Blues Jr Mods

[stratomaster]

I'm starting to suspect there is a problem with the negative feedback in your build, namely the "Meat" control.  Your amp should be almost painfully bright after removing C9.

Would you mind removing the connection at the speaker jack to the negative feedback circuit and seeing what effect this has on your sound?  I'd expect the amp to get noisier, louder, and dirtier.  If it also gets substantially brighter, then my suspicions about the NFB circuit are validated.

[Tbone55]

That's fantastic stratomaster!

Thank you for sharing your knowledge and willingness to entertain what are probably some dumb questions. I honestly didnt think I'd get this deep into this stuff but the more I learn the more I want to learn. It's pretty obvious from the tone stack curves you plotted that I don't want to use the 0.1uf cap for the bass pot. Plotting these curves makes it so much easier to understand the effects of different tone caps and slope resistor values. The knock against the Blues Jr has been that it's boxy sounding because it was very mid rangey.

I'm going to try the 0.047uf cap for the mid pot and the 0.47uf cap for the by-pass mod first as I don't have a 450pf cap right now and see how it goes.  I'll get back to you with my observations.

Cheers!

[Tbone55]

I'm starting to suspect there is a problem with the negative feedback in your build, namely the "Meat" control.  Your amp should be almost painfully bright after removing C9.

Would you mind removing the connection at the speaker jack to the negative feedback circuit and seeing what effect this has on your sound?  I'd expect the amp to get noisier, louder, and dirtier.  If it also gets substantially brighter, then my suspicions about the NFB circuit are validated.

No problem. I'll do that before I do anything else and get back to you.

Thanks.

[Tbone55]

I disconnected the meat pot feedback line from the speaker input jack and am getting no sound from the guitar at all. There's sound from the amp when I turn both the volume and master volume up full that's a bit noisy but not that bad.

I re-checked my connections going to the speaker jack from the OT and they're fine. My feedback wire was connected to the positive terminal of the output jack where the green wire from the OT was connected to. I can't see why there'd be no sound just because I disconnected the NFB from the output jack. I'm puzzled.

[stratomaster]

If you're getting amp noise then something before the volume control is the problem and it likely has nothing to do with cutting the NFB line.

Could be internal to the amp, could be input jack, could be the cable, guitar, etc.

Try plugging in directly. No guitar. Then touch the tip of the cable plug with your thumb. If you get buzzy thumps then suspect your guitar wiring.  Which may have been at the root of your high end problems.

[Tbone55]

Plugged a cable directly into the amp with no guitar. Touched the other end of the cable with my finger and no sound. It's acting exactly the same as when I had my guitar plugged in.

The amp was working perfectly fine the last time I turned it on. Should I try re-onnecting the NFB line back to output jack?

[stratomaster]

There is no failure mode where removing negative feedback causes a problem with signal entering the amp before the first volume control. You said you could hear noise from the amp that corresponds to turning the knobs. This rules out a speaker jack issue.

Try different cables, check the solder joints on the input jack. Make sure the input jack is tight.  Check if you accidentally bumped a wire, left a piece of solder or a lead or an alligator clip, desoldering braid (guilty myself of the last two).  Make sure the loose wire from disconnecting the NFB is tucked away from making contact anywhere or taped off

Push on the jack tip itself with the amp on. You should be able to hear it short/unshort. Use a non conductive probe and be careful with the live amp (all the usual precautions).

[Tbone55]

Problem found.

V1 was not functioning. Somehow, when I swapped tubes, Pin 1 got bent so I had no B+ voltage going to the plate. No wonder there wasn't any sound. I straightened the pin out, put the tube back in and sound was back.

The amps has a bit more noise but sounds much better. The notes have more clarity and the amp sounds even more alive and responsive. I wouldn't say it sounds shrill at all, just clearer. I have to play around with  it a bit more but I like what I'm hearing. Is it the NFB then that's causing a problem? I don't know what it's supposed to do exactly as I haven't read up on it yet.

[stratomaster]

Problem found.

V1 was not functioning. Somehow, when I swapped tubes, Pin 1 got bent so I had no B+ voltage going to the plate. No wonder there wasn't any sound. I straightened the pin out, put the tube back in and sound was back.

The amps has a bit more noise but sounds much better. The notes have more clarity and the amp sounds even more alive and responsive. I wouldn't say it sounds shrill at all, just clearer. I have to play around with  it a bit more but I like what I'm hearing. Is it the NFB then that's causing a problem? I don't know what it's supposed to do exactly as I haven't read up on it yet.

