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Thread: AX84 P1x Build - "Stiff" Sounding Output

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    AX84 P1x Build - "Stiff" Sounding Output

    Hi all,

    Many years ago I built an AX84 p1x amp that uses the single octal output tube. I've since rebuilt the preamp section of the amp several times. I've tried the original design, and then a higher gain preamp and now have rebuilt it again using the front end of the 1974x / 18W design.

    Everything in the preamp section sounds great. I've got the right flavor/sound of the 1974x, but the output feels very "stiff" or almost solid state like to me. I've had this same kind of sound with every preamp I've tried on this thing.

    I'm wondering where I can improve the design. Should I swap the filter caps to lower values (they are currently all 47uF)? Should I change the OT (currently using the Hammond 125ESE)? I'm willing to spend more cash on an OT if it makes a world of difference. Not sure if I've ever been a huge fan of Hammond OTs in general.

    I would appreciate any advice or circuit/component tweaks anyone could recommend to make this thing sound a bit better! I would prefer to not change to a tube rectifier if possible.

    Link to the schematic on Archive.org:
    https://web.archive.org/web/20160531...P1x_101004.pdf

    Thanks!

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    Does it have a tube rectifier? If not, why not just add a sag resistor after the rectifiers, simple and effective.
    Edit: I see there is already a 100R between the rectifiers and B+1, you could try adding another between there and the OT, or try increasing the value of the 100R, see what you like.

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    Quote Originally Posted by g1 View Post
    Does it have a tube rectifier? If not, why not just add a sag resistor after the rectifiers, simple and effective.
    Schematic shows a 100 Ohm sag resistor. But it won't have much dynamic effect with a class A output stage as average power/current consumption is more or less constant.

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    Last edited by Helmholtz; 11-18-2019 at 11:36 PM.
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    I'd knock those 47uF caps down to 22uF.

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    Perhaps it's the EL34? You could try a 6V6 without having to do too many mods. Increase the pi filter resistor R2 to 1k? to reduce B+ voltage and keep the 6V6 dissipation to less than 14W and set the OT impedance to 5k.

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    Quote Originally Posted by loudthud View Post
    I'd knock those 47uF caps down to 22uF.
    Single ended amps require better plate and screen supply filtering than PP amps.

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    Bent Member Chuck H's Avatar
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    Once again Helmholtz in onto the real problem. It's a class A, single ended output. It's the nature of the design to NOT sag. No amount of sag resistors or tube rectification can change the fact that the single power tube has to operate continually somewhat near the max of it's useful current for conducting audio. The same for soft, quiet playing as for hard, loud playing. Which means there's no where to sag to, basically. All the audio current the power amp will use is already being used when it's just sitting there amplifying nothing. In fact it's not uncommon for current to decrease with signal conduction in class A, single ended amps. I think the guitar player community at large has the opposite expectation of "class A" relative to the media hype.

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    Thanks for all of your replies, they are much appreciated!

    I suppose changing the output tube to a 6V6 could be the best option here. I did try the amp with one, but didn't make the necessary changes to the circuit for it to be biased properly. It sounded better but not perfect. I've built other single ended amps (more of a Champ style circuit) and didn't really have this issue. Maybe I'll take some notes from the power supply in that amp.

    I'll take Dave H's advice and make those circuit changes soon.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by bw1985 View Post
    I've built other single ended amps (more of a Champ style circuit) and didn't really have this issue. Maybe I'll take some notes from the power supply in that amp.
    Perhaps there were properties other than "sag" that were giving you the favorable results you're after then. Because the situation is very much as Helmholtz said. You may be chasing the wrong solution because class A, single ended power amps don't really sag.

    Can you provide schematics? One for a single ended amp you like the behavior of and then this one which you don't. We may be able to identify some other parameter that is overtly different about them.

    My guess (only a guess) is that it may have something to do with how much drive you have to the power tube in either deign. A single ended amp will clip more asymmetrical with more drive. Perhaps if you decreased preamp clipping and increase power tube drive for more asymmetry would put you in the zone? So don't be fooled into thinking the 6V6 is offering more "sag" if it works out. It could just be that the 6V6 requires more grid drive to reach the same playing volume levels as the EL34 does now.

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    Last edited by Chuck H; 11-19-2019 at 04:04 PM.
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    Quote Originally Posted by Chuck H View Post
    Perhaps there were properties other than "sag" that were giving you the favorable results you're after then. Because the situation is very much as Helmholtz said. You may be chasing the wrong solution because class A, single ended power amps don't really sag.

