1968 vs 1969 Super Lead sound

[Tazin]

You've definitely covered a lot of bases trying to chase down the early breakup thing. Unfortunately, you have got to a point where my troubleshooting knowledge has just about run out, so I really can't think of anything to add or try. This is a scenerio where you really could use a oscilloscope in tracking this problem down.
The oscillation thing when taking cathode and grid reading on the PI tube is a bit interesting. I know some amps are prone to oscillation when there out of their cabinet/box and you've got them upside down on the work bench....No aluminum sheet shielding the open electronics. I wonder if this is the case with the '68?

[shakti]

I suspect an oscilloscope is in order here. I also spoke to Chris M e r r e n who very kindly offered his insight, but also calls for oscilloscope plots. Hopefully I will be able to borrow one next weekend. In the meantime, all tips are very welcome!

I have no experience with oscilloscopes. Any tutorials out there where I can learn the very basics? I suppose I also need a signal generator, or does an oscilloscope typically come with one? What signal should I apply?

The upside to all this is that I am learning a lot. I spent a lot of time on Youtube watching videos by a user called Uncle Doug. I can not recommend his videos highly enough. Some of it is banal even for me, but I've expanded my knowledge exponentially over the last few days and filled in gaps in my knowledge. His videos are very easy to follow and go into great detail on every aspect of the circuit on a basic level.

Once I get this sorted out, I hope this amp will finally achieve what I've wanted out of it all along - the ability to get both that greasy, gainy, compressed sound of a 68 running flat out, as well as the tighter and slightly cleaner sound of a 69 and on.

[neikeel]

I got an online Signal Generator from a site that Steve Miller advised, it is called Sig-Jennyhttp://www.natch.co.uk/downloads/SigJenny/SigJenny.html
You create your own probe with a stereo mini jack and 0.1uF (I used a 400v short lead mustard with shrink wrap) cap for blocking DC. I use an old Hitachi V212 (ok I am a cheapskate) which is dual trace.
This tutorial should get you started:http://www.soultrampamps.com/wp-content ... _scope.pdf

[shakti]

Thank you, Neil! Something to read over the weekend.

If anyone has any recommendations for a cheap/affordable oscilloscope then I want to look into that. I see that the Hitachi you mentioned pops up on Ebay a lot.

BTW, looks like that signal generator program only works on PC though? I only have Mac available...gotta search a little bit. But an affordable signal generator would be nice too.

[shakti]

I finally got that cheap Chinese cap tester that Carbia mentioned some pages back. I have no idea how accurate that thing is, but it's very easy to use and if it's at least reasonably reliable, then it is a very useful inexpensive tool.

Re our previous discussion on filter caps and ESR, I was amused to find that the NOS Erie 100uFs all have very low ESR, all below 1 ohm each. By comparison a new F&T was about 1.5 ohm. The Eries were all a little below 100 uF (around 85-90) but that's OK. The meter also gives an estimate of voltage loss, which I assume is a measurement of leakage? It gives this figure in percents, so I'm not sure how to interpret it. Typically these filter caps, whether new or NOS, read in the 0.5-0.8% region, so I assume that is good. One of the .1 mustards was up to 3-4% though, even though it sounded fine and replacing it with a Mallory made no difference in a positive direction.

What's interesting though, is that the cap tester claims the NOS Erie 32+32uF is malfunctioning. The meter simply says "no part, missing part or faulty part" or something to that effect, so I don't get any figures. Measuring capacitance with my multimeter gives correct capacitance values for both halves at least. I will take it off the board and try to get a reading with the cap removed from the circuit. Remember, my testing indicated that the problem with the 68 comes from the PI or later, so the preamp filter cap "shouldn't" be the culprit. And when I replaced it with a new F&T 50+50 I did get a little more punch, but the early breakup was still there, so I still don't think it's the main problem.

Hoping to have an oscilloscope and signal generator on hand this weekend. Stay tuned.

[shakti]

Great to have the forum back! I still haven't got to the bottom of this, but it has been on hiatus while the board was down and I was trying to find an oscilloscope. I am picking up an old Hitachi this weekend, and will just use a computer as a signal generator for now. Any tips for a decent, cheap signal generator are appreciated though!

