Amp Tech Thread / Ask a tech Q

[Hemisaurus]

MFD is microfarads. In the power supply, the voltage rating is critical, more critical than a precise Farad rating, it's OK if you put bigger caps in the power supply farad wise, as long as they meet or exceed the voltage rating of the circuit.

I'm saying before you go ripping it to shreds, you can try and figure out if it is the caps, it could be the speakers in the combo, or it could be a bad connection, mine got real quiet when I brought it home, turned out I'd knocked something and the B+ had dropped to half what it should be, hence me saying check the B+.

If you have a solid B+, measured DC of > 700V and there's no AC voltage on your B+ line, say less than 10V AC depending on your meter, that means that the B+ is solid, and the caps on it are doing a good job of controlling the AC ripple. Then find the Bias point as mentioned on the schematic, and set it like it says. Some chicken-hearted tech may have bias'd the amp real cold because he was scared of blowing the tubes.

You can cut the leads of an old cap, but I'd only suggest doing that as a repair method if you were replacing a couple of caps, and don't want to rip it apart, and I would also want to put silicon caulk under the new cap as it won't have a lot of mechanical strength, and they can be physically quite big.

Some people say recap the amp, get it done, some people say don't touch it, it sounded great, just fix it. One of the best stories I heard was here http://www.amp-fix.com/amps.htm read the bit about the 59 Bassman, belonging to a certain person. I love that story, because it shows a tech working hard, and locating and fixing the problems, rather than just throwing a whole bunch of new parts at it.

My suspicion would be the amp may be pretty solid, and just underbias'd, but obviously that's an internet hunch, as I haven't laid eyes, ears or voltmeter on it. So does it have the 12AX7?

[VOLVO)))]

I did the simple shit already, different cabs without the combo at all. It does have the 12ax7. Dumb it down a scad for me, B+? I follow most of this well, im just unfamiliar with the shorthand. The schem I found for it won't load on my computer for some reason. Check the voltage at pin 3, on any socket? 700+ = good to go? Less = replace caps?

[Hemisaurus]

700+ and little AC good
600+ and a lot of AC bad caps
450 or less, probably a bad joint somewhere.

B+ is either the plate voltage OR the voltage applied to the center tap of the output transformer, they get interchanged, but the value will be similar

As to the Darr book, long out of print, take the PDF to Kinko's and make a spiral bound version, good for the bench.

Yes Axial caps. Radials are the ones that stand up. Get the right working voltage though, meet or exceed the previous one.

[VOLVO)))]

See, I catch on quick. I don't have to measure at the tranny, but I can if needed? Pin 3 on any tube socket, load applied, standby off? Checking DCV on the same pin as I'm checking ACV, correct?

If the voltage is low, not much AC involved, the bias pot will adjust the B+, then be measured in mA by my socket probe deal I just ordered, too?

Just ordered the full KT77 retube kit for my Marshall, too, so I'll probably be fiddling around here looking for more answers. Off to find the full PDF of that book. I need it, I thirst for knowledge and shitter reading material.

Thanks, Herb, you know I'll be here groping you for more advice. I might as well learn all of this shit before I go to school for it, so I can breeze it...

[Hemisaurus]

http://www.diyguitarist.com/Misc/J_Darr.htm is as good as it gets, that's the first two sections of the book. Section 3 was a pile of schematics, which was handy in the days before the internet, not so necessary now.

You'd only be checking the output transformer voltage if you thought something was wrong with it.

You're a bit off on the statement below.

Pin 3 is the plate voltage, you want a good steady DC level of > 700V and very little AC, the AC is residual ripple on top of the DC level after the capacitors have done their filtering / smoothing work, the Darr book explains it.

The B+ is not adjustable, it is what it is, it may drop a little as you adjust the bias, but only because of the demand placed on it by the tubes.

The bias adjusts a negative voltage that is applied to the grid of the tube, it's like setting the idle in a car, your engine doesn't start from 0 rpm, nor does your tube start from off.

The socket probe is measuring the idle current of your amp, the current that flows from the plate (anode) to the ground (cathode) when no signal is applied. I suggest in this instance, you do the manufacturers procedure, which does not require a probe.



Click on it to make it bigger.

Yes, load applied, ie. speaker connected else things go boom.

Yes standby off, or amp fully on if your prefer, in standby the plate voltages are switched off, and only the heater voltage is applied to the tubes.

Off on vacation, so don't take offense if I don't respond to any posts for a few weeks

[liquidsmoke]

This is definitely one of those wish it was here moments. Speakers can distort in a lot of ways, but unfortunately so can speaker cabinets. Baffles can rattle, jack panels can buzz, speakers can be loosely mounted, even handles can rattle.

