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? re FX loops in tube amps (warning - on-topic!)
? re FX loops in tube amps (warning - on-topic!)
Hi guys. I hope I'm not spoiling anybody's fun by asking a basic
electronics question, but I'm sort of stumped and I figure I should
ask people before trying anything stupid.
First of all I'd like to thank everyone who advised me earlier, when I
was building my "Miniquad" EL84 amp. It's working great, I like it a
lot. I especially would like to thank Jochen, J.P., and everyone who
gave me circuit advice. I did rebias my phase inverter, that had been
ridiculously cold (0.3 ma), it does sound nice and punchy now. So I'm
happy with it.
I don't think I want to tinker too much with this amp, but I might be
building another someday. Or maybe I might want to put an effects
loop into this one. But I can't figure out how to do that.
My question is this - how do you put an effects loop into a tube-based
guitar amp?
I'm aware that people might say "don't put an FX loop into a tube amp,
put pedals in front of it and run it clean". I appreciate this point
of view. But if you've got a lot of preamp gain it can munch up a
reverb or a chorus really well. Personally, I like a tube amp running
distorted. If I want to use a pedal in front, I do, but I like the
preamp tubes running hot as well. Yes you can still put a chorus or
delay in front - like on Live at Leeds, Pete uses a tape-delay and you
can hear the initial note overdrive his amp while the delay repeats,
at much lower volume, come through clean and undistorted - but if you
want more of a sound like the Edge has, where the distortion comes
before the effect and not after . . . a bit of versatility is fun, is
all. And an FX loop really would be a cool thing to have in a tube
amp.
If it was a solid-state design it'd be a simple thing - I'd just put
it between the preamp and the power amp. Just like a Peavey Bandit,
nothing tricky about it. But I'm afraid to do that all the same. The
voltage levels just don't match solid-state effects there.
For example, my 'miniquad' has 2 12AX7 gain stages, the tone controls
and master volume, another gain stage, a split-load PI, and then the
EL84 power tubes. A line-level signal input injected at the master
volume output is sufficient to run the power tubes to their limits.
If I had a solid-state preamp, I'd just put the FX loop right there.
But since my preamp is also tube, voltage levels there can be quite
excessive for an FX send. There can easily be 70 Vrms at the output
of the tone stack. And I would expect turn-on transients of nearly
the full B+ voltage, which I have to worry about as well. If I put
that into a solid-state reverb unit, it certainly wouldn't sound good,
even if it survived. And I imagine a floor pedal type delay would
just plain melt.
What can I do? I could pad it down for an FX output, but then a lot
of gain will just - - go away when the FX loop is used.
Audio transformers might do the job well, if I could find one I could
run in a 12AX7 plate circuit and do a 100K/600 impedance
transformation there, and have a mate on the other side to bring it
back up, but I don't know where I'd find suitable ones for an FX loop
or how much they'd cost. Do people usually use transformers for this?
The bulletproof way to do it might be to just have a full FX loop
section, which pads the signal down, limits it with Zeners to clear
out turnon transient spikes and other random shocks, buffers it with a
solid-state amplifier (this could be a simple source-follower MOSFET
couldn't it?), and then brings it up to the 70V level. The last
amplification stage would be possible to accomplish with solid-state
but probably difficult. Or use another 12AX7 there. Seems like a
whole lot of work.
How do you guys handle interfacing solid-state effects with tube
circuits?
Hi guys. I hope I'm not spoiling anybody's fun by asking a basic
electronics question, but I'm sort of stumped and I figure I should
ask people before trying anything stupid.
First of all I'd like to thank everyone who advised me earlier, when I
was building my "Miniquad" EL84 amp. It's working great, I like it a
lot. I especially would like to thank Jochen, J.P., and everyone who
gave me circuit advice. I did rebias my phase inverter, that had been
ridiculously cold (0.3 ma), it does sound nice and punchy now. So I'm
happy with it.
I don't think I want to tinker too much with this amp, but I might be
building another someday. Or maybe I might want to put an effects
loop into this one. But I can't figure out how to do that.
My question is this - how do you put an effects loop into a tube-based
guitar amp?
