Any reason this wont work?

[daz]

I have 2 different caps for the NFB on my marshall style build. One is the typical .1uf which i really like because i can use it in conjunction with the treble pot to fine tune the treble to get the right blend of very hi treble and lower treble frequencies. But when i switch to the 2.2uf i love the way the tone gets thick. But then i miss the ability to fine tune the higher end with the .1uf. So i want to use both at the same time. right next to the presence pot is the switch i use to go between the 2 caps. What i want to do is replace that with another "presence" pot so that i have two presence pots, one for the .1uf and one for the 2.2uf. Is there any reaon this wouldn't work? i intend to do it as in the drawing below. If anyone can tell me whether that will work or if not how i should change it i'd appriciate it. Thanks .

[Chuck H]

It would work...Sorta.

One thing is that by using 5k pots you already have the percieved presence setting at about 7 for the "presence" and about 5 for the larger cap. Thats why if you look at the bypassed presence type circuit in the 800's they use a 25k pot. You would be better off with 50k pots for your application. But the controls would have most of their adjustment between about 7 and 10.

Other than that there is the problem of the larger cap swamping the response of the smaller one. In other words, the higher you set the control for the larger cap the less effect you would get from the adjustment for the smaller cap. With the larger caps pot set to 10 adjusting the pot for the smaller cap would do nothing.

You could use an inductor in the circuit for the larger cap so that it only controls the upper mids. Then you could get the same effect you have now by simply putting both controls on the same setting. Or you could bump only the upper mids or only the actual "presence" or any combination. I think that could be a neat circuit.

Not sure what size inductor you would need because I'm formula impaired. But if I figure it out I'll repost.

Chuck

[daz]

Chuck, are you saying the 5 k pot is like having presence at 7 when it's all the way down? The reason that has me scratching my head is that i went to 5 k in the first place because anything bigger had no effect till it was almost all the way up. I used 25k at first but noting changed till it was at about 8 or 9, so i used the 5 k to lengthen the usable throw, and it now just starts to make a difference about 10:00.

Anyways, it's a quick thing to try, tho i don't have another 5k i will use a 10k and try it today.

[Chuck H]

Chuck, are you saying the 5 k pot is like having presence at 7 when it's all the way down? The reason that has me scratching my head is that i went to 5 k in the first place because anything bigger had no effect till it was almost all the way up. I used 25k at first but noting changed till it was at about 8 or 9, so i used the 5 k to lengthen the usable throw, and it now just starts to make a difference about 10:00.

That's what I'm saying. With the 4.7k fixed R all frequencies "see" 4.7k from ground. In an old school type Marshall presence control when the coltrol is set to 0 the .1 cap is completely out of the circuit. With the control on 5 the .1 cap is 2.5k from ground. On an old Marshall you can certainly hear a difference between 0 and 5 on the presence control. I don't use the later 800 style presence so I can't say anything on it's adjustability. But I can say that with the fixed 4.7k R and the .1 cap going through a 5k pot that all the frequencies controlled by the .1 cap are 2.5k from ground with your presence set to 0 (4.7k in parallel with the 5k pot) so this is exactly the same circumstances as a standard Marshall presence control set to 5. With the second 5k pot and the bigger cap, that bigger cap will also allow the frequencies controlled by the .1 cap to reach ground. So now the frequencies controlled by the .1 cap have 3 paths to ground. And you end up at what would be about 7 on a standard Marshall presence control with all your knobs set to 0.

That isn't to say it's wrong. I personally would want to be able to balance my feedback loop closer to flat. But thats just me being finicky. If it works for you, it works. There are no rules in this game. But I am brainstorming a circuit you could try for what you want to do. I'm trying to get around using an inductor because by my findings you would need about 1 Henry. And thats a hard inductor to find if you only want one of them. I'll post a drawing when I have something.