The negative feedback is an out of phase lower amplitude signal that is applied at the 2nd input of the phase inverter.  It is meant to add stability and reduce distortion and noise in the amp.  It works by mixing this mirror (out of phase) signal with the signal from the preamp applied to the phase inverter--essentially subtracting from the signal to prevent it from getting too out of hand.

With the NFB disconnected you're hearing the raw sound of the amp.  In amps not originally designed to forgo NFB this can result in objectionable noise levels and instability at higher volumes. But we can usually get away with it in my experience.

In theory with the "Meat" control set to max the negative feedback is essentially defeated and the response should be nearly identical to disconnecting the feedback.  I have a suspicion that there's a wiring or component value error in your amp OR the fixed presence filter network (7.5k and 0.1μF) is insufficient for your needs, OR perhaps both.

To test this theory you can reconnect the NFB and max out the Meat control. This should sound nearly identical to what you hear with the NFB disconnected. As you reduce the Meat control, the effect of the NFB will become more apparent AND the fixed presence network will be more effective at affecting the high end. 

I'd recommend playing around with this control and examining the components in the NFB loop and PI tail to make sure there isn't a value error.  Particularly the 7.5kΩ resistor. If this is accidentally say a 75kΩ then the amp could feel muted in the high end. One of the Bill M mods was to replace this resistor with a 10k to 25k pot for more control over the Presence.  This is an option for you as well. 

[Tbone55]

Thanks stratomaster.

I'll re-connect the NFB and do as you've suggested with the meat control as well as measuring the component values. I kind of understand your explanation of how the NFB works. It's basically feeding back part of the output signal to the PI cancelling out some of the original signal to keep it from getting out of hand and reducing noise. At least that's how I understand it in the simplest terms.

Will report back with my results as soon as I can. Cheers!

[Tbone55]

Hello stratomaster.

With the NFB re-connected, when I turn the Meat control to it's minimum position (clockwise) the amp sounds pretty well the same as it did with the NFB disconnected. When I turn the Meat control towards its maximum position (counter clockwise) the highs start to go away and the amp becomes a lot more muted sounding. Not good.

I've checked the values of the components you suggested to and they all measured correctly. So right now only the C9/0.001uf cap is disconnected from the circuit and everything else is per the schematic. I'm curious as to why the amp doesn't sound as shrill as you expected with this cap removed. It's actually sounding pretty good as is.

What say yee?

[stratomaster]

Could be a matter of preference: ie what's shrill to me is bright and clear to you.

Could be the speaker you're using is muted in the high end.

Could be a value error somewhere in the build causing additional unintended high frequency roll off.

Could be a build error bleeding off high end.

Without seeing and hearing it, I couldn't tell you for sure. 

My guess was that there was a wiring or component value error in the fixed presence network that was muting your high end.  If there is no error, you could just not like what that filter is doing.  With the meat control set for minimum feedback the effect of that network is essentially zero.  But as you add in the feedback, you're also increasing the signal into that presence filter. 

With negative feedback the effect on tone is the opposite of the effect on the feedback signal. What I mean by that is if you filter the highs out of the NFB signal, then there are fewer highs being subtracted from the overall tone.  The 7.5k resistor is decreasing the effect of the 0.1μF cap it is in series with.  That cap is shunting high frequency NFB content to ground.  If you decrease the resistor, then you should be shunting more high frequency NFB signal and therefore get brighter output at the speaker. 

You can give this a try.  Put a resistor in parallel with the 7.5k.  Start with something between 2.2k and 10k--whatever you have on hand is fine.  Hopefully this gets you closer to the high end you're looking for at higher NFB (meat) settings.

Note: This should not affect your tone at the minimum feedback setting on your Meat control because of reasons discussed earlier.  So you won't lose that tone that you're liking.  We're just trying now to get the amp to be more usable over the entire range of that control.

[Tbone55]

Once again thanks stratomaster.

Tone is very subjective so yes, maybe what's shrill to you isn't to me. I am liking the sound I'm getting right now compared to how it sounded before but I certainly don't mind trying a few more things and seeing where that takes me so I'll try putting another resistor in parallel with the  7.5k one and see how that sounds.

I didn't mention in my earlier post that the volume control has a gradual increase in it and not that sudden jump I was getting between 2 and 3 previously but now with all 12AX7'S in the amp the volume is higher at lower volume levels and breakup is sooner. I might try the 12AY7 in V2 again just to see.

