Marshall w/ melted caps next to tube sockets

[Enzo]

Does what mean that? It is a cap, so when it is working right, it blocks DC, and only allows high freq signal to pass through unimpeded. WHen it shorts, then all kinds of current can pass through it.

[Jasonp]

Does what mean that? It is a cap, so when it is working right, it blocks DC, and only allows high freq signal to pass through unimpeded. WHen it shorts, then all kinds of current can pass through it.

value is what I mean. Basically the smaller the value the less signal will go through the cap?
Thanks for answering my dumb questions, as you can read I'm a total newbb

[Enzo]

Basically the smaller the value the less signal will go through the cap?

Um, not like I think you mean. The impedance of a cap goes down the higher the frequency is that tries to flow through it. Changing cap values is one way to adjust the frequency response of an amp. It is also how you set up the crossover points in a speaker crossover assembly.

Smaller value caos will only let higher frequency signal through. Just the same way the crossover in a speaker only lets treble high frequency stuff through to the tweeter. The simplest crossover is just a plain old cap in series with the tweeter.

There is something we call a brightness cap in some amps. The BRIGHT switch on most any Fender amp is an example. Turn it on and the sound gets a little brighter - not louder, just brighter. Brighter means a little more high frequency than without it. SOme amps don't use a switch for it, some like old Marshalls wired it permanently on one channel and left it off of the other. Instant bright channel and normal channel from two channels that were otherwise identical.

That brightness cap was something like 100pf. The 22pf cap is much smaller in value, so the freqs it lets through are even higher than the extra brightness in our Fender example.

SO it is not like a smaller cap would be turning down the volume, a smaller cap would be like turning up the treble, or in this case maybe turning down eveything else.

But remember, this cap is not in the signal path, this is bypassing some of the amp gain. So it reduces the gain at frequencies much higher than the guitar makes. Think of this cap as turning down the ultra-treble. Sorta...

[Jasonp]

Um, not like I think you mean. The impedance of a cap goes down the higher the frequency is that tries to flow through it. Changing cap values is one way to adjust the frequency response of an amp. It is also how you set up the crossover points in a speaker crossover assembly.

Smaller value caos will only let higher frequency signal through. Just the same way the crossover in a speaker only lets treble high frequency stuff through to the tweeter. The simplest crossover is just a plain old cap in series with the tweeter.

There is something we call a brightness cap in some amps. The BRIGHT switch on most any Fender amp is an example. Turn it on and the sound gets a little brighter - not louder, just brighter. Brighter means a little more high frequency than without it. SOme amps don't use a switch for it, some like old Marshalls wired it permanently on one channel and left it off of the other. Instant bright channel and normal channel from two channels that were otherwise identical.

That brightness cap was something like 100pf. The 22pf cap is much smaller in value, so the freqs it lets through are even higher than the extra brightness in our Fender example.

SO it is not like a smaller cap would be turning down the volume, a smaller cap would be like turning up the treble, or in this case maybe turning down eveything else.

But remember, this cap is not in the signal path, this is bypassing some of the amp gain. So it reduces the gain at frequencies much higher than the guitar makes. Think of this cap as turning down the ultra-treble. Sorta...

Ok, this is starting to make sense.
By gain do you mean volume?

[VintageGrooves]

For what it's worth, there are some folks who won't even wire in a 1-ohm resistor between cathode and ground to simplify checking bias, bc they feel that it complicates the signal path and adulterates the sound or something. I dunno, must take an awfully good hear to hear differences like that...

BTW - the marshall sounded great w/o the caps, so we went with it that way. Many thanx again,
drew

[VintageGrooves]

oops

omg - i'm so sorry - I was looking at page two, and responded to a question that was like 10 comments ago - sorry for the non-sequitor!!!

For what it's worth, in response to the current discussion...
If it doesn't make sense that a smaller cap value could also take away high frequenies, just think of the little cap that's wired to the tone pot in your guitar. It works the same way: when you open the treble pot (which turns down the treble), what you're doing is opening up a "valve" that lets some of the signal bleed off into ground. However, along the way is the cap, and it only lets high frequencies go through it. So, when you turn down the treble, you're letting some of the treble bleed off to ground through the cap, and it is this same cap that also stops any bass from bleeding off the same way. So, depending on where it's at in the signal chain, a cap could either let more treble into the signal path or bleed more treble to ground.
hope that helps,
drew

[Enzo]

There you go, great example.

[Jasonp]

omg - i'm so sorry - I was looking at page two, and responded to a question that was like 10 comments ago - sorry for the non-sequitor!!!

For what it's worth, in response to the current discussion...
If it doesn't make sense that a smaller cap value could also take away high frequenies, just think of the little cap that's wired to the tone pot in your guitar. It works the same way: when you open the treble pot (which turns down the treble), what you're doing is opening up a "valve" that lets some of the signal bleed off into ground. However, along the way is the cap, and it only lets high frequencies go through it. So, when you turn down the treble, you're letting some of the treble bleed off to ground through the cap, and it is this same cap that also stops any bass from bleeding off the same way. So, depending on where it's at in the signal chain, a cap could either let more treble into the signal path or bleed more treble to ground.
hope that helps,
drew

No the more we talk about this stuff the more I can try to understand what you guys are talking about. Pretty cool.

[Jasonp]

interesting thing happened today. I was playing at about 4 1/2 volume wise on the amp.. Super loud on the distortion channel. I looked at the back of the amp and I see the SED power tube on the last socket (power tranny side) start to red plate. So I stop playing and turn the amp off quick, leave it on stand by. So I power it up again all ok, check bias both sides at about 88 mV. Correct me if I wrong here but this tube is on the same circuit as that cap?
Started playing again and was fine. I bet I have a tube going out.. so I changed them out for JJ's and lower the bias to 78 mV a side. Sounds good.

[Enzo]

That is the socket with the cap. Take that tube and swap places with it and one of the others. Now pplay again. If the same tube red plates, then it is obviously the tube. If the other tube now red plates in the same socket, then the amp has an issue there.

I suspect you have a failing tube. The fact it will bias out doesn't mean there is not some little piece inside breaking loose, or just the tube not being able to handle operating under load, regardless of idle bias.

[VintageGrooves]

uh oh....

ok, back again I removed the caps, and the amp sounded great. I advised the customer of the option of replacing the caps or just leaving them out, and he opted to leave them out (the amp seemed totally stable). But, 2 days later, here comes the amp again, this time with a blown filament fuse (the fuse that is in series w/ fil1 and fil2 on the two power tubes and the phase inverter.). The fuse is a 6.3A slow blow, and the one that was in it was the kind with the little bubble in the middle of the filiment. Anyway, I hooked my current meter up to the two sides of the fuse, cranked up my PS real slow, and watched what happened. After confirming that nothing was going to blow up, I powered up quickly and watched the fuse current. When the tubes were totally cold, the current would run up to 8-10A for just a second, and then would settle down around 4.5A.
Is this current normal? If so, should I just assume it was an old fuse? (I realize that this is rare). Also - could this have anything to do with my cutting out C26 and C27?
many thanx,
drew
p.s. I can't tell you if the amp will blow another T6.3, bc my radio shaft doesn't stock anything even close, so I have to order - arghh!