Switching Tube - Solid State Rectification

[Fresh_Start]

I've attached another possible use of the Ground switch hole in my 5E3 chassis.  This uses an on/off/on switch for:


On - Normal NOS 5Y3 tube rectification


Off - Standby


On - Solid state rectification with extra 16uf "reservoir cap" in power rail.


Again, most of the mods I see for Tweed Deluxes seem to be aimed at more clean headroom. I'm guessing that this will bump plate voltage about 60-70 volts. That should delay the onset of preamp distortion noticeably.


The extra power rail filtering may be unnecessary.  My thought was that it would make the power supply stiffer in addition to raising voltages.


Using a double-pole switch would allow me to increase the cathode resistor when in solid state rectification so the power tubes aren't running too hot. Probably will do that but didn't want to confuse the hand drawing any more.


Power transformer is rated for 330-0-330 at 100ma, and I've been told produces plate voltage right around 350 VDC.


So, does this circuit drawing work correctly?  My only real concern is that the extra reservoir cap is in the circuit all of the time. However, it is on the "far" side of the rectifier diodes unless I'm mistaken. And yes, the drawing approach is unusual. Blame Merlin for that.


The output transformer has an 8K primary || 8 ohm secondary and is rated at 20 watts. Am I correct in thinking that 6L6 power tubes would be BAD for that OT?


Thanks for taking a look,
Chip

[Paul1453]

The output transformer has an 8K primary || 8 ohm secondary and is rated at 20 watts. Am I correct in thinking that 6L6 power tubes would be BAD for that OT?


Thanks for taking a look,
Chip

You are going SE OT on this, correct?

From the data sheet attached, @ 350VDC if you connected a 4 ohm speaker to your 8 ohm secondary jack, I think that would put you in the ballpark.

[Fresh_Start]

No Paul, it's a push-pull circuit.

Just know that Neil Young supposedly puts 6L6s in his Tweed Deluxes. Maybe that's why he sounds like his amp is on the verge of self destruction...

Respectfully,
Chip

[Paul1453]

OK, sorry about that.  I saw 20W OT and 6L6 tube so I went SE.

More from the data sheet.  Maybe, it could still be OK with the 4 ohm speaker on the 8 ohm jack.
That should give an effective plate load of 4K, and pretty close to AB values of 3.8K at 360VDC for around 20W output.

Someone with greater knowledge than my rudimentary understanding of these things, like HBP or PRR, 
could probably help you figure out values than could work.
They know about load lines and knees on these data sheets that still baffle me. 

Obviously you cannot run 2 6L6GCs at 450VDC with that 20W OT.
At 360VDC it seems power output increases with plate load,
so an 8 ohm speaker on that 8 ohm jack might cause the tubes to put out more Watts than your OT might be happy with.   

Alternatively, dropping the voltage down to 270VDC in pure class A might work with a 4K load on the plates?   

[PRR]

> 330-0-330 at 100ma, and I've been told produces plate voltage right around 350 VDC.

At full-load with a vacuum rectifier, maybe.

> the extra reservoir cap is in the circuit all of the time.

And when not loaded, approaching 466V. It wants to be something more than 450V rating.

> 6L6 power tubes would be BAD for that OT?

Why? The power output at the same B+ will be a wee bit more than a pair of 6V6 like the OT expected. Assuming the diode rectifier gives 420V under load, the 6L6es can be biased into the 22W Pdiss range (15W at 350V), 22W Pdiss in any modern "6L6" is fine. Power output will be in the 20W-25W zone, which won't hurt a "20W" OT. May give nasty bass, but for guitar, that may be good; anyway it isn't so-much over the rating that I would worry.

> more clean headroom.

350V to 420V is 1.2 times "louder". Can your drummer hit 1.2 times harder to keep up (keep ahead)? I think the "headroom" increase will not be significant.

To go for "6L6 POWER" you want much more B+ or a much lower impedance. 5F6a turf will be a similar 420V but *4K* loading (and double the supply current, and need for fix-bias to balance idle against full current).

> Neil Young supposedly puts 6L6s in his Tweed Deluxes.

Yes, and then it is micced-up through the megawatt house system. I suspect that at many large gigs, the Deluxe can hardly be heard two feet away. (Anyway, didn't you say CLEAN headroom? When Neil rarely plays clean, he's playing the Deluxe at 1 to 10 Watts where it IS a clean amp. OTOH he plays at extrapolated "40 Watts" with 25 of those Watts wild distortion, the poor Deluxe at the ragged edge of collapse.)

[Fresh_Start]

PRR - thanks very much for pointing out unloaded voltage on that reservoir cap. I'll drop that idea.

Your comments about the 6L6 plus 60 volts not making that significant a difference got me thinking. My PT also has 355-0-355 taps which could be connected to the solid state rectification. Obviously that switch would need to be inside the chassis or at least underneath where it couldn't get hit by accident. Might be useless, but I may give it a try.

Chip

[Paul1453]

> 330-0-330 at 100ma, and I've been told produces plate voltage right around 350 VDC.

Doesn't the data sheet indicate significantly more current is needed for 2 6L6 tubes than your 100mA PT can supply?   

[Fresh_Start]

> 330-0-330 at 100ma, and I've been told produces plate voltage right around 350 VDC.

Doesn't the data sheet indicate significantly more current is needed for 2 6L6 tubes than your 100mA PT can supply?   

Yes.

I don't know either.

Need to build amp.

Chip

[PRR]

> significantly more current is needed for 2 6L6 tubes

Depends on idle bias and full-load impedance.

Since he's got a "6V6" impedance transformer, he'll be running the 6L6 far below their maximum ability. Like me today using the 3/4 truck to haul an empty trashcan... well the can and a full bed of snow, say 1/2-ton.

355VAC in silicon gives 500V DC. That with 8K load gets 6L6 into some decent power. Look at the big 807 data-sheet, page 3. Note also that 6L6/807 at 8K load can run G2 very far below Plate, though it can be tough to arrange this. 

[Paul1453]

I'm still trying to learn how to interpret the data sheet info.

I usually try to test out my circuits on my breadboard using my bench PS to see real world current demands in action.
Then I try to make sure the PT I'm going to use in the build is up to the task for the current demands noted on the BB.   
I can't say I've taken good notes yet on how this jives with data sheet numbers.

[PRR]

> PT also has 355-0-355 taps