JCM 800 build output transformer hum

[priesty]

I've been building a JCM 800 based on Kevin O'Connors TUT3 project. After the usual debugging and tweaking based on the teachings of Aiken amps and Merlin Blencowe, I'm left with a perplexing (or should that be perPlexi-ing!) Output transformer hum. I've scoped it and the hum is present on secondary, but not the primary! It's 50hz, seems like mains hum. Mains transformer is 9" from o/t and oriented 90degrees differently bell of o/t pointing at side mains transformer, so flux from mains transformer bells pointing away from o/t. Iron is hammond 1750N and 290GX.

Any thoughts???

[Chuck H]

Is the hum present with no power tubes plugged in?

[jpfamps]

Are the speaker jacks isolated from the chassis?

If the speaker jacks are grounded to the chassis you may have an earth loop.

Ideally you want to run the speaker jack ground to the bottom of the voltage divider that applies negative feedback to the power amp.

[priesty]

Pulled the EL34's, hum has gone. I guess that means the hum must be originating from the primary, and hence power valves? Signal from phase inverter outputs is flat line DC.

Speaker jacks are plastic hence insulated from chassis. I do have a ground wire running from output jack ground to my ground buss, but is terminates at the power tube cathode connection. Should this terminate at phase inverter ground star instead? Think I saw somthing about this on Aiken amp website.

[big_teee]

I just built a 2204, and had Hum issues.
I could pull the el34s and hum was gone.
It turned out to be several things, all in the preamp circuit.
If you Pull V1 & V2 and all is gone, it will be in the preamp circuit.
Terry

[jpfamps]

Pulled the EL34's, hum has gone. I guess that means the hum must be originating from the primary, and hence power valves? Signal from phase inverter outputs is flat line DC.

Speaker jacks are plastic hence insulated from chassis. I do have a ground wire running from output jack ground to my ground buss, but is terminates at the power tube cathode connection. Should this terminate at phase inverter ground star instead? Think I saw somthing about this on Aiken amp website.

You need to double check whether the hum is 50Hz or 100Hz.

50 Hz hum can only come from two sources in the amp (assuming your mains is 50hz...): the mains supply, the filament supply.

This will inductively couple into the audio. From your the description of your problem it sounded like the hum was being induced directly into the secondary of the OT. Have you revised this view?

[Chuck H]

AFAIK you don't NEED to ground that jack at all. It's supposedly ideal to ground the NFB loop and the output jack together though. Isolated would mean that the jack -terminal is connected to the OT -lead only (and therefor the speaker -terminal).

Are you sure the filament winding was done properly. One end of the filament lead should go to all pin #2 and the other to all pin #7. If the two get swapped inconsistently the circuit will not buck hum.

I also agree that it's a good idea at his point to start pulling tubes from the front end one at a time to see if the hum stops in the preamp.

[priesty]

Thanks for all your posts guys!

I pulled V1, 2 & 3 in turn, hum still present. Double checked, filaments are wired pin 2 to 2 and 7 to 7.

I checked frequency of hum against a tone generator - it's 100Hz!!! That means it's been B+ ripple all along!! I can't understand why though, I've got 2x 100uF capacitors in parallel on plate supply node!!! That having been said, B+ ripple is around 1V pl/pk and o/t hum is 2mV pk/pk

[Jazz P Bass]

2mv hum on the output?
I would say that is pretty darn good for a tube amp.
Now if it was 10mv's, that is cause for concern.

[priesty]

Just an update to say I've added a 100ohm 10w flameproof resistor between the 1st 100uF cap and 2nd 100uF cap, taken b+ from second cap, so I've got a C-R-C pi filter if you like. Hum is now barely audible, result! I will loose some B+ voltage, but the 290GX was kicking 463V from rectifier output, so I can afford to sacrifice some.

Thanks again for all of your help.

[big_teee]

Did you filter the B+ before or after the Standby switch.
My Circuit came from the diodes to first Strapped together 50/50, can Cap.
Then it went from the Can Cap to the standby switch, then back to the fuse.
I isolated the B+ leads from the AC Leads, to and from the Standby switch.
Is it possible it's picking up ripple after the Can Cap.
Is it possible to see a picture, or Schematic?
Terry

[priesty]

Big_teee - I've got a pencil drawn schematic, but its a few revisions out of date!!

In words, my set up in terms of electrical flow is as follows:
- HT from transformer
- 3 pairs of IN4007 diodes bypassed with 3 pairs of 10nF 1kV caps
- 1 UF4007 fast switching diode
- standby switch
- 100uF 500V (half a F+T 100+100uF can cap)
- 100ohm 10 Watt wirewound fireproof resistor
- 100uF 500V (other half of F+T cap and B+ take off point)

B+ Supply runs away from all heater supplies to OT Centre Tap.