Amp Techs>>need Help!! (1959 SLP Revisited)

[Mooncusser]

Hi guys,


I need to posta follow up on a help request I started last week.

I have a 200 Marshall 1959 SLP RI (no FX loop)

I just had an new quad of KT-66's installed some moths ago.

This is the original post:


"I need a hand or some opinions...

I have a Marshall 1959 SLP RI (no FX loop).

Tonight I turned it on as usual, power on, standby on.

Let a few minutes go by, getting myself ready for practice. I flip the stand- by to on. I hear a loud pop.....but the led's on the amp are still lighting up as if nothing happened. As soon as the pop occured I noticed that the usual slow hiss that occurs when you flip the standby wasn't audible. Everything is on and working, but ZERO sound out of the amp.

I shut it down, and check both the HT and mains fuse.
Mains fuse is fine, HT fuse blown.

What happened here and is it possible this may have just meant the HT fuse blew and needs a new one, or is there and indicative cause?

(praying it could just be the fuse.......!)

Thank you in advance for all of your opinions and help,
Mass"

So, 99% of you thought it was a bad power tube. I obtained a new power tube and before installing it I installed a new HT fuse as suggested, and watched for the electric arc that would indicate the bad tube. I did so, and man, the#2 Power tube did an lightning dance and blew the HT again. So I said..great!! exactly what the boys thought. I installed another HT fuse, and installed the new power tube that I recieved and the amp came on and hissed as normal. I was glad and powered down looking forward to playing tonight. So tonight when I powered up..mains on/ standby off I let a few minutes go by and flipped on the standby to begin playing and the HT fuse blew again! Only this time the #3 power tube had a lightning dance inside of it.


Can anyone help me out with this?

[jezzzz2003]

Hmm, how old are your old tubes?
You really should change them all to a matched quartet.
I really hopr this is just coincidence,
Id check your screen grid resistors, It's possible for them to self destruct in the case of a bad power tube.

[John Phillips]

Sounds like another bad tube :(.

While not impossible, it's not very likely to be an amp fault causing it since it would most likely blow tubes on the same side. Arcing is not normally caused by an amp fault either - more usually it's a bias runaway (tubes redplate then die).

The most usual cause of arcing other than bad tubes is a bad connection in the speaker circuit causing an intermittent no-load, but that's only likely to happen when the amp is cranked, not when turning the standby on.

It is possible that the amp is self-oscillating due to a fault in the output circuit though.

Given that the set is compromised anyway now, I would remove the tube that just started arcing and the new one, and run the amp again with just the two survivors from the original set. If it blows another, I think you have a problem with the amp...

"Once is happenstance, twice is coincidence, three times is enemy action."

[jezzzz2003]

Whoops, I just saw, a few months ago ey, they should still be ok,
check those screen grid resistors. it's a shame for such a great amp to be down for any amount of time.

[Mooncusser]

OK guys..I'll try that (pulling the #3 and #4 tubes) ..if the #2and #3 tubes are bad, BUT the #1 and #4 if good, then it should still operate because of the push pull circuit...do I understand that correctly?
Thanks...
Also. If you guys think it is possibly the screen resistors where are they and how do you test them.
I am very proficient at electrical application and tsting equipment so dont worry there. My job demands high voltage diagnostics/ wiring and rewiring.
God I hope it's something easy, because I think it would be a while before I could get it to am amp tech.
Thanks, any other suggestions...shoot them at me.
Take care,
Mas

[jezzzz2003]

Cool mate,
Im thinking the resistors might be o.k because the trouble you have now is on the other side,
in any case, theyre the ceramic white things inside your amp, they'll be on your power tube valve bases, probarbly a 5w on each base,
just measure them with your meter set to ohms.
Im no tech so hopefully someone else might be able to chime in and tell you what to look for.
Good luck and may the force be with you.

[Mooncusser]

OK,

I'll try all the suggestions once again.
The amp is biased at a safe 26MA, the quad is a new and tightly matched set.
Let me ask a question...
What are the chances of getting two bad tubes out of a new matched quad?
They are Valve Art KT-66's BTW.
Thanks again for your help,
Mass

[John Phillips]

The amp is biased at a safe 26MA, the quad is a new and tightly matched set.
Let me ask a question...
What are the chances of getting two bad tubes out of a new matched quad?
They are Valve Art KT-66's BTW.
Fairly high, with those tubes. It's really pushing them in that amp too, with the plate voltage as high as it is - it's not the dissipation that's the problem, it's the voltage.

