Blues Jr Mods

[Tbone55]

Thank You Willabe for your book suggestions. I'm going to see if I can get a hold of them and improve my poor understanding of this stuff.

Thanks also for clarifying how the cap value affects frequency. Don't know why I got that one wrong. Caps are passive components and can only filter out frequencies, not add them. They can can only affect frequencies that are already present.

Thanks again for all your help. Cheers!

[Willabe]

Thanks also for clarifying how the cap value affects frequency. Don't know why I got that one wrong. Caps are passive components and can only filter out frequencies, not add them. They can can only affect frequencies that are already present.

Yes. 

[Tbone55]

Hello stratomaster,

Today I placed the .47uf cap across the cathode resistor of V2. As far as I could tell there was a very small boost of the highs but nothing earth shattering. I'm going to leave the cap in place. I'm currently using the 12AY7 in V2. I can't hear much of a difference between it and the 12AX7. Turning up the volume/gain control even to 3 is pretty loud but at least it's more gradual now rather than the volume jump I was getting before.
 I want to try the 12AY7 in V1 and put the 12AX7 back in V2 again before deciding to play with the plate resistor values of V1 to reduce the gain. I've been playing with the amp books frequency response calculator and there's about a 1db difference between using a 12AX7 with 68k or 47k plate resistors vs a 12AY7. The 12AX7 might just be easier so I don't have to have different tubes. I wonder if there's any tonal difference? Is it necessary to create load lines in either case or are the component values already there ok to use?

Also, I got a book, Vacuum Tube Amplifier Basics by EJ Jurich today. I wanted to get the Merlin Blencow book but  I thnk this will do for now. It starts off with the very basics of electronics, what resistors and capacitors are, how to read their values,  what they do etc, difference between AC and DC, Ohms Law formula, calculating power, transformers, eventually getting into vacuum tubes.

I'm waiting for some components I ordered to do a bit more work on the amp. I ordered a 100uf F&T cap to relace the 47uf filter cap, a 100k audio taper pot and some 330k resistors to change out the current 100k linear master volume pot and 430k voltage divider resistor.

I really do want to learn and understand more about how the circuits in the amp work and tweaks to improve them. If I've caused you some frustration in this process I apologize. Your advice, help and patience is greatly appreciated and I hope we can continue to have conversations.

Regards and Cheers!

[stratomaster]

Keep experimenting. You won't harm the amp with preamp tube substitutions. No load lines necessary.

I'm surprised that the 0.47μF cathode cap didn't make much of a difference to you.  Between that and lifting the 0.001μF cap you should be getting another 9-12dB of treble content into the PI. This reinforces my opinion that there's something else wrong with your amp. 

[Willabe]

You said/did nothing wrong, you have nothing to apologize for. 

Glad you found a book. I probably have over 50 different tube amp books I've bought over the last 20+ years. 

[Tbone55]

I'm surprised that the 0.47μF cathode cap didn't make much of a difference to you.  Between that and lifting the 0.001μF cap you should be getting another 9-12dB of treble content into the PI. This reinforces my opinion that there's something else wrong with your amp.

Beats me too. I have no idea what it could be or where to look. I'll keep experimenting with the tube in V1. I guess this would be a great time to get the amp on a scope to see what's going on. If I can find someone that can do that I will. I'm going to check the values of each resistor and capacitor first then go over and trace my layout again to make sure everything is connected where it should be. Will keep you posted.

Cheers!

[Tbone55]

You said/did nothing wrong, you have nothing to apologize for. 

Glad you found a book. I probably have over 50 different tube amp books I've bought over the last 20+ years. 

Thanks Willabe.  Hopefully what I'm reading will actually sink in. 🤣

[Tbone55]

I've been going over my amp to confirm the component values (only resistors and pots) as well as re-trace the wiring from the layout (see attachments).  I did find that I had reversed the inputs from the PI going to V4 and V5 but I'm pretty sure that's not causing a problem. Other than that all the voltages seem to be good.

