Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: luthierwnc on February 05, 2022, 06:06:47 pm
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Hi All, A few months ago I built my version of an HC-30 head with a send/return stereo jack on the triode channel. Sounds just like it should. Lately I've been assembling the pedalboard to go with it and was curious about actually using that loop. Other builds have Dumbleators (RIP) active tube loops inside and on that board I've got a 4-cable set-up. On this one I homebrewed a switch so the board can either be for amps with un-buffered loops or no loop at all.
Attached is a diagram showing the switch and where things go. The loop-switcher has five channels. Four are for front-of-amp FX. The fifth is for an Empress Tape Delay pedal and a Keeley Hydra with reverb and trem. The no-loop position is simply the tail of the chain. In the loop position those effects go to the amp send. I think the Keeley output impedance of 10k should work fine. If not, I have a Neunaber Wet that is fairly loop-friendly. When more cords and gizmos arrive I'll test them.
This is more of a theoretical question. I haven't seen (and can't find) the definitive wisdom on using the Matchless loop. There is also no guarantee that I won't want to put different pedals in the chain that have their own issues. Does it make sense to put a little buffer on the send and return, possibly with level controls, to keep the signal behaved? Cord lengths are relatively short. Right now the Empress Buffer looks promising but I think Radial has a device that will do the same. I expect the Suhr Minimix can too or I could slap together a Klein-ulator.
Thoughts are welcome and thanks for looking. Skip
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updated circuit, sh
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Does it make sense to put a little buffer on the send and return, possibly with level controls, to keep the signal behaved?
Yes, your biggest concern is overloading the input of the first effect after the SEND
A schematic of the amp/loop in question would help.
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A buffer typically refers to a circuit that has little if any voltage gain, but presents a high impedance to its input and has a low impedance output.
It’s therefore beneficial to use a buffer to feed the fx send jack, as it avoids cable capacitance interacting with the typically high impedances in a valve amp, and causing excessive treble roll off.
A buffer per se isn’t beneficial for the fx return, as fx as universally have a sufficiently low output impedance to avoid treble roll off.
What can be useful in the fx return is a voltage amplifier, in case the fx max signal level is less than the amp was intending at the loop insert point.
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Thanks gents,
I received this reply from Keeley within minutes of asking them:
It should work just fine. You'll will have options with/without buffers too. I have both the ES5 and the ES8 (small/large boards) I know the ES8 had buffers on the in and outs that you can toggle on/off. If the ES5 does not have this option (I honestly do not remember lol) you can toggle the Hydra into Trails Mode, that will keep the output buffer of the Hydra online.
The Hydra has a lot of output on tap so you may not even need to buffer it and it may compensate for a colder FX loop, but if you do the option is included especially if you notice some of the top end gets shaved off. If you find that it still needs bumped or high end needs compensated (all FX loops are a little different) AND the ES5 has that option you can crank the output up to +6dB just like the ES8 on said loop. I use this option for a couple quiet effects, or for a provided back-lined amp that has a cold loop on the larger shows.
I hope this helps!
Sean Spears
Service Manager
What I didn't know -- operator error -- is that each loop on the ES-5 already has a switchable buffer. It's new in constructing this board. Figuring that out will tax my pay-grade but I want to get this thing going by spring! Thanks for looking and best on your projects. sh
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Here's the switch I ended-up soldering together, sh