Why does my Elk amp sound like a dishwasher?

[poweredbypoptarts]

Hello everyone,

My name is Boris and this is my first post to the forum.

I recently bought an Elk Custom Amp 30 with a 2x10" speaker cabinet. The amp sounds great and I love the vintage tone. I know these amps are Fender-ish clones in appearance but have their own unique circuit design. My only problem with this amp is all the background noise it has. It sounds like a dishwasher or a record player that has reached the end of the record. Adjusting any of the knobs does not affect the warbly background noise. The noise is always there.

I think my only solution is to slowly rebuild the amp. Some of the caps have seemingly been replaced by the previous owner (or maybe even the owner before him?) but not all of them. Most of the old wire wound resistors are still in place (95%+) but I read that their values can drift over time as the wire undergoes heat stresses and ages. I tried to find a schematic and all I can find is one that is labeled Elk ES-30. This schematic seems to be the right one but I haven't compared 100% of the circuitry against it. A copy is attached to this post. The schematic shows two 7189 power tubes but I have two EL84 tubes. I read that the 7189 tubes are similar to the EL84 tubes but the 7189 tube run a bit hotter. The EL84 tubes look new with fresh silkscreening but the other tubes look older.

I'd like to rebuild this amp in a logical order. First, I would like to get rid of the warbly hiss. What should I attack first? Should I replace the rest of the caps? Should I get real 7189 tubes? Second, I'd like to restore the amp to the right components. What I mean is that the previous owner messed with some parts of the circuit. Some resistors were changed. Where he didn't have the exact value he would take two and tie them in parallel (i.e. needed a 12k so he tied two 28k units in parallel to get 14k, a close value, etc). I was planning to go through the schematic and make a bill of materials so I can order all the replacement resistors in one swoop from a place like Digi-Key. What power rating should I get? Should I get 1/2W resistors? Third, what does "(2L)" next to a resistor value mean? Was that the Japanese way to say 2 Watts?

Thanks!!!

-=- Boris

[kleuck]

Hello First : measure the plate curret of your EL84, it may be too high if your amp calls for 7189. Then replace all the tubes by new ones, one by one, the noise comes most of the time from bad/old tubes. Second : if the amp is old (i don't know these) and the high voltage filtering caps have not been replaced, replace them ALL. After that, the resistors : it's not an issue to use two resistors in parallel, nor to use a slightly different value (except for the cathode maybe) so it's not necessary to replace them all. Carbon comp ones can drift a lot from age, temperature (i had a CC of 220K measured at 370K !!!), moist, and can be noisy, so measure them, and replace all the drifted ones, replace all of them in the first stages of the amp with modern ones and you will have quiet amp.

[poweredbypoptarts]

Hi Kleuck,

Thanks for the reply. I got into my amp a bit more today. Here are some pictures of the guts.

First up is the power supply. It's pretty straightforward. Someone has already installed a 3-prong cord which is nice. I noticed that the fused is wired ahead of the power switch. That means one end of the fuse is always connected to live 120V. After the fuse comes the power switch. I will swap this around so that the power switch is the first component in the path. A few of the wires seem a little longer than they ought to be too. Anyways, here is the picture.



Next up is an example of the some of the soldering done previously. This is one example where someone parallel'ed two resistors plus a trim pot so that they can get 12k. They actually got 14k instead. I'm sure that it is functionally fine but there is something inside of me that wants to clean this up.



In the next picture you can see part of the board with all the components. Yes, there is a bunch of stuff on the other side of the board. Lifting it up will not be easy because there are taunt wires connecting the board to pots, tubes, etc. Some caps have been replaced. Others, no. There are lots of Atlas wire wound resistors. All I see is a value but no power rating. Are these all 1/2 Watt?



Finally, here is the other end of the board.



I have to find a way to cut into the plate signal line and insert my digital meter to read the current. At that point I will probably be ready to flip the board over and see whats on the other side of it.

First, I want to fix up the power supply path and make it a bit safer.

-=- Boris

[Steve Conner]

In the second picture, the large green 100k resistor at the right of the picture seems to be cracked almost in half. I'd try replacing that to start with. Also try cleaning out the soap dispenser.

[g1]

I noticed that the fused is wired ahead of the power switch. That means one end of the fuse is always connected to live 120V. After the fuse comes the power switch. I will swap this around so that the power switch is the first component in the path.

Don't change it. It is wired properly. The first place the AC hot should go is the back end of the fuse holder as your picture shows. Thiis way if the fuse is removed, NOTHING (like the switch for example) is connected to the AC.

[poweredbypoptarts]

g-one,

Thanks for the reply! After I think about it some more I agree with you. It is wired correctly. The fuse would need to protect the switch too. What if there's a funky short between the switch and ground? Yikes! The fuse would need to protect against that scenario.

Steve,

The "crack" is actually some sort of white thermal compound. It's splattered on a few other components and in a few other places. Yes, the amp is filthy! The dirt feels more like an oily film. I have a feeling the best way is to pick a small area and clean it and around it very carefully with a small cloth and some sort of solvent.

-=- Boris

[Dave Curtis, dB AudioTech]

FWIW, those aren't wirewound resistors; the process deposits carbon on a ceramic tube and then a certain amount of it is etched away (in a spiral) to obtain the resistance value. I had/have the same resistors in my old Univox amps. If they were WW, I'd advise keeping them in (low drift, least noise, etc), but they're nothing special. A plate (or cathode) resistor or three might be making the amp impersonate the aforementioned kitchen appliance (if you've ruled out tubes).

That white stuff is put on the screw threads; low-tech LocTite (think: fingernail polish). It keeps them from loosening from vibration.

