Intermittent power drop...

Chickenwings

Chickenwings

Alnico 6/8
Very old twin (1967). It not the tubes cos i have systematically replaced all of em. Its not the filter caps cos they are new.
Ive had a look inside and all the bias feed resistors look okay.
The only think that doesnt is the brown coupling caps. A couple of those look kinda "bulgy" for lack of a better word.
Some of the electrolytics inside look like they have been replaced with newer ones.
I guess im just asking you guys what you think might be causing it before i spend 100 bucks on new caps only to find that it is something else.
Oh yeh....there is a largeish electrolytic cap next to 3 diodes (is it the rectifier?)...that looks old.
Can coupling caps cause an intermittent volume drop?
If so, ill replace all of em before taking the amp to a tech.
If you have any other ideas (transformers maybe?) let me know.
Thanks!
 
Re: Intermittent power drop...

don't do it until someone else advises you on this, but I would meter the outputs of the transformer to see if it's putting out what it should, if it isn't then it likely needs a new one. BUT get a second opinion first.
 
Re: Intermittent power drop...

How much of a power drop are we talking about and is it totally random or does it seem to track with the chassis heating up?

Are there any other symptoms or observations that you can think of?

gibson175 said:
there is a largeish electrolytic cap next to 3 diodes (is it the rectifier?)...that looks old.
Click to expand...

3 diodes? How about 6 in 2 rows of 3 with an electrolytic, 1 watt resistor and another diode next to that up by the pilot lamp. If this is the one, that is the filter for the bias supply and very likely the source of your issues.
 
Re: Intermittent power drop...

gibson175 said:
can i do that with a multimeter, or does it need to go to a tech?
Click to expand...

should be ok with a multimeter, as long as it has protected probes or you keep your hands away from the metal when you are doing you will live.

If you have a digital multimeter then that's even better.

let me know if you have the old needle type.
 
Re: Intermittent power drop...

glassman said:
How much of a power drop are we talking about and is it totally random or does it seem to track with the chassis heating up?

Are there any other symptoms or observations that you can think of?



3 diodes? How about 6 in 2 rows of 3 with an electrolytic, 1 watt resistor and another diode next to that up by the pilot lamp. If this is the one, that is the filter for the bias supply and very likely the source of your issues.
Click to expand...

ive been using it for ages on volume 3 and its fine mostly (it has done the odd random power dive every now and again but its not been too much oif an issue) but ive recently started using it on 5 and blasting it with a fuzz face (lots of bass). It seems okay when the gtr volume is low, but when i crank it, the amp kinda goes AAAAH! NOOOO! and starts acting funny. The volume drops right down, but it comes back again i guess when it cools down i guess. It doesnt seem too worried if i dont hit it too hard volume wise...and sometimes it works okay even when its up louder - i juts noticed this more when i was recording recently and every time i cranked up for the chorus the amp crapped itself. The fuzz pedal works fine with otehr amps, but i think all that low end is just smacking the amp and causing whatever is wrong with it to start acting up. So yeah...its not totally random...it seems to be when the amp is getting hot.

Your second question about the (6) diodes etc near the pilot lamp...yeah thats the one i was asking about. I gues it was 6 in two rows. I closed the amp up again a few days ago so my memory is not great. It looks like its the original cap so it must be like 45 years old! Shall i replace it and report back? ps...to save me opening the amp twice to read the value...do you know the value i should buy? is it .047 at 600v?
Thanks for the reply!
 
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Re: Intermittent power drop...

The original value is 50uF@50v; they generally have 50 to 55 volts across the cap when set properly so "technically" the cap is over voltaged. I put a 50uF@100v in as a replacement. A lot of shops will put in a 100uF@100v electrolytic and call it done. Either is fine and I'd go with whatever is easiest for you to source.

You WILL need to rebias the amp after you replace that cap.

Report the value that's in there now...that may shed some light on what's going on.
 
Re: Intermittent power drop...

ok ill open it up again and have a look. if its 50/50 then ill upgrade to 50/100. ill report back and let you know if it fixes the problem. fingers crossed its not a transformer!
 
Re: Intermittent power drop...

ok ill open it up again and have a look. if its 50/50 then ill upgrade to 50/100. ill report back and let you know if it fixes the problem. fingers crossed its not a transformer!
Click to expand...

Take some pics too, if you can.
 
Re: Intermittent power drop...

ok will do. what do you want to see GV? the whole lot or something in particular?
oh yeh...my band is playing at a burlesque industry night next thus. There will be boobs....

p.s.glassman...the existing cap is a mallory 50uf70v. im no expert with date codes but the cap looks like its a 1969 (same as the amp chassis it seems) I thought this amp was a 1967 based on the transformer codes (all 1966) but some research shows me that the amp is actually a 1969...so the cap is as old as the amp. Ill order a new one in 100v.
 
