Kt120 and class B

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daveweyer
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Re: Kt120 and class B

Post by daveweyer » Tue Jan 06, 2015 4:11 pm

Well I had a chance to look at the MKll amp, and I have a few suggestions if you are willing to do a little soldering.
Replace the power transformer with an Edcore XPWR078. Wire the secondary up as a full wave bridge, like a Marshall 1959 SL, connect the CT of the power transformer to the junction between the first filter caps just like the Marshall; then remove the 470 ohm R38 from the top of the the two 80uf caps C16 and C17 and put it in the middle, right where the center tap connects; then put C18 and C19 in parallel instead of series, taking out the two 100K resistors R22 and R23; then change R26 to a 22K 10 watt.
Your KT 120s will now have 700 volts on the plates, a very safe 350 volts on the screens, and your preamp stages will all have nearly the same voltages, but just a little higher.
Then get an OT which has a 2K P-P primary impedance, Edcore CXPP100 MS 2.2K, and use the FB tap you like for your taste in sound.
KT120s in this configuration can do 150 watts and better. With a 2K P-P impedance you can work them real hard.

Hope this helps.

fakeox
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Re: Kt120 and class B

Post by fakeox » Thu Jan 08, 2015 1:31 am

Thank you so much Dave! Did you play around in your friends amp?

daveweyer
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Re: Kt120 and class B

Post by daveweyer » Thu Jan 08, 2015 4:22 pm

Yeah, I did, at least as much as he was willing to let me. Maybe I could talk him into a mod job, and then post the pics.
Either way, if you need a schematic I can have Xplorer post one of the mod upgrade.
This upgrade is just like one I did for Jimi on one of his SLs. It's pretty simple really, but it does cost you a couple of transformers. I guess a couple hundred is not that much in amp power terms these days. Edcore makes a decent transformer for not much money.

Jazz
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Re: Kt120 and class B

Post by Jazz » Mon Mar 14, 2016 6:57 pm

Hi Dave

This is my second attempt to contact you over this MetroAmp forum , hope that this time I will be more successful ,

I need your opinions of this my OPS and PSU design for one Marshall based DIY amp ,

https://imageshack.com/i/plwWAHP5j
https://imageshack.com/i/pngaUSeij

question , what OPT primary impedance value will be optimum for this AB1 class OPS ? ,

and any other suggestion or critic on this ?

Best Regards
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daveweyer
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Re: Kt120 and class B

Post by daveweyer » Sun Mar 20, 2016 1:16 pm

Whoops, I missed this post amongst all the crap flying here the last week or so.
I'll look at the files and post a reply real soon. Thanks

daveweyer
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Re: Kt120 and class B

Post by daveweyer » Sun Mar 20, 2016 11:25 pm

Jazz,
You should probably look at 3500 to 4000 primary impedance. You might want to be a little more generous with your power supply caps, say 200 to 400 uf on your voltage divided main DC supply. I would also add more on the bias supply.
You might consider using a plate stopping resistor on one of the output tubes of each output pair, i.e. a 50 ohm 5 watt WW from one plate of the upper pair to the OT and another from one plate of the lower pair to the OT. This will help with HF oscillations by staggering the LC constants.

You don't have to use much resistance in the screen circuit for each tube because the screen current will be so much lower, i.e. you don't need much current limiting there because the screen voltage is so much lower. I often use two different values for those resistors too, for the same reason as the plates.

Just make sure you have plenty of driving voltage for the output tubes, their grid 1 sensitivity is lower because of the lower screen voltage.

Hope this helps!

Jazz
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Re: Kt120 and class B

Post by Jazz » Mon Mar 21, 2016 10:41 am

Dave of course it helps ,

Your valuable suggestions will be implemented on this DIY guitar amp ,
BTW ,it is interesting that only you , Bruce De Palma and Telefunken suggest this mode of output power tubes (EL34)operation ,
http://depalma.pair.com/Analog/analog.html ,
seems that this specific mode of operation allow very high output power level and long life of output power tubes in the same time ,
forgot to mention that I will run only EL34 filaments supply ( 6,3VAC/7A ) from non earthed (floating) main transformer separate windings , just to avoid arcing from pin 3 ( anode ) to pin 2 ( filament ) on EL34 tube socket .

:toast:
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daveweyer
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Re: Kt120 and class B

Post by daveweyer » Mon Mar 21, 2016 11:35 am

Just one more thing to keep in mind, you may have problems running in class AB1 because the EL34 doesn't have a lot of extra plate dissipation. At 30 ma of idle current each tube will dissipate over 21 watts, leaving only 5 extra watts for full power. That's more like class A operation. You would probably do better near Class B operation, say with only 15-18 ma per tube if you want them to last a long time.

Jazz
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Re: Kt120 and class B

Post by Jazz » Mon Mar 21, 2016 3:30 pm

daveweyer wrote:Just one more thing to keep in mind, you may have problems running in class AB1 because the EL34 doesn't have a lot of extra plate dissipation. At 30 ma of idle current each tube will dissipate over 21 watts, leaving only 5 extra watts for full power. That's more like class A operation. You would probably do better near Class B operation, say with only 15-18 ma per tube if you want them to last a long time.
Very good point and suggestion ,

for sure that OPS idle plates currents have to be accordingly reduced by shifting power tubes operation points closer to class B1, insuring that EL34 plate and screen dissipation always to stay inside of recommended SOA ,
for example Telefunken EL34 original datasheet suggest 2x20mA as idle plate currents for zero AC volts G1 excitation ,
and 2x96mA as maximum plates currents for full G1-AC excitation ,
all this on 800VDC/B+A and 400VDC/B+G2 and -40VDC for G1, where G2 currents fluctuates between 2x2,3mA and 2X20mA ,
for G1to G1`maximum excitation AC voltage of 56Vpp , recommended Raa is 10K .
Blast from the past...

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