5E3 Compression?

[The Ballzz]

Sorry, was on the phone. Mom says hi.

You can apply a signal generator and check each node along the signal path and at the output. I am pretty sure this design hits is a sweet spot operating at full volume for the output tubes to sing and the somewhat dull eq and lack of tremendous gain before that point helps not to make that fine line turn into scribbles.

When I played early Marshalls I always had to drive them too hard and still hit the input with something to gain them even more. Only after I started building my own amps did I find the best sound is right around where most any amp starts to produce the expected RMS output. I had no idea that there was a balance point about 10-15% inside collapsing the NFB where the amps were fully wrung out and there would be no more excellence on the volume knob. I needed more sustain. The sweet spot is a fairly neutral path to the power tubes and every knob between 5 and 7... if you read this a few times it might start seeming relevant.

It appears the sweet spot on a 5E3 must be wide effin open with the tone control where you like it.

Reeltarded,

When I speak of "COMPRESSION" I'm not referring to the natural compression and tone of the clipping wave form that we all know and love. On most "bone stock" 5E3 amps (especially when using a 12AX7 in place of the 12AT7 in VI) when the volume is fully cranked, the attack of a picked note causes the volume to audibly drop (significantly), along with the higher frequencies more heavily attenuated than the ones below them and then recover to a more normal level. The more powerful the pickup used, the longer it takes for that level to recover. And while it may not technically/electronically be compression, per-se, it certainly is audible as such. It reacts just like a mixing console's compressor would when set at a particular threshold, with fairly high gain reduction and a slow release time, somewhere in the 50 to 200 milli second range.

Ah ha! This is just the kind of discussion I was hoping for! And while I understand and agree that wave form clipping is not compression, "per-se" I'm guessing that whatever it is should be visible on an oscilloscope, as some sort of artifact?
Thanks & Please Keep 'Em Comin'
Gene

I don't agree, Gene. Compression (as used in the studio) can "squash" (attenuate) the waveform without introducing clipping, distortion, or other artifacts.

You may have misunderstood? While the compression may not cause a difference in the "shape" of the wave form, it seems that the scope would at least show a difference in amplitude as that "squash" is occurring? My lack of knowledge and experience may have me making incorrect assumptions though.

Thanks Again Folks,
Gene

[xtian]

You may have misunderstood? While the compression may not cause a difference in the "shape" of the wave form, it seems that the scope would at least show a difference in amplitude as that "squash" is occurring? My lack of knowledge and experience may have me making incorrect assumptions though.

First, you'd have to know what amplitude to expect, in order to notice a difference.

For example, with an idealized compressor pedal or plugin set to 4:1 ratio and appropriate threshold, you might expect to put in a sine wave with amplitude X and get out a sine wave with X/4 amplitude. With a scope and signal gen, you could verify that it was working as expected.

How would you repeat this exercise on a tube amp? You'd have to know the expected gain at each stage. But it is totally complicated by the HT voltage sag that causes compression in the 5E3. You'd have to monitor screen voltage. And I don't think you'd get any compression from a 5E3 that's not also being driven to clipping at one or more stages...so, how do you compare the clean input sine wave with the distorted output, and decide how much, if any, compression you're getting?!?

[The Ballzz]

You may have misunderstood? While the compression may not cause a difference in the "shape" of the wave form, it seems that the scope would at least show a difference in amplitude as that "squash" is occurring? My lack of knowledge and experience may have me making incorrect assumptions though.

First, you'd have to know what amplitude to expect, in order to notice a difference.

For example, with an idealized compressor pedal or plugin set to 4:1 ratio and appropriate threshold, you might expect to put in a sine wave with amplitude X and get out a sine wave with X/4 amplitude. With a scope and signal gen, you could verify that it was working as expected.

How would you repeat this exercise on a tube amp? You'd have to know the expected gain at each stage. But it is totally complicated by the HT voltage sag that causes compression in the 5E3. You'd have to monitor screen voltage. And I don't think you'd get any compression from a 5E3 that's not also being driven to clipping at one or more stages...so, how do you compare the clean input sine wave with the distorted output, and decide how much, if any, compression you're getting?!?

You make some good points @xtian sir! I guess, failing all else, I'll try one change, play my guitar/listen, try another change, play my guitar/listen, rinse and repeat, etc! It would be nice/fun to have and see some technically quantifiable results, but the ears tell the real story anyway!
Thanks Again,
Gene

[strelok]

Here's a few things I might try:

-Change the 5k dropping resistor to something between 1k and 2.5k. Make sure you have some sort of screen grid stoppers installed. 470R is probably fine, 1k is better.
-Change the 5Y3 for a GZ34. Make sure your PT can handle the additional 5v filament current. Would probably be a good idea to check the bias and screen voltages after doing so.
-Excessive bass response can often result in sagging and compression. There's lots of ways to address it but the best IMO is to start from the input and move forward. Try reducing the coupling caps on the first stage to .022 or .047 and/or reduce the cathode bypass cap to something between 1-3uF. If its still too much, move onto the next stage and do the same thing.

[martin manning]

5AR4/GZ34 heater current is actually a bit less than 5Y3, 1.9 vs 2.0A. The difference will be higher Idle voltages (20-30V), and less voltage sag. It would be a good idea to check power tube dissipation and adjust the cathode resistor value if necessary. These changes would be more about reducing the compression and increasing headroom in the power stage than limiting the overdrive of the PI.

