71 Fender Twin Reverb Hum attenuated by knob even W/o tubein

[creaturesleeper]

So you are just pulling the reverb tube when you test this out? I would get a 12ax7 and a 12at7 and swap 'em around with every tube separately and see if that does anything.

Yep tryed that...One thing I did just notice though is when they put the 3 prong plug on they didn't dissconnect the old polarity switch and the ground is going through that and into the fuse plug also that big brown cap is still going to ground from the polarity switch...I read somewhere that you are supposed to disconnect all this when you do the grounded plug mod. This may be part of the problem but I'm not sure without doing it right

[creaturesleeper]

So you are just pulling the reverb tube when you test this out? I would get a 12ax7 and a 12at7 and swap 'em around with every tube separately and see if that does anything.

Yep tryed that...One thing I did just notice though is when they put the 3 prong plug on they didn't dissconnect the old polarity switch and the ground is going through that and into the fuse plug also that big brown cap is still going to ground from the polarity switch...I read somewhere that you are supposed to disconnect all this when you do the grounded plug mod. This may be part of the problem but I'm not sure without doing it right

Oops my mistake thats not the ground...the ground is grounded properly right to the chassis but I think that that capacitor and polarity switch are not supposed to be connected any more

[Scodiddly]

So you are just pulling the reverb tube when you test this out? I would get a 12ax7 and a 12at7 and swap 'em around with every tube separately and see if that does anything.

Yep tryed that...One thing I did just notice though is when they put the 3 prong plug on they didn't dissconnect the old polarity switch and the ground is going through that and into the fuse plug also that big brown cap is still going to ground from the polarity switch...I read somewhere that you are supposed to disconnect all this when you do the grounded plug mod. This may be part of the problem but I'm not sure without doing it right

Oops my mistake thats not the ground...the ground is grounded properly right to the chassis but I think that that capacitor and polarity switch are not supposed to be connected any more

Right you are!

The ground wire from the AC plug goes to the chassis, and the polarity switch and capacitor get disconnected. The neutral line goes right to the power transformer, and the hot line goes through the fuse first, power switch second, and then transformer. Neither neutral nor hot should connect to the chassis through anything other than the transformer.

Nip that old death cap and polarity switch cap out, then see if you've still got a hum problem. You'll certainly have a safer amp if nothing else.

[creaturesleeper]

So you are just pulling the reverb tube when you test this out? I would get a 12ax7 and a 12at7 and swap 'em around with every tube separately and see if that does anything.

Yep tryed that...One thing I did just notice though is when they put the 3 prong plug on they didn't dissconnect the old polarity switch and the ground is going through that and into the fuse plug also that big brown cap is still going to ground from the polarity switch...I read somewhere that you are supposed to disconnect all this when you do the grounded plug mod. This may be part of the problem but I'm not sure without doing it right

Oops my mistake thats not the ground...the ground is grounded properly right to the chassis but I think that that capacitor and polarity switch are not supposed to be connected any more

Right you are!

The ground wire from the AC plug goes to the chassis, and the polarity switch and capacitor get disconnected. The neutral line goes right to the power transformer, and the hot line goes through the fuse first, power switch second, and then transformer. Neither neutral nor hot should connect to the chassis through anything other than the transformer.

Nip that old death cap and polarity switch cap out, then see if you've still got a hum problem. You'll certainly have a safer amp if nothing else.

Hmm it would seem I have a couple problems here...The green wire is going straight to chassis ground but the white wire goes directly to the aux. ac 2 prong outlet which in turn connects to the polarity switch on the side closest to the fuse the black wire does the same thing but on the other output pole and on the other side of the P switch and that big ol cap is still in place...the two black transformer leads which I assume are the primary inputs are connected to the fuse and the on switch respectively. So what you are suggesting is the white neutral wire should go directly to transformer which wire should I connect it to? the one on the on off switch or the one coming from the fuse? Im gonna make a semi educated guess and say the one that goes to the sleeve part of the fuse buss Would that be right? By sleeve I mean the terminal thats not on the end. Also I noticed someone got sloppy with the iron and melted the plastic on two of the twisted heater wires on the 4th 6l6 socket...Im not sure if this affects anything but there is some bare wire on one of em it seems it could possibly cause some interference

[FNM]

The way Scodiddly suggests, then I think the wire from the PT to the fuse would go straight to the white (neutral) wire of the power chord. Then the black (line) wire from the chord would go through the fuse, then to the switch, then to the PT via the other black wire.