Tried putting a 100k resistor to the input of the volume pot but it didnt have much of an effect on how quickly the volume/gain came. There's no jump in volume but the amp still gets loud fast. I would expect the volume to come on more gradually since it's a 10% taper pot. Think I'll remove the 120k resistor attached to the wiper tab and to ground.

I've put the amp back to the original configuration before I started trying the mods. I felt I had l had to in order to hear what the amp was like again and be able to confirm my original observations. The amp does sound boxy and in need of some low end tuning especially with humbuckers. Even with the Meat control dimed the humbuckers felt too bassy. The single coils were a bit better.

After reviewing all of the comments again from the post I've decided to try a few other mods you suggested and a couple you're not so keen on.

1. Change the fat cap from 22uf to 1uf.
2. Put a bright cap across the volume put.
3. Put a  330pf, 470pf or 680pf across the plate resistor of V2 .
4. Change the by-pass cap on V1-A from 47uf to 2.2uf. This will depend on what happens when I do #1.

That's it for now. I'll let you know tge results. Cheers!

[stratomaster]

Good. Glad you're experimenting.  I think you misunderstood the effects of some of the mods you're considering auditioning, but that's ok. You may stumble across something you didn't know you liked.

1. Change the fat cap from 22uf to 1uf.
This will change the frequencies boosted by the Fat switch.  The result will be a more high mid/treble focus boost vs stock.
But only in Fat mode.  This would be a good mod to try if you find you always use Fat mode, or if you find FAT move too loose.  It wouldn't benefit you at all if you don't regularly use that mode.

2. Put a bright cap across the volume put.
You should absolutely try this. Especially if you've confirmed there are no build errors bleeding off high end.

3. Put a  330pf, 470pf or 680pf across the plate resistor of V2 .
This essentially is the opposite of the bright cap.  If you're lacking in high end this will only make it worse.  However, if you add a bright cap and it moves your sound in the right direction but is a bit too aggressive this mod can help tame it to an acceptable level. This is where experimentation will come in handy.

4. Change the by-pass cap on V1-A from 47uf to 2.2uf. This will depend on what happens when I do #1.
As explained above, mod 1 affects only the Fat mode.  The results will be a tighter amp with a but more background noise.  If this is an acceptable trade off to you, then it could work.  However I'm skeptical since you previously installed a 0.47μF cap here and still found the amp to be bass heavy and lacking in highs. Please do try it though.  I have a 2.2μF in V2a on my Super Reverb and love the clarity I get vs a full bypassed stage. I still think the greatest benefit to your amp would be to leave C1 as is and change C2 to 0.001μF which you've yet to try.

On reversing PI leads:  this is a big deal from the perspective of the negative feedback loop that we've discussed as an area of potential problems.  The fact that you preferred the tone with NFB disconnected also points to a problem.  If the grid wires are reversed, but the transformer is wired as shown in the diagram you will get positive feedback and oscillation issues.  Sometimes these issues are obvious with squeals and howls--other times it's more sneaky with phase cancellations and parasitic oscillations. I'd recommend wiring it per the diagram, or at least swapping the primaries of the transformer to see if that improves the sound.

Regarding taper resistor - Removing the 120k will increase the signal out of that volume pot and make your volume problem worse. Draw out the voltage divider as I've shown previously and you'll see this.  But try it anyway.  The best way to learn is to do.

Regarding the resistor into the volume pot - try a larger value, especially if it was moving you in the right direction.  Don't change tactics entirely (erroneously looking at the taper resistor) if the first attempt doesn't achieve the desired results.  I believe I suggested a range, try the you're end, 220k before moving on.  You can even experiment with higher values if you want the amp to operate more in the clean region and are ok with limiting gain/overdrive.

On Meat control - depending on how it's wired "dimed" could mean stock NFB or next to no NFB.  With no NFB the sound is going to be bassy and loose.  To expect otherwise is to not understand what it is doing. 

