Fender Twin Reverb Meets Tektronix 453

[neutronarmy]

Hello,

I am working on a Fender Twin Reverb for a friend who was complaining of a "lack of volume." When I hooked up the amplifier to a four ohm dummy load and monitored its output on a scope. I measured about 70 watts clean output. However, I noticed a rather peculiar waveform at the onset of strong clipping. My first assumption was that the amplifier was suffering from parasitic oscillation. After throwing two other amplifiers under this same test setup I've noticed a similar behavior to varying degrees; the Twin is the most notable offender.

I have started to question whether the issue might be my test equipment/my understanding of it. I'm feeding the amplifiers a 100mV 1KHZ signal from a Heathkit IG-18 signal generator and using a Tektronix 453 scope. Both pieces of equipment were used and rather inexpensive, so I have no doubt that issues in either are a real possibility. I may also be operating the test equipment improperly; my scope knowledge is not nearly as strong as it should be (and fixing that is on a very short list of skills to address quickly). I did try the same setup using a tone generator out of my laptop sound card and got a similar wave form, so I'm ruling the IG-18 out (at least temporarily).

Below are a few quickly shot videos illustrating the situation.
https://www.youtube.com/watch?v=yk4ZAD5oMa8&feature=youtu.be
https://www.youtube.com/watch?v=RVQPiXV_CH8&feature=youtu.be

Thank you so much for any help on this matter!

Take care,
Chris

[Andy Watson]

I think you are seeing the ripple on the amplifier power supply. When you drive the amplifier into clipping - assuming it clips at the output stage - there is nowhere for the signal to go - it hits the power supply rail. (It may not be be the output stage that is clipping - but the same argument applies.) The ripple on the power supply is not synchronised to your signal generator - so it a appears as a blur or fuzz.

Edit: Try setting your scope to trigger on "Line" - the main waveform will become a blur (unless you sync your signal generator) - the ripple on the clipped waveform should become visible.

[neutronarmy]

I think you are seeing the ripple on the amplifier power supply. When you drive the amplifier into clipping - assuming it clips at the output stage - there is nowhere for the signal to go - it hits the power supply rail. (It may not be be the output stage that is clipping - but the same argument applies.) The ripple on the power supply is not synchronised to your signal generator - so it a appears as a blur or fuzz.

Edit: Try setting your scope to trigger on "Line" - the main waveform will become a blur (unless you sync your signal generator) - the ripple on the clipped waveform should become visible.

Andy - thank you so much for your timely response! That makes a ton of sense. I'll play around again with the scope tomorrow and confirm.

[obiwanjacobi]

Fender Twin Reverb is a valve/tube amp, right?

Be VERY careful probing into the power supply. Can have 400 Volts in there. Also beware of the smoothing caps on the power rail - they can hold the 400V charge for quite some time! - even after you have unplugged the amp!

If you start poking into the amp - put one hand in your pocket for safety. That way if you get hit the current wont come near to your heart - it still be as if you got hit by a truck.

On Topic: My guess too, power supply. Could be that the smoothing caps are old and not doing their job. Word of caution: old tube amps are supposed to have a not too stable power rail - that is part of their sound. If you smooth the rail too much (by putting in higher value caps for instance) the sound WILL change - and most vintage lovers will not like it... Just saying, if you're gonna swap caps - make sure its the exact same value (farads - volts may be higher).

Stay safe.

[Grapsus]

Was the reverb effect active during your tests? The reverb tank springs could be picking up mains so you better turn it completely off.

[Keyrick]

1.  Make sure the Vibrato controls (speed and intensity (depth) )are all the way down, or if you have the footswitch, make sure the vibrato is off.  Same with the Reverb control.

2.  100 mV is a little hot.  you may be over driving the front end a little.  Drop down to about 43 mV, which is closer to the output of an electric guitar.

3.  Does the anomaly appear on both channels?

4.  On both channels be sure that all three EQs are set in the middle, around 5.

There are a couple versions of the Twin Reverb.  The original point to point wired ones were rated at 85 watts.  Then there are the Master Volume models that were rated at 100 watts.  Some of those models had what they called an "Ultra Linear" output, whatever that meant.  It was designed to make the output as clean as possible.  The newest models are a modern day replica of the original 85 watt model, but with a PCB instead of point to point wiring.

If you are measuring 70 watts out into a resistive load, the tubes are in good shape, and the owner is complaining of low volume, the trouble could be the speakers.  Take it off of the load and put a different speaker on the output.  Be careful as 70 watts, like that amp, can be stupid loud.  The speakers could just be blown.

Good luck.

Rick

[TimFox]

In vacuum-tube amplifiers, "ultra-linear" usually means that the screen grids are connected to taps on the output transformer primary (typically, 43% of plate swing) for local feedback.  This operation is roughly a compromise between pentode operation (screen grids to DC voltage) and triode operation (screens connected to plates).  It was very common in high-fidelity amplifiers back when stuff was stuff.

[OilsFan]

I'm not sure what is peculiar about those waveforms. It's a tube amp clipping. Am I missing something?