New 18 watt lite build, faint buzz/distortion on low notes- HELP!

[rockybottom16]

I'm sure this issue as probably come up before on the forum, but I haven't had any luck finding a thread. Here's the problem: When you play the low E string, as the note is dying out there is a slight buzz/distortion sound, kind of like when your speaker has a rub and is on it's way out. Obviously, this is only noticeable when playing clean. I've read that some 5e3 builds can have this sound if there's a coupling cap too close to the tip of the input jack, some sort of parasitic. But that is not the case in this amp.

I've built this same circuit before with no issues, so this one has me over a barrel. And I am very careful regarding lead dress. So far there are only two things that I can think of that are different with this particular amp. One is that I only have about 260V on the el84 plates at the moment. I couldn't find the GZ34 I thought I had, so in the mean time I'm using a 5U4.

The other difference is that I used a used output transformer that someone gave me. The numbers indicate it was for a Blues Junior, so it should have the right specs. Maybe it has problems? Maybe that's why it was pulled in the first place?

Has anyone ever heard of a bad OT making noises like this? Any other components that might cause this? (I'm sure it's not the speaker) Other than this pesky problem, the amp sounds very good.

[DrGonz78]

Do you have a copy of the schematic to post? Also, what voltages are on the plates for the preamp tubes?

[mozz]

I don't see how a coupling cap is mounted near the input jack. Also be sure your power transformer can handle a 5U4, they take 3 amps filament while the 5ar4 And 5y3 take 2 amps. Other than that, bad tube, something not biased right, deluxes do have some flab on the low end because the coupling caps are a large value.

Edit, is this a 18watt marshall style or a 5e3?

[rockybottom16]

I guess the contacts in my multimeter were dirty, probably accounting for the low voltage readings. Output tube plates are 335V, first preamp tube plates are 121V, 2nd tube plates are 205V. Output tube cathodes 11V. The schematic I used was drawn by Mark Huss, and I found it years ago at http://18watt.com/storage/18wattLite2b.gif. The only change in my build is that I usually use a GZ34 rectifier or else diodes. I've had great results from this schematic. The only thing I did differently this time was use the second-hand blues junior type output tranny. I normally use a Weber tweed princeton wo22913 transformer.

[rockybottom16]

It's a Marshall type 18 watt lite w/el84s. Right now there's a Weber 5u4 copper cap in there, so current is not an issue. The power tranny is a Weber 02513 using the lower voltage winding. It has plenty of current for this circuit.

[Dave H]

When you play the low E string, as the note is dying out there is a slight buzz/distortion sound, kind of like when your speaker has a rub and is on it's way out. Obviously, this is only noticeable when playing clean. I've read that some 5e3 builds can have this sound if there's a coupling cap too close to the tip of the input jack, some sort of parasitic. But that is not the case in this amp.

I don't know about a coupling cap but it still could be "some sort of parasitic" oscillation. I see it has no grid stoppers on the input. You can get away with that but not always. Try a couple of grid stoppers to see if that fixes it.

[rockybottom16]

I did put grid stoppers on the grids of V1. I usually use a couple 33K, but the closest I had on hand was a couple 22K resistors so I used those. I can't imagine the slightly smaller resistance there would be causing my problem. Of course it is possible. I could add some more resistance there, but I may end up replacing the output transformer before I try altering the circuit.

I did stray from the schematic in another way. I inserted a PPI master volume. Since I use a 5E3 chassis, the output grid wires are kind of long, but I did this on 2 other builds just like this with no issues what-so-ever. Like I mentioned before, the used output transformer is the main wild card here. I just wondered if anyone had experience with a faulty O.T. causing this phenomenon.

[Chuck H]

I've heard the same thing in random amps now and then. Amps that seem stable all the way into full overdrive. Not sure if it's an oscillation or not though. I think it probably is. I haven't had an amp on my bench that does it. It's annoying as hell.

Do you have a scope?

[rockybottom16]

Annoying as hell is the best way to put it. When the other OT gets here, I think I'll loosely wire it up to see if that fixes the problem. If not, I'll commence to moving wires around and hope for the best. I guess it all can't be fun and giggles! Unfortunately I don't yet have a scope.

[Chuck H]

A few things to check out/consider...

The power amp isn't working that hard with the amp played clean as notes are decaying. I've seen a couple of different attitudes WRT trouble shooting snarky problems like this. Some guys start by suspecting the power amp while others start with the preamp. I think you should be suspecting the preamp. The input arrangement is interesting (does the problem happen in both single triode and parallel selection?)

There's twice the current running through the shared plate and shared cathode resistors on the input. Probably fine to use a 1/2 watt on the cathode, but the plate resistor should definitely be a 1 watt.

The two triodes will be interacting and they don't always have the exact same characteristics. Have you tried a different tube in the preamp?

There could be a resonant peak happening. If this is reacting anomalously with typical guitar peaks that might induce a LF distortion.?. Maybe try bumping the cathode resistor to 1k and/or reducing the bypass cap to 22uf (still big enough that you shouldn't notice a tonal difference). I've noticed this phenomenon most in amps with a preamp closer to center bias. So maybe biasing a the preamp a tad cooler could help. And changing that resistor and/or cap would also change any resonant peak. Perhaps mitigating it's reaction with typical guitar characteristics.

Small value caps have terrible LF distortion. You can actually see it on a scope as a spikey ghost wave impressed over the sine wave. This only happens with changes in LF amplitude going into the cap so I suspect it's something to do with charging/discharging. Perhaps try changing the treble cap (the 500pf) to a different type (mica, ceramic, poly). A different type will have different charge/discharge time and that would also change reaction with any resonant characteristics. Perhaps for the better.

Are your input grid stoppers ahead of or behind the grid load? They should be ahead of the grid load right at the grid pins.

[rockybottom16]

Chuck - As always I greatly appreciate your help! The problem does happen with both inputs. I have tried a different preamp tube. And the grid stoppers are mounted right on the tube pins. However, they are only 22K each. The treble cap is a little black mica unit. I have nothing different on hand to try at the moment. I just changed the preamp tube bypass cap to 22uf with no change in the problem noise. Should I assume that you have no suspicions about the second-hand OT that I used for this build?

[Chuck H]

Not yet. But I don't know what the problem is.

Did you try biasing the first stage a tad cooler?

Just for laughs you might also try bumping the preamp coupling cap to .01uf.

[rockybottom16]

Not yet. I don't have a 1K resistor, but I've got a 1.2K I can stick in there. Then I can try sticking another .0047 parellel (piggyback) on the coupling cap which will get it very nearly .01uf just to see if it makes a difference. If it does I can stick a Mallory .01uf in there. I try not to yank things off the board unless I have to. It still confounds me that the last 2 builds using this schematic and layout did not have any problems.

[Jazz P Bass]

Have we ruled out the speaker itself?

[rockybottom16]

Yeah. I've tried 2 different speakers (a Celestion G12H and an old Jensen Concert), both of which sound crystal clear with other amps.