[shakti]

Bump to get some action on this thread again. I should update this thread with what I've been trying in the meantime, but I don't have the time now, so here is the lazy link to a Plexi Palace thread with some updated info (and a summary so far):

http://vintageamps.com/plexiboard/viewt ... 1&t=110976

[shakti]

After a tip from Chris M e r r e n I tried this AM radio trick with my amp; radio is tuned to an empty AM radio frequency and turned up loud. Amp is playing into a load (no speaker).Watch what happens when I play a guitar and hold the radio close to the output transformer:



As I understand it, this is not normal. I haven't had time to try it with my other amps, but AM modulation is not supposed to take place in the transformer.

Secondly, I tried to jumper in the OT from the 69 amp, and it doesn't change the sound of the 68. So the OT by all measurements and tests seems to be good. So what is going on here?

I am still in the dark, but I found this interesting article on rectifier ringing and switching noise:
http://electronics.stackexchange.com/qu ... hing-noise

A couple of quotes:

"The winding of a secondary can be 10s or 100s of millihenries or more, and the rectifiers can have dozens of picofarads of capacitance which could result in a resonant tank oscillating in the hundreds of khz hundreds of MHz, which will radiate substantial EMI."

" Also, I must note that there is nothing special about the diodes in any of that - it's simply that the diodes have some capacitance which can resonate with the transformer secondary and radiate EMI at that frequency. A capacitor across the lines will not snub this, but it will lower the frequency enough that it no longer really matters. There is not much that a 10kHz sine wave will mess with. 90MHz though, and you're right up in FM radio land. AM radio at hundreds of kHz. You get the idea.

It can be difficult to see ringing on an oscilloscope, it will be super imposed on the AC sine wave, and be quite small in amplitude comparatively, and several orders of magnitude faster than the sine wave. It's usually better to use a spectrum analyzer. Any ringing will be a very sharp, noticeable spike. It's also easier to detect it from the radiation than the actual current flow, again due to being superimposed on that 60Hz sine wave."

Could this be rectifier ringing causing distortion artifacts and AM frequency modulation? I only halfway understand the phenomenon, so I don't really know if it is plausible.

[rgorke]

I'll jump in here. I am having, what I think is a very similar issue. When the amp is turned all the way up, the noise is not noticeable. It's when the volume is rolled down or the volume is at 2 or 3 for a clean tone.

It's an "after note distortion" buzzing sound.

I thought I had it fixed and it went away. Now it is back. The presence control really affects how bad it is.

Things I've done:

Changes tubes- all of them
Different cabinets
Reflowed nearly the entire amp...check that, the whole amp, twice!
Addressed lead dress in a couple different ways

I'll try and post a clip.

[danman]

From what Rgorke describes, it sounds like some kind of oscillation that you may need a scope to track down. Check out this thread from TGP where "Fusionbear" has the same issue and was able to track it down with his scope. I believe post #13 and #25 explain it. http://www.thegearpage.net/board/index. ... d.1703127/

[shakti]

In your amp thread you have a sound file, rgorke. I must confess nothign stood out as obviously wrong when I listened to it. There was a slight beating sort of sound in the sustain, if that's what you mean? Try to post a clip of single notes set for a clean sound to see if that illustrates it better.

In any case, it doesn't sound that similar to mine, although the problem could of course be similar still. I have a slightly crackly, fizzy, buzzy sort of high end "static" around each note starting very early on the dial (2 and above). Turning the guitar waaay down makes it go away, but by then there's very little output left. Here's a photo of both the 68 and the 69, when I jumpered in the 69 OT (the forum software won't let me embed it, so you have to follow the link):

http://s1292.photobucket.com/user/soeho ... i.jpg.html

[rgorke]

Is this the clip you listened to?

http://soundclick.com/share.cfm?id=13364631

I get a fluttering distortion after the notes.

I will try and convert another clip shortly.

[rgorke]

Rather than continue to clutter this thread with my issue, I posted in the thread I started about my JTM50.

[shakti]

OK, I may have spoke a little too soon that there is nothing to see on the oscilloscope that seems wrong. I made up a few videos showing oscilloscope views from various spots in the circuit, and what happens when I turn the controls. Have a look and tell me what you think. (Will post the videos below in just a few moments).

What I am planning to do is the rebuild the amp. I have just received some parts that I intend to play around with. I have the OT transformer brackets which I am going to install for looks, and in the process I will rewire the OT. Then I will try a different rectifier and different wiring layout for the HV secondaries. Possibly rebuild the bias circuit and lay everything out in the preamp as cleanly and neatly as I can (although I don't think it's bad right now and the amp is very quiet).

[shakti]