-DC

[poweredbypoptarts]

Dave,

Thanks for the reply! Yes, it must be Loc-Tite because the compound is hard. I just remember the white thermal grease I used to use to attach heat sinks on my processors back the 90s. The thermal grease shouldn't be hard, right?

I have been looking over the schematic more and more trying to figure out what the "L" by the 100k, 12k, and 25k-30k resistors represents. Can someone tell me if I right or wrong here?

L is typically the variable for inductance but the units are Henries, H. You would not put down 2L or 1/2L to represent an inductance value. When I follow the signals from these resistors with the L designation I see that they seemingly set or control a plate voltage or a bias. Then it dawned on me. The value shown is the typical nominal value but it can be adjusted by a potentiometer in parallel and the range of adjustment is given by the L designation. So, the 100k resistor with a 2L should be adjustable by 2x (i.e. up to 200k), the 12k resistor should be adjustable by 1/2x (i.e. up to 18k), etc. I can also adjust downward. That's all I could come up with.

The schematic I am using is attached to the first post of this thread.

Thanks!

-=- Boris

[Dave Curtis, dB AudioTech]

They're not drawn like a variable resistor (like the one in the vibrato ckt); I'd guess the L represents a wattage rating, just a guess, though. It looks like the small trim pot was added to make the bias adjustable.

[poweredbypoptarts]

I found a picture of what I think those resistors are suppose to look like. It's in eBay auction number 190671534016. I made a screen capture of the auction because eBay has a nifty zoom feature for pictures that helps show some details.



I think a potentiometer would be functionally equivalent but back then I bet they drifted. You could set a value, like 102.1k-Ohm, and then after a month it would be 104.9k-Ohm because the wiper drifted. With the locking ring it had less chance of drifting. Knowing that these are hard to adjust I bet Elk/Miyuki used two resistors like this. The 100k unit is the "course" adjustment and the 12k unit is the "fine" adjustment.

What do you think? I was pretty proud of myself for dreaming that up!

-=- Boris

[52 Bill]

What I find interesting is that according to the schematic, the phase inverter is split between two different tubes. Is this the way that the amp is actually wired?

[poweredbypoptarts]

Hi 52 Bill,

Hmmm... I hadn't noticed that before because I was so fixated on the power supply circuitry. Let me see if I can trace the signal path on the schematic first. Then, after this message I will open up the amp again.

My guitar signal comes in on one of the 1/4" jacks with the 70k resistors. The signal gets gained up a bit by the first 12AX7 tube. Then the signal goes through two tone control circuits and the volume adjustment. The signal then bumps into the second 12AX7 stage. After going through the 0.02 uF cap the signal goes to the reverb driver. It looks like the reverb signal is always being generated but the foot switch either switches to ground or lets it go through. The reverb is driver by the first 6AQ8 before being gained up again by another 12AX7. Here's where I get confused and my limitations shine through. I think I see the tremolo circuit in the upper right. I thought the tremolo would be in series with the signal path but it looks like it is mixed in parallel through a 1.5 M-Ohm and a 220k resistor. Then I think I see the phase splitter. It is split between a 6AQ8 up top and another 12AX7 down below. Those tubes feed the 7189 tubes which then connect to the step down transformer.

Did I get that right?

Why are some of the capacitor symbols shaded between their plates and other are not? For exampl, the 40uF 450V symbols have shading while many others don't. Does the shading mean those capacitors would be in one of those multi-cap can packages?

And do I really only have one polarized cap according to the schematic? My amp has a few inside of it.

-=- Boris

[52 Bill]

Then I think I see the phase splitter. It is split between a 6AQ8 up top and another 12AX7 down below. Those tubes feed the 7189 tubes which then connect to the step down transformer.

Did I get that right?

Yes

Why are some of the capacitor symbols shaded between their plates and other are not? For exampl, the 40uF 450V symbols have shading while many others don't. Does the shading mean those capacitors would be in one of those multi-cap can packages?

And do I really only have one polarized cap according to the schematic? My amp has a few inside of it.

The shaded caps are polarized caps (electrolytics), the ones without are low value non polarized types. All of the polarized caps are negative side to ground except the one that is marked as positive to ground in the bias supply. That's why it's the only one marked with a plus and minus sign.

As for the "L" designation on the resistors, I'd agree with Dave that it represents a wattage rating of some kind. That's why some of the designated ones are larger than the others.

[Dave Curtis, dB AudioTech]

"I bet Elk/Miyuki used two resistors like this."

I don't think so. They used the same resistors as they used in the rest of the amp. The 100K is larger, right? 2L = 2W, although I wouldn't think you'd need a 2W dropping resistor for the bias supply. The eBay pic shows wirewound resistors, the ones in your amp are not. I'd change the power supply or at least the plate resistors to flameproof metal types; don't bother with the variable wirewounds. Consider keeping the small trimpot/resistors someone put in the bias circuit (they replaced the 12K with it)...

-DC

[poweredbypoptarts]

Dave and 52 Bill,

Thanks for both of your replies and for helping me understand the schematic better. I started updating and annotating the schematic to reflect what is in the amp now and to add some extra detail. This is all new to me still so I may have made some mistakes in my annotation. Of course, I would like to continue updating the schematic so this is far from done! I want to add the polarization to the shaded caps and that will be another revision.

I'm going to add another attachment to this thread and it will be a PDF of the updated schematic. Hopefully it will come through just fine. I am using Neo Office to update it so some of the graphics and text may look a bit goofy.

elk es30 schem with notes 2012 11 07.pdf

-=- Boris