Noise in SFDR

[David Vinson]

Filter Caps - Size & Brand?

For replacing the filter caps...

--Any brand you think is better than others?
--Should I increase the value to >16uF?

Is there a "sweet spot" for capacitance value that noticeably decreases hum, but doesn't make the bass response muddy, and doesn't fry the 5U4 tube ?

In my silverface, all the filter caps are 16uF, 450V. The two in the last filter stages (after the 2.2K and 10K resistors) are the old brown paper style; the first three are Mallory (I'm guessing these were already replaced?)

Thx for any ideas,

DV

[David Vinson]

Drawings - No layout for the silverface

Hi Enzo,

I've looked at a number of places, but so far have only found the layouts for the AA763 and AB763; the only thing I can find for the Pull-Boost silverface is a schematic (no layout).

The layouts of the AA & AB seem close enough to the SF to be able to do the repairs (so far).

I'm assuming the AA and AB layouts are blackface? Are these layouts drawings all that are needed for redressing to blackface (or are there other resources as well?)

Thanks for your efforts,

DV

[mykey]

[QUOTE=David Vinson;25235]For replacing the filter caps...

--Any brand you think is better than others?

Blackgate capacitors are much better and you can get them from Michael Percy Audio and other suppliers. However these cost more. And yes you can increase the value of the filter capacitor to get less 60 cycle noise, the bigger the capacitor is, the less ripple will be in the power supply. but no, there is no particular sweet spot. marshall amps used between 50-200 mfd caps and fenders used 10-120 mfd or larger, it was a matter of cost and physical size only.
the more modern the cap, the smaller it was made. the muddy sound is determined chiefly by the size of the audio coupling caps, not the power supply filters.

I will try to answer the rest in a couple days, got to go out to the desert and look for a water meter.

[Jag]

There's not a lot to do to convert an SF Deluxe Reverb, even the pull-boost model, to Blackface. I thought we covered all of that in the other thread but I could be wrong. I'll go reread the other thread later and post back.

I haven't seen any layouts for the pull-boost Deluxe Reverb. My guess is that, by the time the late '70's rolled around, the CBS engineers figured out that most other manufacturers were not offering layouts and so they didn't bother either. As I said, there's not alot of difference between the older Deluxe Reverbs and the pull boost version. If you can read a schematic, you could probably use an older layout and note any differences.

I probably would still replace the filter caps even though it looks like several were previously replaced. Unless you date code the caps that have been replaced and find that they are only a few years old.

In an amp with an SS rectifier, I like to beef up the first filter for a tighter bottom. With a tube rectifier I'd stick with stock values.

[Jag]

Here's the nutshell summary, which I posted in the other thread, regarding "Blackfacing" your DR.

The only real changes I made to my DR, other than the recap and other general maintenance I've already mentioned, was to change the resistors on the Phase Inverter to AB763 specs, remove the capacitors going to ground from pin 5 of each output tube and replace the 2.2k resistor between the screen supply and preamp supply to the AB763 value.

If I were really going to pick nits, I suggest eliminating the pull boost and making a minor change to the reverb circuit but I don't really think that's necessary. If you don't activate the pull-boost the amp will be essentially the same as the older DR in that respect.

Here's a link to the original thread in case you want to review it.

[David Vinson]

Yes - we're on the same page

Thanks Jag and others; I appreciate your sharing your knowledge and patience w/ us nubees.

I'm not planning to convert the boost pot - don't use it. I get you re: "blackfacing" -- for this discussion it means changing a few components, basically in the PI and Power stage. Here's my plan:

a) Replace the plate resistors
b) replace the filter caps
c) consider the remaining blackface components in the PI/Power stages
(I've already removed the 1500pF parasites)


So far, the Boost schematic and the AB763 layout are close enough for me to do component replacement.

The Fender Service Bulletin re: Trem Ticking got me to wondering if there were other places that cheap CBS lead dress causes problems... so this was why I asked if AB763 layout is the only lead re-dress reference people need, or whether pictures/real amp are required.

But re-dress is now my last resort (Thanks for saving me that waste!!) I'll report back on results once I get the components swapped...

[mykey]

a) Replace the plate resistors
b) replace the filter caps
c) consider the remaining blackface components in the PI/Power stages
(I've already removed the 1500pF parasites)

get rid of the noise first before you do any other modification. this will build your understanding of what caused the noise and put experience under your belt. you might need the 1500pf. fender put them in there to prevent high frequency oscillation. the amp can break into oscillation under playing
conditions, and not on the test bench. the oscillation can fry the output tubes,
the bias will break down with the oscillations present. you can't hear these types of oscillation, you will need a scope to get an understanding of this.
you may want to invest in a cheap scope to gain some understanding of the
non- hearable things that are present in the audio path.
good amp designers do not add parts to the circuit which are unnecessary, these parts are installed for good reasons, remember this advice.


So far, the Boost schematic and the AB763 layout are close enough for me to do component replacement.

The Fender Service Bulletin re: Trem Ticking got me to wondering if there were other places that cheap CBS lead dress causes problems... so this was why I asked if AB763 layout is the only lead re-dress reference people need, or whether pictures/real amp are required.

But re-dress is now my last resort (Thanks for saving me that waste!!) I'll report back on results once I get the components swapped...[/QUOTE]

[Jag]

good amp designers do not add parts to the circuit which are unnecessary, these parts are installed for good reasons, remember this advice.

When it comes to CBS engineers, I beg to differ on that point. If the amp is built properly, and the lead dress is good, an SR should not need those snubs and they do suck tone.