[Tbone55]

Good. Glad you're experimenting.  I think you misunderstood the effects of some of the mods you're considering auditioning, but that's ok. You may stumble across something you didn't know you

Ain't that the truth 😂.

1. Change the fat cap from 22uf to 1uf.
This will change the frequencies boosted by the Fat switch. 

 Since I don't normally use the Fat switch I'll forego this mod.

3. Put a  330pf, 470pf or 680pf across the plate resistor of V2 .
This essentially is the opposite of the bright cap. 

Of course. It's going to bleed away the higher frequencies from the signal going to the coupling cap.

4. Change the by-pass cap on V1-A from 47uf to 2.2uf. This will depend on what happens when I do #1.
I still think the greatest benefit to your amp would be to leave C1 as is and change C2 to 0.001μF which you've yet to try.

I'll take your advice and do this as soon as I can get a .001uf cap, probably to tomorrow.

On reversing PI leads:  this is a big deal from the perspective of the negative feedback loop that we've discussed as an area of potential problems.  The fact that you preferred the tone with NFB disconnected also points to a problem.  If the grid wires are reversed, but the transformer is wired as shown in the diagram you will get positive feedback and oscillation issues.  Sometimes these issues are obvious with squeals and howls--other times it's more sneaky with phase cancellations and parasitic oscillations. I'd recommend wiring it per the diagram, or at least swapping the primaries of the transformer to see if that improves the sound.

I wired up the power tubes per the diagram and the amp was feeding back and I had to shut it off. Should I reverse the OT leads and see what happens? I'm kind of surprised the amp wasn't howling in the first since I had the PI signal leads reversed. Maybe this is the source of the problem?

On Meat control - depending on how it's wired "dimed" could mean stock NFB or next to no NFB.  With no NFB the sound is going to be bassy and loose.  To expect otherwise is to not understand what it is doing.

 Maximum NFB at 12 and minimum at 1. Only the signal strength (amplitude) is affected by the meat pot. The stronger the signal it lets through the more effect on cancelling lower frequencies and boosting highs. The weaker the signal that gets through the less of an effect on cancelling lower frequencies and the bassier the sound. 

I still have to understand more about the signal path for the NFB signal when it returns to the grid of the 2nd half of the PI as well as how the presence section of the PI tube functions. I thought I had tough a time learning how to use CAD a few years back so I could draw up some guitars and build them on a CNC machine. It seems like childs play next to this. 😁

Thanks again. Cheers!

[stratomaster]

 Maximum NFB at 12 and minimum at 1. Only the signal strength (amplitude) is affected by the meat pot. The stronger the signal it lets through the more effect on cancelling lower frequencies and boosting highs. The weaker the signal that gets through the less of an effect on cancelling lower frequencies and the bassier the sound. 

This isn't right, and I'm having a hard time expressing why. There are a number of erroneous assumptions you're making that are implied by your phrasing but not explicitly stated.

So I'll start here. The NFB is just a resistive voltage divider formed by R27+R60 on the upper leg and R26 on the lower leg.  If you don't know what a resistive voltage driver is, stop and go research.  Meat control is just a variable resistor on the upper leg of the divider. Where this control is set determines the ratio of the negative feedback into the phase inverter. 

At minimum pot resistance you have a 75k:4.7k divider for a ratio of 0.059. If you stopped and reviewed voltage dividers earlier then you'll already know how this was calculated.  If you skipped reviewing dividers and are lost on the calculation, go and read up on dividers.

At maximum pot resistance you have a 1M+75k:4.7k divider for a ratio of 0.004.  This is basically zero and should produce a sound similar to disconnecting NFB altogether.

Minimum pot resistance should be at full counter clockwise on the knob if you wired it as in the Hoffman documentation. This is MAXIMUM NFB as the ratio of the divider is highest.

Therefore Maximum pot rotation results in minimum NFB.