Chuck

[daz]

I see now, thanks Chuck. The 4.7k and the pot together in parallel make for 2.5k to ground. So what you're saying makes sense, but for some reason my NFB loop doesn't sonically seem to reflect that. In other words, when all the way down the amp is muddy untill i turn the treble up quite a bit. As tho the presence has no influence. And it takes till around 10:00 before i can tell theres a difference, and thats subtle. Not till noon does it have a decent effect. But i will try a 25k pot again and see what i get. But as i recall it did nothing till it was most of the way up. however, i want to try it anyways and see how the tone controls work when the presence pot is all the way down because i have noticed the treble and mid controls are very much affected by the presence. One thing i've never been totally happy with is the tone section. I have tried every possible value everywhere in the tone stack and used the duncan TS calculator extensively. But i've always felt i did care for the treble and mid centers, and the bass has never worked the way i wish it would. i can get great tone out of this amp to be sure, but i feel if the tone section were as i imagine it would be phenomenal. And i have noticed a lot of interaction between the tone stack and the NFB.

what i was trying to accomplish with a 2nd pot in the NFB was to get the thick tone the amp has when i turn the pot up about 1:00 with a 2.2uf cap, then have a regular presence pot (.1uf) to dial back in some of the top end sizzle that using the 2.2uf takes away. But i think you're right and this just isn't going to do what i want. I think i will experiment more with other values in the NFB because i haven't done that for a long while, and the amp has been changed so much since i last did. If you have any suggestions for how i could change the NFB's voice somewhat radically form where it is now i'd like to hear what you might come up with. I feel like it's good as is, but i also feel like theres some magic there just waiting to be unlocked. I think the reason amp building and tweaking is so addictive is because all you have to do is make one change and it can throw everything else out of balance requiring you re-tweak many things you have previously tweaked to perfection ! so it's never ending.

[Chuck H]

I see now, thanks Chuck. The 4.7k and the pot together in parallel make for 2.5k to ground. So what you're saying makes sense, but for some reason my NFB loop doesn't sonically seem to reflect that. In other words, when all the way down the amp is muddy untill i turn the treble up quite a bit. As tho the presence has no influence. And it takes till around 10:00 before i can tell theres a difference, and thats subtle. Not till noon does it have a decent effect.

The response of your presence control could have to do with other differences in your circuit. It's all interactive. That's why we have different amp designs to choose from even if we want to clone. Tweak here, tweak there, bada bing, new amp.

But i will try a 25k pot again and see what i get. But as i recall it did nothing till it was most of the way up. however, i want to try it anyways and see how the tone controls work when the presence pot is all the way down because i have noticed the treble and mid controls are very much affected by the presence.

In your case I wouldn't trouble with the 25k pot again. Only because you've already been there. But you could try the older style presence circuit that uses a 5k pot with the cap mounted on it. The ONLY drawback to this circuit is that it can make scratchy sounds while it's being adjusted. And I don't see this as a problem because I don't do much adjusting of the presence control in the middle of a performance or recording.

And of course your tone controls are affected by the presence setting. As I said, it's all interactive. When you change your treble control (for example) you change the signal being "fed back" to the PI tail. This changes the overall response and dynamics of the amp. So the presence setting will effect the treble effect and vice versa.

One thing i've never been totally happy with is the tone section. I have tried every possible value everywhere in the tone stack and used the duncan TS calculator extensively. But i've always felt i did care for the treble and mid centers, and the bass has never worked the way i wish it would. i can get great tone out of this amp to be sure, but i feel if the tone section were as i imagine it would be phenomenal. And i have noticed a lot of interaction between the tone stack and the NFB.

I wouldn't sweat it too much if I were you. Guitar amp tone stacks are very limited in their operation (ie: passive means they only "cut" and never actually boost and they are usually limited in db swing). Lots of amp builders have tried to find unconventional ways around this. Remember those graphic EQ's on the old Boogie's? As long as you can hit a couple or a few sweet spots for the important usable tones you've done well. Adding more circuits just mucks up the tone sooner or later. It's a fine line.

what i was trying to accomplish with a 2nd pot in the NFB was to get the thick tone the amp has when i turn the pot up about 1:00 with a 2.2uf cap, then have a regular presence pot (.1uf) to dial back in some of the top end sizzle that using the 2.2uf takes away.

