Power Tube question

[Johntb]

Hopefully this is the right area to ask.

Are 6L6's and KT66'S interchangeable?

I have an OT from an amp that had 6L6's and I want to make a JTM45 using it but using the KT66'S instead of 6L6's.

The amp was rated at 100 watts (Unicord 1221). I know there has been a lot of debate about that rating. After reading one of the stickers that was originally on the back of the amp it looks like it "consumed" not "delivered" 100 watts of power.

Thanks you,

John

[66Strat]

They are not directly interchangeable. The KT66 heater filaments draw 1.3 amps per tube vs 0.9 amps for the 6L6. The KT66 has a bigger bottle. Space can be an issue.

[shooter]

here's some snippits from datasheets also;

[Johntb]

Thanks for the replies but I am wondering about the OT primary tap being 8.8k (I believe).

[66Strat]

The 1221 schematic indicates the OT to have a 10K ohm primary winding with 16 ohm and 8 ohm secondary taps.
http://el34world.com/charts/Schematics/files/Unicord/Unicord_1221.pdf

Original JTM 45s used a Radiospares OT with a multi-tap primary. Most were configured with a 6K6 primary connection. Later model amps were wired with 8K ohm primary connections. The 8K load carried over to the later model JTM 45s that had the Drake OTs. But the primary winding is just one piece of the puzzle and is not all that significant IMO. With the OT that you have, I would build the amp using 6L6 tubes or 5881s and use the 16oh connection for an 8 ohm speaker load. This will give you a reflected load of 5K ohms to the 6L6s. Keep the B+ voltage to the plates down around 420 volts DC and use 5881s. It will make a very nice JTM 45.

[Johntb]

Thanks. You're right about 10k. I have been looking at all kinds of schematics and wassup going by memory.

[PRR]

KT66 was supposed to be an improved european 6L6.

You have never used a true 6L6, except perhaps in a very vintage amp. Since 1960 we use 5881 and 6L6GC which are significantly improved on original 6L6, in ways that echo what was done in KT66. Except the US types stuck with the 0.9A heater while the KT66 has 1.6A heat.

The Unicord 1221 uses two 6L6GC working at 660V(!!) to make a good 60 Watts output. At this over-voltage, 10K load is appropriate.

However 660V is 32% over the 6L6GC's rating. The tubes we got in the 1970s usually tolerated this. Today it is very risky.

Note that the 1221 ran the Screens at 320V. This is the right thing to do when over-volting the plates and working in a high load resistance.

If you bring Voltage down to 450V-500V (70% of 660V), you can jack-up Current 140%, which makes a happy-load more like 5K. Re-label the "16/8" taps as "8/4" and the 1221 OT will act like 5K to the tubes. This is basically the Fender "50W 6L6" condition, with Screens only lightly dropped from plate supply.

[Johntb]

PRR  Thanks for the tips.
I was not going to use 660v. The amp came in a deal with a bunch PT's & IT's and even a AC30 2X 12 cab.

The guy said he could never get the 1221 working. When I went through it I found a one wrong tube in the 6AN8 and one 12AX7 wired wrong. Then looking at the board someone added caps and resistors and a lot of cap just didn't look right. I had sound through it but maybe 30 watts. I assume the OT is good due to sound being produced and all the issues I found with the board.

Thanks.

[Joel]

I had a couple G.E. 7581A's handy when I built my JTM45 clone (along with a Drake OPT).  They are marked KT66 and 7581A...  which threw me.  According to what I can find on the interwebs the 7581A is an "upgraded" 6L6GC and was marketed as interchangeable with 6L6 and KT66.  I do want to change them out for KT66's one day.  But they are currently working just fine.  And I don't expect to notice much sonic difference.