5E3 Paul C/Ampeg Mod vs. V2B phase inverter grid stopper resistor experiences

DavidP

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I'm revisiting my very 1st build nearly 2 decades ago -- a 5E3 based on the Mission "Tweedy Deluxe" kit offered by Bruce Collins back then.
The amp is basically stock but I would like to address headroom/overdrive ramp-up, flabby bass, and brightness (or lack thereof).

For those who've done it, what's your experience with the Paul C/Ampeg mod change to a fixed biased inverter instead of the self biasing stock setup?

I'm also thinking of simply running a grid stopper resistor to the V2B phase inverter to smooth out the overdrive after Rob's less invasive mod (https://robrobinette.com/5e3_Modifications.htm#Phase_Inverter_Grid_Stopper_Resistor).

Both are straightforward/reversible mods but I'd greatly appreciate hearing about the good, bad, and ugly from those who have done it, before I get under the hood.
 

King Fan

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I'm also thinking of simply running a grid stopper resistor to the V2B phase inverter to smooth out the overdrive after Rob's less invasive mod (https://robrobinette.com/5e3_Modifications.htm#Phase_Inverter_Grid_Stopper_Resistor).

I can only comment on this one. Easiest possible mod, so maybe worth a try first? Really liked it -- takes the rat out of the drive.

1656371016563.png
 

Lowerleftcoast

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brightness (or lack thereof)
The 470K Resistor (control grid stopper) will take away more highs. That can be remedied with a 470pF cap in parallel with this resistor.

According to Hoyle, control grid stoppers should place the body of the resistor very close to the tube pin.
 
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Bendyha

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I hear no discernable loss of highs caused by my 1M resistor, and see no need to add a bypass cap to it. I do hear a better overdrive tone when it is present.
As to the so called PaulC-mod, I have written a critic of it HERE.
Changing the self-bias to a fixed-bias can be done better than the way the PaulC-mod described.
As to the Ampeg-mod, I am not sure if I am aware of what is referred to there, or maybe I've just forgotten, do you have a link? I think maybe it is not so much a "mod" in itself, as a reference to the fact that some Ampegs had fixed-bias cathodyne phase-inverters.
Some Ampegs had direct a coupled cathodyne P.I....... no need for a grid resistor there, as there is also no coupling cap that could cause blocking distortion....and no need for fixed-bias, or self-bias resistors either, but that is a whole other subject, perhaps.
 
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DavidP

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Thanks for the link, @Bendya, lots to digest on that thread! AFAIK, the Paul C mod is the Ampeg mod renamed. Here's the reference I have for it.
 

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bebopbrain

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<pet peeve> Everybody (Dumble, RobRob, Hoffman, etc.) helpfully splits the V1 cathodes and properly doubles the cathode resistors. But they don't halve the cathode bypass capacitors. As if the 5E3 needs more bass! The bypass caps should be 12uF or 6uF if following the 5E5 schematic. </pet peeve>
 

jhundt

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<pet peeve> Everybody (Dumble, RobRob, Hoffman, etc.) helpfully splits the V1 cathodes and properly doubles the cathode resistors. But they don't halve the cathode bypass capacitors. As if the 5E3 needs more bass! The bypass caps should be 12uF or 6uF if following the 5E5 schematic. </pet peeve>
yeah, and nobody likes those Dumble amps, because he didn't know about those bypass caps. Just imagine - if he had known about those caps he might have built a half-way decent amp; people might even want to buy one of his old amps if they saw it in a junk store.
 

cobaltu

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I know this is muddying the waters a bit, but, I think the best way to help the flabby bass and lack of brightness in the amp is simply - cut bass.

Every 'problem' the 5e3 has comes down to - too much bass. The flabby/mushy lows, lack of presence, lack of head-room, nasty distortion at higher levels. Again, ALL of it comes down to the amp having more low frequencies than it was realistically built to handle. A big part of the charm of these amps is their sloshy handling. If you change the biasing of the PI you might find that it isn't really the amp you've loved for 20-odd years.

This is where you have to be careful. This amp has a lot of "ghosts in the the machine." It's honestly a terrible amp that has a combination of idiosyncrasies that many of us find charming, even love. There is a point where you have to decide if you actually like it, or let it go. The amp has an overwhelming reputation that precedes it, and it's reputation often convinces us that it's an amp we all need, and need to love. This really isn't the case. This is not an amp for everyone.

But, you've held on to it for ~20 yrs so it probably just need some tweaking. Sorry for the pontificating.

Anyway... If you want to address the exsesive bass, I would.
1) Lower all the .1uf caps to ~.047uF
2) Lower the PI input cap from .022uF to ~.005uF (I know this one seems weird, but it can actually do a lot to help with the nasty Pi distortion)
3) Lower the 12ax7's cathode bypass cap (V2a) from 25uF to 4.7uF.

You don't have to do all of these at once - in fact I wouldn't - these might all do a better job of retaining the charm of this amp while making it more usable.
 

dankilling

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^^This. I’m a fan of the PI grid stopper, and have done many of the above to my 5e3 builds (Plus a few others) The mods change how the amp behaves, some significantly, but if thoughtfully executed can preserve what is loved about the 5e3 while addressing how raunchy and hard to control it can be. Try one mod at a time, but IMO, changing the .1 caps to .047 and the PI grid stopper are the changes that lose the least of the ‘charming faults’ of the 5e3 while removing the parts that are the toughest to control.
 

slider313

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Ampeg used a 5k-10k negative feedback resistor in their cathode biased Rocket/Reverberocket series of amps with very good results.
 




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