microphonic resistor, or end of shielded cable?

[mikepukmel]

Thanks everyone!! Oh boy, well, I've done a bunch of reading, some from musicians, and some articles like Aiken's and Merlin's sites and the one consistent complaint about blackface circuits was blocking distortion when you turn up the bass, and then turn up the volume. Most of them said "just turn the bass down". One even said "WHY would you turn the bass up that high, you know it will sound lousy" referring to blocking. I wrote Aiken and Merlin emails asked questions a bout blocking. Aiken says that the combination of smaller ohm or no grid stop resistors and larger coupling caps in the blackface Fenders cause the problem to a much greater extent than say the marshalls, that used much smaller coupling caps, had earlier preamp stages that rolled off the bass a lot more, and had bigger grid stops (which some of you wrote above about coupling caps). So, what could I do without changing the tone TOO much, the suggestions were to first add grid stops on the back half of the preamp tubes (after the tone stack), and up the value of the stop on the output tubes. If this wasn't enough, then add grid stops on the phase inverter and reduce the coupling cap on the down side of the phase inverter. Some of this will change the tone of the amp, ok, agreed. But funny that if there is too much blocking distortion, you can't turn up the bass anyway, so ... I guess id rather have the amp as close to designed as possible, but reduce the blocking without going too far. I like to play with a wide variety of settings on the amp, if turning up the bass kills he thing with blocking Id like to reduce that. Merlin and Aiken both suggested 100k grid stops in earlier stages to reduce blocking, but this really does increase audible Johnson noise, but only seems to be a big problem at volume 8 or 9 and I won't go that high.

[Enzo]

Note too there is a difference between potential problems and actually HAVING the problem.

[pdf64]

...one consistent complaint about blackface circuits was blocking distortion when you turn up the bass, and then turn up the volume. Most of them said "just turn the bass down". One even said "WHY would you turn the bass up that high, you know it will sound lousy" referring to blocking...

Actually those responses seem reasonable to me. The flipside of having controls with a lot of range is that some settings will likely sound bad. Perhaps there's blocking in there, but get rid of the blocking distortion and those settings will still likely sound bad.

Some folks can tend to fixate on making their amps sound bad, whereas good musicians concentrate on making the best of what they've got, maybe there's a degree of bias shift in there, on the verge of blocking distortion, they work with it rather than against it.

I think that experience of these things will be your best education, eg build the thing stock and analyse it, then tweak to make it block, oscillate etc, so you get a better idea of what the cause and symptom of these effects are.

...Merlin and Aiken both suggested 100k grid stops in earlier stages to reduce blocking...

I suspect you may have misunderstood / misinterpreted that.
Whatever, it's misapplied, a BF Fender preamp is pretty resilient to blocking distortion.

Can you cite what you read that led you to that understanding?

[stokes]

Note too there is a difference between potential problems and actually HAVING the problem.

In other words,if it aint broke,dont fix it.

[pdf64]

...Grid stops help fix:

1) keeping unwanted RF noise out of amp (e.g. radio stations etc)
2) stability problems, e.g. HF oscillation
3) changing time constants to reduce blocking distortion.
...

As the context to the thread is a classic / vintage design, bear in mind that mitigating blocking distortion was unlikely to have been a consideration, as the amps weren't intended to be overdriven heavily enough for it to manifest.
And the primary reason for the 68k input resistors was likely as input mixers, as they're not mounted at the socket terminal, and with the values apparently most suitable to that purpose.
And that many (earlier) designs didn't have the power tube grid stoppers, likely because parts were expensive and all had to 'earn their keep'.

Grid stoppers don't really change the time constants that relate to bias shift / blocking distortion, rather that by limiting grid current they reduce its effect, see https://www.ampbooks.com/mobile/ampl...ias-excursion/
Not sure how accurate the actual calculator is, but you may perceive that it's more effective to tweak the Cg and Rg values.

...not sure where the blocking distortion occurs, which tube stage(s)...

Blocking distortion is a bias shift that occurs when a capacitively coupled grid is overdriven, ie the preceding stage tries to pull the following grid positive WRT its cathode, thereby inducing grid conduction (ask if you don't understand this).
In a non master volume amp, as with all regular classic guitar amps, as signal level rises from 0, at any reasonable control setting the output tube control grids will be the first place at which overdrive occurs; with most designs, signal levels have to go a lot higher for any stages before that to overdrive (with a BF reverb channel, the reverb driver stage is usually next).
Bias shift / blocking distortion is therefore primarily an output tube grid thing.

As I mentioned, why not just build it stock and get to know it?
If in use, you happen to notice nastiness that you think may be blocking distortion, then test to identify it, and then implement mitigation if it is indicated.
Reducing Cg to 47nF / Rg to 100k is usually sufficient IME.

[mikepukmel]

Thanks, everyone. I will go back and dig up comments about the Fender designs and blocking. Great suggestion to get it built and learn to know it first, before doing stuff. The two technical resources were Aiken's articles and Merlin's book. Both mention as you have above, that Fender never designed or intended his amps to be driven to distortion. And also at least a few other technical articles talk about the preamp designs on Fenders compared to Marshalls, the Fenders didn't cut off the low end like Marshall did. So, this would compound blocking, if you crank the Fenders. Went back and re-read the section on blocking in Merlin's book, and now looking at the grid bias calculator (thanks for the link) After the re-reads, and read, I have a little better idea what causes blocking distortion.

It looks like (please correct me if Im wrong), that the size of the grid stop that would reduce grid current enough to have an effect on blocking, would be much larger than the grid stop designed to reduce unwanted oscillation. Both Merlin and Aiken talk about roughly 100k for blocking, but much smaller < 20k for oscillation problems.

[mikepukmel]

These took a long time to go through, probably 6 months before I started buying parts to build a Deluxe:

https://www.thegearpage.net/board/in...reverb.389128/

There is some chatter at first, but at least a few musicians commented like this:
"That said, the best way around it is to keep the bass knob on 4 or even less, mic the amp and make up for the lack of bass with some EQ in the mains.

Many players overlook this. I think we obsess sometimes over getting lots of bass in our guitar amps, forgetting that it's the bass player's job to play bass. Seriously, I keep my BFDR bass set between 2 to 4 and the amp sounds rich and full in a mix playing in a band. I use a Weber 12F150."

Ive read this from many musicians: keep the bass below about 4. (or it will make the refried bean sound)

I'll dig up some more links, but there are many like that.

So, before I started the build, I emailed a few of the gurus (it wa before I found this awesome site!!!) and thought, well if its only adding a few grid stops and it doesn't kill the tone, and it will help (doesn't have to completely get rid of) blocking, then why not add the grid stops?

[g1]

I always considered the "flabby bass" and blocking distortion as separate issues.
Most common responses I've seen on this forum to flabby bass are usually bass cuts via cathode bypass cap reduction or coupling cap reduction. (aside from "you can't set bass above 4 with a Fender" )
For blocking distortion, larger grid resistors are often mentioned, but as others have mentioned, this is usually with high gain type amps.

[mikepukmel]

I always considered the "flabby bass" and blocking distortion as separate issues.
Most common responses I've seen on this forum to flabby bass are usually bass cuts via cathode bypass cap reduction or coupling cap reduction. (aside from "you can't set bass above 4 with a Fender" )
For blocking distortion, larger grid resistors are often mentioned, but as others have mentioned, this is usually with high gain type amps.

Thanks g1, yeah, and it gets worse when you talk to more musicians (flabby bass vs blocking).
I have to get a setup where I can record some sounds and let you all hear them. I may very well be mistaking farts for flab.