Using a parallel filament tube in a 500-C - is that OK ?

thornev

Super Member
I've been asking most of my questions in my "Now what?" thread, but Catman brought up an issue that concerns me so I thought it warrants its own thread

I put a pair of Tung-Sol 12BZ7 tubes, black ribbed plates with a "fat D-getter on top" in V12 and V13, the left and right phase inverter and driver sections. I've been using 12AX7 tubes in those spots. Catman says the 12BZ7 tubes are parallel filament tubes whereas 12AX7 tubes are series filament tubes. Have I unknowingly created a bad situation that needs to be reversed? The sound is fabulous so I'm hoping it's OK.

Thorne
 
They are both parallel and series filament, depending on whether being used in the 6v setting (series - pin 4 to 5 without using pin 9 ) or a 12v setting (Parallel - pin4 to 9 and pin 5 to 9).

Bookmark this page. GOOD Explanations. http://www.valvewizard.co.uk/heater.html
You're fine.
 
They draw twice the heater current of a 12AX7. If your amp calls for 12AX7 but can handle the extra draw you can use a 12BZ7. Bob
 
In both tubes I'm getting continuity between pins 4 and 9 and between pins 5 and 9 testing the tube disconnected from the chassis. I assume that means the tube is running in the series state.
 
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I'm getting continuity between pins 4 and 9 and between pins 5 and 9 testing the tube disconnected from the chassis. I assume that means the tube is running in the series state.

If you are measuring the tube you will always see that (as long as the heaters are ok) because it's like this.

Pin 4 ---- Pin 9 ---- Pin 5

That's the way the tube is made both 12AX7 and 12BZ7 are same in this regard. But, as mentioned earlier, regardless if the amp uses them series or parallel, the 12BZ7 draws twice as much filament current when both are wired same way.

You have to check the socket on the gear to see how the heaters are actually used - or is that what you did and I just didn't understand?
 
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Crap, no you're right. I measured the tube, not the socket and not with the receiver turned on. So measure with the tubes installed and the receiver turned on, and measure one lead on pin 9 and the other lead on pins and 4 and 5 ?
 
BTW, the 500C schematic I looked at appears to show V12 and V13 have the filaments connected parallel. That is to say that pins 4 and 5 are tied together then power is applied to 4/5 and 9.
 
Crap, no you're right. I measured the tube, not the socket and not with the receiver turned on. So measure with the tubes installed and the receiver turned on, and measure one lead on pin 9 and the other lead on pins and 4 and 5 ?

What is it you are trying to ascertain?
 
I'm trying to ascertain that my system can handle the two 12BZ7 tubes I have installed in locations V12 and V13. Catman brought up that because those tubes can be run in both parallel and series modes, my system maybe cannot handle however they're wired or however my system is wired (I don't know which scenario is the concern).

In regards to schematics, if it matters, my serial number is late - 76944 - so the later schematics would apply.
 
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The 500C appears to run the filaments parallel as previously mentioned. That does not change regardless whether you use 12BZ7 or 12AX7.

What changes is that 12AX7 with parallel filaments those two tubes draw a sum of 600mA.

Plugging in two 12BZ7 running parallel filaments doubles that draw to 1200mA

Whether the transformer can reliably handle an additional 600mA load I don't know.
 
Thanks for your help, Who. There must be some way to determine/lookup what the later 500-C transformers can handle. My hunt begins !
 
Or, save the bz7's for preamp duty. That's where they're gonna have the most sonic effect. I'd not use in the positions where filament voltage is derived from the bridge rectifier (lo-volt.bias supply), but that's just me. Ya just cant keep from molesting that thing, can ya?:whip::)
 
I'm always after the best sound, Cat. That's why I do all these mods (well, and also for the recommendations for extending the life of the unit). I've been tube rolling for a while, started with a headphone amp and now the 500-C. I have to disagree that the 12BZ7's produce better sound in the preamp stage, V14/15. They are rockin' in V12/13. But right now my concern is harming the 500-C with those 12BZ7's... which is your fault.. :blah: (just teasing)
 
Take them out and don't use them in the 500c. They draw too much current for the "Filament" circuit on the SDS board. If you had the original circuit setup with an 8A Bridge, No problem. On the 500c The transformers were the SAME from #10001 to the last one made.
 
(deleted post as I'm learning that 12BZ7's require voltage that my power transformer may not be able to provide. Don't know for sure that's the problem but trying to learn if there is some way I can use those 12BZ7 tubes - an add-on transformer maybe or dumming down the power needs on the 12BZ7)
 

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Hi thorne, I didn't want to rain on your parade re:bz7's, seems i ignited a shiite-storm on that. They do pull more filament current as we have established previously. This may pull your bias voltage to the output toobs down a bit if used in the preamp/tone control (v14-17) circuits.Looking at your sds board pics, it looks like it uses a kbl02 or similar bridge, which may or may not have the extra grunt to accommodate a higher load via filaments. I don't know the available current capacity of the 24 volt secondary which would be the determining factor, that's a spec that's not listed on the schematic. Thats a dave Gillespie question. won't apply to phase inverters as much, as their filament voltage is derived from a different winding on the power transformer. Ymmv. You could always use an equalizer in the tape monitor circuit to play with your freq.response as you see fit. I successfully patched in a pioneer dynamic expander into the reverb Jacks with good results, so you may be able to replicate that with an equalizer. Just food for thought from the ol'cat...
 
Thanks for the post, Cat. I was feeling down in the dumps about not being able to use the tubes and not being educated enough yet to know that they could damage my precious 500-C.

I do have a digital EQ as my signature indicates which is applied to all my signals. My signal path is source device > patch bay/mixer > digital EQ > Fisher 500-C > AR-2Ax speakers + subwoofer. The exception is when I play directly into the AUX input on the Fisher such as with a tablet and phone (both running PowerAmp as my music player). I've started connecting my turntable into the PHONO input on the Fisher which sounds pretty good too.

Anyway, I am researching the 12BZ7 and 12AX7 tubes so I can learn what elements of the BZ7 make the great sound I like (it's not just EQ - there was an overall and significant "umph" of some sort) so that I can look for those elements in other tubes that might be compatible in the 500-C. Or maybe there is some alternative way of using the BZ7's that won't "take down" the SDS board or power transformer "long term". I don't know. If I don't learn anything applicable, I may just resign myself to the fact that the 500C already sounds fabulous as is. I mean even artists (and I'm one myself in the world of music composition) have to know that at some point enough is enough and it's time to start the next project.

I see that the BZ7's transconductance, plate resistance and plate current are twice that of "normal" AX7's. Interelectrode capacitances are also quite different. I don't know if I can electronically "normalize" those specs, for example, or if I can do some kind of add-on transformer to provide the extra current the BZ7's need. From my reading it seems I can add some kind of resistance in the circuit, somewhere just after the power transformer, that satisfies the "thirst" of BZ7's. But even if that worked, I could be ruining the elements of the BZ7's that make it so much to my liking. I'll keep looking for an answer...for a while anyway. Maybe just a pair of Genelex Gold Lions.

PS - I went browsing through my local record store today and they have a 500-C with the older "large numbered" FM dial on it. It is in terrible shape with knobs falling apart, switches that feel like they're locked, just in real need of some work. Price tag? $700 !!! Wishful thinking !
 
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Yep, they're clear nuts! At that price, it oughta have been rehabbed already, with a mint -condition oem wood cabinet. If not, offer $200, see what they say...:eek:
 
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