very rapid low frequency motorboating

[Colas LeGrippa]

Hi, it's the first time I get this problem. The amp is a Traynor reverb mate. The prob is like a reversed plate leads: when standby sw. on, even with input shorted to gnd, a loud sound comes out of the amp. But instead of a loud squeal heard with anode leads reversed, it is a loud low frequency very rapid motorboating sound.

WHAT I DID SO FAR:

-replaced all the tubes and checked bias
-disonnected reverb and trem circuits from the turret board and reconnected everything back
-pulled out rev and trem tubes and disconnected the rev tank
-checked out the voltages, all OK
-replaced the 40uF caps feeding the plates and the screens by 80 uF ones
-pulled out V1 : same problem, then checked resitors values at PI, everything's fine

The prob comes either from the PI circuit ( though all voltages are fine there ), from the audio transformer, or from hell

No, I haven't smoked nothing yet...

Thanks, bros

Colas




 

[SILVERGUN]

Is this a stock amp?
Have any other mods been done?
I ask because this symptom would be common if you had cascaded gain stages that shared a cathode bypass cap...could be a rookie mistake

[Colas LeGrippa]

No other mods than the filtering caps replacement have been performed, the amp is stock. Like I said, the prob appears with only the phase inverter tube and the power tubes connected.

Thanks

[SILVERGUN]

I checked the library for a schematic with no luck.
Have you tried disconnecting NFB? (if it's there)

[VMS]

Might be one of the decoupling caps:


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

[SILVERGUN]

I checked the library for a schematic with no luck.

maybe this one?
http://el34world.com/charts/Schematics/files/traynor/Traynor_YRM-1.pdf

[Colas LeGrippa]

I haven't tried to disconnect the nfb (yes it's still there, the amp is stock) but sure I have checked the two .1uF decoupling caps,  I get minus 17V at both of them, exactly the good bias voltage according to the schems I get here, tacked to the back of the top cover of the amp.
I'll perform more tests gee I am fed up with this particular one
Thanks buddies !!!


Colas

[DummyLoad]

Hi, it's the first time I get this problem. The amp is a Traynor reverb mate. The prob is like a reversed plate leads: when standby sw. on, even with input shorted to gnd, a loud sound comes out of the amp. But instead of a loud squeal heard with anode leads reversed, it is a loud low frequency very rapid motorboating sound.

WHAT I DID SO FAR:

-replaced all the tubes and checked bias
-disonnected reverb and trem circuits from the turret board and reconnected everything back
-pulled out rev and trem tubes and disconnected the rev tank
-checked out the voltages, all OK
-replaced the 40uF caps feeding the plates and the screens by 80 uF ones
-pulled out V1 : same problem, then checked resitors values at PI, everything's fine

The prob comes either from the PI circuit ( though all voltages are fine there ), from the audio transformer, or from hell

No, I haven't smoked nothing yet...

Thanks, bros

Colas

schematic? pics? soundclip?


--pete

[jjasilli]

+1 to VMS.  Also checkout Tubenit's troubleshooting procedures & flowcharts.

[Colas LeGrippa]

It is NOT the kind of motorboating due to insufficient filtering capacitance. I am still troubleshooting other options

Thanks.

Coca Colas

[DummyLoad]

I haven't tried to disconnect the nfb (yes it's still there, the amp is stock) but sure I have checked the two .1uF decoupling caps,  I get minus 17V at both of them, exactly the good bias voltage according to the schems I get here, tacked to the back of the top cover of the amp.

the decoupling caps that VMS is referring to in that link are the power supply B+ caps for the preamp stages. not the coupling caps. e.g. the electrolytic cap(s) for pre-amp power.

--pete

[jjasilli]

-replaced the 40uF caps feeding the plates and the screens by 80 uF ones
Maybe undo that mod.


Noise coming seemingly from nowhere might be caused in the bias supply, but you give no schematic.  Is it cathode or fixed bias?

[Colas LeGrippa]

I have the schematcs tacked onto the top cover but It's too big to take a clear picture of it. The amp is fixed bias with trem and reverb. I ended up replacing all the coupling caps and the ceramic caps (discs),  some were microphonic. When I was knocking on the cabinet, the amp was starting to hum like hell (I had changed the tubes so that wasn't the issue). Now I can play the amp. The trem is working very well but the reverb is a pain in the....neck. I don't know if the tank is defective but when I reversed the input and output leads, it  works just the same but pretty soon a high squeal feedback takes place and I have to unplug the speakers to stop it quick, the s b switch is too long cutting the sound. When the reverb tube pulled out, the reverb knob acts as a treble knob, seems a problem to me. The Fender reverb circuit with the transformer works better, but the Traynor's tremolo is fantastic.


Colas

[VMS]

I have the schematcs tacked onto the top cover but It's too big to take a clear picture of it.

Take two or more pictures, we can put them together then.

[kagliostro]

Are you sure that while you were working the amplifier was not infested by something like this?

click >> Haunter


 


I didn't resisted, sorry


Franco

[Colas LeGrippa]

right on Franco !