[David Vinson]

scope, 1500pF & separate topics

Sounds like I'm getting a scope Any suggestions for something useful, but that won't break my bank$$ (and what are the minimum features I should look for?) Thx for any input!

The sequence of steps in my plan is to separate the issues: kill the noise first, then start looking at mods. It may turn out I prefer the SF tone to the BF tone - I want very clean as well as some occasional dirt; but need a reference point to begin with.

So I'm approaching noise issues one by one: To nail the source of the problem; and to provide issue-specific threads to the forum (when someone is searching on specific problems) ...Not to mention a noobie like me can easily lose track of all the information within a thread that covers overlapping issues and their solutions.

So this is why I started a different thread for the hiss topic. The previous thread ultimately focused on new power tubes install and bias. Once I clean up all the different sources of noise, I'll be exploring opening up headroom. But these will be separate future threads.


Thanks again guys, I've just about assembled my first parts list, and I hope to place an order today. If I get my shit together, I'll document the changes and place on a website.

[David Vinson]

Come to think of it, I did mention this

My opening post in this thread does mention these thoughts; mebbe part of the real value of a thread is solutions AND the unexpected info that surfaces along the way

Thx!
DV

[Enzo]

DO a search, there have been a couple recent threads on scope purchasing.

One quick thought, for audio, and especially limited range audio like guitar amps, you will not find a scope that won't handle it. While they may exist, I've never seen a scope slower than 500kHz, and that is way fast enough for this use. DOn't let anyone distract you with square wave leading edge resolution or some such stuff. It won't come up in a guitar amp. Pretty much ANY scope you find will serve you.

[Jag]

I sort of wonder whether someone who's really only working on one amp actually needs a scope. I hardly ever use mine and, while amp work is not my bread and butter, I work on quite a few amps over the course of a year.

As long as I'm not pushing the limits of the tubes, I prefer to bias based on what my ears tell me. I just calculate the dissipation when I get it sounding good to be sure I'm not going to have a melt down. As far as other uses for the scope, it's nice but I think you can get by without.

That being said, I got my scope off the Bay for less than $90.00. I had to buy probes but still, not alot of cash. If you think you are going to get more heavily into amp mods and repairs then a scope is probably a good idea eventually.

[David Vinson]

Scope needed to confirm parasitic oscillations?

Hey, guys. Thanks again for continuing on this.

So the question is whether I need the scope at all:
We know the SF 1500pF caps prevented "parasitic oscillations" (and we believe those are due to poor dress, but that's another topic ;-)

...but can I tell if I need to put the caps back in without using a scope?
It was suggested earlier in this thread that the only way to know if the amp was now unstable (because of wild but silent oscillations) was by using a scope.

Is this true? Can I take the approach of "well, it doesn't *sound* unstable, so I guess it's OK to leave out" ...or would this risk cooking tubes?

While a scope would be fun to have, it is a whole other side-track to find and then learn how to use. If possible, I'd rather focus now on just getting some parts and replacing them.

Thx mucho,

DV

[Bruce / Mission Amps]

I sort of wonder whether someone who's really only working on one amp actually needs a scope. I hardly ever use mine and, while amp work is not my bread and butter, I work on quite a few amps over the course of a year. ...

Yeah I work on quite a few too.
The amount of time needed to trouble shoot broken amps using my scope is probably 1/20 of "no scope" and "shot gunning" a repair.
It is utterly amazing what I can find QUICKLY using the scope compared to all other test procedures.
I can't imagine not having one and in fact, I have three... all three are dual trace scopes with one being high end 100MHz and the other two, less expensive, PROTEK 20Mhz scopes. One PROTEK I bought in like new condition on Ebay for around $150.
However, I did repairs in my shop for many years using an old gronk vacuum tube, 500KHz HP scope I found at a ham radio swap meet and I only paid $20 for it.
I simply could not live with out an O'scope.
My bench scope(s) are right next in line to my RMS DVMM and my inexpensive PROTEK signal generator, ... my three most "valuable" diagnostic tools.

[mykey]

[QUOTE=David Vinson;25585]Hey, guys. Thanks again for continuing on this.

So the question is whether I need the scope at all:
We know the SF 1500pF caps prevented "parasitic oscillations" (and we believe those are due to poor dress, but that's another topic ;-)

...but can I tell if I need to put the caps back in without using a scope?
It was suggested earlier in this thread that the only way to know if the amp was now unstable (because of wild but silent oscillations) was by using a scope.

The oscillations occur mainly when the "pull boost" is activated, and it's kind of a wimpy boost; rather un spectacular.
no, you don't really need a scope, I suggested that because it's fun to see whats going on in there during and after modifications are made, and expands the understanding of amp maintenance. power supply ripple, bias adjustments, detecting high frequency oscillations all make the scope a great tool to have around.
when this type of oscillation occurs the power tubes can turn red, the bias will break down, the power tubes overheat, the volume drops out, and it's really nice to be able to diagnose a problem like that one. also checking stages for noise is rather handy.
there are a number of PC scopes, some free, some low cost which use an adapter...there is a B&K scope for around $300 which works just dandy for audio applications.
I was lucky to find a used Tektronix 465 many years ago, you would not believe how many times Ive used it (hundreds) to troubleshoot all kinds of problems, both normal and unexpected, so it's something to consider.
but yes to answer the question, a scope is really recommended when trying to decide if parts such as these caps are really needed or not. You are getting the whole story with the scope that otherwise would not be known completely. But depends on how deeply you want the understanding to penetrate.
was it really poor dress or a poor design to start with? in most of these amps the wires to the pull boost were shielded, but that was not enough. there are of course other ways to stop the oscillations besides the caps installed, but the caps were the cheap fast easy solution.