$100.000.000 reward

[floyd]

I am offering a cash reward , ( either in Monopoly or Confederate dollars ) to the person who can figure out HOW to make  a single channel AB763 REALLY sound like the stock two channel.. I've tried EVERYTHING for YEARS. The 270K to ground sounds "OK", but there is something missing in this puzzle. In no way is the tone the same.In fact , the tone is SO unconvincing compared to a two channel. that recently built a two channel AB763 for myself, because i KNEW the current one channel wisdom is flawed.

[sluckey]

I would replace the 270K with a Vibrato channel, complete with reverb circuit and opto tremolo circuit.

 I prefer Confederate dollars.   

[floyd]

Right .. and I did just that. You can't collect your rebel dollars with that answer,, must be ONE CHANNEL. Think about it.

[sluckey]

No fair changing your original post after I responded. I probably would not have even posted if your original post read as it does now. Anyhow, just kidding and trying to make a quick buck.

In no way is the tone the same.In fact , the tone is SO unconvincing compared to a two channel. that recently built a two channel AB763 for myself, because i KNEW the current one channel wisdom is flawed.

There's no flawed wisdom here. Just stripping it down to a Normal channel only. The 270K is just there to simulate to loading of the mixer at the point where the VIB channel would feed the PI. If you don't think it sounds like an AB763 that's OK. Some of us think it does. It's just opinions. Just give it some time and when your ears start failing your opinion may change too.

[floyd]

To get to the point .. the 270K takes a wee more gain away than does the original AB763 .. lets' find out why , and maybe there's a better spot in the circuit to compensate for the missing vib channel.

[sluckey]

To get to the point .. the 270K takes a wee more gain away than does the original AB763 .. lets' find out why , and maybe there's a better spot in the circuit to compensate for the missing vib channel.

Referring to an original schematic, put your finger (or pointer) at the junction of the two 220K channel mixing resistors. Now follow the path of the VIB channel and you'll see that you go thru a 220K mixing resistor to the top of a 50K INT pot and finally to ground. The 270K resistor simulates the losses in that path. I can't see how it can possibly take away a 'wee' more gain.

But, you have a two channel AB763 so let me offer you a challenge. Inject a 200mV 500Hz sine wave signal into the Normal channel. Connect a scope to the input grid of the PI. Adjust the volume and tone controls for a convenient 4 division high vertical display on the scope. Now put the amp on standby and disconnect the 220K mixing resistor for the VIB channel. DO NOT CHANGE ANY AMP CONTROLS. Finally, connect a 270K between ground and the summing point of the original mixing resistors. Flip the standby switch and look at the scope display. What is the amplitude of the signal now? Did it increase? decrease? remain the same?

[floyd]

I will do that.. but have you ?

[HotBluePlates]

Careful... objective measurement is cheating! 

I will do that.. but have you ?

I will venture a guess that Sluckey has.

He does have a single-channel AB763-style build long since done.

[floyd]

Careful... objective measurement is cheating!  

I will do that.. but have you ?

I will venture a guess that Sluckey has.

He does have a single-channel AB763-style build long since done.

And I'll bet his build has less gain than an original two channel.

[floyd]

I've noticed  that the offer of a $ award in the first post , really increases traffic here.

[octal]

Let's not forget the anode resistance of the unused channel's triode, and the 100K resistor to B+ which is also effectively an AC signal ground.  Is it possible that it might present a more complicated load to the active channel than just a shunt resistor to ground?  The level matching test is a good idea, but it would also be interesting to do a THD comparison switching between a shunt resistor and the whole other channel's circuit.

In a real world application, the non-zero AC impedance of the B+ rail due to the filter cap's ESR, and the interaction of the shared cathode cap might create some complicated non-linear behavior due to subtle feedback loops. Or, more likely,  this could all be nonsense.

[sluckey]

And I'll bet his build has less gain than an original two channel.

Well it actually has a little more gain than the NOR single channel we're discussing. I built the VIB channel with reverb but no tremolo.  Three triodes v. two triodes in the preamp. So, not a good comparison since the circuits are so different.

I will do that.. but have you ?

I've never taken the challenge I offered to you because I have never built a standard AB763. I do have a TRRI but it's factory pcb. Way too much trouble to do this simple check.

But you have a homebrew, so you da man! I'll trust your results. Seriously, I'd like to know if there is a gain loss with the 270K. Please post your results.

I wonder if we just have a different definition of gain? I use the electronic definition which says gain is equal to output divided by input and can be easily measured visually, eliminating differences in hearing. I understand that gain may mean something entirely different to guitar pickers.

[sluckey]

Is it possible that it might present a more complicated load to the active channel than just a shunt resistor to ground?

Absolutely! That's why I jokingly suggested replacing that 270K resistor with a complete VIB channel.

The level matching test is a good idea, but it would also be interesting to do a THD comparison switching between a shunt resistor and the whole other channel's circuit.

Getting too hifi for me.

