Hitting the brick wall!

[DarrylC.]

Hi,

I'm new around here but as soon as I saw the contents of the forum I knew I just had to join. I've been a recording engineer and a guitar tech for some years now but have wanted to get into amp building/repair for a really long time. I've been doing minor repairs on recording equipment and effects processors for a while but it seems like it is the same 5 problems over and over and some of what I have learned translates to tube amps and some of it doesn't. I've decided to extend the offer for super cheap(free in most cases) repair work to all of my friends (and their friends) to gain some experience. So far it has been great. Lots of bad switches, toasted resistors, dirty pots, and blown fuses. All of which I can handle with no problems. I'm currently a little over my head with three amps right now. I've got a '65 blackface vibrolux, an early 2000's 60 watt Marshall TSL-60 combo, and a hughes and kettner 20 watter that are all doing the same exact thing. Symptoms are super low output (conversation volume) and the sound is super distorted (like the way a fuzz pedal sounds when the battery is dying). There aren't any visible signs of fried components. I've tested caps and they seem to be ok. I've been reading about what voltage measurements to take at what points to make sure the transformers are ok but I am just getting really confused and starting to pull my hair out. I can't imagine the TSL and the H&K have blown trannys. The 65 fender wasn't treated very well so all bets are off as far as she is concerned for the time being. Any advice on what my next move should be would be much appreciated.

[The_Gaz]

You're on the right track measuring voltages, as they will tell you everything. Take voltage readings at all the pins of the tubes and check them against the schematic to try and isolate the problem. Once you isolate the problem, you've fixed the problem :) Also building a "signal tracer" to hook up to another listening amp is a good tool, or the visual version, which is scoping the different stages with an oscilloscope. Doug has the plans for the signal tracer somewhere on the site. Trouble-shooting is really an art, which I don't think I'll ever master - wholly different from amp building.

[stratele52]

You have to check tubes first with new one , not tube tester, and be sure bias are alright.

2- Check Phase inverter resistor, one bad one and you got low volume and distortion.

3 - Testing Output transformer need signal generator or low AC voltage/ current source.

[DarrylC.]

Thanks guys. I have a homemade audio probe that I use for debugging/troubleshooting effects processors and other low voltage audio gear. Would this be useful for tube amps? I have been afraid of accidentally probing a point that will have high voltage and damaging my "listening" amp.

[sluckey]

I have been afraid of accidentally probing a point that will have high voltage and damaging my "listening" amp.

Put a .1µF @ 630V in the probe to block dc.

[tubeswell]

When tubes amps start to sound 'funny' or unusual, a tube-swap with known good tubes of the same type will quickly tell you whether its the tubes or something else.

Falling volume  and muddy distortion can both be signs of output tubes on their way out. Output tubes typically wear out faster than pre-amp tubes and need more-frequent replacement (excepting situations where pre-amp tubes become microphonic - which it doesn't sound like in your case). With tubes, you should always keep a spare set handy anyhow - they are like lightbulbs and they wear out eventually.

If a tube swap doesn't fix it, report back

[DarrylC.]

Well, I'm decently stocked up on tubes (at least 3 pairs of most common power tubes and a plenty of 12ax7s) so the first thing I tried was tube swapping. One amp in particular has brand new tubes from tube depot (I still swapped them anyway to be sure). Everything looks good on the board too ( I know that is not a good way to test components) and I've not tested every single component on the board put I did test out the caps. Testing the transformers is kinda new territory for me. I would just be amazed that all three of these amps have bad transformers... it seems like it has to be someone simple that is staring me in the face 

[tubeswell]

What are the DC voltages (to-ground) of all the plates, screens, grids and cathodes and the HT (B+ supply rail) nodes?

And what are the AC voltages across the heater and HT (B+) windings on the PT. (If there are tube rectifiers in any of the amps, check the AC supply for those as well).

