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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Tue Feb 05, 2013 5:42 am
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Lets face it guys, this chassis has been drilled and its not exactly original any way.

So What I would like to do is get a new circuit board of high quality and slowly build it into the best Bassman I can with the iron (transformers) it has.

I am even considering a turret style board. (((any suggestions))) ???

Might as well make it a great for guitar head and stop trying to put a bandaid on a huge wound !

A fresh start is what this amp needs !
The filters and filter board should be OK and probabaly the Bias board ???

I wanted a kit to build any way and so, forget its origins for a while.

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Tue Feb 05, 2013 7:20 am
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Here is a little follow up for all those keeping watch.

I installed my bias rite tool and cranked up the Bassman, It went up to areound 41.5 to 42 ma withe the bias pot cranked to wide open, during a fifteen to twenty minute period you would hear a short and quiet burst of hum, the bais readings would fall to around 35, it finally stabilized at around 44ma and so I turned the pot down and have it running at around 36ma.

Sure sounds like a component or a bad solder joint is causing the intermittent noise and change in the reading, again the 6L6 tube sockets are very wobbly.
Possible cause ????

(((Edited for more follow up)))
I've let it run for a while now, to keep the reading at aprox 36ma I've had to make many minor adjustments in the counter clockwise direction of the bias pot, it seems to be creeping up slowly over time.

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Tue Feb 05, 2013 8:12 am
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Chris,

1.) First off... pull all tubes.

2.) measure the bias supply votage at the junction of the two 220k-ohm grid-to-ground resistors attached to pins 5 (grid) ot the 6L6GC.

3.) Must read around (-) 44VDC with bias pot at mid position. This reading has to be very steady and not drift around. Measure this voltage for a good 15 minutes, to be sure.

4.) If the reading is rock steaady (-) 44VDC (+/-) 1 to 2 VDC... then it sounds like you prolly have instability generating between sections (such as, between the V2 and V3 and output tubes).

5.) If the reading is NOT rock steady. You'll we need to go over that bias power supply section again. Check grounding points. Measure resistor values (pull one leg before measuring resistance). Do you have a new 1N4007 (or UF4007) diode, so you can replace the original one? It really doesn't need to be 1N4007. Could be 1N4006 or 1N4005, or equivalent.

If you get to the point of replacing the diode. May want to post. As the direction you place these components onto the board is critical.

Good luck!


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Tue Feb 05, 2013 8:55 am
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Looks like we may be on to part of the problem

I can only get a reading of -45.2 with the bias pot cranked fully on (clockwise)
and at that setting my tubes are running 45 plus ma each

The measurement is rock steady with nearly no fluctuation at the Bias test point
well it fell to 44.9

I also noticed the lamp is very dim so I measured the heater lines at the lamp
they were 3.23 volts ac on both sides

I had a 26K resistor on the bias pot, not sure how that happened
Pays to double check and triple check your work :x


The book Hot Rodding your fender amp has it as 27K on their diagram, I'm guessing that is where I found it.
The layout of an AA864 clearly shows it as 15K

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Tue Feb 05, 2013 10:09 am
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Chris,

On the heater line voltage -- how are you measuring the voltage? At what points on the circuit. Be sure the center tap of the heater line off the PT is NOT grounded. Note how the AA864 heater line has the center tap grounded. And the AC568 does not (but has the twp 100-ohm resistors on each leg)?

Do you see a line off the PT that is green-white or green-black wire soldered to ground?

AA864 heater lines:

Image

AC568 heater lines:

Image


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Tue Feb 05, 2013 11:13 am
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I measured one line at a time and to ground
Then when i measured across both together I got 6.6 volts ac

We have a much more stable bias now with the correct 15K across the bias pot 8)

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Tue Feb 05, 2013 11:16 am
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Chris, You are good to go with that heater line reading. 3.3 VAC from each leg to ground.

I don't know why the lamp seems less bright. Maybe, loose in holder?


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Tue Feb 05, 2013 11:45 am
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Cheap no name bulb (no markings at all) put in a new Wagner 47 and now she shines !

OK first tube after the resistor change on the bias pot
39.4 to 39.7 and stable after 20 plus minutes

Tube #2 39.2 to 39.4 and also stable after 20 plus minutes

Could that one resistor have had that much effect on the entire amp ???
I guess the answer is Yes !

