changing my 36 to use kt66

menger · Post by **menger** » Thu 01/07/10 11:48 pm

iv got a 36 watt tmb im not that inpressed with its a good amp but i like my 18s just as well, but popped a set of kt66 in one of my 6l6 deluxe and was very inpressed i was thiking of sticking the kts in my hardly used 36 watter only thing is the 36 only puts about 330v on the plates and the deluxe does about 415v on the pates, also what changes other than powertube cathiod would be needed, can some one point me to a past thread im sure its been done thanks mark

krx · Post by **krx** » Fri 01/08/10 12:14 am

Well, first you're going to need to put in some octal sockets and probably space them a lot further apart to accommodate those huge KT66 bottles...

KT66s are cool tubes but you'd be better off building an amp around those rather than hacking them into a 36. I'd suggest tweaking the 36 circuit to get the sound you want. What would you like to be different?

dgriff · Post by **dgriff** » Fri 01/08/10 2:46 am

Hi, I am also considering kt66's for my next build. I am thinking of lite 2b front end, kt66 PA, no NFB, Cathode biased, Valve rec,VVR etc......... Any thoughts or suggestions appreciated. Has any one built something similar?
Would standard 36 watt iron be ok, or would I need to look at JTM 45 trannies? Is this into PP watt teritory?
All thoughts welcome
Dave

Post by **zaphod_phil** » Fri 01/08/10 8:40 am

If we're staying with the 18W topology then there's no need to go somewhere else. We've already had several people building 18W derivatives, using 6V6s, EL34s or 6L6s - so why not KT66s? It only takes two or three resistor value changes to run a pair of octal power valves in an 18W. However, I don't recall this having ever been done before with KT66s, so I'm not sure what the OT primary impedance should be, nor what cathode resistor should be. It may be a good idea to do a search for information on the Route 66 by Dr Z.

LeeMo · Post by **LeeMo** » Fri 01/08/10 8:57 am

Wasn't the JTM45 a Kt66 amp? Convert it to cathode bias and you should be set.

LeeMo

krx · Post by **krx** » Fri 01/08/10 12:26 pm

Cathode-biased push-pull KT66 recipe:

* plate ~415 V
* screen ~300 V (could probably go a bit higher and be okay)
* dissipation ~21 W
* cathode resistors 500 Ohms (individual -- data sheets strongly recommend using individual resistors for the KT66)
* output load 8k Ohms (30W transformer)
* grid stoppers at least 10k
* screen grid droppers 2.2k

You'll probably need to tweak the PI to get the drive levels right. Could be a cool amp for sure.

menger · Post by **menger** » Fri 01/08/10 11:10 pm

I think im gonna try rjgtr plexi with the kt66, i have a gds 36 watt ot would that be ok i know it would work for 6l6 so they should be ok for the kt right mark

krx · Post by **krx** » Fri 01/08/10 11:50 pm

menger wrote:I think im gonna try rjgtr plexi with the kt66, i have a gds 36 watt ot would that be ok i know it would work for 6l6 so they should be ok for the kt right mark

See my post above... 8k primary, 30 Watts. The 36W OT should work fine, just double all the impedances in your head. So the 4 Ohm tap becomes 8, 8 becomes 16, etc. This is assuming your OT is designed with a 4K primary.

I wouldn't use a 36W PT though.

dgriff · Post by **dgriff** » Sat 01/09/10 2:00 am

Keep us posted Mark, this looks to be an interesting project

Dave

menger · Post by **menger** » Sat 01/09/10 3:19 am

thanks krx and yes ill whip up a new chassis for this build, the transformer is from graydon at gds its labeled gto-40 rev-1 hts-8307-1 9960849 as i recall graydon said it was for his 40 watt guytrons said it was good for a quad of el84 or pair of 6l6 so id assume its 4k ill have to call him and get the specks and i have a few pts on the bench i was building some 5e5s with that give me about 415 to 440 on the plates so the parts are there im still deciding on a circuit i may even try the jtm18 in the downloads which would basicly be a catiode bias jtm45 ill get some pix up asap thanks mark

Post by **zaphod_phil** » Sat 01/09/10 8:14 am

krx wrote:You'll probably need to tweak the PI to get the drive levels right.

