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Author Topic: peavey heritage vtx snow camo  (Read 4999 times)

ilyaa

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peavey heritage vtx snow camo
« on: March 17, 2014, 04:14:49 AM »
ill keep this one short (schematic attached)

this amp has a weird hum. higher pitched than 60 Hz. i checked power supply - looks okay - except plates are only getting ~363 volts (according to schematic should be 525). other DC stuff looks okay, but i didnt go through everything yet.

i scoped the hum (with a 1K test tone....ignore the shuffling sounds, they are unrelated. the hum kind of blends in with background noise, but you can hear it):

http://dropcanvas.com/#9i6BMjZ3oBMzr4

any ideas?

with no input signal: the amp hums with the controls all the way down. the controls in that case have no effect on the hum. if i scope that (with scope leads across speaker tabs), though, the scope seems to load it down (or ground it?) and it disappears....if i crank the amp up the scope picks up a (quieter hum) that just looks like noise (no discernible waveform) - cant tell if thats just noise floor of amp then.

the bigger problem is that this amp has a snow camo design. i wish there was a repair for that....



Roly

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Re: peavey heritage vtx snow camo
« Reply #1 on: March 17, 2014, 11:11:24 AM »
The following canvas has been suspended - this user has not verified their email address and abuse is suspected. Sorry about that.

I also couldn't get the pix to download, just kept cycling me back in the queue.

Zip for two so I'm not too impressed with dropcanvas.


"Hum" is generally reserved for power frequency, 50 or 60Hz.  "Noise" is aperiodic, such as "hiss" which is high pitched like escaping compressed air.
If you say theory and practice don't agree you haven't applied enough theory.

ilyaa

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Re: peavey heritage vtx snow camo
« Reply #2 on: March 17, 2014, 11:56:04 AM »
EDIT: dont waste your time giving me any advice on this yet. im trying to nail down a clear, repeatable symptom and am getting all kinds of stuff (including a weird octave down effect......). ill post again when i have a more clear symptom description.

okay EDIT EDIT: watch the video i posted.

heres whats up: the hum is not a hum. rather, the issue is, when the amp is played (with a guitar) it makes a weird puttering, almost octave down-y kind of farty noise underneath the note. this is with the preamp shorted, straight to the power amp in.

the video above is a 1K sine wave into the input. the 1K tone comes out fine but it has a lower, buzzy/farty kind of tone underneath it (thats the phantom waveform you see in the video). oscillation?

heres a better clip (this time signal goes into main in): http://dropcanvas.com/#G8A159j2k59a09
« Last Edit: March 17, 2014, 01:30:39 PM by ilyaa »

g1

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Re: peavey heritage vtx snow camo
« Reply #3 on: March 17, 2014, 03:18:20 PM »
  Sounds like a bad filter cap, especially seeing as you have 363V B+ instead of 525V.
Check what the AC voltage is on the B+. (some meters will not be able to measure this properly).

ilyaa

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Re: peavey heritage vtx snow camo
« Reply #4 on: March 17, 2014, 03:28:38 PM »
meter shows AC on B+.....800V?!

its not a super fancy meter so let's hope thats just a little fluke (no pun intended....really). should i scope it there and see what the scope says or go ahead and change the filter cap?

Enzo

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Re: peavey heritage vtx snow camo
« Reply #5 on: March 17, 2014, 06:07:04 PM »
Try connecting a cap in series with your meter lead.  Some largish value like 0.1uf or 0.04u7uf at 600v.   That will block the DC that is confusing your meter.

ilyaa

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Re: peavey heritage vtx snow camo
« Reply #6 on: March 18, 2014, 03:47:21 AM »
the cap in series didnt work - my meter read 0VAC in that configuration....but like i said, its a cheap meter. or maybe cap didnt get it cause its too low frequency (i used an 0.04 uf).....

BUT i scoped it and i got a nasty looking 8 V p-p ~150Hz ripple...asymmetrical trapezoid shape.  thats too big of a ripple, right?? on lo-power its more like 2.5 V p-p - but still too big, right? shouldnt the ripple be quite small, less than a volt?

EDIT: wait a second my probe was on x10 - so thats 80 V p-p!!! (right?)

change the filter caps?
« Last Edit: March 18, 2014, 10:49:31 AM by ilyaa »

Enzo

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Re: peavey heritage vtx snow camo
« Reply #7 on: March 18, 2014, 11:55:13 AM »
Any AC volt meter should be able to measure 60Hz.  Not sure why the cap trick didn;t work.  Hum is 60Hz, and it has no trouble riding through such caps in the amp.


Yes, it sounds like you have excess ripple.  C3 C4 become suspects  And I would be tacking another cap in parallel to find out.   Make sure all the solder is 100% on that power supply board.

ilyaa

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Re: peavey heritage vtx snow camo
« Reply #8 on: March 18, 2014, 06:33:58 PM »
we did it!

replaced the main filter caps and we are all good.

sounds great, actually!

quick question: so i know there is no biasing this amp (i read on another thread here that its got a self-adjusting control grid voltage circuit thing) - does that mean anything for matching tubes? i did not number the tubes when i took them out and im wondering if i should do any measurements to see if i paired them back wrong or not.....

Enzo

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Re: peavey heritage vtx snow camo
« Reply #9 on: March 18, 2014, 08:40:17 PM »
To be picky, like most amps lacking an adjustment, it is not "self-biasing", it simply has no adjustment.

This amp is different from the average amp in that the power tubes have the signal driven into the cathodes.   That means the control grids have about +15v on them instead of -55v.   On the other hand, the cathodes sit at some 80v if I recall, which puts the cathode to grid voltage difference at 65v or so, which is about the same.

Like all commercial guitar amps, they do not NEED matched tubes, and it surely does not matter which tube came from which socket.

Roly

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Re: peavey heritage vtx snow camo
« Reply #10 on: March 19, 2014, 11:47:52 AM »
we did it!

replaced the main filter caps and we are all good.

sounds great, actually!

 :dbtu:

How many does that make now?   ;)
If you say theory and practice don't agree you haven't applied enough theory.

g1

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Re: peavey heritage vtx snow camo
« Reply #11 on: March 19, 2014, 12:06:22 PM »
  Regarding the output tubes bias and balance, there is no adjustment, but there are specs in the notes on the schematic regarding the range of acceptable values for Vi and Vk.  If the tubes are not within the spec you replace them.

J M Fahey

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Re: peavey heritage vtx snow camo
« Reply #12 on: March 20, 2014, 02:32:37 PM »
Agree and add: in this kind of SS cathode driven power amps, tubes are *forced* to pass whatever current the driving transistors send them, until they can't physically do that any more.
These circuits extract up to the last watt those tubes can produce, that's why you see 65W amps with 2 x 6L6 and 130W ones with 4 , and work until the last electron is pulled from that coating.
They are loud and clean for *years* .
In your case, the supply filtering failed, not the tubes.
And when they die, they die for good :(