[The Ballzz]

Thanks to all for the ideas and tips! On the plus side here, I built this amp with a turret board and all connections are above the board. This will make swapping and or adding parts fairly simple and easy! I likely don't want to increase voltages, as going back to my "post-build" survey, I'm seeing plate voltages of 368 VDC, screens at 344 VDC and OT center tap at 377 VDC. I do remember (from playing with different 5Y3s), that when I let the voltage get past 380 VDC, the amp started to sound a bit more harsh and percussive. I can't recall if it was a CP JJ or SOVTEK that popped it up over 400 VDC! I did complete this build a while back and have been rockin' it hard and steady for a couple/few years, so memory is not real precise!

FWIW, even though I claimed this to be "bone-stock," when planning/building, I "averaged" a few things by looking at both the original 5E3 schematic and the one for the repro, Fender 57, such as modern heater layout and adding screen resistors that were not there originally. Given that the original had no screen resistors and the 57 used 470 R, I opted to split the difference at somewhere between 200 R and 250 R, likely 220 R.

After I rock it some more at today's out door gig, I'll dig in and keep you folks posted. First up will be a simple 1 MEG master pot at the PI grid. Next, if needed, I'll start monkeying with caps, but will likely start with the coupling caps to the power tubes, note each difference and work my way backwards towards the input and then decide on whether or not to combine any of the swaps.

Thanks Again Folks,
Gene

[martin manning]

The Sovtek 5Y3 is more like a 5AR4. The JJ is the only modern one that has the right voltage drop, AFAIK.

[The Ballzz]

The Sovtek 5Y3 is more like a 5AR4. The JJ is the only modern one that has the right voltage drop, AFAIK.

That's why I ended up with some sort of NOS, not sure of the brand! At that higher B+, the amp sounded like A$$, for my tastes.

Thanks Again,
Gene

[strelok]

5AR4/GZ34 heater current is actually a bit less than 5Y3, 1.9 vs 2.0A.

Oops I must've been thinking of the 5U4.

Its understandable you wouldn't want higher voltages, amps do tend to sound brighter and stiffer as you go up. I'm definitely a big fan of things being browned out a bit. However maybe still give the resistor swap a try, just lower it a bit, to 2-3k. In my 6V6 2203 build, I had a 1k resistor before the screen reservoir and it had the same problem of just getting way to saggy and compressed when the gain/volume was dimed. If you backed it off just a bit, it would go away. Now its a much different circuit and I think in combination with the cathode follower it was just too much compression. I ended up swapping the resistor for a choke that was around 3H and that largely got rid of it. Granted I'm already at a much lower B+, around 330V, so I didn't have that brightness problem as much. Still that slight change might be just enough.

I think where you really will make a difference here though is in the cap values.

Allow me to elaborate a bit more on why I like to start with input and work forwards rather than the other way around. Then you can decide for yourself how you want to go about it, it is a matter of taste after all. All of these Tweed era Fenders have really large cap values in them, probably because they were copied from audio amp designs, were full bandwidth and linear operation is name of the game. In fact as you progress through the history of guitar amps you'll notice an ever shrinking size of coupling and bypass caps as people started pushing amps into distortion more and more. The reason being blocking distortion and as you push an amp harder into overdrive blocking distortion becomes more of an issue with large RC time constants, such as those found in the 5e3. For instance the 820R / 25uF combination on the cathodes gives a low frequency roll off at about 8hz. This will in part help create that tubby/fuzzy/wooly sounding overdrive that the tweed amps are known for. Now, when blocking distortion is created it isn't just one frequency obviously, there's a rather large spectrum associated with it. So if its occurring early on the amp, say in the second gain stage, and you decrease the cap values downstream what you're left with is the fundamental and maybe a few harmonics being filtered out and all the nasty harmonics associated with it coming through. Personally I hate that sound. Its all the nastiness with out any of the warm low end growl. It can also take out a lot of the low end thump or oomph the amp has if you go to small.

To make matters worse you also have sub-harmonics and other sources of low frequencies that you're not even hearing eating up all your current in each respective stage, and just generally making life harder on the PI. I suspect now that I think about it that all that excessive low end might be the main source of the problem. The 5e3 I built a while back had .022's throughout and a .68 on the first stage bypass, it had no sag when dimed whatsoever, granted it also had a SS rectifier, but still had the 5k screen resistor.

So above all else I would say change the first stage bypass cap to 5uF. That will give you a roll off of 40hz. It won't affect the sound directly, as guitar doesn't go below 80hz, unless you tune down, but it might get rid of some LF crap that's eating all the supply current and not even able to make it through the OT/Speaker anyways. You may have to do it to the second stage as well. Or go to 2.5uF. That'll get you around 80hz.

I wouldn't even bother with the master until I've changed the caps, but take that with a grain of salt/JMO/etc.

[Reeltarded]

That ^^^^^ is a great way to see it.

[martin manning]

Yes great post!

Here’s a page with some specific instructions for changing bypass and coupling cap values: https://www.premierguitar.com/articles/ ... ed-deluxes
And there is tons more info out on the web regarding the 5E3, it’s a popular amp, and easy to tinker with.

Experimenting with the value of the screen node resistor is easy to do, just parallel a larger one across the existing 5k. Another 5k makes 2500, a 10k makes 3300, and a 18k makes 3900.

[The Ballzz]

You folks are the absolute best and have given me a lot of food for thought and experimentation! I'm on a mission and will you you posted.
Sincerest Thanks Again,
Gene

[strelok]

Certainly! Nice to see my late night ramblings turned out to be coherent and useful. Sometimes I write a long stream of consciousness then go to bed and wake up the next morning wondering 'oh god what did I do this time'.