Hondo
278 posts
Mar 09, 2014
8:37 AM
|
I just bought a used VHT Special 6 that, when I play some notes, it has a scratchy sound like a scratchy volume or tone pot. Any ideas?
|
Kingley
3502 posts
Mar 09, 2014
9:02 AM
|
It might help if you can upload a sound file demonstrating the problem. Soundcloud is a good place to use for recording and uploading audio files.
|
Greg Heumann
2634 posts
Mar 09, 2014
10:23 AM
|
Could be a microphonic tube. Remove the rear cover and any tube shields. With the amp on, rap/thwack each tube with a pencil or even a finger. If that reproduces the noise, that's your problem. If it doesn't, it isn't.
----------
***************************************************
/Greg
BlowsMeAway Productions
See my Customer Mics album on Facebook
BlueState - my band
Bluestate on iTunes
|
Hondo
279 posts
Mar 09, 2014
11:08 AM
|
Check this out. I've never seen a black tube or one with a plastic sleeve on it. Does this mean that it's bad and does that plastic come off?
|
Kingley
3503 posts
Mar 09, 2014
11:20 AM
|
That tube is fine. That's the stock tube they use in the VHT. No the plastic doesn't come off. I upgraded mine to a JJ Tesla 6V6 and changed the stock 12AX7 for a JJ Tesla 5751. It gives the amp a nicer tone in my opinion.
|
Hondo
280 posts
Mar 11, 2014
10:36 AM
|
Well my new tubes arrived and a JJ6V6S and an EH 12ay7 were installed. They reduced the volume of the scratchy sound but it still, without a doubt, there. The problem is now mostly limited to 2 double bend or a strong 2 blow. I then tried two different mics and a different cable. The cleaner mic made it more prevalent. This would lead to the speaker. Any other ideas? Thank you for the help.
|
rogonzab
489 posts
Mar 11, 2014
12:23 PM
|
Try this:
- Are you shure that is the amp? Sometimes is another object reacting to that frecuency. Put the amp in another room and try.
- The air moved by the speaker and the vibrations of the cab can "move" (very litle and very fast) the tubes. You can use tube dampers to increase the mass of the tube.
|
5F6H
1754 posts
Mar 11, 2014
12:28 PM
|
Sounds like it is a design fault, try other parts as you get the opportunity, but really I suspect you're looking at reducing gain & high end in the circuit itself.
Some of these changes are very simple if you are a little handy, the VHT drains its caps when you unplug from the wall & put the standby switch in play mode...not that you have any reason to be poking the filter caps & should just work with parts identified...
Maybe moving some of the wires around in the amp may help too, but we'd need photos of the guts. Try some of these tips...
If you have no idea what you are doing, give the amp to someone qualified to carry out the work - certainly do not ever open up the amp whilst still connected to the wall AC (even if turned off), nor powered up.
Definitely do, do's:
C8 - 4.7uf is small, 25uf/25v would be better, no need to go much bigger with a small amp. Observe correct polarity (waisted end to valve pin).
R12 - Bypass with plain wire, this turns off your "treble pot"...which you can't adjust otherwise (whether in clean or boost modes).
R13 - This is the "bass" resistor 33K is quite small a value, I'd look more at 150K to 220K (bass full up).
C11 - remove.
C13 - remove.
Optional/fine tuning:
C15 try removing, this will lower gain at the power tube, if it kills the amp tone, just refit.
R17 - Run a 33K or 47K 1/2W resistor from the valve side of this resistor to the hot speaker jack terminal - this makes a 'negative feedback loop', I normally prefer a loop on SE amps as they can be a bit bitey without. If your amp has a 4/8ohm speaker use a 22K resistor. WARNING: After fitting this loop, your amp may howl as soon as you turn it on, this is because it was built without a loop and OT polarity didn't matter - now you have a loop it does! If you get howling either detach the loop, or switch around the blue & red wires from the output transformer primary, that connect to 6V6 pin 3 & "B+ 1".
R8 - I like 2K to 2.2K 1/2W here to reduce fuzziness with a mic.
R5 - Try larger values to brown off the tone, the bigger you go the less voltage on the preamp valve & grindier the sound, I'd guess at ~47K might be around your useful limit?
The footswitchable "clean/boost" is a resistor that almost disconnects the tone stack, perhaps leaving the amp not quite identical to a 5F2A, but along similar lines.
Observe proper safety protocols (with the amp unplugged, throwing the standby into "play" mode should drain your caps via R25)
----------
www.myspace.com/markburness
http://www.facebook.com/markburness
Last Edited by 5F6H on Mar 11, 2014 12:30 PM
|
SuperBee
1745 posts
Mar 11, 2014
12:48 PM
|
I'd bet on the speaker. Especially if I was gambling with someone else's money and wasn't accountable.
Some speakers more prone than others, some settings and effects make it more likely. And playing style may affect it.
Kind of an expensive experiment though. You might change the speaker and still have the problem even if it is the speaker.
Do you always get the problem or can you change volume/tone settings and make it stop? Adjust your playing and make it stop? Does it do it with all harps? How about other instruments?
|
