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Old February 28th, 2011, 02:22 PM   #1 (permalink)
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SE KT66 Amp Build

I'm finally going to take on my first amp build. It's going to be simple, so no worries, but has anyone ever built a Champ style amp with KT66 instead. I can find more complex SE KT66 schematics with tone stacks and parallel power tubes...but I'm looking for 1xKT66, 1x12AX7 with a volume control only.

Can I use the champ schematic as a baseline???

Any help willl be much appreciated.

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Old February 28th, 2011, 03:15 PM   #2 (permalink)
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okay, here's what you do: start with a weber 5f1a kit to get the larger cabinet with a 10" speaker. make the following substitutions, 1-use the red secondary windings of the power transformer and substitute a 5AR4 for the rectifier tube. 2-change R11 on the weber 5f1 schematic to 27k ohms @2 watts to get the preamp voltages down to the correct territory for a tweed champ. 3-substitute R8 on the same schematic with a 270 ohm 10 watt part and the accompanying C6 capacitor to 25u 50volt. Use the WSE15 output transformer. Plug in a KT66 and enjoy.
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Old February 28th, 2011, 06:40 PM   #3 (permalink)
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That's a simple-minded method of doing this. What you are worried about with the KT66 is giving it enough voltage to put out a decent amount of power. You also have to consider that the screens will need to be dropped to 300V or whatever and so the dropping resistor after B1+ will be larger than your standard champ. You'll probably end up with a larger screen protecting resistor as well. Also you need to calculate the different bias and what your drive requirements are to get full power out (i.e. how much output the second stage has), as well as how much more voltage you need to overdrive stuff. There are many schematics out there using 6L6's in a Champ circuit and so you can easily reference those. Just do some Googling to get started and please ask questions if you get lost or confused!
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Old February 28th, 2011, 09:04 PM   #4 (permalink)
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@ Cliff- first I recommend that he use the higher output of 2 secondary windings on the power transformer plus the use of a different rectifier with much less internal resistance than the stock 5y3 normally found in a champ. This gets the B+ up over 400v, enough to drive a 6L6GC or KT66. The 10k dropping resistor between the plate and screens is sufficient. And I have figured the value for the cathode bias resistor, which by the way is different than that for a 6L6GC. I've built a 5f2a using this setup and it's fine. Are you speaking from experience or just speculating?
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Old March 2nd, 2011, 03:17 AM   #5 (permalink)
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Thanks guys...I'm not going to buy the kit from weber...I have a cabinet ready to go and it calls for a 8" speaker at most. Looks like I've go my research ahead of me. But I guess I'll just keep plugging away. Thanks again.
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Old March 2nd, 2011, 04:23 AM   #6 (permalink)
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Didn't know you already had a cabinet. One of the reasons I recommended the weber kit is because of the cabinet, it's well made and professional looking,something that someone with limited experience in woodworking would appreciate. That said, Weber does offer their kits without the cabinet, and even if you go with someone else's kit or source the parts yourself,you can still use the weber schematic to refer to the changes I suggested.
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Old March 2nd, 2011, 11:27 AM   #7 (permalink)
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Quote:
Originally Posted by ejb222 View Post
I'm finally going to take on my first amp build. It's going to be simple, so no worries, but has anyone ever built a Champ style amp with KT66 instead. I can find more complex SE KT66 schematics with tone stacks and parallel power tubes...but I'm looking for 1xKT66, 1x12AX7 with a volume control only.

Can I use the champ schematic as a baseline???

Any help willl be much appreciated.
here's the lay out I used, I just pulled out the tone section. It works just fine and lots of volume. but, don't tell on me. also I used Hammond Transformers but, I'm just around the corner from Guelph, Ontario so tons of cheap Hammond iron around here. hope this helps you out, cheers
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Old March 2nd, 2011, 02:39 PM   #8 (permalink)
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Originally Posted by GTRLUVR

here's the lay out I used, I just pulled out the tone section. It works just fine and lots of volume. but, don't tell on me. also I used Hammond Transformers but, I'm just around the corner from Guelph, Ontario so tons of cheap Hammond iron around here. hope this helps you out, cheers

Sweet! Thanks. I probably should have just stuck with the plain champ for my first time...but thanks for the schematic! Big help!

