Filament transformer for 8xkt88

[Mr_bibbles]

Hello all,

I have a one-off amp that has never worked properly. After a lot of digging we’ve found that the power transformer does not supply enough filament voltage, so the easiest solution seems to be the installation of a standalone transformer.

There are 8 (!) KT88’s in the amplifier, so if they draw 1.6A of heater current I would need something rated at 6.3V, 13A or higher, right?


Would there be any issues running both the filament transformer AND powering preamp tubes off the original PT’s filament voltage?

[g1]

Correct and no issues with still running preamp tubes off stock PT.
Do you know what the heater winding spec. is on the stock PT? Would it power the KT88's so you could power the preamp tubes with a smaller filament TX for the pre-tubes?

[catalin gramada]

A 13A winding will not be a practical one, more using a small iron core as 80VA transformer is. Better 2x6,5A secondary windings or 4x 4A.

[Mr_bibbles]

Correct and no issues with still running preamp tubes off stock PT.
Do you know what the heater winding spec. is on the stock PT? Would it power the KT88's so you could power the preamp tubes with a smaller filament TX for the pre-tubes?

That's a great question and Idea, I will have to look at it when back in the shop and see what the math says.

A 13A winding will not be a practical one, more using a small iron core as 80VA transformer is. Better 2x6,5A secondary windings or 4x 4A.

Hmm, not exactly sure what you are saying here. Are you saying 13A is a rare value, so better to go overvalue so I can use a more standardized part? Would love to hear a more detailed explanation.

[g1]

I'm not sure if he meant the size would be a problem, or that they are uncommon. But they are available though not cheap. Hammond 167U6 is 6.3V 16A. I would think other manufacturers also make them that big.

The reason I was asking about the heater capacity of the existing unit was that it must have something at least close if it is working at all. If the designer had any sense at all it must have a pretty hefty heater winding. Or some kind of series parallel arrangement of the heaters. If it is close, then maybe you would get away with just moving the pre-tubes off it, which would save expense and weight.
I guess you could take some voltage measurements with it running and see how low the heaters are, and what happens to that voltage if you remove the preamp tube heaters. Also how hot the power transformer gets running the heaters.

[The Dude]

What is the part number of the PT that is in the unit now and aside from the KT88's how many preamp tubes?

[mozz]

I have a monster filament transformer here, let me clear a path to that shelf and will get you the specs.

[Mr_bibbles]

What is the part number of the PT that is in the unit now and aside from the KT88's how many preamp tubes?

The part number is HTS-11415-3, it's a heyboer transformer I've also seen labeled as "pt-500". I haven't been able to find any specifications beyond the wiring diagram which just shows "3.15 / 0 / 3.15 so not much help there.... the designer told me it was designed for 15A but I am unable to verify. I sent an email to Heyboer a while ago and was not able to get a response.

There are 3 preamp tubes in it, so that means an additional .9A of current for a total of 13.7A.... So if this was toeing the line, taking an additional .9A off the PT load and adding a small 1.5A filiment transformer might be the better option?

[g1]

There are 3 preamp tubes in it, so that means an additional .9A of current for a total of 13.7A.... So if this was toeing the line, taking an additional .9A off the PT load and adding a small 1.5A filiment transformer might be the better option?

You said in the first post that you verified that the heater voltage is not holding up. That would have required some measurement. How ever you did it, do it again with and without the preamp tubes.

[Mr_bibbles]

You said in the first post that you verified that the heater voltage is not holding up. That would have required some measurement. How ever you did it, do it again with and without the preamp tubes.

Eek so I have been going back and forth with the designer and he is the one who said he thought it was under-rated, but I had done some reading and it seems like the hunch would match up with the symptoms. I have actually not personally confirmed this but I would love to learn how. The Forum actually went through this amp a while back and eliminated tons of possible culprits, I've attached a link to the thread about the same amp that eats tubes...

Since the Heyboer spec sheet I got isn't helpful, is there a way to measure the maximum amperage delivered by the transformer? We know that we can calculate the draw by the tubes alone, but the Amperage rating is not so straightforward I imagine.

[Helmholtz]

So what is the heater voltage with all tubes in?

[g1]

is there a way to measure the maximum amperage delivered by the transformer? We know that we can calculate the draw by the tubes alone, but the Amperage rating is not so straightforward I imagine.

As Helmholtz stated, measure the heater voltage with all tubes in. If current capacity of heater winding is insufficient, the heater voltage will drop down to unworkable levels.
If heater voltage holds up, you can forget about the power transformer and look for other causes of the amplifier problem.

[Mr_bibbles]

As Helmholtz stated, measure the heater voltage with all tubes in. If current capacity of heater winding is insufficient, the heater voltage will drop down to unworkable levels.
If heater voltage holds up, you can forget about the power transformer and look for other causes of the amplifier problem.

This is great advice, I'll do this today. Would this show up at idle though? The amplifier is only failing under stressed conditions, once it is turned up to practice level by what I imagine is a pretty loud bassist....

EDIT: Errr, heater draw doesn't change with signal, does it...... so idle / usage wouldn't matter.

[Helmholtz]

Measurement at idle will tell..

Do you have a suitable dummy load?

How exactly does the amp fail?

[Mr_bibbles]

Measurement at idle will tell..

Do you have a suitable dummy load?

How exactly does the amp fail?

I do have an attenuator I use as a dummy load. There is a pretty detailed thread about this amplifiers history (and a bunch of attempted fixes) linked below, but the main symptom is that it will cause a tube to fail when played at louder levels. The tube will ALWAYS de-vaccumize with the characteristic white marks around the upper cap of the tube.

HEATER VOLTAGE: it looks like with all tubes installed at idle, it is running at 6.33 off the PT leads and 6.25 at the very end of the KT88 line. Seems like I was wrong again?