I have a Celestion Vintage 30 in my amp that might be contributing to that more muted high end I was experiencing. I had even installed a 0.068ufcap in my guitar to try and coax some highs out. Another factor may be the pickups in my guitar. My neck PU is around 10k and the neck around 12k. The hotter pickups get the more compressed the sound with less highs. Maybe a combination of things between the amp and the guitar contributed to what was going on.

I'll let you know how it goes with that resistor. Cheers!

[stratomaster]

I find Vintage 30s spikey in the high mids and treble, so I don't think that's it.  It could be the overwound pickups.  For that you want to go higher in resistance on the pots, not lower in value of the tone pot. You're attempting to affect the resonant behavior of an RLC circuit, so R gives you the biggest bang for the buck. Plus the tone cap only really comes into play as you roll the pot down.

I think your best bet is to put another 100k in parallel with the 3rd stage plate and stick to the 12AX7, but if you're getting where you want to go with the 12AY7 then have at it.  Ultimately it's your amp. I just want to help you get where you want to go with it.

[Tbone55]

Thanks kindly stratomaster. I haven't tried anything yet as I've been busy with some guests from out of town. I will try putting another 100k resistor across the plate of V2 to cut down the gain of V2 and see how that goes. Is there a tonal difference I might notice?

I believe my tone pot is 500k but I'll have to check. I use a push pull pot so I can split my humbuckers. Unfortunately there's no way to use two different capacitor values for humbucking and single coil without getting into drilling holes and using toggle switches.

Should I still try putting the 0.47uf cap across the cathode of V2 as a by-pass cap to tame a bit of the low end? It's not a big deal to do and it would be interesting to see the results. Should I try hooking up that 0.001uf cap that I currently have disconnected? Am I wrong but do smaller cap values block high frequencies and let lower frequencies pass? I was under the impression that the 0.001uf cap would allow lower frequencies to pass to ground but obviously that's not the case if lifting it gives you more highs. I seem to have the wrong impression.

Thanks again. Cheers! 

[stratomaster]

The only mods I've recommended that will reduce lows are replacing C2 with 0.001μF and increasing the mids cap to 0.047μF.

The 0.47μF cathode bypass cap will actively boost upper mids and highs. It will not reduce the low end content you currently have.

Adding the 0.001μF back in will be counterproductive.  It is forming an aggressive passive low pass filter with the 430k resistor that precedes it. I'm not sure why you'd want to put that in.  The cap is passing high frequencies--but to ground and out of the signal path.

As far as the tonal changes with reducing the 3rd stage plate resistor you'll notice less compression, less gain, and more perceived headroom.

[Tbone55]

Thanks for clarifying stratomaster.

I see how the 0.001uf cap forms a low pass filter with the 430k resistor I just didn't understand how it worked. The high pass low pass filter thing sometimes confuses me but your explanation has made it clear why I wouldn't want to re-connect it. Is the 430k resistor of any consequence because it still remains in the circuit?

The other mods you mentioned for increasing the highs are still doable and even putting the 0.47uf by-pass cap is on the table. Maybe boosting the mid-highs and highs will help. They're easy mods to do and interesting to see what happens.

What kind of speaker do you use in your amp? I can't say I've been impressed with my Vintage 30 and had been contemplating a speaker change but when I started looking I just became more confused. Any thoughts on what might work better with this amp?

Cheers!

[stratomaster]

Is the 430k resistor of any consequence because it still remains in the circuit?
Yes, the 430k forms the upper portion of a voltage divider along with the input lug to wiper of the Master volume pot.
The wiper to ground is the lower leg.  You can modify this resistor to change the maximum amount of signal at the input of the PI.  If you go back to my original mod list I think I mentioned this.  There's a practical lower limit on the resistor though and you'll not want to drop this too much, especially if you're trying to stay clean and less loud.

The other mods you mentioned for increasing the highs are still doable and even putting the 0.47uf by-pass cap is on the table. Maybe boosting the mid-highs and highs will help. They're easy mods to do and interesting to see what happens.
Yes, you should definitely tweak and experiment--especially if you aren't quite there with the amp. It's the best way to learn.

What kind of speaker do you use in your amp?
I haven't had a Blues Jr in over a decade, but when I did I put a Cannabis Rex in it.  Remember I like warmer tones. After my mods I've heard a Texas Heat sound amazing in a customer amp.

I can't say I've been impressed with my Vintage 30 and had been contemplating a speaker change but when I started looking I just became more confused. Any thoughts on what might work better with this amp?
Vintage 30s have a strong following.  I don't particularly like them, but they definitely do the bright and tight tones well.
If I were to recommend something similar but a bit more to my liking I'd pick the G12-H30, but speakers are very subjective and there are almost as many options as there are opinions on each option. I'm going to have to punt on a direct recommendation for your needs knowing that our preferences are so different.

