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Post subject: Cathode resistor on a Bassman?
Posted: Mon May 09, 2011 8:21 pm
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I am doing a cap job on my 1969 Fender Bassman. I noticed on the power tube, there is a 150ohm cathode resistor tied to groud. And a 5mfd 25v cap going from pin 8 to pin 8 on the 6l6 tubes. I don't see this in any of the schematics. My amp is AB165, I believe.

Anyone know why the cathode resistor and the cap? Could it be some sort of bias mod?


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Post subject: Re: Cathode resistor on a Bassman?
Posted: Mon May 09, 2011 10:40 pm
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If your amp is indeed a '69 model it is not an AB165 -- you have the AC568 circuit.

Here's the correct schematic......

http://classic-web.archive.org/web/2006 ... _ac568.pdf

HTH

Arjay

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Post subject: Re: Cathode resistor on a Bassman?
Posted: Tue May 10, 2011 4:57 am
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Some tech suggest to remove those resistors and solder kathode to ground to have more "real "fender sound.

You can do the test yourself; put a jumpers across those 2 resistor and ear the sound. You could need to check and reajust bias.

But in any case, with an old amp, good thing to check bias anyway


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Post subject: Re: Cathode resistor on a Bassman?
Posted: Tue May 10, 2011 8:45 am
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Retroverbial wrote:
If your amp is indeed a '69 model it is not an AB165 -- you have the AC568 circuit.

Here's the correct schematic......

http://classic-web.archive.org/web/2006 ... _ac568.pdf

HTH

Arjay


Thanks! Indeed, it is a AC568. What is the cathode resistor and cap for?


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Post subject: Re: Cathode resistor on a Bassman?
Posted: Tue May 10, 2011 8:50 am
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stratele52 wrote:
Some tech suggest to remove those resistors and solder kathode to ground to have more "real "fender sound.

You can do the test yourself; put a jumpers across those 2 resistor and ear the sound. You could need to check and reajust bias.

But in any case, with an old amp, good thing to check bias anyway

Thanks. I'll try that. The bias is just a balance pot. I might add a trimmer so I can adjust bias as well. I did that to my 78 Vibrolux Reverb and it is quite handy.

Currently, the Bassman is at 425v plate and with a pair of RCA tubes, 28ma. A JJ pair went down to 18ma, too low.


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Post subject: Re: Cathode resistor on a Bassman?
Posted: Tue May 10, 2011 9:38 am
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jazzgene wrote:
What is the cathode resistor and cap for?


Those components were add-ons suggested by the new amp engineers that CBS brought in after Leo sold the company. They were intended to reduce parasitic oscillations within the output stage and increase the amp's clean headroom.

Arjay

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Post subject: Re: Cathode resistor on a Bassman?
Posted: Wed May 11, 2011 10:38 am
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Arjay, I also believe it was a feable attempt at adding a tad of cathode-bias "warmth" to what was then a very non-traditional Bassman tone. Give a the amp some "sag." Didn't work, as most combo fixed and cathode-biased amps don't do either very well.

That idea didn't last too long.... thank goodness. :P


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Post subject: Re: Cathode resistor on a Bassman?
Posted: Wed May 11, 2011 10:53 am
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BMW2002Ti wrote:
Arjay, I also believe it was a feable attempt at adding a tad of cathode-bias "warmth" to what was then a very non-traditional Bassman tone. Give a the amp some "sag." Didn't work, as most combo fixed and cathode-biased amps don't do either very well.

That idea didn't last too long.... thank goodness. :P


+1

However, much of this effluvia was still included in the subsequent AA270 revision. CBS still hadn't "gotten it" yet.

Arjay

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Post subject: Re: Cathode resistor on a Bassman?
Posted: Thu May 12, 2011 11:40 pm
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Gotta say, this amp sounds really good! Just picked up a 57 deluxe reissue. Looking forward to this one.


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Post subject: Re: Cathode resistor on a Bassman?
Posted: Fri May 13, 2011 5:10 am
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jazzgene wrote:
Gotta say, this amp sounds really good! Just picked up a 57 deluxe reissue. Looking forward to this one.

I have one of those Deluxes,it's a good sounding amp....it's one of only two modern amps I own now.


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Post subject: Re: Cathode resistor on a Bassman?
Posted: Sat May 14, 2011 11:48 am
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Rebelsoul wrote:
jazzgene wrote:
Gotta say, this amp sounds really good! Just picked up a 57 deluxe reissue. Looking forward to this one.

I have one of those Deluxes,it's a good sounding amp....it's one of only two modern amps I own now.


Are you using the stock Jensen speaker?


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Post subject: Re: Cathode resistor on a Bassman?
Posted: Sat May 14, 2011 3:34 pm
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No,I put a Weber 12A125 in the Deluxe,it's so much better than the Reissue Jensen...at least to my ears.The Jensen will be okay to most people,but having grown up playing through and hearing American made Jensens,the Weber sounds more like them.
Arjay hates the RI Jensens!


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Post subject: Re: Cathode resistor on a Bassman?
Posted: Sat May 14, 2011 4:58 pm
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Rebelsoul wrote:
Arjay hates the RI Jensens!


"Detests them", actually.

:lol:

Arjay

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Post subject: Re: Cathode resistor on a Bassman?
Posted: Sat May 14, 2011 6:57 pm
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Retroverbial wrote:
Rebelsoul wrote:
Arjay hates the RI Jensens!


"Detests them", actually.

:lol:

Arjay


a bit harsh?


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Post subject: Re: Cathode resistor on a Bassman?
Posted: Sat May 14, 2011 9:29 pm
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If you get the Deluxe and play it the stock Jensen starts to sound better after awhile,and there's some good tones coming out of the amp...I removed the Weber I have in mine to try in another amp and reinstalled the Jensen,I thought that if I left it in I would get used to it,but didn't.
The Weber has a warmer,rounder sound with some great tonal qualities that left the Jensen sounding brittle and somewhat harsh....as I say I can't explain tone very well,but can hear the difference...but I love Weber speakers and currently have them in all of my amps,you would think they would all sound the same but don't...three vintage Deluxe Reverbs I own with the same model speaker,all sound different.


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