5150 Mod Thread

[PaoloJM]

I know what poweramp saturation sounds like. With 4 75w speakers at 3.5 the amp begins to, compress a bit, at 4 you begin to hear more gain as if you turned up the pre gain knob for more distortion. At 5 you being to hear mild crunch, the highs are attenuated greatly and the bass starts to get looser but decrease at the same time. At 6, moderate additional crunch is heard and by 7, most of the highs are gone, and the bass is slowly falling apart. By 8, the amp is in a fair amount of total saturation, sounding as if the amp is about to melt down.

See the difference in volume from 4.5 to 8 is minuscule, tiny and laughable at most, its as loud as the amp is going to get, going beyond 4.5 does not make the amp louder, it just add more total distortion and compression. There is no way that speaker distortion could or would compress the volume to that extreme without a massive amount of audible speaker distortion, and even if it did, I highly doubt that 4 75W speakers with 97db sensitivity would begin to give out with the amp on 3.5.

I did this one day during band practice when we where on break, I chugged some open chords and brought up my post volume, at 4.5 the amp was at its maximum volume, and going beyond that added nothing more than additional distortion as if I was turning up my pre gain, but the added gain was looser and warmer as the amp sounded like it was slowly on the verge of a total meltdown by 7, which there was so much amp distortion that it was no longer musical. The speakers where just beginning to have audible distortion, but the cone excursion was still minimal. The resulting tone sounded as if I had taken recorded clip of the amp on 2, then added either a limiter or a clipper with a 100% softness, clipping the hell out of the signal by a few db. There was the tone of massive tube compression, which cannot be mistaken for speaker distortion as they are two entirely different beasts.

I have pushed solid state amps until they begin to distort the speakers, and I have pushed tube amps until they being to compress and saturate, anyone who has done the same can say the two are although very similar sounding, yet so far away. I know that with my setup, the amp begins to compress at 3.5-4, and after 5 the volume does not increase, you just get more total saturation and more tube compression.

It's quite possible that it does compress further on OD settings, but that is the FX loop return over-driving, not the power amp.
The output from the post is sent to the V3b buffer. The output of the buffer will be the same (practically). This may well have a large enough signal swing to overdrive V3a, the Loop return gain stage. I've yet to specifically test for this, but I will the next time I have one on the bench.
This technically should not happen, but it's likely with the design Peavey used now that I look at it.
If your amp feels like it's compressing after 3.5 then that is what is happening, the power amp is not doing it, I can assure you. My test measurements were taking using a signal generator, oscilloscope and purely resistive load box.
As someone already mentioned; depending on the resonance and presence settings, power amp saturation begins to occur some where around 8 ish on the dial. It's an extremely clean power amp.

There is no requirement for the .1uF cap in your schematic as the tone stack is already de-coupled via C58.

 

[endofall]

is there any way to do this where it would not effect the bass? Also thanks so much to everyone that has posted in this thread!

 

[Chris_P_Critter]

my brain just melted.

 

[TheWinterSnow]

There is no requirement for the .1uF cap in your schematic as the tone stack is already de-coupled via C58.

Wolfe said earlier that he placed the cap there to keep DC off of the pot (he actually said AC, but you get the point).

is there any way to do this where it would not effect the bass? Also thanks so much to everyone that has posted in this thread!

Not really, the sweep also changes how much you can attenuate on the mids as well. The higher the sweep frequency, the more the mids are boosted (not actually boosted but the attenuation is less) and the less you can cut, maybe about 10db. However at a very low sweep frequency, you can get a cut of near 30 db. Its just eh way the tonestack works, its not linear whatsoever. This is why I made a slight modification to the circuit so that you can easily recall the impedance of the stock resistance instead of trying to listen to it on the sweep button.

Hey I have an idea, I took a variation of if someone else's idea and modified it to switch through different circuits, I should patent that shit and call myself Randall Smith

 

[PaoloJM]

Wolfe said earlier that he placed the cap there to keep DC off of the pot (he actually said AC, but you get the point).

There's no DC on the tone stack in a 5150, C58 has already done that job.
It's necessary to use the cap on a Marshall style tone stack, but not with the 5150 style. Won't do any harm using it, just not necessary.

 

[TheWinterSnow]

There's no DC on the tone stack in a 5150, C58 has already done that job.
It's necessary to use the cap on a Marshall style tone stack, but not with the 5150 style. Won't do any harm using it, just not necessary.

I just looked at it again, and yeah, there is already DC coupling beforehand, so the tone stack isn't receiving any DC offset on the signal, so really it isn't necessary.

 

[Omega_Void]

The output from the post is sent to the V3b buffer. The output of the buffer will be the same (practically). This may well have a large enough signal swing to overdrive V3a, the Loop return gain stage. I've yet to specifically test for this, but I will the next time I have one on the bench.
This technically should not happen, but it's likely with the design Peavey used now that I look at it.
If your amp feels like it's compressing after 3.5 then that is what is happening, the power amp is not doing it...