The way I have always done it would be the white wire from the chord straight to the fuse and into the PT with the same black wire it was already connected to. The black wire from the chord straight to the power switch and then to the black wire of the PT it was already connected to.

I believe both these methods would be ok.

[creaturesleeper]

The way Scodiddly suggests, then I think the wire from the PT to the fuse would go straight to the white (neutral) wire of the power chord. Then the black (line) wire from the chord would go through the fuse, then to the switch, then to the PT via the other black wire.

The way I have always done it would be the white wire from the chord straight to the fuse and into the PT with the same black wire it was already connected to. The black wire from the chord straight to the power switch and then to the black wire of the PT it was already connected to.

I believe both these methods would be ok.

Gotcha that makes sense...I don't think this is where my original problem is coming from but it certainly won't hurt things...I have never been zapped by this amp but I don't want to be either!

[Scodiddly]

Doesn't really matter which of the two transformers wires end up being "hot" and "neutral". Just avoid anything else, though the power outlet is OK to keep. The old setup sometimes put the fuse on the neutral, because in those days who the heck knew which was neutral anyway? Now that grounded and polarized outlets are the norm, do it the modern safe way and put the fuse first on the hot side. Just in case the switch goes bad, ya know.

[creaturesleeper]

Doesn't really matter which of the two transformers wires end up being "hot" and "neutral". Just avoid anything else, though the power outlet is OK to keep. The old setup sometimes put the fuse on the neutral, because in those days who the heck knew which was neutral anyway? Now that grounded and polarized outlets are the norm, do it the modern safe way and put the fuse first on the hot side. Just in case the switch goes bad, ya know.

I'm fixing it tommorow! That part of it anyway...need to go buy a couple tools. I tested all the tubes one by one all the sockets sound the same in The power tube section and all tubes sound the same except for one which is quieter than all the rest. I did this with the phase inverter tube pulled.the amp is dead quiet with no power tubes so that eliminates more serious power supply problems and I think rectifier problems The amp was recapped under the doghouse when I got it. I even checked to make sure.There also appears to be new Spragues in the rectifier section and on the hum balance pot as well as several across the board. When I pull the reverb driver tube nothing changes but when I pull the one next to it it goes quiet. Someone suggested the capicitor across the resistor on the Cathode for that tube may be bad. When I ground the middle terminal on the reverb pot it also quiets down quite a bit but I'm not real sure what this means. The pot appears to be grounded quite well to the copper grounding strip underneath it. also the hum gets louder when the reverb pan is disconnected. Thats about everything I have tryed so far...Whew! that was a little long winded. I'm going to get a multimeter tommorow and start checking voltages and resistance etc. Thanks for all the help! By the way this amp has Altec 417 8H speakers...I always thought it was unusually loud even for a Fender Twin this may explain part of that...even just running two tubes it's kind of obscene...I rarely crank it past two or so even at live gigs

[PretendMusic]

And just to add to the confusion the serial number says it's a 67 blackface #A24462 which doesn't really make sense cause it has a silver faceplate with blue TWIN REVERB AMP...it is the AB763 circuit....Ohhh Fender....This doesn't really solve my issue though....just makes me scratch my head some more....Thanks for all the help everyone!

Dude, I checked with my Certified Fender Tech friend, John, again, with your corrected and updated information regarding your amp... (Even though you didn't really seem to appreciate his sage assistance, last time...) And, I gotta figure, that perhaps the reason you love the sound of your claimed "1971" amp, that in his learned opinion, might not have been from one of the years that are considered the best sounding examples of a Fender Twin, is because, as you later noted, it's NOT a 1971 Twin...!!!

