1959 Plexi clone sounds harsh after blowing the HT fuse

[Mrk200]

Last winter I built a 1959 Plexi clone using the Valvestorm kit, and it sounded great until yesterday. I just acquired a Two Notes Torpedo Studio, and accidentally left the impedance switch on the Two Notes at 4 ohms while the amp was set at 8 ohm. About 10 minutes into messing around, the HT fuse blew.

I replaced the fuse, and the amp comes on fine, but there is a high frequency harshness when playing. It's particularly noticeable when striking chords, using a higher output (bridge) pickups. It's subtle when using a single coil neck pickup.

I've heard this exact sound before on this amp, and that time it was a bad bias capacitor. So I replaced both of them, but it didn't help. I've tried replacing all tubes, and the screen resistors, and all the voltages I checked were fine, including the bias voltage.

Before I purchase a oscilloscope and signal generator to track down the cause, is there anything else worth checking?

[nosaj]

Last winter I built a 1959 Plexi clone using the Valvestorm kit, and it sounded great until yesterday. I just acquired a Two Notes Torpedo Studio, and accidentally left the impedance switch on the Two Notes at 4 ohms while the amp was set at 8 ohm. About 10 minutes into messing around, the HT fuse blew.

I replaced the fuse, and the amp comes on fine, but there is a high frequency harshness when playing. It's particularly noticeable when striking chords, using a higher output (bridge) pickups. It's subtle when using a single coil neck pickup.

I've heard this exact sound before on this amp, and that time it was a bad bias capacitor. So I replaced both of them, but it didn't help. I've tried replacing all tubes, and the screen resistors, and all the voltages I checked were fine, including the bias voltage.

Before I purchase a oscilloscope and signal generator to track down the cause, is there anything else worth checking?

What about grid resistors?

It helps prevent high frequency parasitic oscillation in the tube itself
It helps prevent radio frequencies from getting into the input stage, where they can be rectified and lowpass filtered (AM detection) and become audible at the amplifier output
It can limit grid current when the tube is driven into the positive grid region, which helps in preventing "blocking" distortion

nosaj

[Mrk200]

I don't have extra 5.6k resistors to replace the grids, so I'll need to order some. I did measure their resistance and inspect them, and they seem fine.

[Chuck H]

Nothing about playing into a 100% mismatch should have caused your amp to blow a fuse if all the parts were in good condition and properly rated. That said... If you were cranking the amp and the tubes were already stressed the lower load setting on the Two Notes may have caused the tubes to draw excess current. It should be noted that the Two Notes unit is rated for 200W peak and it's entirely possible for a 1959 to exceed that on peaks when clipping.

Replacing the bias caps because that's what it was last time was frivolous. As was replacing the screen grid resistors unless they measured out of spec or showed visible charring. As would be replacing the control grid resistors if they all measure fine. Throwing parts at a problem without knowing what the problem is is a good way to make more problems and confuse the repair effort.

Yes, a scope would be a good thing to have about now. But you don't absolutely need one most of the time.

High frequency harshness is a somewhat subjective failure description considering that the amp has done it before and all voltages and components test fine. If I can glean anything from your problem and post content it would be this...

There is NOTHING about a bias capacitor other than going completely open that should cause harshness ONLY. Even then it's suspect because I would actually expect the opposite effect due to a small amount of NFB between the grid signals. That or just the addition of some buzz/hum. My best guess is that other things were changed in the process last time. Like tubes or the actual bias setting. An inadvertently repaired cold solder joint. Maybe even a small change in bias circuit resistance because of heat applied during the cap change. In any case it's probably the bias that changed. So regarding the problem this time, we could start by looking there. How could an overcurrent condition in the tubes have changed the bias??? It shouldn't. Not much anyway. But it's possible the stress on the tubes altered their ideal bias enough to allow the problem to manifest. A real stretch for me, but I'm rolling with it. Changing tubes and the affect on bias could be hit or miss WRT the problem IF it's bias related. You mentioned that you have good bias voltage, but that doesn't tell us much because the ideal voltage is what may have changed for the old tubes and the new tubes may also have a different ideal bias voltage. What we need to know is the bias current. Chances are good that it's within a safe working range for the tubes, but that's not the whole story. If it is a bias condition causing your perception of harshness you may need to experiment within a safe bias current RANGE with any given set of tubes to determine if the problem gets better or worse.

