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FDP Forum / Amp Mods, Repairs, and Projects / Question on build .


LA , Calif

I try my best
Sep 29th, 2016 08:37 PM   Edit   Profile  

Years ago I took an old fender music master bass amp and converted it into a Princeton AA964 without trem and used to vol and tone setup like a 6G2 Princeton has to use the same face plate.

I used Allen amps PT and OT the PT is one he sells for a Champ yet is larger and the OT is closer to a Deluxe with an 8 and 4 ohm tap.

The questions are I replaced all the electrolytic caps and the PT had a 50 volt bias tap which I used and built an adjustable bias board with good resistors and a 47uf/100 volt Nichicon cap and a good 50KL pot yet I didn't change the cap and it's 10 years old and seems fine. Since it only has 50 volts off the tap would it last? I had Sprague filter caps now F@T and still used Sprague for the preamp. F@T are smaller and the Sprague did last a very long time like 15 years yet the amp is quieter now.

I also used the stock on/off slide switch for the power switch and use both sides or poles . I can't locate any info on these switches as far as amp rating and fender only used them on champs and MM bass amps. I left it in order not to try to fit something else . It seems to work fine and from what I can find the toggle type used on all other fender amps was 3 amp @ 120 VAC. I do have one of those on the rear panel that I used to use as a standby but never use it for that even though I thought I might. I can wire the power to that one only if the switch is better than the slide type. I only used a 1 amp slow blow fuse like a Champ did .

Any thought's?

(This message was last edited by catnineblue at 10:47 PM, Sep 29th, 2016)

Contributing Member

So. Cal. USA

Sep 30th, 2016 11:25 AM   Edit   Profile  

For all the talk about bias caps, and I change them too when performing maintenance, I've never seen one completely fail taking down the bias section. Pots, resistors, yes but the cap....nope. Weak and beginning to grow stuff yes but never totally shot.

So....change it if you feel the need but at 10 years you should be good for a while. If you have a capacitance meter and an ESR meter you can check condition.

I'm not a fan of slide switches either but if it ain't broke why bother brother.

Sounds like a cool little amp.


LA , Calif

I try my best
Sep 30th, 2016 02:34 PM   Edit   Profile  

Thanks Hammond. I don't have an ESR and my DVOM does not have a capacitance function.

I was sort of surprised that the Sprague filter caps I got in 2005 still worked although replacing them with the new F&T's really quieted the amp down, it only had a slight hum you could hear if you were a foot away from the rear.

Another thing is I have read that caps sitting for a long time are not so good yet in order to change this build into a AA964 from a 6G2 I needed another filter cap and dropping resister and used a 15 year old 16uf 475 volt I got and it worked fine. I admit I was a bit tense powering it up the first time and had the chassis covered just in case.

This build started out as a two input 5E3 and I used a Weber eyelet board for a 5E3 yet didn't like the breakup then changed the cathode bias to adjustable fixed and it was better. From there it became a 6G2 and then a AA964.

I did need to do a bit of fiddling to find the proper bias range resistor since most PT's you tap off the one side of the PT at the rectifier @ 340 volts so the stock 1K ohm was far to much , ended up with a 10K ohm since the Allan amps PT has a 50 volt bias tap that works well whether I used 6V6's or 6L6's , can;t recall where I decided on 10K might have been a Weber build schematic or Hoffman site. Point being perhaps the bias cap holds up longer with only 50 volts.

Since this build does not have the bass and treble with their gain losses I need to use a 12AY7 instead of the 12AX7 for the preamp or it's too gainy and harsh. It does make a simple tube swap if I do want low level breakup much like an 6G2.

I built a tall version of a MM bass amp cab so I could fit 2 twelves in parallel for 4 ohms and used the Allan Amps OT 4 ohm tap . It was just too much amp . I built that cab first for a 73 SF champ so I could run that 6 watt into 2 twelves and was amazed how well 6 watts had so much bottom end and since the MM bass amp chassis is the same chassis and a SF champ I decided to put the champ in the MM bass amp cab and made a wooden adapter ring so I could use the Weber Sig 8S alnico that I had in he SF champ . Sounded great . I did try the Weber Sig 10S yet the 8 was actually the same sound by turning the bass pot up one notch. Hated the fender SF champs original 8" alnico since it flubbed out on any bass notes.

I like the Weber sigs so much in this AA964 build I used a Weber Sig 12S alnico . It's 25 watt so it does the job. I like the Sig S because I don't like early breakup . They no longer make the Sig 10S alnico in 4 ohm .

I had to see the 73 SF champ for funds and missed it so much I built one using Weber tranny's and got the Weber Sig 10 S ceramic which is far to bright and the Weber OT can do 4 and 8 ohm . I used an Allen Amps tweed style chassis he no longer makes and built my own cab and use a Jensen C10R which for some reason is not bright. They describe the Sig 12S alnico as bright yet it depends on the amp and it's not a bit bright in my AA964 build. I had a Weber Silver bell 50 watt ceramic in this build yet it made the amp real heavy and was too much bass.

One day I'll find a better 10 for my champ build either a Weber Sig 10s alnico or the Jensen jet 10" falcon. Should have kept the Weber Sig 10S alnico I had.

Well that's my story and the only 2 amps I own other than a pignose 7100.

I do miss Ted Weber , we used to email a lot back in the day , he was a really nice fellow.

(This message was last edited by catnineblue at 04:36 PM, Sep 30th, 2016)

Contributing Member

So. Cal. USA

Oct 3rd, 2016 02:28 PM   Edit   Profile  

I'm not so much a fan of Sprague caps and would suspect some degradation over 10 years or so but...many people change tubes at the time of a cap job and tubes could be a source of hum if mis-matched or worn. Very hard to say.

I've found Sprague's right out of the bag with high ESR and a couple dead shorted. Nice to see another F&T convert.


LA , Calif

I try my best
Oct 14th, 2016 04:16 PM   Edit   Profile  

Another question.

On pin 8 of the 6V6's most princetons had a separate ground to the chassis .

When I built this amp it started out as a 5E3 so what I did was jumper pin 8 of one 6V6 to the other 6V6 pin 8 then to ground at one of the PT studs.

Would it be better to have two grounds or leave it as is?



Oct 15th, 2016 02:42 AM   Edit   Profile  

As long as you have a good solid connection to the chassis then zero difference.
I dislike using PT studs for chassis connection tabs though, as the nuts can work loose over time.
SF Fenders tended to have such tabs soldered to the chassis.


LA , Calif

I try my best
Oct 18th, 2016 07:18 PM   Edit   Profile  

I didn't want to add another post so I'll use this one and refer to my OP for all the builds details.

In this AA 964 build what's different than the real deal is V1 is not shared , I only used 1 triod of the 12ay7 thus the 6G2 tone stack , V2 is the second gain 12ax7 and PI .

Since I only use #1 input jack is it possible to use a 12ax7 V1 , split the inputs and place the full 68k on #2 input as it would be if plugged into and run 34K off the #1 . So input #1 would use 1/2 of 12ax7 and # 2 would use the other 1/2.

Now would this kinda , sorta maybe reduce the 12ax7 gain down to or close to the 12ay7 since there is more resistance to the grid then IF I wanted more breakup use #1 . I would run another cathode resistor and bypass cap just like it has now so not to share cathodes like a 5E3 does.

It's just a thought , I'm quite happy with the headroom the amp has now and I do have a OD if I want to drive things just don't like pedals.

Or is all of this change basically like just turning the guitars volume up (ie) adding resistance or taking it down ?

I'm not sure if they are the same affect so I asked.

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