[phsyconoodler]

The Marshall may have an effects loop jack that's sprung or dirty.Clean them and make sure they ground properly.
  Tubes need to be checked on both amps.If tubes short out they can take out the screen resistors too,so a good check is in order.
 If you are not familiar with the inner workings of a tube amp I would not suggest you doing it.Too many chances to shock yourself,especially if you don't know what you are probing.

[DarrylC.]

Thanks for all of the ideas!  I've taken a breather and tried looking at the H&K and the Marshall again. I am an idiot.While looking at the Marshall I inspected the filter caps again and noticed one of them is swollen and cracked its case. The H&K had an open 1K resistor and another 1K resistor that apparently was so coldly soldered one of the legs came loose. Both of these resistors where tied to pin 9 of the el84s (in series) and maybe go to ground. I am wondering if when the first resistor came loose it resulted in blowing the other...which makes me also worry that maybe it took out a couple of other components out with it. I am hoping all I have to do is replace the 330uf cap in the marshall and we'll be up and running. I still have to track down the schem for the marshall though.

[phsyconoodler]

Just remember that in tube amps resistors and caps don't just randomly fry,there is usually a cause.The biggest cause is bias too hot and shorted tubes.The fuses usually blow in those instances but in some cases they don't and you need to do some investigation.
  Your eyes are indeed the best tool you have.That and poking and prodding,looking for loose or cold-soldered joints.

[sluckey]

http://www.drtube.com/marshall.htm#JCM2000-TSL

[DarrylC.]

You guys are awesome! I know I'm not out of the woods yet, I am about to head to ratshack to see if I luck out and find the caps and two resistors. The '65 Vibrolux Reverb I'm a little afraid of. Not because of voltage (I know how to make things safe to work on) but because it is all original and I don't want to hurt the value of the amplifier by changing things that may or may not need replacing. I'm getting the same problem as far as symptoms go ( super low volume and distorted fuzzy sound).

[DarrylC.]

If a capacitor is swollen but still measures the correct uf is it still considered bad?

[schoolie]

If a capacitor is swollen but still measures the correct uf is it still considered bad?

A capacitance meter won't necessarily tell you whether a cap is bad.

[tubeswell]

The simplest thing that'll tell you if a cap is bad, other than your ears, is the voltages. We still haven't heard about them yet

[HotBluePlates]

... The '65 Vibrolux Reverb I'm a little afraid of. ... I don't want to hurt the value of the amplifier by changing things that may or may not need replacing.

Is this the newer custom shop Vibrolux Reverb? You'll know it from a real 60's blackface amp because there is "Custom" (in quotes) above Vibrolux. That and the p.c. boards inside.

If so, I would not be afraid to replace caps, as that's not gonna hurt value on a modern amp. Even on vintage amps, most people have gotten used to replaced filter caps, as long as you don't butcher the amp in some way.

But is that the problem?

Verify correct voltages at all tube pins. If they're there, you're halfway home.

Verify the onboard speakers are plugged in to the main speaker jack. Most of the time, if nothing is plugged into the main speaker jack, but is moved to the external speaker jack, it will give the sounds you describe.

Verify there's no loose/intermittent connections to/from the board or at input/speaker jacks. Verify connections at the speaker terminals.

It's more common to have something simple like these go wrong rather than a real failure. When there is a failure, most often the general location is revealed by either burned components or wrong voltages.

You mentioned thinking it would be odd for all 3 amps to have bad transformers. I'd think the same thing. While I know people who have encountered failed transformers (even 1 bad one new from the manufacturer), I've never come across on in ~20yrs of amp-tinkering. And they generally don't fail on their own, but because something related to the output tubes failed (either tube, bias supply or accidental short).

[frus]

On Marshall, check the output jacks, because one depends on another making a (ground) contact when nothing is plugged in, and that contact may become dirty/oxidized. Had one with that problem and similar symptoms

[DarrylC.]

... The '65 Vibrolux Reverb I'm a little afraid of. ... I don't want to hurt the value of the amplifier by changing things that may or may not need replacing.

Is this the newer custom shop Vibrolux Reverb? You'll know it from a real 60's blackface amp because there is "Custom" (in quotes) above Vibrolux. That and the p.c. boards inside.