I was looking at a Hammond 270ex PT...I think that should do well...what did you use for choke and OT?
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Old March 4th, 2011, 01:14 PM   #9 (permalink)
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Cool, I used a 270 EX PT, a 125 ESE OT, & A 153 L choke CHEERS
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Old March 13th, 2011, 11:02 PM   #10 (permalink)
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Ok so this has evolved a bit after some research and considerations. This project has now become a SE 2x 6V6 in parallel with 6SL7 preamp.

My question is if the Hammond 270ex is still a good fit for this design? With both the preamp and output tubs at max 300v plate ratings i really only need about 220V RMS for this...so is this over kill? I'm was looking on the Hammond site and still like this PT, but is the 2nd fil wiring of 6.3v @ 4a good enough for the 6V6GT the needs 6.3v @ 4.5a? I know tubes are pretty forgiving except for heater elements:)

My other question..I am using Duncans Amp Tool to design my Power Supply how do I find a PT's "source resistacnce"?

Thanks for the help, yall
Cheers
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Old March 14th, 2011, 06:32 PM   #11 (permalink)
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...... I'm was looking on the Hammond site and still like this PT, but is the 2nd fil wiring of 6.3v @ 4a good enough for the 6V6GT the needs 6.3v @ 4.5a? I know tubes are pretty forgiving except for heater elements:)

My other question..I am using Duncans Amp Tool to design my Power Supply how do I find a PT's "source resistacnce"?

Thanks for the help, yall
Cheers
6V6 heater is rated at .45a, not 4.5a, so the heater winding will be fine.

One finds source resistance by measuring the secondary HV winding with an ohmmeter and record the reading, measure the primary winding with the ohmmeter and multiply that by the voltage ratio of the primary:HV windings, then add that to the reading from the HV winding
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Old March 16th, 2011, 12:56 PM   #12 (permalink)
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The B+ voltages from the 270EX will be higher (probably around 375v straight out of a SS rectifier), but IMO that's a good thing. 144ma of current capability is great. This gives you some room to add another filtering stage, which makes for a huge improvement in hum on single-ended amps. Stick a 40uf cap and a 100-ohm 10W (just to be safe) resistor before your B+ reaches the OT, then follow with the typical cap / choke / cap /resistor / cap arrangement. B+ at the power tubes should be around 360v, perfect for 6V6GTs. Adjust the final power resistor as necessary (4.7K, 10k, 22k?) to get your preamp B+ down under 300V.

A 6SL7 pre into parallel SE 6V6GTs is gonna be sick!

-Laird
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Old March 16th, 2011, 05:10 PM   #13 (permalink)
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Thanks...I hope it sounds sick...time will tell.

Im a first timer so Ive been researching enough to make my eyes bleed. Still working out small details that are harder to find...

i.e. The 6SL7s heater is 6.3V @ .3 ... Will the 270ex 6.3V @ .4a need to be adjusted accordingly? The 6V6 requires 6.3V @ .45a which a poster above said was ok...

I noticed that alot of amps run higher voltages into these tubes than the spec sheets suggest...is there a absolute tolerance? I hope not by trial and error!!!!

And has anyone used Edcor GSXE series OTs? I think the 3.5k ohm 15 watt OT is what I might pickup...reviews?????
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Old March 16th, 2011, 06:38 PM   #14 (permalink)
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.....
I noticed that alot of amps run higher voltages into these tubes than the spec sheets suggest...is there a absolute tolerance? I hope not by trial and error!!!!

And has anyone used Edcor GSXE series OTs? I think the 3.5k ohm 15 watt OT is what I might pickup...reviews?????
The anode voltage rating is not B+, but Vak, the voltage between the anode and cathode, so the actual voltage is lower then you think. Also, the spec's were made to ensure long life, generally that is not much of a consideration, so as long as you do not get arcing at max signal, you are ok.

Edcor GSXE OT's are very nice, and a huge bang for the buck. They are all made to order, so expect a few weeks for delivery.
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Old March 17th, 2011, 03:24 AM   #15 (permalink)
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Please Help!

