Line level valve fx loops

Discuss the stuff that makes your ears bleed.

Moderators: Slowy, Capt. Black

Mattallica
Vintage Post Junkie
Vintage Post Junkie
Posts: 1612
meble-kuchenne.warszawa.pl
Joined: Fri May 20, 2016 9:42 pm
Location: CHCH
Has liked: 558 times
Been liked: 175 times

Re: Line level valve fx loops

Post by Mattallica »

Optical wrote:Is it clipping the input of the H9? Or the H9 clipping the fx return?
Clipping the input of the H9

Optical
Tokai
Tokai
Posts: 419
Joined: Thu Nov 24, 2016 4:34 pm
Has liked: 67 times
Been liked: 142 times

Re: Line level valve fx loops

Post by Optical »

Ok.. replacing the tube with a 12AU7 wont work assuming the circuit is a standard cathode follower send stage. It might work if for some reason it's a rare plate send stage with NFB (highly unlikely).

What you would have to do is change the attenuation circuit at the send point. I dont know what the circuit is as I cant see a schematic anywhere, but you can try putting a 1.5k resistor across the send output. A very easy way to do this is connect a 1.5k resistor across the jack that plugs into the fx send, that will add more output attenuation, if it works great, if it doesnt it wont hurt anything. I think Bruce Egnater had a hand in designing these amps so that might do the trick if the fx send is his design

Optical
Tokai
Tokai
Posts: 419
Joined: Thu Nov 24, 2016 4:34 pm
Has liked: 67 times
Been liked: 142 times

Re: Line level valve fx loops

Post by Optical »

If you can get inside it and post photos of the circuit, an instrument level mod might be more obvious

Mattallica
Vintage Post Junkie
Vintage Post Junkie
Posts: 1612
Joined: Fri May 20, 2016 9:42 pm
Location: CHCH
Has liked: 558 times
Been liked: 175 times

Re: Line level valve fx loops

Post by Mattallica »

Ok.. replacing the tube with a 12AU7 wont work assuming the circuit is a standard cathode follower send stage. It might work if for some reason it's a rare plate send stage with NFB (highly unlikely).

What you would have to do is change the attenuation circuit at the send point. I dont know what the circuit is as I cant see a schematic anywhere, but you can try putting a 1.5k resistor across the send output. A very easy way to do this is connect a 1.5k resistor across the jack that plugs into the fx send, that will add more output attenuation, if it works great, if it doesnt it wont hurt anything. I think Bruce Egnater had a hand in designing these amps so that might do the trick if the fx send is his design
That's in line with what i've come to understand about valve buffers, the mu valve isn't a factor in this particular function right?

When you say across the send output, you mean across the hot and earth tabs of the jack?

Optical
Tokai
Tokai
Posts: 419
Joined: Thu Nov 24, 2016 4:34 pm
Has liked: 67 times
Been liked: 142 times

Re: Line level valve fx loops

Post by Optical »

yes the fx send stage is not configured to produce gain, so the tube mu is not a factor.
Just found the schematic and the resistor across the output wont work very well. You'll need to get inside it and change R55 and R57. If you reckon you can do that i'll tell you what to do.

User avatar
sizzlingbadger
Vintage Post Junkie
Vintage Post Junkie
Posts: 8241
Joined: Sat Jan 15, 2011 7:12 am
Location: Wire Wrapper
Has liked: 1202 times
Been liked: 1398 times

Re: Line level valve fx loops

Post by sizzlingbadger »

You will need boost the gain in the return circuit too unless you don’t mind your amp losing volume.
Tube amp and guitar tones straight from 1958… amazing how believable the sounds were back then, even without the modellers...

User avatar
moliere
Squier
Posts: 315
Joined: Sat Feb 05, 2011 12:13 am
Has liked: 79 times
Been liked: 37 times

Re: Line level valve fx loops

Post by moliere »

I have a similar problem with my RM100 and its line level loop. The quick and easy solution is to turn down the volumes on the individual pre-amp modules and turn up the master. That of course only works if you've still got enough volume after all that. I've got the ebtech level shifter thing, and that sort of works, it still had issues with some gear though. I'd love to DIY up a send level at some point, maybe when I do the dual channel mod.

Mattallica
Vintage Post Junkie
Vintage Post Junkie
Posts: 1612
Joined: Fri May 20, 2016 9:42 pm
Location: CHCH
Has liked: 558 times
Been liked: 175 times

Re: Line level valve fx loops

Post by Mattallica »

Optical wrote:yes the fx send stage is not configured to produce gain, so the tube mu is not a factor.
Just found the schematic and the resistor across the output wont work very well. You'll need to get inside it and change R55 and R57. If you reckon you can do that i'll tell you what to do.
Iv'e taken the amp out of the head shell. I can access R55 & R57 but they are on the main board. It will be a massive PITA to undo all the jacks and plugs and carefully remove the PCB.

Can i go parallel across the existing resistors to change the values?

Mattallica
Vintage Post Junkie
Vintage Post Junkie
Posts: 1612
Joined: Fri May 20, 2016 9:42 pm
Location: CHCH
Has liked: 558 times
Been liked: 175 times

Re: Line level valve fx loops

Post by Mattallica »

sizzlingbadger wrote:You will need boost the gain in the return circuit too unless you don’t mind your amp losing volume.
That's no worries, plenty loud enough and the H9 can boost the output

Mattallica
Vintage Post Junkie
Vintage Post Junkie
Posts: 1612
Joined: Fri May 20, 2016 9:42 pm
Location: CHCH
Has liked: 558 times
Been liked: 175 times

Re: Line level valve fx loops

Post by Mattallica »

moliere wrote:I have a similar problem with my RM100 and its line level loop. The quick and easy solution is to turn down the volumes on the individual pre-amp modules and turn up the master. That of course only works if you've still got enough volume after all that. I've got the ebtech level shifter thing, and that sort of works, it still had issues with some gear though. I'd love to DIY up a send level at some point, maybe when I do the dual channel mod.
I reckon the modules sound naff until they are past 12'o clock though. Thanks for the heads up about the line shifter.