Do the old 15k to 20k PI tail resistor, with 470 ohms PI cathode resistor, in an otherwise stock 18W PI. That should do it.

PCollen · Post by **PCollen** » Sat 01/09/10 9:53 am

zaphod_phil wrote:If we're staying with the 18W topology then there's no need to go somewhere else. We've already had several people building 18W derivatives, using 6V6s, EL34s or 6L6s - so why not KT66s? It only takes two or three resistor value changes to run a pair of octal power valves in an 18W. However, I don't recall this having ever been done before with KT66s, so I'm not sure what the OT primary impedance should be, nor what cathode resistor should be. It may be a good idea to do a search for information on the Route 66 by Dr Z.

Here's my 30 second spiel on this....

Dr. Z Rt 66 is a grid-biased, ultra-linear OT amp and runs 450 Vdc on the plates of the two K66's which are biased at 16.25 (65%) watts stock. Tube spacing for KT66's is 'recommended' at 3 inches on-center. KT66's will draw 1.27 amp heater current PER TUBE, vs. 0.76 amp for an EL84. Likewise, the KT66 is a 25 watt tube, vs a 12 watt EL84, so a 65% bias point will require a lot more current from the PT and through the OT. Finally, I don't think a KT66 will sound 'right' with less than 375 Vdc on it's plates. 330Vdc Vp would be wasting the potential of a great tube, and to get 65% bias point that would require about 50mA bias current per tube, vs about 30-35 mA per tube you are currently drawing.

krx · Post by **krx** » Sat 01/09/10 11:41 am

PCollen wrote:
zaphod_phil wrote:If we're staying with the 18W topology then there's no need to go somewhere else. We've already had several people building 18W derivatives, using 6V6s, EL34s or 6L6s - so why not KT66s? It only takes two or three resistor value changes to run a pair of octal power valves in an 18W. However, I don't recall this having ever been done before with KT66s, so I'm not sure what the OT primary impedance should be, nor what cathode resistor should be. It may be a good idea to do a search for information on the Route 66 by Dr Z.
Here's my 30 second spiel on this....

Dr. Z Rt 66 is a grid-biased, ultra-linear OT amp and runs 450 Vdc on the plates of the two K66's which are biased at 16.25 (65%) watts stock. Tube spacing for KT66's is 'recommended' at 3 inches on-center. KT66's will draw 1.27 amp heater current PER TUBE, vs. 0.76 amp for an EL84. Likewise, the KT66 is a 25 watt tube, vs a 12 watt EL84, so a 65% bias point will require a lot more current from the PT and through the OT. Finally, I don't think a KT66 will sound 'right' with less than 375 Vdc on it's plates. 330Vdc Vp would be wasting the potential of a great tube, and to get 65% bias point that would require about 50mA bias current per tube, vs about 30-35 mA per tube you are currently drawing.

Yep, the Route 66 is a completely different amp. The 5E5 power transformer he mentioned should work well, just make sure there's enough filament current available.

PCollen · Post by **PCollen** » Sat 01/09/10 10:17 pm

krx wrote:
Yep, the Route 66 is a completely different amp. The 5E5 power transformer he mentioned should work well, just make sure there's enough filament current available.

Yes, the 5E5 PT should work fine.

Post by **zaphod_phil** » Sun 01/10/10 8:40 am

PCollen wrote:Dr. Z Rt 66 is a grid-biased, ultra-linear OT amp....

Darn, I'd forgotten that!

OK, let's ignore the the Rt 66 here.

PCollen wrote: .I don't think a KT66 will sound 'right' with less than 375 Vdc on it's plates. 330Vdc Vp would be wasting the potential of a great tube, and to get 65% bias point that would require about 50mA bias current per tube, vs about 30-35 mA per tube you are currently drawing.

krx also recommended about 415V for cathode-biased operation, so you appear to be pretty much in agreement here. However 65% at dissipation would be considered on the cold side for a cathode-biased amp. When dealing with higher voltages in an 18W-based amp you will probably need to adjust the value of the ballast resistors, which drop the PI and preamp rail voltages, to keep those at regular 18W levels.

