Parts Ready For New Build Question

[Ed_Chambley]

I have gotten everything together and have read where dual bias using an dual intensity pot is "not worth the effort."  So I will match tubes and forget about doing this since I want the bias vary tremolo.  The main question I have is how this tremolo actually works?  I have looked over the layout and schematics and I don't quite get it.  I assume there is some fluctuation in the bias circuit which is used.  Could one of you folks explain how it works.  I have searched and have only found references to Leo Fenders genius, but nothing as to how it works.

I guess you don't have to know how it works for it to work, but I would like to understand it better.  I also intend on adding dwell to the reverb so this will be an ongoing string, please have patience with me.  This is my first amp where I am making everything except the chassis and of course the parts.

I am using a PT from Hammond with a bias tap and a Mercury F-Deluxe-FS-OM OT.  The OT will allow for adding additional cabs.   I do not have a choke yet.  I do not see one on the schematic.  If someone has a suggestion as to choke or no choke, please reply.

I have built a pine cabinet and will be using 2, 10's, Alnico recone CTS, paper cones one smooth, the other ribbed. About 20 watts each, 16 ohms.  Tweed covering, Aged shellac and oxblood screen.  The face will be brown.  So far everything looks like it should.

I have read everything about this that I can find on the board about this layout and I know Barry is very happy with his build.  There is lots of praise and I hope mine turns out well.

Opinions vary, but I will ask anyway.  CC resistors all the way?  Combination?  If combination, where?

Also, there is no middle so is the 3 in 1 mod good for this amp?  Also, if there are any other modifications you may have used that you are pleased with, let me know.

I plan for this to replace my main gigging amp, so please don't hold back.  I do not have the experience of you guys and I value it greatly.

Thanks!

[John]

Opinions vary, but I will ask anyway.  CC resistors all the way?  Combination?  If combination, where?

What I have read that makes the most sense to me is, CC drifts with heat, use, and age. Aiken says if you feel you MUST use them, the plates are the best place, where there is a wide voltage swing. Where values are more critical, better to use carbon- or metal- film.

On the other hand, my Harmony has only CC, everywhere, and it sounds pretty sweet. My SE I just got done has nothing but metal film, and it also sounds pretty sweet,  so WTH do I know? 

[drgonzonm]

For a description of how Fenders tremblo works Check out
http://www.freewebs.com/valvewizard1/trem1.html

I believe it does a decent job explaining what Leo did. 

[Ed_Chambley]

For a description of how Fenders tremblo works Check out
http://www.freewebs.com/valvewizard1/trem1.html

I believe it does a decent job explaining what Leo did. 

Thanks for the link.  It is just controlled oscillator circuit using the noise inherent in the circuit.  It is the change in resistance that controls the speed.  But why is it called a bias vary.  Does the intensity pot feeding the bias section actually shift the bias point and the intensity is created by raising and lowering the bias by using this oscillation? I guess what I mean is does the power tubes vary or just the bias of the tremolo tube and this is introduced into the signal path.

[Willabe]

Thanks for the link.

Did you read it?   

[Willabe]

Here Ed, here's some more on trem's. I hope this will be helpfull.

This is a good one, go down to where he lists _ Discreption_ it has info on bias modulation.

http://www.300guitars.com/articles/the-ins-and-outs-of-tremolo/


          Brad        

[sluckey]

.

[Ed_Chambley]

Thanks for the link.

Did you read it?   

Sure I did.  3 caps, 3 resistors, place a 1 meg pot on R1 (Fender) R2 (Vox) and you could use R3 as well. This will control the speed of the 60% created by the 3 caps and resistors.  I got that.  I did not see where it mentions intensity feeding into the bias section.  It may tell me, but it is just probably over my head.

I will read the other one you suggest.  It will sink in sooner or later.

[sluckey]

The valve wizard simply explains how the phase shift oscillator works. He doesn't say how the oscillator signal is actually used to accomplish the tremolo effect, other than, "This is then mixed with the audio signal in some way, so that its amplitude (volume) increases and decreases in sympathy with the trem' signal."

