5e3 coarse/fine volume control

[EW57]

Regarding the all or nothing volume control of the 5E3, is there a reason that two pots couldnt be connected in such a way that would create a "coarse/fine" control, or is this overkill? 

Thanks!

[Fresh_Start]

Do you have linear taper or audio (logarithmic) taper volume pots?

Even with a linear taper pot, it shouldn't be like an on/off switch.  However, an audio taper volume pot would give you more control in the lower range.

HTH

Chip

[stingray_65]

Ive built a few 5E3's and never experienced this all or nothing phenomenon .

Could you please describe this further?

I am curious.

I've also used Lin pots because it was what I had on hand, I did not experience an all or nothing sort of situation, but it did not give the levels I expected in the 1-7 range.


Ray

[EW57]

I probably oversimplified what I have heard/read about the abruptness at lower volume settings. I personally havent built one yet, but am in the planning stages & wanted to make sure to leave myself room for another pot if necessary.

Thanks!

[Fresh_Start]

audio taper pot

[jeff]

Regarding the all or nothing volume control of the 5E3, is there a reason that two pots couldnt be connected in such a way that would create a "coarse/fine" control, or is this overkill?

no and yes
The volume pots for a 5E3 are wired backwards. The advantage is one can be off without shutting the other off. This doesn't work well in my opinion. Wire each channels coupling cap to it's volumes lug, it's wiper to a 220K resistor, the two 220K resistors to the next tubes grid. Connect tone cap A to either C or B. You'll wind up losing some gain. If you only use one channel forget the 220K, reverse the outer lug and wiper then disconnect the other channels outer lug.

The more I look at it the tone control of a 5E3 is wierd. It seems if you plug into chan 2 and have the tone full on chan 1's vol would cut the treb. Never tried one, is that the case?

[Fresh_Start]

The volume pots for a 5E3 are wired backwards.

No, they aren't.  They are wired like a Tweed Deluxe 5E3.  The Volume controls and the Tone control are interactive.  That's the nature of the beast.  You could wire them differently, but it wouldn't be a Tweed Deluxe anymore.

Sorry if that seems discourteous.  Jeff is correct that the Volume pots are wired "backwards" compared to most amps.  The interactive nature of the "channels" is one result.  Another is that the input impedance for the 12AX7 gain stage stays constant instead of varying with a "normal" volume control.  That stage has a constant 500K grid return (or "grid leak" if you prefer) resistor made up of the two 1 meg Volume pots in parallel.  Normally, the grid return resistor for a gain stage right after a volume pot is whatever portion of the pot remains "below" the wiper.  In other words, a normal volume pot acts as a voltage divider on the signal going into that gain stage, but it also varies the value of the grid return resistor.  Gain drops as input impedance drops if I understand it correctly.  (I did not understand this correctly.)

I have a strong suspicion that this unusual setup for the Volume pots is part of the 5E3 mojo.  The gain of the 12AX7 stage right before the concertina inverter is constant regardless of the Volume pot settings.

Jeff - Thanks for questioning the 5E3 circuit.  It got me to look at it again and see it in a different way!

Respectfully,

Chip

[jeff]

 Good point. I never concidered this.
 

No, they aren't.  They are wired like a Tweed Deluxe 5E3. The Volume controls and the Tone control are interactive.  That's the nature of the beast.  You could wire them differently, but it wouldn't be a Tweed Deluxe anymore.

True, if you make a change it's no longer a 5E3 but if you don't change it then you have to live with it. You can't have it both ways: both solve the problem and remain stock. His problem is the aburpt drop in volume. Anything you do to "fix" this is going to change it from being a stock 5E3. I believe his problem, what he's trying to "fix" is caused because the volume pot is wired, for lack of a better term, "backwards".

 If the Mojo is in the grid leak resistor we can use a dual ganged pot. Wire one section as a "normal" volume and the other as a variable resistor from wiper to grid. That way as the wiper approaches ground the grid leak resistance will increase and the two will remain constant.

Of course now you're not lowering the output impedance of the first tube. I don't know if were losing Mojo in that.

[jeff]

OK how about this. In the spirit of his post of using two pots coarse/fine and trying to keep it more "5E3"ey.

We'll use two pots, One controls output impedance of the first tube, one for volume. Use 2M pots maybe? But that doesn't keep the second tubes grid constant.

I don't know, you can't have it both ways. My advice, try the above schematic if you like it, good, if not, you'll have to deal with the 5E3's quirks.

[phsyconoodler]

Quote:"you'll have to deal with the 5E3's quirks"

Or learn how to use them.I use the normal channel at 50% and dime the bright channel.If you plug into the normal channel you get nice cleanish fat blackface-like tone.Now plug into the bright channel and you get ripping overdrive.
  Then put a high-pass filter on your guitar's volume control and use it to get the most incredible cleans from the amp as you turn down.Then simply turn up and get your lead tone.
  You can play a whole set without a pedal and get the best lead and rhythm tones.