No, it is a true 1965. Never been touched. All original everything. I think I am the first person to even see the insides after the amp was assembled and sold in the 60's. He told me his father left it to him after he passed and it sat in an attic for a decade or two. Pretty amazing looking amp.

[tubeswell]

... it is a true 1965. Never been touched. All original everything. I think I am the first person to even see the insides after the amp was assembled and sold in the 60's. He told me his father left it to him after he passed and it sat in an attic for a decade or two. Pretty amazing looking amp.

Well you can pretty much bank on the electrolytics being dodgy, especially if it sat inert for 20 years or so.

[PRR]

> If a capacitor is swollen but still measures the correct uf is it still considered bad?

Swelling is always bad.

The cap will fail. If it works on your test bench, and at customer pick-up, it is liable to fail at the first Big Gig he takes it to.

Cap meters mostly don't meter ALL possible faults in electrolytics (which are prone to odd faults). The leakage resistance may be low (it will run hot and/or load-down the rail). The series resistance can be high (like a dirty contact, only deep inside). It may stand a 1V test fine but go all gooshy with 350V. Some meters don't try to suss this out, others do but must use very loose standards for "bad" or they will condemn too many functional caps.

Cap costs are SO low in context of a professional's essential tool (or a hobbyist's toy) that when in doubt, swap it out. (Unless the amp is mint-virgin AND collectors pay absurd prices for an unmolested museum piece. Probably a 65 BF VL isn't there yet, maybe never.)

[HotBluePlates]

If a capacitor is swollen but still measures the correct uf is it still considered bad?

I have a fan belt in my car with visible cracking and glazing; makes noises when I start up the car but the car still goes when I hit the gas. Should I change the fan belt if it has the right length?

Silly example, but what I'm getting at is signs of present/future trouble.

A possible scenario: a developing failure in a cap may include increasing leakage current. That leakage passes through the cap's equivalent series resistance (ESR) and results in heat dissipation (according to ohm's law and the equation for power). Cap self-heating tends to lead to boiling/loss of the electrolyte, worsening leakage and possibly ESR, leading to more self-heating. Increasing leakage current may or may not be accompanied by a measurable change of capacitance (that's outside the part's original tolerance).

Most modern electrolytic caps have some form of vent for the release of any gas developed as a result of self-heating; if you're seeing bubbling, you're probably seeing evidence of excessive self-heating and future failure. So your best preventive maintenance step is to replace any electrolytic caps with bulging/bubbling.

OPINION TIME: Common Wisdom is to replace all electrolytics in a vintage amp, cause they're all destined to fail eventually. I'd tend to follow that with power supply/bias caps. Cathode bypass caps may have dried up and reduced in value, slightly shaving the bass response. I've noticed folks around here have built clones of vintage amps and desired less boominess than they get with a 1.5k cathode resistor and a 25uF bypass cap in a preamp stage. Often, 1-5uF seems to give them a sound they like. Coincidentally, this is about the range the cathode bypass caps dried up to when I measured the original parts in a '67 Princeton Reverb I used to own. You may take the view that the "dried up" values are part of the vintage sound people like, or you may decide that 25uF represents the "as-new" sound the amp originally had. Your call...

The '65 Vibrolux Reverb I'm a little afraid of. ... it is all original and I don't want to hurt the value of the amplifier by changing things that may or may not need replacing. ...

Then swap electrolytic caps, consider keeping cathode bypass caps if the tube voltages check out okay. Put any parts you take out in a bag and keep with the amp. If someone needs a museum piece, they can put the old, bad caps back in the amp.

Still consider posting tube voltages for this amp. An amp that does not turn on or makes ZERO sound is easy to troubleshoot; a few things checked will tell you what is wrong. An amp that "doesn't sound right" is harder to troubleshoot; many things could result in not-right sound. D.C. voltage measurement of all tube pins (with a.c. measurement of heater pins) is the first step to figure out a problem.