JTM45 style amp with KT66's can't run 6L6's?

[pdf64]

To the original question, the issue might be that the bias circuit doesn't have enough swing to get the bias voltage you need…

The g1 bias voltage required by 6L6 tends to land somewhere between that of KT66 and EL34.
So if the bias supply can be adjusted for KT66 and EL34, then it can also accommodate 6L6.

[johnnyreece]

To the original question, the issue might be that the bias circuit doesn't have enough swing to get the bias voltage you need…

The g1 bias voltage required by 6L6 tends to land somewhere between that of KT66 and EL34.
So if the bias supply can be adjusted for KT66 and EL34, then it can also accommodate 6L6.

Well, there goes that idea, then...

[Matthews Guitars]

Only one resistor in the bias circuit needs to be changed to alter the bias range to suit the tube type. If I remember correctly, change the 220K to something around 120K and that'll do it.

I reset one of my 1959s to run KT66 tubes and that was what I needed to do to make it bias properly.

[Guitarchery]

To the original question, the issue might be that the bias circuit doesn't have enough swing to get the bias voltage you need…

The g1 bias voltage required by 6L6 tends to land somewhere between that of KT66 and EL34.
So if the bias supply can be adjusted for KT66 and EL34, then it can also accommodate 6L6.

Thats exactly what Im thinking!

[pdf64]

I suggest you seek clarification from DrZ.

If the circuit architecture shown in this schematic is correct, you’re under a misapprehension in thinking the amp bears any relationship with JTM45.
https://www.flickr.com/photos/49108115@N04/4501477895

[ChopSauce]

I suggest you seek clarification from DrZ.

If the circuit architecture shown in this schematic is correct, you’re under a misapprehension in thinking the amp bears any relationship with JTM45.
https://www.flickr.com/photos/49108115@N04/4501477895

That seems to be the amp named "Route 66" - or something like that. The OP mentionned an amp named "Antidote" and I'm not sure how these model names relate to each-another...

[stephenl]

Yep, the schematic above is for a RT66.
Antidote is based on JTM45.
From the Dr himself:
https://www.youtube.com/watch?v=PsymtzV7Jhg

[pdf64]

Doh, I would have sworn I read Route 66

[Guitarchery]

I suggest you seek clarification from DrZ.

If the circuit architecture shown in this schematic is correct, you’re under a misapprehension in thinking the amp bears any relationship with JTM45.
https://www.flickr.com/photos/49108115@N04/4501477895

Stephenl is right,that's a Route66 model, the Antidote is very similar to a '66 JTM45.

[Matthews Guitars]

When I decided that ONE of my Marshalls should run KT66s, I had to learn what would have to be done to make that work. (EL34 amp previously.)
Since that amp was going to be a total rebuild anyway, why not?

Long story made short: Bias circuit resistor values were changed to reset the bias range to accomodate the KT-66 tubes. And the output taps have been relabelled, doubling their impedance ratings. Rather than 4, 8, and 16 ohms, they are now relabelled 8, 16, and 32 ohms.

EL34 plate loading is around 3400 ohms, while it's around 8000 for KT66s.

Optimally you'd obtain a new output transformer that's wound for KT66s but using the next higher impedance tap on the speaker jacks is a perfectly good workaround.

For tubes I'm using a well matched set of the excellent Shuguang made KT66s. And with a spare matched set in reserve, I'll be good for a long time.

[martin manning]

EL34 plate loading is around 3400 ohms, while it's around 8000 for KT66s.

Optimally you'd obtain a new output transformer that's wound for KT66s but using the next higher impedance tap on the speaker jacks is a perfectly good workaround.

KT66 are not much different from 6L6, and would really like a 4k Zaa with 400-450V on the plates. IMO the 8k used in early Marshalls was a bad design. 6k6 was better but still too high for the screens to have long life.

[Colossal]

EL34 plate loading is around 3400 ohms, while it's around 8000 for KT66s.

Optimally you'd obtain a new output transformer that's wound for KT66s but using the next higher impedance tap on the speaker jacks is a perfectly good workaround.

KT66 are not much different from 6L6, and would really like a 4k Zaa with 400-450V on the plates. IMO the 8k used in early Marshalls was a bad design. 6k6 was better but still too high for the screens to have long life.

What screen and grid voltage are you suggesting for a 4k load for KT66s, Martin?

[martin manning]

Essentially same as the plates in typical guitar amp fashion. It won't have the same early breakup, of course.

[Colossal]

Essentially same as the plates in typical guitar amp fashion. It won't have the same early breakup, of course.

Ah, ok. Yeah, I would expect much more clean power with low Raa.

[stephenl]

EL34 plate loading is around 3400 ohms, while it's around 8000 for KT66s.

Optimally you'd obtain a new output transformer that's wound for KT66s but using the next higher impedance tap on the speaker jacks is a perfectly good workaround.

KT66 are not much different from 6L6, and would really like a 4k Zaa with 400-450V on the plates. IMO the 8k used in early Marshalls was a bad design. 6k6 was better but still too high for the screens to have long life.

I really like the sound of KT66's with 8K Primary, 400v on plates...
I'm curious now though how the higher primary impedence affects screen life?