AAhh! There's where some of the misunderstanding is. It's important to get your head wrapped around what's happening with the NFB. The 2.2uf cap being in operation doesn't take away the effect of the .1 cap. It swamps it. When the 2.2uf cap is operating in the circuit it is not only increasing "presence" on the upper mids, but also those frequencies normaly effected by the .1 cap. with the 2.2uf cap in service there's nothing for the .1 cap to do. Those frequencies are already in play. Even though your perception is in the upper mid effect.

but i also feel like theres some magic there just waiting to be unlocked. I think the reason amp building and tweaking is so addictive is because all you have to do is make one change and it can throw everything else out of balance requiring you re-tweak many things you have previously tweaked to perfection ! so it's never ending.

Remember your first hot rod? Have you ever owned a boat? Either of these examples would serve. It's a never ending project that never lives up to an ideal. I guess I'm getting old but I use to imagine a perfect amp. Now I try to specialize so that I can be happy with each design. I've stopped expecting that I can build one channel that is great for everything (but I got pretty damn close IMHO ).

Chuck

[daz]

Wow, thats a book, not a post !

When you say...

But you could try the older style presence circuit that uses a 5k pot with the cap mounted on it.

I'm not sure what you mean. I DO have a 5k pot with the cap mounted on it. So i'm wondering what you are saying to do different? Whatever it is it'll try it if you describe it to me.

[Chuck H]

When you say...

I'm not sure what you mean. I DO have a 5k pot with the cap mounted on it. So i'm wondering what you are saying to do different? Whatever it is it'll try it if you describe it to me.

Yeh, That was a pretty poor description... See below. This circuit works better IMO if only because of it's eloquence. Good adjustment swing, fewer parts. easier to implement. My kind of circuit.

You could try the 2.2uf and .1uf on seperate pots with this design too. Just use 10k pots so that the NFB shunt total is still 5k. If you put a 1 or 1.5 henry inductor in series with the 2.2uf cap, each pot would operate completely independant of each other. I'm trying to locate that inductor.

Chuck

[Chuck H]

Just to be clear this time...This is how the new circuit would look.

Chuck

[daz]

Thanks Chuck. I'll try wiring in like the `1st jpg. Don't bother trying to find an inductor tho because if i can't find it locally i'll just pass, as i know i'll end up paying $10 for shipping. But i DO have some pretty good local resources. I just need to know what H value to look for. In particular i'd really like to be able to pump up the low mids like in the 400-800Hz area.

[daz]

I tried wiring it as you showed Chuck, but i think i like the throw as it was before. Maybe just because i'm used to it. However, you helped me out in one way....i found you are correct in that the presence IS indeed allowing some high end with the 5k pot. i had it switchable completely out of circuit and it does add a considerable bit of high end even all the way down. this is a nice find because i can now turn it down and then i'm able to turn the treble up more. this is something i do to get a more fenderish tone, while turning the treble down more and the presence up is more marshall like. So now i can go a bit further towards the fenderish side. Nice !

However, i can't understand how to take the cap out of circuit with a larger pot because when i use my electronics calculator to find what size pot would allow for 4.7k or higher to ground when paralleled with the 4.7k resistor thats there, it takes a 1 meg pot to be close to 4.7k when turned all the way down. yet i don't see presence pots over 25k usually. What am i missing here?

[daz]

Ok, heres the latest. After you (chuck) told me that 5k was not taking the cap out totally i now feel like i may no longer need that mid boost. It seems with the cap out of circuit i can turn the trble up to get enough high end, but it's a thicker tone. With the cap 2.5k away from ground i was getting enough presence to where turning the treble up got nasty and harsh. But w/o the cap the treble gives me the high end i need but at a lower treble freq where the tone is thicker w/o needing that mid boost. So in a roundabout way you helped me get what i was after, at least to a degree if not completely....time will tell as my ears get used to it and make thier decision.

That said, i now have a 25k pot, but the range is now all near the end. But if i go to the next lowest value, 10k, i know there will be enough to screw things up again when i want that thicker tone with the cap at a point where it's not contributing even the slightest presence. I may just go to a 10k tho anyways and cut the trace at one end so turned full counter clockwise it will be open. Chuck, i really appriciate all your help. Truly I do...thank you.

