KT66 Push Pull Output Transformer

[Ed_Chambley]

Questions for some of you helpful folks.  Here is a link to the datasheet: http://frank.pocnet.net/sheets/086/k/KT66.pdf

I was using a EL34, original Drake multitap 50 watt transformer, 3.4k Reflected, 4-8-16 and using the 4 ohm tap to a 8ohm cab yielding ~ 6.8K for the primary.  As far as I can tell the KT66 wants ~8K R a-a so not too far off.  Preferring to get a 4 ohm tap available I ordered and installed a Classictone transformer because a lot of Marshall "gurus" suggested I needed the correct tranny that has a not use 100v tap.  I have no idea why this is so important, but some say so.

Attached is the Transformer spec sheet I have.  The outcome is not as nice as the Drake IMO.  Power increased slightly, but I seem to have lost a fair amount bottom end crunch.  I believe it is as it should be, but I prefer the tone of the lower primary.  I even tried a lower primary of 6K from a Tweed Pro and liked the results.  Going any lower doesn't sound good at all.

I am going to be building a few of these and I am in unfamiliar territory with the KT66.  I have a plate voltage of 460, but it is a tube rectified design and with SS rectification I get 495 plate voltage. 

So my questions are why would get more distortion with the lower Primary winding?  Simply a result of a slight mismatch?

I assume a Primary of 6.8k is close enough not to cause problems with the tubes or Transformer, is this correct?  I may get some wound.

Anyone know of another Guitar Amp that used KT66 other than the JTM45 and Route 66?

[uki]

What amp are you running that stuff ?! 

[JB]

The early JTM45's used the RS "De-Luxe" output transformer which had 6.6K, 8K and 9K primary windings. Marshall, by design or accident, wired them as 6.6K, and used KT66's at around 440-460V.

When the production numbers increased Marshall switched to Drake output transformers.  I think at first running KT66's at 8K before changing the design to EL34's into 3.4K transformer.

A mate of mine very kindly gave me one of these and I built a JTM 45 around it.  I run a pair of KT66's in it at about 450V and it sounds fantastic!  There are re-pro RS transformers available with the same windings and some people use the 8K load but most seem to stick with Marshalls original 6.6K plan. 

So either 6.6 or 8K will work, tone/distortion will change slightly between those two taps on the same transformer and also if using a different make of transformer.  Have a look at a KT66 datasheet, you'll see a graph of distortion vs load with lower distortion for higher load (until the output power starts to drop off).   You've changed both - different brand and, presumably, the Classictone re-pro Drake unit will be 8K.

[tubenit]

Another KT66 push/pull amp schematic here:   http://www.turneraudio.com.au/leakampmods.htm

[Ed_Chambley]

What amp are you running that stuff ?! 

UKI, I have 2, 5879 Pentodes setup.   One on the normal channel of a Marshall Topology and another on the Bright Channel.  Each preamp tube can be ran individually in Triode and Pentode mode.  They also have a Jump switch to allow the mixing parallel and a Cascade switch for series.  I am currently adding relay switching.  Each 5879 has a 1K pot for a cathode resistor.  The coupling caps from the first stage have been increased above standard Plexi from .022 to .01 to achieve a cleaner tone..  Consider it very similar to a early Gibson GA40.

The thinking is like a Plexi where you mix channels, except much more variety and a lot thicker tone when wanted.  So it is 5879-5879-12Ax7 CF, 12Ax7 LTPI to 2, KT66.

Since I am experimenting I only have hand drawn schematics, but will certainly knock one up when I have decided what the final will be.  Right now I am using a Decade Box to determine each cap value and Pots as variable resistors to find the resistor values I like at each stage and watching the scope.

The pentodes allow for much more versatility, but I found rather quickly I had way too much manual adjusting and switching which unless setup as channel switching relay they will not get used much.

All that said, I am extremely happy with the amp at this stage of design and have learned much about the preamp, I did not expect the Output Transformer to make much difference as normally you may hear a slight difference, but here it is obvious.

