So about two weeks ago I practiced and played a show. I noticed my new guitar level was causing an ugly distortion in my amp. I decided to ask some friends on a message board about my ideas to reduce gain by just a touch. This took me for a silly but informative ride. Warning: this could be a long post. Grab a snack.
My new guitar rules. The one thing that’s throwing me is that it’s a lot louder than my previous man guitar. When I plug straight into the amp, or run it with no pedals, it still distorts too much for my liking. I can’t just turn the amp volume down, because it’s distorting the first gain stage, which comes before the volume knob in the amp’s circuitry. The amp is a pro reverb.
The easiest answer is volume pedal. However, I’d never use it for adjusting volume creatively. Plus, I’d like to avoid adding series resistance, avoid removing resistance to ground, as well as avoid a huge pedal. So I’ve come up with a bunch of overly complicated solutions.
1. Plugging into input 2. Tried it last night. Too much loss, too mushy, too much resistance to ground is removed.
1A. Replace the 68k resistor on input 2 to a higher value, like 250k. Plugging into 2 gives me a bit less of a pad than before, but plugging into 1 gives essentially the same sound. Might work.
1B. Install a pot in the hole of input 2 to replace the 1M resistor to ground, making the input impedance continuously variable.
Would probably work, but kind of silly.
2. Tiny unity buffer box with the ability to cut level, placed at the front or back of the pedal tray. Pull down the volume of the guitar in an active fashion.
Would definitely work, but another powered device to worry about.
3. Change the cathode resistor on the first gain stage from 1.5k to 2.2k, 2.7k, or 3.3k. It will give more headroom and less level out of the first stage. I just turn up a pinch to make up the volume lost on my other guitars.
I like this the most, as I like custom tailoring my amp, but is it overkill?
Thoughts? Other ideas?
Twenty four hours later I posted.
4. solid state rectifier plug in. higher voltage might increase headroom on first gain stage
I was getting pretty confident in my assessment. Oh, how I was so wrong.
Erik Miller of Euthymia Electronics posted this.
You should figure out exactly which amplification stage is causing the problem. My guess is that you’ll find it further down the chain. There is a LOT of headroom in that first stage; you have to hit it really hard before it clips.
You’re welcome to bring the amp over and use one of my ‘scopes if you like.
I was too curious to wait to head over to Erik’s shop, so I did a bit more testing myself.
Okay, Erik is right.
I did the trick where I hook up a coupling cap though a couple jumpers and run various points of the amp out to another amp.
So I pulled the power tubes and ran various points of the amp into a SS power amp and cab. After the first gain stage, clean, after second gain stage, clean, after reverb recovery, clean. Then, after the phase inverter, it’s crunchy. Also, though, there is this hard clipping distortion that happens where there is enough level. It hits a certain point and it sounds like a shitty fuzz. Once it gets under a certain level, the distortion just stops. I hooked it up to the both plates of the phase inverter tube, and it’s a bit worse on the side with the 100k plate load resistor, and a little less on the side with the 82k plate load.
So, I tried a different 12AT7. Still there.
I tried a third 12AT7. Still there.
I swapped the .1 cap in the cathode section of the phase inverter. Still there.
I changed the 1M resistor on the non-input side of the PI and the 100k plate load resistor. Still there.
I changed the bias cap, because it was ancient and needed it. No help but I didn’t expect it.
I changed the .1uf coupling cap on the 100k side after the phase inverter, no change.
One thing I didn’t check was the filter cap that feeds that section. I gave it a full cap job about 5 years ago, did a visual inspection tonight, and prodded it with the chopstick, and it never seemed to add or reduce the distortion.
Things i didn’t get to check. The 82k plate load on half of the phase inverter. The 100 ohm, 22k, or 470 ohm resistors in the long tail section.
To be clear. I hook up to the spot on the board where the .001 coupling cap meets the wire that goes to the tube. No distortion, sounds great. I hook up to where pins 1 and 6 meet the board, distortion on both sides. Definitely not the tube.
Finally I got over to Erik’s shop…
So Erik and I scoped my amp today.
We were getting some weird blocking distortion and oscillations, so we changed some coupling caps and the resistors on the grids of the power tube.
Here’s the funny part. After about 90 minutes of futzing with a signal generator into a load, I was playing the guitar through a speaker, and heard some bonus noise. We found out that the reverb return tube had a really dirty socket. We hit it with some deoxit and it cleaned it right up.
I was thinking though, I never got the distortion after the reverb send while testing. It was usually much quieter and less compressed when I was taking the line out post-reverb return. However, it was much louder and more squished when taking the post phase inverter line out. That was causing a vibration when the amp head was on top of my test amp, which must have been causing the rude noise earlier in the signal chain.
