Fender 75 project amp

[phsyconoodler]

I have a gutted Fender 75 amp and I want to run it in a low power class A mode with the stock transformers.
 Am I correct in thinking that I would simply use a bridge rectifier without a center tap to get about 250v out?
  I can't wrap my head around the way Fender gets the low power mode to work in those amps other than lifting the center tap from the full wave bridge.With the full wave bridge it used to put out 500v but I don't want 500v,I want to run it class A at 250v.Possible?
 I've already rewired the rest of the amp as a super reverb and I have an extra tube to add a gain stage if I want later on.
  I call it the Big Bastard. I think it should get 20 watts in class A with a pair of 6L6's biased class A at 250v on the plates.
 Thoughts? Or would it be better to just wire it up 500v and call it a day?It certainly would be loud and clean.I removed the ultralinear taps off the plates already.

[sluckey]

Am I correct in thinking that I would simply use a bridge rectifier without a center tap to get about 250v out?

No. Just wire up two diodes in a conventional full wave rectifier and connect the center tap to ground.

[phsyconoodler]

That should do the trick with the stock 75 PT? The center tap runs to a bunch of resistors and caps in it's stock format and then a switch for the lo-power mode.It's a nightmare of resistors,caps and diodes,a zener.
 It will be interesting to hear what a class A Super Reverb sounds like.I may not like it and revert back to the full wave bridge.

[sluckey]

It's a nightmare of resistors,caps and diodes,a zener.

Not really. Most of that stuff is involved with also switching the bias. When all the extra stuff is stripped out it's really a simple straightforward B+ supply. This pic should make it easier to wrap your head around it...

[phsyconoodler]

Wow! That made it clear!
 Thanks a bunch Sluckey!The full wave bridge is simply what gets lifted and the center tap is always grounded in both modes.Rather simple when it gets all that other stuff stripped away.The thing about the 75 is the WAY the bias and switching supply is implemented on those terminal strips that made it confusing for me.

 You are the best!

Any opinions of what a class A super might sound like?Guess I may find out really soon anyway.Maybe I'll have a class A cathode bias/500v fixed bias switch......nah! I hate too many gadgets in an amp.

[phsyconoodler]

Well fired it up with a whopping 256v on the plates,cathode biased.Didn't have the right cathode resistors on hand so I use a couple of 470 ohm,one on each tube. It's gor a huge disipation of 6 massive watts and 36ma of current. It actually sounds pretty good now,can't wait to hear it when I get the right parts!

 Right now it's saturated distortion and it cleans up when you roll of the guitar.It's plenty loud too.
 The experiment continues............

[phsyconoodler]

Didn't like it cathode biased.Too much grind.So I changed it to fixed bias and it's a lot better.Still very loud at 256v on the plates!
  At least it has some clean now and better as the volume on the guitar gets turned down.
 Still some preamp tweaking to get it right but it has huge promise.
 When you crank it it gets creamy saturated overdrive with the bridge pickup on my strat with texas specials,and I mean creamy and lovely.
  Now to get the tone controls working prperly(used the old pots and didn't check the values)and maybe get some of those pull-pots doing other things like mid and bass boosts.
 Interesting and fun!

[PRR]

> center tap is always grounded in both modes.

NO!!

[PRR]

> center tap is always grounded in both modes.

NO!!

In the 500V/250V form, the PT CT is the *+250V* DC line. Ground is one end of the FWB, 500V at the other end of the FWB.

Note also that the two diodes involved in making 250V are *backward* to the way we usually do them.

__IF__ you _only_ ever want the 250V, then Steve is (of course) correct. Don't get clever, don't follow late-Fender. Ground the CT, connect two diodes the "usual" way, you get +250V.

(You can also get *negative* 250V with two more diodes, useful for your Analog Computer.)

If you "might" go back to 500V, I would wire the PT, CT, and FWB _just_ like Fender, but omit the second layer of 285V(?) caps for now.

15W-17W audio from a PT made to deliver 75W is a real waste of somebody's cash, and a strain on your arm. But the cash is long-gone and if your arm isn't unhappy, it's all good.

[phsyconoodler]

Thanks for the heads up PRR.I alredy found out he hard way it won't wire up conventionally and blew a fuse trying.Lucky for me a I use low amperage fuses for testing.
 I just made it fixed bias and it sounds great.
   The creamiest overdrive ever from a super reverb! Crank it up and it screams! It still has useable cleans too.No choke,just a big resistor for now.
  You have never heard overdrive like this from a super reverb circuit! 255 massive volts on the plates and about -20v on the bias. I rather like it the way it is.

 50 bucks for the amp all burnt up is no huge outlay of cash.Let me tell you it sure sounds better than a stock 75!!  Heavy? Yes. Who knows?I may just get a super reverb PT and convert it later,but for now it's a new flavour to play with.

[phsyconoodler]

ok,after playing with the amp for a while I find that the untralinear OT has to go.It gets too trebly and icepicky.You turn up the treble and your teeth hurt.
  Tweaked just right it sounds ok but it's not the way a guitar amp should be.
So I'm just gonna scap the tranny's and get real Super stuff.I put in the RSC canada 15" speaker and it's lighter and sounds great.
  What the hell was Fender thinking with ultralinear transformers anyway?Probably good for a home stereo.
The reverb is going to need some tweaking cause of the low voltage but it's almost there.
  I ended up cold biasing the first gain stage with a 10k cathode resistor and it sounds much better.No more tweaking til I get a new OT in there cause I might end up reversing everything.
  Cranked Super.Nice.