[black_labb]

jumping in a bit late here. what if you used a single pot (10 or 25k) and tied the wiper to one end of the 4k7 resitor, and the 2 different caps attached to either end of the pot, with the other ends connected to the other end of the 4k7 resistor.

this is assuming you use the 25k, which i think would be better.
in the middle there is 12.5k resistance to each cap (6.25k effectively for all highs, 12.5k for the real highs). when you turn it one way, the resistance drops for one cap and increases for the other, effectively giving you the effect of 2 pots. the only thing it doesnt do is let you turn them both up, but that isnt very useful anyway as chuck described above. also saves you putting any extra holes in for pots.

[Chuck H]

FWIW Daz, you could use the old style presence control and get at least as much adjustability as a 10k pot gives in your current arrangement. It would give you the ability to turn the presence down all the way. You noted how with the typical 800 type presence circuit you always have SOME presence activated. This is true. Your not off your rocker, you just saw it for what it is. Anyway, If you use the older style presece circuit it would allow you to turn your presence down all the way (like a Fender with flat NFB). You would then have the adjustability to use more treble (if it were there). It can be. Just change your slope resistor in the tone stack to 56k or 68k. This would give more upper mids which will thicken your top end and tighten your bottom end. And the presence would now be more like icing on the cake rather than a needed componant all the time.

It's easy to try. Just wire it the way you did before with the 5k pot as the NFB shunt with the cap on it, and change your tonestack slope R to 56 or 68k. 33k is a small slope R for a 50 watter anyhow. Even Marshall only used it on 100W models. And then I think it was to pull back on the top end. At 100W to much top end is like stabbing your ears with a shishkabob swewer.

Chuck

[daz]

Actually i've had a 47k slope in there for the last 3 weeks or so. I tried more but it seemed too thin, and i have made some major changes in the last few days going from a 220k V1A plate back to 100k as it first was and a few other things. that and a few other changes have been great but left my tone rather thin and i'm having a bit more trouble with flabby lows. I know that sounds crazy....made some changes and like it, yet now i have a thin sound and flabby lows !!! LOL! but really, the tone seems better in a way (richer harmonics) if i could just thicken it up and tighten the lows. I tried some .1uf couplers at the EL34's tonite but it was too late to turn it up to tell. Seems thicker at the low volumes i could try it at, but the flab in the lows also got worse i think. I'l know more when i can turn it up tomorrow.

what bothers me is that i fought flabby lows forever with this amp and finally got it under control well enough quite a while back. But now with the changes i've made it seems to have reared it's ugly face again. I may have to change the V1A plate back to 220k, as thats what i think may have done it. Anyways, i like the presence right now since at zero it's the same as switched out. I now need to either figure out this flab thing and the thin-ness which i think would be ok if i could add some low mids. If it's just too much to deal with i may just set everything back to how it was a week ago and had been for quite a while. I just can't stop trying to improve it no matter how good it is ! It's a freakin' disease !

[imaradiostar]

I kinda got lost after reading a bit...but a few quick thoughts.

You can change to amount of negative feedback by varying the value of the 5k pot in a traditional marshall presence circuit- it forms the bottom half of the voltage divider.

You can vary the value of the cap to change the frequencies effected by the feedback loop.

You can place a cap, an inductor, and/or a resistor in parallel with either resistor that forms the NFB voltage divider. This technique is used to help give the bogner uberschall its extremely versatile tone circuit and was used to increase high frequency stability in many classic "example circuits" for hi-fi and PA amps from the golden age of tube audio. I think this could really make for an interesting "mid scoop" tone in an appropriately designed amp because it generates an active control rather than the normal passive control.

Properly set up you could probably insert an LC network that would allow you to boost or cut and (based on changing C or L values) alter the frequency of the boost or cut. A resistor in parallel with the LC network could function as a "Q" control to prevent the mid scoop/boost from being too narrow to be usable.

If you apply the same concept to the original poster's idea you could probably end up with two "presence" knobs- on for each of the frequencies represented. Connect opposing ends of each presence pot so that by setting the knob in the middle there is equal signal to and from the output into the PI (no boost or cut). By rolling the control either direction away from center it either shunts the treble NFB to ground and increases treble or creates a more direct NFB path, increasing feeback and decreasing treble.

