Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: tristanc on November 22, 2025, 03:01:20 pm
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The Princeton Reverb is a classic circuit. It’s fine as is.
But, my understanding is that these were built to as-cheap-as-possible-yet-passible-tone requirements and, thus, BoMs. They struck lucky and their popularity shows that. However, there are mods people do to make them better, and components added in the reissue to make them safer / more suitable for the modern era.
Inspired by Merlin's new book, given cost is no barrier to a DIY clone of a Princeton Reverb, what circuit refinements would you incorporate?
I don’t care about aesthetics / period correctness on the inside. I do care about safety / good engineering practice and, ultimately, the classic PR sound.
Given the constraint of using a ‘traditional’ turret board from a kit, I was thinking:
- MF resistors instead of CC
- bleeder resistor added
- reduce post-PI coupling caps to 0.022uF or 0.047uF
- add 470k stopper to PI
- adding stoppers to grid and screens on the 6V6s
- using a red LED for the tremolo oscillator bias
- add protection resistor on the speaker jack
- add variable bias (and balance?) control
- do my best to ground things appropriately, even if it means running short lengths of wire to connect the various star groupings, and connecting only to chassis at the input jack.
The PRRI schematic has a bunch of these things done already (diodes here and there for protection, fuses, e.g.). I don’t want to go overboard as that would complicate the pre-made turret board.
Have I gone overboard? Not far enough? Destroyed the mojo?
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IMHO, the best mod you can do for a PR that won't affect the mojo of the original blackface amp vibe is to keep the entire circuit as-is, but swap out the filter cap can for separate filter caps at each HT rail supply node and change to a Merlin-style daisy-chained galactic ground return system. This will get rid of some/most of the hum in the noise floor.
Also, if your PT is such that the amp will run at higher voltages (i.e 430-450ish), you could also swap out the PR OT for a DR (6k6 25W) OT and go to a 12" speaker - this opens up the sound and ensures slightly improved power output. And of course, you will probably want to run JJ6V6S if your voltages are going to be that high.
Also recommend a couple of fail-safe protections, namely:
1) 1.5kV protection diodes in series with the rectifier tube plates
2) a 1k load resistor across the reverb transformer secondary and a 2-3kV 100-2000pF cap across the reverb transformer primary to help prevent the RT from failure.
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Reverb HT moved to the unused node with the 18k feeding it reduced to 12k and the reverb driver resistor reduced to 1.5k will give you a nearly silent noise floor from the reverb circuit.
I like to use a large cathode cap (250μF) on the recovery for good measure like the standalone reverb units did.
I also like a 3.3μF cap on the 2nd stage cathode instead of messing with the coupling caps.
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Rob Robinette has a nicely laid out version of the Princeton with his suggested mods here:
https://robrobinette.com/AA1164_Princeton_Reverb.htm#Suggested_Mods
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Rob Robinette has a nicely laid out version of the Princeton with his suggested mods here
Thanks - that's what started me on the journey. Also comparing the 'original' scheme with the reissue. There's a fine balance between copying it verbatim in a new build and making some alterations whereby you end up with a completely different amp.
This is my 2026 amp build challenge - taking my time and, hopefully, using up some of the components I have on hand. I like the idea of making a few small changes (a cap here and there, re-routing the grounding e.g.) and doing it within the existing turret board(s). ie, what's the biggest improvement you can do with the fewest components.
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I have the reverb driver power feed moved to the unused node (what stratomaster has also suggested) and can confirm it did drastically reduce hum.
One more from the Merlin's book that I liked is reducing V1a grid stoppers. Mine is effectively 10k (down from 34k?) on high input, which reduced hiss when the volume is up.
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Thanks - that's good to know. I'll also try the "ultrapath" thing Merlin talks about in his new book.
Attached is where I am currently. It's a bit difficult to see, but components marked are altered or additions. I've modified the power nodes and grounding - whether this will destroy the mojo... :dontknow:
And good point on the grid stopper. I'll likely get rid of the 2nd input (I've never used it when there) and re-purpose the hole for a 'boosted' input (a FET stage in front of the 1st stage). That means I can used a 10k.
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Thanks - that's good to know. I'll also try the "ultrapath" thing Merlin talks about in his new book.
Attached is where I am currently. It's a bit difficult to see, but components marked are altered or additions. I've modified the power nodes and grounding - whether this will destroy the mojo... :dontknow:
And good point on the grid stopper. I'll likely get rid of the 2nd input (I've never used it when there) and re-purpose the hole for a 'boosted' input (a FET stage in front of the 1st stage). That means I can used a 10k.
You'll likely find that the ripple is so low after moving the HT node for the reverb transformer over that the ultrapath is redundant with minimal to non-existent benefits. There was no audible reduction in hum nor visible change on a scope with the ultrapath in or out with my PR.
On the reverb recovery a large cathode bypass cap (100+μF) is beneficial in reducing H-K hum, a log pot is useful for dialing in subtle reverb while maintaining access to Surf City, USA.
Bonus: If you study the same section of Merlin's book that covers the ultrapath you'll come across a frequency response chart of the spring reverb. Use a plate resistor bypass cap on the recovery stage to aggressively roll off above the upper range. This kills hiss while maintaining the full response of the reverb stage. I think I used a 470pF, but I'd have to go back and look.
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You'll likely find that the ripple is so low after moving the HT node for the reverb transformer over that the ultrapath is redundant with minimal to non-existent benefits. There was no audible reduction in hum nor visible change on a scope with the ultrapath in or out with my PR.
Thanks - good to hear that you’ve tried both.
On the reverb recovery a large cathode bypass cap (100+μF) is beneficial in reducing H-K hum, a log pot is useful for dialing in subtle reverb while maintaining access to Surf City, USA.
I’ve been wondering why LED biasing wouldn’t be appropriate for that stage?
And yes, a 100k log pot is on its way - surprisingly difficult to source.
Bonus: If you study the same section of Merlin's book that covers the ultrapath you'll come across a frequency response chart of the spring reverb. Use a plate resistor bypass cap on the recovery stage to aggressively roll off above the upper range. This kills hiss while maintaining the full response of the reverb stage. I think I used a 470pF, but I'd have to go back and look.
Thanks for this - I see he mentions a 100pF grid to ground to kill oscillations, but also refers to hiss just before this. I don’t see any harm rolling off the top end here given its way above where you’d be wanting reverberations?
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Attached is where I am currently. It's a bit difficult to see, but components marked are altered or additions. I've modified the power nodes and grounding - whether this will destroy the mojo... :dontknow:
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Nevermind mojo, the bias supply won't work without any negative voltage; R37 needs moving to the AC side of D3 :icon_biggrin:
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Nevermind mojo, the bias supply won't work without any negative voltage; R37 needs moving to the AC side of D3 :icon_biggrin:
Thanks! Good catch!
So it seems re-doing the power nodes and grounding is likely to make the biggest impact. Smaller things like LED bias are, well, small and nice-to-haves.
