AC filaments too hot!

[Tom Phillips]

…Couldn't I add an auxiliary filament transformer? .

Sure.

Would I have to isolate the two power supplies from each other? .

Yes but actually, if you mean isolate the heater supply from the HV supply, they are already as isolated as they need to be since the heater winding on your existing PT is separate from the other windings. If you disconnect the existing PT heater winding completely it will work fine. If you want to operate the old and the new heater wirings in parallel then you need to make sure that they are hooked up in phase. However, running transformer secondary windings in parallel is not good practice unless the windings are matched sets so I wouldn’t do it in your case. When wiring options allow it many people leave the pre-amp heaters connected to the original PT and run the power tubes from the new auxiliary transformer.

…I could bolt it onto the chassis, run the power lines to the same place as the PT and the 6.3V lines to the same joints where the PT filament lines are soldered in, no? .

Yes, keeping in mind the comments above.

Are there any disadvantages or caveats to this idea?

Pros:..1) Original PT runs cooler 2) You get to keep experimenting.

Cons / caveats: 1) Need to mount the new transformer where it will not cause other problems such as induced hum. (Keep it away from the pre-amp circuits) 2) You will not have improved the light duty design problems associated with the PC board heater circuit so the PCB overheating may continue unless you rearrange the 6.3V feed to the tubes. Since you are using this amp as a learning platform that’s a possibility. You could also move the fuse off the PCB by chassis mounting a better fuse holder.

Those are my thoughts for now.
Tom

[JBeast]

All of the tubes are running off of the same 6V transformer winding but the driver preamp tube V1 and V2 run off of DC - diverted and rectified from the same AC line, the rest of the tubes are running off of AC directly from the PT heater winding. So, correct me if I am wrong: If I want to run just the AC filaments from the supplemental tranny I would need to cut the filament traces leading to the AC fed tubes (Power + PI + Ch2 tone stack driver) and leave the other two (V1 + V2) in tact? In some ways it would probably be easier, albeit 2x the cost for the transformer, to buy the 6.3V, 8A-10A transformer (as long as it will fit inside the chassis), cut and cap the existing filament leads from the PT and utilize the supplemental transformer exclusively for all of the heater connections utilizing the existing filament posts on the board. This way I would eliminate the complications related to parallel heater wiring, cutting traces, etc. I can always hard-wire some teflon wire to those solder joints on the power tube board to bypass the pin header and as you recommended, attach a better fuse holder to the chassis or heat sink of some kind and wire that to the existing posts.

[Enzo]

See the right fuse clip is all bright and shiny? That is a good one. The left one is all dull and gray - it has been VERY hot and is ruined, replace it. What happens is a lot of currenrt flows through, if it makes poor contact, it gets resistive, which heats it up, which makes it worse, and it spirals down to death.


The true specs for the transformer are that it was designed to do what it was designed to do. The test is simply to plug larger tubes in and see if it holds voltage or sags away. Yours didn't hold its voltage. I don't need a spec sheet to know the transformer failed this test.

"How much weight can you lift?
"I don't know... ERRRNNK...but I can't lift that piano."

You bottom photo looks like a row of ribbon connector pins. I'd resolder them just because. I would worry more about the female pins up on the ribbon itself, any brown there?

When Bob at Eurotubes sells you his integrated quad of EL34 and 6L6, he is asserting he has selected them so they both run well on the same BIAS voltages. I assume they will. But that says nothing about whether a particular amplifier power supply can handle the increased heater current. I think that is back to the original misconception that bias and heater were related.

You could mount an extra heater transformer. What I would do for that would be to leave the existing wiring alone, and let it continue to drive the preamp - both AC and DC heaters. The power tubes are in pairs with ribbons back to the main board. I'd pull the wires off the heater supply wires on the ribbons and use my auxilliary tranny for only the power tubes. About 6A total draw, a 10A transformer ought to be ideal. Center tap to chassis. There would be no parallel with the old heater supply, no heater connection in common at all.