[Tbone55]

Thanks stratomaster.

I appreciate the feedback and candor. You're absolutely right about speaker choice. It really is subjective and dependent on the amp and sound you're trying to achieve so I can't blame you for not wanting to make a suggestion. I'll try the few remaining mods in my amp and see how that goes.

I do have a question for you. What are your thoughts on using a 12AT7 in the PI position? The amp is really loud even with master volume turned way down. Would this help reduce the loudness and would I have to change some of the values of the components of V3? I know you recommended me adding another plate resistor in parallel on V2 to reduce the gain. Would this be enough to have the same effect?

Thanks again. Cheers!

[stratomaster]

The real fix is to replace the 50k linear Master Volume pot with an audio taper. 

A reduced plate resistor on V2 will reduce gain. A 12AY7 in V2 will reduce gain. Separating the parallel triodes will reduce gain.  All of these things will result in a quieter amp, but I think the taper of the MV pot will continue to be an issue even with these measures.

I'll have to draw some load lines, but I think you're ok 1 for 1 swapping a 12AT7 into the PI with this design.  It's an easy thing to try and see if you like--especially if you already have the alternate tubes in your possession.

[Tbone55]

Hey stratomaster,

I replaced V2 with a 12AY7 and the there's still plenty of volume available even with the volume control set low. The only time the gain really dropped was when I had the 12AX7 in V1. With the 12AY7 in the V2 position I don't feel like I lost too much gain.

The amp sounds pretty good to me the way it is now so I may not do any of the other mods discussed for now. I would like to try the 12AT7 in the PI position but will wait to hear back from you first. I've read about the load lines for different tubes but haven't tried actually trying to draw one.

Thanks.

[Tbone55]

I think you're right about using a 50k audio pot for the master volume. Been looking for one and not having any luck. It seems the audio taper ones from Apha jump from 25k to 100k. I'll have to see if there's one from CTS I can source.

[stratomaster]

Use a 100k pot and reduce the resistor. A 390k is the closest standard value.

As far as a 12AT7 in the PI, go ahead and try it.  You'll get a slight drop in voltage gain which may not be enough to really notice. You can play games with the plate resistors and bias resistor to try and eek out another dB or two, or better balance the outputs--but that's gilding the lily stuff you can tweak once you're in the ballpark of what you want.

[Tbone55]

Thanks stratomaster. I can get a 100k audio pot no problem and may already have a 390k resistor. I'll give the 12AT7  a try and what happens.

I was trying to understand more about the long tailed pair and the NFB portion of the circuit in the amp. I've read about what the long tailed pair does in the PI and how the NFB works but I'm having difficulty understanding this combo circuit in this amp. Could you give me a short description of how this section of the amp works? Everything I've looked at is completely different than this amp.

Thanks and Cheers!

[Willabe]

I was trying to understand more about the long tailed pair and the NFB portion of the circuit in the amp. I've read about what the long tailed pair does in the PI and how the NFB works but I'm having difficulty understanding this combo circuit in this amp. Could you give me a short description of how this section of the amp works? Everything I've looked at is completely different than this amp.

Looks like a normal long tail pair PI. Doug's has an added meat control.

Print out a copy of the Blues Jr. and Doug's Blues Jr. and compare the PI's/NFB loop/meat control. Look for any extra parts.

Link for Rob Robinette amp site, resonance/meat control;

https://robrobinette.com/Voicing_an_Amp.htm#Resonance_Control

[stratomaster]

Randall Aiken at Aiken Amps has an excellent technical write up on the long tail pair. Just example circuit is nearly identical to the Hoffman BJr. https://www.aikenamps.com/index.php/the-long-tail-pair

[Tbone55]

Thanks for the references stratomaster.

Cheers!

[stratomaster]

Link for Rob Robinette amp site, resonance/meat control;

https://robrobinette.com/Voicing_an_Amp.htm#Resonance_Control

I'm pretty sure the Meat control isn't a Resonance control.  The latter is just an adjustable passive high pass filter similar to what you'd find in a G&L guitar. The Meat control as implemented here looks like a basic pot as variable resistor in series with the 75k feedback resistor.  It's just changing the ratio set by the feedback resistor and the 4.7kΩ tail.  The 22μF is there to keep DC off the pot--and is intentionally made very large to not affect the guitar frequency range at all (this could easily be a 1μF cap and function exactly the same).  There's a DC offset caused by the same 4.7kΩ resistor that would otherwise make the Meat control noisy when adjusting.  One could easily leave that cap out and just deal with a bit of rustling when turning that pot, as I often do when adding an NFB pot to an amp.