Oh, one question I have (being really picky) is where the reverb transformer secondary should be grounded. I've assumed it's acting like a coupling cap so the ground should go to the following (recovery) stage node.
The fun challenge for me is making the changes within the confines of the existing turret boards / chassis. I think I'll move much of the tone stack to the pots, and maybe use some tag strips next to the valve sockets (using existing mounting screw holes) to free up space. I'd like to avoid drilling holes for turrets or in the chassis if I can avoid it.
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I’ve been wondering why LED biasing wouldn’t be appropriate for that stage?
I haven't put pen to paper, but I think you'd end up in a different spot. The LED results in a fixed cathode voltage regardless of current through the triode, whereas the cathode voltage "falls out" from the cathode current and resistor value. I think the former is useful for non-audio stages but may alter the behavior of the stage noticably on an audio stage.
More importantly, however, is I think the shunting of h-K leakage hum relies on the reactance of the bypass capacitor being significantly lower than the plate-cathode impedance. That is, the path to ground through the capacitor is more attractive to the mains frequency cathode input is more attractive than the one to the plate. This demands the presence of a capacitor.
So while you can get the two biasing arrangements to look similar in terms of load lines, the specific behavior we're looking to exploit needs a capacitor in place. If I'm wrong on this point, I'd love a correction.
Thanks for this - I see he mentions a 100pF grid to ground to kill oscillations, but also refers to hiss just before this. I don’t see any harm rolling off the top end here given its way above where you’d be wanting reverberations?
There was a thread on TDPRI that I can't find at the moment where they discussed that a capacitor bypassing the recovery stage grid leak doesn't behave like a simple low pass. Instead it looks a lot more like the frequency response plot of a guitar pickup with a tone control circuit due to the presence of the inductance of the reverb tank output transducer. Conventional wisdom is that this cap shunts hiss and high frequency to ground, but in that thread simulations showed it behaved as a treble peaking circuit before the roll off. It's for this reason that I prefer to do the treble shaping with a plate bypass cap. You can even add a cap across the outer lugs of the Reverb pot for a steeper roll off of the hissy frequencies in conjunction with the plate bypass cap--though I haven't tried this yet. The reverb circuit noise is plenty low on a PR with just a large bypass cap and the cleaner HT node.
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I haven't put pen to paper, but I think you'd end up in a different spot. The LED results in a fixed cathode voltage regardless of current through the triode, whereas the cathode voltage "falls out" from the cathode current and resistor value. I think the former is useful for non-audio stages but may alter the behavior of the stage noticably on an audio stage.
Thanks! I've learned something.
There was a thread on TDPRI that I can't find at the moment where they discussed that a capacitor bypassing the recovery stage grid leak doesn't behave like a simple low pass. Instead it looks a lot more like the frequency response plot of a guitar pickup with a tone control circuit due to the presence of the inductance of the reverb tank output transducer.
And again - learning something new. Thanks.
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I'm passing over the schematic now stripping out additions that may over complicate. So far:
- removing the ultrapath cap given I'm re-doing the power nodes & grounding
- abandoned OT protection diodes (don't think I'd have room, and have never installed them...)
Where I could do with some input: should I bother with a balance pot for the power tubes?
It's a cheap part which means I don't need to pay loads for 'matched' tubes. Albeit only likely to happen maybe twice in the lifetime of this amp. Though it should be an easy addition to the turret board (famous last words).
I've put all my workings here https://github.com/tristancollins/PrincetonReverbUpdate (https://github.com/tristancollins/PrincetonReverbUpdate). There's a branch called "mods" with these circuit modifications: https://github.com/tristancollins/PrincetonReverbUpdate/tree/mods (https://github.com/tristancollins/PrincetonReverbUpdate/tree/mods).
Latest schematic here: https://github.com/tristancollins/PrincetonReverbUpdate/blob/mods/schematic/PrincetonReverbUpdate.pdf (https://github.com/tristancollins/PrincetonReverbUpdate/blob/mods/schematic/PrincetonReverbUpdate.pdf)
I'll do my best to borrow Rob Robinette's layout DIYLC files and update them for this schematic. Maybe even a spice model. We'll see.
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R45 is in the wrong spot. It needs to go directly to the grid. The way you have it now is a voltage divider with the 1M.
I'd also recommend wiring the bias supply closer to what is recommended by Merlin in his site. With 25μF as you've got it, it's under filtered enough to be audible. Instead of wiring the pot at a variable resistor, wire it as a voltage divider and take the bias off the wiper with a 100k safety resistor and a 2nd cap to ground off the wiper or the bias input lug on the Intensity pot.
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R45 is in the wrong spot. It needs to go directly to the grid. The way you have it now is a voltage divider with the 1M.
Thanks - another good catch!
I'd also recommend wiring the bias supply closer to what is recommended by Merlin in his site. With 25μF as you've got it, it's under filtered enough to be audible. Instead of wiring the pot at a variable resistor, wire it as a voltage divider and take the bias off the wiper with a 100k safety resistor and a 2nd cap to ground off the wiper or the bias input lug on the Intensity pot.
Yes, I usually follow his bias supply recommendations - the bias board (original and in the kit) has limited turrets as is. Perhaps this one will need some drilling. Or maybe a new board.
Having put the idea in my head - the extra step to actually just lay out a PCB that fits the mounting holes wouldn't be much more effort....
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Yes, I usually follow his bias supply recommendations - the bias board (original and in the kit) has limited turrets as is. Perhaps this one will need some drilling. Or maybe a new board.
No need. Just a little outside the box thinking. Hang the 2nd cap off the input to the Intensity pot.
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So after many hours trying to clever and shoehorning the circuit changes on to the existing turret board I gave up and laid out a PCB:
https://github.com/tristancollins/PrincetonReverbUpdate/tree/fab (https://github.com/tristancollins/PrincetonReverbUpdate/tree/fab)
They should arrive next week.
I have gone a bit overboard and used multiple ground plane zones to emulate multiple star grounding points. Just tried to keep things logical and compact - and all on one board.
Attached are an updated schematic, a layout (borrowing Rob Robinette's source file) and a quick render of the board. This should be fairly quick to assemble. All the sources are on GitHub in KiCAD 10. I'll be updating things as the build progresses (I'm expecting something to go wrong with the board - I did it very quickly) so at the end should be a verified PCB should others wish to use it.
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After spending many many hours troubleshooting hum on my build, I replaced the reverb recovery cathode bypass cap with a 220u. C9 on your schematic. Dead silent! (Thanks Stratomaster). Now I do that mod in all reverb circuits. The part is dirt cheap. Just add it to your order "in case" ;-)
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You left off the screen node filter cap in your schematic. You'll need that.
Edit:
On closer inspection I see you're adding several board mounted filter caps and splitting the load with a can cap. You do have screen and reservoir filtering, just in an unusual way.
