Got parts - need schematic

[Ty_Bower]

I've got a pair of Hammond 1620 output transformers sitting around doing nothing. I need to build something with them. While the 6.6K primary impedance would probably do best with a pair of 6V6 or 6AQ5, I've got too many 6p14p around not to use them instead.

I'd like to make a pair of mono blocks. I have several pieces of aluminium plate that I intend to use for the top chassis. They're rather small at roughly 9.2" x 5.8", but I figure if a Dynaco Mk3 can fit in a 9x9 footprint I can make a 10 watt amp fit in two-thirds the square footage.

I'm open to suggestions for the power transformer. Right now my best idea is the Allied 6K56VG simply because it is reasonably cheap at $45 a piece. It's a 540VCT unit with 6.3v and 5v filament windings. I expect to use solid state rectification, mostly in the interest of saving space, and I'll probably have a B+ around 360~380 volts. The 5v winding might go to waste.

I want to keep it all in three tube sockets simply because there isn't going to be a lot of space left on the chassis. I need two sockets for the output tubes since the 1620 is PP. I think I can build a LTP phase splitter and skip the first gain stage, kinda like a Baby Huey. I am intrigued by the partial or plate-to-plate feedback scheme. I'd prefer to keep things primitive and forgo the CCS bias on the output tubes. They'll be cathode biased instead, which I believe is a good choice anyway for the EL84 family. I might be convinced to use a CCS on the LTP, but if I can get away with a simple resistor I will. I'd rather build from an easy printed circuit board and a proven tested design, but this is going to end up being point-to-point and mostly pulled out of my backside.

I have no design software, little experience, and even less talent. If anyone wants to offer up a schematic for me to blindly follow I'd be eternally grateful.

[EWBrown]

THis should be relatively easy process.

I'd suggest trying the "Musical Power Supply" power transformers listed on e-bay, either the 275-0-275 or 285-0-285 VAC HV secondary versions. These have sufficient power for a PP pair of 6P14Ps or EL84s, and the primaries have taps for 110, 115 and 120VAC input, so the B+ voltages can be "massaged" as required. I've used these power trannies in a few small amp circuits with decent results. The prices is lower than the Allied power trannies, at around $30 each, at least at the time that I bought mine last year.

The seller is located in Florida, and has the trannies custom manufactured overseas, some from China, and some from El Salvador or Nicaragua.


The lower power supply B+ of around 325 to 330VDC would work well with the 6600 ohm 1620s, and be good for around 12-14 Watts per channel, with Ultralinear mode and a bit more in "pentode" mode.

The best cathode resistor value, with a 325V B+ would be about 330 ohms, 2 Watts for each 6P14P. This would set them around 38 mA, and 360 ohms woould set them around 35 mA. Either value keeps them within their plate dissipation ratings.

For the driver stages, try Tom McNally's "832 PP Amp" which uses a 12AT7 VA / PI circuit, which would be a good match for this basic design.

http://tmamps.com/pics/832sch.jpg

The VA/PI section should have a B+ voltage of around 275VDC, I'll run some TubeCAD scenarios to figure out the best resistance values.

A quick test run indicates 3.3 mA for the PI stage, with the 27K resistors, which is good for a 12AT7. The VA stage can be set for approx 1 mA, and that would change the plate resistor from 270K to 180K, and the 1.8K cathode resistor to 1K, and feed the NFB loop between the 100 ohm resistor and the "cold" end of the 1K cathode resistor This comes out fairly close to an optimal driver circuit for the 6P14Ps. Since the total current consumption is about 4.3 mA, use a 11 to 12 K, 2W resistor to drop the 325-330 VDC B+ to around 275VDC.

HTH

/ed B

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[Ty_Bower]

That transformer sounds ideal, but I had a heck of a time finding the eBay seller. Here he is: http://stores.ebay.com/Musical-Power-Supplies

I'm also thinking about an isolation transformer and building a doubler. Is there any merit to that idea?

I like 12AT7, and I've got a bunch of them. I'll check out that schematic for the 832 amp.

[Ty_Bower]

Better yet - I found his web page:
http://musicalpowersupplies.com/

I sent Matt an email. For the moment, he's out of stock on the part I want.

I just ran out of stock on my PT275. I'm actively working on an improved replacement. It will probably be a couple of months before I have those back in quantity on EBay. I may have a few samples on musicalpowersupplies webpage earlier, but am presently sold out except for some 240v PRI versions.

[EWBrown]

I've used the isolation transformer and voltage doubler approach, it works quite well.

Keep in mind that the windings are not exactly a perfect 1:1 ratio, because of the usual transformer losses, the intended secondary delivers a somewhat higher voltage output when it is loaded below its maximum rating, which can be useful for this application.

When a lower B+ is needed (240 to 250VDC) just reverse and use the secondary as the primary, etc. THis works well with dual primary isolation trannies, use the "output" winding for the primary and connect the two primaries in series and use a four diode full wave bridge.

The resulting B+ voltage will be lower (260 to 280VDC) than is ideal for the 6P14Ps , but you will have LOTS of reserve current in most cases. Just lower the cathode resistors to around 270 ohms for each tube. The output power will be around 10 watts, and the 2HD and 3HD will be very low, which is a good trade-off.

/ed B