It seems like this is the concept behind most active EQ circuits- they use some sort of phase inverting buffer that allows you to pan between adding to or subtracting from the original signal. Because the different frequencies are blended resistively you should be able to boost or cut somewhat independently- a shelved EQ is still a shelf but you should be able to boost or cut the low freq and the higher will be boosted or cut above or below the setpoint of the first.

Wow...that was overly technical and long. I can't wait for someone to point out that I'm wrong or didn't describe something correctly! sorry, it's late and I'm rambling.

jamie

[Steve Conner]

I don't think it's rambling at all, it's the same line of thought that led me to the Ninja Toaster parametric EQ. Turning the EQ knob one way shunts signal away from the tube grid, causing a cut. Turning it the other way shunts the cathode to ground, which causes a boost.

I wussed out of actually connecting it to the power amp's NFB loop, though. I was worried that it might react badly on something, and make the power amp go unstable.

[daz]

You can place a cap, an inductor, and/or a resistor in parallel with either resistor that forms the NFB voltage divider. This technique is used to help give the bogner uberschall its extremely versatile tone circuit and was used to increase high frequency stability in many classic "example circuits" for hi-fi and PA amps from the golden age of tube audio. I think this could really make for an interesting "mid scoop" tone in an appropriately designed amp because it generates an active control rather than the normal passive control.

Properly set up you could probably insert an LC network that would allow you to boost or cut and (based on changing C or L values) alter the frequency of the boost or cut. A resistor in parallel with the LC network could function as a "Q" control to prevent the mid scoop/boost from being too narrow to be usable.

jamie

That sounds VERY interesting, tho i must admit it went over my head as to the wiring you are describing. Any schematics you can point me to that are designed like that aside from the ubershall which i looked for but seems not to exist on the net? also, when you say "with either resistor that forms the NFB voltage divider" are you talking about the 100k from the tap and the 4.7k? If so, are you describing a "resonance" cap? I've tried that but it basically just adds low end and really hindered more than helped, even when used with a pot to vary it. i always ended up leaving the pot down. Mainly because my amp's low end isn't tight enough for a lot of low end.

[Chuck H]

Daz, you sleep with your soldering iron, don't you?

When the bug bites it's hard to stop. I still own the first amp I ever "tweaked to perfection". And even though at this point, some 16 years later, I know there are some pedantic or questionable circuits in there, It still sounds great when I plug it in. Well actually it's in need of new caps and tubes right now. But if it weren't it would still sound great. I WORKED ON THAT AMP NON STOP FOR AT LEAST A YEAR. I had the time and the urge so that's what I did. What a great learning experience.

If your amp is sounding too brash on the top and bottom (and you don't mind a little soldering ) I have a suggetion. I noticed on your schem that you were using a .022 cap for the PI input. Even though the tonestack is fed by a cathode follower the signal is substantially loaded by the time it gets to the PI input. You could try a .0022 cap for the input of the PI. This should still give you your bottom end but the mud factor would be lower. And even though "bleeder circuits" are considered a sin you could try a very low pf cap on V1 from the plate to the grid. Probably 5 or 6pf. Make sure the cap is rated for the plate voltage there. I use little ceramics for this. These two mods are reasonably non invasive to your tone. I fully expect they will improve your OD tone, but if they detriment the clean and in betweens too much you can always put it back the way it was.

Chuck

[daz]

Hey Chuck, listen to this. Two things have happened in the last few hours. Oh, and no....i don't sleep with the iron, but only because i know i'd get burned. Anyways, i tried what you said but it didnt really help. I've tried that before anyways, tho not with that small a value. anyways, two things have happened. One good one bad. the good, i stuck a 33uf cap in place of the 22uf bypass cap on one of the el34's but left the other one at 22uf. the result was great. the tone just got very nice in several ways. Less harsh in the highs, more sponge factor, less mud, more articulation. So i really don't need to lift the caps to get that tone and lower volume anymore. It sound even better than it did lifting the caps i think. Definatly better in some ways.
the bad, and this started BEFORE i did this so i don't recall what i had done that could have caused this, but i'm getting intermittant motorboating. In fact, thats what led me to try the 33uf....i was trying to replace the 22uf on that side because i suspected it might be bad after lifting it to see. When i lifted it the motorboating stopped, so i thought the cap was bad. Turns out it wasn't, it was just that for some reason it stops when i lift the bypass caps. so feeling that cap may be bad i replaced it with a 33uf because i didn't have another 22uf. Anyways, what can cause this motorboating? It only happens either when i change a cap value that just seems to trigger it for some reaon, ot when i put it on standby or turn the power switch off it will do it as it's powering down. any ideas?