You'll want to run the numbers again on the power rating (and voltage rating for that matter) on your bleeder. It's dissipating nearly 2 watts. Replace it with either a 3W or a 330k 1W. Check the voltage rating of any resistor you plan to use on HT. You'd be surprised how many commonly used ones aren't actually up to the task.
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Thanks - funnily enough I’ve been going through ratings just now. Most power rail ones are 3W, and the kit I’ve used uses mostly 1W elsewhere.
But worth doing a check and I’ll also update the schematic with ratings.
And yes, I’m making use of the cap can as a ‘mega cap’ as the returns of A, B and A’ (power valve, screens and LFO) all return to its shared cathode anyway. Rather than replace it.
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After spending many many hours troubleshooting hum on my build, I replaced the reverb recovery cathode bypass cap with a 220u. C9 on your schematic. Dead silent! (Thanks Stratomaster). Now I do that mod in all reverb circuits. The part is dirt cheap. Just add it to your order "in case" ;-)
I’ve a 100u in my stash of random parts ready and waiting!
I think there will be a few components tacked in for the 1st power on, to get a rough picture of the voltages. I was targeting the values on the original Fender schematic, but I’m also conscious the reissue (which people seem to like) varies a bit from those. As I have it drawn, I end up with a 1st stage B+ of 275 (E). The PI and 3rd stage + reverb recover at 290 (D) and the reverb driver at 340.
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It's coming together nicely. Populated the PCB in a little over an hour. Next will be doing the flying leads from PCB -> off board components. I've mostly tightened up the chassis parts. This will likely be a "once it's in, it's in" build...
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It came together nicely in the end. A few hiccups here and there mostly due to impatience, but it's working and sounding pretty good.
Last minute changes:
• removed reverb TX protections - didn't want to spend the time to get tag strips installed.
• removed the FET boost - added way too much noise, but could have been a wiring issue. Will revisit later.
• removed OT protection & didn't wire in extension jack - will add a smaller resistor at some point - the one I have is too big to fit nicely.
• didn't use shielded runs from input -> 1st stage grid or volume pot to 2nd stage grid
Non-obvious (to me) issues:
• my grounding scheme means the reverb tank needed modifying - the input side of the tank was left floating. All I had to do was solder the little jumper pads provided for this purpose. I just didn't realise without the help of Michael at Modulus Amplification... Obvious in hindsight.
• I can still hear some 'thumping' at higher trem settings, and that's with the diode added to the intensity pot. I'll add an audio example soon.
• there is quite a bit of hiss
• there's quite a bit of mains hum (there as soon as I flick the switch, not coming through the speaker, more mechanical I think?)
I've no other PR example to compare against, only my own amp builds and they have been very quiet with regards to hum and hiss - and they tend to be high gain.
[edit] Actually, it's not that bad at all. With fresh ears today, and a less critical state-of-mind, it's fine.
The one thing that does stick out is at high volume settings the onset of what I presume is blocking distortion on lower notes. I reduced the coupling caps and added the stopper to the PI, but there's still an ugly buzz behind the note.
Voltages:
HT: 450
Screens: 440
LFO: 440
Reverb driver: 346
Reverb recovery, PI, Stg 3: 289
Stages 1&2: 262
Biased to 40mA (41.7V at the grids), so a little cold. Will up this to 45mA to get closer to 70%.
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So I've used the amp for a jam session in a pub. It worked well and sounded great.
I did 2 things prior:
- biased to ~70%
- replaced the wire runs to the 1st and 2nd stages with shielded cable (I was being lazy 1st time round)
The latter definitely reduced noise - I can't tell if it's on now. In the pub, with the volume on 10, it was still silent. I'm happy with that. Only the tiniest amount of hiss with the reverb up full too.
I'd consider the PCB tested and validated. If anyone wants one of the remaining boards, just let me know.
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Great build - looks very professional. I've been building a modified AA1164 as well, and have done a number of the same mods, though I'm doing a simplified version with 1-tube reverb and no tremolo (very close to what Lectroid recently built). I definitely picked up some useful info from this thread.
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When I moved the reverb to the unused node I got a problem with the tremolo. It wont run sometimes and then will randomly start up again. Not sure why. In the original location I didn't have the problem. Anyone else experience this?
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When I moved the reverb to the unused node I got a problem with the tremolo. It wont run sometimes and then will randomly start up again. Not sure why.
Tell us exactly how you moved the reverb to the unused node. Sounds like you may have also moved the tremolo too. The tremolo circuit needs to stay on node B.
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I left the tremolo on node B. I moved the wire only for the reverb to the new node. I thought maybe it was the tube so I switched it and it seemed to work ok for a while but now its back to randomly going on and off. When its on it usually stays on though.
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Replace the tremolo tube cathode RC with a red LED as shown on the latest Selmer Zodiac schematic I posted. The tremolo signal will get much stronger and prevent dropouts.
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thats the 3.3k and 25uf connected to pin 3?
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Is there a way to fix oscillation in the princeton reverb without putting in grid stop resistors on the power tubes. I tried it and it worked but I like the amp better without them. Perhaps a snubber cap like they did on the bassman. not sure if its the same.
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Replace the tremolo tube cathode RC with a red LED as shown on the latest Selmer Zodiac schematic I posted. The tremolo signal will get much stronger and prevent dropouts.
I had upgraded the power and output transformer to the deluxe in my princeton. Would that make the tremolo not work right?
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Is there a way to fix oscillation in the princeton reverb without putting in grid stop resistors on the power tubes. I tried it and it worked but I like the amp better without them. Perhaps a snubber cap like they did on the bassman. not sure if its the same.
Did you identify where the positive feedback loop started, eg which circuit stages affected the oscillation, what control settings affected it?
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Does it make sense to be on the volume? I seem to notice oscillation in the volume and possibly when I turn the reverb all the way off.
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Here's a quick recording of the amp. The cleans are glorious. The distorted sounds I'm still struggling with. Help lift me out of the 'hyper critical hole' I'm in.
The attached mp3 is zipped to be able to upload it here.
I can hear a 'raspy' overtone on distorted notes that, to me, is a bit much. Could this be blocking somewhere? The cathodyne grid resistor needs increased from 470k to 1meg? Or bigger grid resistors on the 6V6s?
The amp on 7/10, with a tele with high output pickups, boosted with a clean boost set fairly high (2 or 3 o'clock). So I'm slamming the 1st stage to highlight what I'm hearing. I can also hear it just cranking the amp (guitar -> amp). Also, the raspy note seems to fluctuate with the tremolo setting.
I'm recording via a Boss TAE straight to audacity - the sound in the room is very close.
Am I just listening too closely and this would be inaudible playing in a mix?
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I don't know what the cause of the "raspy" distortion might be, and I do hear some beating in the overtones. I think you're right to go after it.
Having worked with this circuit several times, my personal approach would be to scope your signal through the path to see where your problem originates. This approach can take some time, but it does work.