The fixed presence network (R25 & C13) is just a low pass filter that shunts highs in the NFB signal to ground and prevents them from reaching the PI input. The effect is more highs through the speaker.  However, this only works when NFB is present. The higher the Meat control, the less the effect of the network since NFB is reduced--to the point where it's doing nothing at higher Meat control settings.

So with the Meat control "dimed" there's is (almost) no NFB.  The presence network is doing nothing, so all frequencies from the preamp pass through the PI to the speaker unaffected by NFB.

This results in a looser sound, rawer highs, and more distortion in the signal. There's also more background noise and could introduce instability.

Minimum Meat control is stock Blues Jr NFB arrangement.  This keeps the amp tight, and the fixed presence network brings in an element brightness to the sound.  So if you want a tighter sound, turn the Meat control down, not up.

[Willabe]

He's mixing up the meat control with the presence control again.

Keeps adding frequency to the meat control function description. 

[stratomaster]

He's mixing up the meat control with the presence control again.

Keeps adding frequency to the meat control function description. 

To be fair, there is an element of frequency in the sense that the fixed presence network works most higher NFB.  Perhaps he understands this but articulates it unclearly?  Or perhaps my description of this is adding to the confusion?

[Willabe]

To be fair, there is an element of frequency in the sense that the fixed presence network works most higher NFB.  Perhaps he understands this but articulates it unclearly?  Or perhaps my description of this is adding to the confusion?

I think it is adding to the mix up.

He needs to see and understand the 2 circuits totaly separately. He see's them together.

The Uncle Doug clip shows them separately. 

[Tbone55]

Thank you stratomaster for that clear and concise explanation of the NFB circuit and presence network. I can't express my appreciation for your patience with me. I did read about voltage dividers, I just didn't clue in that's how the NFB was affecting the return signal.

    In summary, the voltage divider determines the ratio of signal being returned to the grid of the PI, thereby determining the amount of NFB.
Wth the meat pot turned fully clockwise only the 75k and 4.7k make up of the voltage divider network. The amount of signal being fed back then is:

4.7k/(4.7k + 75k) = 0.059 or 5.9%  Maximum NFB
Maximum NFB means more lows are cancelled by the NFB signal and the sound is more trebly.

When the meat control is turned fully counter clockwise so it's at it's maximum value (1meg), the ratio becomes :

4.7k/(1meg + 75k)= 0.004 or .4%.  Minimum NFB
Minimum NFB means fewer lows are cancelled by the NFB  signal and the sound is more bassy.

     My meat pot is wired per the diagram but is acting the reverse of your response. When turned fully clockwise (12) I get maximum NFB and the pot acts like when I disconnect the NFB line. When turned fully counter clockwise (1) I get minimum NFB the bass becomes more pronounced and flabby.

     The right outer terminal of the meat pot is connected to the speaker + side of the speaker output jack. With the meat pot turned fully clockwise the signal sees R27/75k and R26/4.7k that makes up a voltage divider circuit. This provides max NFB signal.
       With meat pot turned fully counter clockwise the signal now sees the full 1meg resistance of the meat pot + 75k NFB resistor plus + 4.7k voltage divider circuit and the signal is greatly reduced by the time it passes through the 1meg + 75k and is at the R26/4.7k resistor point. This signal is almost neglibile and will barely have any affect on reducing low frequencies and the sound will be a lot bassier.

I'm hoping I've got this right now and just want to confirm with you before discussing the presence circuit. I've attached images of the information about voltage dividers I read out of the book I got.

A sincere thanks for your help. By the way,  I bought some .001uf caps to try in place of C2 and some 220pf and 470pf caps to try on the volume pots as bright caps. Both places I went to didn't have 330pf caps but I might have some values I can put in parallel to get close.

[Tbone55]

Willabe and stratomaster,

   I appreciate both of your inputs. I was getting frequency and signal mixed up. When I think of the signal coming back from the speaker I see it in terms of all the frequencies coming back and the strength/amplitude of the frequencies. I didn't see how each of them were being handled differently between the NFB and the presence section but contributing to the overall NFB circuit function.

