Tonal and volume differences in Fender amp NORMAL and VIBRATO channels

[Wes]

Is it possible to make the two channels of a classic Fender amp sound alike?

Am I right in thinking that the additional circuitry in any given amp's VIBRATO channel makes that channel different
than the NORMAL channel?

[pdf64]

You need to pick a particular model really, as a generic response may have a lot of exceptions.
If the fx channel includes reverb too, that makes rather a difference, so need to specify that.

[Leo_Gnardo]

Is it possible to make the two channels of a classic Fender amp sound alike?

Am I right in thinking that the additional circuitry in any given amp's VIBRATO channel makes that channel different
than the NORMAL channel?

To answer the second question first, yes the vib channel signal goes through an extra attenuation, filter and stage of amplification, reverb/dry mix, and also has the attenuation/hf rolloff from the vibrato level control. The channels do sound different; often it's the vib channel that sounds stronger but not always.

You could put the dry channel thru identical extra circuitry or do what I do and mix the channels prior to reverb/vibrato. In my case the operation's done to offer the customer different sounding channels but no reason you couldn't make them identical. Tubes count, as well as R's & C's & pick pots with identical tapers too if you want "same numbers = same sound"

[Wes]

Let's say a Pro Reverb. It has a normal channel and then a vibrato channel with reverb. The extra circuitry in the vibrato channel causes the channel to sound different than the normal channel, correct?

Let's say an early 60's Super Amp- while not having reverb, it does have the vibrato circuit that is very different, and more complicated, than all other Fender vibrato circuits. Wouldn't all of that additional circuitry cause the vibrato channel to sound noticeably different from the amp's normal channel?

Ultimately- I have folks telling me that the channels should sound the same in any of these old Fender amps. I am of the mind that that is impossible due to circuit differences in the channels.


super_6g4a_schem.pdf

Is it correct to assume that it is not possible for the channels in an amp like this to sound alike due to their circuitry differences?

[überfuzz]

Kill the Tremolo and Reverb you've got a good start.

Seriously, the frequency response of the reverb circuit is not the same as the rest of the circuit, but given the fact that you're looking for similar sound you're not interested in reverb or tremolo in the matching. I'd say you'd be able to come pretty close, with a little fiddling with the tonal controls.

[Leo_Gnardo]

Let's say an early 60's Super Amp- while not having reverb, it does have the vibrato circuit that is very different, and more complicated, than all other Fender vibrato circuits. Wouldn't all of that additional circuitry cause the vibrato channel to sound noticeably different from the amp's normal channel? <snip> Ultimately- I have folks telling me that the channels should sound the same in any of these old Fender amps. I am of the mind that that is impossible due to circuit differences in the channels.

Brown Supers, Concerts & perhaps a couple others had an unusual "vibrato" where the signal was split into hi and low frequency bands, similar to an electronic crossover, then brought up one after the other when the vibrato was on: first highs, then lows, highs, lows, etc. The slight difference in circuit transit time for the signals resulted in that wooshy almost phaser sound. But when the vibrato's off, that crossover & summing amp are still there contributing their sonic thumbprint - and in these cases it's very obvious. Tough time making a pig out of a sausage. Naturally it does not sound like the non-effects channel.

I am of a mind to think your observations are correct. When you change something, you change something, the channels won't sound identical. At least in the general run of '64-on Fenders like the Pro you mentioned, they did make a reasonable attempt to retain the tone in the vibrato channel. With these, my bet's on the rev/dry mixer stage: with the original signal passing thru a voltage divider involving a 3M3 and 220K resistor, a lot of signal's being thrown away and as observed in voltage dividers of all sorts including volume pots, the high frequencies seem attenuated more than mids & lows. To counteract that, Fender engineers put a 10 pF cap across the 3M3 in an effort to retain those highs and the well-known Fender tone we like, well many of us anyway. If you're really devoted to making the two channels sound near as dammit, I'd recommend fiddling with the values in that mixer stage especially 3M3 and 10pF. Just a smidge either way, you may be able to get 'em close enough so it would be near impossible to tell.

[Chuck H]

Anon: "I am of a mind that the two channels should sound the same with the exception that one of them is affected. Only the effects should be apparent as the difference"
You: "The amps are not made that way. There is signal processing that changes the sound in the effects channel."
Anon: "That is unacceptable. Can you make them sound the same?"
You: "No,"
Anon: "Maybe I need to deal with someone else.?."
You: "Ok. Thank you for stopping at Wes's amp repair."

[cyclone]

but I SO want an apple to be just like an avocado. hey, why doesn't my normal channel on my vibrolux sound as normal as the normal channel on my Pro Reverb? Its, well, not NORMAL!

[loudthud]

It would be interesting to replace the bridge capacitor in a Fender BF with a trim cap. You would need to find one that can handle the Voltage. Perhaps a small Mica compression type 5pF to 20pF.

[Wes]

That cracked me up Chuck! So true!

Thanks for the responses and considerations, guys. I trust your more expert opinions about these things. I am no expert, I am always learning. I find it important to do so as well as check that what I am learning is as correct as possible and/or making sense.