    Can you provide schematics? One for a single ended amp you like the behavior of and then this one which you don't. We may be able to identify some other parameter that is overtly different about them.

    My guess (only a guess) is that it may have something to do with how much drive you have to the power tube in either deign. A single ended amp will clip more asymmetrical with more drive. Perhaps if you decreased preamp clipping and increase power tube drive for more asymmetry would put you in the zone? So don't be fooled into thinking the 6V6 is offering more "sag" if it works out. It could just be that the 6V6 requires more grid drive to reach the same playing volume levels as the EL34 does now.
    The amp that I built was based on this old Valco schematic. It's similar to a Champ I believe but has tons of gain. I built it using an old Bogen CHA-10 that I had sitting around. I am however using a solid state rectifier as I found it worked well for this amp.

    I also swapped the 6V6 for a 6L6 in that amp and it sounded awesome because it actually tamed the gain levels a bit and gave me slight headroom improvement.

    Click image for larger version. 

Name:	valco_gretsch6150_767w.jpg 
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    I've experimented with sag in SE amps and the results can be quite good but adds to the complexity. The idea came from an old Gibson amp schematic and I implemented it with a vactrol driven off the OT secondary, just like an optical compressor. There are a number of issues with this simple approach, the main one being that there's a sweet-spot for operation due to the rather narrow operating voltage for the LED side of the vactrol. It needs a rectified voltage that's trimmed to give the turn-on, and voltage limiting so as not to destroy it (and set the upper operating level), though in low-powered amps this is perhaps less likely. In practice I could get a really nice sag at a particular volume level, but as soon as this was changed the sag circuit needed adjusting to find the new sweet spot. At the time I though it would have some merit in that the sag could be made more or less extreme by adjusting the time factor of the LED circuit. At the most extreme the entire amp behaved like a compressor set to maximum squash.

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    Bent Member Chuck H's Avatar
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    Wow! That's as simple as it gets, huh! It's using grid leak bias for stage one and no cathode bypass on stage two, but still, not much loss with the tone control and basically a shit ton of drive to the power tube grid with what amounts to two series 12ax7 stages.

    You mention having changed the preamp in the current project a few times. To be honest I don't know what you may have going for the preamp in the amp in question so I can't comment on what might be mitigating grid drive there. BUT...

    Notice that in the schematic you provided that the screen node is fed through a whopping 6.8k and that may have an affect on how the screen grid behaves even in a single ended amp. I've read that under clipping conditions that this can create a sort of sag in the form of current transfer between the plate and screen circuits, but understanding this is beyond my pay grade Maybe someone else will chime in on this.

    So in the amp you like there pretty much a straight shot and high grid drive to a power tube which has the screen fed through a higher than typical resistance. This is probably different from the amp in question (no schematic).

    Probably also worth noting is that in the amp you like the power tube response of doesn't have a bypass cap on the power tube cathode circuit.

    You might try bumping the screen node resistance in the power supply (and reduce the preamp node resistance so you don't lose the preamp tone you have) and then lift the bypass cap from the power tube cathode. See if that makes a difference for you.

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    Quote Originally Posted by Chuck H View Post
    Wow! That's as simple as it gets, huh! It's using grid leak bias for stage one and no cathode bypass on stage two, but still, not much loss with the tone control and basically a shit ton of drive to the power tube grid with what amounts to two series 12ax7 stages.

    You mention having changed the preamp in the current project a few times. To be honest I don't know what you may have going for the preamp in the amp in question so I can't comment on what might be mitigating grid drive there. BUT...

    Notice that in the schematic you provided that the screen node is fed through a whopping 6.8k and that may have an affect on how the screen grid behaves even in a single ended amp. I've read that under clipping conditions that this can create a sort of sag in the form of current transfer between the plate and screen circuits, but understanding this is beyond my pay grade Maybe someone else will chime in on this.

    So in the amp you like there pretty much a straight shot and high grid drive to a power tube which has the screen fed through a higher than typical resistance. This is probably different from the amp in question (no schematic).

    Probably also worth noting is that in the amp you like the power tube response of doesn't have a bypass cap on the power tube cathode circuit.

    You might try bumping the screen node resistance in the power supply (and reduce the preamp node resistance so you don't lose the preamp tone you have) and then lift the bypass cap from the power tube cathode. See if that makes a difference for you.
    Thanks for the reply and the information. There is a lot to think about here, and definitely a few changes I could make. For reference, here is the schematic of the preamp I am using:
    Click image for larger version. 

