Is it time for me to finally buy a scope? (SF Twin repair)

[Madison]

Working with this SF Twin.
Dillema.

Things replaced so far-
-Output tubes (good boost in power)
-Filter caps (good idea in theory)
- preamp b/p caps (not sure)

Things I have been playing with and replaced-
-preamp tubes (difference in tone but, not a "wow" moment)
-tone coupling caps.(No real change)

 I get above 6-7 on this amp and it really starts to sound pretty crappy, especially on a neck PUP, lower end chords, on a Strat.
I turn back the bass and it sounds just "okay".
(speakers are okay)

So, sure I could keep guessing and go for an entire cap job just to find out that the OT (my fear) is bad!
Experimenting is expensive.

WTH?
How can I take the guess work out of this work?
Sick of guessing.I can usually spot things down to the preamp section of a new build but these oldies really get my goat.
Never had much luck with the listening amp and a MM will only take me so far.
Someone else must have been at this crossroads somewhere along the line???

Any help and advice appreciated.
Pardon the vent mode.

[Ed_Chambley]

Good investment.  I assume you still have the chassis out.  Just for giggles, try disconnecting the NFB from the board and see what happens.  This is the 100 watter, right?  Master?

[eleventeen]

You can buy a lowish end (100 MHz scope, used) on ebay in the $100-150 range, or even less. Hitachis, Kenwoods, Leaders are fine. Although they were once among the best scopes, I reco against buying a seemingly cheap high-end Tektronix scope for audio. They are not easy to fix. The issue is whether or not you get probes with it and whether or not the trace is bright enough to provide some years of service and whether the trace is burned into the screen (not so bad). Cheap probes are OK, the bandwidth requirements for an audio scope are almost nil. You're interested in whether or not they are good enough condition and have all the clip-on tips such that they can mechanically clamp onto a circuit node and STAY THERE. And if you buy the scope from an ebay goofball, REMIND THEM that it is VERY important to remember to include the probes!

I will repeat again, if your amp is making rat noises with nothing plugged in, replace the plate resistors. I don't care what they measure with an ohmmeter or how they look, physically. 6x 30 cent parts. Once those old carbon comp resistors get old they induce lots of thermal noise and you can't capture it on a scope because it is random noise (won't trigger anything, it'll just fuzz the trace some) and often sits atop relatively big DC volts. IF the plate resistors are bad (noisy) you will NEVER get that hissy unstable rat noise out of the amp without replacing them. I have done this with literally dozens of blackface and silverface amps and the difference is NEVER unnoticeable and is more often dramatic. It is the single cheapest repair you can do to an otherwise working Fender amp. Once they are 30 years old, I consider it an absolute necessity.

[thelonious]

I love my scope. I only use it a couple times a month, and I still think it was totally worth the purchase price. Sometimes it just makes life so much easier, for exactly the reasons you described. Without it, there are some things you just have to guess at.

Back a while when I didn't know any better, I made exactly the kind of purchase eleventeen warned about - a used Tektronix 465B on ebay - but I was one of the lucky ones who, by fortune or grace, ended up getting a fully working one for a good price ($165 shipped). I just had to be patient and careful.

You should also consider USB-based digital scopes that will let you take snapshots of the waveform and save them as image files. The difficult thing is finding an affordable one that will handle the voltages we throw at it in tube world... and that's why a lot of people go with old analog scopes.

[eleventeen]

The 465 series (I used to own one, for many years) is utterly bulletproof...though they have a smallish screen. The much newer ones, the 2235, 2236, 2245 which are much lighter, look much cooler, bigger screen, higher bandwidth, everything....are great scopes, but if they break on you, you're quite possibly done. It is just that they are likely to appear at first glance as a better deal. But they will undoubtedly cost more to fix than whatever you may have paid for one unless you get lucky and get one in babied condition. Many of those were used in outside service use and taken around and possibly beaten up, at least jostled around, to some degree. They were expensive, which means that they were bought to be used, and in most cases they were. That's why I am opining that if you want a $100 scope, you are possibly better off buying a less expensive flavor of scope that was probably *not* used in a really demanding, high-end pro application and run through airport luggage, etc;

[phsyconoodler]