[Willabe]

Link for Rob Robinette amp site, resonance/meat control;

https://robrobinette.com/Voicing_an_Amp.htm#Resonance_Control

I'm pretty sure the Meat control isn't a Resonance control.  The latter is just an adjustable passive high pass filter similar to what you'd find in a G&L guitar. The Meat control as implemented here looks like a basic pot as variable resistor in series with the 75k feedback resistor.  It's just changing the ratio set by the feedback resistor and the 4.7kΩ tail.  The 22μF is there to keep DC off the pot--and is intentionally made very large to not affect the guitar frequency range at all (this could easily be a 1μF cap and function exactly the same).  There's a DC offset caused by the same 4.7kΩ resistor that would otherwise make the Meat control noisy when adjusting.  One could easily leave that cap out and just deal with a bit of rustling when turning that pot, as I often do when adding an NFB pot to an amp.

Ok, when I 1st looked at Doug's schematic, thought that's what it did. So I had it right the 1st time, but took it down when I did a search and found this;

https://el34world.com/Forum/index.php?topic=24439.msg262696#msg262696

Then I saw meat aka 'resonance' and did a search for that and found the Robinette post on his site.

So, it looks like it's the cap values and using 1 cap in series or 2 caps at different uF values with the series -FB pot you get a meat or resonance control.

And you can use both a presence/highs control and resonance/lows control at the same time/in the same -FB loop.   

[stratomaster]

It's simpler than that. A single cap with signal though it will act as a high pass filter much like a coupling cap (value dependent).  A resistor in parallel with that cap gives you the ability to bypass the effect of the cap by giving low frequencies a lower impedance path around the cap.  The resistor attenuates those lower frequencies though--just less so than the cap.  If you make this resistor a pot instead then you can adjust the effect of this high pass filter all the way down to nothing (shorting the cap).

Since it's in the NFB circuit the cut in lows results in an increase in lows to the speaker.

[Tbone55]

Thanks for your input guys.

 You're way ahead of me in understanding what's going on in the Hoffman circuit. Am I understanding this correctly that the Hoffman "Meat" control is acting like both a Presence and Resonance control then? The 22uf cap allows a broad range of frequencies to enter from the speaker but then the individual capacitors split the frequencies up that are fed back to the grid?

After reading the Robinnete link I understand the Presence control is a high pass filter and the Resonance control is a low pass filter and each one blocks out respective frequencies so they don't get cancelled when fed back, resulting in either more lows (resonance control) or more highs (presence control) at the speaker.

I'm sure I've probably got this all wrong. 😁

[stratomaster]

Am I understanding this correctly that the Hoffman "Meat" control is acting like both a Presence and Resonance control then?
No, I don't think it is a Resonance. The capacitor is large enough to be considered a straight wire from an AC perspective.

After reading the Robinnete link I understand the Presence control is a high pass filter and the Resonance control is a low pass filter and each one blocks out respective frequencies so they don't get cancelled when fed back, resulting in either more lows (resonance control) or more highs (presence control) at the speaker.
The opposite. Resonance cuts lows (high pass) in the NFB signal. Presence cuts highs (low pass).

[Willabe]

After reading the Robinnete link I understand the Presence control is a high pass filter and the Resonance control is a low pass filter and each one blocks out respective frequencies so they don't get cancelled when fed back, resulting in either more lows (resonance control) or more highs (presence control) at the speaker.

The opposite. Resonance cuts lows (high pass) in the NFB signal. Presence cuts highs (low pass).

He didn't say in the NFB loop, he said at the speaker, so he's right.

[Willabe]

Am I understanding this correctly that the Hoffman "Meat" control is acting like both a Presence and Resonance control then?

No. I think strtomaster is right, the meat control just kills off the NFB signal going to the presence control.

The 22uf cap allows a broad range of frequencies to enter from the speaker but then the individual capacitors split the frequencies up that are fed back to the grid?

There's 2 different value caps around the pot in the resonance circuit.

After reading the Robinnete link I understand the Presence control is a high pass filter and the Resonance control is a low pass filter.....

No, backwards, presence control is a low pass and the resonance control is high pass.

....and each one blocks out respective frequencies so they don't get cancelled when fed back, resulting in either more lows (resonance control) or more highs (presence control) at the speaker.

Yes.

