Stromberg Carlson ASR 333

[Brycecb750]

Stromberg Carlson ASR 333, kind of a fun project I've been messing around with, I got this years ago looking for a low powered 6BQ5/EL84 PP amp. I didn't like all the controls, like the separate gain and master gain. So I stripped it down a bit, rebuilt the tone controls dual and a simple 3 line input. I used Mallory M150 caps in the tone section, K40y's up front and Astron PIO's going to the 6BQ5's. It was sounding pretty good, but I wanted to be able to take the tone controls out, so a switch will route the inputs directly into the 6U8's. Also I put in pentode triode switches on the 6BQ5. I think I like it with tone section out and in triode mode. It's starting to sound like a pretty good amp!!
I plan on adding a phono preamp where I've put the two empty tube sockets, right now there's nothing wired to them.

 

[analog addict]

Then you'll more or less have a 433. However the phono section on a 433 is nothing to write home about, so you should be ahead of the game. I'd use the RCA phono circuit with 12AX7s....

 

[gadget73]

I did pretty much the same with my 433. Changed the phono circuit to one from a Harman Kardon, removed the various channel selectors, removed the individual channel level controls but kept the master control. I also replaced the ceramic tone caps and at one point I had the tone controls out of circuit entirely, though with better caps they sound pretty decent. I ended up taking it back apart with the intention of building a DC supply to get the last of the hum out of the phono stage and basically it stalled there. Its been in pieces for probably 2 years at this point. It was sounding pretty good about the time I took it apart though.

 

[GordonW]

Yeah, you really need some sort of DC heaters for a phono stage. And yep, the original ASR433 phono stage isn't any great shakes.

Either the aforementioned HK phono stage (the one out of the later model A300, or the one out of the A500, would be good), or a clone of a Dynaco PAS phono stage, would be good. They're pretty similar, both in layout, and in sound.

The aforementioned RCA phono stage (from the RCA tube manuals) isn't bad either.

Regards,
Gordon.

 

[knockbill]

Agreed with the RCA phono stage,,, sounds very good as is,,, no need for tone controls...

 

[gadget73]

I was using one from an HK A500 I think. May have been the A300. Same circuit isn't it? Either way, its whichever model I found a good clear schematic for.

 

[GordonW]

I was using one from an HK A500 I think. May have been the A300. Same circuit isn't it? Either way, its whichever model I found a good clear schematic for.

Slightly different. The A300 phono stage uses a directly-grounded cathode and grid-leak bias for the second gain stage, while the A500 has a cathode resistor bypassed by a cap in that stage. Also, the feedback loop is different, in the way the caps and resistors are arranged (two series loops in the A300, compared to some sort of series/parallel arrangement in the A500).

Regards,
Gordon.

 

[jaymanaa]

I always thought it was odd that they offered that 333 in the LP heyday. I guess it was for guy that wanted to do outboard phonostages, that's all I can figure. However, if you've ever hear the phono stage of a 433, maybe it does make sense. RCA is the way to go, and the RCA tone amp can be handy for some extra gain if needed on a scratch build.

 

[gadget73]

I figured it was more for lower budget console applications that ran a ceramic cartridge.

Gordon, I'll have to look at my notes when I get home. Its all in the same folder, and its been a while since I was inside that thing. Any thoughts on which circuit is better? The thing is in pieces, not like it would be any sort of ordeal to gut it and change it at this point.

 

[Brycecb750]

Now I changed the 6U8 to what was found in the Scott 222d, sounds a little better I think. But my big question is would a 12ax7 amp and phase inverter like found in fisher or some eico be better??? Does anyone know?
Interesting thoughts on building a HK phono pre, I was going to go for a Scott style, but I already have some Scott intagraded amps, so it would be nice to try something different, but I don't know what. I do want to try and run the 12ax7 heaters on the 6BQ5 cathodes.

 

[knockbill]

I figured it was more for lower budget console applications that ran a ceramic cartridge.

Gordon, I'll have to look at my notes when I get home. Its all in the same folder, and its been a while since I was inside that thing. Any thoughts on which circuit is better? The thing is in pieces, not like it would be any sort of ordeal to gut it and change it at this point.

If you want to build one from scratch why not the RCA phono pre? You can listen to mine any time ya want... I think it sounds better than the WA-P2's and they have tone controls... There are mods for it also, if you want some sand in the mix!!

 

[gadget73]

The idea of an SS phono stage did also occur to me. Most of them want a split rail supply, which is not something that is convenient to do in a tube amp. Single rail derived from the B+ line and limited with a zener diode is entirely do-able though. There are so many dual triode phono stages out there that sound good. I also had considered cloning the circuit out of the Fisher since those are pretty dead-nuts RIAA accurate. I'm fairly fond of the phono section in the TA-600.

Now I changed the 6U8 to what was found in the Scott 222d, sounds a little better I think. But my big question is would a 12ax7 amp and phase inverter like found in fisher or some eico be better??? Does anyone know?

I personally like the pentode/triode driver design. It sounds good to me, and S-C cap coupled it so it doesn't get so wanky with variations in the pentode section. Another plus in my book is that 6U8's and 6GH8's are generally cheap tubes to buy, where 12ax7's maybe not so much. "Better" is a highly subjective thing. You can try a dual triode rig, but you'll probably be re-designing the feedback loop in addition to changing the driver design to keep the overall gain of the amp stage right.

Interesting thoughts on building a HK phono pre, I was going to go for a Scott style, but I already have some Scott intagraded amps, so it would be nice to try something different, but I don't know what. I do want to try and run the 12ax7 heaters on the 6BQ5 cathodes.

Not enough current to run the 12ax7 heaters off the cathodes. Needs 150ma to light one of those, and the output stage runs somewhere around 70ma per side if I remember right. Strap both sides together and you've got enough current and about the right voltage to light one. Not real useful since you'd need 2 for stereo.

 

[Brycecb750]

I'll stick with the 6U8's for now, it does sound pretty good. Too bad about the 6bq6's not making enough current to power the phono... I really want them to run off DC

 

[jaymanaa]

HK I'm doing right now has a 12Au7 and a 12AX7 phono stage? I always liked the one in the RCA manual (2 12AX7s), it certainly makes a nice 1 k wave when I run it through my phono inverter. Meaning it is shaped perfectly.

 

[gadget73]

OK, so it was an A300 stage I used. I may have gone that route just because I had the parts on hand to make it work. Its been long enough that the logic behind the choice is forgotten to time, assuming there was any to begin with.

That one at least runs 2 12ax7's, one per channel for the phono stage.

 

[Brycecb750]

I like this A300 phono pre, pretty simple, I think I'll give it a try!

 

[GordonW]

The A300 phono stage sounds GOOD, too. My A300 has been rebuilt with all discrete parts in the phono stage- and boy, does it have LIFE.Great dynamics, and very nice balance.

Regards,
Gordon.