A scope isn't going to tell you anything about what is going on with that amp.
  And a cheap scope won't be accurate either so you will have a cool looking piece of shop gear that will be only useful for locating a break or signal loss.
  Your amp is likely something simple like a bad speaker,improper speaker wiring,too much bass(common Fender issue) or bad driveline coupling caps.
  If you haven't changed all the filter caps,bypass caps then no scope is going to help you there.There is also a learning curve to know what is going on with scope readings as they can lead you down blind alleys and you end up doing all kinds of unnecessary stuff to try and make your scope happy.
   It come down to experience and knowing what pieces cause what symptom and concentrate on that area first.
Voltage readings,current,and substitutions tell you more than a scope will.
   For example: To get good high volume tone on big Fender amps I never turn the bass higher than 3.They just are way too boomy above 3 especially with humbuckers.Almost as bad with single coils.
  Silver face Fender amps have numerous faux fas's that prevent great tone.

[jjasilli]

It is always time to buy a scope.     But the learning curve is steep.    Fortunately there is plenty of help here.     Learning to use it will make you a better techie; and a more humble person -- at least during the learning process. 

But I agree with Psycho that it's not an essential repair tool.

[Madison]

WOW, thanks guys for all the info!
When I get closer to getting one I will check things out over at the tech area.

A friend suggested a Rigol DS1052E which looks super but I am not sure I need or want to spend that much on something like that for my first scope.I don't think I will be using it everyday or even every week at this point.

Would like to find a used one for $100 or something if that were possible.
Just thinking that anything used is going to be pretty well worn out?

For now,I think I am going to just tell this guy to replace the board (that's what I'd do if it were mine)
I reckon , if the caps (and components) aren't bad now......they will be sooner rather than later.

Good investment.  I assume you still have the chassis out.  Just for giggles, try disconnecting the NFB from the board and see what happens.  This is the 100 watter, right?  Master?

I suppose over a lifetime it will be a good idea to have.
Hoping I'd have enough time to study up on how to use it.

Haven't messed with the NFB, I figure it would get more squirrelly without?
100W, MV model.
Chassis back out......wasn't happy with the tone.

[Ed_Chambley]

Haven't messed with the NFB, I figure it would get more squirrelly without?
100W, MV model.
Chassis back out......wasn't happy with the tone.


[/quote]
It may react badly, but easy to do.  My twin runs fine without it, but it is a different circuit.  The last twin I repaired and sold reacted very badly when I did this.  This lead me down a short path of converting the circuit to AB763, and changing the master volume to PPMIV.  It is difficult to get a dual pot in the front.  It was my intention to sell it from the beginning and not "keep it original". 

It sold in less than a week on consignment at a friends store.

If I read correctly, you are not happy with with the bass response and I am thinking in terms of the way my Marshall amps respond.  From 27k through 100K NFB will dramatically change the Bass response.  The more resistance, the more thump.  I will warn you I have not done this with a twin.

[Madison]

Haven't messed with the NFB, I figure it would get more squirrelly without?
100W, MV model.
Chassis back out......wasn't happy with the tone.

It may react badly, but easy to do.  My twin runs fine without it, but it is a different circuit.  The last twin I repaired and sold reacted very badly when I did this.  This lead me down a short path of converting the circuit to AB763, and changing the master volume to PPMIV.  It is difficult to get a dual pot in the front.  It was my intention to sell it from the beginning and not "keep it original".  

It sold in less than a week on consignment at a friends store.

If I read correctly, you are not happy with with the bass response and I am thinking in terms of the way my Marshall amps respond.  From 27k through 100K NFB will dramatically change the Bass response.  The more resistance, the more thump.  I will warn you I have not done this with a twin.

Ah!
Now I see where you are coming from on the NFB.
Adjust it to tame the beast.
Possibly, yes.
Might give it a shot.

I'd like to go the route you described with the amp......if allowed.
We'll see.

[SILVERGUN]

Here's a nice little freebie that I found informative...
Book is called "Troubleshooting with the Oscilloscope", and by clicking on the link it will immediately start to download (2.42MB book)

http://www.tubebooks.org/Books/Middleton_troubleshooting.pdf

[Madison]

Thanks SG, looks about my speed.

[Madison]

Just for giggles, try disconnecting the NFB from the board and see what happens.  

Good call Ed!
I paralleled the 820R, rolled back the bass, and it cleaned up pretty good.(not perfect but pretty darn good cranked, especially in the input 2)
Thanks!