[Mooncusser]

Fairly high, with those tubes. It's really pushing them in that amp too, with the plate voltage as high as it is - it's not the dissipation that's the problem, it's the voltage.

Well, if the KT-66's are vulnerable in the 1959 SLP, maybe the 6L6GC or 6550's would be more in order to avoid the continual popping of tubes. I am going to try out the outer two in a minute or so here and then we will know if it is just a case of the two inside tubes gone bad.
If I power it up with the two outside tubes in and it works fine, could we be safe just to guestimate the two inside tubes were bad?
Generally, when tubes are bad, or they popped due to voltage, is it common for the two corresponding screen resistors to blow out as well, or (hopefully) is that not the case? Ill be back with a report on the operating status in a few minutes.
Thanks so much,
Mass

[Swarty]

I wonder if we're dealing with a flyback issue due to the different plate resistance between EL34 & KT66? If so, turning the output impedence down a notch may be the solution.

[Mooncusser]

I wonder if we're dealing with a flyback issue due to the different plate resistance between EL34 & KT66? If so, turning the output impedence down a notch may be the solution.

To 8 ohms?........this was suggested to me, but it was posed as a suggestion for tone performance not to facilitate the amp/tube voltage issue. If so, please fill me in..I am all ears here.:AOK
Thanks,
Mass

[drfrankencopter]

How about pulling all the power tubes and trying again....just to rule out a problem with the power transformer, or filter caps.

If the fuse still blows with the power tubes pulled, then I'd say that the blown tubes are a smyptom and not a cause.

Cheers,

Kris

[scopeboy]

I honestly do think that these amps will eat Chinese tubes for breakfast. I don't know why Marshall chose that tube type, when it's a faithful reissue of an amp design that Hendrix's techs had to retube with 6550s to make it last for a whole show.

It would IMO be well worth trying some 6550s or KT88s by a better brand like Sovtek or JJ. I tried the Tung-Sol reissue (actually Sovtek) 6550s in one of my amps, and thought they were great. Many people call them "cold" or "Hi-fi" sounding, and while I certainly agree, they suit my amp great. Think less "woman tone" and more "chainsaw tone" :D

If you have the shark jaw tube retainers that won't hold these larger tubes, the Sovtek 5881WXTs (actually a Russian military tube type 6pi3SE) might possibly float your boat. If you decide you don't like the tone of them, well, they aren't too expensive.

Changing tube types will require a rebias. 6550s may need more adjustment than the bias control can give, you should check this before dropping too much cash on a set. Also, your screen resistors may be blown. Once a screen resistor goes, it will put that socket out of action and stress the remaining tube on that side twice as hard. That might explain why you had repeated failures.

[Mooncusser]

OKAY----PULLED THE TWO INNER TUBES, LEAVING THE TWO OUTERMOST INSTALLED AND THE AMP WORKS FLAWLSESSLY
Now what does that tell you folks?....
I really appreciate your help

[John Phillips]

Well, if the KT-66's are vulnerable in the 1959 SLP, maybe the 6L6GC or 6550's would be more in order to avoid the continual popping of tubes.

If I power it up with the two outside tubes in and it works fine, could we be safe just to guestimate the two inside tubes were bad?Yes, if that's the cause. 6L6GCs (real ones) and 6550s do have higher voltage ratings, yes.

Generally, when tubes are bad, or they popped due to voltage, is it common for the two corresponding screen resistors to blow out as wellNot with Marshalls - they use proper wirewound resistors for the screen grids, and they have a HT fuse. I've only ever seen a handful of failed screen resistors in Marshalls (and wrong HT fuse values might have even been involved, I don't remember).

Running at 8 ohms into a 16-ohm load could cause excessive flyback voltages and arcing, but only when running at high power. If the tube arced as soon as you flipped the standby switch, I don't think that's a likely cause.

OKAY----PULLED THE TWO INNER TUBES, LEAVING THE TWO OUTERMOST INSTALLED AND THE AMP WORKS FLAWLSESSLY
Now what does that tell you folks?...That you were most likely unlucky to have got two bad tubes in the same set.

As scopeboy said though, these Chinese tubes do not have a reputation for robustness, and that amp is really running them at (or above) the limit of what they will take.