In a nutshell and bluntly: I have some of those customers who "know everything" about amps and which ones sound best. They claim that an old Fender amp's channels should sound identical at the same settings. They keep on with this even after I show them the differences and explain the differences in the circuitry. Worse yet is that they never test this for themselves until their amps have been worked on.

Most players get an amp, find settings they like, and run with it. Who shows up at a practice or a gig and says, "Hmmm....I think I might just spend a few minutes comparing the characteristics of my amp's channels..."? There is no scrutiny until the device is taken out of use for repairs or service. "It sounds different!" No shit, I replaced your crackly, 40%+ out of spec resistors and leaky coupling caps!

"I miss the crackles and that mud sound."




Leo_Gnardo:

Brown Supers, Concerts & perhaps a couple others had an unusual "vibrato" where the signal was split into hi and low frequency bands, similar to an electronic crossover, then brought up one after the other when the vibrato was on: first highs, then lows, highs, lows, etc. The slight difference in circuit transit time for the signals resulted in that wooshy almost phaser sound. But when the vibrato's off, that crossover & summing amp are still there contributing their sonic thumbprint - and in these cases it's very obvious. Tough time making a pig out of a sausage. Naturally it does not sound like the non-effects channel.


Gotcha. On this particular amp, the vibrato channel has a noticeable scooped out mid/low mid kind of sound. The highs are sparkly and the bass is a little more subdued. The normal channel has a fat mid sound and a bit more bass.

[tubeswell]

Let's say a Pro Reverb. It has a normal channel and then a vibrato channel with reverb. The extra circuitry in the vibrato channel causes the channel to sound different than the normal channel, correct?
...
Is it correct to assume that it is not possible for the channels in an amp like this to sound alike due to their circuitry differences?

If you're talking the AA165, the normal and vibrato channels go through different numbers of gain stages prior to the LTP. The normal channel has 2 triodes, one before and one after the tone stack. The vibrato channel also has these, but then goes through the post reverb recovery mixing stage prior to the LTP. Consequently, the way in which the frequencies are shelved and then re-amplified has different outcomes. So it will be somewhat difficult to get these two channels sounding the same.

[pdf64]

Brown Supers, Concerts & perhaps a couple others had an unusual "vibrato" where the signal was split into hi and low frequency bands, similar to an electronic crossover, then brought up one after the other when the vibrato was on: first highs, then lows, highs, lows, etc. The slight difference in circuit transit time for the signals resulted in that wooshy almost phaser sound. But when the vibrato's off, that crossover & summing amp are still there contributing their sonic thumbprint - and in these cases it's very obvious. Tough time making a pig out of a sausage. Naturally it does not sound like the non-effects channel.[/I]


Gotcha. On this particular amp, the vibrato channel has a noticeable scooped out mid/low mid kind of sound. The highs are sparkly and the bass is a little more subdued. The normal channel has a fat mid sound and a bit more bass.

The particular value of the components used in the crossover section will be subject to their manufacturing tolerance and then potential drift over the decades; this could result in a mid scoop noted in one model but a mid hump in another.

[pdf64]

As it's not been noted yet, the trem channel of many twin channel BF non-reverb amps, eg Tremolux http://bmamps.com/Schematics/fender/...a763_schem.pdf tends to be slightly weaker than the normal channel, due to the heavy loading presented by the 50k trem intensity control to the previous stage.
It may be psycological to do with the slightly lower volume, but it also seems to make the trem channel tone seem a touch 'thinner' than the normal one.

[Chuck H]

Also worth noting WRT to BF type amps is that the two channels are out of phase. If you sit in one spot or stand in basically one place on a stage and plug from one channel into the other you may notice different feedback and harmonic characteristics.

And FWIW the two channels sometimes ARE intended to be basically the same. And they are basically the same. Just not exactly the same. I'm as fussy as anyone and it just wouldn't break my heart if I had to finish a gig through the "normal" instead of the "vibrato" channel.

[Leo_Gnardo]

In a nutshell and bluntly: I have some of those customers who know everything about amps and which ones sound best. They claim that an old Fender amp's channels should sound identical at the same settings. They keep on with this even after I show them the differences and explain the differences in the circuitry. Worse yet is that they never test this for themselves until their amps have been worked on.

Challenge those "know it all" types: tell 'em "Show me a Fender amp that actually does as you say it should, identical tones from channel to channel. And not the PA-100 wise guys." Sounds to me like they learned by gleaning sparkly factoids on the interwebs, never by actually listening to amps, or worse yet (horrors!) reading a schematic.

Then: "Gosh-a-rooties, guess your amp is broken mister know it all, give ya $20 bucks for it." (put devil horned smiley thing here)

If you do get to meet Wild Bill, just for him I got a box of funky old caps from the 50's-60's-70's crammed with UOS and a couple NOS, guaranteed antique electrolytic goodness, everything from bubbly earwax & paper caps up to big ol' 1000 uF 450V Sprague(!) beer cans from 1966, hope he doesn't lose control with the excitement.