[Ed_Chambley]

Another KT66 push/pull amp schematic here:   http://www.turneraudio.com.au/leakampmods.htm

Thanks Jeff!

[Ed_Chambley]

The early JTM45's used the RS "De-Luxe" output transformer which had 6.6K, 8K and 9K primary windings. Marshall, by design or accident, wired them as 6.6K, and used KT66's at around 440-460V.

When the production numbers increased Marshall switched to Drake output transformers.  I think at first running KT66's at 8K before changing the design to EL34's into 3.4K transformer.

A mate of mine very kindly gave me one of these and I built a JTM 45 around it.  I run a pair of KT66's in it at about 450V and it sounds fantastic!  There are re-pro RS transformers available with the same windings and some people use the 8K load but most seem to stick with Marshalls original 6.6K plan. 

So either 6.6 or 8K will work, tone/distortion will change slightly between those two taps on the same transformer and also if using a different make of transformer.  Have a look at a KT66 datasheet, you'll see a graph of distortion vs load with lower distortion for higher load (until the output power starts to drop off).   You've changed both - different brand and, presumably, the Classictone re-pro Drake unit will be 8K.

So are you saying you used a Radiospare with a 6K6 load or did you use the 8K load?  I did see the datasheet and it is also very obvious as I mentioned.  I can see the Classictone Transformer will deliver more clean power, but going from graphs to ears is quite a different thing.

I will say either arrangement sound really good, but the feel changes a lot.  With the 6.8K load I had, if you dig in it makes a very nice crunch whereas the 8K seems to stay much more focused. 

Thanks for your reply as I did not know most use a 6.6k, but I can see why.  Tends to have a more Marshall feel and it does it at a club level volume.

[JB]

My amp is running the RS transformer at 6k6. I haven't tried it on the 8K taps. Those that have say its a little bit cleaner. 

[tubenit]

From London Power

I thought impedance matching was critical. Some designers say the output transformer must be changed if you want to use different output tubes. That seems awfully expensive.

A: It is awfully expensive, and awful that such things would be suggested. There are two issues here, though; one is the notion of "impedance matching", and the other is simple design preference.

As stated throughout the TUT-series, speaker load impedances and reflected loads to the output tubes are all "nominal". An 8-ohm speaker may actually look like anything from 6-ohms to 100-ohms, depending on the frequency, since the reactive impedance changes with frequency. This means that the reflected load to the tubes is varying widely over the frequency range.

A nominal 8-ohm load may reflect 4k to the plates of the output tubes with a given transformer. The amp might be designed to produce its maximum power into this load, with a designed frequency response. This is the "power bandwidth". If we change the load to 16-ohms, the reflected load doubles and the frequency response shifts upward. We lose bass but have a brighter sound, and also lose power. If we change to a 4-ohm load, the reflected impedance drops to 2k, into which the tubes produce less power, and the bandwidth is again narrowed.

The reason for the confusion, I believe, is that people think tubes will try to behave the same way transistors do. Into half the load impedance, a transistor will try to deliver twice as much current. The device may overheat and destroy itself in the process. Tubes, however, simply don't behave like transistors.

The design issue for impedance matching comes into play when a designer takes the approach that "everything is critical". In some circuits, this may be the case. Tubes don't really care. There is no optimum load for a tube unless you are going for minimum THD, and this then depends upon the other operating conditions. For guitar, criticality is purely aesthetic. The designer says "this is good", "this is bad" and in that decree believes it to be so. He is correct in his subjective impression, but should not confuse the subjective and objective.

Ed, I've never worried too much about OT impedence matching.  Unless I know something that would be obviously a safety concern,  I simply use what sounds "good" to me even if it's not on the amp building "culturally approved" list.
 

With respect, Tubenit

[Willabe]

The early JTM45's used the RS "De-Luxe" output transformer which had 6.6K, 8K and 9K primary windings. Marshall, by design or accident, wired them as 6.6K, and used KT66's at around 440-460V.

