JTM 45 build 400v plate voltage, how to calculate OT primary impedance?

[mikepukmel]

Hi All,
I asked a similar question before, the suggestion was to learn the graphical technique. I have Merlin's book, but not sure what Im looking for (what part of the graph). The Amp should have around 400v plate voltage, with KT66 tubes. The output transformer has 9k, 8k and 6.6K primary taps.
What part of the graph would tell me the target OT primary impedance?
Thanks!
Mike

[Helmholtz]

I don't have Merlin's book but I guess it explains graphical loadline construction in PP amps. The primary impedance (Raa, Zaa) corresponds to 4 times the inverse slope of the optimal loadline.
A quick check shows that the 6.6k tap should be closest. Higher Raa will reduce output power and increase screen losses.

This old OSRAM datasheet gives some examples:
https://drtube.com/datasheets/kt66-osram.pdf
You want to look for the fixed bias configuration.

This said, original JTM45 with Drake OTs used 8K. The earlier RS OTs had 6.6K and 8k taps.

[mikepukmel]

Thanks Helmholtz, yes its the graphical load line technique. 6.6k, thanks! I tried picking through the data sheet but a common problem I have is finding definitions for the symbols.

Also, thanks for the link as well..

Is the switch to 6.6K and 8K something to do with switching to EL34? I read someplace that the early JTM45's had Radiospheres OT and KT66, then they switched transformers and output tubes as well.

[bob p]

Maybe Steve Bench's tube pages will help.

Part 3 - Of Loadlines, Power Output, and Distortion

[mikepukmel]

Thanks for the link, Bob. Reading ...

Another question, probably should have asked before: The graphical techniques draw lines on a graph that already has the tubes characteristic family curves drawn. Where can we get good quality images with these curves? I checked the data sheets and they usually don't have graphs as detailed as the ones in the example, like the "Part 3" web page.

[Helmholtz]

Is the switch to 6.6K and 8K something to do with switching to EL34?

No, all the 50W EL34 versions came after the KT66 versions and used Drake OTs (types 784-128 and 784-139) with Raa of 3.2K, which is just fine and provides max. output power with EL34s.

I read someplace that the early JTM45's had Radiospheres OT and KT66, then they switched transformers and output tubes as well.

The early RadioSpares (RS) was a multi-tap general purpose (DIY) OT. There is some good information in Michael Doyle's book "The History of Marshall". The KT66 datasheet recommends Raa=8K for cathode bias operation only. I have no idea why Marshall used 8K with fixed bias and actually don't know if the Radiospares OTs were wired for 6.6k or 8k. Undistorted (THD=10%) output power of originals with 8k was around 30W or lower depending on actual voltages and KT66 quality.

[mikepukmel]

Thanks Helmboltz. I just found some photos of a couple of originals, wow, what a build those things were with that big hole hacked in the chassis and all of those taps so close together on that little board. I read someplace that they had arcing problems since the tap connectors were so close together.

[cerrem]

Having worked on a number of original JTM45 amps with the RS OT over the years...
I would say 70% of them are wired for 6.6K whille the rest wired for 8K ....
These amps were not monkeyed with.... they had the original solder marks and dye from the factory.
Yes arcing was an issue with the pins and TAG board material...had to repair a number of them...
Original JTM45 amps when given the proper mains voltage...and biased the KT66 tubes to like 40mA each with a GZ34 typically makes 450V on the plates..
"Optimum Plate Load" is not always what guitar players like to hear....that "optimum" non-sense works best when designing transformers for Hi Fidelity equipment..
"Optimum" meaning best bang for the buck..essentially you look at your Power Output curve vs Plate Load then you compare it to the Distortion output vs Plate load curve... Between these two curves you get a intersection or best compromise of most output for the least distortion....
Regardless of the actual value of the Plate Load, the full distorted power output will always be the same...because you have a square wave ideally at 50% Duty Cycle hitting the rail voltage...
Increasing the plate load above optimum with these type of Beam Tetrodes and Pentodes will result in less clean headroom.. ie the point at which the sine wave starts to clip will come in sooner...Some players love that others hate it...
After 35 years of curve tracing and writing software to develop complex models, One thing I can pass on is to not waste time doing the full characteristic load-lines...they are an academic exercise and pure wast of time....
The reason is the load is never really linear and the load-line is reactive which makes modeling the load-line more involved......tubes, even NOS tubes are all over the place in their parameters...and then there's FEEDBACK to help linearize the situation that also needs to be accounted for in the transfer function....
Keep in mind that when drawing up a load line you need your reference plate voltage...most textbooks use the idle voltage which makes for errors... You need to use the B+ you would have when the amp is making maximum clean output signal...
Since the B+ sags in many of these amp....if not then the load-line will be off by as much as 50 Volts or more... and your calculated power output will be much higher than reality...

