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

[nevetslab]

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........

[Tom Phillips]

One possibility could be due to the tremolo circuit.
If the intensity control is fully CCW then it will present a 50k Ohm load to the Vibrato channel signal. However, if the intensity is turned up then the load will be the residual value of the pot in parallel with the LDR in the light bug. If the LDR has failed and presents an abnormally low dark resistance or a circuit malfunction is keeping the neon lamp on, then the overall low resistance will load down the vibrato channel signal. This situation is really a long shot but there you have it. Might be worth checking out.

[Jon Snell]

Take a look at the schematic and all is revealed. V4B is the difference.
The 'volume' difference is normal.

[nevetslab]

One possibility could be due to the tremolo circuit.
If the intensity control is fully CCW then it will present a 50k Ohm load to the Vibrato channel signal. However, if the intensity is turned up then the load will be the residual value of the pot in parallel with the LDR in the light bug. If the LDR has failed and presents an abnormally low dark resistance or a circuit malfunction is keeping the neon lamp on, then the overall low resistance will load down the vibrato channel signal. This situation is really a long shot but there you have it. Might be worth checking out.

In all the contol settings normally dialed in, the Tremolo Intensity control is usually set around 2 to 3, with Rate around the same, and Tremolo engaged.

[Jon Snell]

Looks like, yet again, you are chasing a 'fault' that doesn't exist.
I respectfully suggest you learn about schematics and how the circuits work. That will stop you losing sleep over nothing.

[Chuck H]

You're correct that the vibrato channel will have more gain due to the extra gain stage. The reason it's only a couple of dB is the voltage division used to balance the dry and reverb signals before re amping at that extra stage. I think your Twin Reverb, which seems a little high at 5dB could be due to drifted resistance somewhere in that channels circuitry that could raise gain (CC resistors usually drift high) or a higher gain triode for the vibrato channel. And vice versa, could be a drifted resistor or a lower gain triode in the normal channel lowering gain relative to the vibrato channel.

As to the DR and it having lower gain on the vibrato channel I'd be looking for a fault. Low gain triode, drifted resistor, slightly leaky cap skewing preamp stage bias, etc. Also worth considering would be the actual taper of the control pots for each channel. If you don't have them set full up that is. If you don't then retry your tests with all the knobs (except reverb and vibrato controls) full up and adjust/reduce the input signal. Then check respective gain for both channels again.

[Justin Thomas]

I'm with Chuck - pots are hardly precision components & I wouldn't trust any two two act anywhere close to the same at the same "numbers on the dial." Eliminate that variable & see what happens.

Jusrin

[g1]

Looks like, yet again, you are chasing a 'fault' that doesn't exist.
I respectfully suggest you learn about schematics and how the circuits work. That will stop you losing sleep over nothing.

John, he mentioned that this issue is something that has changed with the new amps compared to the older style. It was stated in the first sentence of the first post. He acknowledged the traditional difference and said this is something different than this amp..
If there is something different in the re-issue schematic you posted that is different from the classic versions (AB763); which you think would account for the change in the newer amps, please mention it.

[Jon Snell]

See the circled numbers on the attached schematic. Not difficult to work it out!

[Chuck H]

See the circled numbers on the attached schematic. Not difficult to work it out!

Ok. So then what might cause the anomalous behaviors noted? Which I think was the point of the thread. In fact I don't see where nevetslab said the higher gain on the vibrato channel was abnormal. He mentioned that it was something he'd noted in the past to a greater or lesser degree and that he had an amp on the bench that was contrary. No idea why you said he should learn to read a schematic.

I offered my ideas for what might cause the anomalous behavior in the DR. If you have anything else on the matter we'd all like to hear it.

[Chuck H]

Still on the pots...

As noted earlier I don't know your knob setting for the UUT.

A lot of repairs are done with plain ol off the shelf Alpha pots. I'm sure some are done with OEM Fender parts too and I'm not sure what you do in your own shop. I usually just use what I have on hand. Which is usually an Alpha pot. But I don't run a repair shop. Alpha pots have a 15% log taper and (IIRC) the Fender OEM volume pots for their two channel reverb amps is and was historically a 20% log taper. If the DR on your bench has ever had the vibrato channel volume pot replaced with something other than an OEM Fender pot there's a good chance it's a lower log % than the original Fender part.

[nevetslab]

I have original Fender Replacement pots for when I find they've become worn out and no longer respond to cleaning or the shaft is broken from something hitting it. So, it keeps the same Log Taper as was installed at the factory. I think most of the newer Fender amps in our inventory actually belong to Fender (as are a lot of other mfgr's gear in our rental inventory). So, in that area, I keep with the original parts. Though I still plan on changing to log taper pots for the Reverb when I get around to it.

As to the control settings in this area of having found enough Twin Reverb and Deluxe Reverb amps no longer having the higher gain on the Vibrato Ch over the Normal Ch, I've used both tone controls at '5' settings, as well as at '10', along with the volume pots always at '10'. when making the comparative acoustical measurements. It's not a big deal, but something that makes me wonder WHY am I no longer hearing the higher gain from the Vibrato Channel vs the Normal Channel as I've always been accustomed to hearing. I don't see the answer in the schematics but would assume it to be component differences. I HAVE NOT been finding 12AY7's or 12AT7's in the preamp stages to account for that.

[g1]

See the circled numbers on the attached schematic. Not difficult to work it out!

It shows the opposite of what nevetslab is talking about. If you are going to jump on somebody and criticize like that, please make sure you read the post carefully first.

I was actually measuring lower output level on the Vibrato Ch vs the Normal Ch on a Deluxe Reverb amp

[drewl]

Recently modded a Silver Deluxe Reverb with a pot/switch to remove the trem circuit and without that 50K pot loading there is a nice volume boost.

some of the newer Fender amps have reverb on both channels, so the gain of the normal channel is the same if not more than the trem channel as it doesn't have the trem pot loading.
If you want some insane gain from the reverb channel, jumper across the 3.3M/10pf mixer !

[nevetslab]

I was looking at that circuit loading yesterday on the Twin Reverb schematic, as I had yet another twin on the bench to deal with those cheesy input jacks. They tend to become non-muting after several years of use. I had already replaced the Normal Ch jacks with Switchcraft 12A open frame Jacks with Normal. Those last forever, and muting is typically around 10 mOhms to gnd. The stock jacks Fender uses on the Twin, Deluxe and others similar measure as high as 40 ohms as I was seeing on the Vibrato Ch. I have a growing bag of those jacks, hate the hooped wire around the 1/4" threaded sleeve for the Gnd, and have had issues with that over time. I first tried desoldering and then re-soldering the jacks Tip and Tip Normal, and that got my input to ground (normal closed) down to around 40 mOhms. It did quiet the Vibrato Ch, so it will live for a while longer as far as the noisy channel I was hearing before that resoldering.

I was only seeing about 1dB more gain thru the Vibrato Ch vs the Normal Ch on this particular Twin. I spent some time with schematic and PCB layout diagrams looking for anything obviously different in component values but didn't see anything. Calculating the difference in gain without that Tremolo Intensity control loading down the signal from V2B is definitely a limiting factor, though so is that 3.3M resistor from the plate of V2B. I haven't yet seen any component mod around that 3.3M series resistor in the amps I've had open on the bench, though have only recently started looking when I have to remove the chassis from the cabinet.