Ed, Tone Quest Report did an interview with 1 of the most popular Marshall clone amp builders and he was dead set on that it's a must to use the OT like above with the multi primary taps to get the right sound. He has his own clone/copy OT's wound that way.   

[Ed_Chambley]

From London Power

I thought impedance matching was critical. Some designers say the output transformer must be changed if you want to use different output tubes. That seems awfully expensive.

A: It is awfully expensive, and awful that such things would be suggested. There are two issues here, though; one is the notion of "impedance matching", and the other is simple design preference.

As stated throughout the TUT-series, speaker load impedances and reflected loads to the output tubes are all "nominal". An 8-ohm speaker may actually look like anything from 6-ohms to 100-ohms, depending on the frequency, since the reactive impedance changes with frequency. This means that the reflected load to the tubes is varying widely over the frequency range.

A nominal 8-ohm load may reflect 4k to the plates of the output tubes with a given transformer. The amp might be designed to produce its maximum power into this load, with a designed frequency response. This is the "power bandwidth". If we change the load to 16-ohms, the reflected load doubles and the frequency response shifts upward. We lose bass but have a brighter sound, and also lose power. If we change to a 4-ohm load, the reflected impedance drops to 2k, into which the tubes produce less power, and the bandwidth is again narrowed.

The reason for the confusion, I believe, is that people think tubes will try to behave the same way transistors do. Into half the load impedance, a transistor will try to deliver twice as much current. The device may overheat and destroy itself in the process. Tubes, however, simply don't behave like transistors.

The design issue for impedance matching comes into play when a designer takes the approach that "everything is critical". In some circuits, this may be the case. Tubes don't really care. There is no optimum load for a tube unless you are going for minimum THD, and this then depends upon the other operating conditions. For guitar, criticality is purely aesthetic. The designer says "this is good", "this is bad" and in that decree believes it to be so. He is correct in his subjective impression, but should not confuse the subjective and objective.

Ed, I've never worried too much about OT impedence matching.  Unless I know something that would be obviously a safety concern,  I simply use what sounds "good" to me even if it's not on the amp building "culturally approved" list.
 

With respect, Tubenit

Thanks for that.  I know matching can vary or I would have never hooked up the Drake I have.  What I did not know is the bandwidth narrows either way from "optimal" until I began with this build.  For a minute it threw me, but after some thought it began to make some sense.  That is after thought and multiple sine waves.

I do like the 6K8 and it looks like I will be using the 6K6 as suggested.

Now to find one to test.  Thanks for your interest.

[Ed_Chambley]

The early JTM45's used the RS "De-Luxe" output transformer which had 6.6K, 8K and 9K primary windings. Marshall, by design or accident, wired them as 6.6K, and used KT66's at around 440-460V.

Ed, Tone Quest Report did an interview with 1 most popular of the Marshall clone amp builders and he was dead set on that it's a must to use the OT like above with the multi primary taps to get the right sound. He has his own clone/copy OT's wound that way.   

That is what I am finding.  I thought I knew Marshall amps well and have built some fine EL34's based amps.  I am going out on a limb here and say I think in a 2 tube the KT66 is the way to go.  I did get to play a JTM45/100 with 4 KT66 and it is not usable being so loud and why build something with tubes if you cannot get it loud enough to hear the power tubes?  They distort more smoothly than an EL34, but you have to push them more.  The difference is a lot with the lower primary.  It is enough where you could use the amp in a club, but with the Classictone 8K you couldn't get into the amps wheelhouse without earbleeding and I don't like to use attenuators or master volumes when playing live.

I think I am going in the right direction now.  I wouldn't be so particular about this, but if you played the amp both ways you would agree.  Man it is FAT and CHEWY.  I wish I had read that Tone Quest article, it would have saved me time, but I think I have learned some thing which is always good.

