Input impedance conversion on an old valve amp?

[joegarcia]

So I'm trying to use an old Laney PA valve amp for guitar/bass but it sounds quite weak because the inputs aren't for instruments and are the wrong impedance I believe.

I there a passive unit I can use to boost the level and change impedance into the amp?

I've tried using an overdrive to boost the input but it still doesn't sound right.

[Hamster]

I'm not sure that that's the problem? - have you tried putting a mic into the input to see if there's a difference in output?

[Silent Fly]

[quote name='joegarcia' post='419756' date='Feb 26 2009, 12:38 AM']So I'm trying to use an old Laney PA valve amp for guitar/bass but it sounds quite weak because the inputs aren't for instruments and are the wrong impedance I believe.

I there a passive unit I can use to boost the level and change impedance into the amp?

I've tried using an overdrive to boost the input but it still doesn't sound right.[/quote]

A recommend you a good DI box - It should do the trick.

[joegarcia]

Yea, was wondering if that would work. It only has a jack input though so would have to go xlr-jack which seems to render the whole thing a bit pointless though. Will give it a go though, ta.

This should probably be in the Amps section really, sorry.

[cheddatom]

Some DI boxes have a jack output. A DI doesn't just convert a jack plug to an XLR, it's to raise your instrument level signal to a line level signal, so it won't be pointless at all.

[joegarcia]

[quote name='cheddatom' post='420070' date='Feb 26 2009, 01:52 PM']Some DI boxes have a jack output. A DI doesn't just convert a jack plug to an XLR, it's to raise your instrument level signal to a line level signal, so it won't be pointless at all.[/quote]

No, I know thats what a DI box does, it just seems an alien concept to me going DI-Jack. The jack outputs on DI's are usually just a passive link, so are the same as the input, identical impedance and all.

Will give it a go though, had considered it before posting the thread, in theory it should work so fingers crossed. Just need to find my female xlr-jack lead.

[cheddatom]

I see what you mean, it would be pointless if you used the passive link. Let us know how you get on.

Another thought - surely one of your plethora of pedals has a DI output on a jack socket? I know a lot of Digitech pedals do.

[joegarcia]

Yea, my Bad Monkey has a 'Mixer' out, as does my Blackstar HT-Dual. Tried the Blackstar one which is quieter than the standard out but haven't tried the Bad Monkey yet.

[joegarcia]

Mixer out is the same level with a high end roll off.
Don't have a standard DI at home at the moment but tried it with my Focusrite Trakmaster instrument preamp and it worked a treat.

[bremen]

Valves have a high input impedance so it seems a shame to interpose something like a transistor DI box.

I'm sure it would be an easy matter to change one or two components to restore the 'natural' high impedance. PM me the model number and I'll hunt out a schematic and see how easy it would be. Could be that the input stages have transformers to convert the low impedance of the mic to the high impedance of the valve, and all it would take is to bypass the transformer.

[Mr. Foxen]

Not figured exactly what is is yet, but fairly sure its a Laney L60, Klipp era as it has that logo on it. Looks like the one on a blurry youtube video.

Also from another forum:
[quote]Messin with an 83 Ibanez Roadstar loaded with a HighOrder 8.3 A2. I jumpered a couple channels and added a .003 cap on 1 of the volumes. Also changed all the filter caps.... Stock otherwise[/quote]

Edited February 27, 2009 by Mr. Foxen

[umph]

i don't think it would be an impedence issue, if it was it'd sound like really bad distortion. are the capacitors all still original? if so you might want to replace the reservoir caps as they may have gone a bit wayward making it sound thin or it could be time for a revalving.

[joegarcia]

[quote name='umph' post='421777' date='Feb 28 2009, 04:22 PM']i don't think it would be an impedence issue, if it was it'd sound like really bad distortion. are the capacitors all still original? if so you might want to replace the reservoir caps as they may have gone a bit wayward making it sound thin or it could be time for a revalving.[/quote]

Really? I think it may well be caps and old valve problems too but it sounds like when you plug a guitar or bass straight into a line level input on a mixer, which if it's a PA head is surely what's actually happening.

[umph]

[quote name='joegarcia' post='421779' date='Feb 28 2009, 04:27 PM']Really? I think it may well be caps and old valve problems too but it sounds like when you plug a guitar or bass straight into a line level input on a mixer, which if it's a PA head is surely what's actually happening.[/quote]
they were designed to work with guitar etc aswell, the PA's are very similar to the guitar and bass heads except a few tweaks to make them run cleaner and larger caps but have the same input impedence from looking at the schematic.

I'd look into replacing the valves if they've never been changed and replacing the caps you'd be suprised how much difference it can make if they've never been changed

[joegarcia]

Ah cool, ok then thanks.
Reckon changing the caps will be an expensive job at all?

[umph]

[quote name='joegarcia' post='422173' date='Mar 1 2009, 04:38 AM']Ah cool, ok then thanks.
Reckon changing the caps will be an expensive job at all?[/quote]
well if there the smaller ones which i think they are it shouldn't be to much.