I'm aware that people might say "don't put an FX loop into a tube amp,
put pedals in front of it and run it clean". I appreciate this point
of view. But if you've got a lot of preamp gain it can munch up a
reverb or a chorus really well. Personally, I like a tube amp running
distorted. If I want to use a pedal in front, I do, but I like the
preamp tubes running hot as well. Yes you can still put a chorus or
delay in front - like on Live at Leeds, Pete uses a tape-delay and you
can hear the initial note overdrive his amp while the delay repeats,
at much lower volume, come through clean and undistorted - but if you
want more of a sound like the Edge has, where the distortion comes
before the effect and not after . . . a bit of versatility is fun, is
all. And an FX loop really would be a cool thing to have in a tube
amp.
If it was a solid-state design it'd be a simple thing - I'd just put
it between the preamp and the power amp. Just like a Peavey Bandit,
nothing tricky about it. But I'm afraid to do that all the same. The
voltage levels just don't match solid-state effects there.
For example, my 'miniquad' has 2 12AX7 gain stages, the tone controls
and master volume, another gain stage, a split-load PI, and then the
EL84 power tubes. A line-level signal input injected at the master
volume output is sufficient to run the power tubes to their limits.
If I had a solid-state preamp, I'd just put the FX loop right there.
But since my preamp is also tube, voltage levels there can be quite
excessive for an FX send. There can easily be 70 Vrms at the output
of the tone stack. And I would expect turn-on transients of nearly
the full B+ voltage, which I have to worry about as well. If I put
that into a solid-state reverb unit, it certainly wouldn't sound good,
even if it survived. And I imagine a floor pedal type delay would
just plain melt.
What can I do? I could pad it down for an FX output, but then a lot
of gain will just - - go away when the FX loop is used.
Audio transformers might do the job well, if I could find one I could
run in a 12AX7 plate circuit and do a 100K/600 impedance
transformation there, and have a mate on the other side to bring it
back up, but I don't know where I'd find suitable ones for an FX loop
or how much they'd cost. Do people usually use transformers for this?
The bulletproof way to do it might be to just have a full FX loop
section, which pads the signal down, limits it with Zeners to clear
out turnon transient spikes and other random shocks, buffers it with a
solid-state amplifier (this could be a simple source-follower MOSFET
couldn't it?), and then brings it up to the 70V level. The last
amplification stage would be possible to accomplish with solid-state
but probably difficult. Or use another 12AX7 there. Seems like a
whole lot of work.
How do you guys handle interfacing solid-state effects with tube
circuits?
Re: ? re FX loops in tube amps (warning - on-topic!)
<morris.slutsky@gmail . com > wrote
....
> My question is this - how do you put an effects loop into a tube-based
> guitar amp?
Copy the Fender The Twin, or another later Fender... tube-buffered
send and return, flexible enough for either pedals or rack-mount stuff.
Seriously, spend your commode time reading and understanding schematics.
__
Steve
.
<morris.slutsky@gmail . com > wrote
....
> My question is this - how do you put an effects loop into a tube-based
> guitar amp?
Copy the Fender The Twin, or another later Fender... tube-buffered
send and return, flexible enough for either pedals or rack-mount stuff.
Seriously, spend your commode time reading and understanding schematics.
__
Steve
.
Re: ? re FX loops in tube amps (warning - on-topic!)
On Fri, 02 May 2008 00:50:39 GMT, "Stephen Cowell"
<scowell@sbcglobal . net > wrote:
>
><morris.slutsky@gmail . com > wrote
>....
>> My question is this - how do you put an effects loop into a tube-based
>> guitar amp?
>
>Copy the Fender The Twin, or another later Fender... tube-buffered
>send and return, flexible enough for either pedals or rack-mount stuff.
>
>Seriously, spend your commode time reading and understanding schematics.
>__
>Steve
>.
>
That's funny...I just finished booklet 40 of the 1961 NTS Radio and TV
Electronics course doing this...but don't stay long or you can get the
prep H problems..now I find out I don't have the rest of the course!
Damnmit! That's what you get from Fleabay!!!