The circuit in Uncle Doug's video is different than the Hoffman but are doing the same thing. After reviewing Uncle Doug's video again I understand how that circuit works now and with stratomasters explanation I have a much better understanding of how the Hoffman circuit works (well almost).

Part of my problem is determining how the signal is travelling within the circuits. Especially in this part of the circuit with the long term pair, presence and NFB. I'm probably guilty of over complicating things and making it harder on my self. I'm an overthinker. 😁

I hope I'm headed in the right direction.

[Tbone55]

Sorry, I forgot to add the images from my amp book about voltage dividers.

[Tbone55]

Hello stratomaster,

I've been trying to understand better how the volume pot functions and went back and looked at the circuit example you drew out. I've drawn out 3 scenarios of the circuit, pot full off, pot part way up, pot at max that I'd like you to look at and tell me if I've got them right. I think it's easier for you to see if I'm grasping the concept rather for me to try and explain in words what I think is happening. A picture is worth a thousand words is it not?

I've been spending a lot more time reading and researching and haven't tried any of the mods yet. The Robinette article on amp voicing is deep. Most of it is way over my head but he sure has some different takes on shaping the tone.

 I didn't want to try anything with reversing the output leads of the PI to the output tubes or switching around the OT B+ leads going to the power tubes until I heard back from you. I'm baffled as to why the amp functions properly with the PI output leads going to the wrong tubes but the OT B+ leads hooked up correctly. As I said, reversing the PI output leads to the PT's caused the amp to howl.

Thanks and Cheers!

[stratomaster]

Your understanding of the pot is exactly backwards.  This is why I'm having a hard time taking your word for it when you say the Meat control gives a tighter sound when dimed.

Just look at your last sketch for example: The wiper is grounded.  You even sketched it this way. Where would the signal some from if the wiper is at 0v?

Resistors elevate voltage above ground.  In a voltage divider the further from ground you are the closer to the original signal level you'll get out.

I think you'll see this if you draw an arrow on your sketches showing the direction of the signal path.

Re explanation of your part way on scenario: the signal "sees" all of the resistance simultaneously, not stepwise.  So the input sees 785.7k to ground in this scenario.  This is the principle behind a Thevenin equivalent resistance.  It's also why the wiper to ground resistor affects the taper.  It is always in parallel with the lower leg of the voltage divider. Total resistance to ground as seen by the input signal (Thevenin eq resistance) is less important than the divider ratio in practice.

Also your voltage divider ratio isn't just determined by the pot, but also the tapering resistor in parallel with the wiper to pot resistance.

Redraw this as two series resistors with Rp1 as the top leg and a new R for the bottom leg.  This bottom resistance will be the parallel sum of 120k and 300k, so ~ 85.7k.

I'm going to say this in the most constructive way I know how: your understanding of electronics is profoundly incomplete.  I think it's best if you start at the beginning with a Fundamentals of Electronics type of text or course before diving into the tube amp specific texts.  Otherwise you'll continue to misunderstand and will be dealing with expansions of topics that rely on a strong grasp of fundamentals.

I'm not insulting your intelligence or ability. I'm just making an observation that you dove in the deep end without learning to swim first.

[Tbone55]

I appreciate your honest assessment.  No offence taken.

I'd appreciate you reviewing the revised volume circuits I've made based on different pot settings. In my mind I understood what was happening but I just didn't draw it correctly.

I'd also appreciate your feedback on my previous post about the NFB and presence circuit for my own peace of mind and learning. I think I've got a much better grasp of them bit would like some confirmation.

The one last thing I would appreciate your help with is the issue with the PI output that's reversed to the power tubes. The OT is wired as per the schematic, blue to V4, brown to V5. I tried to reverse the PI output leads to V4 and V5 but the amp just howled.

I'll understand if you don't want to continue providing me help. I'll just try a few simple mods that I haven't tried before and see how it goes.

