Subjective question KT66 vs EL34

[hebaton]

EL34 or KT 66 ? I just got a JTM45 clone ( built from Mojotone kit ) It uses EL34s and an OT with 3.4K primary
I am debating swaping the OT for a Metro spec 8K primary and use KT66s

INterested in views, experience and opinions of KT66 vs EL34 with this circuit. WHat to expect in tone, response, POwer ?

Thanks to all for any input

[Helmholtz]

To me a "JTM45" with EL34s and a 3.4k OT is not a JTM45.
When Marshall changed to EL34s with a different OT, the amp was renamed JTM50 and later JMP50.
So if you want a JTM45 sound, use KT66s with a 7k to 8k OT.
EL34s will give more power (around 50W) while the original amp with KT66s and 8k OT only delivers around 30W clean.

A have a transitional '66 JTM50 which uses the original JTM45 circuit but came with a 3.5k OT (Drake 784-128) and original KT66s.
Not a good match!
I can either use it with EL34s or KT66s (but with doubled speaker impedance) and much prefer the KT66s.

[Roe]

To me a "JTM45" with EL34s and a 3.4k OT is not a JTM45.
When Marshall changed to EL34s with a different OT, the amp was renamed JTM50 and later JMP50.
So if you want a JTM45 sound, use KT66s with a 7k to 8k OT.
EL34s will give more power (around 50W) while the original amp with KT66s and 8k OT only delivers around 30W clean.

A have a transitional '66 JTM50 which uses the original JTM45 circuit but came with a 3.5k OT (Drake 784-128) and original KT66s.
Not a good match!
I can either use it with EL34s or KT66s (but with doubled speaker impedance) and much prefer the KT66s.

actually the first el34 amps had a jtm45 faceplate. These are known as jtm45/50s and jtm45/100s.

I prefer 3k4 w el34s and 6k6 w. kt66s (like a rs dlx ot). 6k8 also works.

[Helmholtz]

The original RS deLuxe OT was rated for 15/7.5/3.75 Ohm secondary impedances.
When used with standard speaker impedances the 6.6k primary impedance increases to just above 7k.
My Marstran RS clone measures 7030 Ohm without and 7550 Ohm with transformer resistance included at the "6.6k" primary.

[Stevem]

It' seems your staying true to form, in that every time you get something new in front of you, you want to change it .

So what is it about the amp as it is now that is lacking in your opinion?

[hebaton]

It' seems your staying true to form, in that every time you get something new in front of you, you want to change it .

So what is it about the amp as it is now that is lacking in your opinion?

I've played other JTM45s in the past, with KT66 and much preferred the tone and response. My question was trying to confirm that would be the main explanation for the differences or if there could be other reasons/changes the make the amp different. I like to play "almost clean" to me, that circuit with KT66 seems to be more "comfortable" to play. More responsive and developing more hamonic content at low gain settings...

I might respecfully add, I get the impression you are somewhat anoyed by my questions, or attitude. If so, you could choose not to intervene. If I got you wrong, please accept my apology.

[hebaton]

Thank you all for the input.
YOu are confirming my impressions. THe orger if going out for a new OT.
My 1st option is to go to heyboer and ask for the "Metro spec" but I am open to suggestion here. Also, if someone has one they would like to sell

Thank you again !

[cdemike]

I've played other JTM45s in the past, with KT66 and much preferred the tone and response. My question was trying to confirm that would be the main explanation for the differences or if there could be other reasons/changes the make the amp different. I like to play "almost clean" to me, that circuit with KT66 seems to be more "comfortable" to play. More responsive and developing more hamonic content at low gain settings...

What specifically about the response did you prefer? This is reading between the lines a little bit, but I tend to associate "more comfortable" and "more responsive" with sag. To a lesser degree, I associate the same qualities with less negative feedback. So I'd check that the amp is using the standard JTM45 filtering values first prior to a major output section rework. The next thing I'd try ahead of a major rework would be decreasing NFB/increasing the value of the NFB resistor, since I suspect something you may have preferred about KT66's is the lower power output which, all else remaining equal, results in less NFB reaching the PI. The 3K4 OT with EL34s will produce more power than 6K8/7K/8K with KT66s, so increasing the NFB resistor's value to 47K or could offset that to bring you closer to a preferred level of NFB.

For non-master Marshalls I think the most important things that set different models/eras apart are filtering, negative feedback, and the tone stack values. Secondarily, the values of bypass cap(s) on V1 and whether V2's gain stage is bypassed make a big difference also, but less than those first items. OT's will make a significant difference as well, but those other things are at least as significant, IMO, so I think you may get more out of the amp if you try those other things ahead of a big expensive rework like replacing the OT.

[pdf64]

Thank you all for the input.
YOu are confirming my impressions. THe orger if going out for a new OT.
My 1st option is to go to heyboer and ask for the "Metro spec" but I am open to suggestion here. Also, if someone has one they would like to sell

Thank you again !

Why not just use twice the intended load impedance, so as to achieve twice the primary impedance?
eg 8 ohm cab with amp set for 4 ohms.

[hebaton]

I've played other JTM45s in the past, with KT66 and much preferred the tone and response. My question was trying to confirm that would be the main explanation for the differences or if there could be other reasons/changes the make the amp different. I like to play "almost clean" to me, that circuit with KT66 seems to be more "comfortable" to play. More responsive and developing more hamonic content at low gain settings...

What specifically about the response did you prefer? This is reading between the lines a little bit, but I tend to associate "more comfortable" and "more responsive" with sag. To a lesser degree, I associate the same qualities with less negative feedback. So I'd check that the amp is using the standard JTM45 filtering values first prior to a major output section rework. The next thing I'd try ahead of a major rework would be decreasing NFB/increasing the value of the NFB resistor, since I suspect something you may have preferred about KT66's is the lower power output which, all else remaining equal, results in less NFB reaching the PI. The 3K4 OT with EL34s will produce more power than 6K8/7K/8K with KT66s, so increasing the NFB resistor's value to 47K or could offset that to bring you closer to a preferred level of NFB.

For non-master Marshalls I think the most important things that set different models/eras apart are filtering, negative feedback, and the tone stack values. Secondarily, the values of bypass cap(s) on V1 and whether V2's gain stage is bypassed make a big difference also, but less than those first items. OT's will make a significant difference as well, but those other things are at least as significant, IMO, so I think you may get more out of the amp if you try those other things ahead of a big expensive rework like replacing the OT.

You are correct in your assertions. I did fing there was more filtering than usual. There were two 32 + 32 uf cans, I replaced the preamp one with a 16 + 16 and that immediately helped.
The amp was also biassed VERY cold, fixing that made a huge difference. You are also correct about my prefrence for lower power. I will loocate the nfb resistor and verify that value.