Help: 6550 / KT 88 powertubes

[Basvarken]

My Orange AD200 needs new powertubes.

This would be a good chance to see if I can get a slightly different sound out of this amp.
It now has a pair of Tung Sol tubes, and a pair of General Electric tubes.
I've always found the amp puts out too strong sub lows, and not enough highs.

I was thinking about buying JJs.

Any opinions or tips?

[SKATE RAT]

thats what everybody keeps recomending to me

[Highlander]

Where can you find a "Psycho" when you need one...? 

[Basvarken]

Where can you find a "Psycho" when you need one...? 

Que?

[OldManC]

Que?

Psycho Bass Guy. He can point you in any direction you need to go regarding tubes and tube amps.

[Psycho Bass Guy]

My Orange AD200 needs new powertubes.

What makes you say that?

This would be a good chance to see if I can get a slightly different sound out of this amp.
It now has a pair of Tung Sol tubes, and a pair of General Electric tubes.
I've always found the amp puts out too strong sub lows, and not enough highs.

Depending on how they're arranged and biased, your sound may simply be suffering from phase cancellation. GE's are known for their midrange snarl, as are vintage Tungsols. Were they installed by a tech who biased and checked the output on a scope to see that it's symmetrical?

[Highlander]

AAAAAARRRGHHHHHHHHH..... BIG SCARY WORDS..... 

[Basvarken]

What makes you say that?

Well, I've been noticing some power loss lately. The amp goes into overdrive faster than I'm used too. And I have to crank it up more to keep up with the rest of the band.
So I asked our guitarist's dad who is a retired airforce radio technician. He checked the tubes with a tester

and came up with the following results

0 - 40: replace
40 - 50: questionmark
50 - 100: good

Test results:
Tube I Tungsol:                    replace              20
Tube II  General Electric:    questionmark     45 going back to 42
Tube III  General Electric:    good               90
Tube IIII Tungsol:       good           90

and

Buis I   Vg2 = 392V   VRg2 = 0,04V (Rg2 = 1KOhm)
Buis II   Vg2 = 392V   VRg2 = 0,07V (Rg2 = 1KOhm)
Buis III   Vg2 = 392V   VRg2 = 0,40V (Rg2 = 1KOhm)   
Buis IIII   Vg2 = 392V   VRg2 = 0,90V (Rg2 = 1KOhm)

Depending on how they're arranged and biased, your sound may simply be suffering from phase cancellation. GE's are known for their midrange snarl, as are vintage Tungsols. Were they installed by a tech who biased and checked the output on a scope to see that it's symmetrical?

Yes that is quite peculiar to have two different brands. That's how I bought it (second hand of course). However the amp had been serviced by a professional before it was sold to me.
To get the amp in better balance, this means balancing the positive and negative phase of the signal in the power amp part, Frank's dad switched tube II and III.
But he advised me to go look for new tubes.


@ Kenny: shut up for once will ya

[Highlander]

(                                                                                                                                                     ) 

[Psycho Bass Guy]

Well, I've been noticing some power loss lately. The amp goes into overdrive faster than I'm used too. And I have to crank it up more to keep up with the rest of the band.

Given what you say, weak tubes certainly agrees with the data, BUT I have some questions, because it looks like the biasing is seriously screwed up, and you'll just kill another set of tubes until it's fixed. Did you notice that on the weak tubes, the bias voltage is very low? That's not a symptom of the tube going bad; it's most likely the cause. Your sound has been boomy because those tubes have been in near runaway and have been burning up and putting out a shitload more current than they're supposed to. I'm quite surprised you haven't blown the mains fuse, and it's a testament to the toughness of those tubes that they still work at all.

So I asked our guitarist's dad who is a retired airforce radio technician. He checked the tubes with a tester

That's a good step one, but to know what's going on in the amp, you really need to check its output on an oscilliscope while taking voltage measurements. One reason is that the tester is nowhere near the voltage of your amp.  The screen voltage of 392 sounds pretty high, but I'd guess and say your Orange probably puts at least somewhere in the neighborhood of 500+ volts on the screens and 650+ volts on the plates and that does make a difference in emission.

Yes that is quite peculiar to have two different brands.

It used to be common when tubes were more common and production tolerances were tighter. Current production tubes can be as electrically strong in terms of emission, but things like internal structure geometry, cathode winding, and general construction techniques have been lost and new tube manufacturers are having to re-figure them out because most of the people who knew how to do those things are dead, and tube QC varies widely by manufacturer.  Ei is an example of a factory that never stopped making tubes from when they were common, but still went under about three years ago because it lost its ability to do good QC and sold a lot of crap, and it was an old Philips factory that was formerly famous for quality!

That's how I bought it (second hand of course). However the amp had been serviced by a professional before it was sold to me.

It's quite possible some part of the bias circuit has failed since then. My Eden VT300 was bought brand new but after a year of constant use had its bias adjustment pot go bad, which had I not caught what was occurring, would have killed half of my output tubes. You may have a similar situation.

To get the amp in better balance, this means balancing the positive and negative phase of the signal in the power amp part, Frank's dad switched tube II and III.
But he advised me to go look for new tubes.

I'd buy a matched quad of JJ KT88's for tone and toughness, and have a good tech install them and check the amp.

[Basvarken]

Thank you.
Frank's dad did a whole lot of testing. I only copied some parts of it in the previous post.

I did blow a fuse recently Which was actually the main reason why I had him check the amp.
We really couldn't figure out why the fuse had blown.
But he did tell me two out of four tubes weren't good.

So are you saying the bad biasing might be the cause of all these problems?

[Psycho Bass Guy]

So are you saying the bad biasing might be the cause of all these problems?

Not necessarily bad biasing, but more likely, a failure in the bias circuitry. Do you have a schematic for the version you have? There are a few different ones, and there may be some significant differences in the designs.

[Psycho Bass Guy]

Frank's dad did a whole lot of testing. I only copied some parts of it in the previous post.

If you don't care, I'd like to see it.

I did blow a fuse recently Which was actually the main reason why I had him check the amp.
We really couldn't figure out why the fuse had blown.

You really need to get someone who knows tube amps then. Unless he worked on radar, and it sounds like he didn't, he can only perform cursory checks. Your fuse blew because half of your output tube were in runaway and pulling more current than the fuse could handle. That's output stage emission 101.

BTW, I hope you're not still using the amp, because you would be risking even more damage.

[n!k]

I personally use JJ KT-88s in my Sunn Model T (along with a complement of JJ's ECC83 and ECC81 tubes in the preamp. I think they sound incredible. Very clear, rumbling bass and plenty of overdrive heat when you push them.

[Basvarken]

If you don't care, I'd like to see it.

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