Hi all.

A friend of mine and I have been working on an amp that we built from the London Power F Pre Fender kit. The power supply is the PSU Pre power supply kit for tube preamp.

We finally got sound to come out of the set up and it does sound pretty good except we have some kind of hum. My friend is MUCH more knowledgeable about this stuff and this is what he wrote. I have attached photos as well. If it is any help, I can record a sound file to upload. but here goes. Thanks for your help:

"[font=Helvetica]Hoping you’re doing well in this time of CoronaPanic. We’ve built one of your kits, and we’re having a hum problem we can't diagnose or fix. [/font]

[font=Helvetica]We’ve combined the preamp and its power supply on a chassis along with a 50 wpc class D power amp and its power supply to make a hybrid.[/font]

[font=Helvetica]Here’s the specifics:[/font]

[font=Helvetica]– Hum is definitely on the preamp side. Power amp, when fed with different source, is dead quiet./[/font]

[font=Helvetica]– Everything is on a star ground except …[/font]

[font=Helvetica]– Hum is at constant medium level and doesn’t change when volume is changed. [/font]

[font=Helvetica]– When playing loud, you can barely hear it, but at low volumes it’s pretty prominent.[/font]

[font=Helvetica]– Hum is not pure 60 hz sine wave, but kind of dirty.[/font]

[font=Helvetica]– We experimented moving wires away from transformers, but no change[/font]

[font=Helvetica]– 12AX7; tried a new tube, but no change[/font]

[font=Helvetica]– Same with iron plate added above transformer as EMS shielding.[/font]

[font=Helvetica]– Only thing we added to preamp is a pot on the output as a level control so we can turn the input up and overdrive the pre at modest sound levels.[/font]

[font=Helvetica]– We bypassed the AC IEC filter, but no change[/font]

[font=Helvetica]– Measured voltages (let us know if we should measure anything else)[/font]
[font=Helvetica]- Heater: 17.4 vdc[/font]
[font=Helvetica]- Hi voltage, 367 vdc[/font]

[font=Helvetica]We’ve fiddled and twiddled, but we’re out of ideas. We’ve put a lot of work into this and would like to get it perfect. Hoping you have some suggestions for things to try. Pictures attached. Let us know if you want other shots."[/font]

   

   

[font=Helvetica]- - - Jonathan Levin[/font]

Hi Jonathan

The PSU-PRE you have is the original version, which for the last few years of its sale included a separate 12V regulator for the heaters. I don't see that in your build and it's the first thing to correct.

The general layout is a bit like a pretzel. Assuming this is the amp with the top cover off AND that the input jack is on the front AND you like the backwards control arrangement, then the innards should be arranged thusly, left to right:
SMPS, PSU-PRE, PA, F-PRE

The controls themselves seem to be in the wrong order even as "backwards" goes, and should be:
AC switch, LED, PA-level, F-PRE Level, Treble, middle, bass, input

The rear connections would be arranged as:
AC input, speaker output

There are two jacks on the rear panel that appear to be uninsulated. What are they?

Star grounding is NOT what you want for audio. TUT3 shows the correct way to wire any amp and is universally applicable regardless of technology.

So, the first thing to do is fix the heater voltage or you'll be burning up tubes. Contact me via email if you need the regulator.

Hi Kevin.

Thanks so much for the quick reply! We could use the regulator. I'll email you. My friend says that explains the high voltage. Was the regulator originally in with the kit. The instruction sheet didn't mention that.

Jonathan

Actually, what is your email address? Thanks.

Hi Jonathan

The 12V regulator has been included with PSU-PRE for a few years until the kit was redesigned - finally used up the original PCBs

Just press the 'email' link on the London Power site, at the top of the page under the banner.

Hi Kevin.



Email sent.



We have really been struggling to get this whole thing together. The instructions that came with the kit(s) were somewhat confusing. I've attached what we have as far as those go. Over the coarse of time, did those get changed/updated?



The top photo in this thread shows the front of the amp. From left to right,  AC power lamp (under that power on/off switch), Mid, Bass, Treble, Gain, Master, and then guitar input.



On the back left to right: AC power, effects loop (yellow) wire, Speakon speaker connector.



We've heard on other forums that the star ground is the best route to go for grounding. The TUT3 book looks interesting but @ $86.00 a bit out of our range. Can you offer any other advice?



Thanks again,



Jonathan

Jonathan

DO NOT scan, distribute or post our kit notes - it is illegal !!!

The basic layout organisation given above is a starting point and it may require that you use a wider chassis to achieve it.

TUT3 is the best guide available for correct wiring, grounding and layout. It details exactly why a star ground is NOT of much use in an audio amp. Many hobbyists have found out that fact for themselves by rewiring a Fender amp using a star and the amp is noisier. They call to ask why that is and I tell them and they have to rewire everything again. Following our Galactic Ground scheme, the circuit will have its lowest noise and best note articulation.

Just because you see parroted information everywhere does not make it correct. At some point, one guy said the Earth is round, now everyone knows that is true but it took a while.

(05-05-2020, 01:22 PM)Jonathan Levin Wrote: Hi Kevin.



Email sent.



We have really been struggling to get this whole thing together. The instructions that came with the kit(s) were somewhat confusing. I've attached what we have as far as those go. Over the coarse of time, did those get changed/updated?



The top photo in this thread shows the front of the amp. From left to right,  AC power lamp (under that power on/off switch), Mid, Bass, Treble, Gain, Master, and then guitar input.



On the back left to right: AC power, effects loop (yellow) wire, Speakon speaker connector.



We've heard on other forums that the star ground is the best route to go for grounding. The TUT3 book looks interesting but @ $86.00 a bit out of our range. Can you offer any other advice?



Thanks again,



Jonathan

Jonathan,
This just my two cents but I think it would be very wise to invest in a quality text like TUT3 especially if you plan on doing projects of any sort with guitar amps.  The London Power instructions I've received have all been very clear but required I had enough background to know how to use the instructions.   When I first started building amps I was using "internet experts" and old layouts and schematics. I was soon was exasperated with the lack of quality and consistency of the information.  Getting some quality texts were HUGE step forward for me.  The best analogy  difference between reading a blog about scientific discoveries and having access to peer reviewed journals.    

All the best with your project!