The first thing you need to do is identify that there is equal bias voltage on all sockets. I think you've done this. You'll need to set up a way to read bias current. There are a few ways to do it and you may already have the amp set up for that or you may have a preferred method. That's it. Set the bias on the cool side and play. See if the amp sounds wrong. Set the bias on the hot side and try again. Note the difference and set the bias accordingly. If this does not turn out to be your issue I only have one other notion.

If you have a filtering issue there may be some ugly interactions happening to the signal that aren't being entirely filtered by the power supply.

Oh, there may be one other thing. I've heard that failing power supply diodes can cause harsh overtones. I don't think any extra current from a mismatched load would fail those diodes, but who knows.

Please don't change anymore parts without knowing that they are faulty. It's like losing your wallet in the living room but looking for it in the kitchen because the light is better.

[bob p]

> ... in the kitchen because the light is better.

I agree that blindly hanging parts is not a useful diagnostic approach. Blindly throwing parts at the problem means that someone does not understand the amp well enough to know where to look for the problem.

OP, I'm having trouble telling exactly what you're saying about the amp because your description is inconsistent. In the first paragraph you say that the amp sounded great until yesterday, and in the third paragraph you say that you've heard the ugly distortion before. If you could provide any additional information to clarify how/when you've heard this distortion before, that might be helpful.

The beginning of your post reads like you're saying that the new fuse is what makes the amp sound different. There are guru men who claim that HT fuses effect tone because the HT fuse is in your signal path. If you think that theory is valid then the first step to debunk that theory would be to check the fuse to verify that is of the proper type and value, clean the fuse, the fuseholder, and try a different fuse.

Later in your post you indirectly admit that the fuse can't be the problem, by admitting that you've heard the same distortion before, when the previous fuse was still in-place. Obviously, the fuse is not the problem. More likely you are stressing other components with your attenuator.

Checking bias is always a good idea. Rather than blindly hanging parts, let's use the scientific method to address the problem. You can still isolate the problem even though you don't have a scope. To do this your best approach would be to divide and conquer -- isolate the stage that is causing the problem by tracing and audible signal through the amp to determine what stage it's coming from. Listen to the tone coming out of each stage by amplifying it. Your ears should be able to tell you which stage is creating the problem. Chances are that the amp is behaving normally and that one of the stages just sounds bad when you drive the shit out of it expecting the tone to continuously improve under ridiculous loads.

Interesting theory about the protection diodes, Chuck. This makes me wonder about whether it would be good practice to bypass them with caps. It's probably not done because any diode switching noise would be buried in signal.

[Chuck H]

Actually, I was talking about the rectifier diodes. A bad one can allow a buzz to impress on the signal and cause sum/difference overtones. At least that's the word. I would expect to hear some kind of noise when idling, but I guess this isn't always the case.?.

[Mrk200]

Thanks for the quick responses guys.

I had the parts already laying around, so it was trivial for me to swap them as a process of elimination. I've put the original tubes and screen resistors back in after confirming they weren't the issue. The only "new" parts still installed are the two bias caps and the diode in the bias circuit.

Regarding exceeding 200 watts - I do have a PPIMV on the amp, and it never went above 75% while connected to the Torpedo.

The harsh sound I had experienced in the past happened around March. I can confirm that I changed absolutely nothing other than one of the electrolytic capacitors in the bias circuit to eliminate this harsh/raspy sound. The sound "sweetened" up with just that change. I did not touch the bias when changing the cap. I mentioned this just because I wasn't sure if it's a common problem, and could help people relate to the sound I'm describing as "harsh", since I don't have a recording of it that I can share.

