Stromberg Weirdnes

[jaymanaa]

Anyone ever see a 433 where they had completely separate single wires going from the Phono RCA negs (Which are grounded to the chassis) to the ground buses for the phono 12AX7s? I've rebuilt a bunch of these and don't ever remember seeing these wires before. Just wondering if some builder knew a trick or something to keep hum out of the phono?? I always do away with the ceramic stage and all that garbage and just stick a 47K R from the mag phono to ground. I was doing that mod when I noticed these wires.

 

[gadget73]

I don't believe mine has that. S-C did some odd stuff with that amplifier. Seems there are a number of variations of it. Just the fact that its star grounded and all is odd for the era and grade of equipment. My S-C PA amps have star grounds in the same manner. I don't know that anyone was doing that with PA amps back then.

 

[jaymanaa]

I have not finished the phono stage on this one yet, but have played plenty of CD player music through it to know it has that wonderful signature S/C sound. I stopped modding anything except the voltage doubler capacitance and phono stages on these long ago, they are as good as they get with the standard S/C circuit IME. What I do with the (sub par IMHO) phono stage is just take the one out of the back of the RCA tube manual and stick it in there. IMHO it's a noticeable improvement to most everyone who's heard one that way. I took the cover off another old barn find 433 to compare and it did not have the extra ground wires, but had jumpers from the pre section to the power section. I've never seen that before, just RCAs up on the chassis with short shielded (for the day) cables connecting the two sections. I may keep that one, as I like running SS preamps into tube power. I wonder what the 433 power stage would sound like behind a Hafler Iris MC phono stage?

 

[jaymanaa]

Just so you know I haven't lost it.....Here are the two wires I'm talking about. Does this look like a factory wired unit? The sockets are riveted, so I'm assuming it is, it almost looks like someone may have added these ground wires. I'm going to try this phono as designed and see how it sounds. I can't imagine the extra wires could do much, but what do I know, not much.

 

[gadget73]

Dunno, looks factory, certainly appears to be the same white insulated wire they used for grounds all over the place in the unit. I'd have to look at mine, but I'm pretty sure I do not have those. Its been apart for years while I was going to get around to doing something to make the phono stage not be horrible, and I never put it back together after fitting the transformer for the DC heater supply. I did clean up a bunch of the front panel wiring to take some of the mess out of the signal path. That seemed to help it. I no longer have the crossover and mono bridging features, and its only got one volume control instead of 3. Functionally all of the stuff you actually make use of works still.

 

[dmag]

I took a look at one I did a while back. It has the same grounds, but one white, one black. It was bone stock, with riveted sockets too, when I got it. The wire is definitely the same cloth jacket wire as the rest of the unit. Now that it's out I guess I'll have to dust it off rotate it back in for a listen. I do remember being very pleased with it. Reminds me, I never did anything with the phono stage, and probably behind the panel either. Oh well...
Just curious, what is the function of cap and resistors?

 

[gadget73]

I think they are a sort of reverse RIAA so you can use a ceramic phono cartridge. It still feeds through the phono amp though.

 

[jaymanaa]

I think they are a sort of reverse RIAA so you can use a ceramic phono cartridge. It still feeds through the phono amp though.

Yes, someone way smarter than me figured out how to make it so you could use a ceramic or magnetic cart. I don't know though, AFAIK 47k has been standard loading for carts like the V15III which I assume would have been fairly TOTL back then,but if you measure the loading in the factory configuration, you don't get that. Also AFAIK, a ceramic cart is much like plugging in a cd Player or other aux component, and you don't get that either?? Nobody I build these for uses ceramic, which I simplify by just using the magnetic and loading it to 47k. Several of the older integrated amps I've seen have this combobulated "mess", so I just load the magnetic to 47 K and they can use the ceramic for another aux. That or I will load the stage to 47k and they can use two turntables and a microphone (that was a bad joke). It depends on the selector switch and if it has two positions or one for phono.
Thanks dmag, at least now I know I'm not just imagining things. I wonder if you would do a test and lift one of the wires and see if you get a hummer. I might add that when I replace the original phono wires with modern shielded silver, hum decreases at loud volume settings with no record, just a cart hanging on the stand. Sometimes these old integrateds can be a real bear to get totally quiet, which is why if I'm building for myself or someone who doesn't mind the considerable amount of time, I lift the entire circuit ground from the cahhsis, meaning if I ohm between an RCA neg to the chassis I get infinite. Then I do a ground wire for each tube to a star grounded at the chassis right next to the main power cap. After that, you can usually stick your earball right into a woofer and can't tell if it's on or off. Phono is without a doubt the hardest of the inputs to get totally quiet. Also, if you want to be real fussy, check the cart data for loading data.

 

[gadget73]

If memory serves on mine, there was a 100K at the input jacks and another 100K at the grid, making for an effective 50k of loading. Close enough for the 47k carts.

The high value resistor drops the signal down so the ~1v output ceramic would work. I won't lie and tell you it likely works very well, but noise would come out of the other end of the thing.

 

[dmag]

Sure I can do that. Ill try to do it tomorrow, otherwise Im not so sure if i will have time this week or next. School finals, ya know. Not easy for an old fart. What exactly do you want? Turntable installed or no?