Inspecting used amps

[atilla]

I dropped into a pawn shop the other day and spotted a Laney tube head. I didn't have time to crank it up and play it, but I just might do that. But I'm also a little leery of pawn shop stuff...are there any warning signs I can look for when checking this amp out to make sure it's in decent condition?

Also, has anybody heard of a Marshall head selectable between 25 and 50 watts? If so, any idea how much they usually go for?

[Neeradj]

Re: Inspecting used amps

Well aside from playing it, make sure everything's held together properly. Switches, jacks, pots, tubes ofcourse, knobs. Try the head at different volumes too, it's a known fact some old amps which aren't used for a long time might blow something or suddenly decrease in volume while playing at high volumes.

I believe the Jubilees were switchable between 50/25 watts. I'm not sure what they go for in the USA but here in Europe they usually go for 750-800 euros. You might wanna hold on to that one, they're increasing in value rapidly

[fett]

Re: Inspecting used amps

Don't turn the amp on unless it is hooked up to a speaker. That is very important. Look for any signs of burns or any indication that it has been altered or repaired. Tube amps with years on them can have leaking filter capacitors. Look for any leaks. Depending on how old it is, be prepared for some hefty repair bills. Twister will probably answer here as well. He is an amp God. [img]/images/graemlins/smile.gif[/img] [img]/images/graemlins/notworthy.gif[/img]

[atilla]

Re: Inspecting used amps

[ QUOTE ]
Don't turn the amp on unless it is hooked up to a speaker. That is very important. Look for any signs of burns or any indication that it has been altered or repaired. Tube amps with years on them can have leaking filter capacitors. Look for any leaks. Depending on how old it is, be prepared for some hefty repair bills. Twister will probably answer here as well. He is an amp God. [img]/images/graemlins/smile.gif[/img] [img]/images/graemlins/notworthy.gif[/img]

[/ QUOTE ]

Yeah, I'm hoping he weighs in with some info! And thanks for the other info.

[CZSER]

Re: Inspecting used amps

The 50/25 switch might be a pentode/triode switch. This decreases the power: remember, 1/2 power doesn't mean 1/2 vol. This is done by connecting the power stage from pentode to triode. It also lowers (just a little) the overall distortion, giving a more vintage sound.
Don't know if they'll let you try this out at the pawn shop, but if you can, crank the amp all the way (vol=10, gain=10 and "50 watts mode" ON) and play as much time as u can (about 1 hour). During the process, pay attention to sound & smells (if you can remove the rear cover, even better). This what I call "a burn in test". I do it always that I mod my amp (pretty frequently). And do check everything the other guys say(pots, jacks, switches, etc.).
My two cents [img]/images/graemlins/wink.gif[/img]

[Twisteramps]

Re: Inspecting used amps

Turn each pot with the amp on - see if any are scratchy. They may be cleanable, or need replacement. If you can, check out every feature on the amp - does it have an fx loop? See if there's something there you can plug into it to check it. Ditto if it switches channels or has anything else that's footswitchable.

DON'T remove any covers while doing a 'burn in' test - you may be giving the amp enough airflow to make it seem ok in the store, but then when you get it home it overheats. I don't know about an entire hour long test... depending on the amp, most pawn shops I know have fairly limited times for testing/trying out gear. An entire hour would stretch their patience pretty thin.

I'd also pull any fuses to make sure they are good - if you see foil or something conductive wrapped in there to make it work no matter what, DO NOT BUY THAT AMP. Also if you see something that doesn't look factory or like a mod - you probably want to run from that too, unless there's a label from the modder. I bought a used JCM800 once that was modded, but it had a voodoo amps label on it. I called trace, verified it was his work, and bought the amp.

Pete

[atilla]

Re: Inspecting used amps

Thanks, Pete. Hey, that head in the ammo can was awesome!

[nonamemx]

Re: Inspecting used amps

[ QUOTE ]


I'd also pull any fuses to make sure they are good - if you see foil or something conductive wrapped in there to make it work no matter what, DO NOT BUY THAT AMP. Also if you see something that doesn't look factory or like a mod - you probably want to run from that too, unless there's a label from the modder. I bought a used JCM800 once that was modded, but it had a voodoo amps label on it. I called trace, verified it was his work, and bought the amp.

Pete

[/ QUOTE ]

Why? Does that mean that somebody is trying to bypass the fuse? (meaning something broke inside?) [img]/images/graemlins/help.gif[/img]

[AlexL]

Re: Inspecting used amps

Yes. Wrapping foil around a fuse will make the amp work, but the fuse will no longer blow like it should.

