cathode bypass and grid stoppers/ when where and why

[Malcolm Irving]

Created a noisier environment by running the amp's own mains cable (230V ac) right over the open chassis near the input stage – noise goes up to 16mV. Then with the foil screen – back down to 7mV.

In the interests of science, I then put the live mains cable so close that it was touching the V1 grid stopper – noise only goes up to 43mV.

[ric]

My experience is that when you get the hum down to the level Malcom has, and it it is fundamental, and you are using ac heating, then the problem is the heaters. But it might be different with the EF86 pentode rather than the 12AX7 triodes I am familiar with.

Speaking of pentodes, has anyone tried using them further along the preamp chain rather than at the input? The idea would be this: Since amps today often have power stages that are too powerful for the application, there is a greater emphasis on preamp distortion. But that is usually all triode, while the output stage is almost always pentode or, similarity, beam power tetrode. I am thinking that an interesting thing to try would be an EF86 phase splitter before the triode phase splitter. Each EF86 output would feed one input of the triode phase splitter through a "stereo" master volume control. The EF86 splitter would also have an "imbalance" control so that you could adjust the ratio of even and odd harmonics. Has anyone tried something like this?

Sounds interesting, ratio/ dial it in and out especially. Then you can really get a feel for what you did and what you think of it.

Please report back if you get around to trying this. Hit or miss, both are informative.

[Chuck H]

I like this better reversed

I then put the live mains cable so close that it was touching the V1 grid stopper

In the interests of science

[SoulFetish]

I used to go to the local tech, some guy who called himself "Enzo". He really seemed to know quite a bit...

I heard 'a that guy. - Word on the street is he knows a thing or two.

[Chuck H]

I heard 'a that guy. - Word on the street is he knows a thing or two.

I dunno... I've heard he drinks cheap beer and makes home made chili. Probably gas you outta the room before you could negotiate an estimate

[Enzo]

Ixnay,,,here he comes...

[SoulFetish]

By the way, I’ve been meaning to ask you.. Is Enzo short for Crescenzo?

[Enzo]

Enzo isn't short for anything, he buys the Magnums.

Enzo comes from Enzo Stuarte, the singer. I adopted it for my air name on the radio in the 1960s. it stuck and became my name.

And now... The Mighty Enzo, and his SHOW!!!

And zaniness ensues.

As we signed off at the end of the show, we played the NAtional Anthem. Here is a vocal version, ours was instrumental, but musically the same:

[g1]

Well, well, well. I learned something new today. The Enzo origin story.

[uneumann]

Sounds interesting, ratio/ dial it in and out especially. Then you can really get a feel for what you did and what you think of it.

Please report back if you get around to trying this. Hit or miss, both are informative.

Used two EF86's as PI in this amp.... https://sites.google.com/site/string.../princeton-aot

There is also a mid-preamp EF86. My goal was not huge OD PI distortion, but rather to get screen voltage compression out of the PI and I managed to get that and it sounded pretty good - a gradual OD transition. Later, (see ver. 3 at page bottom) I added explicit screen compression to the mid-preamp EF86 with an envelope follower. That also works well at clean levels. As for OD distortion from the EF86 - I find it more gritty than from a triode. YMMV since there are so many circuit parameters at play.

[Mike Sulzer]

Used two EF86's as PI in this amp.... https://sites.google.com/site/string.../princeton-aot

There is also a mid-preamp EF86. My goal was not huge OD PI distortion, but rather to get screen voltage compression out of the PI and I managed to get that and it sounded pretty good - a gradual OD transition. Later, (see ver. 3 at page bottom) I added explicit screen compression to the mid-preamp EF86 with an envelope follower. That also works well at clean levels. As for OD distortion from the EF86 - I find it more gritty than from a triode. YMMV since there are so many circuit parameters at play.

Very interesting. Thank you for posting that. Based in some of your useful comments on this site before, I should have already gone to you web site! Your solid state output stage looks to me like a differential input/output compound emitter follower. I think there are many ways to obtain a clean enough output stage. My preference would be the usual national chip set up as a differential amp. The protection circuitry might be worthwhile. (If you have a crazy nephew and he finds a way to plug in a low impedance, your amp might blow.)

Have you considered using a true remote cutoff pentode to get a greater compression range, or do you think that is not necessary?

