Harmony H304-A Troubles

[plook]

I replaced C2 the first time i went in because the original (not very original) was cracked
The 12 AX7 mod was done before i got it

Layout of the 12AX7 attached, along with picture.

[Jazz P Bass]

I replaced C2 the first time i went in because the original (not very original) was cracked
The 12 AX7 mod was done before i got it

Layout of the 12AX7 attached, along with picture.





[ATTACH=CONFIG]25219[/ATTACH][ATTACH=CONFIG]25220[/ATTACH][ATTACH=CONFIG]25221[/ATTACH]

Yes, that is fine.
But did you check it.
How else will we know?

[plook]

Yes, that is fine.
But did you check it.
How else will we know?

No voltage on the other side of C2. Only 90volts on the plate side.

[52 Bill]

I'm going to make a wild guess here that you do not know resistor color codes and have written down measured values on the drawing.

The photos show a couple of things here, the filament for the second half of the tube are powered on via pin 4, I don't know if this is a factor in the hum, but it is drawing current from the transformer.

The input grid (pin 7) is connected to the input jacks with a capacitor. This was needed for the original circuit, but can be removed for this modded tube stage. Removing it may get you a little more volume.

Try grounding pin 7 by shorting across the 2M7 ohm resistor (red/violet/green) and see if the hum goes away. It may help to move the ground connection for the 1K2 ohm cathode resistor (brown/red/red) and the cathode cap to the ground point that the input grid connects to at the center lug of the terminal strip.

Earlier on, I thought that you had said that you had replaced the filter caps. The 20uF Illinois cap in the photo is definitely not new. Did you put the old ones back in?

[plook]

I'm going to make a wild guess here that you do not know resistor color codes and have written down measured values on the drawing.

The photos show a couple of things here, the filament for the second half of the tube are powered on via pin 4, I don't know if this is a factor in the hum, but it is drawing current from the transformer.

The input grid (pin 7) is connected to the input jacks with a capacitor. This was needed for the original circuit, but can be removed for this modded tube stage. Removing it may get you a little more volume.

Try grounding pin 7 by shorting across the 2M7 ohm resistor (red/violet/green) and see if the hum goes away. It may help to move the ground connection for the 1K2 ohm cathode resistor (brown/red/red) and the cathode cap to the ground point that the input grid connects to at the center lug of the terminal strip.

Earlier on, I thought that you had said that you had replaced the filter caps. The 20uF Illinois cap in the photo is definitely not new. Did you put the old ones back in?

1. Yes, i wanted to show what the resistor values had drifted to in case they had a bearing on the hum
2. I'll disconnect pin 4
3. I will try those suggestions now
4. Yes, you're right, i had changed out the old Illinois caps for new ones, including the can cap, but since it did not make any difference and i wanted to try and keep the amp as original as possible, i decided to put them back. I know i might have to change them later, but for now i'll see if these will last.

I'll be back later after the suggested changes. Really appreciate this.

[plook]

Made those changes and there does seem to be an improvement. The rippling hum has gone but there is still a loud general hum which rises with the volume.

[52 Bill]

Made those changes and there does seem to be an improvement. The rippling hum has gone but there is still a loud general hum which rises with the volume.

Does grounding the input or the control grid (pin 7) stop the hum? What happens with a new filter cap at the C node of the power supply?

One other thing to check is the connection between the terminal strip ground and the chassis. It is riveted down there, but I have seen them get corroded and become less than perfect ground connections.

[plook]

I did as above, grounding pin 7 made it quiet, but there was no guitar input at all, but here is an interesting thing. You know how only one side of the tube is being used and how originally the guy had linked pins 4&5 filaments, then i unlinked them. Well i grounded that pin 4 (5 is live with 6.2v) and the hum totally vanished! Plugged in the guitar and bingo, pure golden tone.

Is it safe? Didn't Fender used to ground one side of the filament?

Really appreciate your help here guys.

[Jazz P Bass]

What made you think of grounding pin 4?
(the thought process)
"Desperate situations require desperate measures".
If that's what it takes, do it.
Great fix.

[g1]

I did as above, grounding pin 7 made it quiet, but there was no guitar input at all, but here is an interesting thing. You know how only one side of the tube is being used and how originally the guy had linked pins 4&5 filaments, then i unlinked them. Well i grounded that pin 4 (5 is live with 6.2v) and the hum totally vanished! Plugged in the guitar and bingo, pure golden tone.

Is it safe? Didn't Fender used to ground one side of the filament?

Really appreciate your help here guys.

According to the schematic, one side of the heater winding IS grounded. If you disconnected pin 4 from pin5, you have disabled the heater to that triode half of the tube. So that must be the half that was unused? If so it must have been generating the hum.
Did you try it with pin 4 disconnected but not grounded?

[52 Bill]

Did you try it with pin 4 disconnected but not grounded?

That was what was happening before he grounded it.

According to the schematic, one side of the filament winding is already supposed to be grounded. So, grounding pin 4 either powers the filament on or reconnects the missing filament ground.

Please check to see that one end of the filament winding is grounded to the chassis. Then I would try reconnecting pins 4 and 5 rather than grounding pin 4.

[plook]

Sorry for the delay. It was just an experiment in desperation to ground pin 4, i had no scientific basis other than touch it and hope for the best and the result was beautiful. Not a process i would recommend, a bit like licking a finger and putting it in an antique wall outlet to see if it's still live. Really appreciate all the help. the reason why i've been slow to come back is that i've not got this thread set to notify me of replies.