kt77's in marshall amp?

[pompira]

hi everyone.

i know little about amps in particular and electronics in general
but i'm trying to understand how well, or how badly, kt77's would work
on a jcm2000 series (tsl60 version).

from what i've read so far, it seems that (and please correct me if i'm wrong)
the reason you connect a 8-ohm load to the 8-ohm tap
or a 16-ohm load to the 16-ohm tap
is so that the primary impedance of the transformer
remains adequate to the tubes the amp uses.
on a 4x el34 amp it would be 1.7k
and in my case (2x el 34) it would be 3.4k.

now, it appears many people use kt77's on these el34 circuits and have no complaints.
my question, however, would be regarding the fact that the kt77 tube,
according to what i've read after some "googling around",
is happiest with a primary impedance similar to that one would see
on a transformer meant for a 6l6 circuit.

so, what i'd like to know is if i were to use kt77's on a tsl60
with my 8-ohm cabinet connected to the 8-ohm tap,
would it ultimately be as if were using a mismatched cabinet,
such as a 4-ohm cabinet into that same 8-ohm tap?

or, in other words, would there be any ill effects (and if so, which?)
in having kt77's seing a primary impedance meant for el34's?


thanks in advance
and apologies for my somewhat poor english.

and again, feel free to correct me
if any of my assumptions are untrue.

[overtone]

It depends on the circuit and voltages and the design intentions.
The data sheets call typically for a a higher load Rl(a-a) as the Voltages are raised. But that is just a data sheet and not rock and roll.

I don't know the tsl marshall, but my hunch is that it will be around 450V on the plates and so long as you adjust the bias you will be fine.

Theoretically the mis-match, that you are rightly cautious about, is going in the "harmful" direction, but not much.
Theoretically you coud see a little less tube life. I don't have any experience with the modern re-issues, but the old MO KT77 was very tough.
I cannot comment on the new KT77s or this Marshall amp.

I have various English amps from the 1970s that were designed around the KT77 which have been fine running into the usual load for 2 x EL34 or 4 x EL34.
The 50W OTs present a 3,5k load and the 100W OTs present 1,8k - that is with 465V on the plate and 3V less on the screens with 1k5 stoppers.

Hope this makes sense, if not just ask,
best,
tony

[pompira]

I don't know the tsl marshall, but my hunch is that it will be around 450V on the plates and so long as you adjust the bias you will be fine.

i haven't measured my amp, but tsl60's, from what i understand, tend to be closer to 480V.

Theoretically the mis-match, that you are rightly cautious about, is going in the "harmful" direction, but not much.

about how much, if i may ask?

Theoretically you coud see a little less tube life. I don't have any experience with the modern re-issues, but the old MO KT77 was very tough.
I cannot comment on the new KT77s or this Marshall amp.

the kt77's i had in mind were actually current production ones, such as the JJ.

I have various English amps from the 1970s that were designed around the KT77 which have been fine running into the usual load for 2 x EL34 or 4 x EL34.

i'm not sure i understand.
the amps you mention are fitted with the kt77's they were designed around
or are they fitted with el34's?

The 50W OTs present a 3,5k load and the 100W OTs present 1,8k - that is with 465V on the plate and 3V less on the screens with 1k5 stoppers.

you're saying they were designed around the kt77
and yet they have impedance/load typical of el34 amps?

Hope this makes sense, if not just ask,
best,
tony

it's not that it doesn't make sense.
i'm just surprised to hear that those amps were designed around the kt77
and the primary impedance isn't as high as i would've expected.

[overtone]

quick reply for now:

1. 480V, no problem for a KT77, maybe the screens should be held much lower for reliability. Depends on the designer's choices.

2. "not much in the harmful direction" = about 50% like putting a 6 Ohm speaker on your 8 Ohm connection. Are you familiar with how to calculate the reflected load? I`m thinking that you can just run a 16 Ohm speaker load into the 8 Ohm connection any you will be safe.

3. The JJs have a fairly good rep. just have not tried them myself. I have a pair of GEC re-issues and never tried those either.

4. The amps I have came with the MO Golden Lion KT77. These amps were a development of a proven professional PA amp, which I have a few of, and those ran EL34 as original specification. The designer was very fastidious and only sold to professional musicians and so my take on it is that if there would have been an impedance issue, he would have resolved it before letting these things out of the door. On the road you need reliability. All the transformers were custom winds anyway so I agree it is surprising that no change was made.

Did I mention that the bias will need to be adjusted?

Best, tony

[pompira]

480V, no problem for a KT77, maybe the screens should be held much lower for reliability. Depends on the designer's choices.

i believe that due to design of the amp itself, the voltage for the screens
is nearly the same as for the plates. marginally lower, that is.

