1947 Harmony 200 (Best method for dropping voltage?)

[ylelevier77]

I’m restoring a 1947 Harmony 200 guitar amp. It’s in great condition with only one previous owner who basically kept it in storage and rarely played it. I’ve upgraded all the caps and pots. The amp sounds great but the DC voltage from the rectifier to the first filter cap is reading 360V. The schematic shows a target of 310V (+/- 10%). Any recommendations on how best to bring the voltage down? Voltage drop resistors pre or post rectifier perhaps?

[sluckey]

I would not be concerned about 360V. If you are using a new production 5Y3 then you can easily drop more voltage by using a NOS 5Y3. I would prefer to use a bucking transformer to drop the line voltage. See page 2 in this linked pdf. You could just hardwire to drop 12.6V if you want to keep it simple.

     http://sluckeyamps.com/misc/Amp_Scrapbook.pdf

[ylelevier77]

I’m using the original 5y3 that came with the amp. I’m very intrigued by the bucking transformer. Is there a way to incorporate the original transformer with the “bucking” element? Also, I tried searching for that catalog # part on RadioShack’s site but came up empty.

[sluckey]

You need a separate transformer. Any 12.6V transformer rated for 3 or more amps will do. Here's one...

     https://www.tubesandmore.com/products/transformer-hammond-filament-126-vct-4

I prefer to put it in a small box so it can be used with more than one device. Just plug the amp into the buckaroo and plug the buckaroo into the wall.

[ylelevier77]

Fantastic. This is exactly the route I want to take. Gonna order one today. Thanks for the assist.

[PRR]

What is the 6V6 cathode voltage? (i.e., are the 6V6es sucking appropriately?)

What is the AC voltage across the heater (lamp) wires? Near 6.3VAC or some higher?

What is the Wall Voltage, at your place and the place(s) it may be used?

If you have over 7.0V AC on the heaters, then yes it was designed for the low side of 110V and you are on the high side of 125V, and a 10% drop is quite appropriate. (However it says 117V on the plan.)

If you just have weak 6V6es not sucking 15V across 200r cathode resistor, fix that before you fiddle with mains voltage.

[ylelevier77]

Hey there, thanks for all the suggestions and I’ll try to figure nd time to go through them. Got your message after I’d turned in for the day. But before I ended my day I decided to measure the voltage coming off of the secondary. I connected my DMM to one end of the winding and the other to ground. I also set up my variac to put out 117V. I checked the schematic that came with the amp(It’s in amazing shape!) and it shows a voltage rating of 300V for each end of the primary. Well it turns out each end gave me a voltage of 370! Imthinking that could be a major culprit ;-) The original power tubes that came with the amp are 6V6GTs(12watts) but I decided to replace them during my most recents tests with 6v6GT-STRs(14watts). I was also able to measure the heaters for the output and power tubes. Those read 3V a side so they seem fine. Basically I’m wondering if I need to replace the PT now. One thing however remains consistent no matter voltage I run the amp at. There’s always a huge voltage drop between the center tap and the output tube plates. Usually around 40V or so. The cathode bias resistor is 200ohms and I’ve been able to increase it as high as 370ohms but it doesn’t make much of a difference. But like I said, I’ll try taking some of your suggestions and see what I find. I’ll also try to attach a scan of the schematic. There are notable differences between it and the Harmony 200 schematic that’s been floating around. Most notably the preamp tubes.

[ylelevier77]

I also did a little digging around online and I think the PT may either be non-original or simply a mismatch. Not sure. In any case I took down the PT’s make and model number and I think I found a match that is rated 350-0–350. Again, I’ll try to load some relevant images.

[ylelevier77]

Here’s a pic from an old catalog I found online and I think it’s a match.

[ylelevier77]

@PRR I took some measurements per your suggestions and here’s what I have with the newer 6V6GT-STR rated at 14watts, a 5 watt 120ohm resistor on each HV secondary lead, a 50watt 250ohm resistor between the rectifier and the 20MF 450V cap, and finally a 370ohm resistor in place of the 200ohm cathode bias resistor.

Variac at 112VAC
B+ 315V
Cathode voltage 20.6V
Heaters 6V
Plate Voltage275.5
Plate current 49.5mA
Plate Dissipation 13.64Watts