Weekend Project Thread

[earwaxxer]

Doc - thanks for the correction on that! Granger had me sit in the corner with my 'dunce' cap on for an hour... then I was forgiven. Boy if life was only that easy!

[Alonzo]

Eric,
I've been in that corner before...I think everyone visits there now and then.

John, what are you playing your SR-45 into, I"m playing my 45 into horns, so I"m trying to determine what will improve, I have a hard time defining "dynamics".  Did the sound stage expand, or more depth to voices and instruments?  These chokes are probably a month or so away, I've got to get onto Mike's build list, in the meantime I'm adding BCP-15s hopefully soon.

[Grainger49]

I thought it was my corner.

[2wo]

"I thought it was my corner."

Yea, we are the guys that come over and drink all of your beer and eat all your food ...John

[2wo]

Alonzo, I use the SR-45 mostly with Hornshoppe horns. The grid chokes Give more of a sense of attack and make the amp seem more powerful and alive. It  did not change the tone much. The sound stage did not change in size so much as the space it conveys seems more solid and real. I can hear more of the room in which the recording was made...John   

[4krow]

"IN THIS CORNNNER, IS JAHNNN, THE SOLDER KING(SHOUTING/APPLAUSE). AND, IN THIS CORNER,-, THE RETURNING-BURNNNN  CHAMP! GGGRAINGER! THE BOTTLEHEAD SMASHHHER! ( BOOO! 'YER A BUM!  {SORRY, IT'S NOT YER HOME TOWN}).  JUDGING TONIGHT WILL BE KKKROWWWW...."I DON'T WANNA SEE ANY COLD JOINTS-HOT  LEADS OR BURNING BELOW THE BELT. GENTLEMEN, START YOUR IRONS."


 Somebody make it stop.....

[earwaxxer]

LOL!!! Greg - well done!

[Grainger49]

I thought I was the Corner Sitting Champ.  Just ask my Mom.

[Alonzo]

John,
My Frugel horns are a similar design so it seems that the chokes will be good for me also.  Will give it a try once I have chokes and have fixed the amp.

I've committed a triple sin, I've killed 3 1940 45 tubes for some reason I can't fathom.  While playing the amp and breaking in, about 45 - 1 hour in, one channel will die.  I thought it was a bad solder joint opening when heated so, I re-wet pins 1 and 4, looked for shorts and started again.  About an hour or so later, tube #2 didn't work.  So me, being dumb, took a layer apart to check solder joints, checked voltages on the heaters, plugged in a new tube and a short time later, another tube bites the dust. 
So after 3 tube failures I decide to take the amp down for repairs but I can't see any thing wrong.  I measure 2.9 volts AC on the heaters, the transformer is rated for 5v CT 3A, I'm running the 45 heaters in series with no CT, each measures out at 2.9 volts. 
This has happened on the same side, the side extended from the heater wiring.  I'm thinking of replacing the wiring due to a heat induced short, just to see if that's it, but I don't see any obvious causes.  Does anyone have any ideas??  Oh yeah, tested it with some 2a3's and they played fine...
I'm at a loss, please help...

[Doc B.]

2.9V is a 16% overvoltage. That is quite high, most of the old texts say the filament voltage should stay within +/- 5%, i.e., 2.375 to 2.625V. So that might be at least part of the problem. However, if it keeps happening on only one channel it might be that something else that is isolated to that side is causing the problem. That fact that it doesn't happen for 45 minutes tends to indicate that it is either a thermal issue or maybe it is in fact that the tubes just tend to die after 45 minutes from the filament overvoltage. I'd suggest using a resistor in the filament supply to get the voltage closer to 2.5V and using an expendable tube like a worn 45 to see if it holds up over time.

[2wo]

As it happens my SR-45 uses a single 5V Hammond filament transformer. When I built it the 2.5v were on intergalactic back order. What I did was to cut the tape on the bobbin and separate the center tap so I have two 2.5V windings on one core and I have very close to 2.5 VAC at the 45.

If your 2A3's survive, could it be that your plate voltage and/or current are too high?...John    

edit, I just reread your post. I think it is a bad idea to run the 45 filaments in series

[Jim R.]

I agree with John -- the filaments in series caused me some wonder as well.  With dhts, I've only seen that in push-pull pairs.

-- Jim

[Alonzo]

I think I mis-spoke, actually mis-typed    The heaters are daisy chained but electrically parallel ( I think).  The twisted heater wires go to pins 1 and 4 of the right tube, another twisted set goes from R1R4 to L1L4, R1 is connected to L1 and R4 is connected to L4.  I always get serial and parallel meaning mixed up.  I think this is parallel wiring.

[Grainger49]

Now I am getting confused.  Not from what you mis-typed, but from what is going on inside my mind. 

The cathodes are in the signal path.  With a common power supply that ties the two cathodes together somehow.  My head hurts right now, woke up with a headache.  I'll have to think about this.  If my thought is right all stereo amps with DHT tubes would have to have dual heater supplies.  I'm thinking that this isn't right.  But my head hurts.

More later, less sooner.

[Paul Joppa]

Wait - the filaments are in parallel, so they draw 3 amps at 2.5 volts, and you are feeding them from a 5 volt/3amp winding? And you only see 2.9 volts? There is more than one thing wrong here. The transformer should happily make 5 volts at a 3 amp draw, and the 45s should die quite rapidly with 5 volts on the 2.5v filament.

Perhaps you are using just half the 5vCT winding, and ignoring one of the end leads? And you power line might be much higher than the transformer was designed for? Those things together might make the moderately excessive voltage. If you are really getting only 2.9 volts from a 5v winding there is some other problem.

If the filaments are not independently biased, you may have one tube that is hogging the current, leading to excessive dissipation and even cathode stripping.

Once you get the power supplies straightened out, it is possible the cathodes can be re-formed and there may be some life left in those "dead" 45s. No way to be sure, of course!