Well, let's just call it a "paranoia resistor". It probably does not need to be there at all.
My concern was that it's possible to have the secondary of the OPT unloaded, either when no speaker is connected, or as you switch between speakers and headphones. The plate resistance of the 6EM7 is so low - it should control the primary voltage swing well even without any load - that I can't envision any transients big enough to hurt anything, but for the price of a couple resistors, I put them there.
Pete
It has been stated that running a tube amp with no load can blow it up. This can be very true, I have seen guitar amps catch fire when the speaker gets unplugged or blows (open voice coil).
In my experience an SE amp under normal conditions does not experience this type of failure since there is always current flowing through the primary. However driving an SE amp to clipping with no load causes the primary current to abruptly cease, which generates a kilovolt level spike, possibly zapping an OPT. A resistor across the secondary is good insurance at the cost of a few milliwatts of output power.
A simple resistor across the speaker terminals is good insurance especially in a low or moderate powered SE amp. It is not sufficient to save the OPT in a high powered P-P amp that is capable of being severely clipped. This means most guitar amps over 40 watts and maybe Petes other red board when someone hot rods it to 125 WPC. I have seen diodes, zeners, transient suppressors, gas discharge tubes, and resistors used alone or in combination in attempts to save OPT's from their users. Nothing is 100% foolproof though.
I'm still debating which output transformer to get. Anyone know any other brand that offers affordable(around $100/pair) 8K SE output transformers? Transcendar only have 3K and 5K versions available. Electraprints are out of question since I wish to make this amp under $300(excluding chassis etc).
Good luck. The only guys I might think would be able to deliver something in that price range would be vt4c.com. Look in their "Mars" single ended line. I don't see anything on their web site, but an email might be worthwhile. They're made in China, and I'd expect they could wind up anything you might need if sufficiently motivated.
1N4007 will work fine. It might induce some noise... if you hear a buzz, replace them with SiC rectifiers.
You might not need to do anything - if it's 150 ohms you will be pretty close. Try it!
Pete
Not such a stupid question! The left terminal of the TO-220 diode as you look at it from the front is the cathode. There's also a tiny dot on the PCB silkscreen next to the cathode lead.
It might, though sometimes diode recovery noise gets around by capacitive or magnetic coupling... if that happens the only way to get rid of it is to spread things out, or remove it at the source. Quite often it couples across the power transformer to the filament winding, then capacitively to the cathodes...
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