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#1 2013-01-05 21:17:24

Redwood
Member
Registered: 2012-12-10
Posts: 19

Still struggling to get this V4 running right

I've been working on this dang distortion model V4 for weeks and just can't figure out where the problem is.  I've traced the entire circuit top to bottom and verified every value both by markings and measurement.  Even after replacing all the coupling caps I've literally pulled them and retested them out of circuit with an old Pyramid CRA2 cap analyzer to make sure none are leaking, and yet my problem persists - OK output at low volume but severe clipping distortion at any volume over 9 o'clock or with bass or treble cranked.

While I have anomalous DC voltages at a few points on the schematic the most serious appears to be at pins 2 and 3 of V4, It's a cathode follower so supposed to have some fixed positive voltage at the grid but it's nowhere near spec.  According to the schematic, with no signal I'm supposed to see 95V at V4-2 (grid) and 45V at V4-3 (cathode).  I have nearly the inverse of that, around 45V at V4-2 (50% of spec) and over 60V at V4-3 (more than 50% *over* spec).  I have a scope (though my skills with it are weak) and this is where I see the issues with the signal start to arise. 

I've done extensive testing of my output transformer and it appears to be OK based on resistance readings, and I also disconnected the center tap then sent 1VAC backward through it using a variac and got what appeared to be appropriate readings on the primaries, around 12.5VAC at each power tube plate to center tap and exactly twice that plate to plate.

However, I did notice that when sending a tone generator signal through the amp while on a dummy load, as I increase the volume I get a notably audible tone from the OT itself, like something is mechanically moving a lot inside it.

For what it's worth, I own a FLIR infrared camera and my 6CG7 reverb tube is running hotter than hell, especially at pin 8, though the 5W 10ohm resistor tying it to the rail appears fine.  It's right at the solder socket pin solder joint where I see the heat, despite having cleaned it up and resoldering it a couple times.  That particular point is another one where I've got some wonky voltage, getting 4VDC where I should only see 0.6.  Not coincidentally, C9, a .47 cap tying this pin to the green reverb input, does act funny - I don't have a high Z analog VOM but with my digital meter I get some substantial voltage there that drains off really quickly once the probe is applied, even if the reverb cable is not hooked up.  Again, I've pulled that cap and checked it and it does not appear to be leaky, and even replaced it with yet another brand new cap but I get the same condition.  In any event even the scope image of the signal looks OK through V3 (reverb), and it's not until V4 (12DW7) where it starts to deteriorate.

Is it possible I just have a bad OT and that's causing all this weirdness upstream?

Conversely, even though I've just installed all new filter caps and diodes, and the DC voltages at my rails look good, could excessive ripple be messing with my preamp tubes?

I've also stripped out the PEC tone module and rebuilt it with discrete components.  I'm not 100% certain I got that part right but it made no difference either way.

It appears likely the mid-EQ amp section is the source of the problem but I don't understand either the cathode follower application nor the use of the three little inductors very well.  In particular I can't figure out why my V4 cathode DC voltage is so high when all the resistors around it are exactly to spec, and it's got coupling caps C14, C15, and C21 between it and any other source of positive voltage.  It's like somehow that tube is running backwards but I have no idea why - and yes, I have tried swapping different tubes in there and I get the same thing regardless.

I'm at the point where I'm almost ready to strip ever last component out of this chassis and start from scratch.  If  it was my amp I'd shelve it and forget about it for six months but I'm fixing it for somebody else and we've already invested nearly $400 in parts so I really need to get this thing out the door.

Unless it's the OT - or even if, given that I've convinced myself it's OK - there's little doubt this will come down to some stupid mistake on my part.  I just wish I could figure out a logical way to track it down.

Last edited by Redwood (2013-01-05 21:18:44)

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#2 2013-01-05 23:14:14

hangman
Banned
From: Seattle Washington
Registered: 2006-09-04
Posts: 1848

Re: Still struggling to get this V4 running right

Do you know anyone with an oscilloscope?   The problem should be simple to find if you can see how a test signal looks going through the amp.

Honestly,  I think you are kinda firing away in the dark,  but what you need is more information.   The o-scope will give you that.