[Tbone55]

Well I knew I had something wrong. 😂 Thanks for straightening me out.

So the meat pot is controlling the amount of NFB by passing or blocking different frequencies as you present either more or less resistance to the A/C signal coming back from the speaker acting as a variable NFB control. The 22uf cap is large enough so that that no frequencies passed by the meat control will be blocked and can make their way to the secondary grid of the PI.

So do the two capacitors, C12 and C13, comprise the presence control along with R25 and R26? I'm trying to wrap my head around the signal flow from where it enters into the first grid and the long tail pair as it passes through the circuit tied to the 2nd grid an NFB.

Thanks.

[stratomaster]

The Meat control is independent of frequency. It's essentially a resistive voltage divider.  It acts indiscriminately across all frequencies and only affects amplitude. It's a volume control for your NFB circuit.

C13 is the capacitor responsible for the frequency response of the Presence circuit.  R25 and C13 alone.

There is some interaction between these parts of the circuit, but it's easiest to see them this way to get the concept.

C12 is the input capacitor for the 2nd input of the PI.  If it's sufficiently small it can reduce low end. More info on this cap is at the Aiken link above.

[Tbone55]

Thanks stratomaster for the clarification. So the meat control only controls the amplitude of the feedback signal either letting it aĺl pass through or reducing it by turning the meat controll. When it's fully open only the 75k feedback resistor is in play but when its fully closed the full 1meg plus the 74k resistor are in play.

Is the meat control interacting with the presence section then to determine which frequencies will be part of the NFB signal going back to the second grid then? How is the presence section determining what frequencies will pass to ground and which will be passed through with the NFB? I know C13 determines the frequencies because of its value but how does this value determine the cut off point to ground and the rest of the circuit?

Thanks.

[Willabe]

Thanks stratomaster for the clarification. So the meat control only controls the amplitude of the feedback signal either letting it aĺl pass through or reducing it by turning the meat controll. When it's fully open only the 75k feedback resistor is in play but when its fully closed the full 1meg plus the 74k resistor are in play.

Yes.

Is the meat control interacting with the presence section then to determine which frequencies will be part of the NFB signal going back to the second grid then?

No.

Meat control is independent of the presence control. It's been said already.

How is the presence section determining what frequencies will pass to ground and which will be passed through with the NFB? I know C13 determines the frequencies because of its value but how does this value determine the cut off point to ground and the rest of the circuit?

You need to go back and re-read about presence controls in NFB loops.

https://robrobinette.com/Voicing_an_Amp.htm#Presence_Control

https://www.aikenamps.com/index.php/what-is-negative-feedback

Uncle Doug;

i=2ubxWulfch_V0MDj

Do some searches here and on line for NFB loop and presence controls.

[Tbone55]

Thanks Willabe.

The presence control in the Aiken model has a 5k pot and a capacitor connected to its wiper. If I understand its function, the 5k pot is a shunt for the high frequencies coming from the NFB circuit, passing them through the capacitor to ground.

The more high frequencies shunted to ground the less the negative feedback and less the effect on the high frequencies from the original input signal. This boosts higher frequencies at the speaker. The fewer high frequencies shunted  to ground mean more negative feedback is applied against the original input signal with more high frequencies cancelled out. This reduces the amount of high frequencies at the speaker.

If I understand the Hoffman circuit correctly, the meat control is varying the strength/amplitude of the return signal which affects the amount of negative feedback. The C22uf cap allows all the frequencies to pass but high frequencies are shunted through the R25 C13 presence circuit. Since the majority of high frequencies are shunted to ground, the NFB consists of all the other frequencies. The strength of this NFB is dependent upon the amplitude of the signal determined by the meat control.  This signal is applied against the original input signal being fed into the 2nd grid and cancels out a portion of the same original input frequencies thus boosting the highs at the speaker.

Have I got it right?
Thanks.

[Willabe]

Uncle Doug explained all this in a very simple and clear way.

If I understand the Hoffman circuit correctly, the meat control is varying the strength/amplitude of the return signal which affects the amount of negative feedback. The C22uf cap allows all the frequencies to pass.......

Yes.

...... The C22uf cap allows all the frequencies to pass but high frequencies are shunted through the R25 C13 presence circuit. Since the majority of high frequencies are shunted to ground, the NFB consists of all the other frequencies. The strength of this NFB is dependent upon the amplitude of the signal determined by the meat control.  This signal is applied against the original input signal being fed into the 2nd grid and cancels out a portion of the same original input frequencies thus boosting the highs at the speaker.