An alternative easy thing to try would be working your way down the gain chain by subbing in various tubes into V1. 5751<12AT7<12AY7. (If you get down to a 12AU7, you really have problems! But, you won't. . .) It's easy to get rid of distortion; the hard part is to preserve the tone and your desirable harmonics.
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I don't know what the cause of the "raspy" distortion might be, and I do hear some beating in the overtones. I think you're right to go after it.
Having worked with this circuit several times, my personal approach would be to scope your signal through the path to see where your problem originates. This approach can take some time, but it does work.
Thanks - would you use a sine wave (or triangle?) to spot the extra ‘note’? Any ideas what that might look like on the trace?
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anyone ever added treble and bass controls for the reverb?
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anyone ever added treble and bass controls for the reverb?
I've never heard of anyone adding that level of tone control to a reverb circuit in an amp or outboard unit. The most I know of is the single tone control in the Fender 6G15, which I suppose you could get close to by adding it in before the reverb mix pot.
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would you use a sine wave (or triangle?) to spot the extra ‘note’? Any ideas what that might look like on the trace?
Sine wave every day for this problem. You're looking for unexpected waveforms at first, primarily to identify where the problem lies. But I would run your FFT function after each triode -- even if the waveform looks perfect -- to look for the introduction of weird harmonics or oscillation that gets amplified later in the signal path.
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The raspy distortion could be due to the cathodyne phase inverter. Have you read Merlin's page on that? https://www.valvewizard.co.uk/cathodyne.html - he notes that the cathodyne phase inverter can lead to unpleasant distortion.
So you might try upping the grid resistor to 1M like you were thinking and see how that goes.
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Here's a quick recording of the amp. The cleans are glorious. The distorted sounds I'm still struggling with. Help lift me out of the 'hyper critical hole' I'm in.
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I can hear a 'raspy' overtone on distorted notes that, to me, is a bit much. Could this be blocking somewhere? ...
The raspy distortion could be due to the cathodyne phase inverter. ...
I will offer that you're "turning up to distortion" but "concerned about rasp"...
I point that out to say "maybe don't worry so much" but I also acknowledge I heard what you heard, and I have perhaps been too lazy to chase it down in the past (mainly because I was playing real 1960s Princeton and Princeton Reverb amps, and I wasn't about to be modifying them). So:
- Yes, the cathodyne/split-load inverter is known to have some technical issues that lead to a harsher distortion.
- You can leverage Merlin's suggestions to tame the cathode/split-load inverter (I personally have not tried them yet, but very likely will add them to an upcoming Princeton Reverb build).
- Yes, grid-blocking at the output tubes could be an element of the sound/rasp.
- Consider the "arc-protection diode" in Merlin's cathodyne fix:
The 1N4007 conducts if the Grid is ~0.6-0.7v more-positive than the Cathode.
When a tube-Grid is more-positive than its Cathode, the tube acts like a Diode, and also draws grid current from the preceding circuit.
The sudden-onset of grid-current changes the apparent-load to the preceding stage from ∞Ω to maybe-couple-kΩ (!)
This change from "no load" to "heavy load" clamps the output-voltage of the preceding stage and contributes to "buzz/rasp."
- So the "arc-protection diode" prevents the Grid (of the inverter in Merlin's usage) from going much-positive of the Cathode.
"Grid Blocking" is a reaction to grid-current of a tube, which then charges the coupling cap preceding that tube's grid, and shifts the bias of that tube more-negative.
- You could apply a pair of diodes across the output tube "bias feed resistors" (that are 220kΩ from the Intensity pot wiper to the 6V6 grids in the Princeton Reverb (https://www.18watt.com/storage/18-watter_buzz_info_311.pdf)). They would be a Zener diode (making up most of the clamp-voltage) and a 1N4007 (adding another ~0.7v to the clamp-voltage).
We are mimicking the "Paul Ruby mod" to eliminate buzz (https://www.18watt.com/storage/18-watter_buzz_info_311.pdf). Scroll to Page 7 of the linked PDF for a schematic-view of the change. The example amp has a bias of "Cathode 7.5v positive of the Grid" which we mentally equate to "Grid at -7.5v relative to the Cathode." The Zener diode (9.1v) and the 1N4007 (0.7v) conduct when the Grid goes negative of the cathode by 9.1v + 0.7v = 9.8v.
This total (9.8v) is a bigger-swing in the negative direction than the original Bias Voltage (7.5v) implies we should need... except that the people actually testing this idea noticed "a little grid-shift" due to grid-current when the grid is driven positive of the cathode is okay, and they were also cont=strained by available Zener-diode-voltages.
How you gonna apply this to a Princeton??
- You bias up your output tubes using whatever method you prefer.
- Measure the 6V6 Grid Volts with the tremolo off. If your 6V6s operate near data-sheet values, you're probably not too far off from the "-34v" shown on the schematic (if your 6V6 plate & screen voltages are also near-400v as shown on the schematic).
- You would pick "Zener(s) plus 1N4007" that yield something like 35-38v at conduction. A 400v power supply with a 100kΩ (plate- or cathode-) load resistor implies current cannot exceed 400v / 100kΩ = 4mA, and 40v x 4mA = 160 milliwatts, so our Zener doesn't need an especially high power-handling (the Zeners in the example are 500mW devices).
- If your output tube bias allows, maybe you use 36v 1N5258B (https://www.digikey.com/en/products/detail/vishay-general-semiconductor-diodes-division/1N5258B-TR/4810521) in-series with a 1N4007 (and this 1N4007 may not be needed). Normal voltage-clamping happens on the positive-going side of the input signal due to 6V6 grid current when the grid is driven positive of the cathode. The diode(s) across the 220kΩ bias feed resistors provide voltage-clamping on the negative-going side of the 6V6 input signal, which prevents excessive bias-shift that creates crossover distortion and buzz/rasp.
It's worth a shot! And if you report it works in your amp, then I know I should use it in my Princeton reverb build!! :icon_biggrin: :icon_biggrin:
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A Possible Problem: Deep bias-vary Trem while playing clean.
Right after posting I thought, "Bias-vary trem when playing clean might try to push the grids far-negative. What will the Zener do to that signal?"
I don't know. I would want to try it out and find out. But I also think about the possible need of a DPDT switch to lift one side of each of the Zeners to disconnect them if they negatively impact the trem-sound when played cleanly.
There is also an interesting effect that may happen (or be disrupted) when playing a bias-vary trem amp loud & into distortion: when the loudness fades, then trem becomes more intense (https://youtu.be/crjP8RcQkKs?si=ilF8yobifTy8VLlE&t=34). I do not know whether the Zeners would monkey this effect as well (assuming you even use it, as it is probably easier to use in the pedal demonstrated in the link).
