Pro Jnr diode replacement question

[CharvelBlue]

I have a 1994 Fender Pro Jnr amp and I recently came across a webpage where the factory rectifier diodes where replaced with quick recovery MUR1100 diodes and this greatly improved the buzzing in this amp. Its odd that the author says his PJ has 1N4007 diodes, as my 1994 PJ and every model up to 2005 is shown to have a 1N5062 on the circuit diagrams. I also don’t have the 0.01uF caps across the diodes like the later models do.

https://jpfamps.com/fender-pro-junior-h ... stigation/

So I’d like to try the UMS1100 diodes (my amp is currently apart being recapped and also has quite a bit of buzzing) but was wondering if these are also a good replacement for 1N5062’s, which have 800V peak reverse voltage, 1A forward current, 4uS (4000nS) reverse recovery time, compared to the MUR1100’s which have 1000V peak reverse voltage, 1A forward current, 0.075uS (75ns) recovery time. They seem to be compatible but as I’m fairly new to working on amps I just wanted confirmation as I might be missing something. Thank you. Data sheets here:

https://www.mouser.com/datasheet/2/68/1 ... -14584.pdf
https://www.digchip.com/datasheets/part ... UR1100.php
or the MUR1100EG as the MUR1100 appears to be obsolete:
https://au.mouser.com/ProductDetail/?qs ... WQdw%3D%3D

1994 Pro Jnr schematic here:
https://schematicheaven.net/fenderamps/pro_jr.pdf

[martin manning]

Interesting article on viewing the switching noise. UF4007 is another fast recovery diode option, 1kV, 1A, 75ns.

[CharvelBlue]

Yes I haven't seen the diodes as a source of buzzing mentioned anywhere else. So as long as the new diode is 1KV, 1A and around 75nS it would be ok? No other critical specs?

[ChopSauce]

Well, I'm not sure I understand the conclusions of the article(*) very well...
(maybe one or two sentences recalling them would have been ideal)

Doesn't the author observed that MUR1100 (that is SLOW recovery diodes, rather) with NO snubbers being the least noisy option?
(which would contradict most of my beliefs in that area!)

At least, It seems that the author mentions too hot biasing of the power valves, heater hum and poorly isolated connections to V1 grids as other obvious sources of noise.
___

(*) https://jpfamps.com/fender-pro-junior-h ... stigation/

[CharvelBlue]

His conclusions seem to be that the buzzing is greatly reduced with the new diodes, but there are other sources of noise like hissing and humming which need to be addressed in other ways.

I wonder why you say the MUR1100 are slow recovery when their recovery time is so much less than the std diodes. I agree it seems counterintuitive that there is LESS noise with no snubbers but thats what he found. Unfortunately I don't have the equipment or knowledge to confirm or refute. Then there are soft recovery diodes like MSRF860G which are still much faster in recovery than the std ones, but who knows what their effect on the buzzing would be.

[ChopSauce]

Another option is to try some soft recovery diodes in the rectifier. We installed MUR1100 diodes instead of the stock 1N4007s

Sorry if I have misunderstood the word soft. I'll let you care of the details...

[gktamps]

MUR1100EG datasheet specifies the part as an ultrafast recovery diode, and is not described as soft recovery per se.

Buzzing can come from many sources in an amp. As far as I'm aware, even standard recovery diodes are not generally associated with audible buzzing in an amp, although avalanche diodes do make a lot of RF switching noise (which can be alleviated). Martin's suggestion of UF4007s would be an easy thing to try, and I use UF diodes in all of my amp builds, but you may find you have other causes related to the buzzing in your amp.

[CharvelBlue]

ChopSauce: no problem.

gktamps: yes it's described as UF but not soft so not sure where Frank got that from. Anyway it's a quick mod so I will add some UF diodes to my PJ and see what it does.

[ChopSauce]

On a related matter: some shops advice the 1N4007 they sell as ultra-fast ones - like there are no more other models. Any comment?

[CharvelBlue]

I would go to another shop. A standard 1N4007 has a reverse recovery time of 2uS whereas a fast recovery diode like the MUR1100 has a recovery time of 0.075uS which is 100's of times faster. Tell them to read the data sheets. You can also buy a UF4007 which is an ultra fast diode.