[Ed_Chambley]

.

Thanks Steve.  The intensity IS created by shifting the power tube bias.  So if you did have dual bias, you would have to have the ability to vary the bias to BOTH bias setups or you would have only one tube varying bias.  You would get a half baked tremolo unless you controlled the 2 precisely.  Which means if you did use a dual intensity pot it would be $$$$$, like a PEC.  Even the you run the risk of the pot degrading over time.  That's why in the other thread I read most confirmed "too much trouble."  Much easier to match tubes.

[Willabe]

The valve wizard simply explains how the phase shift oscillator works. He doesn't say how the oscillator signal is actually used to accomplish the tremolo effect, other than, "This is then mixed with the audio signal in some way, so that its amplitude (volume) increases and decreases in sympathy with the trem' signal."

My fault, sorry Ed.   

[sluckey]

That's why in the other thread I read most confirmed "too much trouble." 

Which thread? Link please...

Alpha makes a readily available dual pot that is ideal for Hoffman's style bias vary trem. Cost is $2.60. If I wanted bias vary trem AND dual bias pots, I'd use it in a heartbeat. No reason not to do that if you can wire it properly. A schematic is very handy when implementing these kind of circuits.

[Ed_Chambley]

That's why in the other thread I read most confirmed "too much trouble." 

Which thread? Link please...

Alpha makes a readily available dual pot that is ideal for Hoffman's style bias vary trem. Cost is $2.60. If I wanted bias vary trem AND dual bias pots, I'd use it in a heartbeat. No reason not to do that if you can wire it properly. A schematic is very handy when implementing these kind of circuits.

I would like to do it.  I found the thread, but I don't know how to link.
Adding dual bias pots to Princeton
Raybob

this is the topic.  I will find out how to link

[Ed_Chambley]

http://www.el34world.com/Forum/index.php?topic=7487.msg66454#msg66454

Here is the link Steve.  I sort of made the decision because of your comment.  Let me know if you have changed your mind because I would like to do it.  Single NOS tubes are much cheaper and I got a lot of them.

[Ed_Chambley]

That's why in the other thread I read most confirmed "too much trouble." 

Which thread? Link please...

Alpha makes a readily available dual pot that is ideal for Hoffman's style bias vary trem. Cost is $2.60. If I wanted bias vary trem AND dual bias pots, I'd use it in a heartbeat. No reason not to do that if you can wire it properly. A schematic is very handy when implementing these kind of circuits.

I have been looking for a schematic to do this.  I'll bet you know where one is?

[Willabe]

Ed, here's what I came up with from KOC books. Single pot's, not dual.

Will this work?

Steve how does a single/dual pot for this, instead of 2 pots work?    

      
   Brad      



 

[Ed_Chambley]

Here is an attempt.  Since I am using the AA1164, I was wondering if you could just duplicate the Bias pot sending to the DUAL 250L pot and separate the 220k grid resistors.

And could you connect both halves of the dual pot to the same cap .1uf cap?

[sluckey]

Here is an attempt.  Since I am using the AA1164, I was wondering if you could just duplicate the Bias pot sending to the DUAL 250L pot and separate the 220k grid resistors.

And could you connect both halves of the dual pot to the same cap .1uf cap?

That's exactly how I would do it. You must connect the left side of both INT pots to that .1µF cap.

[Ed_Chambley]

Here is an attempt.  Since I am using the AA1164, I was wondering if you could just duplicate the Bias pot sending to the DUAL 250L pot and separate the 220k grid resistors.

And could you connect both halves of the dual pot to the same cap .1uf cap?

That's exactly how I would do it. You must connect the left side of both INT pots to that .1µF cap.

I hope you are serious because it means I finally got something right.
Thank you!

[HotBluePlates]

... The main question I have is how this tremolo actually works? ... I assume there is some fluctuation in the bias circuit which is used. ...

In case you hadn't found a satisfactory answer in your other reading, here's something that might help.

Any tremolo (not vibrato) circuit which does not use an optoisolator (or transistor/JFET) to ground the signal alters the bias of a tube somewhere in the amp to create a trem effect.