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Many thanks all - so I think I have a plan of action:
• inject sine wave and scope until I ‘see’ something at the speaker out (and hopefully hear it - I’ll use my Boss TAE with headphones and a volume control)
• probe each stage to see where it starts
• make some circuit changes.
I’ll go easiest -> hardest. Or most obvious -> brute force. So, depending on what the scope shows, something from the below:
• swap 470k -> 1Meg on the cathodyne grid
• raise the grid stopper value on the 6V6s to 10k (or more)
• reduce the coupling caps between the PI and 6V6s further from 47n -> 22n
• clip in a 470pF cap across plate load resistors to bleed some HF (if that’s a cause) then go down in size until it stops.
• desolder one end of the diode I placed across the intensity pot (maybe that’s the cause / I have it in backwards?)
I’m not sure on the zeners on the 6V6 grids given the bias wiggle - but could be something to try. I did something similar years ago in a SE build as an experiment.
I’ll report back.
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Checked my princeton today and it works good, no oscillation except for the reverb control. i seem to get oscillation, a swooshing sound right when i turn the reverb off and when i turn it to max. i have a newer accutronics tank, could it just be a bad tank?
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• swap 470k -> 1Meg on the cathodyne grid
• raise the grid stopper value on the 6V6s to 10k (or more)
• reduce the coupling caps between the PI and 6V6s further from 47n -> 22n
• clip in a 470pF cap across plate load resistors to bleed some HF (if that’s a cause) then go down in size until it stops.
• desolder one end of the diode I placed across the intensity pot (maybe that’s the cause / I have it in backwards?)
1st two possible tweaks done as I had a few mins spare -- no difference to the raspy tone, and if anything it's made the onset of it more abrupt (but that could be all in my mind).
Another thing occurred to me - I've a B+ of 450V, screens at 440V and biasing the pair to 40mA. Which should be ~70% Pdiss (I'm conscious that the load line spends most of it's time above the Pdiss line, but good enough for rock and roll). Maybe I'm hearing crossover distortion? Coupled with the breakdown in GFB? I'd expect the raspy noise to pulse with the bias wiggle trem, if I have that right?
I've not broken out the scope yet, which I hope will reveal a lot.
So I'll try lowering the bias a bit and take a look.
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I've not broken out the scope yet, which I hope will reveal a lot.
I had the chance earlier to do a basic output scope. The PDF attached has plots & FFTs at various gain settings;
• 1W (4/10 on vol)
• onset of clipping / kink (6-7)
• shark fin (8)
• up full
• input burst to look at blocking
There's a 'side kink' that moves up and down the wave with volume setting, as well as 'almost a nipple' forming on the lower lobe (I didn't get a trace of that but if I'd boosted the input a bit more may have appeared).
I confess to throwing this at AI - one suggestion amongst others was OT saturation.
To some extent I'm reassured that it's not blocking - that plot looks just like Merlin's example of when blocking is minimized.
Would be great to get additional interpretations.
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Can you temporarily solder a 3.3k resistor in parallel with the 1.5k cathode resistor on your cathodyne?
There's a thread on TDPRI where the role of this resistor was investigated thoroughly. The conclusion was that the 5E3 value of 1k gave a much better PI balance than the 1.5k used on the 1164.
I wonder if that'll make much of a difference for you.
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have you tried that mod strato?
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Yes. I run a 1M input resistor and a 1.2k bias resistor. Its a much smoother overall sound to my ear, but I don't have an A/B comparison.
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a 'side kink' that moves up and down the wave with volume setting
those can be "transients", an AC signal riding on your fundamental Frequency
does your scope have a FFT/spectrometer mode?
IF so;
look for a spike that comes n goes, usually it's a harmonic of the fundamental you're using for testing
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Yes. I run a 1M input resistor and a 1.2k bias resistor. Its a much smoother overall sound to my ear, but I don't have an A/B comparison.
do you have a pi grid stop resistor?
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Can you temporarily solder a 3.3k resistor in parallel with the 1.5k cathode resistor on your cathodyne?
There's a thread on TDPRI where the role of this resistor was investigated thoroughly. The conclusion was that the 5E3 value of 1k gave a much better PI balance than the 1.5k used on the 1164.
Thanks - I think I've just found that thread. In the spirit of trying to alter the bias point, I quickly swapped in a 12AT7. Messes up the tremolo but the distortion / fuzz is still there. I'll have a think about what that could mean. Next time I've the chassis out I'll clip in the 3.3k and report back.
a 'side kink' that moves up and down the wave with volume setting
those can be "transients", an AC signal riding on your fundamental Frequency
does your scope have a FFT/spectrometer mode?
IF so; look for a spike that comes n goes, usually it's a harmonic of the fundamental you're using for testing
The FFTs are in the PDF along with some harmonic values. I've pulled the graph out (attached).
I also spotted another thread on here from a few years ago. Still wading through it, but it was suggested issues with the OT could explain things like I'm hearing. I may have a spare OT so temporarily hooking that up may be another option to try.
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looks like there's a low freq oscillation riding on your fundamental or something is breaking down with signal load. since it's affected by volume, it's probably creeping in before the PA section.
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looks like there's a low freq oscillation riding on your fundamental or something is breaking down with signal load. since it's affected by volume, it's probably creeping in before the PA section.
Interesting - I'll check the cap can, wiring around it and do a bit of chopsticking. Could the oscillation be cancelled out by the push-pull output until it starts to break up, when the feedback collapses and the rogue signal appears?
I'm using the standard JJ cap, and would hope 40uF would be enough filtering. But another thing I could do would be to clip in another big cap to see what happens.
Thanks
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Some progress. I pulled out the chassis to get a 4 channel scope on it. In doing so I didn't bother connecting the reverb tank. "Hmmm, that's strange, why's the trace changed from yesterday..."
Then I realised that probing the reverb driver grid(s) could make the waveform change. A lot of playing about made me realise without the reverb tank connected the distorted tone was great. Plug it in and I get this high 'hash' behind the notes. I also could see some ringing on the reverb send.
Great - a line of inquiry. So I added some capacitance from the grid to ground. That seemed to help a bit. Doing some googling suggested adding some stoppers - so I put a 1k from the PCB to the 1st grid, then bridged to the 2nd grid with a 470ohm.
That 'seemed' to partially resolve it. However, putting everything back in the cabinet and listening just now the overtone is still there... Darn it.
The traces captured suggested I've had an impact but perhaps not enough? Channel 1 is input, 2 is speaker output (resistive load), 3 is reverb out, 4 is reverb return. State 2 is just as the ringing appears and clipping about to be seen. 3 is clipping, 4 is amp up full. A very different picture to the previous traces, so something has changed.
This post was useful: https://www.tdpri.com/threads/silverface-princeton-reverb-reverb-related-distortion.1171963/
He seemed to sort it by moving the 10pF cap a little bit.
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Recent Fender amps use a 10k grid stopper at the socket of the reverb driver. You can try this.
I've also seen 560pF caps from plate to cathode on certain reverb driver tubes.