Anyways, I think i'll try an even bigger cap on that one side....maybe it'll sound even better ! I guess it's just throwing the signal balance off thats giving me richer tone.

EDIT: no cigar. But still it sounds great with the 33uf on one side. I wonder also if the voltage rating could have anything to do with it. Both are rated high enough since theres only 30v there and the 22uf is a 50v. But the new 33uf is a 160v. Wonder if somehow that could have anything to do with it. I think not, probably the imbalance of AC signal with the different UF ratings huh? Gott say tho, this has made a truly big improvment. I was pretty sure the needed improvments were in the PA and now i know thats true. the thing is just so much sweeter. the #2 strat position just quacks like crazy, which is one of my tests for great tone. This may be the best it's been so far. also fogot to mention....i put the 220k back on V1A's plate. I now know it just sounds much better like that and that the 100k was partially responsible for that thinness i was getting.

[Chuck H]

Anyways, I think i'll try an even bigger cap on that one side....maybe it'll sound even better ! I guess it's just throwing the signal balance off thats giving me richer tone.

I suspect that's just what's happening. You could also try an imbalance at the PI. Imbalance has been discussed here as a possible componant of good overdrive. I'm still on the fence since I try to balance everything and I'm getting good results. It could be in the "different strokes for different folks" category too.

As far as the motor boating I have a clue. Motor boating is an oscillation. When you lift the caps it stops. That tells me that when the amp is putting out less power that whatever is causing the motor boating no longer has the strength to induce the oscillation. So consider lead dress on any circuit that would increase in voltage when the amp is at full output. Anything in the NFB loop and OT secondary leads are the most suspect. Be sure to move all preamp leads as far from the output and NFB loop as possible. Likewise, move all leads related to the NFB loop and OT secondaries as far from the preamp as possible. After that you would need to consider your board layout and even your grounding sceme.

Proximity... Consider that word... It's the biggest reason proponants of hand wired amps tout for the inferiority of printed circuit boards. I'm still on the fence on this too. But it's true enough that all the componants in your amp, even or especially lead wire, has a radiant field. If two componants radiant field are sharing space that can induce a reaction in each other. Two componants of like phase in too close proximity results in an induced positive feedback. This can cause squealing or motorboating. Depending on the size of the radiant field and the sensitivity of the effected circuit these two enemies don't need to be that close to each other for there to be a problem. Two radiant fields of opposite phase causes NFB. It's these small NFB's that occur with the tight spaces on PCB's that give them their bad rep. I once modded an amp for a friend that had incredible OD but occasional fits of oscillation. I moved the OT on the chassis as I felt it was poorly located. The oscillation stopped but the OD tone slipped from hairy and grand to mediocre. I put the OT back where it was and the OD tone was back. As were the fits of oscillation. The OT location was introducing positive feedback that was increasing the preamps gain in a very specific way. He ended up with another amp and I now own this odd ball. Whenever I get around to it I hope to solve it's issues and learn a little about how to intentionally induce minor instability in a good way. The late Ken Fischer was said to use this practice. But it seems like such an obscure art that I can't imagine ever being able to do it at will with a new design or even repeat it consistently considering componant tolerances and all. Anyhoo, there's a place for you to start and some info to digest.

Chuck

[Steve Conner]

High frequency squeals may be caused by lead dress, but motorboating isn't. The formal explanation for it involves a lot of math (and can be found in RDH4) but what it amounts to is that, hidden in your circuit, is something similar to Fender's old tremolo oscillator.