This is where your tripping up. Your making this over complicated. Uncle Doug went through all this, go back and watch it again.

The meat control has nothing to do with the presence control except that their both in the NFB loop. They function independently of each other.

You can have an amp with just a presence, or just a meat control, or just a resonance control, or an amp could have any 2 of these or all 3 of these.

[Tbone55]

Thanks Willabe. I do tend to over complicate stuff.

Is this the way the two circuits function?

1. NFB controls only the amplitude of the signal coming back.
2. Presence circuit allows high frequencies to pass to ground.
3. Remaining frequencies travel through to the grid and combine with the same input frequencies to cancel them because they are out of phase.
4. Result is higher frequencies from the input signal are boosted at the speaker because lower fequencies were cancelled.
5. The meat control is there to determine how strong of a signal is sent back to mix with the input signal. This determines how much frequency cancellation will occur and therefore the boost of the higher frequencies to the speaker.

In the Uncle Doug video the NFB was returned to the cathode of the second half of the 12AX7 Champ but in the Deluxe Reverb it went back to the cathode of the first half of the 12Ax7. Is this because one amp is single ended and the other push pull or does this have to do with the presence control in the Deluxe?

Thanks again.

[Willabe]

Your not getting this.

Uncle Doug went all through this stuff. Watch it again until you get it.

And the way your phrasing things is hard to understand what you mean. And your still mixing up parts of presence and meat controls do.   

1. NFB controls only the amplitude of the signal coming back.
2. Presence circuit allows high frequencies to pass to ground.
3. Remaining frequencies travel through to the grid and combine with the same input frequencies to cancel them because they are out of phase.
4. Result is higher frequencies from the input signal are boosted at the speaker because lower fequencies were cancelled.
5. The meat control is there to determine how strong of a signal is sent back to mix with the input signal. This determines how much frequency cancellation will occur and therefore the boost of the higher frequencies to the speaker.

1. No.
2. Yes, but your saying it backwards. A presence circuit set to 0 has no effect on the NFB loop, as you turn up the presence knob it sends the highs to ground removing them from the NFB loop. So those highs are not in the NFB loop anymore. So because their not canceled by the NFB loop the sound from the speaker has more treble.
3. Yes, their still in the NFB loop.
4. Nope. Nothing was boosted, hi's were just not cancled in the FB loop, lows are still in the loop. Re-read #2.   
5. Nope. That would be a presence circuit. Meat control just takes out the amount of NFB fed back to the PI from full FB all the way down to 0 FB. 

In the Uncle Doug video the NFB was returned to the cathode of the second half of the 12AX7 Champ but in the Deluxe Reverb it went back to the cathode of the first half of the 12Ax7. Is this because one amp is single ended and the other push pull or does this have to do with the presence control in the Deluxe?

No and no.

Has to do with the PI. SE or PP doesn't mater, it just has to go back to at least the tube before the power tubes, doesn't mater if their SE or PP.

On the Deluxe Reverb amp, the PI is a 12AT7, and the NFB signal gets injected into the 2nd have of the 12AT7, Vb.

You need to buy some books on tube amps.   

[Tbone55]

At this point I'm totally confused. Time to get some books or just leave my amp as is and forget about understanding any of this stuff. 🤣

Thanks.

[Willabe]

Merlin's books and Kevin O'Connor's TUT books are very good, you could look for a used book.

And there are many other good books on tube amps now a days.

Not everybody can understand everything, we all have different gifts of areas of understanding for different things. Don't be too hard on your self.

Find what gift's God gave to you and work with them.   

[stratomaster]

At this point I'm totally confused. Time to get some books or just leave my amp as is and forget about understanding any of this stuff. 🤣

Thanks.

For what it's worth, your summaries of your understanding were improving and pointing towards a grasp of the concepts that were missing earlier in the thread. I think you've learned a lot and show the capacity and desire to want to know-- just not yet the initiative to seek out the information on your own.  That's the missing ingredient.

As far as giving up on the amp, that's a baby--bathwater scenario.  While you may not yet have a deep understanding of the why, the what has been thoroughly documented on this thread.  There are plenty of recommendations and explanations of what these recommendations will accomplish.  I'm sure you'll find the ingredients for what modifications take your build closer to your ideal within the posts here.

I think you should set aside some time to try them and experiment.  So long as you're following best safety practices no harm can come to you or the amp by following these recommended component value swaps.
You're probably not going to like the effect of every one of the recommendations, but you'll learn what they do, and that could be useful on a different amp in the future.  Learning what voicing changes are suitable to you is a valuable piece of info. And so long as you're willing to put the time in, you can always go back to stock. Once you experiment for a while, come back with your observations and learnings.