An overlooked place for tweaking the hash out of Fender reverb is the recovery triode plate resistor. A fairly aggressive 470pF cap can be used without losing much of the useful reverb information due to the mid-focused and bandwidth limited nature of the circuit.
You can also temporarily install a dwell pot to see if attenuating the input to the reverb driver kills the hash. If so, then warming the bias or slightly raising the plate voltage (or a little of both) may be in order.
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Recent Fender amps use a 10k grid stopper at the socket of the reverb driver. You can try this.
I've also seen 560pF caps from plate to cathode on certain reverb driver tubes.
An overlooked place for tweaking the hash out of Fender reverb is the recovery triode plate resistor. A fairly aggressive 470pF cap can be used without losing much of the useful reverb information due to the mid-focused and bandwidth limited nature of the circuit.
You can also temporarily install a dwell pot to see if attenuating the input to the reverb driver kills the hash. If so, then warming the bias or slightly raising the plate voltage (or a little of both) may be in order.
Thanks - this is really useful and gives me a few things to try.
With fresh ears this morning I gave another listen. Still there as before. However, unplugging the tank (something I hadn't done before yesterday) removes the annoying tone almost entirely. So it gives me a sole area to focus on.
Annoyingly, I had planned on putting a 100pF 2kV cap across the reverb transformer primary (I assume the equivalent of plate to cathode?) and a 1k across the secondary following Merlin's book. But got lazy when the layout would get messy and left them off... So:
1) I'll try plate to cathode with the cap I have.
2) add in the 1k at the reverb send
The driver is running at ~343V B+, with 6.2V at the cathode - so 4.2mA going through both triodes. A quick look at the tube curves shows this is pretty cold (and/or a weak JJ tube from the looks of it). Maybe too cold?
3) clip in more capacitance across the recovery plate resistor
4) clip in another resistor across the 1.5k Rk? Get it down to 1k perhaps?
5) Increase the stopper(s) to 10k (and the 'bridge' stopper too, if appropriate)
Will report back.
[EDIT] thinking about it and looking at the later 019320 schematic - could this all be due to the driver being biased too cold? c.v. the ~ 3.2V on the later version, 6 is close to cut off. If I'm feeding it a hot signal that could be causing something to 'leak' back in to the clean signal line? Hence hearing the rattiness when the reverb is connected but turned to a minimum.
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How hot of a signal? If your RMS AC voltage at the driver tube grid is ≈70% of the cathode voltage you could have grid clamping and that would affect the dry signal.
The part that has me scratching my head is that disconnecting the tank eliminates the problem. This would imply it's a problem that is fed through the tank and mixed back into the signal through the recovery stage OR that the effect of eliminating the load of the tank changes the overall plate load in a favorable way to reduce grid clamping. I'm leaning towards the latter.
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How hot of a signal?
I'll try and take a reading tomorrow.
The part that has me scratching my head is that disconnecting the tank eliminates the problem. This would imply it's a problem that is fed through the tank and mixed back into the signal through the recovery stage OR that the effect of eliminating the load of the tank changes the overall plate load in a favorable way to reduce grid clamping. I'm leaning towards the latter.
Agreed - that is odd. Unless it's parasitic coupling to a close-by wire? Or something silly like having the reverb transformer oriented the 'wrong' way round? Or wires reversed?
I'm wondering what order I do the experiments in. I've limited time tomorrow to do some play testing, figuring I can always scope things properly later on when I have more time to investigate and tidy up any bodges.
1) clip in another 1.5k across the 1.5k Rk to get closer to the 680r value I see on other schematics
2) clip in plate to cathode 100p (or higher - 560p)
3) clip in the 1k at the reverb send (protection - likely no difference to this case unless it's impacting plate loading when unplugged. Could be red herring.)
4) clip in more capacitance across the recovery plate resistor
5) Increase the stopper(s) to 10k (and the 'bridge' stopper too, if appropriate. A bigger job - one for later)
I could go one by one until it's, hopefully, resolved, or it could be a sum of a few of them. Who knows...
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I think the most useful test isn't listed. Add a Dwell control temporarily to see if you can induce/cure the effect by attenuating the input to the driver.
Also, I don't understand why you have a "bridge" grid stopper.
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Good shout - I’d need my iron out for that. Perhaps I can do the opposite - add capacitance to let more signal in and see if the issue gets worse?
The 2 stoppers thing was from a google - there being 2 potential gain loops that could pick up interference, so belt and braces approach was to add a 2nd resistor. Overkill maybe, but also removed the length of wire from one grid to another. I can remove on the tidy up.
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I'm about to give up... Just to make sure I'm not chasing a figment, attached is a sound sample. Should I just settle for it?
Amp at 7/10, bit of reverb & trem, high output tele bridge pickup. A low E plucked with guitar volume up full, then down a bit to clean up, then back up again. Then some high notes with IMD. Some power chords. Recorded via Boss Waza TAE. Sound in the room is a bit more muffled and the rattiness more prominent at living room conversational volume.
Today I tried:
- a 12au7 in the reverb driver slot,
- changed the bias of the 12at7 (two 1.5k in //, 5.1V on cathode, 320V on plate),
- grounded out the reverb entirely,
- removed the tube,
- added in a cap to the reverb transformer primary and 1k to the 2ndary.
No difference. Which makes me think the reverb may not be the source.
Oh, I'm hitting the driver with 11.8Vrms.
I also tried various caps in places (plate resistors, e.g.), increasing existing cap values, and basically experimented seeing if I could filter off the tone I'm hearing. No joy.
I took the opportunity to solder in a 10k grid resistor to the reverb driver and the 3rd stage grid. Latest schematic attached.
Should I just call it done and play it?
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Hi guys, been following this thread with interest, a lot of great info here, thanks to everyone who contributed! I have recently completed a similar project to update my PR. I originally built it with the Hoffman board and layout probably 10 years ago. I had some issues with a horrible blocking distortion from the reverb driver at higher volumes along with a fair bit of hum. I also wanted to tidy up some of my old sloppy wiring and less-than-ideal grounding, so decided to do some updates as well with some of the mods in this thread.
The Hoffman layout was a bit easier to modify in terms of power supply and grounding, I added a couple extra power nodes with axial caps along the board, moved the reverb driver power to node C and split the grounding as discussed here among other things. Was a bit tricky in spots but managed to do it just using existing turrets.
End result is great, MUCH better noise floor overall, the reverb is silent, and the only hum is mechanical from the PT. As-built schematic is attached, it ended up very similar to what you have Tristan. This also fixed the horrible distortion I originally had which was much worse than what's in your recording, it sounds like what was described in the TDPRI thread you linked (https://www.tdpri.com/threads/silverface-princeton-reverb-reverb-related-distortion.1171963/).