On Fri, 02 May 2008 00:50:39 GMT, "Stephen Cowell"
<scowell@sbcglobal . net > wrote:
>
><morris.slutsky@gmail . com > wrote
>....
>> My question is this - how do you put an effects loop into a tube-based
>> guitar amp?
>
>Copy the Fender The Twin, or another later Fender... tube-buffered
>send and return, flexible enough for either pedals or rack-mount stuff.
>
>Seriously, spend your commode time reading and understanding schematics.
>__
>Steve
>.
>
That's funny...I just finished booklet 40 of the 1961 NTS Radio and TV
Electronics course doing this...but don't stay long or you can get the
prep H problems..now I find out I don't have the rest of the course!
Damnmit! That's what you get from Fleabay!!!
Re: ? re FX loops in tube amps (warning - on-topic!)
On Thu, 1 May 2008 15:22:28 -0700 (PDT), morris.slutsky@gmail . com
wrote:
>The bulletproof way to do it might be to just have a full FX loop
>section, which pads the signal down, limits it with Zeners to clear
>out turnon transient spikes and other random shocks, buffers it with a
>solid-state amplifier (this could be a simple source-follower MOSFET
>couldn't it?), and then brings it up to the 70V level. The last
>amplification stage would be possible to accomplish with solid-state
>but probably difficult. Or use another 12AX7 there. Seems like a
>whole lot of work.
>
>How do you guys handle interfacing solid-state effects with tube
>circuits?
Morris, If the circuit is the one that I remember, you probably don't
have extra tubes to work with, right? Not sure if you can punch the
chassis, or if the transformer will handle extra filament current (it
probably can).
So my first suggestion would be to get some gain back, probably by
bypassing your cathode resistors (I don't think you had bypass caps)
and build a small attenuator/mixer network something like what's in a
Twin, as Steve has suggested. Except maybe with lower resistor
values. Then send the signal out to a small digital reverb, and back
in after the attenuator. You'd have to make sure you don't have dry
signal or it could feed back directly (depending on circuit impedances
-- I'll have to take another look). The digital reverb will sound fine
and will be fun to tweak. You are right about placement in the chain.
Kevin O'Connor's book "The Ultimate Tone" has some FX loops, and I
believe he used Mosfets somewhere (don't remember what circuits), so
maybe you could get some ideas if you can find that.
Confirm whether you're still working with your previous schematic and
I'll try to check it again. I did have some suggestions regarding
gain scaling and component values but didn't get around to posting
those.
On Thu, 1 May 2008 15:22:28 -0700 (PDT), morris.slutsky@gmail . com
wrote:
>The bulletproof way to do it might be to just have a full FX loop
>section, which pads the signal down, limits it with Zeners to clear
>out turnon transient spikes and other random shocks, buffers it with a
>solid-state amplifier (this could be a simple source-follower MOSFET
>couldn't it?), and then brings it up to the 70V level. The last
>amplification stage would be possible to accomplish with solid-state
>but probably difficult. Or use another 12AX7 there. Seems like a
>whole lot of work.
>
>How do you guys handle interfacing solid-state effects with tube
>circuits?
Morris, If the circuit is the one that I remember, you probably don't
have extra tubes to work with, right? Not sure if you can punch the
chassis, or if the transformer will handle extra filament current (it
probably can).
So my first suggestion would be to get some gain back, probably by
bypassing your cathode resistors (I don't think you had bypass caps)
and build a small attenuator/mixer network something like what's in a
Twin, as Steve has suggested. Except maybe with lower resistor
values. Then send the signal out to a small digital reverb, and back
in after the attenuator. You'd have to make sure you don't have dry
signal or it could feed back directly (depending on circuit impedances
-- I'll have to take another look). The digital reverb will sound fine
and will be fun to tweak. You are right about placement in the chain.
Kevin O'Connor's book "The Ultimate Tone" has some FX loops, and I
believe he used Mosfets somewhere (don't remember what circuits), so
maybe you could get some ideas if you can find that.
Confirm whether you're still working with your previous schematic and
I'll try to check it again. I did have some suggestions regarding
gain scaling and component values but didn't get around to posting
those.
