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Post subject: Re: 1970's Bassman 10 project just came in
Posted: Sat May 23, 2015 4:28 pm
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I just finished converting to a true bias control
one more joint to solder and .........................
The RCA's are idling at 34ma and quiet

Still gonna change the filter caps (on order)

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Post subject: Re: 1970's Bassman 10 project just came in
Posted: Sat May 23, 2015 6:25 pm
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Chris, did changing over to bias pot control also fix the dropout issue? Like Shimmy, I think it maybe the carbon comp resistors --- esp those in the power supply. Metal oxide would be the way to go in the PSU rail. IMHO.

Also, check the resistors on the 6L6GC sockets. The grid stopper and screen resistors. Flameproof metal oxides, here would be nice, too.


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Post subject: Re: 1970's Bassman 10 project just came in
Posted: Sat May 23, 2015 6:30 pm
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Metal oxides riding over the 6L6GC's is next while I wait for the lytics to come in
and no I still have drop out and noise
We shall overcome !!! Eventually 8)

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Post subject: Re: 1970's Bassman 10 project just came in
Posted: Sat May 23, 2015 7:30 pm
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Anode plate voltages on 6L6GC, 12AT7, & 12AX7 within specs? Cathode voltages on the 12AT7 and 12AX7?

Steady voltages?


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Post subject: Re: 1970's Bassman 10 project just came in
Posted: Sun May 24, 2015 4:22 am
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Any idea why there are two small ceramic caps from pin 1 to 8 then to ground on both 6L6 tubes ???
nothing I've seen in a standard Bassman circuit
Image

and a shot of the true bias conversion
Image

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Post subject: Re: 1970's Bassman 10 project just came in
Posted: Sun May 24, 2015 4:54 am
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Pin 8 of the 6L6 is connected to ground (using one lead of the cap), that's standard. Trace the red wire from the other side of the cap (pin 1) to see where it goes. Pin 1 on the 6L6 is not used, so it is just being used as a terminal for another part of the circuit.

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Post subject: Re: 1970's Bassman 10 project just came in
Posted: Sun May 24, 2015 6:07 am
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here ya go
Image

You'll probably have to click on the picture and enlarge it to see i made notes on it in red
so it looks like some of the signal is being run to ground through those small ceramic caps from pin 1 to pin 8 to ground.
I've never seen that before, cure for oscillation ???

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Post subject: Re: 1970's Bassman 10 project just came in
Posted: Sun May 24, 2015 7:09 am
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Chris,

Those are anti high-frequency parasitic oscillation caps. Seen in many CBS era Fender amps. If you look carefully, I think that you'll see them off pin 5-to-ground (after the 1500-ohm grid stoppers). Usually, the grid stopper resistor is soldered across pin 1 (unused, as Shimmy stated) and pin 5.

If they are soldered from pin 1 to ground --- this just means someone put them in, electrically, before the grid stoppers. Not the usual convention and prolly done by a tech post production.


Image

Image


Last edited by BMW2002Ti on Sun May 24, 2015 8:03 am, edited 2 times in total.

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Post subject: Re: 1970's Bassman 10 project just came in
Posted: Sun May 24, 2015 7:31 am
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The one on the first 6l6 is very dark
Like it's been hot
Snipping should be ok ?
Does pin 8 need a resister to ground or
Is straight to ground ??

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Post subject: Re: 1970's Bassman 10 project just came in
Posted: Sun May 24, 2015 8:01 am
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Chris, every amp I've worked on, that had those caps, I've clipped off with no problems. However, if one of them looks burnt, that maybe a bad sign. These caps are attached to the signal grid. And very little current should be passing though them. If one is burnt, that is a sign of a bad short. Prolly a shorted 6L6GC. Or a leaky coupling cap from the phase inverter (0.1mfd).

Might be worth investigating. Did the amp work fine, before this recent cut-out issue? Could be you have an unstable amp. Just a WAG.


http://bmamps.com/Schematics/fender/bas ... _schem.pdf


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Post subject: Re: 1970's Bassman 10 project just came in
Posted: Sun May 24, 2015 11:28 am
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Looks like the burn marks are from it touching an exposed wire over time
Removed the caps off of pin1 and left the lead going from pin 8 to ground

Also removed the .047 that went from the treble pot on the Studio channel to the volume pot as per the
how to make your bassman 10 sound nearly like a Super reverb article I read, supposed to open up the tone

Right now on the bench (connected to the cab) I get 501 and 502 Volts DC from the pin 3 of both 6L6GC's
Beemer requested the voltages

And dead quiet ! no pop when the standby was engaged
Gonna button her up and give it a good 20 minute workout
Updates to follow

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Post subject: Re: 1970's Bassman 10 project just came in
Posted: Sun May 24, 2015 12:19 pm
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Update
Since removing the caps from the 6L6's she rocks out and no more pops crackles etc.......
taking that cap off the treble and volume pot (studio channel) really opened up the tone, gonna check into the rest of the mods (very simple) after the new filter caps
http://www.premierguitar.com/articles/S ... Bassman_10

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Post subject: Re: 1970's Bassman 10 project just came in
Posted: Sun May 24, 2015 1:40 pm
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Those caps did not connect to pin 5 of the 6L6, they connect to pin 1 and a red wire from pin 1 to..... the other side of the grid resistors. :?:

Here is the schematic for the Bassman 10, note the two caps, .002 µF, connected on the input side of the grid resistors. Those are factory installed, and although possible, the ceramic caps rarely fail. I would put them back, or put two new ones back. The purpose is to keep the output tubes from red plating (, unstable oscillation, with maybe no sound problems to warn of impeding failure). :wink:

https://drive.google.com/file/d/0B_UmuX3ZWQWnZEhfNVNpU1NXSkE/view?usp=sharing

Let me know if the link doesn't work and I can email a copy to you.

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Post subject: Re: 1970's Bassman 10 project just came in
Posted: Sun May 24, 2015 2:21 pm
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Now that its converted to a true bias adjustment I doubt red plating will be an issue

however the amp was bold and sounded great, i played for a half an hour or so , no issues, turned it off had some coffee went back down and it went BZZZP on start up and dropped out , crank the volume and it overcomes whatever is stopping the power, for a while till it drops out again

Dude what a jerk of an amp ! are ya fixed or not ??? obviously not !

PS the schematic won't print Shimilou I have a copy but ya can't really read the little stuff

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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: 1970's Bassman 10 project just came in
Posted: Sun May 24, 2015 3:51 pm
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This is one WEIRD ! Amp :shock:

How about a wire connected to the filter caps that goes nowhere !
Image

here is where it kinda enters the chassis
Image

Here is where it is connected to one of the 20uf 500Volt caps

Image

Very strange, however i saw a picture of a UL model that had two such wires to nothing

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