[simonallaway]

Quote:"you'll have to deal with the 5E3's quirks"

I use the normal channel at 50% and dime the bright channel.If you plug into the normal channel you get nice cleanish fat blackface-like tone.Now plug into the bright channel and you get ripping overdrive. Then put a high-pass filter on your guitar's volume control and use it to get the most incredible cleans from the amp as you turn down.Then simply turn up and get your lead tone. You can play a whole set without a pedal and get the best lead and rhythm tones.

Perfect! If only there was a page somewhere with useful summaries such as this for every amp   

[HotBluePlates]

True, if you make a change it's no longer a 5E3 but if you don't change it then you have to live with it. You can't have it both ways: both solve the problem and remain stock. His problem is the aburpt drop in volume. Anything you do to "fix" this is going to change it from being a stock 5E3. I believe his problem, what he's trying to "fix" is caused because the volume pot is wired, for lack of a better term, "backwards".

 If the Mojo is in the grid leak resistor we can use a dual ganged pot. Wire one section as a "normal" volume and the other as a variable resistor from wiper to grid. That way as the wiper approaches ground the grid leak resistance will increase and the two will remain constant.

Of course now you're not lowering the output impedance of the first tube. I don't know if were losing Mojo in that.

The first tube does not have its output impedance lowered by the stock arrangement (or the proposed modified arrangement).

What is interesting regarding the first stage and the volume control is that the actual gain of the first stage is increased as the volume control is turned up.

A tube stage sees not only the plate load resistor as its load (which does set the d.c. condition for the stage), but it also sees the following stage's grid reference resistor through the coupling cap as a parallel path to ground. That means the coupling places the 2 resistors in parallel, making the load seen by the tube smaller. A smaller load means less voltage gain developed by the first stage.

So as you turn up the volume, less signal is shunted to ground and the gain of the first stage increases.

Assume the volume is turned up halfway, which corresponds to 100k to ground for a 1M audio taper pot (assuming 10% taper). For frequencies not rolled off by the coupling cap, that reduces the apparent a.c. load of the first stage to 50k. With a 12AX7 stage (and assuming the internal resistance is 56k as the operating point; it could be higher or lower), the gain is 100*[50k/(56k+50k)] = 47. With a typical volume arrangement, the gain is 100*[100k/(56k+100k)] = 64 (and constant with volume rotation).

Further, because the coupling cap is in series with the wiper, the bass roll-off point changes with the position of the volume control. The bass rolloff is determined by both the plate load resistor and the following stage grid reference resistor (or the portion of the pot resistance from wiper to ground) in series and the cap value. Fortunately, this effect happens at a low enough frequency that it doesn't matter to us, even if you use a smaller-than-stock coupling cap.

[Fresh_Start]

Thanks HBP! 

I'm confused about something though - where's the variable voltage divider for the output of the first stage(s)?  Is the upper "leg" of the voltage divider the plate resistance of the triode?  IOW is it possible that most of the effect of the volume control is due to the Volume pot's impact on the gain of the first stage?

Also, would you mind either confirming or debunking my theory about the gain of the 12AX7 stage described in post #3?

This discussion has been great for me.  I've been slogging through Merlin's preamp book and suddenly some of the theory and formulas are making sense (I think... )  In addition to figuring out the nuances of the 5E3 volume controls, this has given me great insight into how a normal volume control works.

Respectfully,

Chip

[HotBluePlates]

... where's the variable voltage divider ... ?

The voltage divider is made up of 2 legs: the internal plate resistance, and the "load resistance." But at a.c., the plate load resistor is in parallel with the grid leak resistance of the following stage. The portion that determines the gain is the load resistance, which is why you get gain closer to mu with a bigger plate load resistor.

Also, would you mind either confirming or debunking my theory about the gain of the 12AX7 stage described in post #3?

It is confusing as written.

Forget any external parts. As long as the grid is negative of the cathode, the impedance looking into the grid is very, very large. So for normal conditions, we look at the tube as having an infinite input impedance, and concern ourselves with only the external resistors.

Volume controls in 99.9% of amps are wired so that the previous tube stage sees a non-varying resistance to ground; the varying resistance to ground is seen by the tube grid looking backwards toward the previous gain stage. If the resistance seen by the tube is 1M or less, most small-signal tubes don't behave any differently regardles of a change in value. So the common form of volume control has no effect on the 2nd stage, and the non-varying portion is being seen by the first stage.

In the 5E3 Deluxe, the common arrangement is reversed. Common volume controls would need isolating resistors in series with the wipers to prevent interaction (see the 5F6A Bassman as an example; there is very little interaction compared to a 5E3 Deluxe). Fender saved the cost of 2 resistors, by having the ungrounded end pot be a series isolating resistance if the pot is set below maximum. We know it doesn't work that great, but does give some interesting options.

