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 Post subject: Fender 75 thoughts and Biasing
PostPosted: Sat Apr 06, 2013 7:40 am 
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Hey everyone. I got myself a (new) vintage amp today and its the Rivera-era Fender 75 amp with some bad power-tubes and only chinese 12AX7's. When i have the amp on "High-power" mode, both power-tubes glows a hot red and starts to hum really loud.

I bought a whole set of Tung-Sol 12AX7s, Jan phillips 12AT7's, a JAN 5751 and a pair of TAD 6L6GC and now it sounds really really good.

May i ask you all how to bias a Fender 75? The thing is; i cant find the bias pot on the inside of the amp. It only has a hum balance and a output tube matching pot. I have biased Deluxe reverbs and Blues deluxes before, but its really easy to bias those because of the already installed pots.

Besides the whole bias thing; what are your thoughts on this amp?
All help is appreciated!


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 Post subject: Re: Fender 75 thoughts and Biasing
PostPosted: Sat Apr 06, 2013 10:54 am 
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You can't find a bias pot in this amp because it doesn't have one. Unless you modify the amp and replace the output tubes matching circuit with a bias circuit, you would have to replace bias resistors. A real PITA.

http://www.bazziamps.com.ar/diagramas/fender75.png

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 Post subject: Re: Fender 75 thoughts and Biasing
PostPosted: Sat Apr 06, 2013 11:04 am 
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bluesky636 wrote:
You can't find a bias pot in this amp because it doesn't have one. Unless you modify the amp and replace the output tubes matching circuit with a bias circuit, you would have to replace bias resistors. A real PITA.

http://www.bazziamps.com.ar/diagramas/fender75.png


Okay.

Thanks for clearing that up!
Are you the same Bluesky that is on TDPRI? I liked the thread about the different tube swaps in the BDRI! That 12AY7 really fits in that V1!

This one: http://www.tdpri.com/forum/archive/inde ... 57230.html


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 Post subject: Re: Fender 75 thoughts and Biasing
PostPosted: Sat Apr 06, 2013 11:11 am 
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Feelyfeels wrote:
bluesky636 wrote:
You can't find a bias pot in this amp because it doesn't have one. Unless you modify the amp and replace the output tubes matching circuit with a bias circuit, you would have to replace bias resistors. A real PITA.

http://www.bazziamps.com.ar/diagramas/fender75.png


Okay.

Thanks for clearing that up!
Are you the same Bluesky that is on TDPRI? I liked the thread about the different tube swaps in the BDRI! That 12AY7 really fits in that V1!

This one: http://www.tdpri.com/forum/archive/inde ... 57230.html


Yep. Don't spend a lot of time there anymore. If you liked that old thread, you will like this one even more:

viewtopic.php?f=13&t=80142&p=911995

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 Post subject: Re: Fender 75 thoughts and Biasing
PostPosted: Sat Apr 06, 2013 12:38 pm 
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bluesky636 wrote:
Feelyfeels wrote:
bluesky636 wrote:
You can't find a bias pot in this amp because it doesn't have one. Unless you modify the amp and replace the output tubes matching circuit with a bias circuit, you would have to replace bias resistors. A real PITA.

http://www.bazziamps.com.ar/diagramas/fender75.png


Okay.

Thanks for clearing that up!
Are you the same Bluesky that is on TDPRI? I liked the thread about the different tube swaps in the BDRI! That 12AY7 really fits in that V1!

This one: http://www.tdpri.com/forum/archive/inde ... 57230.html


Yep. Don't spend a lot of time there anymore. If you liked that old thread, you will like this one even more:

viewtopic.php?f=13&t=80142&p=911995


Gold. 8)


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 Post subject: Re: Fender 75 thoughts and Biasing
PostPosted: Sat Apr 06, 2013 7:07 pm 
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I used to own one. A retube always helps these amps. Just use the balance pot and screw it back and forth until you find the spot where the amp is quietest.

FYI: These amps may be "Rivera-era" in that he started running the amp division when these came out, but Rivera himself had no hand in this amp. The 75 was designed by Ed Jahns.

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 Post subject: Re: Fender 75 thoughts and Biasing
PostPosted: Sun Apr 07, 2013 8:08 am 
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I changed out the original EV 12F speaker with a Vintage 30.
Dont know if its a downgrade or upgrade price-wise.
But my ears says V30!!!!

Maan what a sound with a new speaker! Its really bass-heavy (not the boomy bass) and smooth nice highs.
How much do you think i can flip the EV 12F for? Or should i keep it?

I wonder if my Fender blues deluxe likes the EV 12F...


