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Post subject: Reverb On Normal channel
Posted: Wed Feb 11, 2015 5:29 am
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Aspiring Musician
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Location: australia
hi my DRRI amp made in 2004 has reverb on the Normal channel and it,s dead stock unit./You can here it when the normal Volume knob is at two and the reverb turned up at five.But when turned up past four The reverb Goes away Is this normal ..Has any one Have the same thing on there amp.s Cheer,s Have a nice day...


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Post subject: Re: Reverb On Normal channel
Posted: Sun Feb 15, 2015 4:37 pm
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It could be normal. I've seen bleedthrough on a scope, but never had anyone complain about it. There is a common cathode capacitor on V1 and V2 in the preamp that causes it.
It is wired that way to allow 2 guitars to be plugged in at one time. One in the normal channel and the other in the Vibrato channel. The common cathode acts to resolve phase differences between the two channels. It's kind of a mixer circuit that is found on most fenders with normal and Vibrato channels.
Since few pickers ever plug in two guitars, you can separate the channels. Take out the 820 ohm resistor and the single 25/25 capacitor. Replace it with two 25/25 caps and a 1.5K resistors. One on each cathode.
This slightly improves the detail and responsiveness of both channels.


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Post subject: Re: Reverb On Normal channel
Posted: Mon Feb 16, 2015 12:08 am
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Not quite TimsAudio. There is one extra half of a tube for the Vibrato channel, which is one more phase shift than the Normal channel, making the two channels out of phase, as the signal comes from the Plates of the tubes, not from the Cathodes. You can still plug in two or more instruments, or use both channels jumped together, but the Normal channel will always be out of phase with the Vibrato channel, unless the circuit is modified.

My guess is that the reverberation that you're hearing is either the tubes ringing, maybe even the springs in your guitar, or something else in the room, maybe not from the amp.

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Last edited by shimmilou on Mon Feb 16, 2015 12:14 am, edited 1 time in total.

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Post subject: Re: Reverb On Normal channel
Posted: Mon Feb 16, 2015 12:13 am
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shimmilou wrote:
Not quite TimsAudio. There is one extra half of a tube for the Vibrato channel, which is one more phaes shift than the Normal channel, making the two channels out of phase, as the signal comes from the Plates of the tubes, not from the Cathodes.


Correct! Should not be significant interaction between channels. Crosstalk maybe due to coupling, from failing filter caps. How old are the main caps under the doghouse?


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Post subject: Re: Reverb On Normal channel
Posted: Mon Feb 16, 2015 10:06 am
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I wasn't suggesting that the channels were in phase.
You can observe bleedthrough at the common cathodes. While the signal path is through the plates, if one channel is passing a positive wave and the other channel is passing a negative wave, the common cathodes will sum those two waves.
The result is that the plate of one channel will not be able to fully swing negative if a positive wave is present on the other channel. It's not a true mixer, but it does resolve phase differences.
Why else would Fender incorporate this circuit on so many of their amps? It's not because they're cheap, its because there is a specific function for the circuit.


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Post subject: Re: Reverb On Normal channel
Posted: Mon Feb 16, 2015 10:35 am
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Aspiring Musician
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Joined: Thu Feb 26, 2009 2:55 am
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Location: australia
Hi All well its not The tubes.or the guitar,s it does it with acoustic guitar,s played thru Normal channel as well, Or One at a time with my Electric Guitars with No trem as well .The reverb Is There you can here it as i say On the normal channel with just one guitar plug,ed in to it .The amp is nearly ten year,s Old but i have used it every week for that time and has been,to a Tek every 12 month,s or so,the cap,s in the dog house seem fine,The amp has been doing the same thing from new as well .you know you find out these thing,s playing around with a new amp..Its not a problem To me just wondering why it does it. i did a show with a friend a few weeks ago and we used acoustic guitar,s played thru both channel.s And we herd the reverb coming thru The normal channel when just turned up to two.,i played with the reverb Knob up and down,and it came thru loud a clear,Till you turned up the normal channel past four then it goes away.Its all good Fun, Have a nice Day,,Cheer,s fellas...


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