Harmony 415 Build

[Rontone]

I've cleaned up a Hammond A0-43 to turn into a Harmony 415, Power Transformer tests OK, I get 332v-0-332v unloaded, 7v on the heaters and 5.8v on the rectifier, the AO-43 comes with a 5U4, and can get a 5Y3 to try in there

The Harmony spec is 320v out of the rectifier on the schematic, is it fine to add a dropper resistor and filter cap before the anode to get it down a bit? I understand there are C.max limits on rectifier tubes, so looking at 5U4 and 5Y3 I was thinking a 22uF - 32uF in that position, but are there other factors to look out for?

Also C10 on the Power Tube cathodes is a 50uF, is that due to the tremolo being attached to the bias point?

[bmccowan]

I've long wanted to build a 415, despite the fact that I am not a fan of the Marshall 18. I have an AO-35 chassis that could donate the iron. 50mfd is not that unusual for the power tube bypass cap - many Valcos use 35mfd there. Voltage - I suggest you search the forum for AO-43 info - I seem to remember lots of discussion of higher voltage than what you want. I've used that iron for a 2x6L6 amp - but those tubes can take the juice.

[Rontone]

50mfd is not that unusual for the power tube bypass cap - many Valcos use 35mfd there

Ah, I see now, I hadn't thought a bypass cap would be put there but after reading a bit more now see why,


I'm trying to work out the point to point layout with these tagstrips and can get most of it in a good position now, but are there any issues to look out for with the 6AU6 tremolo tube?

Harmony put theirs near the rectifier and power tubes, maybe for short wires and a simple build, but I would ideally like to fit it into an existing free hole, its quite far away from my inputs and first gain stages, but I do have to run a wire from power supply node to trem circuit and then back to power tube cathodes, I was thinking put those wires across the rear, its fairly close to the heater wiring, but far away from the more sensitive parts of the circuit

[Rontone]

On the Harmony schematic there is no 6.3V heater centre tap, the heater wiring is also connected to the tremolo circuit and has a path to ground, so I will omit the CT I have, or is there another way to wire it up?

Why are the heaters connected to that part of the tremolo circuit?

Also V3 and V4, the power tubes have their heater pins swapped on the wiring, is there a reason for this, or can I hook them up in phase with each other?

[sluckey]

On the Harmony schematic there is no 6.3V heater centre tap, the heater wiring is also connected to the tremolo circuit and has a path to ground, so I will omit the CT I have, or is there another way to wire it up?

Why are the heaters connected to that part of the tremolo circuit?

They are not connected to the tremolo circuit. They are connected to chassis ground. The draftsman simply chose to use that chassis ground symbol since it was in a convenient location on the drawing. He could have used any convenient chassis ground symbol or even drawn a separate ground symbol. It's all the same electrically.

Since your PT filament winding has a real center tap I suggest connecting that to chassis ground instead of connecting one side of the filament winding to chassis. Or, if you prefer to use an artificial center tap, just tape off the real center tap wire and install two 100Ω resistors as shown in the attachment. My choice is to use the real center tap wire.

Also V3 and V4, the power tubes have their heater pins swapped on the wiring, is there a reason for this, or can I hook them up in phase with each other?

I like to connect pin 4 to pin 4 and pin 5 to pin 5. It's your choice.

[Rontone]

They are not connected to the tremolo circuit. They are connected to chassis ground

Thanks, I see that now, I was overthinking that the connection to the heaters had some influence over the tremolo somehow, I need to remember that a path to ground is simply....a path to ground,


I have added a few pics of what I've got so far and an annotated scheme, preamp grounds in green, power amp grounds in red

Attaching the tremolo circuit grounds to the preamp ground bus is ideal as it will be close, would this be fine?

[sluckey]

Attaching the tremolo circuit grounds to the preamp ground bus is ideal as it will be close, would this be fine?

Since the tremolo signal connects to the power tube grids I suggest connecting the tremolo circuit grounds to the power ground point.

Will you be using those two dual cap cans? If not, I have a couple suggestions on your ground scheme.

[Rontone]

Will you be using those two dual cap cans? If not, I have a couple suggestions on your ground scheme.

I was planning on trying to keep them, if they are bad in the future I can keep them for 'looks' and mount axial filter caps inside, I will be putting a smaller value for the first filter cap for the C-max of the rectifier and then use these 50uF originals for the rest

Were you thinking mount to them, as the can cap mounting/grounding tabs have a hole that is just right for mounting a ground rail

[sluckey]

Were you thinking mount to them, as the can cap mounting/grounding tabs have a hole that is just right for mounting a ground rail

No. I was thinking that since the dual cap cans have only on ground tab for the two caps, you have to compromise your ground scheme. But if you use individual axial (or radial) caps you have a negative lead for each cap and the ground scheme can be optimized to split the preamp and power grounds better.

[Rontone]

Thanks, that is something I forget about can caps, can't separate those grounds for series connection either,

I will try it first with these caps, and if its not great then redo in the future, can't do much more soldering/parts work on it right now, as a faceplate needs making before I commit to fitting hardware on the front, almost forgot to put an on/off light on the panel!