In any case, here's what he told me, first of all:
"Does he know that you can kill yourself doing this, even unplugged? The repair tab has got to be cheaper than his funeral, if it's a few bucks that he's worried about."

Then, he said:
"If it was born with an AB763 circuit, it's not a 1971. The AB763 is the jewel of circuits - Second revision, July, 1963. (My '69 Silverface Super Reverb has a tube chart that says AB763 and has no date code stamp. The use of these date code stamps ended sometime in 1968, but Fender was still using up their old stock, which is a very good thing. Originally, my Super was actually an AA1069 circuit, internally, with very thin yellow wiring throughout. What a mess. After I did a complete rewire to color-coded, cloth-jacketed wire, and a few parts changed and rearranged, it is now an AB763.)

An AA763 is considerably more "growly" than an AB763, but somewhat "unmanageable". But, both are like, WOW!!! He may have to use the transformer codes to properly date the amp, as I did. If they are all close to the same date, it is likely that that's roughly the date of manufacture. If any are off by a large margin, towards the future, that means that that transformer has been replaced. Go with the earliest date, as it is rather unlikely that any parts would have been replaced with NOS or used parts.

Keep in mind, in my earlier comments, I was bitching about the AA270 circuit and not his amp, because I don't know what it really is..
This might help you, though: http://www.fender.com/support/amplifiers.php

Wish him a Happy Holiday season..."
-John

- - - - - - - - - - - - - - - - -

So, creaturesleeper, there ya go... Congratulations! You now seem to have the "jewel of circuits"!!! Sweet.
You no longer have "CBS at it's worst", but, instead, it's a fucking JEWEL, man!!! (Which, I guess, still hums.)

I hope you enjoy this posting better than our last attempt at helping you.
I also hope that you didn't kill yourself since your last posting, a week ago, tinkering inside your amp, or it WAS your LAST POSTING!

And, though I don't know shit about the workings of amps, myself, I'd say, If all else fails, turn down the reverb! And, stop pulling parts out of it, and then wondering why it hums!!!

In the mean time, Rock on, man!

Have a good holidays!
-Chris (and John)


("Hell hath no fury like a Fender scorned." -From The Leo of Fender, by W. Shakespeare, I believe...)

.

[creaturesleeper]

And just to add to the confusion the serial number says it's a 67 blackface #A24462 which doesn't really make sense cause it has a silver faceplate with blue TWIN REVERB AMP...it is the AB763 circuit....Ohhh Fender....This doesn't really solve my issue though....just makes me scratch my head some more....Thanks for all the help everyone!

Dude, I checked with my Certified Fender Tech friend, John, again, with your corrected and updated information regarding your amp... (Even though you didn't really seem to appreciate his sage assistance, last time...) And, I gotta figure, that perhaps the reason you love the sound of your claimed "1971" amp, that in his learned opinion, might not have been from one of the years that are considered the best sounding examples of a Fender Twin, is because, as you later noted, it's NOT a 1971 Twin...!!!



In any case, here's what he told me, first of all:
"Does he know that you can kill yourself doing this, even unplugged? The repair tab has got to be cheaper than his funeral, if it's a few bucks that he's worried about."

Then, he said:
"If it was born with an AB763 circuit, it's not a 1971. The AB763 is the jewel of circuits - Second revision, July, 1963. (My '69 Silverface Super Reverb has a tube chart that says AB763 and has no date code stamp. The use of these date code stamps ended sometime in 1968, but Fender was still using up their old stock, which is a very good thing. Originally, my Super was actually an AA1069 circuit, internally, with very thin yellow wiring throughout. What a mess. After I did a complete rewire to color-coded, cloth-jacketed wire, and a few parts changed and rearranged, it is now an AB763.)

An AA763 is considerably more "growly" than an AB763, but somewhat "unmanageable". But, both are like, WOW!!! He may have to use the transformer codes to properly date the amp, as I did. If they are all close to the same date, it is likely that that's roughly the date of manufacture. If any are off by a large margin, towards the future, that means that that transformer has been replaced. Go with the earliest date, as it is rather unlikely that any parts would have been replaced with NOS or used parts.