"not much in the harmful direction" = about 50% like putting a 6 Ohm speaker on your 8 Ohm connection.

i see. i might not feel very comfortable with that.

Are you familiar with how to calculate the reflected load?

i read about it. but the problem is i don't know anything about the OT on my amp,
besides the fact that it is whatever they fit stock at the factory.
i'm simply assuming my 8-ohm cabinet connected to the 8-ohm tap
will reflect 3.4k at the primary because it's a 2x el34 marshall,
but i can't say i know that for a fact.

I`m thinking that you can just run a 16 Ohm speaker load into the 8 Ohm connection any you will be safe.

unfortunately i don't have a 16-ohm cab.
and there's no 4-ohm tap in the amp to run my 8-ohm cab.

Did I mention that the bias will need to be adjusted?

i would adjust it even if i were to replace the current set of tubes
with another set of the same type and brand (=C= el34)
35ma @ plate a.k.a. 40ma @ cathode per tube
is what i'd use for those - should i use different values for kt77?

[overtone]

Yes, you can bias as you would for EL34.
But to get there, the bias voltage will, in my experience, be quite different.

Biasing for say 70% of 25W will you get a longer life out of the KT77. It can be pushed to 28 Watts, but I don't see the point of that. Go for longer life and reliability.

For reliability keep them running, don't switch them on and off like fairy lights,
best, tony

[pompira]

For reliability keep them running, don't switch them on and off like fairy lights

i usually switch the amp on, play for an hour or so, then switch it off.
i do this no more than 2 or 3 times a month.
the stock =C= el34 tubes are still sounding good
despite being 4 or 5 years old.

perhaps, all things considered, i should drop the kt77 idea,
and stick with the same brand and type.

don't think, however, that your valuable replies were wasted.
i very much appreciate that you've shared your knowledge,
and i'm happy that you took the time to help me clear my doubts.

[overtone]

my pleasure

[pompira]

still regarding the tsl60, is it safe to replace the 12ax7 in v4 (phase inverter) with a 12at7?

[Bob S]

It should be ok.
The AT7 has less gain, so it will change the sound.
Clipping will change.
What are you looking to do ?

[pompira]

It should be ok.

should? does that mean there's the possibility it might not be ok?

The AT7 has less gain, so it will change the sound.
Clipping will change.

i don't mind if there's a perceivable change in the sound,
as long as i'm sure the amp isn't being harmed.

What are you looking to do ?

i have a 12at7 tube that i bought for an amp i don't own anymore.
rather than throwing out a brand new tube, i'm trying to find out
if that tube could be used in the amp i currently own.

[Bob S]

It'll work - just not sure you'll like it.
One way to find out.
Same pin out with lower gain & higher current capabilities.
I like 12at7's as reverb drivers.
Useful tubes.
Bob

[pompira]

It'll work - just not sure you'll like it.
One way to find out.

i gave it a try, out of curiosity, and didn't find much of a difference between the 12ax7 and the 12at7.
it certainly worked, which is good, but red-plating power tubes also work - for a little while.
my point here being "just because something appears to work at first, doesn't mean there's no impending disaster".
for that reason alone, after testing, i put the 12ax7 back in place.
still trying to find if i can use it with the certainty it won't harm the amp in any way.

Same pin out with lower gain & higher current capabilities.

many tubes, both preamp and power, have identical pin layout,
but their characteristics differ greatly from one another.
that's why i chose not to assume the 12at7 was an acceptable replacement
solely based on the fact that it physically fit in the v4 slot.

I like 12at7's as reverb drivers.
Useful tubes.

unfortunately for me, the reverb on the tsl60 doesn't seem to be tube-driven.

[Bob S]

Bob S wrote:

Same pin out with lower gain & higher current capabilities.


many tubes, both preamp and power, have identical pin layout,
but their characteristics differ greatly from one another.
that's why i chose not to assume the 12at7 was an acceptable replacement
solely based on the fact that it physically fit in the v4 slot.

OK - you have a lot of reading to do.

[pompira]

OK - you have a lot of reading to do.

did i say something wrong?
are tube characteristics identical just because the have they same pin layout?
i would think not, but you seem to disagree.

anyway, in my first post i made clear that

i know little about amps in particular and electronics in general

so i don't need you to tell me about my ignorance on the subject as i'm well aware of it.

i have, however, requested to be corrected in case i said something wrong.

feel free to correct me if any of my assumptions are untrue.

and that, of course, is applicable to any of my posts.

still, thanks for your input.



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anyone else in the forum could tell me whether it's safe
for the amp to use a 12at7 in v4 of a tsl60 or not?