AX84 P1x Build - "Stiff" Sounding Output

[HaroldBrooks]

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.

[Helmholtz]

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.

[bw1985]

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

[bw1985]

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?

[Chuck H]

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.

[HaroldBrooks]

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 !

[Helmholtz]

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.

[Chuck H]

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)

[Helmholtz]

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.

[Chuck H]

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?

[Helmholtz]

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.

[bw1985]

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!