Have someone play your guitar, or feed a walkman or ipod into the fx loop of your amp, and get it to distort, and try feeling your way round the amp, does pressing on anything cure the distortion? Try the joints, the handles, the jack panel, the rear panels, front panels, and check the screws holding in your speakers are tight. Do they screw into wood or does it use t-nuts, etc.

The obvious answer, if it's not the cab would be to pull 2 speakers from your 4x12 and swap them in, you can use your tone knob if they sound too bright, then you'll have a 250W 2x12 and still handle 300W safely on your 4x12, so tell us about the impedances

I just played through a 15" bass cab and got the same speaker distortion at high volume levels, actually I can start to hear it at around 2 on my Laney which is minimum practice volume. It's not quite as fuzzy sounding because the tone of the bass cab is darker but it still happens big time. I'm thinking it has to be the elusive and mystical power amp tube break up I always read about, only I wasn't on a quest to find it. But is it possible that my tubes are just about shot and need to be replaced? Or is a 50 watt amp just inferior to a 100 watt amp for trying to dial in more of a regular heavy metal high gain sound at high volume levels? Is tube break up hard on the tubes or amp itself?

[Hemisaurus]

Is this in different spots, a lot of rooms have their own resonant items, drywall rattles, lightswitch panels buzz, ducts vibrate etc. I have the advantage where I can drag stuff out into the open air, and the only bouncing I get is off the barn across the yard.

If you're tubes are shot, you won't get me describing it, like Zappa said, it would be like dancing about architecture.

Tube breakup is hard on tubes, as you are running them full on, it's like running your cars engine at maximum revs, it's gonna hurt it, even if you are only in first gear and going 30 mph, it's also hard on the amp, as the rest of it works hard to support the tubes.

If you want a heavy metal sound, and you don't want tube worries, Randall and Peavey both do some very nice solid state heads.

50W to 100W is 3dB I think, I believe a mouse may fart louder than that

This is the sort of problem you'd definitely be better getting a local expert in on, someone that can actually hear what is going on. Failing that experiment yourself, different cabs, different locations, different tubes, different amps.

If you hear the same rattle with two different heads on the same cab, it's not the head.

If you hear the same rattle on two different cabs with the same head it's not the cab.

But if you drag that suspect cab/head to a different spot, and it sounds different or better, you've been chasing phantoms, I spent a while doing this with one of my cabs, tightening down the driver, going round the cab looking for squeaks, finally realised I was just vibrating the ducting on my furnace

[VOLVO)))]

Herbis, If I wanted to hop into speaker reconing, Is it any more complicated than replacing the cone, and voice coil? Magnets don't go bad, oi?

[liquidsmoke]

Is this in different spots, a lot of rooms have their own resonant items, drywall rattles, lightswitch panels buzz, ducts vibrate etc. I have the advantage where I can drag stuff out into the open air, and the only bouncing I get is off the barn across the yard.

If you're tubes are shot, you won't get me describing it, like Zappa said, it would be like dancing about architecture.

Tube breakup is hard on tubes, as you are running them full on, it's like running your cars engine at maximum revs, it's gonna hurt it, even if you are only in first gear and going 30 mph, it's also hard on the amp, as the rest of it works hard to support the tubes.

If you want a heavy metal sound, and you don't want tube worries, Randall and Peavey both do some very nice solid state heads.

50W to 100W is 3dB I think, I believe a mouse may fart louder than that

This is the sort of problem you'd definitely be better getting a local expert in on, someone that can actually hear what is going on. Failing that experiment yourself, different cabs, different locations, different tubes, different amps.

If you hear the same rattle with two different heads on the same cab, it's not the head.

If you hear the same rattle on two different cabs with the same head it's not the cab.

But if you drag that suspect cab/head to a different spot, and it sounds different or better, you've been chasing phantoms, I spent a while doing this with one of my cabs, tightening down the driver, going round the cab looking for squeaks, finally realised I was just vibrating the ducting on my furnace

This was the second cab in the same room. This is distortion coming out of the speakers for sure, both cabs, not anything in the room vibrating. I can't see how it's not the head at this point. I'll take the head in and have them check into it. Thanks for the info.

[Hemisaurus]

Herbis, If I wanted to hop into speaker reconing, Is it any more complicated than replacing the cone, and voice coil? Magnets don't go bad, oi?

They can age, and get slightly weaker, but no, not bad.

[VOLVO)))]

Tryin' to learn.

Biased my Marshall, turned out great. I wanna do that bright cap mod, but it's not where It's supposed to be, according to all the pictures. Everything I have is board mount, including the pots, no caps connected to the preamp... shit, I guess I could follow the trace to the wiper of the preamp pot, and that would be my bright cap, oi? 

Haven't touched the MM yet, that comes ASAP, though...

[Hemisaurus]

Tryin' to learn.

