Gain Difference between Vibrato & Normal Ch on Fender Twin & Deluxe Reverb

[g1]

I agree with Tom's assessment in post #2 about the LDR resistive element. Have you tried disconnecting one end of the LDR resistive side?

[nevetslab]

I agree with Tom's assessment in post #2 about the LDR resistive element. Have you tried disconnecting one end of the LDR resistive side?

I haven't tried that yet, but as I don't have any plans to modify the amps to switch off the Tremolo's Intensity control, I can see from calculating the attenuation that 50k pot introduces, I have no doubt it would substantially increase the Vibrato Ch's gain.

No, the issue that still stands is WHY have I been seeing this difference in gain between Vibrato Ch vs the Normal Ch over time. I see in all of the schematics I have of both Deluxe and Twin Reverb amps the same resistors used in the tube stages, tone circuits, reverb circuits and vibrato Intensity circuits. In time, I may find what it is. This is mostly just one of those things I've come to notice and isn't a problem to any of our clients using our rental stock of amps. I have way too much to do to keep up with the repairs/maintenance demands than treating this observation as a hot project to solve. As stated before, just one of those things that make ya go hmmmmmm.......

[Chuck H]

... the issue that still stands is WHY have I been seeing this difference in gain between Vibrato Ch vs the Normal Ch over time.

Can I assume you're talking about your more recent notice of the vibrato channel being lower in gain with the newer production amps? Because I think you recognized why the vibrato channel had higher gain in the older amps.

[Chuck H]

re: The trem roach:

The reverb mix triode plate is always playing into the 50k pot load. Leo reports that the trem may be set to two or three in the testing. All that needs to be done to see if the trem roach resistance is responsible would be to set the vibrato intensity to zero and recheck gain for either channel. At that point the roach is entirely eliminated from the circuit.

[g1]

Do the AC voltages under test conditions match the re-issue schematic? (2.7VAC vs 3.8VAC)
I'm wondering if it may be just poor tolerance on the newer volume pots. You would have to find a way to control for pot variance in your comparisons.

[Chuck H]

I'm wondering if there isn't a difference in the roach resistance for newer units? That's why I'd like to know the results of testing with the vibrato channel intensity set to zero. If Leo has been testing with the vibrato channel intensity set at two or three (considering it's a reverse ausio taper pot) a low roach resistance for newer units could make a considerable difference. And...

From what I've read the "off" resistance for the older trem roach was about two megs. So where do the newer units read? And are they prone to low resistance as a failure mode? Possibly an early failure mode?

[nevetslab]

I'm wondering if there isn't a difference in the roach resistance for newer units? That's why I'd like to know the results of testing with the vibrato channel intensity set to zero. If Leo has been testing with the vibrato channel intensity set at two or three (considering it's a reverse ausio taper pot) a low roach resistance for newer units could make a considerable difference. And...

From what I've read the "off" resistance for the older trem roach was about two megs. So where do the newer units read? And are they prone to low resistance as a failure mode? Possibly an early failure mode?

That COULD well be where the issue is, taht of the "off" resistance of the olter Tremolo roach. I haven't looked that that thus far. I think there's another Fender Twin out in the collection of road cases due to come onto the bench, though don't yet know what its' problem is yet, or what vintage it is. This will take some time to sort thru, as we have 28 Fender Twin Reverb's and 21 Deluxe Reverb's in our inventory.

I didn't notice any significant difference in the Intensity setting of the Tremolo being turned on, set to around '2 to 3'. Granted, I'm looking at the RMS average acoustic level (RMS SLOW) to Pink Noise, so there's a fluctuation of around 1dB during the burst 'ON' time during the measurement. Amp chassis is normally in the chassis during these measurements, though sometimes, it may be on the service cradle when dealing with the issues it came in for.

I will look into this issue on the range of resistance of the LDR portion of the roach, as well as the effects of the loading as the Intensity is turned up. I rarely have it up to levels like we heard in that aged tune ''Rumble" from the 50's or early 60's.