Ampeg SVT-CL

[bsco]

Looking for some advice....I have this amp for repair.....back in August of 2013, this amp had all 6 of the 22 ohm resistors replaced...they had measured correct value but showed signs of overheating...so I replaced them...one side could not be biased properly so I had to replace 3 of the 6 output tubes in order to get both sides balanced...It is back now with one tube bad and the 22 ohm resistor associated with the tube was open.....I have replaced that again and was going to install another new tube.......however, I am concerned that this might happen again....within the next 5 or 6 months or so....Is there anything else I can look for before I proceed?? I was thinking about a new tube socket although I can't see anything wrong with the present one..... I also noticed that there were two spade connectors(flag shaped), that were badly corroded from overheating over time......I have two new ones here now to install as well.....Any thoughts out there in repair land...as always you guys are great in giving advice....
Cheers,
Bernie

[Enzo]

You had a tube failure, probably nothing more than that. it is common for the screen resistor to burn out when the power tube fails.


I know it costs the same as a month's rent, but it is preferable to replace all the power tubes, not just half of them.


There is a service bulletin telling us to replace the 22 ohm resistors with 220 ohm resistors, and I recommend you do that.

[bsco]

Thanks Enzo...I'll have to pick up some 220 ohm resistors as I don't think I have any here on hand at the moment...I'll have to double check....to be quite honest there were 6 brand new tubes replaced at once......then a few months later one tube gave out.....tried to find a single tube which would match the ones that were there already rather than install a brand new set again....Couldn't find one so I had to install three on that side so that I was able to get it biased...( I used the bias lights as a guide).... I am not 100% sure but I think the tube in the previous location is gone again...they did use this for about 6 months or so before it happened originally...then I swapped out three tubes to keep that side balanced with the other side....then about 6 months later the same tube location went out....so that was why I was wondering if there was something else I should check...maybe it was just a bad tube.......also I have noticed that when I get a matched set of six tubes there is always one that doesn't work well with the other five...and I have run into that problem a few times now.....
Cheers,
Bernie

[MarkusBass]

It think that it was just a bad tube. But you may take a closer look at the socket of this valve. Are there any problems with the soldering?
Before you replace the 22 Ohms resistors I suggest that you find this service bulletin. It's due to the fact that it also suggested to remove diodes that are connect parallel to the resistors. It does not make sense to apply only half of what is needed.
The reason why one valve of matched six is not performing as you would expect is simple; the tubes are mached with catalogue voltages - usually much lower than you have in the amp. Quite often it is 250V instead of eg. 600V that you have in the amp. Some companies match tubes with 500V but you have to ask to it.

Mark

[Enzo]

You are correct, the parallel diodes should also be removed. I should have included that.

That is the sum total of the bulletin - remove the 22 ohm and diode at each tube, and install a 220 ohm.

[MarkusBass]

There are two more issues worth mentioning:
- you have to warm-up the amp for at least 20 minutes to set the bias. Otherwise it will be too high and this may make the life of valves much shorter.
- you shouldn't use the LED indicators for bias setting. They are highly inaccurate. Instead, you should measure voltages on 10 ohms resistors. It should be something like 240 mV if I remeber correctly.

Mark

[bsco]

Ok. Thanks Markus Bass and Enzo....just a quick question...if the diodes are removed, why were they there in the first place?? I did a google search for this bulletin but came up empty....
Cheers,
Bernie

[MarkusBass]

I did a google search for this bulletin but came up empty....

This is hard to believe. I typed "Ampeg SVT CL Service Bulletin" in google and I've got a lot of results. This one is second from the top: http://music-electronics-forum.com/t34501/
And the service bulletin we are talking about is also on this page: http://music-electronics-forum.com/t34501/#post325689

Mark

[bsco]

Thanks Mark...I didn't find this in google...a lot of sites did come up but I guess I clicked on the wrong ones....Sorry about that.....
Cheers,
Bernie
P.S. Just downloaded the info.....I never saw this stuff on google...I'll have to check the serial number range to see if the one I have falls within these numbers...
Cheers,
Bernie

[soundguruman]

The problem is almost always a bad tube,
or blown screen grid resistors. (the 22 ohm resistors)
When you plug a bad tube in, it blows the 22 ohm resistor (again.)

Replace all the 22 ohm with 1 watt, for starters.
this is helpful.

Then with good tubes and good screen grid resistors, you "should" be able to bias the output...
without the protection shutting the amp off.

What you need to keep in mind, is that this is a Crate amp, not an Ampeg.
And if you "think like Crate" you will start to understand the warped concept.

I suppose you can replace 22 ohm with 220...
but it makes the plate less sensitive.
Drops more voltage, and makes the 220 more likely to blow.
Variations on a theme...

