Blackstar series 1 200 Dropping ht voltages on output tubes

[Nicale19]

Hi guys, I bought this used amp from a guy
It is a first series built in Korea
The amplifier has an anode voltage on pin 3 of the kt88 of about 690v.
It has burned a tube and tends to "ignite" the other tubes when switched on
Basically it does not have a standby, like other blackstars, but when the power switch starts, the tube immediately receive all voltages
Yesterday, to avoid "burning" starts, I used a variac.. And somehow the tubes didn't get glowing in a bad way.. Bringing the tension to the right, then everything was ok
Suurely the tubes need to be changed, But before I do that I would like to ask you if you have any suggestions on how to lower the voltage a bit, if there is a way for a soft start.

Any advice or experience is welcome

[Nicale19]

Pcb 2

[AlNewman]

It has burned a tube and tends to "ignite" the other tubes when switched on.

Suurely the tubes need to be changed, But before I do that I would like to ask you if you have any suggestions on how to lower the voltage a bit, if there is a way for a soft start.

Yes.  Use a light bulb limiter.

[shooter]

Any advice or experience is welcome

a digital amp that complex requires a schematic, please post yours, otherwise it's like shooting ducks inside your home, blind, with 12 kids n a wife running around just to make it fun.

[jjasilli]

Please excuse this rant, but I suspect amps like Blackstar are not meant to last (longer than the warranty period), or to be repaired.  Yes they sound good; but it's the build quality. KT 88's drawing high current, mounted on PC boards, probably with thin traces, and cement (bathtub) power resistors sitting on the board, is not a proper recipe for long life.  An amp like that with proper build quality would likely cost at least 2x as much new. 

[Nicale19]

Please excuse this rant, but I suspect amps like Blackstar are not meant to last (longer than the warranty period), or to be repaired.  Yes they sound good; but it's the build quality. KT 88's drawing high current, mounted on PC boards, probably with thin traces, and cement (bathtub) power resistors sitting on the board, is not a proper recipe for long life.  An amp like that with proper build quality would likely cost at least 2x as much new.

Agree

It's a Chinese quality (no offense, Chinese build for what you pay), In the guise of a valuable product.
From an engineering point of view it is quite refined, but from the point of view of quality, we are on a sufficient/mediocre level.

[rake]

Want a run around? Try dealing with Blackstar. One of my customers bought a HTStage60 212 MK2 and after 6 mos the clean channel crapped out.
It went to their US repair center for over a year. He finally complained and they gave him his amp back still broken. He contacted Blackstar directly and
was told we can't help you, it's now out of warranty! 

When I got involved Blackstar told me to piss off and go thru the US dealer for parts (Korg USA) They said "we just sell them"! They did give me a link to a site
that had some parts. Luckily they had a main board on sale for the 112. Not the same but close enough to rob all the effected parts needed.
here's the link:

https://www.partsisparts.net/catalog/blackstar-parts

[Nicale19]

Want a run around? Try dealing with Blackstar. One of my customers bought a HTStage60 212 MK2 and after 6 mos the clean channel crapped out.
It went to their US repair center for over a year. He finally complained and they gave him his amp back still broken. He contacted Blackstar directly and
was told we can't help you, it's now out of warranty! 

When I got involved Blackstar told me to piss off and go thru the US dealer for parts (Korg USA) They said "we just sell them"! They did give me a link to a site
that had some parts. Luckily they had a main board on sale for the 112. Not the same but close enough to rob all the effected parts needed.
here's the link:

https://www.partsisparts.net/catalog/blackstar-parts

Same treatment

Useless and uncooperative assistance.

I think they are not sailing in good waters and trying to cover up Mud
Their old forum has been disabled due to excessive complaints

Ps, out of curiosity, what exactly had broken?

[AlNewman]

Well, if they won't give you a schematic...  The pictures you shared look pretty clean, no signs of heat damage, or damaged parts.

2.  You could trace the circuit and build a schematic so us dummies could understand the amp.

You could go through with a dmm and test all resistances compared to the ribbons on the resistors, and see if something's out of spec.

You could take voltages at all pins of the tubes, as well as all voltages on the power rail, and mark them at your schematic you built at point #2.

You just looked for point #2.

When in doubt, refer to points 1, 3, and 4.

[Nicale19]

Does anyone know any method to lower b+?

[mresistor]

yes   get a 50W B suffix chassis mount zener diode   if the amp has a center tap on the high voltage secondary winding tie it to the zener. You can find them in various voltages to drop however much voltage you want to drop. If no Center tap on the HV winding then that's a different story.
Can yuou post photos of the HV secondary winiding? What kind of recitification is used in this amp?

https://www.newark.com/solid-state/1n3336b/zener-diode-50w-68v-do-5/dp/10P4952   as an example 

What is under that large heat sink?

[jjasilli]

Just blundered into this: 

[jjasilli]

There are other ways to drop B+, which possibly can be used in combination.  Outboard Variac; brown box; or bucking transformer.  Internal bucking transformer; dropping resistor.

[Nicale19]

There are other ways to drop B+, which possibly can be used in combination.  Outboard Variac; brown box; or bucking transformer.  Internal bucking transformer; dropping resistor.

Damping transformer?

[jjasilli]

A damping transformer is a transformer that has been designed to reduce vibrations, noise, or inrush currents.

A bucking transformer reduces voltage.  Search this forum.

[Nicale19]

Thanks for the answer, I'm looking for information to reason a bit.
In the meantime, I received some new tubes and I carried out tests that I had previously done without tubes, that is to connect two 3k3 25w resistors, each on an ac branch that goes to the diode bridge that then brings voltage to the kt88
The marquee, without tubes, dropped to 560/570v from the original 700v
The problem is that with the tubes mounted, the sound comes and goes, as if there isn't enough power to push the power amp
What happens, technically in your opinion?

[SEL49]

connect two 3k3 25w resistors, each on an ac branch that goes to the diode bridge that then brings voltage to the kt88

3.3Ks are unreasonable. They will cause way too much sag on the B+ rail.

[Nicale19]

connect two 3k3 25w resistors, each on an ac branch that goes to the diode bridge that then brings voltage to the kt88

3.3Ks are unreasonable. They will cause way too much sag on the B+ rail.

To lower by about 100/150v, those were the values
With about 500ohms, nothing happened.

In any case, I solved it with a danping transformer with double outputs from 20 to 60v.
Set to 0/50v, I drop to about 540v
In un are in variac mode.. Substantially !

[AlNewman]

Think about idle current at your KT88's.  Fixed bias@ 60% @ 500V is 42mA/tube, so 82mA over 1600R in parallel.  (Just plate dissipation, disregarding screen and preamp draw.)

That's 131V drop at B+.

Now when you push your amp, the current at the KT88's will go up considerably.  That's increased current over the (fixed) 1600R resistance dropping voltage between your PT and B+ rail.  Sooner or later, the current will pull down the voltage to cutoff, which likely explains the cutting in and out.  You were literally starving the tubes of voltage.  And I bet those resistors were getting pretty toasty as well.

[kagliostro]

In un are in variac mode.. Substantially !

In Bucking way I'll say

Franco