Tremolo circuit missing resistor

[Mat Cortex]

Hi
Lately, my 1965 Gibson Falcon GA-19RVT tremolo faded away. In the repair process it appear that R44 is missing, the resistor right before the depth pot. I repair the amp ( cold solder on C19) and every thing were back to normal (R44 still missing). BTW, I would like to salute Ampgeek whose helped me trought the troubleshooting process.

Here's my question: why someone would removed that specific resistor or change its value ?

Can some one help me (newbie) understand this ?

Thanks
Mat

[martin manning]

R44 determines the maximum depth of the tremolo. With the pot R4 connected directly to C2, the greatest possible bias voltage swing is available.

[JazzGuitarGimp]

If R44 were just removed, the tremolo will not work. Was R44 removed, and replaced with wire (short)?

Also, if R44 has been replaced with a short, as Martin suggests, this may explain why the tremolo doesn't always work, because the load on the output of the oscillator is increased significantly, which may cause the oscillator to shut down.

[statorvane]

Is it possible VR4 was also replaced with a 500K pot?

[JazzGuitarGimp]

Is it possible VR4 was also replaced with a 500K pot?

That is an excellent question, and a possibility I hadn't considered. However, if you run the numbers, you'll find that changing the 250K pot for 500K doesn't really buy much in the way of a lighter load on the LFO:

- With the stock circuit, the maximum load on the LFO is when the intensity is all the way up = 270K in series with the parallel combination of the 250K intensity pot and the two 220K grid leak resistors; so 270K + 76.4K = 346.4K.

- With the stock circuit, but R44 jumpered out, the maximum load (again, with intensity pot all the way up) = the parallel combination of the intensity pot and the two 220K grid leak resistors = 76.4K

- With R44 jumpered out, and the intensity pot replaced with 500K, the maximum load (intensity pot all the wayup) becomes the parallel combination of the 500K pot and the two 220K grid leak resistors = 500K || 110K = 90.2K.

Not to mention the intensity pot needs to be reverse audio taper, which are difficult to source.

[martin manning]

...but the 220k grid leak resistors don't go straight to AC ground. The power tube grid impedance is too high to worry about, and on the input side each has the 0.02 PI coupling cap and 100k plate load. At 10 Hz the 0.02's look like 800k ohms, so I don't think there is any problem. For this type of tremolo to sound good I would be looking for the p-p voltage of the LFO signal at the junction of the 220k's to be about equal to the power tube bias voltage.

[Mat Cortex]

Hi
thanks to all of you for your replys. the cap have been solder directly on the pot. So maximum depth control range were changed. Actually the depth controle goes from almost none to clear cut. I dunno how to explain the sound i'm listenig to.

here's the photo of the cap and pot

Regards
Mat

[JazzGuitarGimp]

...but the 220k grid leak resistors don't go straight to AC ground.

I stand corrected. Thank you, Martin. So the difference in LFO load is 520K (regardless of intensity pot position) and 250K if R44 is jumpered. This still might be enough additional load to make the oscillator shut down - especially if a weak tube were used for the LFO. If the LFO is intermittent, I think this is the first place I'd look.

[Mat Cortex]

Sorry guys for my next question but.... what mean LFO?

Why the designer of this amp doesn't make it to be that way (R44 jumped)?

by changing R22 value, can i change the frequency range? lower the value, lower the range?

[xtian]

what mean LFO

low frequency oscillator. usually a sine wave.

[Mat Cortex]

thanks !!!