Thanks for all your advice and patience.

[stratomaster]

See the attachment. Your understanding is still backwards but somehow also muddled.

It's not that I don't want to help you (see the last 3 pages of posts with suggestions and detailed explanations) it's that the help I can provide is most useful with a strong grasp of the fundamentals.  Until you put in the time to learn these things well enough to catch your mistakes on the basic building blocks of analog circuits we'll continue to talk past each other.

You need to be able to look at your diagram and intuit things like 'the max and min pot diagrams couldn't possibly be identical'. By not catching that you went on to try and solve an impossible voltage divider and further clouded your understanding in the process.  Once you get the fundamentals down come back and reread this thread.  I assure you it will make more sense.

As far as the PI.  The correct orientation due OT primaries is the one that doesn't howl.  There's a 50/50 chance of getting it right with the fix being to just swap the wires if it's wrong.  Seems like my guess was wrong that you had some phase cancellation occurring that was reducing high frequency response.

[Tbone55]

Hi stratomaster,

I will make one last effort to redeem myself. I've gone back to basics with just a simple volume pot connection for a guitar and have attached images of the sketches I've made.

With respect to the PI issue, while it's been some time since I did the conversion of my Blues Jr (2019), I do remember that I originally had the PI outputs connected as per the schematic/layout and the amp howled. Someone on the forum told me to reverse the PI output leads and tte howl would go away, and it did. I'll probably reverse the OT leads and the PI ouput leads so the layout is correct.

While it seems I don't have a grasp of the fundamentals, for myself, I feel that I have learned quite a lot. Before this I had no idea what a NFB circuit was, what a by-pass cap does, what load lines were and why they're important, what a long tailed pair phase inverter was or how the presence circuit worked and so much more. Most of this is thanks to you.

Cheers!

[Brownie]

Did you consider using that unused triode on V2 for a cathode follower?

[stratomaster]

Did you consider using that unused triode on V2 for a cathode follower?

There is no unused triode on the Hoffman version

[Tbone55]

I've  tried several mods on the amp.

1. Replace C2 with a .001uf cap.
- provided a slight increase of highs

2. Lifted C9 from ground
- realy increased the highs, maybe too much
- Meat control has more useable NFB range than before. I only have to keep at just over the halfway point.

3. Put a 330k resistor on imput of volume pot.
- volume doesn't ramp up as quickly as before 

4. Replaced R19 Master Vol pot with a 100k audio taper pot and R18 with a 330k resistor.
- master volume now has a much more gradual increase than before.

This is probably a dumb question but why would you use C9 to bleed off so much of the high frequency to ground?

Thanks.

[stratomaster]

I've  tried several mods on the amp.

1. Replace C2 with a .001uf cap.
- provided a slight increase of highs
It did no such thing. It limited low frequency content (as has been discussed several times before)

2. Lifted C9 from ground
- realy increased the highs, maybe too much
- Meat control has more useable NFB range than before. I only have to keep at just over the halfway point.
Yes, when I first mentioned this mod I said it was a defeating an aggressive low pass filter. That's why I was surprised at your reports that you needed more highs than this. It's also why I recommended a plate bypass cap to coincide with it.

3. Put a 330k resistor on imput of volume pot.
- volume doesn't ramp up as quickly as before 
Good. This is exactly the intended effect.  You are losing a good bit of gain at max volume with that value. Not a problem if you like it. Just be aware.

4. Replaced R19 Master Vol pot with a 100k audio taper pot and R18 with a 330k resistor.
- master volume now has a much more gradual increase than before.
Yes the linear stock pot is a poor choice for this application.  The 100k buys you a bit more signal at the upper end, but nothing earth shattering.

This is probably a dumb question but why would you use C9 to bleed off so much of the high frequency to ground?
I think it's a carryover from the Fender design.  Some versions of the Blues Jr have both a bright cap at the volume pot and this cap to ground. I think they were intended to balance each other out. The Hoffman version deleted the bright cap, but kept this cap.  A popular Blues Jr mod was to clip the bright cap and leave the cap to ground in place.  This could be why Hoffman chose to keep this cap.

