johnnyreece wrote: ↑Tue Mar 08, 2022 3:59 pm
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 wrote: ↑Tue Mar 08, 2022 3:59 pm
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.
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.
johnnyreece wrote: ↑Tue Mar 08, 2022 3:59 pm
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.
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...
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.
Matthews Guitars wrote: ↑Thu May 19, 2022 10:58 amEL34 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.
Matthews Guitars wrote: ↑Thu May 19, 2022 10:58 amEL34 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 wrote: ↑Thu May 19, 2022 6:24 pm
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.
Matthews Guitars wrote: ↑Thu May 19, 2022 10:58 amEL34 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?