Question about Fender Twin...

[Mbira]

I posted this at a more "guitar amp" specific site, but no luck yet...


I'm repairing a twin reverb for a friend-it's one of the "master volume" ones. I think from '74. Anyway, the tremolo circuit isn't working, and I'm not seeing the little light flicker in there, so that may be some of the problem with that.

His big concern is that the amp isn't delivering enough power (isn't loud enough). My question is, how do I know if there is enough power or not? I have a 100w 4ohm power resistor to use. The B+ seems good to all the tubes . Don't get me wrong, the amp is still plenty loud-There is nothing obvious to me, but he says that "his ears weren't bleeding at full volume".

Also, I am having a hard time getting decent readings with my scope when I turn the bass eq knobs. They seem to give me basically an on or off setting with the wave-I'm not getting any kind of graduation in the gain (I did sweep my signal generator down to low frequencies, though I don't know exactly what frequencies I should use to check each pot). This happens on both the vibrato and normal input though, so it's probably my problem and not the amps.

Any thoughts?
Thanks.
Joel

 

[jrmintz]

[quote author="Mbira"]
His big concern is that the amp isn't delivering enough power (isn't loud enough). Don't get me wrong, the amp is still plenty loud - There is nothing obvious to me, but he says that "his ears weren't bleeding at full volume".

Any thoughts?
Thanks.
Joel[/quote]

There's your problem - maybe his ears have bled once too often...

:green:

 

[peterc]

I had one of those once as well, but I couldnt turn the amp up loud enough to check, it was just way too loud.

I eventually put a 12AX7 in place of the 12AT7 (??) & the guy loved it! I preferred the sound of the 12AT7 but he was happy.....

Peter

 

[pstamler]

The first question to ask the client is: "Was there a time when the Twin gave you the kind of volume you wanted?" If the answer is "yes" then there's probably something wrong with it. If "no" then he's just got a case of owner-dissatisfaction.

But I'm betting there's a real problem. If you have a fresh 12AT7 around try replacing the one in the amp; these seem to die in Fender amps.

Peace,
Paul

 

[cayocosta]

The Tone Stack calculator is designed to help you check out the design and response curves of a variety of tone stacks used in popular guitar amplifiers:

http://www.duncanamps.com/tsc/index.html

Low Power or Loss of Volume:

http://www.geofex.com/ampdbug/lowpower.htm

 

[Test Point]

for the trem problem, this may be obvious but make sure you have shorted the tremolo footswitch connections on the rear of the amp if you don't already have a footswitch installed. If that is done and still no trem, it's probably the little cockroach (the cds cell/neon lamp assembly) or one or all of the three disc caps near the roach. If the roach is bad, they can be had for around $20...I think antique radio supply has them.

As far as the level of sound...I had one of these in the shop a few weeks back and the customer was complaining of it sounding thin and the "in your face" sound was gone. After checking power out and all voltages (which were normal), I saw that someone had somehow changed the wires on one of the speakers so they were now out of phase. I moved them back to where they were supposed to be and the amp was back to normal and I was the amp owner's hero once again. Sometimes, it's just the simple things...

Test Point

BTW, I should add as an edit, that if it still has the original filter caps and bypass caps in it, they should all be changed as a matter of course, after all, the amp is over 30 years old and that means so are the caps.... also check (or change) all the 100k ohm preamp plate resistors as they usually go high in value or they at least get noisy if nothing else (the dreaded cracklies). Also change the 470 ohm screen grid resistors and the 1.5k control grid resistors under the power tube sockets for good measure...they are probably a little toasty.....all part of the cost of owning an older amp.

Good Luck

 

[CJ]

Lay the amp flat on the carpet and measure the ac volts across the speaks.

 

[Gus]

I have tested old caps in some fenders. Sometimes they are fine. I used a lc102 and check value,DA, ESR and leakage at working voltage.

Now there might be something to some players complaining about the tone going away after the cap replacement. The old stuff often can still have LOW ESR. Some new caps are crap.

Remember the first two B+ taps have alot to do with the tone Plate and Screen voltage taps and caps used there.