That said, after all these mods I still have a buzzy distortion when cranked, the exact same as your most recent sound sample. The general consensus I've seen is that PRs are not great when cranked, and truly fixing the issue kinda turns it into a different amp...so I think I'll just leave it. Happy to play it clean and crank my 6v6 Plexi instead :icon_biggrin:.
Just my 2 cents. Big thanks to you for documenting your project so thoroughly, it has been a big help to me and I'm sure many others :thumbsup:
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tristanc - I don't think you need to give up yet. It might help if you did an audio file where you play it loud with the umpleasant distortion and then the same with the reverb tank unplugged, which you said removed it. Then we could really be able to understand what you are talking about. Also, play some chords, and tell us what guitar you are using and what pickup. I recently built a PR with 1-tube reverb and no trem - when I put that back together I will test it some more and compare to your sound files - when first testing that amp, the distortion wasn't very pleasing either, but it may have improved with some of the tweaks I did.
And on another note, I built a Princeton 6G2 a couple of years ago (my first tube amp build), and I didn't like the distortion on that either, but apparently I had biased it too low (calculating based on 12W for a 6v6GT instead of 14W), and when I upped it some, it sounded a lot better. Cold bias can produce unpleasing distortion for sure, so you might want to tweak yours a bit higher to test out.
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And on another note, I built a Princeton 6G2 a couple of years ago (my first tube amp build), and I didn't like the distortion on that either, but apparently I had biased it too low (calculating based on 12W for a 6v6GT instead of 14W), and when I upped it some, it sounded a lot better. Cold bias can produce unpleasing distortion for sure, so you might want to tweak yours a bit higher to test out.
This is a good point, I'll try this too. One downside I can foresee with a hotter bias is it might weaken the trem somewhat.
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Oh, I'm hitting the driver with 11.8Vrms.
This is wild. This number should be safely below the cathode voltage if you want to keep the driver stage happy.
The fact that you are getting an issue without the tube in place though tells me that you're hearing 2 different problems, though. And your most recent observations seem to contradict your prior reports. I think some is the cathodyne rattyness (a fixed bias arrangement or the "Stokes mod" may be worth a try) and the rest is grid clamping from slamming the driver stage.
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End result is great, MUCH better noise floor overall, the reverb is silent, and the only hum is mechanical from the PT. As-built schematic is attached, it ended up very similar to what you have Tristan. This also fixed the horrible distortion I originally had which was much worse than what's in your recording, it sounds like what was described in the TDPRI thread you linked (https://www.tdpri.com/threads/silverface-princeton-reverb-reverb-related-distortion.1171963/).
Thanks for the kind comments and also the comfort that what I have may be 'typical'
That said, after all these mods I still have a buzzy distortion when cranked, the exact same as your most recent sound sample. The general consensus I've seen is that PRs are not great when cranked, and truly fixing the issue kinda turns it into a different amp...so I think I'll just leave it. Happy to play it clean and crank my 6v6 Plexi instead :icon_biggrin:.
This point is key - perhaps this is just 'the sound' of a PR. I'm asking too much to cover both the fender cleans and higher gain power amp distortion. I have a 6V6 plexi (also with 2204 preamp) for higher gain or boosted front end. Stick to the clean + a pedal for OD.
The PR's sweet spot may just be a tad below things start clipping, where the compression kicks in, but not where this annoying buzz / rasp comes in. Other amps the conditions are different. Topologically, jumper out the reverb and turn off the trem, you have a simple amp that should distort nicely / in a way that is pleasing.
I've tried to find examples on youtube where people are playing straight in with the amp on 10. But they tend to have pedals / other things going on.
And on another note, I built a Princeton 6G2 a couple of years ago (my first tube amp build), and I didn't like the distortion on that either, but apparently I had biased it too low (calculating based on 12W for a 6v6GT instead of 14W), and when I upped it some, it sounded a lot better. Cold bias can produce unpleasing distortion for sure, so you might want to tweak yours a bit higher to test out.
That is something I could try - I am using JJ 6V6S tubes, so 14W. Worth experimenting with.
This is wild. This number should be safely below the cathode voltage if you want to keep the driver stage happy.
I think this is just a fact of the circuit. You turn it up and have a high output guitar you'll hit the reverb hard. But it _should_ take it, to a point. Or at least, this is a factor of the sound.
The fact that you are getting an issue without the tube in place though tells me that you're hearing 2 different problems, though. And your most recent observations seem to contradict your prior reports. I think some is the cathodyne rattyness (a fixed bias arrangement or the "Stokes mod" may be worth a try) and the rest is grid clamping from slamming the driver stage.
Yeah - I may have a couple of things going on. To add to things, it had a bit of a drop / rough handling this week and, unless I'm imagining it, the noise floor has increase - hearing a lot more shot noise.
I used the amp on Tuesday night, it sounded great but I didn't play with it much beyond 4/10. So before the onset of the issue.
I'll report back when I have time to experiment. Thanks again for the comments.
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This is wild. This number should be safely below the cathode voltage if you want to keep the driver stage happy.
I think this is just a fact of the circuit. You turn it up and have a high output guitar you'll hit the reverb hard. But it _should_ take it, to a point.
That IS the point. When the grid voltage exceeds the bias offset is when it can no longer take it.
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What stratomaster said.
The Fender reverb circuit was never designed to handle that much input voltage. Some form of "Dwell" control is needed to allow this clean amp handle dirty levels of signal.
Bright Idea!??? What if you replaced the Volume control with a dual pot. Use the second section wired in reverse to replace the 1 meg dwell resistor. That would automatically reduce the reverb driver input signal as the volume was increased. No, I have never tried it. Just trying to avoid adding another hole and knob to twiddle.
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The driver is running at ~343V B+, with 6.2V at the cathode
Oh, I'm hitting the driver with 11.8Vrms.
Stratomaster is right: the Driver's limit is 6.2v x √2 / 2 = 6.2v x 0.7071 = 4.38v RMS. The driver is certain to distort after that.
I think this is just a fact of the circuit. You turn it up and have a high output guitar you'll hit the reverb hard.
When you have 11.8v RMS to the Driver, how much voltage do you have at the Input Jack? Is it similar to the ~100mV a guitar pickup might deliver?
Or are you landing in a range of voltage like a heavily-boosted signal post-pedal-boost?
Normally, the Tone Stack and Volume control impose more than enough signal-loss that "high signal to the Driver" is not a big deal (though Reverb does get more-intense with high Volume settings, because there is more-input to the 12AT7 driver). If your use-case envisions slamming the snot out of the front-end with boosts (which wouldn't sound too great with a blackface Fender unless you retrace all of Randall Smith's steps, and turn it into a Boogie), then maybe ac427v is right & you need a Dwell control to dial-down the signal to the Reverb Driver tube.
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When you have 11.8v RMS to the Driver, how much voltage do you have at the Input Jack? Is it similar to the ~100mV a guitar pickup might deliver?
Or are you landing in a range of voltage like a heavily-boosted signal post-pedal-boost?
