Sc20/sv20 Power Tubes...what Is The Deal Exactly?

[tallcoolone]

I’ve read musings that these amps can take any octal tube but can they really? Haven’t seen anything from Marshall about it. Can I just slap a pair of KT88s in here?

And do pairs of power tubes need to be matched?

 

[Chris4189]

I’ve read musings that these amps can take any octal tube but can they really? Haven’t seen anything from Marshall about it. Can I just slap a pair of KT88s in here?

And do pairs of power tubes need to be matched?

The whole matched tube thing comes from the HiFi world. Tube matching, at least closely, IS over rated. To be within 5 or 10mA is good because it keeps hum at a minimum and allows the tubes to wear out somewhat evenly. Even with proper burn in before testing, tubes will drift in use and likely drift apart. So a "perfectly matched" pair of tubes won't stay perfectly matched for long. Starting off as closely matched helps to insure that they won't become "grossly mismatched" over time.

 

[tallcoolone]

The whole matched tube thing comes from the HiFi world. Tube matching, at least closely, IS over rated. To be within 5 or 10mA is good because it keeps hum at a minimum and allows the tubes to wear out somewhat evenly. Even with proper burn in before testing, tubes will drift in use and likely drift apart. So a "perfectly matched" pair of tubes won't stay perfectly matched for long. Starting off as closely matched helps to insure that they won't become "grossly mismatched" over time.

Sure that makes sense, I try to buy in pairs anyway, but I do wonder if my amp would still sound it’s best with 2 random el34s in my box here...maybe a brand new Chinese one and a well used Siemens from the early 80s?

More interested in the ‘any octal tube’—kinda fascinating to me that there is so much discussion on these amps yet zero answers as far as replacing a critical user replaceable part.

 

[K2JLX]

The amplifier is cathode biased so in theory you should be able to put in octal tubes other than el34s: I haven’t seen anyone do that yet.

If I were to do it (I’m not) I would go with a pair of KT66’s.

 

[tallcoolone]

I would love to step beyond ‘theory’ here...

 

[rich88uk]

Marshall have confirmed to me, over the phone, that all of the studio range are cathode bias and are designed to run with EL34s.

Now, as KT77 and EL34s are very similar in power range etc then you could in theory try KT77s. I would be interested in hearing this option with the studio vintage.

KT88s and 6550s run a lot more power so you would blow the amp and I was advised not to do this.

I was under the impression, the same as you, that you can run any octal valve in there, but this does not seem to be the case.

 

[Kinkless Tetrode]

I have ran KT66s in the mini Jubilee. No problems encountered. Also 6L6GC with no problems.

 

[K2JLX]

Marshall have confirmed to me, over the phone, that all of the studio range are cathode bias and are designed to run with EL34s.

Now, as KT77 and EL34s are very similar in power range etc then you could in theory try KT77s. I would be interested in hearing this option with the studio vintage.

KT88s and 6550s run a lot more power so you would blow the amp and I was advised not to do this.

I was under the impression, the same as you, that you can run any octal valve in there, but this does not seem to be the case.

Great info. Thanks.

 

[purpleplexi]

My plexi has a bias pot for each tube so I can get them both spot on. That said whenever I've put non matched tubes in it sounds like arse. It might be that there are 2 different 'magic' tubes out there but I've never found them.

 

[Chris4189]

Sure that makes sense, I try to buy in pairs anyway, but I do wonder if my amp would still sound it’s best with 2 random el34s in my box here...maybe a brand new Chinese one and a well used Siemens from the early 80s?

More interested in the ‘any octal tube’—kinda fascinating to me that there is so much discussion on these amps yet zero answers as far as replacing a critical user replaceable part.

Sure that makes sense, I try to buy in pairs anyway, but I do wonder if my amp would still sound it’s best with 2 random el34s in my box here...maybe a brand new Chinese one and a well used Siemens from the early 80s?

More interested in the ‘any octal tube’—kinda fascinating to me that there is so much discussion on these amps yet zero answers as far as replacing a critical user replaceable part.

Actually perfectly matching "Push Pull" power tubes effectively cancels out even order harmonics. So a little mismatch is desired tone wise. Tubes that are greatly mismatched will be less desirable since one tube is really cold and the other is too hot

 

[tallcoolone]

Marshall have confirmed to me, over the phone, that all of the studio range are cathode bias and are designed to run with EL34s.

Which is literally the only info you already had haha!

Sounds like the Studio series amps are el34s or their very close variants only. Good to know—lots of internet experts with misinformation out there.

 

[rich88uk]

Which is literally the only info you already had haha!

Sounds like the Studio series amps are el34s or their very close variants only. Good to know—lots of internet experts with misinformation out there.

If in any doubt it's always worth phoning Marshall. I've found them to be incredibly helpful over the years on the phone.

 

[Kinkless Tetrode]

Here is what Santiago had to say on the mini Jubilee and what power tubes you could use:

it is cathode biased, you can use any octal tube you want without having to do anything. It is the nature of that circuit (same as preamp tubes)
Fixed bias is a different thing, they rely on an internal voltage which is "fixed" and that can be adjusted or not. 100W Marshall amps are fixed adjustable bias, Boogies are fixed non-adjustable bias

Marshall will be officially conservative and that is understandable.

 

[tallcoolone]

Here is what Santiago had to say on the mini Jubilee and what power tubes you could use:



Marshall will be officially conservative and that is understandable.

Don’t understand why they would be conservative if there is no risk as Santiago suggests. It seems a bit odd that an ex-employee is the only person who can give a straight and honest answer to this pretty significant question.

 

[K2JLX]

Here is what Santiago had to say on the mini Jubilee and what power tubes you could use:



Marshall will be officially conservative and that is understandable.

Ah! So I could put KT66’s in the SV20h if I desired. Perhaps?

 

[Georgiatec]

Ah! So I could put KT66’s in the SV20h if I desired. Perhaps?

Yep....they will either work or not. They are basically the same set up as the Astorias and SL-5. KT66 Doesn't work in the SL-5 but everything else I have does. Anything works in the Astoria.

 

[Jethro Rocker]

Don’t understand why they would be conservative if there is no risk as Santiago suggests. It seems a bit odd that an ex-employee is the only person who can give a straight and honest answer to this pretty significant question.

They're covering their ass for some dumb ass hat move someone will make I would guess. They don't talk about all the impedance choices either just to make it simpler and again, cover their ass.
I will take Santiagos advice above some office dude at Marshall.

 

[tallcoolone]

They're covering their ass for some dumb ass hat move someone will make I would guess. They don't talk about all the impedance choices either just to make it simpler and again, cover their ass.
I will take Santiagos advice above some office dude at Marshall.

Well, I want to put KT88 tubes in my sc20 and I just read above:

KT88s and 6550s run a lot more power so you would blow the amp and I was advised not to do this.

So does putting KT88s in my amp constitute a ‘dumb ass move’?

 

[Jethro Rocker]

Well, I want to put KT88 tubes in my sc20 and I just read above:



So does putting KT88s in my amp constitute a ‘dumb ass move’?

Fucked if I know for sure. Just looking at what @santiall said....maybe he will chime in. I know @Kinkless Tetrode has tried quite a variety jncluding 6L6 in his mini Jubilee

 

[Jethro Rocker]

Addendum not being a tech or such, I wonder as well if the plate voltage is so low on these amps to deliver only 20 watts - someone measured theirs at about 230V - would it matter which tubes? Would KT88s Run roo much power at that rate?