Name:	18watt_SE_Nov-9.jpg 
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    However, I am using a 220k resistor to ground instead of 470k after the 0.01uF coupling cap.

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    Quote Originally Posted by Mick Bailey View Post
    I've experimented with sag in SE amps and the results can be quite good but adds to the complexity. The idea came from an old Gibson amp schematic and I implemented it with a vactrol driven off the OT secondary, just like an optical compressor. There are a number of issues with this simple approach, the main one being that there's a sweet-spot for operation due to the rather narrow operating voltage for the LED side of the vactrol. It needs a rectified voltage that's trimmed to give the turn-on, and voltage limiting so as not to destroy it (and set the upper operating level), though in low-powered amps this is perhaps less likely. In practice I could get a really nice sag at a particular volume level, but as soon as this was changed the sag circuit needed adjusting to find the new sweet spot. At the time I though it would have some merit in that the sag could be made more or less extreme by adjusting the time factor of the LED circuit. At the most extreme the entire amp behaved like a compressor set to maximum squash.
    I wish I could like that twice! I've never considered that...... and it's brilliant!

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Mick Bailey View Post
    I've experimented with sag in SE amps and the results can be quite good but adds to the complexity. The idea came from an old Gibson amp schematic and I implemented it with a vactrol driven off the OT secondary, just like an optical compressor. There are a number of issues with this simple approach, the main one being that there's a sweet-spot for operation due to the rather narrow operating voltage for the LED side of the vactrol. It needs a rectified voltage that's trimmed to give the turn-on, and voltage limiting so as not to destroy it (and set the upper operating level), though in low-powered amps this is perhaps less likely. In practice I could get a really nice sag at a particular volume level, but as soon as this was changed the sag circuit needed adjusting to find the new sweet spot. At the time I though it would have some merit in that the sag could be made more or less extreme by adjusting the time factor of the LED circuit. At the most extreme the entire amp behaved like a compressor set to maximum squash.
    Love it! I've contemplated a way to get controllable compression (of a desirable nature hopefully) out of an overall design without the need to build a "compressor" into the circuit. Since you seem to be "on the path" as it were, my idea was to use a voltage divider off the top of the power tube cathode circuit (in a cathode biased amp with a full AC bypass) to feed voltage to an earlier preamp cathode (also AC bypassed). That way when voltage rises at the power tube cathode the drive signal is analogously reduced from the preamp. I haven't tried it because I'm doing other tricks with the cathode circuits in my cathode biased amps. But here it is for the world to see now. In the "public domain" as it were now. As is your idea. Just report back if you try it I'd love to know how it works out.

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    Last edited by Chuck H; 11-20-2019 at 03:04 AM.
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    Quote Originally Posted by bw1985 View Post
    Hi all,

    Many years ago I built an AX84 p1x amp that uses the single octal output tube. I've since rebuilt the preamp section of the amp several times. I've tried the original design, and then a higher gain preamp and now have rebuilt it again using the front end of the 1974x / 18W design.

    Everything in the preamp section sounds great. I've got the right flavor/sound of the 1974x, but the output feels very "stiff" or almost solid state like to me. I've had this same kind of sound with every preamp I've tried on this thing.

    I'm wondering where I can improve the design. Should I swap the filter caps to lower values (they are currently all 47uF)? Should I change the OT (currently using the Hammond 125ESE)? I'm willing to spend more cash on an OT if it makes a world of difference. Not sure if I've ever been a huge fan of Hammond OTs in general.

    I would appreciate any advice or circuit/component tweaks anyone could recommend to make this thing sound a bit better! I would prefer to not change to a tube rectifier if possible.

    Link to the schematic on Archive.org:
    https://web.archive.org/web/20160531...P1x_101004.pdf

    Thanks!
    Might I also suggest trying a larger value for the power tube screen resistor (or add one if none present right at pin #4) ? I did this recently on a Hi-Fi Zenith to a guitar amp conversion, and I am super happy I did. The amp I modded is a push pull 6V6, and not the same as your single ended gem, but if you "crush" the screen voltage using a large ohm value resistor, say 2.7k ohms or more, you will add some compression by pushing the screen voltage lower on the power peaks, and perhaps it will soften the stiff quality for any tube you want to use.

    By the way I also used a 6L6 in my Gretsch 6150, and at first I liked it, but grew tired of the increased clarity LOL, and went back to the more abrasive sound the 6V6 was giving.

    I will try adding a large series screen resistor to my 6150 amp, now that the idea is in my head !

    Just another thought out of the Ether.

