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Post new topic KT66 Deluxe Reverb Style amp
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Author Topic:  KT66 Deluxe Reverb Style amp
Mike Scaggs


From:
Nashville, TN
Post Posted 23 Aug 2017 7:53 pm     Reply with quote

Well I usually talk steel guitars but is another amp I just finished for my good customer in KS. It is a Deluxe Reverb style amp but with many mods. Now for you amp guys you will get this right off.

Big big iron to support the current draw of those tube
Big output transformer pushing 2x10 Celestion Golds 8Ω
2 Gold Lion KT66 Power Tubes
5AR4 tube rectifier
All vintage blue mallory coupling caps (yeah!)
NOS preamps tubes at a whopping 240vDC on the plates (nice and strong)
420vDC on the plates B+ @ 117vAC
Midrange control in the back
Power section coupling caps are .047uF instead of the flabby .1s

Theres more but you guys get the picture.

For you non-electronic guys, this amp will blow your head off! Great guitar amp but never tried it on steel yet.

Anyway, thought I would show you something you might never see anywhere else...

Cheers,







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Godfrey Arthur


From:
Philippines
Post Posted 23 Aug 2017 9:00 pm     Reply with quote

Juicy...
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Don Mogle


From:
Fort Worth, TX, USA
Post Posted 27 Aug 2017 3:20 pm     Reply with quote

Mike,

Your work in impeccable! Beautiful work on the electronics.

Man, I sure wish you lived in the Dallas/Fort Worth area.

Don
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Tim Heidner


From:
Port Arthur, TX
Post Posted 27 Aug 2017 6:13 pm     Reply with quote

Not sure what makes that a Deluxe Reverb other than the name plate, but I bet it sounds great!
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David Gertschen


From:
Phoenix, Arizona
Post Posted 27 Aug 2017 7:20 pm     Reply with quote

If that amp sounds as good as it looks, I'd bet it's a killer...Although your dog doesn't seem all that enthused about it, lol!
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James Quackenbush


From:
Pomona, New York, USA
Post Posted 29 Aug 2017 11:54 am     Reply with quote

N-I-C-E !!!!!!!!!..........Jim
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Michael Butler


From:
California, USA
Post Posted 29 Aug 2017 2:45 pm     Reply with quote

great build! i have an original 66 that is amazing.

what change does the .047 caps do vs. the .1?

play music!
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Mike Scaggs


From:
Nashville, TN
Post Posted 31 Aug 2017 5:31 am     Reply with quote

Michael Butler wrote:
great build! i have an original 66 that is amazing.

what change does the .047 caps do vs. the .1?

play music!


It fattened up the Bass response and makes it tighter to mine and others ears...
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Mike Scaggs


From:
Nashville, TN
Post Posted 31 Aug 2017 5:40 am     Reply with quote

Tim Heidner wrote:
Not sure what makes that a Deluxe Reverb other than the name plate, but I bet it sounds great!


Absolutely right Tim. This is more like a super hotrod Vibrolux Reverb in most ways. The trannies are way bigger however to cover the filament current on the KT66s and a matching big Output tranny too running at 4ohms. There was a bit of metal work on the chassis to get it all to fit, careful planning is needed. This chassis with the big iron will not fit a 1x12 standard DR cabinet so 2x10 was my choice, it was a great choice IMHO. The circuit is what you would expect AB763 with some tweaks. The screen resistors are bumped up to 1K and the control grid resistors are bumped from a 1.5K to 5.6K to avoid some parasitic oscillation. Another easy mod was wiring the power tube sockets to accept EL34s, 6L6s, LT66s, 5881s, 6550s, etc etc ... You builders know how to do this.

This is one of those cases where the right combination of circuit, cabinet size, speaker choice, tube choice, etc all came together in a fantastic way. In fact, I loved it so much I actually kept one for myself LOL. The fact I can build whatever I want shows how much I love these.

You builders out there should give one a whirl, I think you will be very pleased in the end.
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Last edited by Mike Scaggs on 6 Sep 2017 4:08 am; edited 1 time in total
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Steve Pawlak


From:
Arizona, USA
Post Posted 31 Aug 2017 7:52 pm     Reply with quote

Beautiful work Mike!
As a novice I recently built a JTM-45 clone from a kit from the Tube Depot
It was my my first build and it fired right up.
Sounds terrific I may change power tubes to KT66


Last edited by Steve Pawlak on 9 Sep 2017 5:22 am; edited 1 time in total
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Mike Scaggs


From:
Nashville, TN
Post Posted 1 Sep 2017 3:35 am     Reply with quote

Great job! Now it's time to move on to the next phase and scratch build an amp, you can do it...

