Harmony H304-A Troubles

[Jazz P Bass]

How about a few basic voltage measurements.
Static. No signal.
Mains draw in watts.
B+.
Each preamp tube plate & cathode.

[plook]

Here are some voltages with volume turned all the way down

V1 (12AX7)
pin 1 not used
pin2 not used
pin 3 not used
pins 4, 5, 9: heaters6.2v
pin 6 (plate) 230v
pin 7 grid 0.2mV
pin8 cathode 4.7v

V2 (6AT6)
Pin 1: screen grid 0v
Pin 2: cathode 1.7v
Pins 3 & 4: heaters 6.2v
Pin 5: .1v
Pin 6: .1v
Pin7: plate 120v

V3 (6V6)
pin 1: not used
pin 2 & 7: heaters 6.2v
pin 3: 221v
pin 4: 228v
pin 5: 3mV
pin 6: not used
pin 8: 10.7v

V 4 (5Y3)
pin 1: not used
pin 2: 238v
pin 3: 19.5v
pins 4 & 6 heaters: 5v
pin 5: 2.5v
pin 7: not used
pin 8: 238v

B+ from filter cap 238v

Appreciate anything you might have on the above. It's killing me, but i am wondering if it is pins 3 & 5 of the MT

[Jazz P Bass]

Pin 3 & 5 are not used on the 5Y3.
Pin 6 & 4 should be the high voltage Vac reading.
Pin 2 & 8 are technically the heaters.
Anyway, send a siganl through the amp.
At full volume , what signal level is at the 6V6 grid? Volts ac.

[52 Bill]

The 6V6 plate and screen grid voltages seem to be reversed, is this correct or mis-noted here?

When the 12AX7 was subbed for the original input tube what was added as the cathode resistor value? Cathode bypass cap?

[plook]

I want to thank you so much for helping me with this. 52 Bill you got me looking at the bypass cap and resistor on the cathode and the resistor was hidden underneath the 22uF@25v cap. One end had snapped off. I put a new cap and 1.5k resistor in and the amp is alive, though it has terrible hum.

[g1]

the amp is alive, though it has terrible hum.

Your B+ seems like it may be low (Megatron had 300V). And you have bad hum. I would suspect either the filter caps are bad or the output tubes are biased very hot.

[plook]

I plan to swap out the filter caps tomorrow.

What should the bias be in there? It's cathode biased. I'll check the value of the 270 ohm resistor on the cathode of the 6V6 tomorrow

I'll re-check the voltages now that it's working right. How would i up the B+

[Jazz P Bass]

If the filter capacitors are bad, they will be putting a large amount (relatively) of ripple on the B+.
That ripple will give a false B+ reading.
Once the new caps are installed you will most probably see the B+ rise.
Then, knowing the true B+ voltage, you can calculate the proper bias point.

To set the bias point you will need:
1: The actual plate voltage (pin 3 to pin 8)
2: The voltage across the cathode bias resistor (pin 8 to ground)
3: The value of the cathode bias resistor (in ohms)

To find the current flowing through the tube divide the cathode voltage (#2) by the cathode resistance (#3)
To find the plate dissipation, multiply the plate voltage (#1) by the current.
This will give you the dissipation in watts.
A 6V6 will run nicely at about 10 watts.

[plook]

If the filter capacitors are bad, they will be putting a large amount (relatively) of ripple on the B+.
That ripple will give a false B+ reading.
Once the new caps are installed you will most probably see the B+ rise.
Then, knowing the true B+ voltage, you can calculate the proper bias point.

To set the bias point you will need:
1: The actual plate voltage (pin 3 to pin 8)
2: The voltage across the cathode bias resistor (pin 8 to ground)
3: The value of the cathode bias resistor (in ohms)

To find the current flowing through the tube divide the cathode voltage (#2) by the cathode resistance (#3)
To find the plate dissipation, multiply the plate voltage (#1) by the current.
This will give you the dissipation in watts.
A 6V6 will run nicely at about 10 watts.

That is fantastic information, thank you. Looking through my capacitors I see I don't have the can 16/16/16@550v. I could order a JJ 40/16/16/16 or could I get away with using some 20uf@500v. Leave the old can in place but anchor 3 x 20uf inside. Or will that extra smoothing alter the tone too much?

[plook]

Swapped out all caps and it didn't make any difference to the hum or the B+ Ignore the question about the 16/16/16 above, it's 20/20/20 in this amp.

[Jazz P Bass]

Number one, is the amp humming without a guitar plugged in?
(I am thinking, oh, it could be single coil hum.)
Number 2: you did not measure anything before you swapped the caps.
Looking for a change, of coarse.
You may have tried measuring the actual Volts AC ripple on the points marked A, B & C Source.
This can still be measured with the new caps in.
The 'B' & 'C' Source should be relatively ripple free.
It would also be a benefit if you could measure the hum at the speaker.
A bench mark so to speak.
Set your meter to read Volts AC.
If nothing untoward is noticed in the ripple measurements, try turning any controls.
Do any of them have an affect on the hum.
Try pulling the tubes one at a time.
Do any of them affect it?

[52 Bill]

Number one, is the amp humming without a guitar plugged in?
(I am thinking, oh, it could be single coil hum.)

For that matter, plug the guitar in and then turn the guitar's volume control to zero, what happens to the hum?

Do the amp's controls change the hum volume or tone?

[plook]

Voltages measured the same before and after cap swapping.

Using your instructions this is what i found:

1. Normal level of amp hum when volume is all the way down, increases as vol pot is rotated
2. Bad hum when vol put up with or without guitar plugged in, guitar vol has no impact on it
3. 0Vac on speaker
4. Bad hum goes when the 12AX7 is pulled out but i can hear a low rippling pulse when vol pot on full
5. When the V2 6AT6 is pulled you can only hear a normal low level of background hum which is not effected by the vol pot.

It's a bugger!

[52 Bill]

1. Normal level of amp hum when volume is all the way down, increases as vol pot is rotated
2. Bad hum when vol put up with or without guitar plugged in, guitar vol has no impact on it
3. 0Vac on speaker
4. Bad hum goes when the 12AX7 is pulled out but i can hear a low rippling pulse when vol pot on full
5. When the V2 6AT6 is pulled you can only hear a normal low level of background hum which is not effected by the vol pot.

You have proven that the source of the hum is at the 12AX7 circuit. This is not the original wiring, so there is no way for us to know what is going on there. Please describe what was done or draw up a simple schematic so that we can suggest fixes.

[Jazz P Bass]

Check C2.
It is there to block the dc voltage from the plate.
On one side there is plate voltage
On the other there shoud be none.
If it is leaking, it should be replaced.