Artificial Center Tap Resistor Wattage?

[dbishopbliss]

I am restoring an Harmony 1482 amp that has no center tap on the heaters. I thought I would create an artificial center tap using the 100 Ohm or 220 Ohm resistor method.


A couple of questions:


What wattage resistor should I buy? I have 1/2 watt resistors already, but wasn't sure if I should use something with a higher rating.


Should I use 100 Ohm or 220 Ohm. I read somewhere that adding the artificial tap creates additional draw on the power transformer. I also read that the Harmony (Danelectro) amps were made cheaply so I want to use whichever value would stress the PT the least.


Does it matter where I put the center tap?  I was thinking of elevating the heaters by connecting the ground to the cathode of the 6V6 tubes (shared cathode resistor). Can I just connect the two resistors to pins 2 and 7 to pin 8? Or should the resistors be at the end of the heater chain (or at the very beginning).


Thanks for your help.

[sluckey]

I would use two 100Ω @ 1/2 watt. Two series connected 100Ω resistors across 6.3VAC only draws 31.5mA. That's a negligible current draw on the PT. Mounting them on the 6V6 socket as you describe is a neat and convenient way to do this. No need to put them at the end of the chain.

[SnickSound]

I would use two 100Ω @ 1/2 watt. Two series connected 100Ω resistors across 6.3VAC only draws 31.5mA. That's a negligible current draw on the PT. Mounting them on the 6V6 socket as you describe is a neat and convenient way to do this. No need to put them at the end of the chain.

Which is 0.2W for anyone wondering, or a mere 0.1W per resistor since there are two. So 1/2W resistors would have a x5 safety factor. No problem.

[pompeiisneaks]

You're alive!  Your videos disappeared from Youtube and I'd not seen you anywhere, where have you been hiding snick? :)

~Phil

[pdf64]

Problem is that for some power tube failure modes, HT current finds a path to 0V via the heater resistors; most any tech will have come across buzzy amps where the heater balancing resistors have blown. When the power tube short happens and the heater resistors blow, the entire heater line is pulled up to HT voltage, which has the potential to damage the heater to cathode insulation of every tube in the amp.
Hence I prefer to use beefy 3 to 5 watt wirewound heater balancing resistors, which tend to hold up during such shorts until a fuse pops, thereby protecting the other tubes.

[John]

I had thought that the 1/2 watt resistors made a good "fuse" against shorts though, maybe saving tubes and/or transformers?

[SnickSound]

You're alive!  Your videos disappeared from Youtube and I'd not seen you anywhere, where have you been hiding snick? :)

~Phil

Haha, still alive. I have decided to stay away from Youtube for a bunch of personal reasons. Haven't really built anything since the "Supro" (which is now basically a Deluxe Reverb with 6973s), although I'm making a two channel high-gain thingy for a friend right now

[2deaf]

Sometimes people shunt each leg of the heater winding to ground through diodes.  I guess that would keep the heaters from being pulled up to HT during a short, but it seems like it would burn the heater wires and stress the power supply until something gives. 

[pdf64]

Sometimes people shunt each leg of the heater winding to ground through diodes.  I guess that would keep the heaters from being pulled up to HT during a short, but it seems like it would burn the heater wires and stress the power supply until something gives. 

I think that special SIDAC devices may be used to achieve that, regular diodes wouldn't be suitable.
Unfortunately, when a tube shorts, something has to give; as the short mentioned above involves HT current, HT fusing is a good idea.

[2deaf]

I think that special SIDAC devices may be used to achieve that, regular diodes wouldn't be suitable.

Wouldn't shunting a heater lead to ground through a diode hold that lead at 0.6V_dc or so in the event of a short?  Is there something else about diodes that makes them unsuitable?

How long does it take to damage the heater-to-cathode insulation?  Longer than it takes for a fast-acting fuse to blow?

[trobbins]

The example fault has been seen as arcing from plate to heater terminals on an output stage tube socket. 

Preventing that type of fault from happening is a great start.  Keep the socket surface clean between pins 2 and 3.  Some have put heatshrink or glued sleeving over the pin 3 wire/terminal.  Some add flyback diodes from the anodes to ground, and some put a MOV across each half-primary winding, as further preventative measures.

The fault current can be simulated using PSUD2 if you are keen.  The fault current passes through the output transformer half-primary winding, and then through the two humdingers (if you use them), and so the elevated heater voltage can be estimated, as well as the current through a PT secondary fuse, and the current through the humdinger resistors.  The smaller the humdinger resistance, the lower the elevated voltage seen by the heaters.  Whether the fuse or the resistors would pop first, or the arc would extinguish or smoulder away, would depend on your amp, but it can be fairly easily estimated if you are keen.

PS. I can't see that using diodes for humdinger function is going to work appropriately.

[2deaf]

I can't see that using diodes for humdinger function is going to work appropriately.

The diodes aren't used for the humdinger function.  The usual resistors are in parallel with the diodes.  The cathodes of the diodes are connected to ground, so there is no heater current through the diodes.

[jjasilli]

It seems a simple solution to the fault condition mentioned by pdf64 & trobbins is to put a fuse in the heater supply, as recommended by the Valve Wizard.  Anode diodes, as trobbins mentions, may be another bullet-proofing solution, but some claim they affect tone. 

[pdf64]

It seems a simple solution to the fault condition mentioned by pdf64 & trobbins is to put a fuse in the heater supply, as recommended by the Valve Wizard.  Anode diodes, as trobbins mentions, may be another bullet-proofing solution, but some claim they affect tone.

