Kustom K200B-4 hisssssssss

[Mars Amp Repair]

Hey folks,
I have this Kustom amp and I've not worked on this particular model. this apparently is the Cadillac of the tuck & roll models for it's time.

The 2nd channel has mondo hiss with the volume control full down & the boost switch off. The boost switch off completely bypasses the tone switch stages.

I'm almost at the point of deciding that these just were 'hisssssy', as I can't find any heat/cold sensitive transistors & trying to signal trace just seems to show the signal getting amplified as you'd expect.

I have replaced a few of the transistors that were heat sensitive. They're no longer heat sensitive, but the hisssss is at the same leve..

anyway, has anyone had experience with these to know if they are just hissssy?

I think I'll take the cab home & sleep on it tonight glen

[sonic_tooth]

Glen,

I have a K200 with a similar problem. I found this thread over at vintagekustom.com. Have not really had time to check it out myself though. Excuse the paste here, but I could't figure out how link to the thread.

K200 A5 hiss problem solved [message #8215] Fri, 09 February 2007 09:00
daveo@zrgmail.com
Messages: 48
Registered: January 2007
Location: Missouri Member

I finally found and fixed the primary hiss problem / source in my K200 A5.

As I suspected, it was in the summing / reverb board mounted to the floor of the chassis. There are some stages on that board with extremely low levels. That, coupled with marginal transistors was making for a lot of noise.

Replaced the transistors in the middle of that board last night, and the hiss level dropped by almost 15db!

Next I’ll give the reverb pre-amp a similar treatment…as it’s now the noisiest thing in the amp. With the reverb shut-down, you can hardly tell the amp is on now.

Others suffering with the persistant hiss of the A5 PA chassis may wish to try this as a fix. I just used fairly generic, 2N3904 NPN.

Dave O.



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Re: K200 A5 hiss problem solved [message #8216 is a reply to message #8215 ] Fri, 09 February 2007 12:37
stevem
Messages: 1427
Registered: June 2004 Senior Member

Thats a great thing isn`t it,! with 2n3904 costing at most 15 cents each, it makes you wonder how much better a whole amp would sound rebuit with them?
15 DB is almost half the total gain that the whole amp can provide if I remmember right so thats a hugh difference in hiss.
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Re: K200 A5 hiss problem solved [message #8218 is a reply to message #8216 ] Fri, 09 February 2007 17:47
daveo@zrgmail.com
Messages: 48
Registered: January 2007
Location: Missouri Member

I'm gonna try to at least change-out all the transistors on the summing/reverb board over the weekend. There are a bunch.

I also spotted one or two of the same type on the power amp board...the levels through there might be high enough not to make much difference however.

Wish I understood the feedback circuit routing through this thing. I'd really like to limit the bandwidth a little more. In stock form, this A5 is only 3db down at 100khz...and that's all the way through the input pre-amps!

I put a 330pf cap across (to ground) where the recording output comes off the power amp board. This brought the 100khz response down to about -6 or -7db...without affecting the response at 20khz by more than a half db. There's just no reason for a guitar amp to have this kind of high-end response. I'd think it could only cause trouble...with picking up RF, and adding noise. I'd rather control it in the feedback loop however.

Dave O.



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Re: K200 A5 hiss problem solved [message #8236 is a reply to message #8215 ] Mon, 12 February 2007 06:50
stevem
Messages: 1427
Registered: June 2004 Senior Member

It can be touchy thing to start messin with the feedback of those first two stages in the driver, less feedback would result in more gain and the amp may fly off into oscilation.
Mabe something along the lines of what you tryed with A L/C circuit is worth giving a shot. Most of that hiss is in the 5K TO 8k range it think.
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Re: K200 A5 hiss problem solved [message #8239 is a reply to message #8236 ] Mon, 12 February 2007 09:12
daveo@zrgmail.com
Messages: 48
Registered: January 2007
Location: Missouri Member

Here’s my reasoning…
The amp seems all too likely to pick-up interference from the florescent shop light on my work bench. This light generates a bunch of noise…mostly around 50khz. Any equipment that’s not very well shielded, or has a fairly high cut-off, picks up interference from that thing. This Kustom is no exception to that. In fact, that light can cause this K-200 to wanna start into an oscillation through the reverb tank at around 1.6khz.

My thought was to increase the feedback at higher frequencies, to reduce the bandwidth…as long as the phase doesn’t twist around up there…and the feedback stays negative. Similar idea to putting a small cap across the feedback resistor of an op-amp. I’ve used this trick successfully in lots of other amps which weren’t all that stable at super-sonic frequencies.

