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  #1  
Old 01-03-2011, 06:34 PM
Trotter Trotter is online now
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How do you install a 68k grid stopper resistor on the input?

I have a new VHT Special 6 combo amp... Its great little practice amp but it really picks up the radio signals (at least in my home).

I noticed a guy on the Telecaster Forum has the same problem but was easily able to fix it with a simple mod (68k grid stopper resistor on the input).

BTW... The amp has 2 inputs (High and Low).

Can someone please walk me through how to do this mod? It sounds easy, but I need a step-by-step outline for dummies so to speak! A pic would really be great!

Oh, and yes... I will drain the filter caps first (I do know how to do that)!

Thanks in advance!
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Old 01-03-2011, 07:31 PM
TweeDLX TweeDLX is offline
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Trotter,
With two inputs, they should already be in place, but you can always mount one directly to the 2nd pin of the first pre-amp tube. There will be a wire (possibly shielded) from the inputs to the first tube. It may go to the board first, so follow it to the tube from there. Disconnect the wire from the pin and solder your 68K resistor where the wire was. Solder the other end of the resistor to the wire and cover with heat shrink or electrical tape. Done.
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  #3  
Old 01-03-2011, 10:39 PM
Trotter Trotter is online now
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Quote:
Originally Posted by TweeDLX View Post
Trotter,
With two inputs, they should already be in place, but you can always mount one directly to the 2nd pin of the first pre-amp tube. There will be a wire (possibly shielded) from the inputs to the first tube. It may go to the board first, so follow it to the tube from there. Disconnect the wire from the pin and solder your 68K resistor where the wire was. Solder the other end of the resistor to the wire and cover with heat shrink or electrical tape. Done.
Thanks TweeDLX! I will give it a shot my friend. Wish me luck
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Old 01-03-2011, 11:09 PM
TweeDLX TweeDLX is offline
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One thing to remember. If it IS a shielded wire, make sure your resistor doesn't contact the shielding. Good luck!
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Mike

"Yes...I was having a cup of tea with Mr. Roccoco here, when suddenly this madman burst through the door. Honking wildly, at the last possible second, he stopped on a dime. Unfortunately, the dime was in Mr. Roccoco's pocket..." . Good Deals here.
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  #5  
Old 01-04-2011, 07:21 AM
CaptainJake CaptainJake is offline
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If there are already resistors on the input 10k will be a plenty high enough value, even then you don't really ever need to go higher than 33k
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  #6  
Old 01-04-2011, 03:17 PM
Trotter Trotter is online now
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Quote:
Originally Posted by soiajake View Post
If there are already resistors on the input 10k will be a plenty high enough value, even then you don't really ever need to go higher than 33k
Good to know. Thanks! I'll try starting with a smaller value first then.
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Old 01-16-2012, 08:53 AM
momomo momomo is offline
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hello,
i'm new to the gear page. i just bought a vht special combo & when I returned to my home I noticed extensive radio noise / signals. I tried various different cables, but no luck.

so my question here:

did this fix work?

can you post a picture, of where to change what? I have no experience with this kind of modding..
thank you in advance!
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  #8  
Old 01-16-2012, 01:58 PM
TweeDLX TweeDLX is offline
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Like this:
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Mike

"Yes...I was having a cup of tea with Mr. Roccoco here, when suddenly this madman burst through the door. Honking wildly, at the last possible second, he stopped on a dime. Unfortunately, the dime was in Mr. Roccoco's pocket..." . Good Deals here.
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  #9  
Old 01-16-2012, 06:31 PM
momomo momomo is offline
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Thank you TweeDLX.
I think I will bring my amp together with this schematic to some amp repair shop & ask them to do it.
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Old 01-16-2012, 08:10 PM
Billm Billm is offline
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If you study the schematic, you'll find that the signal flows through two 68K resistors, in parallel. The resulting 33K is enough to kill most spurious radio signals. If you look at the construction:


you can see that the wire from the jack down to the 12AX7 is shielded. About the only other thing you can do inside the amp is to build additional shielding around the plastic jacks, a little cage inside the amp.

Does the amp pick up radio with nothing plugged in? With just a cord? With a cord and guitar? Plugged into the high input? Into the low input?

Dirty contacts, including the shorting switch inside the jack, can cause a "diode effect" that detects radio signals. So can a crystallized solder joint or even a dirty tube pin.

How's your electrical ground? Do you have one of those $10 outlet checkers that tell you if you have a proper ground and if the outlet is wired correctly?

The amp needs an overall health check before you mod it. Of course, if you live under a big ol' radio tower, all bets are off.
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  #11  
Old 01-17-2012, 02:21 PM
guitarcapo guitarcapo is offline
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An extra resistor before your guitar signal hits the first preamp tube might make your tone muddy too. It's not like your signal from the guitar is amplified any yet so basically some of your guitar signal is just winding up heating that little resistor you installed instead of getting amplified.
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  #12  
Old 01-18-2012, 04:40 AM
momomo momomo is offline
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thank you all for your help!
the guy in the amp shop installed the resistor & now the radio problem is solved!

Last edited by momomo; 01-18-2012 at 06:32 AM.
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