And what the trem oscillator is, is an amplifying stage with a negative feedback loop around it containing three RC filters. Each RC filter contributes some phase shift, and at some frequency, the three shifts add up to 180 degrees. This flips the negative feedback into positive, and if the tube provides enough gain to make up the losses in the RCs, the circuit oscillates.

In the case of a power amp, the OT already contributes one time constant, so you only need another two RC networks to get your 180 degrees and start it motorboating. The input coupling capacitor to the PI, with the PI grid leak resistor, is your second time constant. (Yes, this is part of the feedback loop.)

So you just need one more RC to kick it off, and I guess Daz's undersized cathode bypass caps are it.

The worst oscillations occur when all three time constants are the same, as they are in the trem circuit. If the amp is only just oscillating, then changing any one of them should help.

Even if the NFB loop isn't actually oscillating, it can still be underdamped, if this is the case you can sometimes see the speaker cone flapping in and out more than it should.

Old hi-fi designs like the Williamson etc. have one more gain stage inside the NFB loop than guitar amps, which adds one more RC, so they naturally just want to oscillate. Tuning all the coupling cap sizes for good bass without motorboating can be a nightmare. (And again, the magic incantations for doing it can be found in RDH4.) I think this is why the audiophools give them a reputation for poor bass.

Preamp stages can also motorboat, again the circuit is just like the old trem oscillator, but one of the three RCs is the supply dropping resistor and electrolytic bypass cap. That's why dried up electrolytics cause motorboating: if the cap loses enough capacitance, the RC time constant gets shorter until it's similar to the preamp coupling caps, and off it goes.

[daz]

I think you nailed this steve. Because while it had already been doing this occasionally just a little, when i tried chuck's idea to change the .1uf cap on the NFB side of the PI thats when it went crazy and began doing it constantly instead of just a few little putts when i turn the standby or power switch off. So i guess i'll just have to change most any component in the NFB or PA in general that can be changed in value slightly w/o affecting the tone but will change the balance of things enough to stop it, eh?

Today i intend to try a few things that i liked before i discovered the imbalanced cathode caps but didn't keep because they contributed to the flabby lows. Now that those are no longer an issue i want to try a few things to beef the tone up. For example, someone elsewhere recommended i try .1uf couplers in front of the el34's in place of the .022's, and it seemed nice but because my lows were muddy this made that worse, tho otherwise it seems to sound really nice an beefy. So i will try that and some other things. I find that when something isn't quite right like flabby bass, many potential improvements don't work when they otherwise would have if that issue didn't exist. So once you fix that issue you can then go back and re-try those things. Thats what is so addictive about the design process.....one little change makes the entire rest of the amp now very different in terms of what tweaks will or won't work. so you can literally make one change that un-does all the things you perfected in the past ! It's crazy stuff and thats why i've been at this amp for a probably a year now !

[daz]

Chuck, how would one go about unbalancing the PI? Right now the plates are 00k/82k, but from what i've read that actually balances it. So i tried adding a 15k r to the 100k so it's now 115k/82k. It seems to cut some more of the brightness which is good because the amp has more than i wat anyways and other methods of cutting it haven't left me with very pleasing results. But is the way i just did this good or are there rules i should go by such as WHICH plate gets the lower resistor and how to know how much difference to use between them, or things like shoul i drop the 82k further or add to the 100k as i did? Any suggestions as to how to do this correctly instead of blindly as i am now.

[Chuck H]

High frequency squeals may be caused by lead dress, but motorboating isn't.

Well I personally never had a problem with it. Nor am I an EE. But I thought it was true because of accounts I have read about and (perhaps wrongly) was trying to relay helpful info. One account in particular that is in print comes to mind. Now I can't speak to the technical competance of Gerald Weber but he does have an excellent reputation for repairing and restoring vintage Fender amps. In one of his books he writes that an early Deluxe model (can't recall which) has a tendancy to motor boat when overdriven. The common fix is to turn the PI input cap 90* on the circuit board. Perhaps this is a rare example but I have read other accounts of PFB due to layout causing motor boating. Like I said, I have never had a personal experience with it so anything I read about may have been a lie or an incorrect assesment of the problem.

It's worth noting that some of the changes Daz made to the amp in this process would not have altered the phase relationships but somehow made the problem worse.

Chuck