menger · Post by **menger** » Sun 01/10/10 12:52 pm

Yes there will be alot of tweaking zp, im still researching the right pre amp not so sure a tmb or even a light would really be that great i may have to go out of the realm of 18watt, the best im thinking would be the jtm18 or maybe a ef86, should have chassis fab in a few days mark

krx · Post by **krx** » Sun 01/10/10 1:18 pm

So I'm still wondering why you don't try tweaking your 36 to get it where you want instead of building a whole new amp... The circuit is more important than the tubes.

menger · Post by **menger** » Sun 01/10/10 2:43 pm

the 36 is fine, i just dont think its any better than a 18watt IMO , i like my 18s more, and ive bult them all so its kinda of a waste, its not much louder iv had it awhile iv got it to sound good, and yes the circuit is more inportant than the tubes but we build amps right, so im building amps and tell truth you only go so far with el84, there great but i think about 30 36 watt is the perfect amp, in six months when i got a bunch of 18 watt iron sitting around ill get back to them, but right now im loaded with iron for bigger bottles and 400+ plate voltages , and i really liked the kt66 in the 5e5 so now i guess im looking for right preamp, with the 5e5 at hi platevoltage, the amp is awsome wideopen almost jtm45 id like to be able to get that tone with out cranking up so hi i really need to try many preamps to get whats in my head, the amp im thinking very well maybe the first marshall a bassman only cathiod biased which i like better than fixed iv built a jtm45 they sound kinda like my 5e5 with kt66 but i want better tones lower volumes tweakable with controls ( sorry zp for getting off topic) my intents are a 36 type marshall amp mark

PCollen · Post by **PCollen** » Sun 01/10/10 2:58 pm

zaphod_phil wrote:
PCollen wrote:Dr. Z Rt 66 is a grid-biased, ultra-linear OT amp....
Darn, I'd forgotten that! OK, let's ignore the the Rt 66 here.
PCollen wrote: .I don't think a KT66 will sound 'right' with less than 375 Vdc on it's plates. 330Vdc Vp would be wasting the potential of a great tube, and to get 65% bias point that would require about 50mA bias current per tube, vs about 30-35 mA per tube you are currently drawing.
krx also recommended about 415V for cathode-biased operation, so you appear to be pretty much in agreement here. However 65% at dissipation would be considered on the cold side for a cathode-biased amp. When dealing with higher voltages in an 18W-based amp you will probably need to adjust the value of the ballast resistors, which drop the PI and preamp rail voltages, to keep those at regular 18W levels.

I'm not sure if a standard 36 watter OT or PT could handle the current flow of two KT66's biased close to Class A operation (90%), if that is the intent of cathode biasing, which with 400Vdc on the plates would be around 75 mA each. Add another 7mA from the pre-amp and PI and the rectifier would be pushing around 160mA. That's likely also above the aforementioned 5E5 PT current specs. I'd go with the 18-watt TMB preamp and PI, and grid-bias the KT66's at 16-17 watts with 400-415 Vp.
That could be nice, and without EL84's it's going to sound very different regardless of biasing method.

krx · Post by **krx** » Sun 01/10/10 3:53 pm

menger wrote:the 36 is fine, i just dont think its any better than a 18watt IMO , i like my 18s more, and ive bult them all so its kinda of a waste, its not much louder iv had it awhile iv got it to sound good, and yes the circuit is more inportant than the tubes but we build amps right, so im building amps and tell truth you only go so far with el84, there great but i think about 30 36 watt is the perfect amp, in six months when i got a bunch of 18 watt iron sitting around ill get back to them, but right now im loaded with iron for bigger bottles and 400+ plate voltages , and i really liked the kt66 in the 5e5 so now i guess im looking for right preamp, with the 5e5 at hi platevoltage, the amp is awsome wideopen almost jtm45 id like to be able to get that tone with out cranking up so hi i really need to try many preamps to get whats in my head, the amp im thinking very well maybe the first marshall a bassman only cathiod biased which i like better than fixed iv built a jtm45 they sound kinda like my 5e5 with kt66 but i want better tones lower volumes tweakable with controls ( sorry zp for getting off topic) my intents are a 36 type marshall amp mark

Sounds like one for PPWatt.com then

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changing my 36 to use kt66

changing my 36 to use kt66