[uki]

UKI, I have 2, 5879 Pentodes setup.   One on the normal channel of a Marshall Topology and another on the Bright Channel.  Each preamp tube can be ran individually in Triode and Pentode mode.  They also have a Jump switch to allow the mixing parallel and a Cascade switch for series.  I am currently adding relay switching.  Each 5879 has a 1K pot for a cathode resistor.  The coupling caps from the first stage have been increased above standard Plexi from .022 to .01 to achieve a cleaner tone..  Consider it very similar to a early Gibson GA40.

The thinking is like a Plexi where you mix channels, except much more variety and a lot thicker tone when wanted.  So it is 5879-5879-12Ax7 CF, 12Ax7 LTPI to 2, KT66.

You know I'm not that smart about amps/electronic but I can tell you have something that seen to be unique, and the tone as well is probably unique,
I didn't even know about this 5879 tube and that it can be used in tube amps,

What LTPI means? Something to do with phase inverter?

How many watts your amp does? (trying to find what the next build will be  )

Since I am experimenting I only have hand drawn schematics, but will certainly knock one up when I have decided what the final will be.  Right now I am using a Decade Box to determine each cap value and Pots as variable resistors to find the resistor values I like at each stage and watching the scope.

The pentodes allow for much more versatility, but I found rather quickly I had way too much manual adjusting and switching which unless setup as channel switching relay they will not get used much.

Keep good documentation/notes of your findings and in hand, it will prove worth in the long run. That is one thing I can tell by experience.

All that said, I am extremely happy with the amp at this stage of design and have learned much about the preamp, I did not expect the Output Transformer to make much difference as normally you may hear a slight difference, but here it is obvious.

I bet !! Indeed just by building an amp is a big learning curve even for those that are already experience, specially when doing something that isn't the usual thing.
Some guy around here that also build amps, was saying something about OTs. The way the coils are done, the type of the metal in the core, does some difference in the results. Crossing coils and grain oriented metal for the core, I got a cheap OT for my amp, it does sound good but not THAT good, imagine why now ...   also I've fond out the OT primary coils are not balanced, it does 19/21.

I did like a lot the little you said about this amp, got me curious, some pics maybe ?!!   

[Ed_Chambley]

LTPI- Long Tail Phase Inverter.  Same as in your build.  Watts about 33 with 6.8K clean.  Did not measure the 8K yet.

Did you say photos?
Sure, work in progress

[Ed_Chambley]

More

[Ed_Chambley]

More

[Ed_Chambley]

more again, sorry if some look the same.

It looks nothing like this now.

Once I get everything worked out, I will rebuild most everything.

[Willabe]

Well, lookie lookie there at the different brands/types of coupling caps. 

[Ed_Chambley]

Well, lookie lookie there at the different brands/types of coupling caps. 

That is very common for me.  The Blue Sozo's are no longer there.  They have been replaced with NOS Aerovox.  And yes, I am nutty about them and can easily hear differences.  This amp has some Sozo yellows in it they do not even make.  John made some samples of different values when he gave me all those.

If you look close you may see 2, .033 Sozo Vintage Mustards neat the PI.

I do have enough Philips Mustards NOS to build 4 of these thing which is what I will probably end up using.

[Ed_Chambley]

Well, lookie lookie there at the different brands/types of coupling caps. 

Lots of different resistors as well.  It is very sloppy, but I am prototyping.

[uki]

Man this looks good !! Thanks for sharing!!

LTPI- Long Tail Phase Inverter.  Same as in your build.  Watts about 33 with 6.8K clean.  Did not measure the 8K yet.

Did you say photos?
Sure, work in progress

I like clean sound, I don't like when the amp drive at low volume, takes away the possibility to play clean.
My amp does well in the low input until 6-7 nice clean and loud sound, now in the high input it start having some saturation at 4-5.

Thanks , yes I mean photos, pic is a short for picture.

[Paul1453]

What we could see of your bench setup looks good too, IMO.   
Got to have those little, see through drawers of parts handy.
Mine has my 1/2-1W and 2W+ Ohmite resistor drawers stacked next to it.