[phsyconoodler]

I've built one channel AB763's and they all sound great.It's all about gain structure to get them 'exactly' the same as the two channel versions.I prefer the single channel ones quite a bit.I have built ones with trem and reverb and they sound delicious.I have also built ones with no trem or reverb.Also delicious sounding.
  What is it specifically that you are after here?

[tubeswell]

Its not just about trying to simulate the mixing resistor load. The AC load at the mixing resistors is also defined in part by the plate resistor and internal plate resistance of the 'other'/(passive) channel.

[phsyconoodler]

It's all about the gain.With two tube sharing a common cathode cap and resistor the gain is less.I don't see how it translates into much at all,cause guys have been removing V 1 for years anyway to get a slight bump in gain.Simply try a 5751 or 12AY7 and see if that gets you back to your magic spot.
  I simply turn down my guitar's volume a touch to get that same feel.

[kagliostro]

$100.000.000 reward

Dead or alive ? 

K

[octal]

I wrote:

The level matching test is a good idea, but it would also be interesting to do a THD comparison switching between a shunt resistor and the whole other channel's circuit.

 

sluckey:
 

Getting too hifi for me.

I wasn't suggesting that lower THD is a good thing in this application (or any guitar amp), I was just offering a scientific way of determining if there's more going on (vis-a-vis nonlinearities) than a simple level shift when you're back feeding an unused triode.

[HotBluePlates]

... HOW to make  a single channel AB763 REALLY sound like the stock two channel..

How do they sound different?

I don't currently own any 2-channel blackface-style Fender amps. I don't currently own any 1-channel blackface-style Fender amps, either. I used to own several different blackface Princeton Reverbs, and a '67 Super Reverb. They all sounded like Fender amps to me (yeah, volume and breakup was a little different).

I once had a 6G2 Princeton that I wanted to like but hated cause it didn't sound warm like any of my other (tweed or blackface) amps. Dunno if the speaker was to blame, I sold it too fast.

My point being, I've seen/heard different amps both have the "Fender sound" as well as a Fender amp that didn't.

What is it you're expecting to hear and not getting?

[terminalgs]

I'm here for the reward money:

"stage 1":

You'll need an extra triode, 6AV6 or 1/2-12AX7: take your pick  PLUS 1 cap, and 3 resistors.




"stage 2": now add the absurd MOJO

the 25/25 under the shared cathode at "C" should be 60's vintage, somewhat leaky and non-functioning so that unknown frequencies will not pass such that some instrument signal can live on the shared cathode and gets amplified by the 'dummy triode'.

"stage 3": the truly absurd MOJO requirement:

use an NOS tube (mullard,telefunken,GE,etc..) and  .1 blue molded cap.


"optional mod:"   instead of grounding grid of dummy triode, tie to center lug of volume control.

[SILVERGUN]

What is it you're expecting to hear and not getting?

Guys getting pissed off and arguing that he's wrong.... 

He seems to have lit the fuse and ran far, far away........unfortunately it was a DUD
Unless he's over @Ebay looking for a signal generator, a scope, and some mojo......then he'll definitely be back to prove that everyone is wrong 

[darryl]

I'm here for the reward money:

It's probably denominated in freshly printed eleven dollar bills . . .

[floyd]

What I'm saying , is the single channel 270K to ground mod in the NON-REVERB AB763 does not sound like a two channel. The single channel 50K to ground mod in the AB763 w/ reverb does not sound like a two channel...a bit harsh/gainy. Just trying to find help to figure it out.. sorry to upset you "SILVERGUN" , you obviously feel strongly about this topic for some reason.

[HotBluePlates]

The single channel 50K to ground mod in the AB763 w/ reverb does not sound like a two channel...a bit harsh/gainy.

So if the amp in this case had a little less gain, you'd be happy with it?

There are a bunch of ways to reduce gain. It would help to know exactly what you have now; if you have a schematic, that would help. Knowing the starting point, there are probably dozens of things to suggest to lose a bit of gain.

"Harsh/gainy" could be preamp tubes as well, so I wouldn't rule out trying a few different ones. Changing the input tube will probably have the most impact on the amp's overall sound.

[SILVERGUN]

sam, I'm here for the laughs...I'm sorry if I insulted you...
You kinda came off "a little confrontational" with sluckey in your original posts (read it back from an outsider's view)

When sluckey takes the time to address your concerns it's comparable to going to Trump Tower and having the Donald valet park your car for you....and then responding like you did is comparable to trying to mess the Donald's hair up, instead of tipping him....

I knew you would be back to answer the 10 unanswered posts since your last....

I'll leave it there, and again, I do apologize for my sense of humor. I honestly came to this thread in an attempt to learn something,,,and the title had nothing to do with it.....I have this habit of following sluckey and HBP's posts in an attempt to learn stuff.

Sincerely, good luck and happy holidays

[HotBluePlates]

...I have this habit of following ... HBP's posts in an attempt to learn stuff. ...

I'd follow PRR's posts, as I'm still learning and get stuff wrong at times.