Keep in mind, in my earlier comments, I was bitching about the AA270 circuit and not his amp, because I don't know what it really is..
This might help you, though: http://www.fender.com/support/amplifiers.php

Wish him a Happy Holiday season..."
-John

- - - - - - - - - - - - - - - - -

So, creaturesleeper, there ya go... Congratulations! You now seem to have the "jewel of circuits"!!! Sweet.
You no longer have "CBS at it's worst", but, instead, it's a fucking JEWEL, man!!! (Which, I guess, still hums.)

I hope you enjoy this posting better than our last attempt at helping you.
I also hope that you didn't kill yourself since your last posting, a week ago, tinkering inside your amp, or it WAS your LAST POSTING!

And, though I don't know shit about the workings of amps, myself, I'd say, If all else fails, turn down the reverb! And, stop pulling parts out of it, and then wondering why it hums!!!

In the mean time, Rock on, man!

Have a good holidays!
-Chris (and John)


("Hell hath no fury like a Fender scorned." -From The Leo of Fender, by W. Shakespeare, I believe...)

.

Hey Thanks! no I haven't killed myself I know how to drain the caps etc. and actually solved some of the noise issue...One end of a cap was lifted in the rectifier circuit but it was hard to see it looked still soldered but there was no connection there. After further testing I found significant AC voltage where it shouldn't be and so the rectifier may have some bad Diodes although it looks like it's been rebuilt or one of the Filter caps may be going bad but they are all new too and hard to test right. I also removed the cap from the old polarity switch. So I got rid of quite a bit of noise and now it sounds more like it should...But still not quite running up to spec. My teacher at college has a scope and is gonna take a look under the hood...Oh and it has the original trannys..the serials start with 606 and I believe those date as original. The AB763 is actually the Blackface circuit with a couple modifications which are very easy to reverse.... Thanks for all the help and I'll keep ya posted!
Happy Turkey!

[PretendMusic]

Thanks for all the help and I'll keep ya posted!
Happy Turkey!

Sounds like it's slowly getting better. Continued good luck!
May the light of Leo shine down upon you.
I've just been playing my trusty '73 Tele Thinline...! ( Thank you, Leo! )

Now, if only I could get my Workstation PC back up and running, I'd be good, too...
I use Sony ACID Pro, and I guess it (my PC) has been trippin' out on me! Fuckin' technology!

Everyone create something, every day.
-Chris

PS, Turkey? Man, I'm still stuffed!

PPS, John responded:
The 606 on the transformer means the amp was built in the 60s, either '68 or '69 in his case, which is good.
It depends on the numbers following: 606-821 or 606-9-42, for example, which would be the 21st week of 1968 or the 42nd week of 1969, respectively.
It usually took them a month or two to put them in the amps and render them finished.

He seems like he has it all under control now.


.

[creaturesleeper]

Thanks for all the help and I'll keep ya posted!
Happy Turkey!

Sounds like it's slowly getting better. Continued good luck!
May the light of Leo shine down upon you.
I've just been playing my trusty '73 Tele Thinline...! ( Thank you, Leo! )

Now, if only I could get my Workstation PC back up and running, I'd be good, too...
I use Sony ACID Pro, and I guess it (my PC) has been trippin' out on me! Fuckin' technology!

Everyone create something, every day.
-Chris

PS, Turkey? Man, I'm still stuffed!

PPS, John responded:
The 606 on the transformer means the amp was built in the 60s, either '68 or '69 in his case, which is good.
It depends on the numbers following: 606-821 or 606-9-42, for example, which would be the 21st week of 1968 or the 42nd week of 1969, respectively.
It usually took them a month or two to put them in the amps and render them finished.

He seems like he has it all under control now.


.

