Epiphone Pacemaker 50T PT short

[angelodp]

Yes that will work electronic wise, ( don’t know about physical wise ) but you will need to add another dropping resistor before the first filter node to bring down the 360 volts that you will have off of the 275-275 windings down to the stock schematic reading of 235.

I would say that if you get the V+ down to even 250 that will be pretty good, but you will then have to increase the resistance of the output tubes cathode bias resistor which is now 270 ohms.

Looks like a added 1 ohms per volt, or a total of 285 ohms should do it to get the outputs to idle back where they should.

Looks like the original PT was a 185-185 volt deal.

I do not have time right now to figure out the ball park resistance you will need for the new added V+ dropping resistor, but it looks like the amp is idling at about 95 ma.

To keep things nice and cool I would go with a 10 watt resistor for this.

Steve, how did you determine that the original was 185-185 volts on the PT

[angelodp]

So the last measurements of the existing PT had 460v across the Secondary and 6.3vac for the heaters.... that's all there is.

All the heaters are off of the same winding at 6.3v so Steve, it makes sense to continue in that fashion.

[martin manning]

Here’s the power supply of one of my Univox B45 amps that use the 6X4 recto and it’s filaments are run off the same 6.3 volt winding as all the other 3 tubes in the amp.

The risk there is a rectifier h-k short, but the subject Epiphone was wired that way too

[angelodp]

Is it plausible that an h-k short caused the failure of this PT?

[Stevem]

No, not very likely!

Your condition as I have seen in the countless failed PTs I have replaced over the decades is very simply long term high current/ high heat loads due to bad filter caps.
Long term high current loads that are right on the edge of blowing the AC mains fuse for hours of use.

Even the best made power transformers once the AC mains fuse has blown 3 times or more your well on the way to a failed PT.

Having a output tube short along with the amp running on bad filters is another easy way to get a failed PT.

Adding a fuse just for the heater winding and another for the B+ will keep the new PT alive for ever.

[angelodp]

Steve, ok great info and makes so much sense. And yes this amp got run hard and put away wet (for many years), so it reached its limit. The Power bias cap was spewing white goo. Maybe suggest a new Recto tube and power tubes to the owner.

[maxkracht]

Adding a fuse just for the heater winding and another for the B+ will keep the new PT alive for ever.

Apologies for going slightly off topic, but what are your opinions on HT center tap fuse vs two HT fuses. Merlin makes the case that current can still flow if a diode fails short and the CT fuse won't always help. Is this scenario just too rare to worry about with a valve rectifier?

He also doesn't seem to think heater fuses are necessary when running on AC.

[martin manning]

GE 6X4 Datasheet snip. Note max h-k voltage.

[sluckey]

GE 6X4 Datasheet snip. Note max h-k voltage.

The heater will never be positive with respect to the cathode so I would think the only concern is to use with the cathode at less than 450V. I have two amps (Vox AC4 and Hammond AO44) that use EZ80 and EZ81 connected to a common filament string.

[martin manning]

Exactly.

[angelodp]

May be able to source a Dynaco PT that has dual 6.3v taps and 205/0/205 secondary. It's tad bigger than original but it may just fit. Looking it over

[Stevem]

That will work even though it's a tenth of a amp short on the heater voltage you should still end up with 6 volts or maybe 6.1 volts.

However you will have to make a artificial center tap for the heaters which is no big deal.

[maxkracht]

Unless I am misunderstanding, two 6.3v taps, so should be more than enough and you get a 1A winding for the rectifier. Not a huge deal to enlarge the mounting holes and file down the cutout if it is close.

[angelodp]

Yes seems ideal...lucky break. Gonna see about sizing today.

A

[angelodp]

Have you guys bothered to do a de-rust on an amp like this, or is it too risky... loose the screened type and graphics?