Can't work out if I have a problem with my amp?!

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I did some recording the other day to compare my stock 72 Tele Custom pickups with the Oil City replacements which I just received and noticed in the recordings that the clean tone has a bit of rustle-y distortion riding on top of the notes, especially when I play hard (that's the best description I can think of).  I recently returned the amp (a 5E3 clone I built) to stock - I had added a reverb circuit to it and got rid of the normal channel to free up one triode of the preamp valve - and so I'm not sure/can't remember if this distortion is normal or not.  I've tried re-flowing the pre-amp solder joints, and adding a 1M grid stopper to the PI, as well as a 10K grid stopper in front of V2 pin 2.  None of these has got rid of the sound.    Have a listen and tell me: is this just normal onset of distortion for a 5E3, or am I inventing this problem?  

I'm grateful as ever for the collective knowledge and advice of the 'Board.





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  • Further observations:

    1. The rustle-y distortion over the clean sound occurs with other speakers.

    2. The Bright channel's volume pot rustles and scratches when turned, but only when there is a signal passing through it.

    3.  The problem occurs with both channels.

    4. Maybe some larger grid stoppers on the power valves?

    5.  This thread should maybe be in the Amps section.
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  • Have yuou overloaded the input on the DAW? Try turning down a bit, or fiddling levels.  Seems to only occur on higher notes played loud.
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  • That's what I thought it might have been - or distortion from the very sensitive condenser I was using - but it wasn't.  Since re-wiring the amp with both channels I can play much cleaner than before (by turning the unused channel volume up past 10 - a 5E3 quirk as I'm sure everyone knows), but it also means I notice this distortion riding on top of the note.  The audio clip isn't the best example as it's only very slight, but it's the best example I have recorded.  I can reproduce it in the room, so to speak.  In fact, I can't avoid reproducing, which is the problem!
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  • ICBMICBM Frets: 33952
    edited April 2014
    I can't listen to the clip where I am at the moment, but scratching noise when turning a pot is often a sign of a DC leak onto the pot, either via the coupling cap from the previous stage or from the grid of the following valve. Either of these could cause unwanted distortion too. Try changing V2 first and see if it stops or changes.
    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone."
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  • There's no DC on the pot - it snuffles and snorts when being turned between 0 and 1.5 (when the sound suddenly comes in in true 5E3 style) unless the strings aren't vibrating, in which case it's silent.  I'm not even sure if there's a problem - I think I may be nit picking.  Maybe I should get out the house for once...

    Thanks for the suggestions though both of you.
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  • ICBMICBM Frets: 33952
    Try changing V2 - or at least swap it with V1 if they're both 12AX7s and you don't have a spare - scratchiness at the very bottom end of the track could still be a leak from the following valve. (Rather than the coupling cap, which affects the whole track.) You probably won't be able to measure the DC voltage easily if it is that.

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone."
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  • Swapping the valves was the first thing I tried - sorry, should have said. I'm away till Monday but will investigate further when I return. Cheers.
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  • Update on this damn amp.  I have tried it with a different speaker, with different valves and without the attenuator on the current speaker.  I noticed that without the attnuator the E played at the 9th fret of the G string made some really nasty noises - very jagged distortion.  This was the same with the same note played on the D and A strings, but not on the B and E.  So it didn't like the thicker tone of the heavier strings played high up the neck.  (The distortion decreased a lot at Eb and F.)

    I measured for DC on the first coupling caps (between the first gain stage and volume/tone controls, and was getting up to half a volt when a signal was applied.  Is this normal, or is my DMM just getting confused with an AC signal?  I was getting the same reading on the bright and normal channels when plugged in to those channels and playing.  Please help!
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  • ICBMICBM Frets: 33952
    That could indicate heavily asymetrical distortion, which will introduce a DC component. (Probably caused by incorrect bias on the preceding stage.) The cap isn't significantly leaking if you can't measure a voltage with no signal. Do you have a scope?
    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone."
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  • Unfortunately not. I've been thinking about getting one. Can you recommend something affordable?
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  • ICBMICBM Frets: 33952
    edited April 2014
    No idea what's current, sorry! I bought mine (an old Panasonic dual-trace job) about ten years ago from a repair shop that was closing down, to replace my even older 1960s valve one which had died irrepairably.

    For audio amp work it's hard to go wrong though, you don't need anything very sophisticated - although for valve amp work it's advisable to get one rated for 400V minimum on the inputs, and preferably 600V.

    You also ideally need a signal generator, but anything that will produce a clean sine wave between 10Hz and 10KHz is enough.
    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone."
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  • Blimey, trying to buy an oscilloscope is more complicated than building an amp.  I looked them up on Mouser, and the first one was priced at £14,500.  And you can't search by max input voltage.  Maybe I'll phone the local vintage radio and TV people and see if they have one spare they want to flog.
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  • ICBMICBM Frets: 33952
    There are a few on Ebay. Search between £50 and £100 and you'll find something half decent.

    This one looks OK, although you'll need to get probes separately.

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone."
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  • I couldn't find any others rated for over 400V max input, so I got the Beckman one.  Thanks @ICBM.  Now I just need to learn how to use it - thankfully the user manual was easy to find and download.
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