Biased my Marshall, turned out great. I wanna do that bright cap mod, but it's not where It's supposed to be, according to all the pictures. Everything I have is board mount, including the pots, no caps connected to the preamp... shit, I guess I could follow the trace to the wiper of the preamp pot, and that would be my bright cap, oi? 

Haven't touched the MM yet, that comes ASAP, though...

Doncha mean AZAP!!

Seriously 725V may jump a bit, be wary.

Dunno, you know I don't know those PCB mounted JCM800's, Marshall used to be pretty good about making things pretty clear, so yeah if the circuit is identical, and the bright cap is on the track of a pot. What do you mean bright cap? JCM800s don't have a bright switch, are you talking about futzing with some of the coupling caps for more bass?

Long as your careful taking the part out, and replace it with a cap of equal or better voltage capability, the worst that should happen is it makes bad noise, just make sure and save the old part

[VOLVO)))]

There's a cap that if you just snip one leg out, and bend it away, it'll cut the brightness of the amp (which it is, almost unbearably so.) It can be done on every 2203/2204/etc... Danny G, and Worthless Willie did it to theirs, and said it made a world of difference.

http://www.tdpri.com/forum/amp-central-station/13035-jcm800-bright-cap-mod-success.html

[Hemisaurus]

What you mean you don't run it with the vol on 10 all the time

Yeah trace the circuit, look for a 1nF cap, looking at the 2203 schematic, there's only one 1nF cap on it, so should be easy to narrow down. Of course there's a thing on your guitar called, err, a tone control

[VOLVO)))]

It doesnt effect the master. Which is usually 7+, only the pre, which is on about 5. 4-6 is where the mod is supposed to shine. So, It needs to be done.y

[justinhedrick]

It doesnt effect the master. Which is usually 7+, only the pre, which is on about 5. 4-6 is where the mod is supposed to shine. So, It needs to be done.y

do it. do it. a buddy of mine has 2 800s that has 1 w/o the cap and 1 with. night and day difference. for realz. both are PC board amps too.

[Hemisaurus]

For shits and giggles you should ground the cap at the wiper end, after disconnecting it from the wiper first of course.

Then you have a LF boost instead

[liquidsmoke]

If you want a heavy metal sound, and you don't want tube worries, Randall and Peavey both do some very nice solid state heads.

Just took my Laney into the shop today. Most heavy metal guitarists who prefer a high gain sound use tube amps it seems but if the amp guy says there isn't anything wrong with my head and I am able to determine that I wasn't experiencing speaker breakup I will look into getting a solid state head, it just needs to not sound like static crap. I like a dark smooth high gain sound, not brittle and trashy and super trebly.

[Jor el]

Hemi & Sunno}}}

Avoid shit like this?

http://www.ebay.com/itm/Sovtek-Mig-60-Chassis-/250875965587?pt=LH_DefaultDomain_0&hash=item3a695f7093#ht_500wt_1136

[VOLVO)))]

Not necessarily, well, yes, at that price, but if you just need parts, I buy cooked amps all the time. The caps look brand new, and I can't see anything burned in the picture. Probably a fuse, or something dumb... or a dead PT. Either way, not worth 200 bucks.


You need parts?

[Jor el]

No.
Just buy curious.

People are asking 650+ for Sovteks on the 'bay.

[VOLVO)))]

If you're looking to buy it for yourself, I'd probably pass. too many variables with a sight unseen amp.

[Hemisaurus]

I'd do the math, and see if it still sounds worth it.

Retube kit (using Eurotubes for a 50W Marshall as a guide) $61
New Power Tranny (Magnetic Components again using 50W Marshall) $82
New Output Tranny (same as above) $53

So you're talking the buying price, plus the $40 shipping, plus $61 for a new set of tubes, so an extra $100 to start.

If one or both of the trannies are bad, add that on.

If you can't do your own repairs, figure tech bench fees probably a $40 minimum if you're lucky, more likely $80 minimum and another however much $40-80 per hour.

You can get a Bugera for $300-$350 new, with tubes, that works, and is 100W, you can pop out two of the tubes to make it like the 60, Freightshakin' Bob runs his with two tubes, and I have a story about that for another time, but it sounds good, he's happy. I remember the Peavey Windsor used to knock about for $399 as well, again 100W again with tubes, again works out the box.

It's scarey but these are both budget tube amps, which really is what the Sovtek is too, except they are new and come with warranty. I can't fathom people paying a fortune for dodgy knock-offs, when you can get new knock-offs for cheaper???

[justinhedrick]

No.
Just buy curious.

People are asking 650+ for Sovteks on the 'bay.

jesus, i feel bad about selling mine for $250 about 3 months ago . . .

[VOLVO)))]

Which of those Bugeras ended up on your bench because it couldn't handle the heat? I've had several people ask me to replace those molex plugs that just melt to the board...