On a side note: I had yet another so called custom shop fender show up for repair. Geeze those things are terrible. They do make me coin constantly repairing them 😆😆😆
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J Fletcher


From:
London,Ont,Canada
Post Posted 1 Sep 2017 3:49 am     Reply with quote

Nice work on those amps, so tidy!
Mike, did you mean you put 5.6 k grid stoppers on the KT66's?
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Mike Scaggs


From:
Nashville, TN
Post Posted 1 Sep 2017 5:05 am     Reply with quote

J Fletcher wrote:
Nice work on those amps, so tidy!
Mike, did you mean you put 5.6 k grid stoppers on the KT66's?


These resistors, which are commonly called "grid stoppers", are not put on the control grid of the tube for signal level attenuation purposes; rather, they act as a very high frequency low-pass filter in conjunction with the input capacitance of the triode (which is the sum of the grid-to-cathode capacitance and the Miller capacitance, and can get as high as 100pF or more). In the normal operating mode of a vacuum tube, the grid is biased negatively with respect to the cathode. Because of this, there is no current flow into the grid element, and it looks like a very high impedance circuit node. This means that there can be little or no midband attenuation of the input signal, because the voltage divider formed by the series resistor and the high input impedance of the tube is very small. For all practical purposes, the attenuation is negligible at midband, so there is no "increase in gain" by removing these resistors. Attenuation only occurs at the higher frequencies, above the frequency breakpoint caused by the series resistance and the input capacitance.
The grid resistor accomplishes the following things:

It helps prevent high frequency parasitic oscillation in the tube itself
It helps prevent radio frequencies from getting into the input stage, where they can be rectified and lowpass filtered (AM detection) and become audible at the amplifier output
It can limit grid current when the tube is driven into the positive grid region, which helps in preventing "blocking" distortion

So..... yes some people call them "Grid Stoppers"
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J Fletcher


From:
London,Ont,Canada
Post Posted 1 Sep 2017 4:29 pm     Reply with quote

I only mentioned the grid stoppers , because your post originally said "plate load resistors". I knew you meant grid stoppers , but some may have been confused. Just trying to clarify. Nice work on that amp.
Must sound killer.
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Tom Wolverton


From:
San Diego, CA
Post Posted 1 Sep 2017 5:29 pm     Reply with quote

Mike, what is the wattage of the amp?
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Mike Scaggs


From:
Nashville, TN
Post Posted 1 Sep 2017 7:52 pm     Reply with quote

Tom Wolverton wrote:
Mike, what is the wattage of the amp?


I didn't measure it and do the math to be honest. If I had to take a guess it would be 50 watts... It is louder than I need really but I love it
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Chris Boyd


From:
Leonia,N.J./Charlestown,R.I.
Post Posted 2 Sep 2017 5:11 am     Reply with quote

Superb work... what a cool amp ! Very Happy
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Mike Scaggs


From:
Nashville, TN
Post Posted 6 Sep 2017 4:09 am     Reply with quote

J Fletcher wrote:
I only mentioned the grid stoppers , because your post originally said "plate load resistors". I knew you meant grid stoppers , but some may have been confused. Just trying to clarify. Nice work on that amp.
Must sound killer.


Ha! I see it now, I meant control grid resistors which of course is the same as grid stoppers. Thanks for busting me Smile
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John Goux


From:
California, USA
Post Posted 8 Sep 2017 5:09 pm     Reply with quote

KT-66's sound great in a Vibrolux Reverb. J
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Mike Scaggs


From:
Nashville, TN
Post Posted 8 Sep 2017 5:32 pm     Reply with quote

John Goux wrote:
KT-66's sound great in a Vibrolux Reverb. J


It would except a Vibrolux Reverb power transformer is to small for the filament current of a KT66.
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Steven Paris


From:
Los Angeles
Post Posted 11 Sep 2017 5:17 am     Re: KT66 Deluxe Reverb Style amp Reply with quote

Mike Scaggs wrote:
NOS preamps tubes at a whopping 240vDC on the plates (nice and strong)

Why the extra-high voltage on the preamp tube plates?
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Mike Scaggs


From:
Nashville, TN
Post Posted 11 Sep 2017 5:20 am     Re: KT66 Deluxe Reverb Style amp Reply with quote

Steven Paris wrote:
Mike Scaggs wrote:
NOS preamps tubes at a whopping 240vDC on the plates (nice and strong)

Why the extra-high voltage on the preamp tube plates?


Well that was a product of the old Mullards I stuck in there. I had a new TAD 12AX7 in V2 that measured out at 175vDC, 170vDC is pretty normal. Stuck that Mullard in at 240vDC and that amp came to life. Ultimate tube tester, the amp itself Smile
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