Fuses don't blow because the fuse is bad; they blow because there is a circuit getting overloaded. These can be rare occurrences, but if someone has "foiled" a fuse, it may mean that it is happening a lot, and they are tired of swapping it out. And frequently blowing fuses is definitely a sign of a problem.


- E.

[Neeradj]

Re: Inspecting used amps

[ QUOTE ]
The 50/25 switch might be a pentode/triode switch. This decreases the power: remember, 1/2 power doesn't mean 1/2 vol. This is done by connecting the power stage from pentode to triode. It also lowers (just a little) the overall distortion, giving a more vintage sound.

[/ QUOTE ]

I don't think that's completely true though, no offense. The half switch on the Marshalls is almost the same thing as pulling out (a) power tube(s), it heats up the tube(s)more and gives you a more expressive, responsive and harmonically rich sound which could normally only be obtained by the head at high volumes. It does decrease in measurable volume, it just will be very hard to hear the difference. You see 50W is only 5 dB less loud than 100W, but the common ear won't hear the difference though it's there.

It also won't decrease in distortion, since the amp will break up much faster. Whether the changes will be a more vintage sounding amp depends on the amp. A really cool guy I know from the Seymour Duncan forum pulled out two tubes to compare the 100W Rivera Knucklehead at a 50W level. The results were actually the amp sounding more modern [img]/images/graemlins/scratchhead.gif[/img]

[atilla]

Re: Inspecting used amps

From what I've heard, you're pretty dead-on. The decrease in power and volume won't make it sound more vintage, but will give more distortion at a slightly lower volume.

That bit on the decibel difference between 100 and 50 watts was very interesting. My understanding is that 100 watts is twice as loud to the ear as 10 watts, so that seems to fit. This all assumes that we're comparing tube amps to tube amps, because solid state power isn't as loud at the same power.

[Twisteramps]

Re: Inspecting used amps

[ QUOTE ]
[ QUOTE ]
The 50/25 switch might be a pentode/triode switch. This decreases the power: remember, 1/2 power doesn't mean 1/2 vol. This is done by connecting the power stage from pentode to triode. It also lowers (just a little) the overall distortion, giving a more vintage sound.

[/ QUOTE ]

I don't think that's completely true though, no offense. The half switch on the Marshalls is almost the same thing as pulling out (a) power tube(s), it heats up the tube(s)more and gives you a more expressive, responsive and harmonically rich sound which could normally only be obtained by the head at high volumes. It does decrease in measurable volume, it just will be very hard to hear the difference. You see 50W is only 5 dB less loud than 100W, but the common ear won't hear the difference though it's there.

It also won't decrease in distortion, since the amp will break up much faster. Whether the changes will be a more vintage sounding amp depends on the amp. A really cool guy I know from the Seymour Duncan forum pulled out two tubes to compare the 100W Rivera Knucklehead at a 50W level. The results were actually the amp sounding more modern [img]/images/graemlins/scratchhead.gif[/img]

[/ QUOTE ]

If it's a 50 watt amp going to 25 watts, it's most likely a pentode/triode switch like the original poster stated. Most 50 watt amps use a pair of tubes in push/pull... yanking one tube out isn't going to have the same effect as pulling two tubes from a four tube amp.

Pete

[atilla]

Re: Inspecting used amps

Thanks again for the tips. I might have to a little used amp shopping. Depends on whether or not my current head is going to continue misbehaving, though.

[nonamemx]

Re: Inspecting used amps

[ QUOTE ]
Yes. Wrapping foil around a fuse will make the amp work, but the fuse will no longer blow like it should.

Fuses don't blow because the fuse is bad; they blow because there is a circuit getting overloaded. These can be rare occurrences, but if someone has "foiled" a fuse, it may mean that it is happening a lot, and they are tired of swapping it out. And frequently blowing fuses is definitely a sign of a problem.


- E.

[/ QUOTE ]

Ah, ok, Thats what I suspected. Just getting the facts straight, thanks.

[Neeradj]

Re: Inspecting used amps

atilla, yes I was talking about tube amps compared to tube amps. It suprised me at first when my old JCM800 2210 (100W) could be turned up as much as a Koch Multitone (50W) and I didn't hear any difference volume wise. I was also talking about a Rivera Knucklehead 100 which I've ordered, to a friend of mine, and worrying about whether it'd be too loud for my bedroom. He then said there's only a 5dB difference between 100-50W

Twisteramps, I've read about the Jubilee and heard Marshall's halfswitches on those amps almost had the same effect. It might have only related to the 100W Jubilees so maybe I was misinformed, if you say so I believe you, I'll see if I can fetch that article up somewhere [img]/images/graemlins/smile.gif[/img]