Kustom K50 Main PCB Mounted 'Can' Style Diode(?) Help

[El Músico Loco]

I have a Kustom K50 amp that I am replacing the volume pot on and I see what looks like a Ge transistor with two legs mounted to the main pcb and the body-'can' is mounted in one end of a fuse holder on the power transistor heat sink. The legs are broken off of the body. Can anyone help me identify this part and what I can use to replace it? I have put a schematic request up on the request thread. Once I get these two things sorted any help to reduce the crazy hiss coming out of it will be very welcome.
Thank you,
Chris

[J M Fahey]

Very probably it's a bias diode (hence the heat sink attachment) and Silicon.
I think a plastic 1N5402 may have about the same diameter so as to re-use the "fuse clip", or you can just epoxy a 1N4002 there.
What I don't get is why that amp is not overheating because of this.
To know which leg goes where, consider that diode forward biased, so the white band or strip should point towards the most negative of original pads where the former one was connected.
Is it really hiss (as in a tube losing steam through a crack) or hum ?
Does that hiss respond to any control?
Can you reproduce music anyway?

[olddawg]

More than likely it is a "stacked" diode which are just a few silicon diodes in series in the same package. Without a schematic or a picture it's hard to say. If it is a stacked diode you can replace it with a few modern rectifier diodes in series. If you use large enough diodes you don't have to worry about heat disipation. You need to look at the schematic, and if you are lucky the symbol will tell you how many junctions there are in the device and consequently how many diodes to use. If it was an old stereo amp you could just measure the voltage drop across the device in the other channel. BTW, does it have a number on it?

[Mark Black]

I'm thinking it's one of these:

1N3754, Tube 1N3754; Röhre 1N3754 (1N3754)

Kustom used those in other amps, and manufacturers often stick to what they know works, so maybe?

If so just about any single-junction silicon diode with a 100V rating would probably be pretty close, leaving only details of the mounting to thermally couple to the heatsink. Crest audio used to use what appeared to be a 1N4002 mounted into a solderless non-insulated ring terminal (where the wire would normally go) in order to get thermal coupling for a similar use (gotta watch the lead dress & maybe use some heatshrink to avoid shorting with that concept).

Last time (quite a while ago) I worked on a hissy Kustom it was a noisy FET near or at the input stage. Found it by using some freeze spray to see what changed...

[El Músico Loco]

Hi Everyone,
I replaced the diode with a fat SY228 that has the same .784 reading as the original. It is not even getting hot. I replaced the volume pot and now is working enough to troubleshoot the hiss. I still have not been able to find a schematic for this yet so I am winging it. The hiss definitely is associated with the treble control. I tried using freeze spray but was unable to make any change. I also noticed now that the that the tremelo/vibe is not working. Thanks for all the help so far. I will search around to see if this has been tackled before but any more help would be great.
Cheers,
Chris

[J M Fahey]

It is not even getting hot.

Aaaaaaaaaaaahhhhhh !!!!!!!!!!!!!!!!!!!!!!!!!
So you got uan of dem' fancy "diodos fríos" , eh ?????
Let me tell you sumthin' , it should get as hot as dem sheat shink !!!!!!!!!!!!!
El Técnico Loco.

[Enzo]

Well, I certainly do HOPE your bias diode is not getting hot.

[olddawg]

Hi Everyone,
I replaced the diode with a fat SY228 that has the same .784 reading as the original. It is not even getting hot. I replaced the volume pot and now is working enough to troubleshoot the hiss. I still have not been able to find a schematic for this yet so I am winging it. The hiss definitely is associated with the treble control. I tried using freeze spray but was unable to make any change. I also noticed now that the that the tremelo/vibe is not working. Thanks for all the help so far. I will search around to see if this has been tackled before but any more help would be great.
Cheers,
Chris

Sounds like the voltage drop of a single silicon junction to me. If the trem isn't working it may be injecting some noise. Just a thought. Does it require a foot switch or a terminating pug to work?

[Mark Black]

These guys claim to have the schematic. I have ordered from them before (when I just couldn't find something for free or from the manufacturer) and they have always been straight-up with me:

Kustom Schematics

Sometimes it's really worth a few bucks to speed up the repair process, and you can always pass the cost along if it's a retail job.

[El Músico Loco]

Hi and thanks for the help. I was confused about the diode getting hot as the original is mounted to the power transistor heat sink. The amp is working now and I finally scored a schematic (Thanks Mark Black for the link). I am getting lots of hiss associated with the treble control so I will go thru the section today and report back. I have a foot switch to control the reverb & tremolo functions. The reverb works (though does not sound great) but the tremolo does not come on. Any ideas?
Thanks again to all.
Chris

[Steve Conner]

I think the diodos are more likely to be "fritos" than "frios" around here.

For starters, test the footswitch with an ohmmeter, to make sure the cable is intact and both switches actually work.

[Enzo]

Just in case:

The diode is mounted on the heatsink so it can sense the temperature of the heatsink. it is not there because the diode gets hot. The diode has a natural voltage drop end to end, and that changes with temperature. By mounting it on the heatsink, as the power transistors get hot, it allows that diode to change voltage drop and compensate the bias of the power stage.

[km6xz]

The bias diode should be the same temperature as the heatsink for the output transistors. The diode temperature coefficient is the same as the that of the output transistors so the bias changes to track, the conduction of the transistors. As transistors heat they will conduct more current at the same bias so you want the bias to be adjusted to compensate for that increase in sensitivity. This happens only if both the transistors and the diodes are at similar temperatures, forming a stabilizing negative feedback loop, opposite of the positive feedback look internally in junction transistor temperature. Make sure the diode is mounted on the sink so it can work properly as the thermal protection. Otherwise, the output can go into thermal runaway, conducting more current, generating more heat, which increased the current which further increase the temperature, a classic positive feedback loop.
That is something tech assure before the unit is returned to the customer; thermally stressing the amp and monitoring the tracking of that negative feedback loop. Otherwise it might work fine lightly loaded on the bench but burn up after getting hot in the middle of a gig when stressed by temperature..
Take care of that before worrying about noise.

[El Músico Loco]

Thanks for the insight Enzo & km6xz. I will mount the diode to the heat sink now and report how I get along with the rest of the problems. It is going to sound lovely very soon I am sure.
Chris

Edit... I have checked that the footswitch is changing states on both buttons and the tremolo still does not come on. Will let you know more soon.

[El Músico Loco]

Hi,
Ok got the diode mounted and ready to tackle the hiss now. Finally got a schematic too. I found some info about the hiss from this model and it recommends starting by replacing R2/22K & Q1
2N5033/P-JFet. was wondering if anyone here has done this yet and what other P channel Jfets might work. I cannot find one in my area of Spain. I tried a 2N5460 but it does not work. Any other substitutes recommended?
Thanks,
Chris