Recto Hum - Jemsite
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post #1 of 17 (permalink) Old 01-18-2003, 11:59 PM Thread Starter
rgr
 
Join Date: Dec 2000
Location: Longmont, Colorado
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Recto Hum

I have a question for all you Rectumfrier users out there. I recently picked up a Single Recto head, 50W is plenty for me and it's a loud beast as it is. But... It has a totally annoying AC hum, really loud, supposedly the shop that got it in changed the 6L6s in it (the new ones don't have a name on them so I don't know what they are) because the other ones weren't working. It sounds great but the hum is loud enough when you aren't playing to be totally annoying and pretty much make it unusable for live playing, much less recording. I actually called Mesa and got a new output transformer for it, didn't help, fortunately it didn't cost very much and Tim at Mesa was really cool, even e-mailed me the schematics. I guessed at the output tranny right off since it came into the shop used and the power tubes weren't working.

So, do you guys experince alot of noise with your rectos? I expect some hiss because of the gain, but not the low frequency AC hum. Gonna get it checked more Monday, I'm giving up after checking and touching up all the solder connections on the filter caps and "virtual" center tap on the heaters. Guess I just want to know if the amps are just that noisy or if I still haven't found the problem.

Thanks,
Roger
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post #2 of 17 (permalink) Old 01-19-2003, 12:05 AM
 
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Funny, when I saw your subject line, I glanced at it, omitted a few letters and just saw "Rectum".
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post #3 of 17 (permalink) Old 01-19-2003, 12:42 AM
 
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Re: Recto Hum

Quote:
Originally Posted by rgr
I have a question for all you Rectumfrier users out there. I recently picked up a Single Recto head, 50W is plenty for me and it's a loud beast as it is. But... It has a totally annoying AC hum, really loud, supposedly the shop that got it in changed the 6L6s in it (the new ones don't have a name on them so I don't know what they are) because the other ones weren't working. It sounds great but the hum is loud enough when you aren't playing to be totally annoying and pretty much make it unusable for live playing, much less recording. I actually called Mesa and got a new output transformer for it, didn't help, fortunately it didn't cost very much and Tim at Mesa was really cool, even e-mailed me the schematics. I guessed at the output tranny right off since it came into the shop used and the power tubes weren't working.

So, do you guys experince alot of noise with your rectos? I expect some hiss because of the gain, but not the low frequency AC hum. Gonna get it checked more Monday, I'm giving up after checking and touching up all the solder connections on the filter caps and "virtual" center tap on the heaters. Guess I just want to know if the amps are just that noisy or if I still haven't found the problem.

Thanks,
Roger
A couple of more things to check.
1. Unplug your guitar. Does it hum? I know this sounds stupid, but that's the first thing you have to check
2. Make sure both "shoulders" of the power amp work OK. Check the biasing, make sure both tubes work. I had an amp with a defective current sensing resistor (used to bias the amp) and it had some serious hum until I found it out. Basically what happens is when this resistor goes bad (if there is such a resistor in rectos), a half of your power amp doesn't work. Usually what an inexperienced owner will do, he will bias another half too hot, and get himself an almost "class A" single-ended power amp. Under the normal conditions, two parts of a push-pull cancel the hum for each other. Take one shoulder out - the hum of another comes out unsupressed.
3. Depending on how old this amp is, it _might_ need a cap job. But this is really the last resort.

Try the first two things, and let us know what you find out.
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post #4 of 17 (permalink) Old 01-19-2003, 12:51 AM
 
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Its likely a tube issue...
search for an online rectifier manual or read a copy of one at GC if they let you..
it will tell you what to check for...
but thats what it sounds like..
as for it being a biasing issue...
thats not possible on boogies...
their patended engineering on tube amps is for that very reason...
there are no bias adjustments in them...
so its likely a tube issue or something else...

First thing I would do is go buy some new tubes from Boogie(recommended)
Esp since you bought the amp used. general rule of thumb to retube used amps... and always keep and extra set of power tubes handy..
if its the newer boogie model and it has EL 34's in, try switching to 6L6's.. they are strenuously recommended over the EL-34's by Boogie..

just some tidbits to chew on...
As for my Dual Rec solo head... No noise.. even with it cranked to the max....



scott
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post #5 of 17 (permalink) Old 01-19-2003, 01:12 AM
 
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This "patented engineering" is just usual design, but tubes are biased really cold and the plate voltage is really high. So in a wide range of transconductance and gain your tubes are not biased too hot.

The importance of having the output tubes carefully matched is very overhyped. Think about it. It is IMPOSSIBLE to wind a perfectly balanced output transformer. Either inductance or impedance or both will be different in 2 halves of the primary. So your carefully matched tubes will be working in different conditions anyway due to this often overlooked factor. I'm not saying that you can mix new and old tubes in the amp. I'm just saying that two brand new tubes can be used without matching.

