Scrap Build, Sluckey Dual Lite, planning

[CrocMule]

Howdy folks.  Russ from northern New Mexico here.  I'm new to the Hoffman Forum.

I've got a scrap build in mind that I would love to get some input on.  I want to use the Foster transformers out of a '59 Newcomb TR-1640M record player to build a Dual Lite.  (Thanks Steve!  I was basically trying to invent your circuit when I discovered your schematic.)  Both PT (TR-209, 394920) and OT (TR-214, 394926) tested good.

With the 6AN8, the Newcomb circuit looks like a heavier load than the Dual Lite -- so I'm guessing the PT should be fine (both circuits use a 5Y3).  And the OT will see the same 2x EL84s as the Newcomb circuit it came from, so that should be fine too. 

The big difference I can see is that the 360-0-360 Foster PT is a little hot for the Dual Lite, so the B+ may be 30-40V higher than the 325V spec.  I don't foresee any problems with that in terms of safety, since the Newcomb was running at 362V -- so I think I'll just build it before trying to adjust the B+ in the circuit.  To compare tone, I can variac the amp down to spec after it's built.

I just noticed that the OT is ultralinear.  I assume it can be used in a non-UL circuit like the Dual Lite -- just ignore the UL taps, right?

Does anyone see any red flags?  Any advice?

It'll be housed in a dead Ampeg SS combo chassis and cat-scratched cab, so tinkering with the circuit after the build isn't going to mess up any pristine aesthetics.  This'll be a jalopy.  I may even use tag strips inside, to facilitate future changes.

I'm not diving into the build right away, just getting more and more distracted by planning for what should be a mid-winter project.

Thanks in advance for any help!

[mresistor]

Well my thought is you could run the screens off of a power supply node and tuck the ultralinear leads from the OT away.   

[shooter]

what he said ^^^


or
since they're there, maybe try it UL, then change after you played it 10-20 hours

[CrocMule]

Hey, thanks @mresistor and @shooter.

maybe try it UL, then change after you played it 10-20 hours

That's a very interesting suggestion.  I'm guessing the conversion from a UL circuit to a non-UL circuit isn't as easy as just switching the screen inputs.  Otherwise people would be putting that on a switch already.  What else would be involved?  Couldn't be too much of a change.

But actually, clean headroom isn't what I'm wanting from this amp.  I'm intrigued by what it would sound like with a UL output, but I think I should just build it non-UL first.  I could always change it in the opposite direction later.  My practice space at the moment is TINY, so I already have to wear earplugs a lot of the time. 

Thanks guys!

[mresistor]

Hummmm....  not really sure how you would implement switchable screens on the amp in question.  Need a screen node on PS and a good HV rated switch.

[CrocMule]

Hummmm....  not really sure how you would implement switchable screens on the amp in question.  Need a screen node on PS and a good HV rated switch.

Ha!  That's a great screen shot.  Thanks!  So I'm not the only crazy one (and not in bad company either!)

There is a screen node in the Dual Lite circuit.  It's "B"

The Dual Lite schematic is attached to post #1, at the top of the attachment list, it just doesn't have a thumbnail since it's several pages long, so it's really easy to miss.

I'm trying to keep the initial build relatively simple -- I think it'll be enough of a challenge without adding features at this stage.  I imagine I'll want to tinker with it after it's working, so I can always experiment later

There are several reasons why I don't want a standard AC15 circuit, why the Dual Lite is attractive.  Getting rid of the vib/trem from channel 2 is a big one, since it looks like that will save me a bunch of parts and work.  I'm shooting for a relatively simple initial build just to get it working first -- tone is the primary goal.

IIRC, the Ampeg chassis has plenty of room.  I might punch a couple extra holes for noval sockets, just in case I want to try adding reverb later...  I think there's enough PT to add a 3-triode reverb circuit.  The Ampeg cab has a tank in it.

[mresistor]

Oh sorry I wasn't quite awake this morning..  forgot you're converting to a dual lite.   I built a dual lite and it is one of my favorite amps, especially the AC15 lite - it has great sound


Be sure to get a "shorting" switch (make before break)   


Like Shooter suggests try the amp UL and non UL first before implementing any switching, you might not like one or the other. 


Reverrb would be a great add on.

[CrocMule]

I built a dual lite and it is one of my favorite amps, especially the AC15 lite - it has great sound

Be sure to get a "shorting" switch (make before break)   

Yeah, I'm really looking forward to this build.  Good to hear that the circuit sounds great.  I've got several 40-50W heads, but only one combo (and that's a giant ice-picky Gibson that needs some work).

I have a couple of monstrous multi-pole rotary switches that I'll have dig out to test if they're MBB.  Fingers crossed -- I'm trying to use as much stuff that I already have for this as I can. 

I just discovered the one-tube reverb schematics -- who came up with that?  Tubenit?  It's brilliant!  That might be what I'll try adding later -- I'm pretty sure the PT can handle another 12AX7.

[CrocMule]

I'm looking at adding one tube reverb to the Dual Lite circuit, but I'd like it to be universal (both channels).  I'm sure someone has done this already -- does anybody have a schematic to show how to add reverb to a Dual Lite?

Many Thanks!

[sluckey]

That is a very tall order!

[CrocMule]

That is a very tall order!

In looking at the schematic and there's no point before the PI where the two channels come together for the reverb output to feed into.  Similarly, there's no single point for the signal from both channels to feed the reverb input.  Is that why it's a tall order -- major circuit redesign to get reverb on BOTH channels?

Thanks Sluckey!

[sluckey]

yes

[CrocMule]

Thanks.  I'll leave the reverb off then -- I wouldn't be able to choose which channel to put it on!

I added up the current loads of the tubes in both circuits, to compare what the PT was putting out in the Newcomb to what it will be asked to put out in the Dual Lite (I have no specs for it).  There's plenty of room on the B+, but the Dual Lite with another 12AX7 for reverb would put the heater current 50mA over the load it saw in the Newcomb.  I don't think this would be a problem, since I'm sure the PT isn't rated for the exact load it saw in the Newcomb.  I doubt an extra 50mA would push it over the line.

But that made me wonder if there's such a thing as sag on the 6.3V windings.  What happens when the heater windings of a PT are pushed to just below their limit?  Seems like there may be some "filament starvation," or some such thing.

[mresistor]

Reverb on the 18W lite isn't a bad idea.