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Over the years, it seemed like there was always about a 5dB gain or level difference between the Vibrato Ch and the Normal Ch on Fender Deluxe or Twin Reverb amps, both being set the same. Lately, I've been finding that hasn't been the case on their recent production amps. I was actually measuring lower output level on the Vibrato Ch vs the Normal Ch on a Deluxe Reverb amp that's been in our rental inventory for years.
Looking at the schematics, I'd expect to see higher output level thru the Vibrato Ch with it having the extra gain stage following the gain make-up stage after the tone circuit, and does have the Reverb circuit getting mixed in, then the two channels are summed together ahead of the LTPI driver stage.
I have a vintage Bruel & Kjaer 2130 Frequency Analzyer/SPL meter set up with a 1" mic that I usually have fired up when I'm burning an amp 20 ft away running at stage levels, being fed burst USASI Noise from my GenRad 1382 Noise Generator/1396B Tone Burst Gen, fed thru an HP 353A Step Attenuator/Xfmr-coupled to drive the input to amps. Stage level acoustically at the amps is typcially 110dB SPL wideband, and at my desk where the 2130 sits, I usually will set the level for around 94dB to 100dB SPL.
Dropping the send level from the burst noise generator 10dB (-50dBV input to the amps instead of -40dBV) can definitely take the amp out of overdriving the input stage, though usually I use -40dBV as my test signal level.
Looking at the Schematic for a '65 Fender Deluxe Reverb, where they show input signal level at the grid of V1A and V2A of 37mVAC, and then see 2.7VAC at the plate of V1B and V2B, both channels circuits up to that point look the same. Then, following the additional gain stage of V4B for the Reverb mixing into Vibrato Ch, we see 3.8VAC. That equates to 2.65dB higher signal level. Both channels mix together thru 220k resistors, though Normal Ch goes thru a 47nF coupling cap while Vibrato Ch goes thru 100nF.
On the amp I had on the bench, I was actually seeing 1dB lower output acoustically feeding same signal level thru the Vibrato Ch.
I just had a slightly older Twin Reverb here in the shop last week, and on it, I was measuring 5dB greater output thru the Vibrato Ch than thru the Normal Ch. I dropped the input signal level to -50dBV, and still had the same difference.
I haven't started pulling chassis' out to scrutinize the resistors being used in the two channels to look for where this difference in gain is. I just had always been confortable with there being higher gain thru the Vibrato Ch, until recently finding that not to be so.
65_Deluxe_Reverb_Schematic.pdf
65_Twin_Reverb Sch.pdf
GenRad 1382 Noise Spectrum.pdf
I inserted a graph of the GenRad's 1382 Noise Generator's White, Pink and USASI noise spectrums. For bass amps, I normally use Pink noise though for guitar amps, I find the USASI Noise spectrum more typically related to a Guitar's response range.
Has anyone noticed this level difference deviation over the years of Amps produced by Fender to show this. Just one of those things that make me go hmmmmm........
Looking at the schematics, I'd expect to see higher output level thru the Vibrato Ch with it having the extra gain stage following the gain make-up stage after the tone circuit, and does have the Reverb circuit getting mixed in, then the two channels are summed together ahead of the LTPI driver stage.
I have a vintage Bruel & Kjaer 2130 Frequency Analzyer/SPL meter set up with a 1" mic that I usually have fired up when I'm burning an amp 20 ft away running at stage levels, being fed burst USASI Noise from my GenRad 1382 Noise Generator/1396B Tone Burst Gen, fed thru an HP 353A Step Attenuator/Xfmr-coupled to drive the input to amps. Stage level acoustically at the amps is typcially 110dB SPL wideband, and at my desk where the 2130 sits, I usually will set the level for around 94dB to 100dB SPL.
Dropping the send level from the burst noise generator 10dB (-50dBV input to the amps instead of -40dBV) can definitely take the amp out of overdriving the input stage, though usually I use -40dBV as my test signal level.
Looking at the Schematic for a '65 Fender Deluxe Reverb, where they show input signal level at the grid of V1A and V2A of 37mVAC, and then see 2.7VAC at the plate of V1B and V2B, both channels circuits up to that point look the same. Then, following the additional gain stage of V4B for the Reverb mixing into Vibrato Ch, we see 3.8VAC. That equates to 2.65dB higher signal level. Both channels mix together thru 220k resistors, though Normal Ch goes thru a 47nF coupling cap while Vibrato Ch goes thru 100nF.
On the amp I had on the bench, I was actually seeing 1dB lower output acoustically feeding same signal level thru the Vibrato Ch.
I just had a slightly older Twin Reverb here in the shop last week, and on it, I was measuring 5dB greater output thru the Vibrato Ch than thru the Normal Ch. I dropped the input signal level to -50dBV, and still had the same difference.
I haven't started pulling chassis' out to scrutinize the resistors being used in the two channels to look for where this difference in gain is. I just had always been confortable with there being higher gain thru the Vibrato Ch, until recently finding that not to be so.
65_Deluxe_Reverb_Schematic.pdf
65_Twin_Reverb Sch.pdf
GenRad 1382 Noise Spectrum.pdf
I inserted a graph of the GenRad's 1382 Noise Generator's White, Pink and USASI noise spectrums. For bass amps, I normally use Pink noise though for guitar amps, I find the USASI Noise spectrum more typically related to a Guitar's response range.
Has anyone noticed this level difference deviation over the years of Amps produced by Fender to show this. Just one of those things that make me go hmmmmm........
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