Anyway, thanks Erik; it was really neat putting the amp on the scope and checking it out!
Okay. I thought it was fixed. Here’s where it gets fun.
just spent a couple hours befuddled by my amp.
Here’s my amp.
When Erik and I left, he had put in new coupling caps between the phase inverter plates and the power tube grids. However, he left the leads very long on purpose. Since I had to pull them anyway to clean it up, I decided to do some more testing tonight. So I pulled the coupling caps and power tubes and did some more audio probe testing.
Remember the phase inverter is not connected to the power amp. Essentially, it’s just the preamp.
I hooked up the synth, held down a G with tape, and plugged it in to the amp. When I hooked the audio probe to the plates of the first three gain stages, I got clean signal or a natural mild overdrive. when I hooked up to the plates of the phase inverter, it sounds clean until the volume hits a point, and then it gets kind of a very hard clipped distortion. However, after a couple more numbers on the volume knob, the harsh distortion/static stops. Here’s the funny part. On the plate connected to the 82K plate load, it distorts heavily from about 7-9, but on the side with the 100k plate load resistor, it distorts from about 5-7. So the lower value plate loads is bleeding off more audio signal to ground through the filter cap, making me have to turn the volume higher to get the distortion.
Also, when I turn the volume up on the keyboard, then the distortion happens lower on the volume knob.
So the distortion is frequency dependent, as well as level dependent to something in the phase inverter circuit. However, once it goes past a certain level/voltage, the ugly static/distortion stops. Also, adjusting the tone controls will adjust when the distortion happens.
I wanted to simplify the circuit. I pulled out all the tubes except for the normal channel and the phase inverter. I still got the harsh distortion, and only on the phase inverter plates. Also, I found that if I hoo up to pin 7 grid of the phase inverter, I can hear a similar static. So whatever it is, it’s not on the pin 2 grid signal input, but I pick it up on the other grid of the PI. In addition, it seems in phase on both sides of the phase inverter. So whatever is causing the noise, it’s modulating the current through the PI in phase on both side, and is not coming in from previously in the circuit.
Since the distortion happens at different levels on each plate, and is in phase on both sides of the PI, it gets somewhat cancelled out when running through the power amp and output transformer. I hear the difference when running through the amp in normal operating mode.
I thought maybe there was a problem with the filter cap on the PI section. Perhaps at a certain voltage level it will get weird. I swapped the filter cap, and the problem did not change at all.
My next test is to pull all the tubes except for the PI, find a way to run a loud enough input signal into the grid, and check the output on the plates. I want to make sure it’s a defect in the PI circuit, and not multi-stage interaction.
I’ve heard stories about fender circuit boards becoming semi conductive, or flux on the board acting like a capacitor with other flux on the board. The board is 45 years old, and very warped. I can’t help but want to put a new board in and just prepare my amp for the next 45 years.
So I’m pretty stumped. I can’t help but think that it’s one bad part that fails at a certain frequency or ac voltage. Also, it could be some kind of low frequency oscillation that happens at a certain voltage/frequency, and then intermodulates the audio.
The other idea is to just pull and replace EVERYTHING in PI section (caps, resistors, socket wires, etc.) see if that fixes it. I’d clean the board when everything was pulled as well.
And then tonight I have this little experience
onight I decided to try and fix my weirdo amp problem.
I hook my amp up like I had before, with the power tubes pulled and an audio probe hooked into a power amp from the phase inverter. I ran my keyboard into the amp to play the note that would fart out the phase inverter. Guess what? It works fine. I can NOT get the amp to get all weird on me. It was a little disconcerting, actually.
So I put the amp back together and fire it up. There is a loud hum and very low volume. I’m messing with knobs and I smell burning. I look in the back and the power tube closest to the rectifier is bright fucking red!
I turn it off, swap the power tubes, fire it up really quick, and the problem stays with the socket. Something must have happened with the bias supply. I take apart the amp again. I measure the bias. It seems to be fine. I put in my spare tubes. They’re running fine. I prod the new grid stopper and pop, tube goes crazy and the bias has cut out on that tube. Shit, I must have done a cold joint on the resistor.
I re-solder the resistor, and the tube is still ramping up, but there is -16v on the grid instead of -45 (correct) or 0 (the way it was wrong before when I almost had a Chernobyl. I swap the replacement tubes and now the problem follows the tube! I must have damaged my replacement tube while testing the bias. Balls. I put the original tubes in, and it works and sounds great. I put the amp back together and no meltdown. Hooray?
So, let’s see how long this lasts. It really sounds wonderful right now, though.
Did you make it through? I hate and love my amp. Cheers.