V3a comes after the Post gain, so all signal into the poweramp passes through it.

If V3a is clipping at the level TheWinterSnow is applying with Post at 3.5 and the poweramp is not, then V3a will clip before the poweramp at any Post setting. It simply has less headroom -- in this scenario poweramp clipping can never occur, surely?

If V3a is not clipping when TheWinterSnow has his level at 3.5, but the poweramp is then V3a has more headroom than the poweramp -- and poweramp clipping is possible. If it took post setting of 8 to get poweramp clipping for you, this implies only that your level was lower coming out of the tonestack.

Unless I've missed something.

 

[sprack]

If V3a is clipping could you run a 6922 in its place? It'd be cold, but it would have loads more headroom than a 12ax7.

 

[Omega_Void]

If V3a is clipping could you run a 6922 in its place? It'd be cold, but it would have loads more headroom than a 12ax7.

No idea. But the point I was trying to make is if PaoloJM did achieve poweramp saturation, then V3a cannot be clipping before the poweramp, in his 5150 at least.

I do notice that the cathode of V3a is bypassed, though (by C50). This means that at high frequencies the gain of the stage is more dependent on the individual valve's gain, since it defeats the cathode-to-grid NFB.

For the values shown, half-boost is ~10kHz. Full boost is the difference between the gain set by external R, and the valve's intrinsic AC gain.

Given the wide range of gains seen in nominally-identical valves, this is another reason why the amp's clipping character may be different from one head to another, and change with tube-rolling even when the replacement tube is the same make/model and even batch.

 

[Harry Hughes]

my brain just melted.

Yeah no kidding bro.
I just read this entire thread, and I cannot for the life of me understand 99 per cent of all the technical stuff these guys are talking about.
I just wanna hear clips of these mods, not read pages and pages of technical stuff I can't understand

 

[Omega_Void]

my brain just melted.

Yeah no kidding bro.

Sorry.

 

[Omega_Void]

For a less-technical version of my last post:

There's a shelf boost in the highs. Putting a different 12ax7 in V3 socket will change the amount of that boost, maybe by just a little, maybe by a lot depending on the individual tube.

If you're okay messing with your amp's guts, removing the 0.0047 capacitor would remove that high boost completely.

That capacitor is C50 on the schematic -- but might be differently numbered in your amp!

No idea if this would sound "better" or "worse", but it's a possible mod.

 

[PaoloJM]

V3a comes after the Post gain, so all signal into the poweramp passes through it.

If V3a is clipping at the level TheWinterSnow is applying with Post at 3.5 and the poweramp is not, then V3a will clip before the poweramp at any Post setting. It simply has less headroom -- in this scenario poweramp clipping can never occur, surely?

If V3a is not clipping when TheWinterSnow has his level at 3.5, but the poweramp is then V3a has more headroom than the poweramp -- and poweramp clipping is possible. If it took post setting of 8 to get poweramp clipping for you, this implies only that your level was lower coming out of the tonestack.

Unless I've missed something.

When you're measuring for power output you use a clean signal in order to verify clean output power.
When the 5150 is fed with, and processes, a clean signal up to the post control, it takes all the way to 8 ish, again depending on setting, to see the signal compress or clip. The power amp is putting out a full, clean 120 Watts at this point. The mean there's headroom to process preamp signal right the way up to 8 ish on the dial.
I did feed clean signal directly into the phase inverter at one point and it took a large signal swing to get the power output clipping. It was beyond 120 Watt by that stage.
That's why I think the power amp has slightly more headroom available than the loop can provide, though the loop can provide enough to reach 120 watts.

I've no reason to doubt TheWinterSnow and I was trying to think what else might produce the compression he was hearing. In the original 5150 I was thinking that the ultra channel, and the crunch might just have enough signal swing at the post control to overdrive the loop itself. As in these channels were already putting out 120 Watts by 3.5 and were simply over-driving the loop after that.
This would effectively mean that the crunch and ultra would need to have generated as much signal swing as the clean chanell on 8, but at 3.5 on the post.
I've not measured it or anything. It's just a theory. It was all I could think of the explain what he was hearing.

The 5150II has pretty heavy signal attenuation on the ultra channel (and the drive setting on the clean) tone stack so I doubt it would happen there.
All channels should be close in volume on it and the loop shouldn't be over-driving. Again I haven't specifically measured it.

 

[TheWinterSnow]

V3a comes after the Post gain, so all signal into the poweramp passes through it.

If V3a is clipping at the level TheWinterSnow is applying with Post at 3.5 and the poweramp is not, then V3a will clip before the poweramp at any Post setting. It simply has less headroom -- in this scenario poweramp clipping can never occur, surely?