I know at least 2 people who have checked those "matched" tubes from Mesa in their tube testers. They were so unmatched it wasn't even funny.

And there there should be a biasing trimpot buried in the guts of that recto somewhere. No matter what they say about their design, they have to adjust the bias at the factory.

Having said that, I want to add that I wasn't talking about biasing. I was saying that one half of his power amp might be completely turned off due to resistor failure. That's exactly the reason why they've suspected the output tranny was a problem. The same symptoms would exist if one part of the primary doesn't work.

It's also worth checking the screen resistors while you're at it. In case of tube failure thy burn out before the fuse.
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post #6 of 17 (permalink) Old 01-19-2003, 01:19 AM
 
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oh no, i agree with you on the resistor and tube stuff..
I was merely pointing out some basic info for him...

Thats good research tho
and i have heard the same story locally here on the 'matched tube' issues.. ...
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post #7 of 17 (permalink) Old 01-19-2003, 04:03 PM Thread Starter
rgr
 
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Thanks for the suggestions, and I know tube amps pretty well, I'm an EE and have built amps in the past but this one is driving me nuts.

A) I may pick up some tubes tomorrow but I don't think that's the problem. BTW, I prefer Svets, probably the best power tubes you can get today, followed by JJ, don't know where the Mesas come from. Tube matching is way overhyped, kinda like true bypass in pedals (when I get my pedal business going I'll put up a dissertation on that on my website, although I do use true bypass, there are times where it is good and times when it is bad for tone). The tubes in the amp are new, according to the shop, but they have no markings so I don't know where they came from (potential source of questions).

B) Mesa uses fixed resistors for the bias, no bias adjustment pot, I can make an arguement for either, the fixed bias makes tube changes easier but I am going to check the bias circuit, and yeah, they do run the bias a little on the cold side. Since it's fixed, I guess they want to be on the safe side.

C) Yes, it hums like a cheap hooker on Sunset even without a guitar cable plugged in (Mesa uses a shorting plug that shorts the input jack to ground when a cable isn't plugged in).

D) I have changed the output tranny, since the shop got it in with bad tubes I didn't want to take a chance that the previous owner had accidentally fried the output section by not having a speaker load hooked up, hence, why he got rid of it so cheap.

E) After some surfing around, this is apparently not that unusual an occurrence on these amps. Some of the construction techniques Mesa has used have me asking "What the h**l were they thinking?" The main board is a pain to get to, so servicability sux. I may layout a board for them to mount the controls to and send it to them. It's insane what you have to go through to get to the solder side of the board. This is also an older model so I don't know if they have rectified (no pun intended) some of these issues. I'm going to check for ground loops on it today, may put in a central ground and run lines to different ground points on the board. The next issue would be possibly the choke but I'm leaning more toward a layout/ground loop problem right now.

F) Twohands,
I know what causes Rectum Hum... Mexican food.

Thanks for the ideas, I'll let you know what I find out.

Roger
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post #8 of 17 (permalink) Old 01-19-2003, 04:28 PM
 
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Nah, I'm almost 100% sure it's not ground loop. If it was ground loop, every single recto would have had this hum, it's PCB construction, so if there is a problem it's 100% repeatable. If there are ground wires going to different parts of the board (like in my TOL100), it's good idea to check for oxidation though if ground wires are connected using some kind of solderless connectors.

Another thing you might want to try is the "divide and conquer" methodology. Start at the first preamp tube and try pulling them out one by one (turn the power off before removing the tubes!) and see if hum remains. If you pull all preamp tubes and phase splitter tube (the one that's closest to the power tubes) and the hum still remains it's either filtering or power amp failure. If it ceizes after you pull a tube, try replacing this tube with another one. The hum should stop. Now go backwards and put the tubes back.
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post #9 of 17 (permalink) Old 01-19-2003, 04:35 PM
 
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BTW, the worst serviceability I've ever seen is in TOL100 head. You have to unscrew around 3 dozen screws and completely disassemble the chassis to see the back of the PCB. I had to do that to replace two screen resistors (fried) and current sensing resistor. Even biasing requires the removal of one (small) PCB. Now all four screen resistors are mounted on standoffs and are replaceable without removing the PCB and current sensing resistors are replaced with 1W metal film types.
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post #10 of 17 (permalink) Old 01-19-2003, 06:04 PM Thread Starter
rgr
 