The trem signal is attached to a point in a gain or output stage where the trem signal combines with the bias of the stage. The trem signal is alternating, while the bias is steady. As a result, the trem signal alternately adds to, and subtracts from, the standing bias.

The varying bias turns the tube more-on and more-off at the rate of the trem signal. The guitar signal is then modulated at the rate of the trem signal, which we hear as a varying volume.

The trem signal is slow enough (often 2-8Hz) that we perceive it as a pulsating volume, rather than as an audio signal. That's simply because the trem signal is well below the audio range.

The "genius" of the output stage bias vary is that it is applied to both sides of the push-pull output stage at the same time, making it a common-mode signal. The push-pull stage responds almost exclusively to difference signals, meaning the two sides of the output most have different or opposite polarity signals. Since the same trem signal is fed to both sides, it can alter the volume of the output, but does not result in a "pumping" noise.

This is to be considered an upgrade over trem applied to a single-ended preamp stage, which might have continuous background noise at the trem rate. Leo Fender first patented a trem signal applied to a shared cathode resistor of a paraphase inverter stage (tweed Tremolux) to achieve this effect, then later it was applied to the output stage bias feed resistors when the amp's phase inverter was changed.

[sluckey]

I hope you are serious because it means I finally got something right.

I am and yes you did. I'm glad to see you start with a schematic. That should be the first step of any amp project.

All that said, it is just a Princeton. Dual bias pots on 6V6s seems a bit excessive to me, especially since it complicates the trem circuit. Leo's Princeton works well and it doesn't even have a bias pot. I don't think there's any performance value to dual bias pots in the Princeton. It's kinda like a 'fixed/cathode' bias switch or pentode/triode switch. Hey, it works. That's neat. But you'll eventually quit flipping the switches or twiddling the pots. (I know, I've done all three of the mods I just mentioned.)

The real value of the Princeton dual bias, is the learning process. The thinking, drawing, implementing, and finally the 'ah' factor when you finally see it works as you initially hoped/planned. And from my hobbyist viewpoint that's plenty of reason to do it, so go for it. Just proceed in a logical fashion and enjoy the 'ah' factor when it comes together.

[Ed_Chambley]

Thanks Guys for the input/info.  I am getting an understanding of the bias tremolo.  At the same time I also understand why the optoisolator became of common use.  It is a less maintenance approach, at least that's the way I see it.  It does not rely on keeping proper bias or as closely matched output.  If it is on tho, I can always hear it as background.  I use tremolo a lot, but my Super Reverbs all have the roach and are modded with switch pot.  There is a drawback in doing this because of the increased gain.  I had the opportunity to play a brownface with this type of tremolo and IMO, for me this is superior.

Still looking for other suggestions.  I have drawn a new turret mask for adding the additional lugs for the dual bias.  Before I start drilling the board, I am looking for other modifications which may need to be added to the board.  The layout contains no middle control and there probably is no need for one, but I do plan on adding the 3 in 1 switch.  

Also, I am not certain how to add a dwell to the reverb.  Does any allowance need to be made to the board for this?
 
I also do not want the reverb tank in the bottom of the cabinet.  Can a tank be mounted to the side making the tank vertical.  I would think the springs need to be horizontal.  If that is the case, I will simply make a false bottom to cover the tank as I tend to store a repair kit in my amps.  Additional set of tubes, fuses.........etc.

The PT I have for this is 272DX, 123va, 115vac, 60hz, 600V C.T. @ 144ma., 5V @ 3A, 6.3V @ 4A
It seems like it may be a little on the low side, but it seems like every Hammond I use measure more than stated.

[Ed_Chambley]

I hope you are serious because it means I finally got something right.

I am and yes you did. I'm glad to see you start with a schematic. That should be the first step of any amp project.

