Hum debugging, effects loop buzzing [SOLVED]

6

6v6

New member
Hi,

I have a Egnater Rebel 20 that I recently bought off CL. Overall I'm really happy with this amp, but there are two things really bothering me:

1. Constant hum/buzz: There is a background buzzing that seems to always be present.
2. Effects loop seems broken: if I connect anything into into the return the amp starts feeding back which rapidly gets louder and louder.

I'm pretty new to guitars, amps and especially tube electronics but I am an engineer and I know my way around DMM and O-scope. I also have bench top power supply and function generator if any of that equipment can help.

I'm looking for good tutorials, training tips etc that can help me solve the two problems above and improve my general understanding of the workings of guitar amplifiers. As above, I can poke and prod around things but I'm not one to play cowboy with electronics. I'd rather learn a bit an proceed safely.

Thanks
 
Thanks for the response. I have gotten a recommendation from http://www.dougstubes.com/ but I don't want to buy until I'm sure this amp will work.

I have recently replaced v1 with a 'groovetube' (only thing I could find locally) because it died.

I don't want to spend money on a bunch of tubes only to find out that isn't the cause, or that the amp is toast, or ....

So before I wanted to isolate the problem first, is there a way I can do that without buying a new set of tubes?
 
Please excuse the necro-bump.

yes, this is still broken. no i don't have a lot of time to deal with it.

Here is what is happening:
Connecting anything (even a patch cable) between the send/return and I get super loud buzzing. I mean louder than the master knob is even set to and it seems like it would get even louder if I kept it going.

I've tried:

1. Patch cable between send & return
2. Place new 12ax7 in each of the preamp tube spots

No luck.

I'm the second owner, so of course there is no warranty.

I have seen the 'mod' that is supposed to be done but I haven't applied it, for the life of me I don't see why swapping r10 and r13 should fix this. I don't want to confound the situation.

I'm not going to take this to a tech, so I'd appreciate any advice.
 
The 1° owner could have tried to do the "mod" by himself and could have made a mistake,no :confused: ? You could check R10 & R13 ...the mod do make a difference for pedals ,eliminating the buzzing.I would check the two R's...Good luck !
 
Thanks for the response.

I did the mod tonight. The first owner definitely had not applied the modification. After doing the mod I measure 44.8kOhms at R10 and 9.8k on R13.

It makes no difference what pedals are tried. Even a straight patch cable causes the buzzing. But I have tried: tc ditto, arion sch-1, boss ds-1, russian big muff pi and a KoT clone (not that i'd use the last three in my loop). It is always the same.

This effects loop is a pretty simple circuit, so I think next I'll have to try replacing the op-amp. The op-amp used is about the cheapest version money can buy so that is my next suspect. I'll probably put a socket in where the op-amp goes so I can try different ones.

The downside here is I'm not sure how well this thing will hold up to my abuses. I don't have the best rework tools and these boards were definitely not intended for rework - real cheap POS. Terrible soldering, are these hand soldered? I haven't ever seen such crappy work from wave soldering.
 
Well I replaced the op-amp but no luck. Same buzz... At least i have a TI op-amp now? lol

Not sure what to try next. I've looked and checked there are definitely no blown caps.

The buzzing happens as soon as the 'return' side is closed, if that is any clue.

From the loop schematic I don't see anything else obvious, I haven't digested the full schematic yet, which has no values listed on it and even the ref numbers don't match the 'mod fix' schematic I was given.

Any ideas?
 
Hmm.. Looking at the schematic and I think the only place that uses the -12v rail is in the effects loop op-amp.

Could it be the -12 supply that causes it? I haven't checked the voltages...
 
Since from all the responses I can tell people are waiting, with baited breath, for the dramatic conclusion to this tale I shall continue reporting my findings.

I measured voltages on U301 and U302 (these are a pair of 79L12 regulators that generate the +/-12V rails for the effects loop. Near as I can tell they are not used by anything else.

U301P1 = -.7
U301P2 = 0
U301P3 = 16.3

U302P1 = 0
U302P2 = -17.9
U302P3 = -11.6

All values are DC voltage, while in standby.

This means that the effects loop amp is getting no positive rail. So I'm thinking to replace the 79L12 but I don't have one (of course) The output cap C301 looks pristine but my DMM cant measure ESR. The input caps also look great. I may just order everything anyways..

So, if any egnater-experts are actually listening it would be great to get some input/guidance.

++edit
But I did find an LM7812 in my junk bin..

If I put it so the 16.3 is coming on its input then it should output that +12v I need. I think ? It should go in backwards of the CJ79L12 because it is a -12v part. I'll go read some data sheets and ponder this.

thoughts?
 
/facepalm

HEY! I think they put the wrong effing part in here for U301!!! It should have been a 78L12, which would match up with what I'm measuing and put out a + supply for the 5532...
 
ring a ding ding! we have a winner!

Dear Egnater,

It is not so good to use a negative voltage regulator when trying to make a +12v rail.

:p

(just kidding)

So I swapped in my 7812 (all i had laying around) and it is working crystal clear now.

By the way, I love this amp, which you can plainly see by the effort I extended to resurrect it.

Thanks for making a great product.
 
viking22":2m9ajol6 said:
Congratulations,6V6 :thumbsup: ! Well done !
Click to expand...
Thanks viking22.

Very satisfying to have fixed it myself and kept it from a landfill.

Also reverb sounds much better in the loop.
 
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