How to get preamp overdrive with fx loop?

[Jacew]

This may sound a weird, dumb or downright pointless question, but currently makes a perfect sense for my situation and I can't apparently figure this out myself. And I currently don't have time to sensibly explain why I have this problem. (long story...)

So, how does one attenuate the signal of preamp before passive loop without losing the preamp overdrive. The amp in question has preamp volume (gain) right before singal goes to PI. Signal from preamp is too high for any effects in loop. But adding attenuator in loop works exactly like the gain pot and cuts out the preamp distortion! So the whole fx loop becomes pointless...

How is this generally made work in amps? I know loops should be usually buffered, but wouldn't attenuator still work like in this case.

Or is the attenuator circuit used in buffer for fx loop somehow different? I don't see how it could be...

[Joey Voltage]

Re: How to get preamp overdrive with fx loop?

This may sound a weird, dumb or downright pointless question, but currently makes a perfect sense for my situation and I can't apparently figure this out myself. And I currently don't have time to sensibly explain why I have this problem. (long story...)

So, how does one attenuate the signal of preamp before passive loop without losing the preamp overdrive. The amp in question has preamp volume (gain) right before singal goes to PI. Signal from preamp is too high for any effects in loop. But adding attenuator in loop works exactly like the gain pot and cuts out the preamp distortion! So the whole fx loop becomes pointless...

How is this generally made work in amps? I know loops should be usually buffered, but wouldn't attenuator still work like in this case.

Or is the attenuator circuit used in buffer for fx loop somehow different? I don't see how it could be...

That sounds like an odd situation. typically in amplifiers that have a lot of preamp gain and a loop, the signal is padded at the end of the preamp with a fixed potential divider that takes the place of where the master volume typically is (this divider acts as if the master volume was turned down very low). All the overdrive has already happened at that point, so the attenuation only brings the signal level down, not the distortion level. This is usually buffered by the send stage to present a reasonable z to drive cables, and ensure no additional loading or degradation of the signal. The signal from the fx then gets re-amplified via the fx return to bring the signal up about where it was before attenuation, and from the loops return you have a master volume feeding the PI. It sounds like the amp you have relies somewhat on overdriving the PI to get any kind of overdrive, else the placement of the gain pot as you describe wouldn't really be acting like a gain pot more than just a volume pot. if It has some sort of PPIMV, honestly there is no good placement for the fx loop with in the amplifier, and it is best to use a line out to drive the fx, and then use something else to reamp the signal afterwards essentially using the amplifier as one big slave preamp. What amp is this? It may be easier to help you if we knew at least that part.

[Boogie Bill]

Re: How to get preamp overdrive with fx loop?

What amp?

What pedal(s) are you trying to put in the loop?

Specifics are helpful.

Bill

[VinceT]

Re: How to get preamp overdrive with fx loop?

What amp?

What pedal(s) are you trying to put in the loop?

Specifics are helpful.

Bill

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[Jacew]

Re: How to get preamp overdrive with fx loop?

It sounds like the amp you have relies somewhat on overdriving the PI to get any kind of overdrive, else the placement of the gain pot as you describe wouldn't really be acting like a gain pot more than just a volume pot. if It has some sort of PPIMV, honestly there is no good placement for the fx loop with in the amplifier, and it is best to use a line out to drive the fx, and then use something else to reamp the signal afterwards essentially using the amplifier as one big slave preamp. What amp is this? It may be easier to help you if we knew at least that part

It's Tiny Terror modded with passive fx loop. And it is designed to overdrive the PI, so that's probably the problem. It just never occurred to me that so much of the distortion amp is capable of comes from that PI tube. It's not really "overdriven" tone without that, more like "slightly grainy"...

Does PPIMV mean Post or Pre PI master volume? (it's dumb abbreviation if you ask me...)

Yesterday I did some experimentation with attenuators, some pedals I had around and Mustang modeler. It seems it could be made to work with a booster (or preamp) that's capable of amplifying the signal at least at line level, preferably slightly over line level.

