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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Thu Jan 31, 2013 5:40 pm
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OK
I went back and touched up some solder joints I wasn't crazy about, i removed a few of the ultra skinny wires and replaced them with cloth 22 gauge wire.
swapped out all the pre amp and power tubes.

Now I have sound, its amazingly faint and distorted.
Only if you crank it to 10

I went with the 100, 22, 16 F&T's and changed all the resistors under the cap cover to spec and with metal oxide for heat protection.

what Am I missing ???????

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Thu Jan 31, 2013 5:45 pm
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How do the voltages check-out, with no tubes? A continuity test( amp unplugged and discharged) will have to be done. I would start by checking all grounds, even those that are accomplished through mechanical connections. Art

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Thu Jan 31, 2013 6:05 pm
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Art, removing the single point of the pin 8 grounds and giving each one its own ground solved a lot of the humming.
I also replaced some of those extremely thin wires.

I am beginning to think those pin 8 grounds are still an issue.

So I'll reconstruct them extra heavy duty, I was using solid wire 22 gauge cloth wire and it may not be enough, What do you think ???

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Thu Jan 31, 2013 6:06 pm
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PS the voltages checked out so far

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Thu Jan 31, 2013 7:43 pm
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I'm at a loss guys :cry:

I have checked every solder joint, added heavier wire in every place i worked, made the grounds better etc...........

I get sound like the volume knob is on zero, fuzzy guitar.

What did I do wrong ???
Every time I hit the standby to try and hear it i get a shorted speaker kinda sound super loud gas attack :roll:

The 12AT7 seems much brighter than the rest of the tubes
was there something I missed in the heater wiring ??? is there a polarity I should have tracked ???
HELP !!!!!

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Thu Jan 31, 2013 7:57 pm
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Chris,

Check the voltages off the PS rail to the 12AT7 and 12AX7's. Check to be sure that the heaters of all the tubes is getting voltage. You can do both checks with the 12AT7 and 12AX7's pulled. And the 6L6GC pulled. Heater is AC voltage. All other checks are DC voltages.

Really go over that 12AT7 PI section. Check for continuity. Proper grounding. Good solder points to the coupling caps. You can just plug in a "bogey" 12AT7 and measure the anode and cathode plate voltages.

See if this finds the culprit.

BTW... you went back to BF GNFB (global negative feedback) of 820 ohms, right? This section isn't completely reverted back to BF standards. Remember, the 12AT7 does not have the 100-ohm grid-to-ground resistor of the AA864 circuit. If you get all the proper voltages in this section... TRY THIS...

1.) Pull one leg of the 820-ohm GNFB resistor. Get clean volume now? You'll prolly need to go back to the original AC568 GNFB network (47k-ohms, bypassed iwth 100 pico-farad + 0.1 mfd cap).

2.) Alternatiuvely... try reversing the secondary leads off the output tranny (OPT). The ones that go to pin 3 of each 6L6GC. Just to see if the OPT was wired out-of-phase WRT the OPT that came with the amp.

Do #1 OR #2. But, not both together. As reversing the OPT leads = changing the polarity of the GNFB from negative to positive voltage. If you try both test together, you won't know if it's the leads of the OPT or the voltage of the GNFB that is the issue.

Good luck! Keep us posted.


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Thu Jan 31, 2013 8:10 pm
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Gotcha

12AT7 = 445 volts

V-2 12AX7 =230 volts

V-1 12AX7 =225 volts

Red lead from the diodes to the board = 450 volts

I'll try pulling the 820 ohm resistor leg and let ya know

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Thu Jan 31, 2013 8:24 pm
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Just a question, You're getting 445VDC at the12AT7's pins #1 and #6 ? Art

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Thu Jan 31, 2013 8:28 pm
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Hey

I pulled one leg of that 820 and..............................

IT WORKS !!!!! and even sounds GOOD !!!!!


still a slight pop when i flip the standby to play.

OH MY !!!!

Looks like I'll have to rebuild that .1 and get a nice silver mica for it too

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Thu Jan 31, 2013 8:29 pm
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Art I got that reading from pin #1

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Thu Jan 31, 2013 8:58 pm
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Chris,

Try this amp without GNFB. It may work just fine. Just check the idle bias and the 6L6GC (for orange plating, etc).

There are many amps without GNFB. And they sound very good.

Worse gets to worse, put the original AC568 GNFB network back into the circuit. Its 47k-ohm resistor (plus the 100 pico-farad and 0.1 mfd cap) supplies LESS feedback voltage to the 12AT7 PI tube, than the AA864 GNFB.

Remember, we didn't really fully revert that 12AT7 PI section --- 100% back to BF AA864 settings, right?

Try Deoxit GN5 (or D5) on that STANDBY SWITCH.

Good luck! Keep us posted.


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Thu Jan 31, 2013 9:44 pm
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I buttoned her up and gave her a spin !

I'll rebuild that old part of the circuit just to keep her stable, I have plenty of Brown sound to tap into with the new JTM-45 clone 8)

Something with POWA and Headroom is kinda nice too !!!

The Traynor may have a new home soon, its a bad boy for sure but.......
I'm only one guy and my son is playing bass more and more.
Image

Image

A little tolex work and it will be Super Sweet !!!!!

Thanks to all !!!
Beemer I know I drove you crazy !!!!
Art Thanks again Brother !!!
Arjay Moral support and helping to keep it real !

Next ! :lol: :lol: :lol:

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Thu Jan 31, 2013 10:30 pm
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Chris, very cool!

Just to be anal... may want to check the bias on those 6L6GC, say twice in the next week --- to be sure things aren't drifting around. Forget the GNFB. You are going to like the amp without that network.

Good tranny and good 6L6GC tubes love to run free without global feedback constraints.

:D :mrgreen: :D


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Thu Jan 31, 2013 10:44 pm
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BMW2002Ti wrote:
Good tranny and good 6L6GC tubes love to run free without global feedback constraints.


WORD!

Glad it's all coming together now, Chris. A bit of a nail-biter when it failed the initial "smoke test" but I think you got a handle on things now.

Rawk on!

8)

Arjay

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Thu Jan 31, 2013 10:45 pm
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They are hanging tight at around 34ma
Right now
There is a little noise still
Could still be that switch ???

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

Visual Sound Route 66
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