All that said, it is just a Princeton. Dual bias pots on 6V6s seems a bit excessive to me, especially since it complicates the trem circuit. Leo's Princeton works well and it doesn't even have a bias pot. I don't think there's any performance value to dual bias pots in the Princeton. It's kinda like a 'fixed/cathode' bias switch or pentode/triode switch. Hey, it works. That's neat. But you'll eventually quit flipping the switches or twiddling the pots. (I know, I've done all three of the mods I just mentioned.)

The real value of the Princeton dual bias, is the learning process. The thinking, drawing, implementing, and finally the 'ah' factor when you finally see it works as you initially hoped/planned. And from my hobbyist viewpoint that's plenty of reason to do it, so go for it. Just proceed in a logical fashion and enjoy the 'ah' factor when it comes together.

I agree to a point.  The main value I see is 3 fold.  First is the learning process.  Next is my tube collection.  I don't care what anyone says, NOS tubes sound better.  I cannot explain why, but they do.  The largest thing I want to accomplish is using the same rig with the different people/styles I play.  In old country, tremolo is a must and I have actually been using a pedal I made from the Bluesbreaker circuit instead of the tremelo in my Super.  It runs 100% wet to another amp.  Mixing it into the front of the Super I lose all of the pop and twang, but when I played a Brownface a while back it seemed as if engaging the tremolo caused no loss in tone.  Probably just in my mind, but it seemed like if I had this I could reduce my setup for these gigs.

That being said, I appreciate your help.  I know this will not be my only amp need as I also play with folks where Marshall is a necessity.

[sluckey]

The layout contains no middle control and there probably is no need for one, but I do plan on adding the 3 in 1 switch.

Mid control wont need a change to the board. Just replace the 6.8K on the Bass pot. What is the "3 in 1 switch"?

Also, I am not certain how to add a dwell to the reverb.  Does any allowance need to be made to the board for this?

No board change needed. Just remove the 1MΩ resistor from V2-2/7. Look at the attached pic.
 

I also do not want the reverb tank in the bottom of the cabinet.  Can a tank be mounted to the side making the tank vertical.  I would think the springs need to be horizontal.

Accutronics makes tanks to mount in almost any position (see their website). I have my spring assembly mounted in the bottom of my cab. It's also inside a heavy duty vinyl bag just like the original Fender combos. And I have spares in a zip lock bag, plus a footswitch just thrown in the bottom of the cab. No problems at all. I wouldn't put a brick on top of the tank bag though!

The PT I have for this is 272DX, 123va, 115vac, 60hz, 600V C.T. @ 144ma., 5V @ 3A, 6.3V @ 4A
It seems like it may be a little on the low side, but it seems like every Hammond I use measure more than stated.

That PT sounds about perfect to me. You'll have plenty of B+, but if you still think you need more, just use SS rectifiers.

About the dual bias...

The main value I see is 3 fold.  First is the learning process.  Next is my tube collection.  I don't care what anyone says, NOS tubes sound better.

Back in the day when 'NOS' was just plain 'new', players did not know about or cared about biasing and tube matching. They just changed output tubes as needed, often only one. When an amp went to the shop for repair, the tech 'might' measure the negative bias voltage on the output tube grids IF the problem was related to the output stage. He might even adjust the bias pot to set the voltage to the value listed on the schematic. If there was no bias pot, then close was good enough. We really obsess way too much about bias today, especially on little amps. BTW, I don't recall ever seeing a Princeton Reverb used in a band in the '60s. They were just too small for a decent sized club and nobody had the fancy PAs of today, so nobody was miking a little amp.

[Ed_Chambley]

The layout contains no middle control and there probably is no need for one, but I do plan on adding the 3 in 1 switch.

Mid control wont need a change to the board. Just replace the 6.8K on the Bass pot. What is the "3 in 1 switch"?

Also, I am not certain how to add a dwell to the reverb.  Does any allowance need to be made to the board for this?

No board change needed. Just remove the 1MΩ resistor from V2-2/7. Look at the attached pic.
 

I also do not want the reverb tank in the bottom of the cabinet.  Can a tank be mounted to the side making the tank vertical.  I would think the springs need to be horizontal.

