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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sun May 12, 2013 11:38 am
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I had recently cleaned the shorting contacts on the jacks.
This is weird like its picking up a signal from my hand.
I wondered about the 12AX7 in v-2 since its in that area where it seams to pick up the sound.
Odd that the Bright Channel is dead quiet.
I'm not beyond believing that The input jack on the normal side is messed up and beyond deoxit and some scraping.

Hey Happy Mothers Day by the way !
Pretty sure none of you are mothers so tell your wifes and or Moms !!!

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sun May 12, 2013 12:53 pm
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blues bondsman wrote:
I had recently cleaned the shorting contacts on the jacks.
This is weird like its picking up a signal from my hand.
I wondered about the 12AX7 in v-2 since its in that area where it seams to pick up the sound.
Odd that the Bright Channel is dead quiet.
I'm not beyond believing that The input jack on the normal side is messed up and beyond deoxit and some scraping.

Hey Happy Mothers Day by the way !
Pretty sure none of you are mothers so tell your wifes and or Moms !!!


You are getting capacitive coupling between the amp and your hand causing your body to act as an antenna and pick up noise. It could be a tube and V1 would be the most likely choice as it is the most sensitive. V1 is shared between the normal and bright inputs so one triode could be bad. I would look closely at the components from the normal input jacks (both high and low sensitivity inputs) to the normal half of V1 including the tube. Check for a bad solder joint at each jack and the 1 meg input resistor and the 68K grid stoppers in the normal channel. V2 is common to both channels (voltage amp and cathode follower driving the tone stack) and would affect both channels so it is unlikely that it is the problem.

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Wed May 15, 2013 10:07 am
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Looks like I may have to install new input jacks, swapping tubes didn't have much affect.
The noise went away if I touch the chassis, and so I assume its a grounding problem.

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Wed May 15, 2013 11:36 am
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Chris,

Like Bluesky said, I believe you have a capacitance-coupled stray RF problem. You touch the chassis and the hum goes away --- this is because the capacitance in your hand is altering the the inherent capacitance of the amp. And altering how your amp is picking up the stray RF frequencies. Kinda like loading an antenna with a different LCR circuit. Anyhoo...

To test your jack --- get a spare plug. Short the patch cord end so the two points are soldered together. Like a shorting plug. Now, plug this shorting plug into the input. Any change in hum? If not, the jack is ok. If the shorting plug makes the hum less, you have a bad jack.

If the shorting plug didn't change the hum issue, you prolly are picking up stray RF. Maybe move the wires around and see if this helps. Be sure that the shielded cable running from the jack to both grids of the 12AX7 input tube have good grounding on the jack side. The tube side should not be grounded.

If this wire is not a shielded cable, you may want to replace it with shielded coax wire.

Good luck! This can be a very vexing issue. But, in my experience the cause is most often between the input jacks and the grids (pins 2 & 7) of input tube.


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Wed May 15, 2013 12:10 pm
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Cool !
So I took a short 1/4 cable and plugged it in the 1st jack of the normal channel
I took a wire with alligator clips and shorted the cable at the other end, one clip on the neg and one on the poss.
Zero hum ! not a drop !
So its safe to say that the most used input jack may be in need of replacement.
Makes perfect sense.
Wish I had a points ignition file !!! I'm betting I could get that contact up and running with a little touch of a super thin file.

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Wed May 15, 2013 12:18 pm
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It's worn out, I would replace it. You could bend it and clean it, but a new one wold be much better.

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Wed May 15, 2013 12:36 pm
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Funny thing is, I go out to the garage and in the top of my tool box in with other misc. tools is none other than an old points ignition file !
Probably from my youth :mrgreen:
My first car was a 67 or 68 Impala, i was under the hood as much time as I was behind the wheel :lol:
I paid 50 bucks for it and drove it for years ! See a pattern here ????

So now I have zero hum !!! :mrgreen: :mrgreen: :mrgreen:
I did however bend the contact slightly to give it a better contact, so filing and bending did wonders !
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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Wed May 15, 2013 2:02 pm
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blues bondsman wrote:
Funny thing is, I go out to the garage and in the top of my tool box in with other misc. tools is none other than an old points ignition file !
Probably from my youth :mrgreen:
My first car was a 67 or 68 Impala, i was under the hood as much time as I was behind the wheel :lol:
I paid 50 bucks for it and drove it for years ! See a pattern here ????

So now I have zero hum !!! :mrgreen: :mrgreen: :mrgreen:
I did however bend the contact slightly to give it a better contact, so filing and bending did wonders !
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And the wives laugh at us when we hang on to that useless junk. :lol: :lol: :lol:

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Wed May 15, 2013 3:35 pm
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I've had to "adjust" several input jacks to get rid of noise and hum. :lol: :lol: :lol:

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Wed May 15, 2013 3:42 pm
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Lesson learned !
Deoxit alone isn't enough !!!

It takes physical contact with the contact points of the input jacks to get the kind of connection that stops noise.

I noticed the back two jacks have nearly zero bend in the shorting contact piece and both fronts were slightly bent to add pressure on the point of contact.

She is whisper quiet now, even with the volume way up there.

Thanks !!!

I'll order some new jacks for future replacement as I'm sure I only bought a little time with this.

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Wed May 15, 2013 5:41 pm
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Chris,

Good news!

Love that points file. :D

I have used a needle nose pliers to adjust the contact angle. Just like old ignition points. Use a dab De-Oxit on the filed contact. This will help keep them from oxidizing, again.


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Wed May 15, 2013 6:33 pm
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Well.........
Clark Piedmont troubles #2

Then the stupid thing blows a fuse and smokes one of the 5881 power tubes.
Lucky for me I have a nice matched pair of Sylvania's here at the house.

However to make a long story kinda short, same tube tested in both sockets, one side it reads 25ma the other side reads 37ma, How is that even possible ?????
so the tubes both show 25 on one side and 37 on the other, in other words its not the tubes, its the sockets.

One thing after the other with this amp !

PS the side reading 12ma more is where the tube fried.

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Keepn' the Blues Alive

2004 50th Ann. Limited edition AMSE Stratocaster
2017 50's Baja Telecaster BSB

1968 Bandmaster with 2X12 cab C-rex speakers
VHT Special 6 Ultra combo

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Wed May 15, 2013 8:25 pm
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bias meter went to zero I heard sizzle and there was a poof of smoke from the standby switch.

Thats it I'm done !
there is a whole boat load of money out the window !
I'm playing acoustic from now on !!!!!

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1968 Bandmaster with 2X12 cab C-rex speakers
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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Wed May 15, 2013 11:37 pm
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Chris,

See my email to you about bias type & plate voltages? You need to do a step-by-step methods to find the culprit(s). But you already knew that. :D


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Fri May 17, 2013 1:53 pm
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I decided to let Mr Clark have a look at it, too much value for me to be in there possible causing more harm than good !

Hopefully he will treat me right being that its his creation.

He said there were updates to the piedmont he would do (safety stuff)

We shall see

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Keepn' the Blues Alive

2004 50th Ann. Limited edition AMSE Stratocaster
2017 50's Baja Telecaster BSB

1968 Bandmaster with 2X12 cab C-rex speakers
VHT Special 6 Ultra combo

Visual Sound Route 66
Cmatmods Signa Drive


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