I don't see that heater fusing would prevent the heater line (and so all tube heaters) being pulled up towards HT, thereby potentially damaging their h-k insulation.

Flyback diodes on the anodes may help with arcing external to the valve, but perhaps not with an internal short.

Wouldn't shunting a heater lead to ground through a diode hold that lead at 0.6V_dc or so in the event of a short?  Is there something else about diodes that makes them unsuitable?

How long does it take to damage the heater-to-cathode insulation?  Longer than it takes for a fast-acting fuse to blow?

Yes, I think that in the fault condition, diodes would work, I'm just not sure how the diodes would act in normal operation  Possibly either they would conduct heater current to 0V to some degree, or if not, then the balancing action of the resistors would get messed up.

I've no idea how long the h-k insulation can hold out for, but hopefully an F fuse stopping HT would help.

[2deaf]

Yes, I think that in the fault condition, diodes would work, I'm just not sure how the diodes would act in normal operation. Possibly either they would conduct heater current to 0V to some degree, or if not, then the balancing action of the resistors would get messed up.

Ahh, I see what you mean.  For each phase it would be a conducting diode in series with a balancing resistor for the ground reference instead of a balancing resistor in series with an equal balancing resistor.  So the positive side of the heater signal would only be 0.4V above ground while the negative side would be 5.9V below ground.  DIAC's and SIDAC's block in both directions so that they don't come into play for the balance, but they will break over for the HT fault.

I've seen the diodes used over the years, sometimes by reputable builders.  I've never tried it, myself.

[jjasilli]

pdf64:  I don't see that heater fusing would prevent the heater line (and so all tube heaters) being pulled up towards HT, thereby potentially damaging their h-k insulation.


I was thinking that the short condition would cause enough current flow to blow a fuse in the heater circuit, thus turning off the heater circuit.

[pdf64]

pdf64:  I don't see that heater fusing would prevent the heater line (and so all tube heaters) being pulled up towards HT, thereby potentially damaging their h-k insulation.


I was thinking that the short condition would cause enough current flow to blow a fuse in the heater circuit, thus turning off the heater circuit.

Blown heater fuses stop the heater current, but wouldn't stop the HT current that would be pulling the heater circuit voltage up. The heaters all could be cold, but the shorted tube (eg anode to heater short) could result in damage to the h-k insulation of all the other tubes.

[jjasilli]

Ok, so there would be HT voltage in the heater leads, but no current.  Isn't that a good thing?   

[John]

Well I don't know much, but honest to God I don't know how there's any 50 year old Fenders or Marshalls still out there. 

[jjasilli]

Yes, these are rare occurrences and aren't a bother to most of us.  But amps on tour get roughed up and are more likely to fall victim to all kinds of faults.  Tech's may be concerned to make amps "bullet-proof".  The show must go on.  Maybe it's good info to be aware of.

[pdf64]

Ok, so there would be HT voltage in the heater leads, but no current.  Isn't that a good thing?   

Tube info usually notes a limiting value for h-k voltage, 100-200Vdc is typical. If a tube short pulls the heater line much higher than that (eg up near HT voltage), then the tubes h-k insulation may become damaged.

As no fault current then flows, it won't damage the amp particularly, other than popping the balancing resistors (and/or heater fuses in some modern amps). Replace those and the tubes and the amp should be fine. It would be tough however if some of the damaged tubes were valuable vintage types, though they may still be usable in equipment with dc heaters.

[jjasilli]

Ok, thanks.  Good points.

[pdf64]

Well I don't know much, but honest to God I don't know how there's any 50 year old Fenders or Marshalls still out there. 

Yes, these are rare occurrences and aren't a bother to most of us.  But amps on tour get roughed up and are more likely to fall victim to all kinds of faults.  Tech's may be concerned to make amps "bullet-proof".  The show must go on.  Maybe it's good info to be aware of.

Yes, for an individual tube amp user, such power tube failures (ie that result in heater balancing resistor / heater fuses blowing) are unusual, and we won't personally experience it more than once or twice (if at all), unless we're using tube amps hard and frequently, eg on a pro/semi pro basis.

However, I suspect that for tube amp techs, it's fairly common to be given buzzy amps to sort out, and 9 times out of ten it's due to a power tube failure blowing those heater balancing resistors , and then even when those are replaced, still finding that the amp buzzes until most/all the tubes are replaced (ie due to the heaters being pulled way above their h-k limiting voltage.

So, in the light of that, if we're involved with building / repairing tube amps, it makes sense to me to implement mitigation for common tube failure modes such as these.
Hence my suggestion to fit heater balancing resistors that are much beefier than is necessary for regular operation; fitting a suitable SIDAC between a heater line and 0V (on the tube side of a heater fusing if fitted) is an alternative.

The point being to make amps more resilient to common failure modes, ideally so that all that's needed to be up and going again is to replace the bad tube, rather than have to make good collateral damage / send it to a tech, in order for proper operation to be restored.

[pdf64]

Doh, double post

[John]

However, I suspect that for tube amp techs, it's fairly common to be given buzzy amps to sort out, and 9 times out of ten it's due to a power tube failure blowing those heater balancing resistors , and then even when those are replaced, still finding that the amp buzzes until most/all the tubes are replaced (ie due to the heaters being pulled way above their h-k limiting voltage.

And that makes sense, good points all around. My thinking was  was that the 1/2 watt resistors "fusing", saves/might save transformers. But you're saying that just because the heaters shut off, doesn't necessarily mean less damage. So now, even though I'm still pleased with my little funny    I will go back and re-read this thread again till I understand the ins and outs better.


Thanks!