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PostPosted: Tue Jun 26, 2007 5:15 am 
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Hi everyone

I've been reading intensely here for a while but this is my first post. I have to say it's seems to be a great resource and populated by great people.
I'm hoping I might be able to get some thoughts from you guys on an issue that is coming up on the amp i'm currently building.

First I'll briefly describe the project, then detail the issue in question.

It's a DC30 circuit but with tremolo and the tube effects loop from 'The Ultimate Tone Vol.1' (titled 'The Best All-Tube Effects Loop'). I have a 3 position switch on the front which will allow Series, Parallel or OFF....totally switched out of the circuit at both ends and restoring the circuit to the normal non-loop configuration.
Each pair of EL84's has it's own standby switch and triode/pentode switch, so every combination is possible. There is no master volume.

The chasis is aluminium and the transformers are a pair of torroidals i had built to my spec's. I am using a choke and GZ34 rectifier tube. I will try to post a couple of pics as soon as i can work out how to do it.

Okay...now....here's the issue. I want to have both channels run through the effects loop. This will mean summing them just before the PI, the point where the loop is inserted. It looks like the simplest method will be take the signal from the EF86 channel volume pot wiper through a 220K resistor to the PI end of the 220K resistor in the TB channel heading into the PI. I wonder whether there will be loss of gain through the added 220K. I am also concerned about the 100K resistor to ground at that point, if i leave it in it could pull too much gain from the EF86 channel, if i remove it it might create too much gain from the TB channel. Any thoughts?

The second issue is the issue of phase. It looks to me like the 2 channels are out of phase, but as they normally feed each side of the PI, they will end up in phase at the output. If I sum the 2 channels , they will both feed one side of the PI and they will be out of phase, and i imagine will show up that way at the output. I had hoped to avoid that because sometimes it's great to play through both channels together. Basically i can't think of any way i can resolve this, it looks like if i want them in phase at the output, i'd have to run only 1 channel through the FX loop (no room now to install a second loop) and keep to the original 1 channel per input of the PI configuration , or, have the 2 channels through the loop and live with the fact that i have to have the channels out-of-phase.

I think the gain issue is more pressing at the moment.

Any thoughts or comments anyone?

(sorry for the long post, this stuff has been brewing in my tiny brain for a LONG time, hehe).

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PostPosted: Wed Jun 27, 2007 1:17 pm 
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For the Effects loop, do you have to sum them? Why not use a multiple pole jack?

There could definitely be a gain issue here. You might want to increase those values. you will have to experiment a bit I imagine.

Two channels out of phase wont work. How did the original DC30 do this?

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PostPosted: Wed Jun 27, 2007 2:47 pm 
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Hi

Thanks for the response.

Yes, i need to sum the 2 channels as the effects loop is active, it uses a 12AT7, 1/2 for the buffered output, and the other 1/2 for the return. It features 2 level controls, one each for the send and return. The return input can be switched between guitar and line level. My main purpose for the loop will be to run a tape echo in there, and i would like to have it available to both channels. The active loop location is between the pre-amp and the phase inverter.

The DC30 doesn't have the problem because although the 2 channels are out of phase, they are fed to opposite sides of the phase inverter rather than summed.
The VOX AC30 Top Boost models however do sum the TB and Normal channels, and they are out of phase to one another. (The Normal channel uses only 1/2 a 12AX7 before the PI, so it has a 180 degree phase shift exactly the same as caused by the EF86 in the DC30). They feed one input of the PI (the other side is fed by the Tremolo channel).

The gain issue is something i will have to wrestle with by trial and error, as you say.

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PostPosted: Sat Jun 30, 2007 2:21 am 
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When I speak of 2 channels being out of phase to one another, the Fender amps also come to mind....the models with reverb. They have an extra 1/2 of a 12AX7 in the reverb channel, and hence another 180 degree phase shift before they sum and go to the Phase Inverter. (2 stages for the normal channel, 3 for the reverb).

I did read once that Brian May of Queen used his famous booster into one channel of his Voxes rather than using the old trick of jumping the guitar into the Top Boost and Normal channels with a short lead because he was aware that those channels were out of phase and didn't care for the sound whn they were combined.

Of course it would be entirely possible to play into both channels but feed the input to one of the channels through a 1 transistor booster, set to unity gain, to take advantage of the transistor's 180 degree phase shift and hence bring it back into phase with the other channel.

I will be experimenting with this stuff as soon as this amp is going, which is not far away now.

I have been trying to find a way to get some pics up here for you....haven't been succesful yet though.....

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