Thanks.

[Tbone55]

Thanks stratomaster.

1 What I meant about the C2 cap was that I'm hearing more highs because it's limiting more lower frequencies. My bad for phrasing it the way I did.

2. I now understand why you recommended a plate by-pass cap to go with removing the C9 cap. I had reading about this recently. It'll tame some of the highs. I'll check your post to see what cap value you recommended.

3. I'm not seeing a huge drop in max volume with the 330k resistor. I'm having to raise the volume/gain a bit more for breakup but I prefer having more room at the front end to use my pedals.

Thank you kindly for your other reply and for answering my question about the C9 cap. I've been spending a lot of time reading about basic electronics to get my understanding and knowledge level up. It'll require me reading the material over quite a few times.

In the meantime I'll put that by-pass cap around the plate resistor and see how it goes.

Cheers!

[Tbone55]

I didn't havea 680pf cap to bypass the plate resistor so I put a 220pf and 470pf cap in parallel to give me 690uf. Close enough for rock'n'roll. 😁

It did as advertised and removed some of the highs. The Meat control still has a good working range and it really helps especially when switching between humbuckers and single coils.  I may play around with some cap values for the treble and mid controls. Overall, the amp is sounding much better.

I'm not sure if there's anything else I should try at this point but I'd be happy to hear your thoughts.

Thanks.

[stratomaster]

You can try just the 220 or 470pF.  With the parallel triode it would make sense to use lower values across that plate resistor. My original recommendation was for a single triode.

If the amp is doing what you like then there's no need for tweaks unless you just want to experiment and learn.

[Tbone55]

Thanks stratomaster. I'll give those cap values a try on their own. I'd certainly be willing to experiment some more with your guidance and I certainly want learn some more. Maybe something with the presence section? Whatever you think you would be the most logical thing to try next. I'm game. 😁

Would it make any difference at this point if I changed out C8/.0022uf for the C9/.001uf I removed?

Cheers!

Cheers!

[stratomaster]

Everything makes a difference. Either try it and see what difference it makes or look into passive filters in your electronics studies. Or both.

These are questions you can answer for yourself. You don't need me for that.

[JB]

One of the biggest improvements you can make on a Blues Junior is a new cabinet. With the exception maybe of the Mk4 they have a tendency to sound boxy. You’re fighting against that inherent trait with all the mods. Plug it into a different cabinet and see what you think. Different speakers help too but the cabinet is the main culprit holding back what the electronics can do.

[Tbone55]

One of the biggest improvements you can make on a Blues Junior is a new cabinet. With the exception maybe of the Mk4 they have a tendency to sound boxy. You’re fighting against that inherent trait with all the mods. Plug it into a different cabinet and see what you think. Different speakers help too but the cabinet is the main culprit holding back what the electronics can do.

It's definitely worth a try. Thanks.

[Tbone55]

This is the final tally of the mods  I wound up doing if anyone's interested.

1. Changed C1 by-pass cap from 47uf to  22uf.
2. Changed fat cap from 22uf to 2.2uf.
3. Removed low pass filter before master volume. (R18 & C9)
4. Replaced master volume with 1M audio pot
5. Put a 330k resistor on the input terminal of the volume/gain pot, R5.
6. Replaced R17 cathode resistor with 10k resistor cold clipper.
7. Replaced V1 with a 12YA7. Waiting to try a 5751.
8. Replaced V3 with a 12AT7.
9. Replaced B+ 47uf cap with a 100uf cap.
10. Replaced screen node 22uf cap with 47uf cap.

Thanks to all that chimed in, particularly stratomaster. Cheers!

[stratomaster]

This is the final tally of the mods  I wound up doing if anyone's interested.