[JBeast]

Oh man, replacing that clip is gonna be such a pain in the arse, hahaha.
Removing the board in this thing means disassembling the rear and front plates of the chassis including all of the knobs, jacks, pots, wiring, etc.
The pcb is so thin I'm always concerned it will snap in half, shoot me now, Argh!!!

Yes, those solder joints are on 13 pin headers - male to female - no ribbons, the male pins on the power tube component board just mate directly to the female header on the main-board. Is that still considered a ribbon connector if it's just pins and header? I'll flow some new solder on those for sure. That's a great idea to pull those filament pins on the headers and mount the filament lines for the new filament tranny directly to the power tube board. I was getting complicated in my head thinking I had to isolate the AC section from the DC and cut traces etc. Then again I'm one of those people who makes math harder than it is by over-thinking it, hahaha.

I have space on the chassis next to the PT for a 10A transformer but it will be mounted directly over top of the filter caps, is that a problem or potential hum issue?
What if I run the filament wires through the same chassis hole as the PT filament wires, will that be an issue with regards to added noise?

The dimensions of those 6.3/10A Hammond transformers are too large to fit inside the chassis, I only have 2 1/8" height/depth clearance inside the chassis to fit a transformer but I have plenty of space on top.
This is where a toroidal transformer would be ideal (to fit inside the chassis) but I'm not finding any toroidal filament transformers just toroidal PT's.

[Enzo]

OK, header then, no, not a ribbon without the ribbon, just an inline connector. Solder it anyway, whatever we call it.

A filament transformer IS a power transformer, just a single voltage one.

I been servicing Crate amps professionally over 25 years, and have yet to break a pc board yet. I don't sit on them, or grab them with a wrench.

Then again I'm one of those people who makes math harder than it is by over-thinking it, hahaha.

transformer but it will be mounted directly over top of the filter caps, is that a problem or potential hum issue?

What if I run the filament wires through the same chassis hole as the PT filament wires, will that be an issue with regards to added noise?

You are still doing it, too.

FIlter caps are hardly sensitive to radiated fields, they are usually closer to power transformers than most of the rest of an amp. Same with heater supply wires. You are not proposing that a pair of wires carrying 6vAC is going to pick up 6vAC from the wires next door, are you? Signal leads that are going to an amplifying stage can pick up noise, that is why they should cross AC suppy wires at a right angle where possible, and certainly do not put 6v heater wires parallel with signal wires when crossing the chassis. But the impedance of that 6v winding is so incredibly low, nothing is going to get picked up by it, and has little place to go if it got there.

So, stick it out on top. I see no advantage to packing it inside, especially if room is tight.

[JBeast]

OK, header then, no, not a ribbon without the ribbon, just an inline connector. Solder it anyway, whatever we call it.

Inline connector, ribbon connector, duck billed platypus, whatever - Solder it - Done!

A filament transformer IS a power transformer, just a single voltage one.

Got it.

I been servicing Crate amps professionally over 25 years, and have yet to break a pc board yet. I don't sit on them, or grab them with a wrench.

I'll quit whining now and put down the vice grips (kidding about the vice grips)

You are still doing it, too.

Old habits die hard

FIlter caps are hardly sensitive to radiated fields, they are usually closer to power transformers than most of the rest of an amp. Same with heater supply wires. You are not proposing that a pair of wires carrying 6vAC is going to pick up 6vAC from the wires next door, are you? Signal leads that are going to an amplifying stage can pick up noise, that is why they should cross AC suppy wires at a right angle where possible, and certainly do not put 6v heater wires parallel with signal wires when crossing the chassis. But the impedance of that 6v winding is so incredibly low, nothing is going to get picked up by it, and has little place to go if it got there.

So, keep all power lines away from the amplified output lines and cross @ 90deg whenever necessary, otherwise, fuggetaboutit!

So, stick it out on top. I see no advantage to packing it inside, especially if room is tight.

I'll order that today. It's cheaper than buying a new set of tubes and in the long run I'll have gained something from the experience besides merely taking this thing apart again (argh - under my breath).
Thanks again Enzo, as usual your advice has been succinct and valuable, I appreciate it!