[bsco]

I replaced all the 22 ohm 1/2 watt with 22 ohm 1 watt resistors as this is what was called for on the schematic..the service bulletins however say to remove the diodes and replace the 22 ohm with 220 ohm 1 watters....There is one tube that when inserted in the tube tester does not light up.....although it gets hot...and when tapping the tube it shows as having an intermittment short so I do know that I have a bad tube...the reason for this post was I just wanted to know f there was something else I should check as these are very high power amps with very high voltages.....and take a lot of abuse...just because the knobs can go to ten, they put them on fourteen......if you know what I mean....I am not familiar with Crate very much.....never realized that AMPEG was a Crate amp under the hood....I always thought AMPEG made their own line of amps....and Crate had theirs and SLM was only a distributor as they supply more than one brand of amp....
Cheers,
Bernie

[MarkusBass]

You had a bad tube and this was a reason for the failure. The change advised by Ampeg is aimed to extend life of the valves. You can apply it or not. I do it with every SVT Classic I get.

Mark

[soundguruman]

I replaced all the 22 ohm 1/2 watt with 22 ohm 1 watt resistors as this is what was called for on the schematic..the service bulletins however say to remove the diodes and replace the 22 ohm with 220 ohm 1 watters....There is one tube that when inserted in the tube tester does not light up.....although it gets hot...and when tapping the tube it shows as having an intermittment short so I do know that I have a bad tube...the reason for this post was I just wanted to know f there was something else I should check as these are very high power amps with very high voltages.....and take a lot of abuse...just because the knobs can go to ten, they put them on fourteen......if you know what I mean....I am not familiar with Crate very much.....never realized that AMPEG was a Crate amp under the hood....I always thought AMPEG made their own line of amps....and Crate had theirs and SLM was only a distributor as they supply more than one brand of amp....
Cheers,
Bernie

Yeah, you did the right thing.
Take diodes out, good...replace 1/8 watt with 1 watt 22 ohm, good.

The output tubes are arranged in two triangles.
1/2 output is one triangle.
the other 1/2 is the other triangle.

I like to put TWO tubes in, one in each triangle. (provided the 22 ohms are all GOOD)
Now test, does it pass a clean sine wave, does it trip off the protection?
If it trips OFF, one of the two tubes is BAD. put another tube in...(or one of the 22 ohm is OPEN.)

Until it passes a sine wave, and the protect does not trip OFF. With 2 tubes...

Now move those TWO tubes to another PAIR of sockets, repeat the test...
did it pass a sine wave, did the protect trip OFF? see above again...

By doing this with TWO tubes, you can test all the sockets individually...
You can test the tubes individually...
You can spot a bad socket, or a bad tube...

Now that THIS has passed, proceed to FOUR tubes. Two in each triangle.
Does the amp pass sine wave, does the protect trip OFF?

Now proceed to SIX tubes. Repeat the tests.
With SIX tubes installed it "should" bias correctly.

However it will NOT bias correctly, if one tube is bad, or if the screen resistor(s) are blown.

This allows testing the tubes, and the sockets, one at a time.
And allows you to eliminate a bad spot, by process of elimination...

Even a non-tech can spot a bad tube or a bad socket, using this simplified method. Process of elimination...

Designed by the same guy who designed all Crate amps...
Built in the same factory, with the same parts as crate, by the same people as crate.
It's a Crate on the inside, and says: "Ampeg" on the outside.

"Crate-Peg."

Later, the factory closed, and production moved to China. But, still same amp...

Of course it's nothing like a real Ampeg.
It's more like a home stereo amplifier...
And there was never a complete schematic for it.

REAL Ampeg SVT: Made in USA.
The amp was designed, so it could blow 2 output tubes, and keep right on playing, no matter what.

BUT the Crate and the Chinese one won't work like that.
So, the fact that it has 6 output tubes, is where the similarity ENDS.

There is no real resemblance to the REAL SVT. Not in design anyway...
The "Pseudo-Ampeg" and the REAL Ampeg, are two different amps.

[soundguruman]

You had a bad tube and this was a reason for the failure. The change advised by Ampeg is aimed to extend life of the valves. You can apply it or not. I do it with every SVT Classic I get.

Mark

I do it to every one also.
And far less problems in the long run.

You call them: "Ampeg." that's funny.
They will always be: "Crate-Peg."

[bsco]

Thanks Markus.....I will do that...and Soundguruman....I'll test out the tubes and sockets etc using the procedure that you have suggested....I do know for a fact that I have one bad tube so I'll replace that one and then carry out all the checks...Thanks guys for the help....
Cheers,
Bernie
Now if only I can get rid of this damm flu....