Mmm not a lot space on either side for send/return level knobs. I don't know the practicalities of mounting them in the jacks if they are board mounted.

Optical
Tokai
Tokai
Posts: 419
Joined: Thu Nov 24, 2016 4:34 pm
Has liked: 67 times
Been liked: 142 times

Re: Line level valve fx loops

Post by Optical »

Mattallica wrote:
Optical wrote:yes the fx send stage is not configured to produce gain, so the tube mu is not a factor.
Just found the schematic and the resistor across the output wont work very well. You'll need to get inside it and change R55 and R57. If you reckon you can do that i'll tell you what to do.
Iv'e taken the amp out of the head shell. I can access R55 & R57 but they are on the main board. It will be a massive PITA to undo all the jacks and plugs and carefully remove the PCB.

Can i go parallel across the existing resistors to change the values?

Just a note - the FX return stage is pretty much at max gain already, so boosting it further wouldnt be possible.

Can you post a photo of the board? You might be able to work from the top

Mattallica
Vintage Post Junkie
Vintage Post Junkie
Posts: 1612
Joined: Fri May 20, 2016 9:42 pm
Location: CHCH
Has liked: 558 times
Been liked: 175 times

Re: Line level valve fx loops

Post by Mattallica »

Optical wrote:
Mattallica wrote:
Optical wrote:yes the fx send stage is not configured to produce gain, so the tube mu is not a factor.
Just found the schematic and the resistor across the output wont work very well. You'll need to get inside it and change R55 and R57. If you reckon you can do that i'll tell you what to do.
Iv'e taken the amp out of the head shell. I can access R55 & R57 but they are on the main board. It will be a massive PITA to undo all the jacks and plugs and carefully remove the PCB.

Can i go parallel across the existing resistors to change the values?
It's a bit tight but doable:




Just a note - the FX return stage is pretty much at max gain already, so boosting it further wouldnt be possible.

Can you post a photo of the board? You might be able to work from the top
Attachments
20180716_193228.jpg

Mattallica
Vintage Post Junkie
Vintage Post Junkie
Posts: 1612
Joined: Fri May 20, 2016 9:42 pm
Location: CHCH
Has liked: 558 times
Been liked: 175 times

Re: Line level valve fx loops

Post by Mattallica »

Mattallica wrote:
Optical wrote:
Mattallica wrote:
Iv'e taken the amp out of the head shell. I can access R55 & R57 but they are on the main board. It will be a massive PITA to undo all the jacks and plugs and carefully remove the PCB.

Can i go parallel across the existing resistors to change the values?





Just a note - the FX return stage is pretty much at max gain already, so boosting it further wouldnt be possible.

Can you post a photo of the board? You might be able to work from the top
R55 is bottom right and R57 is tucked in behind the big cap

Optical
Tokai
Tokai
Posts: 419
Joined: Thu Nov 24, 2016 4:34 pm
Has liked: 67 times
Been liked: 142 times

Re: Line level valve fx loops

Post by Optical »

I've just had a thought after looking at the circuit more - you are probably distorting the fx loop return stage (and maybe the send stage too) when you have the channel masters high. If that's the case, I dont think that lowering the send signal is going to fix your problem because you wont keep the tone of the channel masters above 12, because essentially the fx loop mod is going to do the same thing as turning the channel masters down.

I might be wrong and the extra gain from the channel masters being high actually causes the fx send stage to clip/add gain. In which case lowering the send level might keep some of the gain, but definitely not all of it because the fx return wont clip then.

You might just need to turn the channel master down so the H9 input doesnt clip, then turn the H9 output up to as loud as it can go to try and get the fx return stage to clip again. Have you tried that?

If that's not enough gain then it might still be worth modding the fx loop to keep the send stage clipping, but you wont be able to do anything about the return stage which wont clip. Although you could mod that for more gain too. But you are then going to start altering the compression and feel of the amp overall...

Mattallica
Vintage Post Junkie
Vintage Post Junkie
Posts: 1612
Joined: Fri May 20, 2016 9:42 pm
Location: CHCH
Has liked: 558 times
Been liked: 175 times

Re: Line level valve fx loops

Post by Mattallica »

Optical wrote:I've just had a thought after looking at the circuit more - you are probably distorting the fx loop return stage (and maybe the send stage too) when you have the channel masters high. If that's the case, I dont think that lowering the send signal is going to fix your problem because you wont keep the tone of the channel masters above 12, because essentially the fx loop mod is going to do the same thing as turning the channel masters down.

I might be wrong and the extra gain from the channel masters being high actually causes the fx send stage to clip/add gain. In which case lowering the send level might keep some of the gain, but definitely not all of it because the fx return wont clip then.

You might just need to turn the channel master down so the H9 input doesnt clip, then turn the H9 output up to as loud as it can go to try and get the fx return stage to clip again. Have you tried that?

If that's not enough gain then it might still be worth modding the fx loop to keep the send stage clipping, but you wont be able to do anything about the return stage which wont clip. Although you could mod that for more gain too. But you are then going to start altering the compression and feel of the amp overall...
I see what you're saying. I haven't tried bringing down the channel masters with the H9 yet. Worth a shot for sure. Back in she goes then!

Post Reply