I do have a 1 ohm resistor going to pins 1 and 8 on the power tubes, so when I measured the bias, it was the voltage across that resistor. Here are the measurements (using JJ EL34):

B+ at pin 3: 491V (same across all tubes)
Pin 5: -44.15v (same across all tubes)
V4: 34.4mV (Pin 1, across 1 ohm resistor)
V5: 32.9mV
V6: 32.8mV
V7: 32.9mV

I tried lowering the the bias to 30mV (measured at V4), and the harsh sound persisted.

I did test the rectifier diodes with my multimeter, and they tested fine. But I'm not sure if they could test as working, yet still cause harsh sounds.

If it's helpful, I've uploaded pictures of the amp. I built it using the Metroamp build guide, and added the PPIMV, along with a 3PDT switch since I have a dual power PT (hence the two bias pots).

[bob p]

Actually, I was talking about the rectifier diodes. A bad one can allow a buzz to impress on the signal and cause sum/difference overtones. At least that's the word. I would expect to hear some kind of noise when idling, but I guess this isn't always the case.?.

Oh, the diode switching noise is definitely there if you look for it. Because of the bathtub frequency response of the PSU caps it really pays to bypass them to get rid of HF noise. But it's better to address the noise at the source. For rectifier diodes i will always do one of two things: FRED or capacitor bypass on slow diodes like 1N400x. Slow rectifier diodes do create HF hash on the supply and snubbing them at the source is the best way to eliminate it. On a bridge type of setup I'll use 4 caps, one across each diode.

[bob p]

If it's helpful, I've uploaded pictures of the amp. I built it using the Metroamp build guide, and added the PPIMV, along with a 3PDT switch since I have a dual power PT (hence the two bias pots).

Looking at that document, some of the photos spoke volumes. I have to ask -- Is the Metroamp build guide widely regarded among hobbyists as a good reference on how to build a guitar amp?

[Mrk200]

Looking at that document, some of the photos spoke volumes. I have to ask -- Is the Metroamp build guide widely regarded among hobbyists as a good reference on how to build a guitar amp?

I'm not sure if I can answer that question accurately. I can say based on the searches I performed on how to build a 1959 clone, and looking for a build kit to simplify part souring, it seemed to be common. Even now, searching "marshall 1959 kit" in Google yields Valvestorm as the second result after Mojotone.

Edit: The build guide and photos were also the most comprehensive that I could find, which is what won over me.

[g1]

It doesn't sound like the bias is overly cold, but why not at least try going the other way with the adjustment a bit.
You're at around 33 or 34 mV, how about up to 38 or 40? Any change in the problem, better or worse?

[Mrk200]

I went up to 39mV and the harshness remained.

[Mrk200]

I hooked up my Bassman to the same cabinet I've been using with the 1959, and the harshness was also there. So at this point, I'm wonder if something is going on with one of the speakers. Either way, it does not appear to be the amp.

Thanks everyone for your help.

[Chuck H]

Always good to suspect anything in the "chain". Good find. But has this cabinet been "harsh" before? Usually speakers break in in the opposite way. They get more mellow. Unless a speaker is blown, as in scraping it's voice coil or has a torn cone I don't know how a speaker would do what you describe. A bad speaker usually makes itself obvious on close inspection. Compare the speakers in the cab by ear, try your rig as it was used when that cabinet sounded right. It could be that the cab is just brighter than ideal for some NEW circumstance. So is there anything else added to the chain that might be causing this? Like that new Two Notes Torpedo Studio you "just acquired"? If you had an older system that was working for you before, try it. All good again? Isolate the problem before assuming that a cabinet that was good strangely isn't now.

[bob p]

Like Chuck says, speakers tend to sound softer/sweeter as they break in, and it's sort of a typical for them to grow more harsh, assuming that the driver isn't damaged (like being blown or dragging a coil). Have you tried checking the drivers for rubs?

Unless a speaker is blown, as in scraping it's voice coil or has a torn cone I don't know how a speaker would do what you describe.

I would add that pushing a perfectly good speaker too hard can result in the kind of problems the OP has described. IME these kinds of harsh metallic sounds can come along as a warning sign when a speaker is being pushed to it's excursion limit. It'd be interesting to know what kind of speakers/cab the OP is referring to and what kind of signals he's driving them with, if the attenuator was in play, etc.