum. (The substitute OT wasn't bolted to the chassis, so I could easily move it around, and even this did not influence the hum.) Also tried a third set of matched EL84s, with no change.

So I went ahead with plan B: installed an attenuator based on RJ's Power Broker. You can see it in the photos. The bothersome hum disappears along with the noise floor when running thru the attenuator. Problem masked.

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didit
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Re: Monkeymatic Rocket 64

Post by didit »

Just a thought. Expect your power transformer is flux band free. No personal experience, but some hi-fi DIYers claim success retrofitting copper foil bands when it's missing. Carefully. Or just leave that towel wherever it landed and move on.

Best .. Ian
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xtian
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Re: Monkeymatic Rocket 64

Post by xtian »

I fixed it.

Image

I added NFB to my Rocket. (Yes, I know I may lose my membership over this.) This sorted the hum nicely.
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