Jumpering Channels with Blackface Reverb Amp

[Charlie Wilson]

OK I have a brain teaser for you that I can't figure out. A friend of mine has a Blackface Deluxe(new reproduction) and wants to be able to jumper from the vibrato channel to the normal channel. Yes, I know they are out of phase and doesn't work very well in the normal manner. On a couple of other forums, it was hinted at that it could be done with a two conductor cable. It was also hinted at that the second input does not need an earth ground just a negative signal. So maybe this involves isolating one of the jacks from ground? No one was explicit about how to do it. Any ideas? Also, he wants to do it with a cable not a splitting phase reversing box.
CW

[xtian]

Nope. Sorry. Won't work.

Our guitar cables have two conductors. One is the shield, also called ground, and has ZERO AC (signal) voltage on it--it also provides a voltage reference for the signal. The other conductor is for the AC signal. If you try reversing the wires, you'll get zero sound.

You need to get the signal into the correct polarity, and this can only be done with an inverting gain stage. That means electronics (tube gain stage, FET gain stage, isolating transformer with phase switch, etc.).

[Tony Bones]

You could mod the amp so that the two channels mix before the existing third gain stage. That way you'll end up with reverb and trem on both channels.

[Stevem]

Why does he want this, as the normal channel is sounds just the same as the Vibrato channel but for a bit less gain, or in other words cleaner cleans!

For the Money it would take to do the mod for such you might as well set the Vibrato channel for good cleans and then get a stomp box that will drive the channel harder for solo work.

Also since its a reproduction amp they use a circuit, a cheap cheap circuit board that if you just look at it for too long the dam circuit traces start to lift no less it you touch it with a hot iron and then there's the start of 3 times the normal labor cost to do any repair or mod to the amp.

These type of service / work these amps where made to have done on them is to change out tubes when needed and adjust the bias when needed and if you want them to sound better then it's time to rip that cheap ass board out of there and install a good old tag board!

[martin manning]

OK I have a brain teaser for you that I can't figure out. A friend of mine has a Blackface Deluxe(new reproduction) and wants to be able to jumper from the vibrato channel to the normal channel. Yes, I know they are out of phase and doesn't work very well in the normal manner. On a couple of other forums, it was hinted at that it could be done with a two conductor cable. It was also hinted at that the second input does not need an earth ground just a negative signal. So maybe this involves isolating one of the jacks from ground? No one was explicit about how to do it. Any ideas? Also, he wants to do it with a cable not a splitting phase reversing box.

I wonder if these forum guys are misinformed or speculating about the how this is done, and it is actually some version of the known channel mixing mod that Tony B mentions. The reverb on both channels mod as seen in the '68 Custom Deluxe Reverb is very easy to do on the '65 DRRI, if your guy wants to go that way. What that arrangement does is fully parallel the second stages of both channels (they already share Rk and Ck), making a tube mixer.

[Charlie Wilson]

Thanks guys. Yeah I suggested the reverb on both channels thing and he is considering it. The vibrato channel on a blackface amp can be a bit thinner and brighter than the normal channel, especially on a Deluxe reverb. He just wants to fill it out a little mixing in some of the normal channel. The amp is one of those Blankenship vintage style builds so it is easy to work on.
CW

[JD0x0]

The 'thinning out' is due to the 3.3Meg/10pF network, if I'm not mistaken. This resistor is meant to attenuate dry signal, but the large value cuts highs, so to compensate for that, they add the bypass cap. The 10pF value, especially if drifted a bit high, creates a perceived treble 'boost' as it overcompensates a bit, so you essentially have something similar to a 'bright switch' on. You could adjust that network to remove the thinness on the vibrato channel or just add snubber caps to smooth the highs out a bit more.

Putting the channels in phase is nice with a re-voiced channel. You can then blend the tones easily.

[jjman]

Also the Vib channel has the secrete bright cap and the Normal does not.