1. Changed C1 by-pass cap from 47uf to  22uf.
This does nothing in the guitar range.  The way to tighten the signal from this stage is putting a 0.001 in C2 as I tried to advise numerous times.
2. Changed fat cap from 22uf to 2.2uf.
I like this change. 1 to 2.2μF are great cathode values for boost functions.
3. Removed low pass filter before master volume. (R18 & C9)
4. Replaced master volume with 1M audio pot
This drastically alters the voltage divider in the design, and can slam the PI input provided the preamp is putting out adequate gain. I'd recommend going back to your 100k log pot with a 330k or 390k at the input.
5. Put a 330k resistor on the input terminal of the volume/gain pot, R5.
We've already discussed the significant cut in gain from this value.
6. Replaced R17 cathode resistor with 10k resistor cold clipper.
This is a parallel triode. That 10k is meant for a single triode. You'll need a 5.1k resistor there to get the intended effect.
Also it will cut a lot of gain and is meant for adding asymmetric clipping to preamp distortion.  If you're after clean then it's counterproductive and can sound thin.
7. Replaced V1 with a 12YA7. Waiting to try a 5751.
Neither of these are good choices for taking advantage of the cold clipper. If you're after clean, then keep these tubes and revert the 3rd stage to stock
8. Replaced V3 with a 12AT7.
9. Replaced B+ 47uf cap with a 100uf cap.
10. Replaced screen node 22uf cap with 47uf cap.

Thanks to all that chimed in, particularly stratomaster. Cheers!

[Tbone55]

Thank You Kindly Stratomaster.

1. I did put a .001uf cap in as you suggested but I forgot to mention it. The 22uf cap is a BillM mod. The frequency response
    calculator shows a small db drop across the frequencies. I thought it would help tame the front end a bit.

2. The fat cap change to 2.2uf works well as a boost for a lead. Helps cut out some of the bottom end. Another BillM mod.

3&4. Changing the master volume pot is entirely on me (as is everything else  ). I did it based on the some of the
        Fender schematics I saw and some stuff I read. Of course I had no idea of the consequences. Should have asked here before
        trying it. will correct that.

5. Yes, we did discuss the gain drop of putting a 330k resistor on the volume pot. I honestly can't hear any significant gain drop
    but will try a 220k anyway.

6. Really screwed up with the cold clipper not thinking about it being a parallel triode that would affect the value of the cathode
    resistor. I'm pissed at myself because I'm pretty sure I was aware that I had to undo half of the triode in order to use the 10k
    resistor. I know I don't have a 5.1k resistor so maybe I will put another 10k resistor in parallel. Apparently this was another
    BillM mod. I'd like to hear how it sounds when it's set up properly because it doesn't sound good right now.

7. I was experimenting with the 12AY7 in V1 but of course had no idea that it would affect how the cold clipper would work. That
    and the fact that I had the wrong resistor for the cold clipper really made things worse. Will be putting the 12AX7 back in to try
    with the cold clipper. If it's a bit too much I'll revert back to the original 1.5k resistor and try with the 12AY7.

I've tried to order the Blencowe books but Amazon US won't ship one of them to Canada for some reason. Amazon CA does carry them. Can't understand why they would ship one of the books no problem but not the other one. Ordered them from a different source but will be some time before they get here.

Thanks again for your advice. Cheers!

[FrankAmps]

Wow, lot's of info here. Thanks all. I have a new to me 2010 cream board BJ and want to mod it. I have done the BillM mods to many BJs and will do to this one too, but the Fat cap reduction to 2.2uF and 3rd stage coupling cap change to .022uF interest me as well. I found a IV version schematic and it appears Fender has made these mods to the amp as well. Interesting. They made it a cathode bias amp surprisingly, a radical change. The IV is being applauded so maybe it will be an amp that is acceptable  right out of the box. A pine cabinet will hopefully finish my project, if they ever go on sale.

[uki]

Here a better link for the same doc on reply #11

http://ax84.rru.com/media/ax84_m35.pdf

great thread btw!