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I've tried replacing tubes, yeah, the noise is lower when you remove the first preamp tube, but it doesn't change when you replace it with another. Since that is the high gain tube, it makes sense if it is noise induced from AC on the DC line. The PCB layout sucks on this one, and like I said, I'm not the first person with this problem, read some of the HC reviews on the Single Rect. The boards are also different on the Dual and Triple Rect as well as the new Single so, I can't rule out the board, yet. They could have laid the heater traces too close to the ground or DC lines, maybe that's why they changed the board, and since it's gotta change, they went ahead and added another resistor and/or cap for the "Raw" setting on the high gain channel. Stuff like that happens all the time, "we fixed a problem but let's add something new so it looks like we added a feature." MicroSoft anyone? The ground on the PCB makes a big C around the board on mine, a ground plane would have been much, much better, the "C" layout is asking for trouble (I used to lay out 6 layer mircoprocessor boards). A ground plane would have been a minimal extra charge on the board but well worth it. I'm going to check the bias circuit out tonight and if that's not it, call Mesa tomorrow to try to get the real scoop.

Thanks for the help though, I'll let you know,
Roger
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post #11 of 17 (permalink) Old 01-19-2003, 06:33 PM
 
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So far it sounds like a bad cap in the power supply or something like that. The symptoms you described suggest poor filtering in the power supply. Another thing to try (just to make sure it's not filaments/tubes) is to temporarily disconnect the filament wires from the PCB and connect a 6V lantern battery as a filament supply.
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post #12 of 17 (permalink) Old 01-19-2003, 08:38 PM Thread Starter
rgr
 
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Yep, I'm starting to agree with you, the bias voltage is fine, and even if I don't agree with the way the PCB is laid out, it should have made nasty noises when they checked it out in the factory. The amp was built in '98 or '99, so it's very early for a filter cap to go out, if it was an old Marshall I would've probably replaced those even if it didn't hum.

I'll try to find some caps tomorrow, thanks again.

Roger
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post #13 of 17 (permalink) Old 01-20-2003, 08:49 AM
 
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As far as i know that is somewhat normal for high gain amps.
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post #14 of 17 (permalink) Old 01-20-2003, 11:26 AM Thread Starter
rgr
 
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High gain or not doesn't really affect the filter caps, and, yeah, it's quite early for one to go bad, if it is rated properly. But, the voltage across the caps is at least 350V and the caps are rated for 300V, hmmm, not a good choice in my book, so yeah, they probably have failed.

Roger

****Okay, ignore that rant, looking at the schematics the caps are stacked in series so their voltage handling is doubled. That's what I get for looking at this stuff while the Raiders are tromping my homeboys.
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post #15 of 17 (permalink) Old 01-25-2003, 03:19 AM Thread Starter
rgr
 
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Okay, I found the problem, here's an e-mail I sent to a guy in Europe having the same problem, I'm cutting and pasting so I don't have to type it twice:

I found the hum problem on the Recto, annoyingly simple, also annoying that it is allowed to happen in the first place. After replacing the filter caps, output transformer and a complete retube I was really fed up with the amp. So I started checking individual components on the board, there was one resistor that was reading like a short to ground, even though it didn't look burnt or anything. So, begrudgingly, I took all the pots, jacks and switches off the front panel and unsoldered the LEDs so I could get to the other side of the board (don't worry, you don't have to do this). When I started checking the same resistor, it checked out fine, I was totally aggravated by this time. So I started putting it back together and noticed that the footswitch jack had a black washer on the back, on closer inspection it was a phenolic (non-conductive) washer. Normally all jacks ground the ring to the chassis as well as ground on the board. So I pulled out the schematics I got from Mesa and started looking at the switching matrix, I hadn't looked at this one at all, didn't think it would have anything to do with the problem. Slight backtrack, to keep the first gain stage quieter, they run the heater of the first preamp tube off of DC, across a +3V to -3V line they have made off of the 6 VAC heater lines. They also use the +3VDC and -3VDC in the switching circuits. Turns out the -3VDC line runs to the ring of the footswitch jack, the plastic washer on the back keeps the jack from touching the chassis but they use (at least on mine) a metal washer and nut on the front of the jack. If you tighten the nut too much it will ground out just enough through the powder coat paint to ground out the -3V line and introduce a ton of noise (the 3V supplies aren't super clean DC) into the first preamp tube.

So the fix is to make sure the footswitch jack is not grounding to the chassis, I would recommend using plastic washers on both sides of it if you can find some that fit. In a pinch you can probably wrap some electrical tape around the jack where it goes through the hole in the chassis, and most importantly, don't overtighten the jack. I know sometimes it's nice to go through and make sure things are nice and snug but don't overtighten the footswitch jack. My amp is fine now, it's a high gain amp so it's going to be noisy but it doesn't have the overpowering hum now.
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