[Tbone55]

Thanks Willabe and stratomaster for your advice and words of encouragement.

After having read the Robinette and Aiken articles plus watching the Uncle Doug video I thought I had a better grasp of NFB and presence circuits. Seemed simple enough what they're both doing but obviously it's not that simple in my case. I really wanted to understand how they're working in the Hoffman amp. Guess it's time to get some books like the one by Merlin Blenclowe that's been recommended.

As far as my amp is concerned there are a few things I'm going to try.

1. Replace my master volume 50k linear pot with a 100k audio pot to see if I get a more gradual increase in the overall volume. I'll be replacing the 430k resistor that's in series with the pot with a 390k one.

2. Maybe try a 12AT7 in place of the 12AX7 PI tube. Will probably do this before I try replacing the master volume pot.

3. Replace the .022uf cap at the mid pot with a .047uf cap.

4. Place a 0.47uf by-pass cap across the cathode resistor of V2 to reduce some low end.

5. Replace C2/.0022ufcap with C9/.001uf cap to allow a bit more highs through.

The amp is sounding pretty good to my ears right now but maybe these few extra tweaks will improve it a bit more. I'm not going to attempt anything as far as the presence circuit or NFB goes since I haven't got it right in my head yet.  Your feedback on these changes would be appreciated.

I do have a question for you about capacitors and their frequency response. How does its value affect what frequencies will pass? My understanding is that a smaller value, say .001uf vs .1uf , the smaller value allows highs through and the larger value blocks highs. A capacitor can't just give you more highs or more lows, it can only reduce one or the other from getting through. Is this correct?

Thanks and Cheers!

[stratomaster]

Reporting back stratomaster.

Only thing left for me to do is take the C9/.001uf cap I removed and put it in place of the C2/.0022uf to see if I can get a bit more treble.

There are a number of ways to get more treble. The coupling cap isn't one of them.  It will roll off low end. This might give the effect of more treble, but it's roundabout.

To directly affect treble response here are a few options: You can try a bright cap on the volume pot. You can put the 0.47μF cap you used to have in C1 in parallel with the 1.5k cathode resistor for the 3rd stage.  You can put a 330-470pF cap in the tonestack in place of the 250pF. 

From way back around post 41. We already covered that you will not get more high end by changing that coupling cap.

You can absolutely find the answer for your question about how cap values affect frequency response for yourself.  This is the missing ingredient I referenced earlier.

I think you have everything you need in this thread to figure your amp out and tons of additional resources to help with learning the technical aspects.  Put in the time for yourself.  It is rewarding. 

[Willabe]

Merlin's books for guitar/bass amps are divided into; preamp design and  power supply design. Get the 2nd edition if you get 1.  I both these books, but 1st  edition. I have not held either 2nd edition in my hands. I don't know what he might have changed or added. I did see where Merlin said he got a lot wrong in the power supply 1st edition book. 

Kevin O'Connors TUT 1 covers preamps, power amps and power supplies and more. They are expensive. You can find them used. I have this book, I think it's it's a great book to start with. 

[Willabe]

I agree with stratomaster.

But here's 1 more last reply for now. (You can still ask all the questions you want here. I'm just out until you do some more on your own.)

I do have a question for you about capacitors and their frequency response. How does its value affect what frequencies will pass? My understanding is that a smaller value, say .001uf vs .1uf , the smaller value allows highs through and the larger value blocks highs. A capacitor can't just give you more highs or more lows, it can only reduce one or the other from getting through.

No, as the cap gets smaller in value, it keeps shaving off bottom end low frequency. .01 > .02 > .047 > .1, starting with the .01 lets the least bass through, then each next cap lets more low end frequency through. It doesn't change the highs.
But if you add bass it can sound the same as removing treble. Think of a stereo or guitar amp. Put the treble and bass knobs both at 12:00. Turn the treble full up, the high end now swamps out the bass end, with out having turn the bass down.. Do the same with the bass, turn it up full, it swamps out the treble, without having turned the treble down.

[Tbone55]

Thank You stratomaster.

I should have looked back through the post about swapping C1 and C13. I'm waiting for a 100k audio taper pot to try on the master volume control so I won't be trying that yet. No one around where I am had one.

I'll probably try putting the .47uf cap across the cathode resistor of V2 first since this seems the more logical way of fine tuning the sound. Will let you know as soon as I can the result. I'd also better look into getting some books. Turns out I know a lot less than I thought.

Cheers!