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#3 2013-01-06 02:25:55

Redwood
Member
Registered: 2012-12-10
Posts: 19

Re: Still struggling to get this V4 running right

Apologies for the long rambling post.  I mentioned in the second paragraph, I do have a scope, though my skills with it are not the greatest.  I don't have a proper tone generator but have an app on my phone that appears to do the trick.  I can trace the signal at the grids and cathodes through V4 and that's where they start to get funky on me.  I would be ever so grateful for some pointers as to exactly what points I should be looking at and how to interpret them.

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#4 2013-01-07 23:39:58

hangman
Banned
From: Seattle Washington
Registered: 2006-09-04
Posts: 1848

Re: Still struggling to get this V4 running right

What does it looke like on the plates of v4?

With your phone run a 1k sine wave into the amp with eq set at 12 and volume one set to 9 o'clock and volume 2 set to zero.

Make sure the volume on your test tone generator (phone) is not too high,  we don't want the preamp to clip before the power amp.

Hook the output of the amp up to a dummy load,  and monitor that on one of the scope channels.

Take a look at pin 1 and pin 6 of v4,  and what do you see?

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#5 2013-01-09 22:31:34

Redwood
Member
Registered: 2012-12-10
Posts: 19

Re: Still struggling to get this V4 running right

Thanks for the pointer, Hangman, but I'm afraid before I saw your post I was poking around with the scope already and while I was able to look at those points earlier now my scope is acting funny.  At the grids my signal trace moves off the screen after a click or two of the V/div and after three or four clicks I can't find it at all.  At the plates I can never find it to begin with.  I think I must have popped a cap either in my probe or else at the AC/DC switch on the scope.  The trace also behave as the manual says it should when switching from DC to AC when I go through the probe calibration procedure.  I just ordered a new pair of probes (only had one to begin with anyway).  If that doesn't get it working right I'll crack the case and see if there's something connected to the AC/DC switch that I smoked while poking around inside the amp.

In the meantime, I just noticed I have about 2.5VAC at the plates of the power tubes with no signal - is that normal?  I also noticed at very fine resolution I have a sort of sawtooth pattern running along the entire length of my sine wave signal as early as the grid of the first preamp stage.  Starting to wonder if it hasn't been my brand new filter caps all along.

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#6 2013-01-10 08:50:47

hangman
Banned
From: Seattle Washington
Registered: 2006-09-04
Posts: 1848

Re: Still struggling to get this V4 running right

The power tubes will have a fair amount of ripple on them,   This is not important in a push/pull amplifier because the ripple is the same on all four tubes,  but the outputs are out of phase,  so the ripple gets cancelled out.   
Almost All push pull guitar amps have a fair amount of ripple on the plates.

I wouldn't get too worked up about the sawtooth wave on the grid of your first preamp tube.   Look for it on the plate.   That will tell you if your power supply has problems.  On the plate the ripple in the preamp should be very small.  You should have to turn your scope to the lowest volt settings. 

It would seem odd that both of the dc blocking caps in your scope would be shorted,   Set the ground to zero,  then set it to AC,  on the highest voltage setting.. I you can't see the signal,  you should at least be able to see the signal trace at ground... Turn the volt setting down until the signal can be seen.

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#7 2013-01-10 16:00:52

Redwood
Member
Registered: 2012-12-10
Posts: 19

Re: Still struggling to get this V4 running right

I had a typo above; meant to say the trace does NOT behave as the manual says it should when doing the probe calibration.  I'm supposed to stick the probe in a little port on the front of the scope and it puts a square wave on the screen, then adjust the probe with a little screw that can cause the square wave to deflect one way or another.  That part works fine, but when I align the top of the square wave along the centerline at the DC setting, it's supposed to move up to centered over the centerline when I switch to AC and it doesn't do that at all. 

As I said, I was able to locate the trace at both grids and plates a couple days ago, and I haven't made any substantive changes to the amp which is why I suspect I may have damaged the scope.  I did put the probe to some pretty hot spots inside the amp, including the plates of the power tubes.  It was set to AC at the time but could those 500+ DC voltages have overwhelmed it?  The scope itself is a Tektronix T922 that I've had for over a decade but hadn't even powered up for probably 3 or 4 years before last week.

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