Filament transformer for 8xkt88

[Helmholtz]

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?

No link showing.

So you have something like a 400W attenuator (wow)?

Heater voltage looks good. Weak heater supply cannot explain the power tube failures. Makes me wonder about the designer's competence.

Did you check idle currents? Are tubes redplating at medium to high power?

Do you have a schematic?
Can you post pictures?

There's a possibility that power tube bulb temperature limits ( 250°C) are exceeded in this amp.
In this case you need to install a fan.

[g1]

No link showing.

Here: https://music-electronics-forum.com/...100-clone-woes

[Helmholtz]

Thanks.

That old thread is confusing. I don't feel like reading it through completely.
Just post all relevant info here, like OT specs, number of tubes used, idle currents etc..

A pair of KT88s can deliver up to 75W at a loaded plate voltage of 450V, depending on OT primary impedance.
So a 250W OT looks underrated.

Also you can't just pull output tubes without changing the load impedance.

If the output tubes see too low a load impedance, plate dissipation with signal will increase.
OTOH, if load impedance is too high, the screens will suffer.

Need to know the rated primary impedance of the OT.

Is there a schematic?

[Mr_bibbles]

Thanks.

That old thread is confusing. I don't feel like reading it through completely.
Just post all relevant info here, like OT specs, number of tubes used, idle currents etc..

A pair of KT88s can deliver up to 75W at a loaded plate voltage of 450V, depending on OT primary impedance.
So a 250W OT looks underrated.

Also you can't just pull output tubes without changing the load impedance.

If the output tubes see too low a load impedance, plate dissipation with signal will increase.
OTOH, if load impedance is too high, the screens will suffer.

Need to know the rated primary impedance of the OT.

Is there a schematic?

So this looks very promising in terms of diagnosing the issue; the OT is indeed 250W, it's a heyboer HTS-12541. If KT-88's are producing 75W *per tube* than that means even with 6 installed we are maxing at 450W? And with 8 it's 600W?! Would that produce flyback voltage capable of repeatedly de-vacumizing a tube?

Some basic information

8 jj kt88s
I.5k, screens 2.2k grids.
Pt500 heyboer
250w OT heyboer
The amplifier has switchable impedance at 4/8/16.


No tubes installed voltages:
Plate: 496 VDC
Screen: 396 VDC
Grid: -47 VDC

[Helmholtz]

No it's up to 75W per pair. That makes 300W with 8 tubes.
But only if B+ is at least 450V measured at full output and if the OT primary impedance is properly chosen (around 1.1k with 8 tubes).
Or is this a UL OT?

Lower B+ and higher primary impedance will result in less output.

This said, an underrated OT is not likely to cause power tubes to draw air - unless if has a problem and tends to arc at high power.
How many tube failures were recorded?
How do tube sockets look?

(That corrupted link is really annoying.)

[g1]

(That corrupted link is really annoying.)

Yes it is. This is the second thread it has happened to. Only pg.2 has the corrupt links in both threads. (the recent 5150EVH thread was the other)

[The Dude]

......(That corrupted link is really annoying.)

Yes it is. As said, this is the second one (that I know of on the site). I sure hope this doesn't become a constant thing here.

[g1]

Boss if you click any of the post #'s on this page, it will redirect to another thread. Same issue as in pg.2 of the EVH5150 thread ( https://music-electronics-forum.com/...evh-fuse-f3-f4 )

[Mr_bibbles]

No it's up to 75W per pair. That makes 300W with 8 tubes.
But only if B+ is at least 450V measured at full output and if the OT primary impedance is properly chosen (around 1.1k with 8 tubes).
Or is this a UL OT?

Lower B+ and higher primary impedance will result in less output.

This said, an underrated OT is not likely to cause power tubes to draw air - unless if has a problem and tends to arc at high power.
How many tube failures were recorded?
How do tube sockets look?

(That corrupted link is really annoying.)

1. There have been 3 total tube failures I Believe; one before I saw it and 2 after doing some changes.
2. It WASN'T the same socket each time.
3. It had arced once on the TUBE SIDE of the socket, but I replaced that one so it was fresh.
4. The arcing was from Plate to Heater

I was reading over the old thread and saw this:

One thing to look out for with KT88 tubes is how pin 1 is connected. With many tubes this connects to the shell - even if the spec sheet doesn't show it or refers to it as NC. It's best to measure and check to find out. Many custom builders will then use pin 1 as a tie point for the grid stopper. So, now all the tube shells are at bias potential and there's a metal ring hanging off each grid. I don't have any evidence to support this, but my conjecture is that this can cause stability issues. I've had mysterious faults where removing the grid circuit off the shells has resolved problems. My fix is to install new grid stoppers without tying them to pin 1 and then connect this pin to ground.

Another issue with having the shells connected to the grids is that anything that shorts any one of the shells to ground immediately kills the bias to all tubes. A spring retainer could make contact with the shell, for example. Ok, it kills the drive as well but the tubes will very quickly red plate.

In this case, the amplifier has 1 Ohm resistors to ground attached to pin 1. They all measure within spec, but is it possible this is interacting with the base sleeve in an unintended way?

[catalin gramada]

Why bias check resistor attached to pin 1? Put the bias resistor in the tube cathode (read pin8), pin1 should run directly to the ground and nowhere else for kt88/6550 tubes. I just heard a guy striked by a tube shell just because 1&8 was tied together and an open cathode resistor leaved the base shell at full HT potential in open circuit. That's happen in an Orange AD200 designed perhaps by stupid kids. Please don't mess with that !

[g1]

Boss if you click any of the post #'s on this page, it will redirect to another thread. Same issue as in pg.2 of the EVH5150 thread ( https://music-electronics-forum.com/...evh-fuse-f3-f4 )

Problem fixed. I hope globally. Thanks Boss .

[g1]

In this case, the amplifier has 1 Ohm resistors to ground attached to pin 1. They all measure within spec, but is it possible this is interacting with the base sleeve in an unintended way?

It's not impossible.
I would for sure set it up like Catelin outlined in post #25 above. Your earlier drawing showed pins1&8 tied together, I think that is bad with these and 6550 tubes.

[Mr_bibbles]

It's not impossible.
I would for sure set it up like Catelin outlined in post #25 above. Your earlier drawing showed pins1&8 tied together, I think that is bad with these and 6550 tubes.

Eep, Pin 1 and 8 ARE tied together with a single bias-measuring 1 Ohm resistor leading both pins to ground.... should pin 8 be tied to ground through the 1 Ohm Resistor, and then pin 8 to ground directly? Wouldn't the difference in impedance between the two pins be very minimal after this change? I'd like to understand this route of thinking better.

Again, all of these resistors are measuring well.

[catalin gramada]

Is nothing here to understand. Just follow the pin chart instruction: pin8 is cathode pin1 is base shield. There is no reason to tie it together other than get in trouble. So, pin8-bias resistor-ground / pin1-solid wire-ground.

[Mr_bibbles]

Is nothing here to understand. Just follow the pin chart instruction: pin8 is cathode pin1 is base shield. There is no reason to tie it together other than get in trouble. So, pin8-bias resistor-ground / pin1-solid wire-ground.

Thanks Catalin, I will do this!

...but isn't there a technical explanation as to why having the base connected through a 1 ohm resistor could cause de-vaccumized tubes? We know that it IS wrong, but I would like to know specifically which electronic repercussions are occurring... From what it sounds like, there is potential being stored in the base shell which is arcing to the chassis, causing a break in the tube through damage to the base?

Could this be exacerbated by an OT that is underrated?