[DummyLoad]

What the hell was Fender thinking with ultralinear transformers anyway...

steel guitar and C&W twang tone. fender never did seek "rock market" until the rivera years with the pull distortion controls. e.g. evil twin etc.. the fender amps made in the CBS era, IMO are less than desirable, except to gut. <evilgrin>

btw, consider doug's multi-tap SR OT - nice sounding chunk of iron, unless it lands on your foot.    

--DL

[Willabe]

What the hell was Fender thinking with ultralinear transformers anyway?

What the hell was Fender thinking with ultralinear transformers anyway...

steel guitar and C&W twang tone. fender never did seek "rock market"

I think DL's right.

UL, cleaner and more power.

I've brought this up here before and nobody said anything one way or the other.

UL taps add feed back to the output stage. So try taking out the NFB loop from the OT and leave the UL taps hooked up and see what that does.


       Brad          

[phsyconoodler]

steel guitar and C&W twang tone

 Not really.The Fender 75 has the pull pot nonsense and the distortion channel.Gained up and cranked it sounds great.It just doesn't do a nice semi-clean blues tone well.
 At 256v to get close to 70% disipation I need 80ma current.I put in a bias pot and tried it anywhere from 50ma to 85ma and it sounds best around the 75ma mark.
  Turn that treble knob up and it gets screetchy treble.I didn't think there was that much treble available in that amp!Same with the mids;it gets screetchy when you turn the mids up too far too.Bass is pretty normal.
  There is one happy place but even move the knob(treble) one digit and it gets out of control.
  I have removed ultralinear taps before and not had this happen and I use Hammond untrlinear OT's all the time without hooking them up with no issues either.So if I were to hook them back up it might help that?

[PRR]

> What the hell was Fender thinking with ultralinear transformers anyway

It is not safe to get 75 Watts from a pair of 6L6GC with pentode connection.

UL will turn-down G2 voltage when working hard, and is mildly more abuse-proof.

There is no need for a new OT. Just tape-up the G2 taps and feed them normally.

The OT "may" be "better" than some guitar iron, but I don't think that's a problem. The VT40's OT was astonishingly good, over 20KHz with no NFB, yet sounds fine, even for Jazz (when de-rated down to 20W). If it's ice-picky, drops some caps here or there.

[phsyconoodler]

I'm going to have to dig a bit to find out where all this treble is coming from.I did re-use the coupling caps from the old build as well as resistors to make it economical.I may have overheated some components removing them.
 
 

[phsyconoodler]

I turns out I had a cold solder joint on the grid of one power tube.The tone is normal now.Just gotta get a better 15" speaker and she is done.There is serious cone cry on this old speaker.Sounds ok clean.
  Nice reverb even at 250v.
Rather dirty after 4 on the dial....not everyones cup of tea with a Super.......but a really useable amp.
 I'm going to get a Warehouse 15: and try that out soon.

Still going to get another PT and raise the voltage;I'm a big fan of the stock Super Reverb tone.In this case,more a Vibroverb with the 15" speaker.
  I am having trouble figuring out how to get the stock PT to go back to the 500v mode.I just haven't seen this type of configuration before and haven't figured it out yet.
  Sluckey's probably rolling his eyes right now..........damn kids he's thinking! I'm a 57 year old kid.

[phsyconoodler]

Ok,I'm looking at the diagram Sluckey gave me and it looks like the 250v comes from the center tap.So if I want 500v again I just need to wire it up as drawn and it should get me there.I think.


 Simply lift the center tap and use the full wave bridge? Mental block

[sluckey]

Simply lift the center tap and use the full wave bridge?

That'll work. However, leaving the CT connected to the junction of those filter caps will force the voltage across the caps to be equal and you'll still have your 500V. You would not have to use the two resistors across the filter caps unless you just want then for draining purposes. So, either way, flip a coin.

[phsyconoodler]

Ok,wired it as a full wave bridge rectifier and it blows the fuse.I removed the lead from the first filter caps just to te it.Blows fuse.Center tap removed from transformer ground.

 Odd.Going to make a new rectifier board,maybe something grounded out underneath?But with two diodes removed and the center tap going to ground it works fine for 250v.
  Hmmmm.........

[smackoj]

does the Fender 75 amp have the same circuits as the Super Reverb?  If is does then I want to find a cheap one and follow in your footsteps noodler!

 

[sluckey]

Ok,wired it as a full wave bridge rectifier and it blows the fuse.I removed the lead from the first filter caps just to te it.Blows fuse.Center tap removed from transformer ground.

Please say you didn't have the center tap connected to chassis ground!

[phsyconoodler]

No I lifted it off ground.That's what's confusing me here.I've wired up tons of full wave rectifiers and full wave bridge rectifiers and I'm willing to bet it's just something stupid like a bad diode in the bridge.
  I assume diodes are good when they are brand new.I may have to rethink that and check every one I use.

And this power supply is different for me;how it comes up with 250v at the transformer center tap is weird ....to me.
  I think I understand it now.
I wired it like this when it blew the fuse.I had it disconnected from the filter caps first.
  The transformer center tap was lifted.It must have a bad diode.

[phsyconoodler]

Yup.Bad diode. It's working at 500v now.

Thanks again for all your help Sluckey.