If V3a is not clipping when TheWinterSnow has his level at 3.5, but the poweramp is then V3a has more headroom than the poweramp -- and poweramp clipping is possible. If it took post setting of 8 to get poweramp clipping for you, this implies only that your level was lower coming out of the tonestack.

Unless I've missed something.

While I didn't think of that, it is very true, if V3 saturates before the power tubes, then the power tubes will never be able to reach saturation.

To add, It doesn't help that I have my bass at 8, and resonance on 10 (with the presence at 7). The bass especially in the feedback is going to kill the headroom a lot quicker without being perceived as loud.

As I promised for those interested, here are some screenshots of the response of the tone stack for the schematics that I posted

Mid Sweep Effect

Mids @ 0, this is the effect of sweeping the mid sweep pot:

Mids @ 5, same effect with the mid sweep pot:

Mids @ 10, same effect with the mid sweep pot:

Mid Effect:

Mid Sweep @ 0, this is the effect of moving the mid knob from 0-10

Stock 5150 without the MS, this is the normal range of a stock 5150's mids knob (this is still obtainable with the mid sweep mod, you just have to listen for it):

Mid Sweep @ 10, same effect with the mid knob:

As you can see, sweeping the mid sweep knob has more drastic tonal changes than the mid knob itself. It also allows with some settings for the mid knob to effect the bass or highs quite similar to how the poweramp and tone stack settings work on the Uberschall (not identical but to the same effect).

 

[TheWinterSnow]

When you're measuring for power output you use a clean signal in order to verify clean output power.
When the 5150 is fed with, and processes, a clean signal up to the post control, it takes all the way to 8 ish, again depending on setting, to see the signal compress or clip. The power amp is putting out a full, clean 120 Watts at this point. The mean there's headroom to process preamp signal right the way up to 8 ish on the dial.
I did feed clean signal directly into the phase inverter at one point and it took a large signal swing to get the power output clipping. It was beyond 120 Watt by that stage.
That's why I think the power amp has slightly more headroom available than the loop can provide, though the loop can provide enough to reach 120 watts.

I've no reason to doubt TheWinterSnow and I was trying to think what else might produce the compression he was hearing. In the original 5150 I was thinking that the ultra channel, and the crunch might just have enough signal swing at the post control to overdrive the loop itself. As in these channels were already putting out 120 Watts by 3.5 and were simply over-driving the loop after that.
This would effectively mean that the crunch and ultra would need to have generated as much signal swing as the clean chanell on 8, but at 3.5 on the post.
I've not measured it or anything. It's just a theory. It was all I could think of the explain what he was hearing.

The 5150II has pretty heavy signal attenuation on the ultra channel (and the drive setting on the clean) tone stack so I doubt it would happen there.
All channels should be close in volume on it and the loop shouldn't be over-driving. Again I haven't specifically measured it.

Wow you cleared up a lot of things here. For starters, a single frequency signal has much more headroom before clipping. If you had an audio signal that has a larger bandwidth, the saturation point would be reached much sooner. The more frequencies there are, the sooner/quieter the amp will be before it starts to saturate. I have seen this many times before in my study with AM radios and other amplifying semiconductor devices. So wile a single sine wave will clip at 8, adding another sine wave of differing frequencies at the same voltage will result in you dropping the post gain down 3db to be at the point of saturation, or something close to it.

I don't doubt what you say as well, if the loop's tubes have less headroom, then V3 will saturate first, and if V3 has a much smaller headroom, then the power tubes will never saturate. However, you also have to take into effect the differential amplifier with NFB, as this increases the gain of V4 regardless of the incoming signal from V3. If you recall correct, due to the NFB to the differential amplifier, the greater the signal coming from the secondary coil of the OT, the much greater V4 amplifies the voltage going to the tubes. This increases the strength of the incoming signal to the tubes by an expotential factor compared to if the amp did not have a NFB. With that in mind, the V3 can have less headroom than the power tubes but the power tubes can still saturate first. Then if they do saturate first, V3 can still saturate if the post gain pushes them into V3, in which the total volume of the power tubes will be compressed or limited.

Also, not that it is too important, but the 120watts the amp is rated for, is the idle dissipation at the maximum quiescent point. This is also where the tube becomes a Class A amplifier. In reality when an amp is say biased to 70%, when no signal is being sent to the tubes, they are still dissipating 42mA of current, and dissipating 84W. That means as a signal is going through the tubes, they are dissipating much more than that. A 5150's PSU is capable of supplying a total of 400W and the power section eats up more power than anywhere else in the amp by a very large margin. This would imply that 5150s eat up damn near 400W. I don't know if it is true or not, but I have heard before that the power section alone when it is being driven into saturation is pulling ~300W

 

[Omega_Void]

...When the 5150 is fed with, and processes, a clean signal up to the post control, it takes all the way to 8 ish, again depending on setting, to see the signal compress or clip.