 

[Test Point]

[quote author="Gus"]I have tested old caps in some fenders. Sometimes they are fine. I used a lc102 and check value,DA, ESR and leakage at working voltage.

Now there might be something to some players complaining about the tone going away after the cap replacement. The old stuff often can still have LOW ESR. Some new caps are crap.

Remember the first two B+ taps have alot to do with the tone Plate and Screen voltage taps and caps used there.[/quote]

I agree to a point. what I see alot of are amps that haven't been played in years and they (the owners) turn the amp on and it hums....usually the caps have dried out and are non-reformable at that point for the most part. If the amp is played alot, usually the caps are OK and then it's usually just resistor change time as the old carbon comp resistors get micro-cracks and cause alot of noise, especially the preamp plate load resistors and the screen grid resistors get toasty...and you're right, it's getting harder to find good caps anymore. I just had to recap a Marshall Plexi and customer wanted LCR caps installed (that was OEM for those amps) and I had a devil of a time finding them here in the states (I finally found some at Angela Instruments). I guess they aren't being made any longer...I have heard that J/J (formerly Tesla) is making direct replacements but I haven't tried them yet. I do use their tubes alot and they seem to be made pretty well and don't sound too bad.

For what it's worth,

TP

 

[Butterylicious]

Excuse me Mr. Test Point,

You say that the "Dreaded Cracklies" are often caused by pre-amp plate resistors going high. Would this problem hold true for a Soldano SLO-100? A buddy of mine has brought this same amp to me like 3 times in the past 2 years. Each time I work on it, it lasts for a few months and it's back to the same old problem.

The first time I worked on it, I ripped out some mods that had been put on it to lower the gain which he was told, "you want it this way & it will fix your problem." Just pulling the mod eliminated the cracklies. The second time, one of the inputs had quit working and that was 1/2 a dead 12AX7. The third time was a cap job which I told him was about due in the first place. That time it lasted better than a year. I was kinda bummed when he called. I don't charge my friends to fix their stuff, but I am disappointed on a personal level that this amp keeps coming in for visits.

 

[Test Point]

Butterylicious...
Is the problem cracklie/staticky (sp?) noises that happen intermittently? It could be a tube (any of them for that matter) and the way to find out is to replace each tube with a known good one one at a time and see if the noise goes away with the replacement...what is really hard is when you have several noisey tubes...at that point, I just put a whole new "known good" set in and see if noise goes away. If not, than you probably have a, or some, bad plate resistors. The value going high isn't necessarily going to cause a noisy signal...the way it's been explained to me is that with the hundreds (or maybe thousands) of heat/cold cycles that a carbon comp resistor goes through, it starts to get "micro" fractures in the resistive substrate which arc (micro-arcs if you will). That is what causes the noise. I have a friend that calls the noise static bombs and sometimes, that's what it sounds like. I suppose that the net effect of the microcracks is that the value of the resistor will creep up because there is more resistance between the carbon particles. I usually replace the carbon comps with flame proof resistors but there are some good carbon comps made today if one wants to stick with that kind of resistor. The flame proofs are made of metal oxide or metal film...I'm not sure how quiet they'd be in a high gain circuit but I've never had complaints using them and to my ears they are very quiet. That's not saying much.... :shock:

good luck

TP

 

[Mbira]

I posted a reply a couple days ago-but I don't see it...I'm out of town on a different computer.

The amp is from a music store, so they aren't comparing this amp to how it used to be, but rather other Twins from this era...

Hey CJ-what am I looking for when I measure the AC across the speakers? Would I be putting thru a signal at the same time?

I will check the polarity-that may well be the problem.

The caps are the originals so it may well be time for a cap job. The thing is that the B+ is good. Also, the signal doesn't look different before and after audio caps (there doesn't seem to be any noise going into the signal)How do you know if the caps are bad if B+ is good and there doesn't seem to be really excessive noise. Tapping around doesn't seem to reveal any specific bad parts or resistors-Though there may be in the trim section.

Joel

 

[mikka]

Old or dry cathode bypass caps on the pre valves can drop gain in the pre stages causing low volume. Sounds like the amp needs a complete cap job anyway........ new valves......new plate resistors..... time for some maintenance.