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 Post subject: Re: Fender 75 thoughts and Biasing
PostPosted: Sun Apr 07, 2013 11:14 am 
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I wouldn't push a V30 too hard in a 75-watt amp......

Just sayin'.

Arjay

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 Post subject: Re: Fender 75 thoughts and Biasing
PostPosted: Sun Apr 07, 2013 2:01 pm 
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Retroverbial wrote:
I wouldn't push a V30 too hard in a 75-watt amp......

Just sayin'.

Arjay


Ofcourse not.. Im just playing it on bedroom level, master only at 1.5-2.

What would be a good substitute for the EV 12F to put in the 75?
Should the speaker have more watts than the amp itself?


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 Post subject: Re: Fender 75 thoughts and Biasing
PostPosted: Sun Apr 07, 2013 4:31 pm 
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Powdered Toast Man wrote:
Just use the balance pot and screw it back and forth until you find the spot where the amp is quietest.


I think there is something wrong with it.
It does not change the amount of hum at all and if i screw the pot to the middle it buzzes like CRAZY...


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 Post subject: Re: Fender 75 thoughts and Biasing
PostPosted: Sun Apr 07, 2013 4:35 pm 
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Feelyfeels wrote:
Should the speaker have more watts than the amp itself?


Yes. I prefer a 50% margin of safety when possible.

Arjay

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 Post subject: Re: Fender 75 thoughts and Biasing
PostPosted: Sat May 11, 2013 8:11 pm 
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Putting the pot in the middle does not mean the tubes are balanced. Depending on the tubes and how closely they're matched, it could be far from the middle. Use a DVM or bias rite type device to measure current on each tube. Then turn the pot until each is drawing the same current.


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 Post subject: Re: Fender 75 thoughts and Biasing
PostPosted: Sat May 11, 2013 10:21 pm 
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Feelyfeels wrote:
Powdered Toast Man wrote:
Just use the balance pot and screw it back and forth until you find the spot where the amp is quietest.


I think there is something wrong with it.
It does not change the amount of hum at all and if i screw the pot to the middle it buzzes like CRAZY...


Look at the schematic and your amp closely. There are two balance pots. One is a bias balance pot (10K ohms) the other is a hum balance pot (100 ohms) which is part of the 6.3 volt heater circuit. The bias balance pot can be easily replaced with a true bias adjustment pot. Just Google "How to Blackface a Silverface amp" or something along those lines. The hum balance pot is the one that is adjusted to set minimum hum level. No test equipment is needed, just your ears.

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 Post subject: Re: Fender 75 thoughts and Biasing
PostPosted: Sun May 12, 2013 5:44 am 
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bluesky636 wrote:
Feelyfeels wrote:
Powdered Toast Man wrote:
Just use the balance pot and screw it back and forth until you find the spot where the amp is quietest.


I think there is something wrong with it.
It does not change the amount of hum at all and if i screw the pot to the middle it buzzes like CRAZY...


Look at the schematic and your amp closely. There are two balance pots. One is a bias balance pot (10K ohms) the other is a hum balance pot (100 ohms) which is part of the 6.3 volt heater circuit. The bias balance pot can be easily replaced with a true bias adjustment pot. Just Google "How to Blackface a Silverface amp" or something along those lines. The hum balance pot is the one that is adjusted to set minimum hum level. No test equipment is needed, just your ears.

Image


Thank you Bill!

Found this: http://www.diyguitarist.com/GuitarAmps/TR-BiasMod.htm
Ill try that out!

Im thinking about removing the master volume out of the circuit. Do i just need to unsolder all the connections to it or do i need to replace something?

Is it even worth removing the master volume?

Thanks again man!


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 Post subject: Re: Fender 75 thoughts and Biasing
PostPosted: Sun May 12, 2013 6:52 am 
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Feelyfeels wrote:
Thank you Bill!

Found this: http://www.diyguitarist.com/GuitarAmps/TR-BiasMod.htm
Ill try that out!

Im thinking about removing the master volume out of the circuit. Do i just need to unsolder all the connections to it or do i need to replace something?

Is it even worth removing the master volume?

Thanks again man!


The component values are different, so the mod may not be exactly as shown, but it should be similar. Also, I didn't notice that this amp has a half power switch which also effects the bias so I'm not sure what the impact might be without further study. Perhaps Arjay or BMW2002Ti can weigh in on this.

Looks like the 100K ohm master pot could just be replaced with a 100K ohm, 1 watt resistor. Connections would remain the same. Not sure it is really worth it, though. Just turn it all the way up.

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Last edited by bluesky636 on Sun May 12, 2013 8:12 am, edited 1 time in total.

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