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Re: Intermittent power drop...

gibson175 said:
ok will do. what do you want to see GV? the whole lot or something in particular?
oh yeh...my band is playing at a burlesque industry night next thus. There will be boobs....
Click to expand...
Underneath, components , capacitors and the state of their covering and ends. One good under chassis shot should do it.

Old and worn caps usually create heat. It will shrink and/or discolour the cap covering [ plastic with print on it ] Easy way to spot a stuffed cap.
Expanding Electrolyte from heat will push out the end stopper too, or if it's a radial cap [ both wires at one end] the top will bulge and crack.


oh yeh...my band is playing at a burlesque industry night next thus. There will be boobs
Click to expand...

We will need pics:fingersx:
 
Re: Intermittent power drop...

gibson175 said:
p.s.glassman...the existing cap is a mallory 50uf70v
Click to expand...

Ah...the sealed white ones. No way to visually tell if they are on their way out either. Check under the dog house, there may be another one of these under there, particularly since it's a 69. There are lots of variations in the late silver face amps.

And I agree with GV, photos would be nice...you can throw a few pics of the amp in too if you want.

One of my regulars got a gig at a nudie bar...good thing it was a three performance deal...if they had gone by their initial performance, they never would have been called back.
 
Re: Intermittent power drop...

Gman....yeh it was the cap you mentioned! It was a silver mallory tho. ANyways...there was a little hole in the end of it where i guess some of the innards burner their way out, or corroded or dried up or whatever. Ive taken pics but for some reason my phone and computer wont talk to each other. ill post them when i work it out.
Anyway...i had a couple of 70uf100v caps here, so i soldered one in while im waiting for new 50uf100v caps to arrive. The amp is working nicely now, although i have not cranked it up cos i finished at midnight last night.

the coupling caps are a blend of the brown things and white mallorys. im considering replacing them all too...thoughts?
 
Re: Intermittent power drop...

Everybody else seems to hate the brown coupling caps...enough to term them "brown turds". I, on the other hand like them. They are PVC caps and really smooth things out; they have dissipation losses that are very similar to paper in oil. I'd keep those.

As far as the white electrolytics go, those are all cathode bypass caps and have faded in value by now; if you replace them with new 25uF@25v electrolytics, the tone and feel of the amp will change. You can put 20uF@25v electrolytics in to approximate the current value but the new cap will have a "better" coupling and while the frequency slope will be similar to what you have now, the amp will "feel" different. If you like the relaxed state that amp has evolved to, I'd stick with what you got in there now; anyone can make an old amp sound new but it takes a lot to make a new amp sound old. If you want to replace some of these, only do it on the channel you don't use so that if one of them decides to give up the ghost you have that channel to ride out the rest of a gig. They rarely fail anyway and with <2 volts across them, they don't cause any damage when they do.
 
Re: Intermittent power drop...

cool....got the pics working....(the doghouse caps are all new so i have not photographed them)
here is the toasted cap:Ill just put a new 50uf 100v electros in to replace the dead one (i put a 70uf in while im waiting for the new 50)
DSC_0033.jpg

here is where it came from:
DSC_0031.jpg

the rest of the guts:
DSC_0030.jpg

Ill leave the brown turds in as you suggest glassman...if its working fine and sounding good i guess there is no need for new ones (although the last brown turd on the left is going a bit "bulgy"). I do wonder abpout the blue ons in there tho (the 50uf100v above and the 2 small blue ones in the shot below)...i guess something has gone bad and someone has put them im as replacements. Perhaps the rest of the white mallorys are getting close to dying too.
DSC_0029.jpg

and the chassis:
DSC_0028.jpg

and a gratuitous shot of my rig (the pedal on top is a hartman NKT275 fuzz. Its almost always on. I use the guitar knobs to get various sounds):
PB190018-1-1.jpg
 
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Re: Intermittent power drop...

Thanks for the pics.
You've nailed your problem cap. The other 50uf/100V there is definitely a replacement. The covering/labelling is of the type that discolours and shrinks with heat. The blue goes very pale and the ends shrink so they no longer cover the end. Either says heat damage. That cap is probably from around the 80's and I would replace it as well as it is probably a bit tired by now and works in conjuction with the other 50uf/100V. Best to have the two in the same range of age and performance.

The Brown [greencaps as they're known] caps don't always have a consistent physical form so a slight difference in size and shape is not uncommon.

Where are you getting your caps from , may I ask? 50uf is not a very common type these days. Mostly 47uf are available. Same with the 25uf or 20uf. Mostly available is 22uf.

Do I see an empty valve socket there ? [ far left ]
 
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