[uneumann]

Very interesting. Thank you for posting that. Based in some of your useful comments on this site before, I should have already gone to you web site! Your solid state output stage looks to me like a differential input/output compound emitter follower. I think there are many ways to obtain a clean enough output stage. My preference would be the usual national chip set up as a differential amp. The protection circuitry might be worthwhile. (If you have a crazy nephew and he finds a way to plug in a low impedance, your amp might blow.)

Have you considered using a true remote cutoff pentode to get a greater compression range, or do you think that is not necessary?

Thanks Mike - right - the output stage is implementing a pretty simple idea. I shy away from IC solutions since they prevent tinkering with internals. As to shorts ... Either side can drive separate loads to ground (at ~1/4 power ea). If one output is actually shorted (has occurred intentionally and unintentionally ), the PS current limit kicks in (4-5amps depending on which supply I use) without device damage. The output caps are also good protection against DC shorts. The amp works fine without them, but they're good spkr and amp protection.

As for the pentode - I've only used the ef86. The compression range of the ef86 circuit is far higher than anything I'd normally use, but it adds another dynamic to the circuit behavior. This amp was sort of an experiment in preamp dynamics. I didn't expect dynamics from the SS output stage, so they had to come from the preamp. The ideas worked pretty well so they're among my design options for future amps.

[Mike Sulzer]

Thanks Mike - right - the output stage is implementing a pretty simple idea. I shy away from IC solutions since they prevent tinkering with internals. As to shorts ... Either side can drive separate loads to ground (at ~1/4 power ea). If one output is actually shorted (has occurred intentionally and unintentionally ), the PS current limit kicks in (4-5amps depending on which supply I use) without device damage. The output caps are also good protection against DC shorts. The amp works fine without them, but they're good spkr and amp protection.

As for the pentode - I've only used the ef86. The compression range of the ef86 circuit is far higher than anything I'd normally use, but it adds another dynamic to the circuit behavior. This amp was sort of an experiment in preamp dynamics. I didn't expect dynamics from the SS output stage, so they had to come from the preamp. The ideas worked pretty well so they're among my design options for future amps.

Tinkering is fun and mostly very useful. I think you could argue that for the SS output stage you just want perfectly linear power gain. Ideally I suppose you should use a design by someone such as Douglas Self. The distortion measurement has almost as many zeros after the decimal point as the dollar value of the national debt has before it.

[uneumann]

Tinkering is fun and mostly very useful. I think you could argue that for the SS output stage you just want perfectly linear power gain. Ideally I suppose you should use a design by someone such as Douglas Self. The distortion measurement has almost as many zeros after the decimal point as the dollar value of the national debt has before it.

Yes - the argument for zero-distortion from SS amps makes sense, but that often brings with it a context of high-gain, NFB-stabilized circuitry. My simple circuit has pretty low-distortion (to my ears - I don't have a good means of measuring it precisely) and it has almost no NFB other than that provided by the .22 emitter resistors. There is a difference when I compare it to a SS Hi-Fi bench amp (fed by one PI output). I wish I could quantify it better, but qualitatively, its "sweeter" so it does add some color/distortion. I'm open to suggestions/speculations as to why. Currently thinking it may be tied to output impedance. Without NFB, Zo may be high enough to matter and likely is nonlinear with amplitude. Anyway - it's been interesting to pursue this periodically over several years (up to ver 5.1 now). Ideas and insights keep coming and it sounds pretty good.

[Mike Sulzer]

Yes - the argument for zero-distortion from SS amps makes sense, but that often brings with it a context of high-gain, NFB-stabilized circuitry. My simple circuit has pretty low-distortion (to my ears - I don't have a good means of measuring it precisely) and it has almost no NFB other than that provided by the .22 emitter resistors. There is a difference when I compare it to a SS Hi-Fi bench amp (fed by one PI output). I wish I could quantify it better, but qualitatively, its "sweeter" so it does add some color/distortion. I'm open to suggestions/speculations as to why. Currently thinking it may be tied to output impedance. Without NFB, Zo may be high enough to matter and likely is nonlinear with amplitude. Anyway - it's been interesting to pursue this periodically over several years (up to ver 5.1 now). Ideas and insights keep coming and it sounds pretty good.

Each of your four two transistor output circuits is a simple feedback amplifier where the driver transistor provides current gain into the junction of the output transistor which has voltage gain. The gain of the circuit is reduced to unity with feedback to the emitter of the driver transistor. As a result, the output impedance is low, similar to a simpler emitter follower.

Apparently you have found a way to make use of SS distortion in a power circuit to achieve a desirable sound! That is good news.