Buying: Checks and Trial

[paul, the]

Hello all,

I'm trying out an old head and cab tonight.

I was wondering if anyone could recommend any checks that should be performed, just anything that should be looked/listened out for when buying gear.


I was told over the phone that the amp had some hum - which would be fixed by a re-valve.

Also, one of the four speakers in the cab is mismatched and has a different sized dust-cover (I don't know the implications sound wise of this).

Any help/advice would be greatly appreciated.

Cheers,

paul.

[Oxblood]

Which amp is it? Geez the make and model, and I'll have a delve in my drawers (if you'll pardon the expression) for relevant info.

Here's a bit of general rat-sniffing procedure:

1) Make sure the amp is plugged into the cab and get the seller to assure you that the output impedance is set correctly for that cab.

2) DON'T plug in an instrument yet.

3) Switch on the amp and let it warm up for a few minutes on standby before switching it on fully (assuming it has a standby/ON switch anyway - some don't).

Then just stand and listen to it while it's sitting there. If the seller is rabbiting on, politely ask him/her to be quiet while you listen. Obviously, if it starts making any scary loud buzzing, squealing or 'motorboating' noises, switch it off at once and either walk away from the deal or start negotiating a total bargain-basement price, 'cos it clearly needs some major attention. If it's pretty quiet, keep listening while you approach the amp and peer inside. If the output valves are visible, check to see if they look normal. What's normal? A few points of bright orange light are OK: that's just the heaters (also, with some valves you can see right inside and see the heater filament itself glowing -that's OK too), but if the anodes themselves (the large grey plates) are glowing red like a cooker hotplate on a low setting, switch off the amp. Red anodes mean that the valves are passing far too much current, which will shorten their life and can even cause damage to the output transformer. It could be occurring simply because they're not properly biased, but it can also indicate a more serious underlying fault that will need repair.

Assuming all is well so far, let's move on.

RUSHING/CRUMBLING SOUNDS: it's likely that the coupling capacitors need replacing (no big deal - easy and cheap to fix).

HUM: In a totally healthy amp, there shouldn't be any noticable hum coming from the speakers. 50Hz hum (sounds like you get from a single-coil pickup) may suggest that there's a bit of a screening problem in the signal path (not a big worry). If there is a constant 100Hz hum that doesn't really change regardless of what you do with the controls, it suggests that one or more of the big power supply smoothing capacitors needs replacing (easy to fix, though new caps will cost £3 - 5 pounds each, so the price should be reduced to reflect this).

CRACKLY CONTROLS: No big deal. Might just need a squirt of contact cleaner, or a replacement pot. Pot crackle can also be caused by DC leaking into the signal line (those pesky coupling caps again!)

Finally, if it hasn't already blown a fuse or started to fill the room with smoke, plug in your bass and play!

ADDED LATER:
[quote name='paul, the' post='20025' date='Jun 19 2007, 08:04 AM']I was told over the phone that the amp had some hum - which would be fixed by a re-valve.[/quote]
It's [i]very[/i] unlikely that replacing valves would get rid of hum, unless that hum is being caused by poor inter-electrode insulation between the heater and cathode of one or more valves - but unless the seller has access to a valve tester, he wouldn't know that. He's giving you a bit of the old BS there. You can bet he hasn't a clue why it's humming. Armed with the checklist above, though, you might! I'd love to see his face when you give a sharp intake of breath and say [i]"No, mate, that's your smoothing caps gone west. Fixable, of course, but it'll cost....."[/i]

Edited June 19, 2007 by Oxblood

[paul, the]

Cheers Oxblood!

I read that over many times on the way up to the guy's house.


Unfortunately, the amp didn't work. It was very faint at full volume and the hum was really quite loud.

The valves looked good and the plates weren't red. He even had some spare German valves. I didn't think to try a different lead to the cab until I got home though - I'm really hoping that that wouldn't have solved the problem. He said he'd tried it the previous night and it worked fine, although he hadn't played it regularly for 3 years.


It was a Fender Bassman 100 with its 4x12 cab, circa 1976.

...A bit of a shame - although he prefers the sound of his ABM500, 115+210.

[Oxblood]

[quote name='paul, the' post='20406' date='Jun 19 2007, 08:35 PM']Cheers Oxblood!

I read that over many times on the way up to the guy's house.
Unfortunately, the amp didn't work. It was very faint at full volume and the hum was really quite loud.

The valves looked good and the plates weren't red. He even had some spare German valves. I didn't think to try a different lead to the cab until I got home though - I'm really hoping that that wouldn't have solved the problem. He said he'd tried it the previous night and it worked fine, although he hadn't played it regularly for 3 years.
It was a Fender Bassman 100 with its 4x12 cab, circa 1976.

...A bit of a shame - although he prefers the sound of his ABM500, 115+210.[/quote]
Sorry, mate. One thing: you can rest assured that a different speaker lead wouldn't have helped. Wish I could have been there with you. I always see a faulty valve amp as a glorious opportunity to nab a bargain - but obviously that's easy for me to say, 'cos I can usually fix 'em! How much was he asking, BTW?

Oh yes: was the hum there all the time, regardless of what you did with the controls, or was it affected by them?

[paul, the]

[quote name='Oxblood' post='20435' date='Jun 19 2007, 09:27 PM']Sorry, mate. One thing: you can rest assured that a different speaker lead wouldn't have helped. Wish I could have been there with you. I always see a faulty valve amp as a glorious opportunity to nab a bargain - but obviously that's easy for me to say, 'cos I can usually fix 'em! How much was he asking, BTW?

