Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dusty712 on July 01, 2025, 10:39:42 am
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I've been working on finishing up my amp and the last thing remaining is the effects loop that I wanted to install. I'm using the Kevin O'Connor kit (https://londonpower.com/electronics/tube-effects-loop/) and find that it works well with my old t.c. electronic GMajor where I can control the send/receive levels on the unit (in addition to the loop's send/receive levels). The problem I'm running into is with regular pedals that take instrument level signal. I have to keep the send signal level really low, otherwise they clip the signal; the side-effect to this is that the return stage of the loop doesn't seem to be able to bring the signal level back to a strong enough signal to match the level originally coming out of the preamp.
I was hoping to get some help and some ideas on any potential solutions. The simplest solution I can think of is to throw a clean boost or eq pedal in the loop to make up for the volume loss. I'm hoping that there are better solutions to this, maybe even something that I could add to the amp to give me a simple volume boost (which could for example be tied to a switch of instrument/line level).
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The return stage has a 1M feedback resistor which limits the return gain to less than 10x. If you remove that resistor you should get the gain you need.
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Thank you very much Merlin for your reply. I hope it's as simple as that, I'd just like to make sure I know what you're referring to (apologies for the beginner questions).
I hope it's okay to include a hand-drawn version of part of the schematic, but if it's not I can remove it. The resistor you're referring to is R3 in the attached diagram? Also, when you say remove it, you actually mean to remove it and break the connection (so there's no feedback) or replace it with a straight wire?
The return stage has a 1M feedback resistor which limits the return gain to less than 10x. If you remove that resistor you should get the gain you need.
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Yes just remove R3, no feedback. If you then have too much gain you can always replace it with a higher value, like 4.7M or something.
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Thanks again Merlin for the advice. No feedback definitely brought up the volume a bit; it's still not quite at unity, but it's better. It also looks to have had an effect on the parallel mix where the dry signal appears to be louder; I'll have to double check if this is indeed the case, or if I'm just hearing things.
Yes just remove R3, no feedback. If you then have too much gain you can always replace it with a higher value, like 4.7M or something.
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You've got the return pot set to max, and the pad switch set to -20 right?
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Yep, return pot is set to max, but the his kit doesn't seem to have a pad switch (just a parallel/serial switch and send/return levels). I don't know if the pad switch is something that he discusses in his book, but didn't make it to the kit (or I'm just completely not seeing it)
You've got the return pot set to max, and the pad switch set to -20 right?
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Hmm, I'm surprised you're not getting enough gain then. A pedal should be able to give you 1V, so you should be getting close to 40-50V after the return stage :w2:
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The voltages help, I'll take a look. Thanks again for your help, really appreciate it.
Hmm, I'm surprised you're not getting enough gain then. A pedal should be able to give to 1V, so you should be getting close to 40-50V after the return stage :w2:
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3M is highish for a pre-amp triode grid leak (unless they're shooting for some weird hybrid grid-leak-bias/cathode-bias setup). Could be the bias of the stage isn't optimal - maybe replace the 3M grid leak with 1M (if you've taken out the local NFB resistor)
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BTW this is the circuit (found on the other forum), but it shows a 12AT7 which will have less gain than I assumed. Is that what you're using?
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I am indeed using a 12AT7 which I didn't even think about. I'll try a 12AX7 as well, maybe that will be enough for what I need.
Thanks for sharing the schematic. It's close to the kit's PCB layout, but the kit has the send/receive level pots outside the circuit (pots are tied to the respective jacks), no -20db toggle (it looks like a 47.5k is used) and some of the component values are quite a bit different. I'll take a closer look.
Also I just want to make sure that my post isn't taken as me saying that the loop isn't working well. It's likely just my configuration since it works very well with line level signals.
BTW this is the circuit (found on the other forum), but it shows a 12AT7 which will have less gain than I assumed. Is that what you're using?
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This seems to be the fx loop kit on offer, 'use any 12A_7 tube type' and no level switching, just send and return level controls
https://londonpower.com/electronics/tube-effects-loop/
https://theultimatetone.com/thread-39.html
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Yep that's the one.
This seems to be the fx loop kit on offer, 'use any 12A_7' type and no level switching, just send and return level controls https://theultimatetone.com/thread-39.html
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Swapping to a 12AX7 should give about 5dB more return gain https://www.thegearpage.net/board/index.php?threads/updated-tube-comparison-table.2279173/
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Thanks again everyone for the help. Using a 12AX7 gave me the last little boost that I needed. That combined with removing feedback gives me better range when using stompboxes.