Weekend Project Thread

[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

 

[jrebman]

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  ;D  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!

 

[2wo]

I have a breadboard DRD amp set up that will run any 4 pin tube. Once I was changing from 300B to 45. I was so focused on dialing in the B+ and driver voltage, I forgot to set the filament to 2.5V. Needless to say it didn't last very long...John

 

[4krow]

John,

  Now that I have to pay more attention to these things myself, how much different was the old filament voltage setting as compared to the new? I am working with various tubes and power supplies, and had better watch myself! Musical Fidelity makes a buffer amp that I am presently working with, but there isn't enough information for a guy like me to really go to far with it. Not even a schematic as far as I know.

 

[Grainger49]

How do they propose that you assemble it without any instructions?

 

[2wo]

I had it set for 5VAC for the 300B.

 

[4krow]

Well, actually, the Musical Fidelity is a complete/assembled product that some like to hot rod. Of course, you can buy a kit for changing out caps, diodes, and wiring, but, I wanted more. A power supply, for example in addition to the rest of the upgrades just made good sense. So, I am waddling through this stuff with one hand behind my back. I'm gettin there, but not like others who probably have a better understanding of the circuit in the first place. One way to start out in a better position was to buy an additional unit, and measure the voltages per normal operation. Too many fires on the brand, err, brands on the iron, or something like that. I have got to get some of this stuff out of the way...one, at , a , tiem.  errr, thyme.

 

[Alonzo]

Started to poke around in my amp, check resistances and voltages and do a connection by connection comparison (since one side worked and the other didn't) to find my mistake.  Haven't found anything different yet but I did find some wiring showing burned insulation (see pic).  This is the wire connecting the the power supply choke to the dropping 1.3K 10W resistor.  The cap at this connection looks fine. 
So I'm off to my parts bin, looks like I may have 2 or more problems, not just an out of spec heater current/voltage.

Sorry for the flash glare, you can see the burned red wire center of the picture.

 

[Doc B]

Looks to me like the wire insulation is just getting fried because that big metal resistor is cooking it. It's probably PVC and replacing it with Teflon insulated wire is probably the best solution. If it's already Teflon wire - yeow - that sucker is getting really hot!

All that said, if this is a single high voltage supply it's not likely that that melted insulation is causing a problem on only one channel.

 

[Alonzo]

Finding out more and more....replacing all the power supply wiring with teflon insulated wire.  The C-7 choke on the nonworking side reads 151 ohms, 247 for the working side so I'll be replacing that choke also.  Lots to do this weekend...

 

[Alonzo]

Pinging Paul J., pinging Paul J.,
You mentioned back a couple of responses ago that my 45 tubes cathodes may be re-formed.  While I wait on filament transformers and plate chokes to be delivered, is there a method of bringing these tubes back to life?

 

[Paul Joppa]

Don't do anything until you have the correct filament voltage. Then you can run the tubes in the amp, monitoring the plate current, and see whether the current returns after some burning in. If it doesn't come back in 50-100 hours then they are probably well and truly dead.

 

[Alonzo]

Got my filaments transformers in the mail today (yaaaaay!!) so since the chokes are still out there somewhere, I'm going to install the filaments and work on burning in and hopefully reforming the cathodes on my blown 45's.  Now the question is;  is it an advantage to install a switch to turn these on first for normal operation?  I don't want to just run the filaments ( I think) for the re-forming but it should be better to warm them for normal operation.

 

[Paul Joppa]

Yes, re-forming requires both correct filament power and some plate current through the tube.

And yes, warming the filament before applying high voltage will normally extend the life of a tube. There is little consensus about how much difference it makes - it's not huge, or everyone would agree - but I think there are some old statistics from the fifties to support the theoretical notion.

 

[Alonzo]

I'm starting my rebuild of the amps power supply and I came across an item I'd like some opinion on, actually a couple of items.  I'm changing the PS layout and form, going to a full wave rectifier with UF4007's and I thought about adding another LC phase to the filters.  I have some Hammond 155J chokes (1K ohms 15H 30ma) that I thought of using.  Since I have to add a 2K resistor to drop voltage, why not use a choke/cap and a smaller resistor in it's place. When I run it in PSUD the numbers are pretty much the same, would there be a big sonic difference?

BCP-15 have arrived so in they go...
Also have some MCaps (47uF 400VDC),  would they be wasted as PS caps at the last cap of the PS filter before the BPC-15?
Pic's attached of the new parts...and of the PSUD scenarios

 

[Alonzo]

Finally think I'm done (at least with the mechanical portions, it still could use some "beautifying" and a right angle power cord.  Added plate and grid chokes, increased the value on the cathode bypass and the coupling cap.  Gonna let it burn in for 6-7 months before I look at it again.  A better power transformer is needed and to hopefully get rid of the filament transformers.
It's playing the local jazz radio station now, sounds pretty good, the real test is tomorrow hooked to the "regular" system of turntable=>seduction=>quickie=>horns.  Hopefully there's not much hum, right now I can't hear any at my listening chair.
Some pics of the "blivit".

 

[jrebman]

Alonzo,

Congratulations!  That re-desines the long weekend if you ask me .  Looking forward to your impressions in the regular system.

-- Jim