FRED Diodes For A Kustom 200B?

[earache]

Hello all
I have read lots of pros and cons about FRED diodes as replacements for stock diodes - some claim their amps sound better with them - others dispute this.
How well do FRED diodes work as a means to reduce hiss in an amp? Specifically I have a Kustom 200B head that hisses like a steam kettle and I am wondering if I replaced the bridge rectifier with FREDs, would it be an improvement? Has anyone done this before?

[Gtr_tech]

Boo-teek rectifier diodes won't help a noisey front end. Metal film resistors would have more effect as far as noise reduction. Been a long time sinse I was inside one of those old Kustoms...I don't remember how noisey they were.

[earache]

Signal Tracing Inside The Amp

Hi Gtr-Tech

Thanks for the thoughts on the FRED diodes. Other posts about Kustom amps here and at Welcome to VintageKustom.com!!! have additionally stated that the tantalum caps are noisy and can drift out of spec...your opinion on tantalums?

I have tried tracing a signal thru this amp using an oscilliscope - my first try at this kind of troubleshooting. I bought a DMM that has an integrated square wave generator and so I made a cable that has banana plugs on one end for the meter, and a 1/4" jack at the other end, so I can plug the signal source into the input jack and work from there. The square wave immediately goes all shark-fin shaped right at the input jack. I guess this isn't a very valid approach to tracing a signal... but I wonder why the square wave immediately gets misshapen right at the input jack? Does it have to do with getting a good ground return for the scope?

So many questions! Thanks in advance, you guys are so very helpful!

[Jazz P Bass]

Square Wave

Running a square wave through an amplifier is useful at times.
You can check slew rate, rise & decay times.
It is useful after the amplifier is functioning.
You really need to apply a sine wave.
With an adjustable amplitude.

[Enzo]

Whatever improvements FREDs might make, it will be subtle. If you get loud white noise in the amp, it is most likey a noisy semiconductor.

In the case of a noisy amp, one should be thinking "fix this amp," not "let's re-engineer this circuit."

[gbono]

Uultrafast soft recovery diodes will not help your problem. I do not see how controlling over shoot will reduce your noise problem. In most audio applications this would be waste of money (assuming you would pay more for this type of rectifier).

[earache]

Shifting Focus

Thanks for your opinions everyone - I'll shift focus away from the rectifier and search for the source of the hiss elsewhere, and I'll let you know what I find.

[J M Fahey]

Please post the preamp schematic, that's where the noise comes from.

[Jazz P Bass]

Kustom 200 preamp

Preamp Schematic:

[Tom Phillips]

Earache,

Some thoughts.

I know you said in your original post that your Kustom 200B head "hisses like a steam kettle."
Have you ever heard one that hisses less to your ears? I ask this because:
1) It's impossible to quantify the descriptions that we give when describing a problem. Especially in situations like this.
2) Those Kustom amps were always hissy compared to the tube amps of the day.
3) It's a good idea to be sure that you are chasing a real problem rather than trying to make the amp better than it ever was.

Regards,
Tom

[Mark Black]

Just curious - do any of the controls affect the hiss? This to help narrow the problem a bit.

A technique I have used with decent success on amps that work but are noisy:

First I might warm the entire board with a heat gun (not too hot - just warmer) to see if the noise changes drastically, followed by a global cool-down with freeze spray (again to see if there is a drastic change in noise level).

If either or both of those actions cause a drastic change then I start narrowing down by alternating board heating and individual component cool-down.

If neither extreme has much effect then it's off to other troubleshooting methods.

Of course as Tom pointed out those amps always were a little noisy...

Best of luck -
Mark

[J M Fahey]

Thanks.
Now a few questions:
Volume on "0", bright off, treble and bass on "5": ¿ does it hiss unbearably/audibly?
Now treble on 10 :¿?
Now Treble on 5, volume on 5: ¿?
Now Treble 10, Vol 5 :¿?
Now Treble 5 , Vol 10:¿?
Now Treble 10, move Vol slowly from 0 to 10: ¿does the hiss increase smoothly along the scale or it does (as I suspect), "jump" a lot on the last part, say 8 or 9 to 10?
Note: when I say 0, 5, 10, whatever, it's on a 0(leftmost) to 10 (rightmost) scale, with 5 meaning the middle spot.
The amp design is good, but it's compromised by the very low voltage supply it gets.
We'll tackle the noise issue first and later mod it to a better supply, if allowed by the components used.
As an extra curiosity: plug your bass (fully open) on the high input, put bass and treble on 10, bright on, volume 0.
Start playing normally and slowly, by half-points , start rising the volume, until it just starts to clip/fart/growl/whatever. Post the Volume position.
Close your bass volume and tone: ¿does the Kustom hiss unbearably/bearably?
Thanks.
Good hunting.

[earache]

Earache,

Some thoughts.

I know you said in your original post that your Kustom 200B head "hisses like a steam kettle."
Have you ever heard one that hisses less to your ears? I ask this because:
1) It's impossible to quantify the descriptions that we give when describing a problem. Especially in situations like this.

Thanks Tom. Yes I have heard a Kustom K200 amp that was much quieter than this particular one. The "hisses like a steam kettle" description is admittedly vague, sorry. This amp has a hiss with a slight sputter almost like bacon frying (there I go trying to quantify the description again... ). It certainly is louder than you would like in an amp for it to be usable.

[earache]

Just curious - do any of the controls affect the hiss? This to help narrow the problem a bit.

Thanks for the help Mark - actually the hiss is always there, even with all the controls turned off.

A technique I have used with decent success on amps that work but are noisy:

First I might warm the entire board with a heat gun (not too hot - just warmer) to see if the noise changes drastically, followed by a global cool-down with freeze spray (again to see if there is a drastic change in noise level).

There wasn't any noticeable change with the application of the heat gun, unless the noise of the heat gun covered it up or I didn't leave it pointed at the board long enough. I don't have any freeze spray so I couldn't try that.

[earache]

J M Fahey's questions (in italics) are being addressed in line below
Thanks.
Now a few questions:
Volume on "0", bright off, treble and bass on "5": ¿ does it hiss unbearably/audibly? Hiss is quite audible
Now treble on 10 :¿? Hiss is there but doesn't increase.
Now Treble on 5, volume on 5: ¿? Hiss is there and hum starts
Now Treble 10, Vol 5 :¿? More hum along with the hiss
Now Treble 5 , Vol 10:¿? Louder hum, starts to feedback
Now Treble 10, move Vol slowly from 0 to 10: ¿does the hiss increase smoothly along the scale or it does (as I suspect), "jump" a lot on the last part, say 8 or 9 to 10? Always a sputtering hiss, hum appears at level 5 of volume and increases when volume turned up
As an extra curiosity: plug your bass (fully open) on the high input, put bass and treble on 10, bright on, volume 0.
Start playing normally and slowly, by half-points , start rising the volume, until it just starts to clip/fart/growl/whatever. Post the Volume position. Starts to clip at about a volume of 7 - 8
Close your bass volume and tone: ¿does the Kustom hiss unbearably/bearably? Hiss is audible and is at a level that renders the amp not useable

J M - In all, the hiss, which has a slightly sputtery quality, is always there and is unaffected by control knob changes.
Thanks for the thought and time invested in these troubleshooting questions, your approach to fault finding is very helpful and informative