KT77 a direct replacement in a JCM800 converted to EL34?

KFBR392

Silver Supporting Member
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I have an absurdly bright late 80s JCM800 that was converted to EL34 from 5881s. I hear that KT77s will bring in some bottom end as opposed to more mid-focused EL34s, and the amp needs new tubes so it’s time to switch anyways. Worth noting: I have the bass dimed on this amp and it still doesn’t have a lot of low end. Sounds okay in a mix but it needs more bass even in a band setting.

Will the KT77s be a direct drop in replacement for EL34, and if so, do you have any recommendations? I’m assuming I’ll need to bias, which is kind of pain in the ass but I can do on my workbench if needed.

Thanks in advance.
 

alivegy

Member
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1,176
What speaker are you using, and where do you run the preamp volume? I can’t turn the bass on my 800 above 4 without it causing the preamp distortion to get muddy and flubby. With the eq on the 800 youre more setting what frequencies you want to break up first than setting an overall output eq.
 

KFBR392

Silver Supporting Member
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3,521
What speaker are you using, and where do you run the preamp volume? I can’t turn the bass on my 800 above 4 without it causing the preamp distortion to get muddy and flubby. With the eq on the 800 youre more setting what frequencies you want to break up first than setting an overall output eq.
Speaker is an old Rola G12-65. I run both volumes pretty high on the dirt channel and control with master since I’m in a tiny apartment, but live they get turned down to about halfway.
 
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7,422
Biasing necessary?
Always.

KT77s were actually designed with slightly higher reflected impedance in mind, which is part of why the don't seem as middy.

Then they are 32W absolute max which should be taken advantage of to soften their sometimes glassy quality.

And I'd try the GL over the JJ any day of the week.
 

Sirloin

Silver Supporting Member
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17,574
Speaker is an old Rola G12-65. I run both volumes pretty high on the dirt channel and control with master since I’m in a tiny apartment, but live they get turned down to about halfway.
Which model of JCM 800 is this?
What cab?
 

Sirloin

Silver Supporting Member
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17,574
4210 combo.
So there are a number of factors at play. Marshall tone controls don’t do much. That 112 combo is naturally mid range focused with not much bottom. G12-65 is an M magnet speaker and will not have much low end in a combo. There is a bright cap on the gain potentiometer. The higher you set the gain, the less bright you will perceive the amp to be.

An H speaker will add more low end than power tube swap. But you’re still limited by a relatively small open back combo.

Have you ever plugged the amp into a good 412 and bypassed the internal speaker?
 

Gridlock

Silver Supporting Member
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2,202
Speaker is an old Rola G12-65. I run both volumes pretty high on the dirt channel and control with master since I’m in a tiny apartment, but live they get turned down to about halfway.
The G12-65 is the best speaker for a JCM800. It should add some base and warmth. The KT77’s should also add some thump.

I don’t like bright amps. I’ve sold so many beautiful and rare vintage amps in the past, because I could not tame the brightness. I’ve recently tried going the EL84 route again when I bought a new Marshall 1974x. I always sold my EL84 amps because they were too bright.

A member here recommended trying an EQ with a Mesa Boogie Mark IIB that I owned. I sold the Boogie but kept the EQ.

This time I tried the 7-band EQ in front of my 1974x and set it to a “V”. The low volume brightness issue with my 1974x was solved. The EQ added just the right amount of missing low-end (more bass on tap if needed) and added fullness to my sound. Plus the 1974x sounds great with or without the EQ at band volumes.

You might try an EQ with your JCM800, for around $100 new, and a lot less used, it’s worth a try.
 

drbob1

Gold Supporting Member
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30,113
I used to own a small box, JCM800 from Canada, so EL34s. That SOB was cut your head off bright! The bright cap on mine was actually on the volume pot IIRC, so lifting it was easy. It was still bright but not as damaging. I played it into a bunch of different speakers, the 65s were still pretty bright. G12M-25s were a little better. Jensens seemed a little less harsh. But I didn't shed a tear when I moved that amp on. Strange that my 82 vertical input is a completely different sounding amp.

Also worth saying that the MV Marshall JCM800s are NOT designed as a "get the distortion from the preamp" amp. They're designed for you to get the sweet spot at slightly less stage volume. I'm never happy trying to get distortion/overdrive on the 82 with the preamp knob. For that purpose, just get a pedal and set the amp fairly clean. In fact a REALLY cool 2 channel way to use the JCM800 is to plug into the low input and set it up for the typical warm, clean tone. Then do all the distortion with pedals!
 

sickboy79

Member
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14,053
Always.

KT77s were actually designed with slightly higher reflected impedance in mind, which is part of why the don't seem as middy.

Then they are 32W absolute max which should be taken advantage of to soften their sometimes glassy quality.

And I'd try the GL over the JJ any day of the week.
Love the GL KT77. I have a set in my 2553 Jubilee head and Top Hat Emplexador head. Really cool tubes and a great alternative to the traditional EL34 tone. I can't speak for the JJs but, I would absolutely recommend the GL.
 




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