Since I don’t have any kind of a usable schematic for this amp, I don’t know what the feedback circuit looks like…so I’m still shooting blind.

Are some of the other, later K-200’s similar to this A5 chassis? I could purchase one of the other K-200 manuals which are available if anyone thinks it would be similar. I’d guess much of the power supply and power amps were pretty similar in the A series?

The pre-amp boards might be a little different, and that darn summing/reverb board is probably different.

Thanks,
Dave O.



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Re: K200 A5 hiss problem solved [message #8243 is a reply to message #8215 ] Mon, 12 February 2007 12:37
stevem
Messages: 1427
Registered: June 2004 Senior Member

I do not have much experience with messing around feedback in a SS amp, but in a tube amp, more feedback produces a wider! frequency range, so you might get just the opposite result of what you are looking for.
The driver/output stage of the A serise should be the same as the latter B serise amps.
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Re: K200 A5 hiss problem solved [message #8245 is a reply to message #8243 ] Mon, 12 February 2007 14:26
daveo@zrgmail.com
Messages: 48
Registered: January 2007
Location: Missouri Member

Steve:

True enough, flat negative feedback will flatten out the frequency response. What I’m considering however is “frequency contoured” negative feedback.

If we apply more negative feedback as the frequency increases (by placing a small cap across the feedback resistor), the gain of the amp will decrease as the frequency increases...creating high-frequency roll-off. Think of it as a treble control, at 100khz.

This was the bandwidth limiting I was proposing. Without a full understanding of the feedback circuit in these amps however, it’s hard to figure out where to start.

This “contoured” feedback must by applied with some caution however. Many amps (especially tube amps) do not exhibit very linear phase response...especially up near 100khz. As the phase rotates, you could wind-up applying positive feedback, instead of negative...just because of the non-linear phase response.

(I’m more of a tube guy myself BTW...probably because I’m such an old fart)

I’ll just buy a service manual for the other K200 amps. Thanks for the tip.

Dave O.



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Re: K200 A5 hiss problem solved [message #8251 is a reply to message #8215 ] Tue, 13 February 2007 06:46
stevem
Messages: 1427
Registered: June 2004 Senior Member

Dave.
You might just like the early 200s(the frank) heads better.that phase inverter transformer makes for less hiss and a bit more mellow of a an amp, as seeing as you a tube guy like me, these amps with out the output stage protection circuit clip far more like a tube amp by noy shifting the duty cycle around.
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Re: K200 A5 hiss problem solved [message #8255 is a reply to message #8251 ] Tue, 13 February 2007 08:49
daveo@zrgmail.com
Messages: 48
Registered: January 2007
Location: Missouri Member

I was thinking about getting a frank head. The schematic you sent me looks interesting...I think it was for one of those...that got me thinging about one.

Were they also 100 watts RMS?...Thougth I heard once that they were more like a 60 watt head...but I could have been dreaming.

Finally ordered a manual for the B series K200...from Kustom Dan I think...or whoever is selling them at Vintage Kustom.

Dave O.





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Re: K200 A5 hiss problem solved [message #8256 is a reply to message #8215 ] Tue, 13 February 2007 12:41
stevem
Messages: 1427
Registered: June 2004 Senior Member

They where 60 watts RMS,but putting in a k200 size PT or a new torridal with higher current output might let you sneak up to 80 watts if you need the extra clean headroom.
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[Mars Amp Repair]

thanx sonic,
in this particular model, the reverb circuitry is integral to the channel 1 board. The hiss in this amp is coming primarily from the 2nd channel. I do believe most of it is from these inherently noisey transistors, but I can't replace them all. I am using a 2n2222, pretty much a generic npn like the 2n3904 mentioned.

I suppose one could just try to reduce the hi-end response, but I'm not certain that would be an effective solution.

thanx for taking the time to find that thread...glen

[Enzo]

So scope it out or signal trace it to find where the hiss is coming from. The input stages will typically add more noise than later, but you never know. There are plenty low noise transistors that will drop in for a 2222.

[Mars Amp Repair]

yeah,
I know it sounds like it should be that simple, but the hiss is just incrementally amplified as you go from stage to stage. There seems to be no stage where it all of a sudden appears.

Even if you isolate the stages by shunting the signal, the last stage before the power amp seems to also add plenty of hiss on its own. Replacing that transistor has no effect.

Seems to be a design thing as the power amp stage adds its own fair share of hisssss, too.

Fortunately, this guy is ok with it. glen