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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Thu Jan 31, 2013 10:51 pm
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Chris, 34mA is a little cold for 465 VDC plate volts = about 15 watts per 6L6GC. Try turining up the bias to around 38-39mA (18 watts per tube). Check tubes for orange-plating.

What kinda noise? Static? Humming? Popping? Ticking?

If the STANDBY SWITCH causes similar noise when turned on-&-off, prolly need a new switch. Did you try contact cleaner on this switch?


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Thu Jan 31, 2013 11:05 pm
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Ill go over it again tomorrow
It's not hum and its not sizzle
Kinda like its got an extra hot signal
Input noisy overdrive pedal kind of noise
And it's very slight
Ill adjust the bias tomorrow too
Not even sure which pair of 6l6's to run
In there

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Fri Feb 01, 2013 7:27 am
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The sound is like moving air even with the volume pots on zero (1)
there is a very slight hum in the background of the noise

The bass side has a volume pot that spikes hard at about 2 on the dial

however the overall tone is warm and sweet !

Do you suppose the noise is the result of loosing those caps to ground on pin 8 of the 6L6's ?

A tweaking we will go :mrgreen:

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Fri Feb 01, 2013 12:51 pm
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I doubt it's the lost of those 2000 pico-farad grid-to-ground caps that were once on the 6L6GC. "Hissy" or blowing air noise is usually generated in the input, gain, phase inverter stages. Try some different 12AX7s and 12AT7s and see if that helps. Also, the wiring layout hanging off those sockets can pick up or magnify radio frequency (RF) noise.

Sometimes, changing the position of some wires stops this noise. If they cross over each other --- try to have them cross at right-angles.

Usually, tube(s) and/or layout dressing of the wires stops this problem.


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Fri Feb 01, 2013 9:25 pm
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Yep, I agree about the lead dress!!! Also, you might want to try a lower gain tube in the V2 position. I have a 5751 in my Bassman. Trying different pre amp tubes and PI tube, can be very benefical. Seems like the amp is coming into focus, Chris!!! Art

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sat Feb 02, 2013 6:56 am
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I'm going to spend some time with it today, The power tubes have a lot of movement in the sockets too.

I see a few more of those extra thin wires on the board I would like to replace while I'm in there.

When you get past the wind noise, the tone is everything you would expect from a
a sweet old Fender tube amp.

I do have a couple of those 6060 Brimar 12AT7's and a 5751 for v-3 too

Looks like New 6L6 sockets would't hurt either !

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sat Feb 02, 2013 7:52 am
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Here is an interesting find on the board of the bassman

See the marked in red two black wires leaving the cluster of small mallory caps and resistors ?
both leave at different angles but go to ground one on the brass plate that holds the controls (right wire) and one to the left side of the volume pot on the normal channel (the ground lug)

Question isn't that a ground loop ??? and therefor no ground at all ???

Image

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sat Feb 02, 2013 12:31 pm
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The only way to know if it's a ground loop is to try different grouding arranements. I'd use a continuity check of the original line (resistance DC on the DVM, with amp off) --- to be sure those lines don't past through other ubseeb comoponents.

Did you try different 12AX7 and 12AT7 to rule-out tube noise, first?

Chris, you have mail! re: way I go step-by-step to isolate noise and that original GNFB arrangement of the AC568.

Good luck!


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sat Feb 02, 2013 1:48 pm
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Excellent strategy !!!
I'll report back
Got the mail also.

Most snow and cold all year and I've been at the jail like its summer time :roll:

Don't they know I'm busy with important stuff :mrgreen:

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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: Sat Feb 02, 2013 6:07 pm
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Chris, that volume pot is usually grounded straight down to the brass ground buss plate. And the cathode bypass circuits usually take a reasonably straight path to same brass buss plate. that system probably works, to ground the pot, but it's kind of a lazy way of doing it. JMO, Art

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sat Feb 02, 2013 6:23 pm
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OK
Update rewired V-2 which helped cut some noise.
While testing live I bumped V-3, it made that plugging into a live amp sound
and sent a flash through the V-5 6V6.
Just happened to have a two pack of Fender nine pin sockets :mrgreen:
I used windows paint and a picture of the socket to keep my wires straight and boy did it help !!!
The heater wires were easy to remember and so I didn't list them
HUGE Difference in noise !!!
I had also hit deoxit on the pots and switches earlier.
I have one socket left, I'm thinking about giving it to the `12AT7 PI
AKA 6060 Brimar that I have in there 8)
Image

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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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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sat Feb 02, 2013 6:58 pm
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Chris,

You are getting close to solving the noise issue.

1.) Bad V3 socket. Replace.

2.) Check the soldering of pins 3 and 8 of that phase inverter (12AT7 or V4). This solder point often gets brittle with time. And is critical for balanced PI output (connects the two cathode halves of that PI).

Suck off the old solder. Rebend the two tangs together with a needle-nose plier. Resolder.

http://www.webphix.com/schematic%20heav ... _schem.pdf


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sat Feb 02, 2013 7:00 pm
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Cool!!! The PI would really dig a new socket. Glad you're gettin' that noise level down!!! It always amazes me, how the synergy of these guys can work out. Keep at it!!!....
yeah been dealing with the snow and ice up at work today. OOPS! sorry about stomping on Beemers post! Art

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sat Feb 02, 2013 7:09 pm
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Art,

No problem. It's hard to "long distance" repair amps. So, as many opinions to "fixes" and experiences are always welcomed.

BTW... Chris, you may want to look at that white Mallory bypass cap with the "solder hickey."


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sat Feb 02, 2013 7:41 pm
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Just had to notice that didn't ya !!! :cry:

I put the 47 K back in and got the original blast when i hit the standby to play.
Pulled it again and now i have a motor boat sound that wasn't there :( :(

Too many florescent lights computers modems within feet of where I'm working, all that could be influencing the whole mess.

Gonna give the PI a new socket as well, looks like whatever corrosive got inside this amp hit those two sockets pretty well so, a new one is in order.

I did notice that in the two books I have that describe the "blackfacing" they both mention using an 100K and an 82K to replace the 47K's in the Phase inverter
any thoughts here ???

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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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