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    Last edited by HaroldBrooks; 11-20-2019 at 05:01 PM.

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    Might I also suggest trying a larger value for the power tube screen resistor ?
    Good idea!

    Screen stoppers produce some automatic screen compression at higher output level.
    It's an instantaneous compression that causes soft clipping and distortion.

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    Last edited by Helmholtz; 11-20-2019 at 06:52 PM.
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    Quote Originally Posted by HaroldBrooks View Post
    Might I also suggest trying a larger value for the power tube screen resistor (or add one if none present right at pin #4) ? I did this recently on a Hi-Fi Zenith to a guitar amp conversion, and I am super happy I did. The amp I modded is a push pull 6V6, and not the same as your single ended gem, but if you "crush" the screen voltage using a large ohm value resistor, say 2.7k ohms or more, you will add some compression by pushing the screen voltage lower on the power peaks, and perhaps it will soften the stiff quality for any tube you want to use.

    By the way I also used a 6L6 in my Gretsch 6150, and at first I liked it, but grew tired of the increased clarity LOL, and went back to the more abrasive sound the 6V6 was giving.

    I will try adding a large series screen resistor to my 6150 amp, now that the idea is in my head !

    Just another thought out of the Ether.
    Thanks for the advice! So from what I understand, I can just connect this directly on Pin 4 of the tube in series?
    Should this be a 1W or 3W resistor? EDIT: Just realized I have a 1K/5W coming from Pin 4 connected to B+2. I will swap it with a 2.7k

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    I removed the bypass cap on the power tube cathode and increased the value of the power tube screen resistor to 2.7k. Also tried using a 6V6 tube and it is sounding better.

    One thing to note is that there is an extra filter cap on this amp compared to a Champ or the Valco 6150 schematic. Perhaps I can remove it and see what happens?

    Also, is there any reason I should remove the power tube grid resistor? Will it have any effect on the tone?

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    Bent Member Chuck H's Avatar
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    The extra cap is part of a "pi filter". It's a good way to mitigate hum in single ended designs. Since the gross current use in the power tube doesn't increase with conduction I don't think removing the pi filter arrangement will help. Right now we're manipulating the screen grid current characteristics. Which are a relatively small share of the power tube current compared to the plate.

    You can try removing the grid stop resistor, but I don't expect much joy. Maybe some degree of cap charging cooling the bias is part of the charm you hear in the other amp. No harm in the attempt. The worst that can happen is that the amp may become unstable or sound too harsh in the extreme HF.

    You should have tried increasing the plate to screen resistance on the HV rail first before increasing the screen resistor IMO.

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    Quote Originally Posted by bw1985 View Post
    I removed the bypass cap on the power tube cathode and increased the value of the power tube screen resistor to 2.7k. Also tried using a 6V6 tube and it is sounding better.

    One thing to note is that there is an extra filter cap on this amp compared to a Champ or the Valco 6150 schematic. Perhaps I can remove it and see what happens?

    Also, is there any reason I should remove the power tube grid resistor? Will it have any effect on the tone?
    The screen grid resistor I believe effects "tone" indirectly, in that it makes the screen voltage drop more and more rapidly, so that adds a type of compression to the power tubes ability to create power. It's audible, and will change the dominance of some frequencies in the signle as a result, but overall it's subtle until you start raising the values past good sense. It's also a protective element incase of a short of the screens, so I wouldn't be so quick to remove it all together, regardless of what else you change to effect tone. And I wouldn't be quick to remove any control grid resistor either just like Chuck recommended. A control grid resistor is there to stop RF interference and blocking distortion, and rarely has an effect on tone or response unless it's a very large one.

    I also spent some time changing the screen power rail resistors on my latest amp project, and lowering the resistor values raised the screen voltage, and made my particular amp that much more aggresive in sound. I am assuming you wouldn't want that by the OPs description, but here's the catch, you might !

    When it comes to mods, nothing is off limits in my book, unless it leads to a near-term unsafe condition for the amp. So try something and reverse it if you don't like it (aligator clip leads are great for testing different values). Going up or down on the screen voltages by chanign the power node resistor changes the character and content slightly of the tone, and you may also find that effective. If you raise the power rail node resistor to a higher value you are on safe ground as you will drop the screen voltage a bit, but if you lower it and increase screen voltages markedly, be on the lookout for screens potentially being overloaded and start glowing.