I don't really have a specific thing I need the fx loop for. I'm just expanding (and exploring) the versatility of TT.

Already found out that it can do brilliantly singing cleans with reverb in fx loop after the attenuator Preamp probably drives pedal bit harder, and cutting the gain there saves a lot more high end than cutting the gain from amp (gain pot has two sliders, so it isn't just cutting the preamp output) or guitar.

[Jacew]

Re: How to get preamp overdrive with fx loop?

it is best to use a line out to drive the fx, and then use something else to reamp the signal afterwards essentially using the amplifier as one big slave preamp.

I've been weighting between this and modifying the signal in fx loop into useful condition. I decided to start with the cheaper option... If this doesn't work out, I might try that in the future...

I just have doubts that solid state power amp driving speakers wouldn't sound nearly as good as TT, despite the preamp section. Another tube amp is not an option budgetwise.

[jeremy]

Re: How to get preamp overdrive with fx loop?

ppimv is post phase inverter mv. it allows you to clobber the pi then bring the signal down

[Jacew]

Re: How to get preamp overdrive with fx loop?

ppimv is post phase inverter mv. it allows you to clobber the pi then bring the signal down

Ok. Thanks. So the entire problem is just that the preamp and PI works as a "single unit" rather than the preamp shaping the tone and power amp just amplifying it.

I haven't been able to get the level right after the loop, and since the "get the effects after distortion" thing is obviously not going to work, I think I've done with this. Obviously installing an effects loop in Tiny Terror was one of those things everybody says is not going to work, but you have to try it out to find out why exactly...

At least I can stick volume pedal in that loop for a easy access to gain knob. Perfect way to clean up the amp

[Boogie Bill]

Re: How to get preamp overdrive with fx loop?

Normally, wouldn't you run an attenuator between the power amp and the speaker? This allows the GAIN to be set for optimal preamp distortion; the MASTER VOLUME sets the overall level, and then the attenuator sets the overall acoustical volume, so that the power amp can be pushed hard, into power amp distortion, if desired...yet maintain a reasonable SPL.

Loops are good for post-preamp effects like reverb, delay, EQ and some modulation effects. Using attenuators or boosts in the loop is often an iffy proposition, IME, dependent on amp design.

Bill

[Joey Voltage]

Re: How to get preamp overdrive with fx loop?

It's Tiny Terror modded with passive fx loop. And it is designed to overdrive the PI, so that's probably the problem. It just never occurred to me that so much of the distortion amp is capable of comes from that PI tube. It's not really "overdriven" tone without that, more like "slightly grainy"...

Does PPIMV mean Post or Pre PI master volume? (it's dumb abbreviation if you ask me...)

Yesterday I did some experimentation with attenuators, some pedals I had around and Mustang modeler. It seems it could be made to work with a booster (or preamp) that's capable of amplifying the signal at least at line level, preferably slightly over line level.

I don't really have a specific thing I need the fx loop for. I'm just expanding (and exploring) the versatility of TT.

Already found out that it can do brilliantly singing cleans with reverb in fx loop after the attenuator Preamp probably drives pedal bit harder, and cutting the gain there saves a lot more high end than cutting the gain from amp (gain pot has two sliders, so it isn't just cutting the preamp output) or guitar.

Yes, that particular amp is PPIMV, or post phase inverter master volume, and yes a lot of its overdrive is coming from the phase inverter in that design. If you put a loop in the typical place (between the preamp, and PI) in that amp, especially a passive one, it will really bring your signal levels down, and may not heavily overdrive the proceeding circuitry. All is not lost though, but you would either have to use something active, or slave it. You do have some space issues in that amp, so the active part would be a tight fit if you could even muster it, you would also have to have a little more complex of a loop design if you wanted to reap the benefit of post distortion loop placement, other wise the loop placement between the preamp and PI would be less than ideal, but could still be made to overdrive the PI, although at the expense of having the fx signal over driven as well which wouldn't be much better than just putting them through the input.