I figured out the numbers after I posted that and John is totally right 606 69 24 so July of 69 which seems right. My rectifier and caps are all good I did some wrong measuring! It's a learning curve right! I forgot that the diodes reverse voltage each stage in the rectifier so I didn't measure things right everything there is as should be. The cap that was lifted was the one in the bias circuit right before the bias pot and right after the one diode which makes the negative bias current there. I was losing 20 VDC there with it unsoldered like that so was running extremely cold bias It's supposed to be around -51 spec. wise. and I was getting less than -30. Not to mention all the extra filtering I lost...no wonder she sounded funny! I just wonder how long it was like that...Hmmm. Im gonna shorten and retwist the heater wireing cause it's all sloppy and poorly dressed and someone got lazy with the iron and burned insulation off on a couple spot's so that should clean thing's up quite a bit more.
In regards to your PC I run Vegas Pro 8 which is kinda like Acid but it's more geared towards video editing...it's very frustrating as far as latency and stuff goes cause without alot of buffering I get alot of drop out's and crap like that but If Iturn the buffering up my Latency goes way up. I need an external drive and soundcard for it I think. This computer is pretty old and has a built in soundcard which sucks kinda. Iv'e just been 4 tracking cause theres no latency issue there and than dumping into the comp. for editing if I want to. I just want old solid stuff me and computers don't get along very well as convenient as they can be sometimes.
I bet that Tele sounds rad!
I have a 89 HM Strat I picked up for pretty cheap...I initially liked it cause it's ugly as sin but than I played it and loved it! it's a great guitar and pretty versatile...I usually just use the two single coils though.
Anyway good luck with your Comp. and thanks for all the help!

[PretendMusic]

In regards to your PC I run Vegas Pro 8 which is kinda like Acid but it's more geared towards video editing...it's very frustrating as far as latency and stuff goes cause without alot of buffering I get alot of drop out's and crap like that but If Iturn the buffering up my Latency goes way up. I need an external drive and soundcard for it I think. This computer is pretty old and has a built in soundcard which sucks kinda.

Yeah, I use Vegas 4.0, for making Tripped-out Videos for my Chill-Space Music...
It's a wonderful thing, Vegas. I wonder if you might get better results with your older PC, if you were using an earlier version...? with less CPU-sucking features.
Check the Bit-Torrent, perhaps, for v.3.0 or 4.0, if you do such things... (Or PM me...)

Also, for the latency, if you have an analog mixer, like a Mackie or such, you might be able to run your DAW's output into the mixer, and monitor it along with the Live part before it goes into the Computer... don't hook it up so that the mix goes back in to the DAW, though... OR, just listen to Vegas through your phones, and listen to your guitar amp, in the room, with one ear... But, you're on your own, there. Just a couple of less-than-well-detailed ideas...

Actually, I sometimes record into Forge, while monitoring the track in Acid...
And, half the time, I also get clicks throughout the recording, too. Play for 20 minutes, and it's all for fuckin' shit! (Sometimes. I'll save it by doing a Click-Removal, in Forge, and then running it through some heavy distortion and Chorus Plug-ins...!) And, Vegas often seems to give me the least trouble with clicking. Go with the bigger buffer, and monitor your amp live.

Or... Yeah, 4-track it, and bounce it to the computer... Great idea!!!

Whatever!
Have fun,
-C

PS, LIFE IS JUST A BUNCH OF FUCKING "WORK-AROUNDS"! Good Luck!


.

[PretendMusic]

So, Sleep, ya got him goin', now!

Here's what "Fender Tech" John had to add:
"You might pass this on, albeit a bit lengthy and technical:

So this is probably what your amp was, originally, unless it was built before July, 1969:
http://www.ampwares.com/schematics/twin ... _aa769.pdf

And this is what you want it to be, I think:
http://www.ampwares.com/schematics/twin ... _ab763.pdf


Notice that the AA769 circuit has a bias-balance control, more or less, and that the bias level is actually fixed. The AB763 circuit has a true, bias-level adjustment, and requires a pair of 5% tolerance resistors, both 220K, rather than 68K/10%. This makes all the difference. Also, there are a few other resistors in the amp that vary between the circuits and need to be changed, as well. And, the bias control needs to be re-wired as a bias-level adjuster, rather than a balance. You'll have to carefully compare the schematics, along with your amp's actual circuit, to know how it's currently wired and what needs altering. It is important to make sure the entire amp is rebuilt according to the AB763 schematic and layout. Exactly! Be thorough... and double-check. And check again. Or, it will be AB763 in name only.