So I've been trying to figure out the Duncan PSUD2 program.
Here is a screen capture of what I've got so far. Please let me know if I"m way off.
This is planned with Hammond 270EX 275-0-275 secondary 144mA. I tried my best to figure out the source impedance...not sure if I"m close.

But anyway I need 360V for parallel 6V6 plates and 300V for 6SL7....so am I to be looking at the RMS column at V(C1) etc?

There are no real instructions for this and I am a noob...so please have mercy...but I need your input and instruction.

Thanks!
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Old March 17th, 2011, 04:17 AM   #16 (permalink)
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Instead of the 5k load resistor, I like to use the built in current sink. You can change this by right clicking and changing the type to a current sink. Then you just add up the total quiescent (no signal) as well as the full power current draw (it's alright to estimate these numbers!) and setup the current sink to have a stepped response. Setup the sim times so that at the quiescent state the output voltage has enough time to settle to it's final value before you step up the current. This will show you how much voltage you can expect to drop at full load. You can get even more accurate by adding a current sink after the rectifier (acts like the KT66 plate), after the first dropper (KT66 screens) and after the last dropping resistor (all preamp stages) and stepping the current in each of these at the same time so that you can see the voltage drop at each stage at the quiescent current and max current draw. Let me know if any of this needs to be clarified.
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Old March 17th, 2011, 09:59 AM   #17 (permalink)
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Instead of the 5k load resistor, I like to use the built in current sink. You can change this by right clicking and changing the type to a current sink. Then you just add up the total quiescent (no signal) as well as the full power current draw (it's alright to estimate these numbers!) and setup the current sink to have a stepped response. Setup the sim times so that at the quiescent state the output voltage has enough time to settle to it's final value before you step up the current. This will show you how much voltage you can expect to drop at full load. You can get even more accurate by adding a current sink after the rectifier (acts like the KT66 plate), after the first dropper (KT66 screens) and after the last dropping resistor (all preamp stages) and stepping the current in each of these at the same time so that you can see the voltage drop at each stage at the quiescent current and max current draw. Let me know if any of this needs to be clarified.
Actually, I was hoping at least to learn how to read the results before I did anymore with this program. That way I know what I'm doing I'll try what you spoke of and see what I come up with. I've also read on some sites that it is best to start with a 10uf cap for a cleaner filter(but that may follow a choke)...but I've noticed that the way I have it set up above, it looks cleaner at with a 47uf or 100uf cap to start. This is why i second guess my effort....I was hoping someone would tear it apart so I would learn something
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Old March 17th, 2011, 12:51 PM   #18 (permalink)
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For 2 x 6V6 in SE parallel, your choke in a whole-of-supply CLC filter should be rated at 120mA minimum to ensure reliability (altho' 100mA may work, but don't blame me if it doesn't). Your OT reflected load should be around 3k5 to 4k. FWIW
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Old March 17th, 2011, 02:32 PM   #19 (permalink)
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For 2 x 6V6 in SE parallel, your choke in a whole-of-supply CLC filter should be rated at 120mA minimum to ensure reliability (altho' 100mA may work, but don't blame me if it doesn't). Your OT reflected load should be around 3k5 to 4k. FWIW
Yeah I calculated the impedance to be about 3700ohms I think I'm going to pickup the Edcor GXSE15-8-3.5k should work nice with a 12"weber alnicoS...but I was playing through a Reverberocket II last night with a new Celestion that sounded like a million bucks!

Anyway, I should have full schematic up later for all to tear apart. Working on some final details :)
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Old March 17th, 2011, 02:50 PM   #20 (permalink)
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If you get a 8k PR Z OT with a couple of taps (say 16R and 8R) on the secondary winding, you can use your (say 8R) speaker in either tap depending on whether you want to run 1 or 2 x 6V6s. (You would also set up the 6V6s with something like a 470R cathode resistor and a 2nd switchable 470R in parallel with that to get the 'right' amount of bias). What's more (if your PT is rated for 2 6V6s) you could also run 1 x 6L6 instead. (But if you were thinking to run 1 x KT88 in SE with a choke in a CLC supply filter, you would want the choke DC current rating to be about 200mA)
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