[BrownIsound]

Valve arts KT66s have a 25% infant mortality rate in my JTM45...I only use GECs now.

[RedMan]

Fairly high, with those tubes. It's really pushing them in that amp too, with the plate voltage as high as it is - it's not the dissipation that's the problem, it's the voltage.

I always get a kick out of it when people think the big bad 1959 Marshall has high plate voltage. Its no higher than any typical Fender of the same ilk.

[scopeboy]

Yeah, but Fender used 6L6GC tubes. When people say the 1959 super lead has high plate voltage, they really mean "Dangerously high screen voltage for the EL34 tubes it was shipped with". The 6L6GCs are happy with high screen voltages.

[Swarty]

I don't think folks "really mean" screen voltage when they say high plate voltage ??? Also, not sure of your sources, but EL34s are rated for like 800V plate voltage and 500V screen voltage. 6L6GCs are 500V and 450V ????

FWIW, if I recall the SLP is about 460V, which I don't consider all that high. It is the earliest 100s that were high ( 600+), and these dropped to around 500V after a year or so.

[John Phillips]

I always get a kick out of it when people think the big bad 1959 Marshall has high plate voltage. Its no higher than any typical Fender of the same ilk.It is - unless you're comparing it to an Ultra-Linear Fender, which have the same sort of voltages (520V or so typically, compared to around 460V in a pre-UL Fender Twin). You wouldn't be very safe putting Chinese KT66s in one of those either. The Reissue SLP does have a little lower voltage, but still probably a bit high for those tubes.

[scopeboy]

EL34s can take 500V on the screen IF they have 800V on the plate. The higher voltage on the plate makes the screen run cooler. With 500V on the plate, they really want 350V on the screen, or failing that, a 1.5k screen resistor.

[RedMan]

It is - unless you're comparing it to an Ultra-Linear Fender, which have the same sort of voltages (520V or so typically, compared to around 460V in a pre-UL Fender Twin). You wouldn't be very safe putting Chinese KT66s in one of those either. The Reissue SLP does have a little lower voltage, but still probably a bit high for those tubes.

Not comparing to an UL Fender but to a typical BF Fender with 460VDC on the plates. Thats exactly what you'll find on a typical 1959. Its not that KT66's can't take high plate voltages either. The early Marshalls with the mistake on the PT giving them over 600 volts on their plates ran KT66's just fine. The problem is new production tubes aren't as robust as NOS tubes. That, my friend is where the problem is.

Talk to Mike at KCA, he'll fix you up with some good KT66's.

[Mooncusser]

Not comparing to an UL Fender but to a typical BF Fender with 460VDC on the plates. Thats exactly what you'll find on a typical 1959. Its not that KT66's can't take high plate voltages either. The early Marshalls with the mistake on the PT giving them over 600 volts on their plates ran KT66's just fine. The problem is new production tubes aren't as robust as NOS tubes. That, my friend is where the problem is.

Talk to Mike at KCA, he'll fix you up with some good KT66's.

Thanks for all the info guys. The KT's were worth a try, but in the end the 6L6GC is going to prevail. At least I got to hear them and come to know them in the short time I used them. Even if this hadn't happened, I would still have come to prefer the 6L6GC's.

Mike is by far the premier tube dealer in my book. He helped me beyond the call of duty.
I think some SED =C='s might be on the list next. I was toying around with trying out some 6550's, but I dont know much about them, or what mod would have to occur to accomodate them. So, if it isn't broken, don't fix it. I love the 6L6GC's in my Plexi circuit. But..if any of you want fo fill me in on the 6550's, the info is more than welcomed.

Take care,
Mass

[John Phillips]

Not comparing to an UL Fender but to a typical BF Fender with 460VDC on the plates. Thats exactly what you'll find on a typical 1959.Typical reissues yes (although I would actually say 470, but that's probably splitting hairs :)). But original 100W Marshalls aren't as low as that... or at least I've never come across one (out of dozens I've worked on) - 490 to 550V is the range I've seen, and mostly around 520 if the supply voltage is accurate.

I agree about old KT66s - although even then I'm not sure I would say that they ran 'just fine' in early 100W Marshalls (probably at least part of the reason Marshall changed to EL34s) - but this is Chinese ones we're talking about... 450V seems to be about the limit, which is fine in a JTM45 of course, but pushing it even in a SLP RI.