Oh yes: was the hum there all the time, regardless of what you did with the controls, or was it affected by them?[/quote]


It [i]was[/i] on ebay - but has now been taken off. Although I have no idea what it's worth and I doubt I'd have the money spare to fix it. I hate not knowing if it would be a £20 job or a £200 job - besides the fact I haven't heard it yet

The hum was almost silent on stand by, but when it was switched on it was very loud - and increasingly so as the Master was turned up.

It's a real shame, it was nice looking and the grill hadn't gone brown. But after some bad experiences in the past, I tend to run a mile from anything that isn't working right.

Here's a picture of a Bassman (this isn't it, this is a 50 and a dif' cab):




Although after seeing the blonde blackfaced ones, I'd be left GASsing and feeling second place:



fwar.

[Oxblood]

[quote name='paul, the' post='20545' date='Jun 20 2007, 01:29 AM']It [i]was[/i] on ebay - but has now been taken off. Although I have no idea what it's worth and I doubt I'd have the money spare to fix it. I hate not knowing if it would be a £20 job or a £200 job - besides the fact I haven't heard it yet

The hum was almost silent on stand by, but when it was switched on it was very loud - and increasingly so as the Master was turned up.

It's a real shame, it was nice looking and the grill hadn't gone brown. But after some bad experiences in the past, I tend to run a mile from anything that isn't working right.

Here's a picture of a Bassman (this isn't it, this is a 50 and a dif' cab):


Although after seeing the blonde blackfaced ones, I'd be left GASsing and feeling second place:



fwar.[/quote]

I guess the seller withdrew it on account of the fact that it's now obviously got a fault. Can't blame the guy. We all know what happens if you list an amp on eBay as 'faulty': no bids until the last 2 minutes, then sold for a tenner to some leary geezer with a glint in his eye and the whiff of soldering iron smoke about him (no names, no pack drill - you en't seen me, right?)

As for that blackface... Fwar indeed!

[lukeward2004]

This is a very informative thread - im going to Pin this.

[Ted]

[quote name='Oxblood' post='20185' date='Jun 19 2007, 01:47 PM']It's [i]very[/i] unlikely that replacing valves would get rid of hum, unless that hum is being caused by poor inter-electrode insulation between the heater and cathode of one or more valves - but unless the seller has access to a valve tester, he wouldn't know that. "[/i][/quote]

I just had an Ampeg B-15N in which hummed like a bastard - it had two completely different types of 6L6 in the output stage, one of which was on its last legs. A pair of new, matched valves got rid of the hum. Push-pull output stages rely on valve matching to cancel hum - although the matching doesn't have to be very tight for it to work. This was an extreme case.

TW

[MIJ-VI]

[quote name='paul, the' post='20406' date='Jun 19 2007, 02:35 PM']Cheers Oxblood!

I read that over many times on the way up to the guy's house.


Unfortunately, the amp didn't work. It was very faint at full volume and the hum was really quite loud.

The valves looked good and the plates weren't red. He even had some spare German valves. I didn't think to try a different lead to the cab until I got home though - I'm really hoping that that wouldn't have solved the problem. He said he'd tried it the previous night and it worked fine, although he hadn't played it regularly for 3 years.


I[u]t was a Fender Bassman 100 with its 4x12 cab, circa 1976.[/u]

...A bit of a shame - although he prefers the sound of his ABM500, 115+210.[/quote]

To anyone who is interested in buying one of these rigs:

I once (briefly) owned a used Bassman 100 with its matching (and infamous) cross-fire 4x12.

That guitar speaker-loaded, cheap, particle board POS cab buzzed like a beehive as soon as I turned the amp up--which I didn't get to do at the seller's house due to the presence of "a sick family member".

If one wants the Fender amp tone at a usable volume level, then these days the Tech 21 Blonde Character Series pedal-->a proper modern bass rig is worth an audition.

Blonde sound clips: [url="http://www.talkbass.com/forum/showthread.php?t=610156&highlight=blonde"]http://www.talkbass.com/forum/showthread.p...ighlight=blonde[/url]

As of this scribbling, the upcoming NAMM show may yield a revamped line of CS pedals which (hopefully) includes DI functions and pre-sets.

[tonyquipment]

Wow what a great thread. Lots of good tips and tricks here.

I'm now slightly better than a 'cab kicker' hah!

[Twincam]

To add a few more tips. That's not been said. Some of this may seem obvious but it's worth saying.

If the amp makes a buzzing noise with the gain and volume right down then. It's probably the transformer, these can often be tightened up on the base and on the sides to tighten the plates. However it may also need a rebuild or total replacement. I've seen this on a few valve amps now and most of them are able to be tightened and are ok. Use this to haggle the price down just in case it needs replacement if they won't budge walk away.

Test all the controls! Buttons, sliders everything. They should feel reasonably firm not loose and they If crackle they will just need a squirt of contact cleaner as long as they work in there full range of motion.

Switch the amp on and off several times there should be no Loud Pops! some amps do have some pop on turning on or off but it should only be a little bit. Loud pops are often component failure in an circuit that prevents this. Again to note that some amps naturally do have pops it's worth checking on forums if this is the case before hand or during the test (it's handy having a smart phone)

And speaking of which, always check forums for reviews and people posting about particular faults, common or not too the amp.

If it's a combo then do also give the speaker a look over if possible. Look for any cracks or repairs. Ask if it's the original speaker. Listen for any cab rattles. Does the tweeter work if it has one, does the tweeter control work if it has one.

Don't be afraid to ask questions. Has it been serviced, If so who by and any proof? What's original and what isn't? If it's had any new valves has it been biased? If not then is it self biasing, if not does it have a adjustment pot etc? if no then it will need biasing always re bias a valve amp when replacing any valves, there are a few exceptions to this, so again know your amp.

Check basschat, eBay, gumtree for the current prices never assume you know or that the seller knows, unless it's cheaper then great.

[ebozzz]