Bandmaster 5e7 excessive hum

[909one]

Hey there,
I'm sure there are tons of posts on here to cure hum problems, while I have found one or two threads of these, the search feature makes it impossible to search for the word 'hum' being that its under 4 letters. (This a very stupid search parameter IMO, I'm sure we'd all be very happy if it was removed)

Anyway, This is a new bandmaster 5e7 clone.
I have tried everything within my limited knowledge to alleviate this hum.
The amp hums without the guitar plugged in, both gain knobs all the way down. I have replaced all of the tubes, one by one. I have double checked the schematic with the amp. All of the connections seem to be correct.
I used metal sheilding on the back panel, even tried grounding it the chasis, no help.

I made an audio file so you can hear the hum.
http://www.atundra.com/ampnoise/

Hear's what you are listening to:
I used a very noisy Gibson Skylark from the 60's as a basis for comparision.
It hasn't been maintained in at least 10 years and still has a two prong plug on it. I figured this was a good base example for noise floor on vintage amps. Most people's amp are in this condition.

Ok, the audio file is sliced into 5 sections, with a bit of silence in between.
These are how the slices are divided up:

1. Microphone on and recording, no amp on. (This shows the nominal mic preamp hiss)
2. Gibson Skylark on. Gain know at zero. No guitar plugged in.
3. Bandmaster on. Both gain knobs at zero. No guitar plugged in. Presence, Treble, Bass all set at 12 o'clock.
4. Gibson Skylark on. Gain set at 9 o'clock. Guitar plugged in.
5. Bandmaster on. Gain set around 8 o'clock on Bright channel. Guitar plugged in.

(the last two tests are not totally fair, because I couldn't match the gain of the amps exactly)

Things to note:
I didn't change any mic preamp settings or move the mic at all between sections. I used the same speaker for both tests. It was rated the correct imnpedance for both amps. The guitar being used has humbuckers, to help rule out single coil hum.

Hopefully this audio file will provide some clues to some experts as to what it might be. And you can also see that the hum is definitely pretty excessive, even for a vintage amp build. I am very friendly with a soldering iron and I am open to any suggestions for adding/removing components.

Thanks!

[paulg]

Couple of thoughts: You said it's a new amp, is this a build from a kit? If so, grounding is always an issue. Check Randall Akiens site for a read on multi point star grounding. Seperating the power supply grounds from the preamp can have a drastic (positive) effect. Also, heater filiment wiring is a likely culpret. Some times moving the wires around using a wood chopstick can reveal the cause of the hum.

[909one]

Yes its a new amp.

Its a clone from Ceriatone. I bought the amp pre-built.
(Just for the record Nik has been wonderful with answering questions about it. I probably have 20 email replies from him. I just don't want to send the thing out for repairs, I'd like to learn myself.)

Nik suggested moving the wires around with the chopstick.

There are 2 or 3 star ground points in the amp.

I'll read Randall Akiens site about grounding.

I'll try your other suggestions.
Thanks!

[phsyconoodler]

Input jacks.Make sure the grounding lug on all the jacks are actually making contact.If it isn't,the amp will hum with no guitar plugged in.

Also the heater wiring...do you have a centre tap for the filament wires or did you use two 100 ohm resistors for a virtual centre tap?

[909one]

I'll check the input jacks. As for the heater wires, I don't know enough about circuits to know what the heater is... I need to get a good book on this stuff.

[Trout]

I'll check the input jacks. As for the heater wires, I don't know enough about circuits to know what the heater is... I need to get a good book on this stuff.


I would also try to confirm the jacks are closing properly, They are supposed to short with no guitar plugged in.

The biggest thing is as you mentioned in your first post,
The amp hums without the guitar plugged in, both gain knobs all the way down.

This is almost certainly a ground issue. You might start by comparing the actual Fender Layout (http://www.ampwares.com/ffg/schem/bandmaster_5e7_layout.gif). Notice the differences in the grounding scheme.

I have worked on several 50's 5E7's and one thing I can tell you, they are very quiet. At idle with no guitar plugged in there should be no hum and the volume knobs can be turned up with only minimal hiss.

Often when using the " Star grounding scheme" long lengths of wire contribute to the noise more than they solve anything. Looking at the Ceriatone Layout (http://ceriatone.com/images/layoutPic/fenderLayout/5E7Ceriatone_311207.jpg) there are several noteworthy differences as far as the grounding goes. That would be a area requiring attention.

[909one]

Thanks for that confirmation, i have never owned a vintage style tweed amp.
I was afraid that they were just noisy amps.

I will look into grounding issues between the schems.

I really appreciate the effort in comparing the two schems.
Thanks for your help.