Normally, the Tone Stack and Volume control impose more than enough signal-loss that "high signal to the Driver" is not a big deal (though Reverb does get more-intense with high Volume settings, because there is more-input to the 12AT7 driver).
For reference, a stock 64 Super Reverb I serviced last week had about 8.4V on the driver cathodes and a signal of about 3.6Vrms on the grids at the onset of clipping at the speaker (maximum clean output). Input signal was about 120mV.
11V is just straight up punishment.
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If you haven't resolved the issue yet, it may be worth looking at the voltages on the reverb recovery and subsequent triodes. Going back to your schematic and your posted voltages, it looks like your voltage on the reverb recovery is way over the 160V on the Fender schematic, and the final gain stage and the PI are also at quite high voltages. So you may need to up those voltage dropping resistors.
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Recent Fender amps use a 10k grid stopper at the socket of the reverb driver. You can try this.
I've also seen 560pF caps from plate to cathode on certain reverb driver tubes.
An overlooked place for tweaking the hash out of Fender reverb is the recovery triode plate resistor. A fairly aggressive 470pF cap can be used without losing much of the useful reverb information due to the mid-focused and bandwidth limited nature of the circuit.
You can also temporarily install a dwell pot to see if attenuating the input to the reverb driver kills the hash. If so, then warming the bias or slightly raising the plate voltage (or a little of both) may be in order.
Does adding a 10k grid stop do anything to the tone or overdrive? could it be smaller then 10k?
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No. The input capacitance of a 12AT7, even in parallel, is such that you'd need 100k+ to begin to hear an effect. Plus this is the reverb circuit which is bandwidth limited, so maybe something like 330k would be ok before you hear it.
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why 10k? how do you determine the value. on the power tubes they use like 1.5k
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why 10k? how do you determine the value. on the power tubes they use like 1.5k
Read this: https://www.valvewizard.co.uk/gridstopper.html
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ok will read. will that stop oscillation in the reverb section? I get some when i turn the reverb pot.
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ok will read. will that stop oscillation in the reverb section? I get some when i turn the reverb pot.
Probably not. You likely have another problem. Could be any of a number of things. Is your amp also a Princeton Reverb style?
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yes. i have a new accutronics tank that i heard are not that good anymore. could that be it?
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also.
before i added grid stop resistors to the power tubes and that seemed to fix the problems. but i didnt like the effect on the overdrive so i removed them. it seemed to make the amp clean and then overdrive at the louder volume instead of having the overdrive mixed from the beginning.
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yes. i have a new accutronics tank that i heard are not that good anymore. could that be it?
It's possible the tank has an issue, or the problem is the placement. Make sure the connections are facing the speaker baffle. You can also try moving the tank outside of the amp temporarily to see if it's coupling to the speaker or cab. Sometimes the issue is the transducers being loose. A dab of E6000 or even neutral cure silicone can take care of that. Or you may need a padded reverb bag.
also.
before i added grid stop resistors to the power tubes and that seemed to fix the problems. but i didnt like the effect on the overdrive so i removed them. it seemed to make the amp clean and then overdrive at the louder volume instead of having the overdrive mixed from the beginning.
It's unlikely that you heard a real effect or you have some other issue. You can measure the output power at the onset of clipping with and without the grid stoppers and compare them to see if your ears were lying to you or not.
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not sure it seemed to be different. like i said the amp sounded more clean less hairy.
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i have a new accutronics tank that i heard are not that good anymore.
What do you mean not good, how so?
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An update from me - I've done little-to-no tweaking and just used the amp.
What I did do was listen to this video: https://youtu.be/CMps8xUBC2E?si=lwB2VXWQnwNyEN4e (https://youtu.be/CMps8xUBC2E?si=lwB2VXWQnwNyEN4e)
Specifically these sections where the amp is on 10.
Strat: https://youtu.be/CMps8xUBC2E?t=1811&si=eG0lHkTMcCMhaVDI (https://youtu.be/CMps8xUBC2E?t=1811&si=eG0lHkTMcCMhaVDI)
LP: https://youtu.be/CMps8xUBC2E?t=1918&si=oZ0681HI2L_hF9NB (https://youtu.be/CMps8xUBC2E?t=1918&si=oZ0681HI2L_hF9NB)
Tele: https://youtu.be/CMps8xUBC2E?t=2030&si=Sk4LZ4Rhrec5Qms9 (https://youtu.be/CMps8xUBC2E?t=2030&si=Sk4LZ4Rhrec5Qms9)
Playing along at home, living room volume, through my attenuator, I think I get a very similar sound / distortion characteristic. I'll try and record some parts to do a comparison. So perhaps this is just The Sound of the Princeton and I'm over thinking it. And maybe the sweet spot of the amp is at 5/10 and drive should come from somewhere else.
To that extent, I bought a BD-2 clone to add a bit of drive at lower volume and the combo sounds great - especially with the Celestion Neo Creamback speaker I swapped in.
One finding was that at 'playing with a drummer' volume (4/10) I had to turn the gain on the pedal way down, compared to playing at home through an attenuator - perhaps the speaker coming in to play.
I'm still loving the amp. Love the form factor, the cleans at the point that things start compressing, and the reverb / trem. I wish I had built some of my other amps in to this form factor - particularly the higher gain ones.
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i have a new accutronics tank that i heard are not that good anymore.
What do you mean not good, how so?
they say you get a weak reverb from it. they use a press on wire for the transducer i guess. not sure why but thats what i hear.
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You're unlikely to get the deep drip of an old Gibbs pan with a new one from any manufacturer. That's just the way things are. I don't futz with old broken reverb pans anymore. The juice is not worth the squeeze.
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You're unlikely to get the deep drip of an old Gibbs pan with a new one from any manufacturer. That's just the way things are. I don't futz with old broken reverb pans anymore. The juice is not worth the squeeze.
I agree with this statement. At least in my hands, I can't seem to get what I want out of a reverb unit built into my amp builds. It may well be my own lack of expertise, but I've given up on the idea of true old school reverb in favor of using pedals to enhance my tone.
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I couldn't leave it alone....
• Reflowed basically all the joints. No difference.
• Tried playing with the bias again, on the assumption that perhaps there is cross-over distortion. Potential improvement. Instead of aiming for 70% I went to 80% That _seems_ to have made the issue less noticeable.
• Modified the reverb mixing components - the 3Meg and 10pF - based on a forum post of someone describing a similar distortion issue. Potential improvement. I clipped in a 68pF across the 3Meg (so 78pF) and to my ear, the extra treble made things sound better at 10/10 and provided just a bit more bite at lower volumes. The extra bite can be taken away with the treble control. It may be masking the harsh distortion, though.
• Clipping in a 1Meg in parallel just made things louder and the reverb not so good.
So I'm going to leave it biased hot for a few days and see how I get on. Next up, replacing the 10pF with a 68pF - may have to simulate that to see the implication...