Also depending on input level. If you give it 0.1V it will take half the Post setting to reach clip than if you give it 1.0V.

...All channels should be close in volume on it...

But only with the same Pre Gain and BMT settings.

I expect TheWinterSnow is just hitting the front end harder than your test signal was (Tube Screamer), and probably his Pre Gain and tonestack settings dialled in less losses than yours.

...if PaoloJM did achieve poweramp saturation, then V3a cannot be clipping before the poweramp, in his 5150 at least.

I should say that the clipping is not a hard limit, it's only so if V3 clips a lot before the poweramp.

 

[Omega_Void]

Also, not that it is too important, but the 120watts the amp is rated for, is the idle dissipation at the maximum quiescent point.

Um, no. It's the maximum power it can deliver to the correct load.

...I don't know if it is true or not, but I have heard before that the power section alone when it is being driven into saturation is pulling ~300W

My back-of-the envelope calc says this ain't too far off.

In class A, power dissipation is shared about equally between tubes and load. Once into class B, half the tubes are dissipating nothing as they're in cutoff. Ergo at point of saturation total poweramp consumption is 180W (120 in the load, 60 in the tubes). Go an extra 3dB and that hits about 255W. Plus about 5.7 watts per 6L6GC.

 

[PaoloJM]

Also, not that it is too important, but the 120watts the amp is rated for, is the idle dissipation at the maximum quiescent point. This is also where the tube becomes a Class A amplifier. In reality when an amp is say biased to 70%, when no signal is being sent to the tubes, they are still dissipating 42mA of current, and dissipating 84W. That means as a signal is going through the tubes, they are dissipating much more than that. A 5150's PSU is capable of supplying a total of 400W and the power section eats up more power than anywhere else in the amp by a very large margin. This would imply that 5150s eat up damn near 400W. I don't know if it is true or not, but I have heard before that the power section alone when it is being driven into saturation is pulling ~300W

The 120 watts refers to the output power of the amplifier and is not the power dissipation rating of the power amp. Those are two different things, but frequently have the same value in guitar amps.
A large portion of the 400 watts required to run a 5150 gets used for the valve heaters.
Valve amps are very inefficient when to comes to a power in / output power ratio.

The greater the NFB to more linear the power amp and the less gain it exhibits.
Remember the resonance and presence controls are reducing NFB in the frequency ranges they work as they are dialed up.
I suppose it's possible that the power amp will clip itself despite a clean signal being fed to it if the presence and resonance were both on 10. I didn't measure that. Do people play with those settings?



Omega, the input signal I used was just enough to over-drive the preamp and the backed off slightly. It's the max signal to still achieve a clean signal at the output. There's no variance with input signal as you describe as I'm using the max signal. It's the max clean output I was looking at.
A distort signal with the same Vp will not drive the power amp any harder in terms of sturation, but obviously the Vrms or a distorted signal is equal to V peak whereas the Vrms of a clean signal is only .707 of V peak and so it may seem quiter. There may well be slew rate distortions but I doubt it at guitar frequencies.
What I'm thinking in TWS case is that for the original 5150 Vp out on the crunch/ultra might be more than Vp out for the clean channel because of the nature of the design. The 5150II schematics does seem to have addressed that, which is why I offered the theory.

Vp-p for all channels should be the same as I described assuming that the clean gain setting is right up to it's max before clipping. Gain settings for crunch and ultra shouldn't matter at they are already clipping at most settings. TMB settings can affect levels at specific frequencies, I was testing at 1kHz. I'd no reason to test at any other frequency for the work I was doing but the next time I have one I will out of interest.

 

[TheWinterSnow]

I think we have discussed enough at this point and should probably start simplifying what we have been talking about and the possible mods that have resulted from the discussions. I have made the posts about the mid sweep, but other things that where quickly talked about included altering the miller capacitance of V5b to reduce the fizz of the preamp and improved tightness, bypassing C15 for better feel, piggybacking a 120K resistor across R9 for tightness, removing C26 to remove some of the low end from the rhythm channel and improved tightness, chokes, and stabilizing the power tubes by increasing the screen grid resistor values. We should definitely discuss those further and lay out how to do the mods. I am in the process of getting the parts to do a few of these mods and will most likely post a how to video soon.

 

[Omega_Void]

Omega, the input signal I used was just enough to over-drive the preamp and the backed off slightly.

Oh, sorry. Didn't get that you were hitting Vp in the pre.

TheWinterSnow's probably right though that we're getting into stuff which is only marginally related to the mods. I probably should leave this thread alone and go solder something.