Now that the volume control arrangement is reversed, looking backward from the 2nd stage grid, the tube sees a constant resistance. But that doesn't do anything beneficial for the 2nd stage. Instead, now the first stage sees a varying load resistance, because a smaller resistance to ground after the coupling cap is now in parallel with the plate load, which makes the total load smaller. That, in turn, makes the actual gain delivered by the 1st stage smaller.

But I find this a cool feature. I sure don't mind it in my 5E3 copy; when I need more clean volume, I use my 5F4 Super copy.

And the real issue people have is that they feel the 5E3 lacks headroom because it's distorting by the time you turn up a bit beyond halfway. That's because the amp has more gain than needed to drive the 6V6's. You can make changes to give the illusion of a wider range of clean volume, but the actual SPL from the speaker won't be any more because the clean power from the amp to the speaker is any larger.

But seriously... if you wanted more clean power, you'd have stepped up to the Super or Pro (or later the Bandmaster and Bassman).

[Fresh_Start]

... where's the variable voltage divider ... ?

The voltage divider is made up of 2 legs: the internal plate resistance, and the "load resistance." But at a.c., the plate load resistor is in parallel with the grid leak resistance of the following stage. The portion that determines the gain is the load resistance, which is why you get gain closer to mu with a bigger plate load resistor.

Also, would you mind either confirming or debunking my theory about the gain of the 12AX7 stage described in post #3?

It is confusing as written.

That's because I am confused.  Please consider my theory debunked.

GE 12AY7 data sheets shows 25K plate resistance for each 12AY7 triode.  Resistance between what two points?

Assuming 25K is the "upper leg" of our voltage divider, would the lower leg would be 100K || wiper-to-ground of Volume pot? Does the attached drawing represent the location of the plate resistance correctly?

Sorry for beating a dying horse,

Chip

[jeff]

But at a.c., the plate load resistor is in parallel with the resistance of the following stage.

How are the plate resistor going to B+ and the grid leak  going to ground  parallel at a.c.?
Does this mean as far as AC is concerned the value of the load is equal to the value of the resistors in parallel?

So if you wanted a coarse/fine volume with two pots would you do this?

[sluckey]

B+ rail is at AC signal ground simply because of the filter caps. So, as far as the ac signal is concerned, the plate loade resistor is effectively connected between plate and ground.

[HotBluePlates]

You've got it right Chip. For everyone else that might be mislead, the 25k resistor between the tube plate and the coupling cap is simply a way to represent in internal resistance of the tube at the operating point.

[HotBluePlates]

So if you wanted a coarse/fine volume with two pots would you do this?

I think feeding the 2nd volume control from the first control's wiper might work better.

But EW57's initial question is, I think, based on the commoents of others in forums like this, rather than a past negative experience with a 5E3. The coarse/fine control is not really needed in my opinion.

I say that because a 5E3 distorts starting a little above halfway on the volume control. On a blackface amp, for instance, breakup might start at closer to 6-7. I don't see this characteristic as a defect, but some folks perceive it as a lack of headroom. That is, they "hear with their eyes" and see a lower volume setting as less clean headroom. My position is that clean headroom is determine by SPL at the speaker before distortion, and that you can tweak the volume control to look as though it stays clean at a higher setting.

[jeff]

I say that because a 5E3 distorts starting a little above halfway on the volume control. On a blackface amp, for instance, breakup might start at closer to 6-7. I don't see this characteristic as a defect, but some folks perceive it as a lack of headroom.

I think I misinterpred the problem. I thought the problem occured when the volume pot was set very low(around 1).  I though the problem is as you turn down you're also reducing the gain so the volume drops quicker as you get lower. So it's hard to play quietly and turn it up just a little between 1-2 without dumping the volume.

I once had a guitar pot that was burned out or something so that one lug was dead. I wound up wiring it as a varible resistor until I could replace it. I experinced this "all or nothing" mentioned in the first post when the volume was low.

I think feeding the 2nd volume control from the first control's wiper might work better.

If you wanted two volumes I agree, but this was more of an attempt to lower the load of the first stage with the var.res. then fine tune the volume with the second. If the second pot was fed by the wiper we couldn't adjust the load of the first stage and keep that good ole "mojo" that was spoken of. The first pot would act in a similar fashion to the original volume pot and the second would allow for fine tuning at low volumes.

[stingray_65]

I think one of the REALLY neat features of these tweed Fenders (besides the AWSOME mojo when fed with a single coil pickup) is that they are so easily modded to get YOUR desired tone.

If you got the scratch to build a tweed amp, any of them, just do it!

Just remember, there is a huge likelihood you may get the itch to tweak it a bit, especially after reading a few great ideas in a thread.

The 5E3 has been built by so many builders so many times and in so many ways and I've only read once in a forum of someone who was dissatisfied with it.

Ray