Accutronics makes tanks to mount in almost any position (see their website). I have my spring assembly mounted in the bottom of my cab. It's also inside a heavy duty vinyl bag just like the original Fender combos. And I have spares in a zip lock bag, plus a footswitch just thrown in the bottom of the cab. No problems at all. I wouldn't put a brick on top of the tank bag though!

The PT I have for this is 272DX, 123va, 115vac, 60hz, 600V C.T. @ 144ma., 5V @ 3A, 6.3V @ 4A
It seems like it may be a little on the low side, but it seems like every Hammond I use measure more than stated.

That PT sounds about perfect to me. You'll have plenty of B+, but if you still think you need more, just use SS rectifiers.

About the dual bias...

The main value I see is 3 fold.  First is the learning process.  Next is my tube collection.  I don't care what anyone says, NOS tubes sound better.

Back in the day when 'NOS' was just plain 'new', players did not know about or cared about biasing and tube matching. They just changed output tubes as needed, often only one. When an amp went to the shop for repair, the tech 'might' measure the negative bias voltage on the output tube grids IF the problem was related to the output stage. He might even adjust the bias pot to set the voltage to the value listed on the schematic. If there was no bias pot, then close was good enough. We really obsess way too much about bias today, especially on little amps. BTW, I don't recall ever seeing a Princeton Reverb used in a band in the '60s. They were just too small for a decent sized club and nobody had the fancy PAs of today, so nobody was miking a little amp.

[Ed_Chambley]

The layout contains no middle control and there probably is no need for one, but I do plan on adding the 3 in 1 switch.

Mid control wont need a change to the board. Just replace the 6.8K on the Bass pot. What is the "3 in 1 switch"?
3 in 1 just for switching different values in the 6.8k.  Nothing really big, it just allows for multiple resistors to be switched in-place of the stock mid resistor.  It is just something I put on a JTM-45.  With the princeton it will not be necessary to have the caps, but you could.  Like I said, just a switch for 3 values.

[Ed_Chambley]

Here is a little deal where I parsed the princeton files and combined them.  It does not have the dual bias, but when I finish the mask and the board and revise the schematic completely, I will post it.  This is a low resolution version of a 36 x 22 document.  I print these out full size in high resolution at 100% on a large format printer.  It is a great reference to hand above your work bench.  The mask can be cut out and used as a drill template.

[sluckey]

That Princeton Reverb project was developed by several members right here a few years ago. That final schematic was probably drawn by tubenit using PCBexpress. The editable file is available on the forum. And the layout drawings were created by bnwitt using Visio. He'll probably give you the Visio files if you want them.

It will be much easier to edit the drawings in their native format rather than use a graphics editor.

[Ed_Chambley]

That Princeton Reverb project was developed by several members right here a few years ago. That final schematic was probably drawn by tubenit using PCBexpress. The editable file is available on the forum. And the layout drawings were created by bnwitt using Visio. He'll probably give you the Visio files if you want them.

It will be much easier to edit the drawings in their native format rather than use a graphics editor.

I cannot find a member bnwitt.

[John]

http://www.el34world.com/Forum/index.php?action=profile;u=66

[sluckey]

I cannot find a member bnwitt.

Look again. You must be doing something wrong on the search.

[Ed_Chambley]

I cannot find a member bnwitt.

Look again. You must be doing something wrong on the search.

Found it with john's link, then I found it in the members list.  I am just not familiar with the forum.  I sent him a message.  Visio is easy stuff.  Opened it up, made a list of my favorite shapes including some I downloaded from your page.  It is like Autocad and Illustrator mixed.  Very useful.  Now I have to find a Windows machine for home.  I picked up 2 broken ones less than a year old yesterday.  I will get one out of the 2.  Problem is I would rather it be portable.  I have been looking at netbooks with Atom processors, but I can get a 1 year old notebook with way more processing power for less money by the time you get the atom loaded the way you need it to run applications.  Right now all I can do is use Visio at work which is a conflict.  BTW, the other amp is really coming along.  Once I get the small kinks worked out and get the tone stack more mid scooped and increase the bass response, I got something really nice.  It does need a cap on the standby as it pops a little.