 

[NewYorkDave]

I think CJ was just having some fun with you. Taking an AC voltage measurement across the speakers would be little better than useless. You need to use a resistive load to perform any kind of meaningful power output test. And besides, your ears and possibly your speakers would give out before you reached the full power output of the amp.

There's a zillion things that could cause reduced power output. Are the tubes old? Have you checked the voltages? Is the bias set too high? Anyway, step one is doing a proper output test to eliminate the whole subjective variable from the equation. For instance, the amp might be fine but the speakers might be inefficient/****** up/wired up wrong, etc.

90% of the time, if the trem oscillator fails to oscillate and the tube isn't bad, it's the caps in the phase-shift network. Occasionally it's the resistors. And sometimes the neon lamp inside the "bug" goes bad. You can just cut the thing open and build a new bug with a fresh NE-2 neon lamp and black shrink-tubing. The LDR itself rarely goes bad.

Crackling in old Fender amps is often caused by moisture that's been absorbed by the eyelet boards. This sometimes goes away if you let the amp bake for a good long time and do not store it in a damp place. I've heard of some extreme cases that required dismantling of the board and dipping in solvent to drive out all the moisture :shock:

 

[CJ]

I believe I read the volts on the speaker thing a long time ago in Jack Darr's book. I still have the book, I will re read tonight. I think another method mentioned was using a 100 watt light bulb and seeing how bright it gets. But that is a resistive load.

 

[CJ]

power to the people.

 

[CJ]

OK, I remember now. There was a root or square in there that I remember, so I was wondering what I forgot.

Measure the ac volts. Square it, and divide by the speaker impedance.

Procedure:

Borrow an analog Simpson meter from an old timer.

use alligator clips so you can play and measure at the same time.

wait till the neighbors are at work.

wear ear plugs or phones and play Snowblind thru a stomp box..
(compression helps keep needle still)

Try to get an average reading off the ac voltmeter.

eg.

24 volts of full tilt boogie measured on the terminals.



24 times 24 is 576.

576 divided by 8 ohm now cooked Celestion G-12-30 pre bulldog alnico is 72 watts rms.

this way, you are using the reactive load, which is more real than a pwr resistor.

And, you are using a real git, not a sig gen.

the low end of snowblind is better than trying to get 100 watts off an Eruption hammer on.

this does not tell you much about the speakers, which can be very inefficient unless they are D-150's, which are highly unlikely to be in that amp unless someone knew what they were doing.

 

[NewYorkDave]

(Sigh)... yes, it IS reactive, and that's the problem when it comes to taking a measurement. And have you ever looked at an impedance vs. frequency graph for a loudspeaker? The simple volts-squared-over-resistance method you'd use for a resistive load goes out the window: the speaker is a MOTOR, reactive, with back EMF and all that good stuff. And telling the guy to pound his speakers (and his ears) with the full output power of his amp is just irresponsible.

What do you mean by "more real?" Resistors are imaginary? They don't dissipate power? Come on. I think you're getting mixed up because you read somewhere that reactive load boxes sound better than resistive, which could very well be true, but we're talking about taking a measurement, not building a "power brake" device.

It's time for some basics on the problems of measuring power in a reactive load:

http://en.wikipedia.org/wiki/AC_power

 

[CJ]

Well, I said wear plugs, maybe you caught the post while I was editing.

Is going to a rock concert irresponsible? Maybe.

Power measurement at what frequency?

100 watts of 2000 hz will make you deaf.
100 watts of bass won't.
so where do you spec your power?
Where do you spec a transformer's pri impedance?

You could get a piece of nichrome wire and measure a white noise signal's temp increase in a fish tank, or you could send the amp to a calibration lab?

:guinness:

 

[NewYorkDave]

[quote author="CJ"]What's the matter, did you lose the hot dog eating contest?[/quote]

What's the matter, you can't refute anything I wrote on a technical basis, so you have to insult me personally instead?

The test you described will not give him any kind of meaningful figure that he can use to determine if the amp is putting out what it should, unless he performs the exact same test with all conditions/loads equal on another identical amp known to be good. And then a load resistor, signal generator and scope starts to look like the "easy" method, at least to me...