Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Ed_Chambley on July 24, 2012, 01:31:53 pm
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Anyone ever installed an effects loop on a Princeton. I've been thinking of a simple loop, maybe even a mojo board as I would like to get some modulation further down the circuit, but the bias vary will affect this as well. There is probably a very simple way, but I have not been able to find where anyone has done it. As you can see from the layout, the chassis is fairly full, but I do have some room. I seem to be getting a capacitance loss in the effects chain caused by delay/echo that cannot be really be overcome by eq and a booster. Specifically, I am speaking of a MXR Green analog delay and a Wampler Faux Tape Echo.
I am getting a very good James Burton sounding cluckin tone when plugged into the amp, but it disappears when delay is added. Everything else sounds great just in-front, but there is something about getting a quick, strong slapback that kills the snappy sound. I have looked a various loops, just asking for a little experience here before I begin as I would like to rule out what others may have tried they were not pleased with.
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I have NOT installed an effects loop on a PR. However, if I were going to .......... I would try a passive effects loop 1st.
I have used a MXR Carbon Copy Delay and a Boss Digital Delay in passive effects loops like this & I have been very happy with the results. In fact, when I built a D-lator (active effects loop) to use in the passive effects loop ........... there was not a substantial enough improvement in tone to warrant keeping it.
So, both of my current amps have just a passive effects loop & I like the delays in them.
With respect, Tubenit
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I have NOT installed an effects loop on a PR. However, if I were going to .......... I would try a passive effects loop 1st.
I have used a MXR Carbon Copy Delay and a Boss Digital Delay in passive effects loops like this & I have been very happy with the results. In fact, when I built a D-lator (active effects loop) to use in the passive effects loop ........... there was not a substantial enough improvement in tone to warrant keeping it.
So, both of my current amps have just a passive effects loop & I like the delays in them.
With respect, Tubenit
Thanks, I have to install the jacks anyway, this looks simple and that is what I was looking for. I'll give it a go.
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Seems like a MOSFET buffer before the Send jack would make sense but I have not played with this yet. Without a buffer there, any resistance to ground in the effects loop would be in parallel with the 470K plus 100K pot which make up the ground leg of a voltage divider with the 3.3M resistor. I learned the hard way to be very careful when tinkering in this area of a Blackface Fender.
Another question is whether you want the effects loop before, after, or in parallel with reverb. I don't know the answer, but if you took the Send before the 3.3M resistor and Return after it then you'd need to knock the signal down to match the dry signal.
Hope that helps,
Chip
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Depends what you want the loop to handle. Geetar type FX pedals handle signal levels of around -10dBV, Rack-mount equipment 0dBV, CD players +4dBV.
The output impedance of the send section ideally needs to be low in order to prevent unwanted coupling into the send cable. if you want a simple line out, then taking the send signal from a plate 9 via a voltage divider, will have a relatively high output impedance. Whereas attaching the send to an unbypassed cathode will give a really low output impedance and a low signal strength that is low enough to ensure that the signal hitting your FX pedal will not distort unwantedly. You can even put a 5k pot in that type of send circuit to further cut the signal, without noticeably upsetting the output impedance. And if you use a tip-switching jack on the send, you can combine it with a bypass cap to ensure that the stage cab be bypassed when the FX send is not in use. This is an idea from Merlin's 1st book (but this is my own schematic of it)
(http://farm8.staticflickr.com/7115/7657260914_aa635144b1_b.jpg)
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Adding to Tubeswell's post, the input impedance of the send buffer needs to be high to avoid messing up the dry/reverb mix in this circuit. A tube triode or a MOSFET set up as a cathode follower the way tubenit does in his amps would serve nicely.
Respectfully,
Chip