With my '69 Superverb, and other retro-mods I've done, I replaced any other resistors/caps that were out of tolerance, and removed any ceramic disc caps that aren't shown in the AB763 chart. It's good to use carbon composition resistors, rather than metal-oxide or metal-film, as they will drift in value a bit, producing the sound for which these old amps are known. Some might argue this point, but I seriously believe it. I also replaced my power cord with a 3-wire, grounded type, but left the 3-way polarity switch and the cap. If you get into a weird room, where the house wiring is backwards, you will be glad you have that. Or you will have to use a 'lift', which will cause a serious shock hazard.

The electrolytic capacitor you mentioned, which was loose near that single, negative-biasing diode, is typically 47 uF/50-70 V. I've recently found that using a 100 uF/100 V in its place takes out almost all of the residual noise. A few bucks... It makes 'em really quiet. And be sure you mind the polarity - negative is hot... Clean tube sockets, along with clean and straight tube-pins are a good plan. WD-40, a wire brush and a needlenose are good for that. You can isolate microphonic preamp and driver tubes by tapping them, gently, with a metallic screwdriver tip, while the amp is on, volume knobs on zero, shields removed.

Regarding biasing, there are so many opinions on the subject. The current trend (no pun) is to try measuring bias current (plate current) by installing a 1 Ohm/1-2 Watt resistor, between the cathode of each 6L6/5881 and ground. Then, by measuring the voltage drop across these resistors, one can calculate the current, dividing by 1. mV=mA. Sounds exciting and dangerous doesn't it? And why do it? It ends up wrong!

One of the best articles I've ever read on the subject is by Randall Smith, the founder and owner of Mesa Boogie. He refers to the latest bias current craze, and all of it's variations, as the sprinkling of Holy Water on amps by technicians, basically to con customers into thinking...(something quite mystical!) In other words, bullshit... He says that, in all of his years working on Fender amps, he never once heard Fender mention bias current. Neither have I. On each schematic, there is a listed value for bias voltage, usually -52 Volts, idle VOLTAGE. New tubes, old tubes or no output tubes at all, it's -52 V., period. Let the tubes warm up & go where they want, is the idea. I used to do dynamic biasing, with a scope and a 1-Volt p-p input signal, speakers disconnected, and a dummy load of proper impedance, but I do that no more. I've attained true enlightenment that way. They sound so good when they're done right.

I hope I didn't go on too much. Old Fenders get me all exited!

A great place for tube amp & guitar parts:
http://www.tubesandmore.com/
Sale on until Monday. They have it all.

Best of luck,
-JM
"


And, also, check out the Behind The Gear, (in T.O. issue #56), interview with Groove Tubes company founder Aspen Pittman... He's "The Man", when it comes to Tubes, it seems!
BTW, I love the Behind The Gear section! It's often the first thing I turn to, when my new issue arrives...
And, I find that it's often WAY TOO SHORT!!!
WTF? They're just gettin' goin', and it ends! WTF?
Still, great stuff, Tape Op, as usual!
-C


.

[creaturesleeper]

So, Sleep, ya got him goin', now!

Here's what "Fender Tech" John had to add:
"You might pass this on, albeit a bit lengthy and technical:

So this is probably what your amp was, originally, unless it was built before July, 1969:
http://www.ampwares.com/schematics/twin ... _aa769.pdf

And this is what you want it to be, I think:
http://www.ampwares.com/schematics/twin ... _ab763.pdf


Notice that the AA769 circuit has a bias-balance control, more or less, and that the bias level is actually fixed. The AB763 circuit has a true, bias-level adjustment, and requires a pair of 5% tolerance resistors, both 220K, rather than 68K/10%. This makes all the difference. Also, there are a few other resistors in the amp that vary between the circuits and need to be changed, as well. And, the bias control needs to be re-wired as a bias-level adjuster, rather than a balance. You'll have to carefully compare the schematics, along with your amp's actual circuit, to know how it's currently wired and what needs altering. It is important to make sure the entire amp is rebuilt according to the AB763 schematic and layout. Exactly! Be thorough... and double-check. And check again. Or, it will be AB763 in name only.