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Tue Feb 05, 2013 12:31 pm
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First run through is a huge thumbs up !
hiss is gone, tone is very good, now a little bass heavy compared to yesterday, not in a bad way at all.

Couldn't see orange pulsing however the V-6 last 6L6 had a lot of blue glow and the blue did pulse a bit on harder hit cords, notes etc.....

Can't get to full volume right now due to napping here at the house
(the wife watches a friends little guy while she works).

The pins are good and tight on the 6L6 sockets but the entire "plastic" part of the socket wobbles, the screws to the chassis are also tight.
Can all that movement have some effect on the tube ? or contact ?

Its getting fun again :mrgreen:

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Tue Feb 05, 2013 1:16 pm
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Chris,

1.) No. The wobbling is prolly not an issue. As long as the amp doesn't change tone when you wobble it. Usually one of both of the socket's hold-down screws are loose or stripped.

2.) Ok.. there are difference kinds of purple haze in output tubes. :lol:

If it's a lazy purple glow between the plates and glass, it's ptolly contaminates burning off. Is this a new tube? Common occurence in new tubes.

If the glow is bright purple-white, intense and can be seen within the plates of the tube -- -could be a bad sign. Esp if the idle bias jumps around on that tube. Maybe a gassy tube. Or one with conduction issues. In this case, best to replace it.


Just to refresh. What did you do?

3.) Change bias power supply resistors? What is the bias now? Do you need to crank the bias pot to one side full CCW or CW or get this reading?

OK.. didn't see your first post here. Yes one dropping resistor in the bias pwoer supply can make a world of difference. That voltage at the junction point of the 220k-ohm grid-to-ground resistors is MUCHO critical. Has to be rock steady and (+/-) 2 VDC of value on schematic. Or you'll never get the output tube to bias properly. At least within the range of the bias pot.


4.) Replace or alter the GNFB network?

Good luck. Hope that you are getting close. How's the OD?


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Tue Feb 05, 2013 3:33 pm
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GFNB is gone and never to return !!!!
I hope !

Now for a little more power, full boar test coming up !

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Tue Feb 05, 2013 8:29 pm
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sounded pretty stink'n good !

But still the 6L6's (fourth pair I tried) were acting kinda funny
Dancing blue glow 8)

Here is what i found
Image

Ya that should have been changed a long time ago !
Sitting right on top of V-6 the last 6L6

But now that I re soldered the two tube sockets I now have more noise again :x
I'm guessing the heater wires do have to follow the same path (pins ) on the 6L6's ????

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Tue Feb 05, 2013 8:54 pm
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Chris,

That is prolly a fried screen grid resistor (yellow-purple-red, before incineration). Soldered between pins 4 & 6 of the 6L6GCs.

If I were you, I'd replace all of the screen resistors and grid stoppers (the 1500 ohm resistors soldered between pins 1 & 5). Sometimes when one resistor pops, the others have been stressed. Use metal ocide, flame-proof puppies.

If V6 6L6GC had the dancing purple glow --- could be it lack voltage on the screen grid, due to toasted resistor. Screen grid helps control emissions within the tube and prevent runaway.


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Tue Feb 05, 2013 9:04 pm
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I replaced them all and now have more noise
And more tube flashing
Arghhhhhh !!!!!
The other resistors were close to spec
But I changed them any way

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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: Tue Feb 05, 2013 10:57 pm
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Chris,

You replace them with metal oxide, flame-proof resistors? Should be quieter than carbon comp (even new CC's). Double check to be sure the 470-ohm resistors are between pins 4 & 6. And the 1500-ohm resistors are between pins 1 & 5.

Ok... The lead to pin 5 (grid stopper resistor) and to pin 4 (screen resistor) must be as short as possible. The leads to pin 1 and pin 6 for the two resistors can be longer to accomodate. This is because long resistor leads to the grids can pickup noise and induce instabities.

Sometimes when you solder leads or resistors onto the sockets, the tangs that hold the tube can loosen up. May need to toghten the socket tangs.

You you are going to do this. DISCHARGE main caps. Use a small screwdriver, awl, or dental probe to bend the socket sleeves.

Good luck! Hope this works.


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