Single Ended OT question

[bal704]

I'm building a SE 6L6 amp. I've got an Allen OT, and was wondering about the wiring. On the Primary side, does it matter which wire goes to B+ and pin 3? On the fender schematics, the secondary side with common corresponds to the primary side going to B+ (brown in this case).

se_ot.jpg

[martin manning]

If your amp will have a feedback loop from the secondary then the primary-secondary phase matters. There are no phase markings on the diagram so you really don’t know which primary lead is in phase with the secondary common.

[bal704]

No feedback on this one.

[sluckey]

I'm building a SE 6L6 amp... On the Primary side, does it matter which wire goes to B+ and pin 3?

No... Unless there is a global NFB loop such as found in a Fender Champ. If that's your case, don't cut the leads until you know you have them connected correctly. The amp may howl or squeal or produce some other not so obvious bad behavior if connected wrong.

There was a time when most US transformer manufacturers of simple SE OTs used a color standard. Primary blue goes to the plate. Primary red goes to B+. The "color standard" was never a requirement and some manufacturers had their own ideas about colors, for example, your Allen OT.

[Stevem]

It's odd that they draw out the diagram with the black / common on the top of the secondary side, which is near the total opposite of other brand OTs wiring drawings!

This would lead me to say that the Brown wire was intended to get fed off of the power supply node.

[R.G.]

As Martin pointed out, if there is not a feedback loop around the transformer, the winding phases make no difference at all. None.

And as long as the wires go the right places, no matter how inverted or convoluted, how a schematic is drawn makes no difference at all. Unless there is some kind of additional note about the layout of the schematic (which is possible, but rare), then which wire is near the top has no meaning at all.

It is easy to find out what the phasing of a transformer really is, but without feedback around the transformer, there is no need.

[bal704]

I ended up hooking the brown to B+, blue to the tube.

The reason I was asking is because I've been fighting this thing trying to understand the voltages. I've got 2 multimeters hooked up, one to the B+, the other to the 6V6 cathode resistor (560 ohm/22uf). I can't get a steady voltage on either meter under load. The cathode voltages have been insanely high, like 200% of power level at times according to my bias calculator web site. The B+ has varied from 360v to 450v, and seems to depend on nothing I can find. I watch the meters and power it off quickly as the cathode voltage starts ramping. Part of my problem is late yesterday I discovered one of the spade connectors on my speaker wire had come off the speaker, leaving me with no load on the amp for the last few power ups yesterday. I have fixed that however, and the amp wasn't on for long when I saw the voltage ramping up.

I just fired it up, and saw 450V on the B+, 20v on the cathode. That's a bit over 100% of the max for a 6V6, but not heart attack inducing. The amp sounds fine for about 10 seconds, but starts getting crackly. I watch the B+ remain steady at 450v, and the cathode voltage start dropping towards zero as the sound fades away. I just put a new 6V6 in the amp, so I'm pretty sure the tube is good.

I measured the OT, and I've got resistance on both primary and secondary.

Any thoughts?

[bal704]

An update:

This is basically a Bassman AB165 normal channel feeding the SE power section of a AA764 Champ power section. There is a resistor to ground from pin 5 of the 6V6 that I had left off. I assume this is a ground reference. I tacked on a 1M resistor I had laying around and the B+ stabilized at 450V. I used a 1K5 Cathode resistor to get 38V across it. This gets me about 10W by my math, which is better than I had before.

My only concern at this point is the 450V B+. Normally I figure a 1.25 bump factor to estimate the final voltage once the amp is loaded up. This is an Edcor 325-0-325 PT, so I was figuring around 400V from my Diode rectifier when it was loaded up. That's why I used 450V caps. The amp hits about 470V on power up, and drifts down to 450V as things heat up. That takes about 10 seconds or so.

How much concern should I have about the voltage level on those caps?

[thetragichero]

325-0-325 is 650vct
650vac * .7 (assuming full wave rectification) = 455vdc
450vdc on the plates is what I'd expect

edit: I'd be using 500v caps if it were my build, possibly even a totem pole on the first node, but I'm the kinda guy who likes to be extra cautious

[Stevem]

Your concern about the filters holding up with that 400 plus volts need to extend to the outout tube running in class A single ended like your building for also, as that voltage level is very high and will exceed the 6L6 DC wattage rating at idle with the mormally called for 300 ohm cathode resistor, or in the least not idle the tube close enough to its mid point .

My 80s Champ had 390 volts on its plate even with a 5Y3 recto in there and when you minus out the idle cathode voltage and calculate the idle wattage it was running that 14 watt 6V6 at over 15 watts!

I had to replace the stock 470 cathode resistor with a 580 ohm resistor to get it all happy.

[bal704]

I set this up to run either one or two 6V6's, or one 6L6. When I run two 6V6's in parallel I'm getting 436V at steady state, 37V each on the separate cathodes. That's about 10W per tube.

I'm going to install a standby switch with a 600v cap in front of it just to be safe.