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

[Guitarchery]

Hi guys, new member here. I've got a Dr Z Antidote, its a JTM45 style amp that runs KT66's (ultra linear OT) and I recently contacted DrZ customer support about putting a set of 6L6GC's in there but they replied that it can't run 6L6GC's but can take EL34 with proper bias changes. This sounds strange to me, KT66 need 1,3A EL34's 1,6A and 6L6's only 0,9A so what could be the difference from an original JTM45 that could cause a problem from running 6L6 tubes in it? Can you tell from the photo included?

20220225_143636.jpg

[martin manning]

I wouldn't think there would be any problem as long as you can bias them properly. Heater current won't be a problem as you say. Note that 6L6GC has lower transconductance (6 vs. 7 mA/V at the same 250V Va and Vg2), so they will need more drive to get the same output power.

[Guitarchery]

Output power is not a problem for me, this amp is plenty loud for my needs. Could it be some different than traditional wiring at the tube bases that would not work for both kind of tubes?

[martin manning]

Pinout is the same for KT66 and 6L6GC, 6L6GC should be fine with the UL connection too, but depending upon the OT primary impedance it may or may not sound good. The heater voltage might be a bit high given the lighter load of the 6L6.

[Guitarchery]

Would you say high enough to prematurely wear out the tubes? I guess if I like the sound better with 6L6's and leave them in permanently I could put some resistors in line to get them to see 6,3V right?

[martin manning]

Would you say high enough to prematurely wear out the tubes?

That depends on the power transformer. You'd have to measure the heater voltage with all tubes installed.

I guess if I like the sound better with 6L6's and leave them in permanently I could put some resistors in line to get them to see 6,3V right?

That's one solution, back-to-back diodes is another.

[Guitarchery]

Thanks so much Martinl!

[stephenl]

If your interested in trying it, you could add another power supply node for the screens and change to pentode operation. You might find you like the sound of KT66's better that way. I'm not a fan of UL for guitar amplifiers. I built one that had selectable UL / Pentode with KT66's into an 8K primary. UL just sounded kind of "meh" compared to pentode. I also built a Route66 clone and ended up converting it from UL to pentode for the same reason.

[Guitarchery]

If your interested in trying it, you could add another power supply node for the screens and change to pentode operation. You might find you like the sound of KT66's better that way. I'm not a fan of UL for guitar amplifiers. I built one that had selectable UL / Pentode with KT66's into an 8K primary. UL just sounded kind of "meh" compared to pentode. I also built a Route66 clone and ended up converting it from UL to pentode for the same reason.

A selectable UL / Pentode mod seems too good to ignore! what would you say are the tone benefits of pentode for KT66's? More harmonics maybe?

[stephenl]

I wouldn't bother with making it selectable unless you really want to A/B the different sounds. I'd predict that you'd end up just leaving it in Pentode mode.

This is just my opinion, but I think UL sounds very sterile. Others have described it as sounding like there was a blanket over the speaker cab, lacking dynamics, etc...

Making it selectable adds a little complexity - you'll need to add relays rated for the screen voltage and related switching circuits. It's really the only way to prove to yourself what you like the sound of though.

[Guitarchery]

Got it, making it selectable goes deeper than I'd go... Thanks!

[pdf64]

I’ve used a simple DPDT switch to change from UL to pentode operation (on a KT88 pair bass amp).
It’s no more stress on the switch really than the regular triode - pentode switch.
In this case, with 470ohm 1W mounted between pins 4&6 of the socket, pin6 connects to the switch common, one throw to the UL tap, the other to a 1k 5W shared screen grid resistor.

[martin manning]

I’ve used a simple DPDT switch to change from UL to pentode operation (on a KT88 pair bass amp).

Was there already a screen supply node in the power supply? A UL power section may not have one.

[pdf64]

I’ve used a simple DPDT switch to change from UL to pentode operation (on a KT88 pair bass amp).

Was there already a screen supply node in the power supply? A UL power section may not have one.

Correct, I added a HT node for the pentode mode. stephenl mentioned doing that earlier when the idea was proposed, so I assumed it was a given.

I used a twin reverb choke and a series pair of 47uF 400V ecaps, as the HT is up near 600V.

[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. I remember on my JTM45-ish clone, I had to do a little work to the bias section in order to accommodate both KT66 and 6L6. Just a thought!