[Jacew]

Re: How to get preamp overdrive with fx loop?

Normally, wouldn't you run an attenuator between the power amp and the speaker? This allows the GAIN to be set for optimal preamp distortion; the MASTER VOLUME sets the overall level, and then the attenuator sets the overall acoustical volume, so that the power amp can be pushed hard, into power amp distortion, if desired...yet maintain a reasonable SPL.

Loops are good for post-preamp effects like reverb, delay, EQ and some modulation effects. Using attenuators or boosts in the loop is often an iffy proposition, IME, dependent on amp design.

Bill

Volume is no problem. I needed the attenuator because the signal level of TT preamp made any pedal in the loop to sound like it's going to explode... After which I'd need a boost to get the level back up.

[Jacew]

Re: How to get preamp overdrive with fx loop?

Yes, that particular amp is PPIMV, or post phase inverter master volume, and yes a lot of its overdrive is coming from the phase inverter in that design. If you put a loop in the typical place (between the preamp, and PI) in that amp, especially a passive one, it will really bring your signal levels down, and may not heavily overdrive the proceeding circuitry. All is not lost though, but you would either have to use something active, or slave it. You do have some space issues in that amp, so the active part would be a tight fit if you could even muster it, you would also have to have a little more complex of a loop design if you wanted to reap the benefit of post distortion loop placement, other wise the loop placement between the preamp and PI would be less than ideal, but could still be made to overdrive the PI, although at the expense of having the fx signal over driven as well which wouldn't be much better than just putting them through the input.

Indeed. Idea was to just add jacks between preamp and power amp, and keep all the rest needed to make it work outside of the amp, so it's easy to modify.

BTW: I was using Fender Mustang with load box and attenuator in speaker out for boosting the return signal, as I currently don't have suitable preamp or booster for the job. My DIY attenuator blew up the output rheostat way before I got the level high enough.

Is the preamp really pushing PI that hard, or have I missed some technical detail here why it doesn't work? Am I just going to fry the PI, before getting the level as high as without the attenuator?

How much output tube preamp does generally produce (Isn't the upper limit for ECC83 output something like 1 watt)?

Of course it's possible that that rheostat was just specced for very small wattage... (It didn't say and I salvaged it from an old broken radio.)

[Joey Voltage]

Re: How to get preamp overdrive with fx loop?

Indeed. Idea was to just add jacks between preamp and power amp, and keep all the rest needed to make it work outside of the amp, so it's easy to modify.

BTW: I was using Fender Mustang with load box and attenuator in speaker out for boosting the return signal, as I currently don't have suitable preamp or booster for the job. My DIY attenuator blew up the output rheostat way before I got the level high enough.

Is the preamp really pushing PI that hard, or have I missed some technical detail here why it doesn't work? Am I just going to fry the PI, before getting the level as high as without the attenuator?

How much output tube preamp does generally produce (Isn't the upper limit for ECC83 output something like 1 watt)?

Of course it's possible that that rheostat was just specced for very small wattage... (It didn't say and I salvaged it from an old broken radio.)

There are a couple of things you can do especially as you indicated you "interrupt jacked' it for an external loop or unit. how far do you want to go? I can post some ideas for you, but the good ones require active circuitry that need HV supplies.

[Jacew]

Re: How to get preamp overdrive with fx loop?

There are a couple of things you can do especially as you indicated you "interrupt jacked' it for an external loop or unit. how far do you want to go? I can post some ideas for you, but the good ones require active circuitry that need HV supplies.

Active circuitry is no problem. I'd just like it to be as simple and cheap as possible.

But as said: Preamp adds so little of the actual distortion in the signal, there's really no point of taking this further. I might get a reactive load box and power amp in the future, but currently it works fine as it is.

[Joey Voltage]

Re: How to get preamp overdrive with fx loop?

Well you can design the loop to go after the PI so you can get the over drive from it. slaving is the best if you also want to dime the mv and make the el84's clip too. But I'll give you a few ways to accomplish putting the loop after the distortion generation in that amp.