With my '69 Superverb, and other retro-mods I've done, I replaced any other resistors/caps that were out of tolerance, and removed any ceramic disc caps that aren't shown in the AB763 chart. It's good to use carbon composition resistors, rather than metal-oxide or metal-film, as they will drift in value a bit, producing the sound for which these old amps are known. Some might argue this point, but I seriously believe it. I also replaced my power cord with a 3-wire, grounded type, but left the 3-way polarity switch and the cap. If you get into a weird room, where the house wiring is backwards, you will be glad you have that. Or you will have to use a 'lift', which will cause a serious shock hazard.

The electrolytic capacitor you mentioned, which was loose near that single, negative-biasing diode, is typically 47 uF/50-70 V. I've recently found that using a 100 uF/100 V in its place takes out almost all of the residual noise. A few bucks... It makes 'em really quiet. And be sure you mind the polarity - negative is hot... Clean tube sockets, along with clean and straight tube-pins are a good plan. WD-40, a wire brush and a needlenose are good for that. You can isolate microphonic preamp and driver tubes by tapping them, gently, with a metallic screwdriver tip, while the amp is on, volume knobs on zero, shields removed.

Regarding biasing, there are so many opinions on the subject. The current trend (no pun) is to try measuring bias current (plate current) by installing a 1 Ohm/1-2 Watt resistor, between the cathode of each 6L6/5881 and ground. Then, by measuring the voltage drop across these resistors, one can calculate the current, dividing by 1. mV=mA. Sounds exciting and dangerous doesn't it? And why do it? It ends up wrong!

One of the best articles I've ever read on the subject is by Randall Smith, the founder and owner of Mesa Boogie. He refers to the latest bias current craze, and all of it's variations, as the sprinkling of Holy Water on amps by technicians, basically to con customers into thinking...(something quite mystical!) In other words, bullshit... He says that, in all of his years working on Fender amps, he never once heard Fender mention bias current. Neither have I. On each schematic, there is a listed value for bias voltage, usually -52 Volts, idle VOLTAGE. New tubes, old tubes or no output tubes at all, it's -52 V., period. Let the tubes warm up & go where they want, is the idea. I used to do dynamic biasing, with a scope and a 1-Volt p-p input signal, speakers disconnected, and a dummy load of proper impedance, but I do that no more. I've attained true enlightenment that way. They sound so good when they're done right.

I hope I didn't go on too much. Old Fenders get me all exited!

A great place for tube amp & guitar parts:
http://www.tubesandmore.com/
Sale on until Monday. They have it all.

Best of luck,
-JM
"


And, also, check out the Behind The Gear, (in T.O. issue #56), interview with Groove Tubes company founder Aspen Pittman... He's "The Man", when it comes to Tubes, it seems!
BTW, I love the Behind The Gear section! It's often the first thing I turn to, when my new issue arrives...
And, I find that it's often WAY TOO SHORT!!!
WTF? They're just gettin' goin', and it ends! WTF?
Still, great stuff, Tape Op, as usual!
-C


.

As far as I can tell someone has already done alot of these changes... or it's already a AB763.Those two resistors after the bias pot are 220K and that cap is 100uf/100v as well. the bias voltage is-52 and the pot appears to be wired as a true bias pot with one wire headed out of it rather than the two wires you see on AA769. One difference is the two 100k resistors on the heater wires coming off the pilot lamp but I think those are there to lower hum. Also on AA769 there is a lead going to ground with a capacitor on pin one V4 which my amp doesn't have. It appears someone once put those resistors to ground to measure bias but they are no longer there just some solder smudges. I havent checked more thoroughly than that but everythings lokking good so far! I much appreciate the wealth of info you are sending my way! Thanks! One really good site I found is el84world.com there is a wealth of info there for amp builders. Oh I did my math wrong again...June 69 not July...I was a month off....