    This only generally happens however if the current on the screens was high to begin with, and... Ta da ... the 2.7k screen resistor you installed is there to limit too high a current. I think there is a difference between the node resistor's action of current limiting and the series screen resistors ablilty to limit current, and I'm not quite sure why, but I believe it has something to do with the capacitors in the power circuit in concert with the node resitor that act as a filter. The series screen resistor mounted on the pin is more or less on it's own, and that's why the values don't have to be too high to be effective at limiting screen current and causing screen current sag, and a lot of it with a 2.7k resistor.

    One last thing, a different speaker might change the character of your tone bigger than an amp mod. This is often the case, but you have to spring for another speaker or cabinet. Ebay makes this a workable experiment, as you can buy a used speaker, and re-sell it for a small loss if you don't like it. Never under estimate the ability of a different speaker or cabinet to suddenly solve your amp problems ! I recall vivedly the first time I ran a small l tube head through my 4 x 12 Marshall greenback cabinet years ago. It sounded an awfull lot more like my Marshall ! I realised at that point how important speakers and cabinets are !

    Bear in mind I am new to all this amp mod stuff, and while I found out a thing or two working with my own amps, your best bet is to post questions and get answers from the excellent and experienced crowd of guys on this board, and then try a lot of things on your own, like you seem to be doing. Good luck with your mods, keep trying and you will get it !

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    I think there is a difference between the node resistor's action of current limiting and the series screen resistors ablilty to limit current
    The screen node resistor mainly lowers screen and preamp supply voltages by its voltage drop depending on average DC screen + preamp currents. The node's filter cap prevents fast voltage changes. Lower screen supply voltage means lower screen power/dissipation and lower power tube gain.

    A screen (stopper) resistor instantaneously reacts to increased screen current demand by dropping more voltage, thus counteracting high screen currents. In this case screen voltage and tube gain directly change with screen signal current - meaning instant compression and smooth signal distortion.
    In a single ended power amp only one side of the signal is affected , so asymmetry increases.

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    Last edited by Helmholtz; 11-22-2019 at 10:01 PM.
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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Helmholtz View Post
    The screen node resistor mainly lowers screen and preamp supply voltages by its voltage drop depending on average screen + preamp currents. The node's filter cap prevents fast voltage changes. Lower screen supply voltage means lower screen power/dissipation and lower power tube gain.

    A screen (stopper) resistor instantaneously reacts to increased screen current demand by dropping more voltage, thus counteracting high screen currents. In this case screen voltage and tube gain directly change with screen signal current - meaning instant compression and smooth signal distortion.
    In a single ended power amp only one side of the signal is affected , so asymmetry increases.
    Are there any considerations about a higher value screen grid resistor also increasing screen grid circuit impedance?

    (I'm actually asking because I wish to know)

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    Quote Originally Posted by Chuck H View Post
    Are there any considerations about a higher value screen grid resistor also increasing screen grid circuit impedance?

    (I'm actually asking because I wish to know)
    Not sure what you mean?

    The screen series resistor is the screen termination impedance.The filter cap/node it connects to can be considered zero AC impedance to ground.

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    Quote Originally Posted by Helmholtz View Post
    The screen series resistor is the screen termination impedance.The filter cap/node it connects to can be considered zero AC impedance to ground.
    Right. So the resistance in between the HV node and the screen grid would be the screen circuit impedance, right? That being the case, what are the advantages/disadvantages of higher or lower screen circuit impedance (DC resistance not withstanding) WRT tube operation?

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    Quote Originally Posted by Chuck H View Post
    Right. So the resistance in between the HV node and the screen grid would be the screen circuit impedance, right? That being the case, what are the advantages/disadvantages of higher or lower screen circuit impedance (DC resistance not withstanding) WRT tube operation?
    Higher impedance screen termination allows for more modulation of screen voltage (and consequently instantaneous tube gain) by the varying screen current and thus causes more (asymmetrical) signal compression/distortion.

    It's a kind of non-linear local NFB/degeneration effect.

    Total screen circuit impedance as seen by the screen current is tube internal screen impedance + screen resistor. Internal screen impedance varies with operating conditions but is typically much higher than the external circuit resistance.

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    Just wanted to follow up on this with a THANK YOU to everyone who responded or offered advice, it was all very helpful!

    What I ended up doing was decreasing the filter cap values down to 22uF and 33uF, changing the screen resistor to 2.7k and switched back to an EL34 tube. I found the 6V6 ended up not having enough volume or headroom after the other mods and changes.

    Overall the amp sounds totally different now and has much more of the sag feel of the original 1974x (luckily I have one here to compare to!). I even added a switch to toggle different bright cap values. Now its time to give the amp a proper housing!

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