[mikepukmel]

Having worked on a number of original JTM45 amps with the RS OT over the years...
I would say 70% of them are wired for 6.6K whille the rest wired for 8K ....
These amps were not monkeyed with.... they had the original solder marks and dye from the factory.
Yes arcing was an issue with the pins and TAG board material...had to repair a number of them...
Original JTM45 amps when given the proper mains voltage...and biased the KT66 tubes to like 40mA each with a GZ34 typically makes 450V on the plates..
"Optimum Plate Load" is not always what guitar players like to hear....that "optimum" non-sense works best when designing transformers for Hi Fidelity equipment..
"Optimum" meaning best bang for the buck..essentially you look at your Power Output curve vs Plate Load then you compare it to the Distortion output vs Plate load curve... Between these two curves you get a intersection or best compromise of most output for the least distortion....
Regardless of the actual value of the Plate Load, the full distorted power output will always be the same...because you have a square wave ideally at 50% Duty Cycle hitting the rail voltage...
Increasing the plate load above optimum with these type of Beam Tetrodes and Pentodes will result in less clean headroom.. ie the point at which the sine wave starts to clip will come in sooner...Some players love that others hate it...
After 35 years of curve tracing and writing software to develop complex models, One thing I can pass on is to not waste time doing the full characteristic load-lines...they are an academic exercise and pure wast of time....
The reason is the load is never really linear and the load-line is reactive which makes modeling the load-line more involved......tubes, even NOS tubes are all over the place in their parameters...and then there's FEEDBACK to help linearize the situation that also needs to be accounted for in the transfer function....
Keep in mind that when drawing up a load line you need your reference plate voltage...most textbooks use the idle voltage which makes for errors... You need to use the B+ you would have when the amp is making maximum clean output signal...
Since the B+ sags in many of these amp....if not then the load-line will be off by as much as 50 Volts or more... and your calculated power output will be much higher than reality...

Cool, thanks cerrem! Re 6.6k vs 8k, are the differences for different output tube types, or are these all KT66? What would the effect be if you ran the amp, switched to 8k, then ran again, just in volume, or tone character as well?

[cerrem]

Cool, thanks cerrem! Re 6.6k vs 8k, are the differences for different output tube types, or are these all KT66? What would the effect be if you ran the amp, switched to 8k, then ran again, just in volume, or tone character as well?

The 6.6K was intended for Ultra-Linear operation using the 43% taps.....that is a "classic" optimum value.....used for 6L6, KT66 and 5881 807 family of tubes...
I believe the 8K is a "classic" value used for TRIODE connected 6L6 family of tubes...
8K is also "classic" value for pair of EL84 or pair of 6V6 tubes in pentode wiring...
Of course set plate loads need to be at specific operating voltage in order to properly operate...
As for the 6.6K vs 8K ....6.6K is cleaner and punchier and 8K is like AC/DC Highway to hell sound..

[HTH]

As for the 6.6K vs 8K ....6.6K is cleaner and punchier and 8K is like AC/DC Highway to hell sound..

That’s interesting - the closest I’ve heard to that tone is the mid 70s 50w Marshall’s with the lower B+. I’ve had a couple of Lead & Bass combos with that tone, but I believe that some heads were like that too.

Will certainly have to try an 8k OT with a JTM45 preamp some time. Did they have around 450v B+ too???

Cheers

[mikepukmel]

The 6.6K was intended for Ultra-Linear operation using the 43% taps.....that is a "classic" optimum value.....used for 6L6, KT66 and 5881 807 family of tubes...
I believe the 8K is a "classic" value used for TRIODE connected 6L6 family of tubes...
8K is also "classic" value for pair of EL84 or pair of 6V6 tubes in pentode wiring...
Of course set plate loads need to be at specific operating voltage in order to properly operate...
As for the 6.6K vs 8K ....6.6K is cleaner and punchier and 8K is like AC/DC Highway to hell sound..

Awesome, thanks for the info. Do you know of any amps that have a switch, even if inside, like with jumpers, or do you expect to wire the amp, set the bias and leave it?

[cerrem]

Awesome, thanks for the info. Do you know of any amps that have a switch, even if inside, like with jumpers, or do you expect to wire the amp, set the bias and leave it?

Nowadays, I tend to dial an amp in and then just leave it the hell alone and focus more on playing music with the band...
Switches on OT's can lead to arcing and extra parasitic inductance that can spell trouble...
I learned my lesson back when I was young and eager to constantly modify amps...One night back in the 80's I was playing a show with a Marshall Major upside down and open chassis on top my 4x12 cab... I would solder different parts in and out between songs experimenting... Well I must have had way too many beers and whiskey shots and I forgot which way the STAND-BY switch is suppose to be when the amp is upside down... Well last thing I remember I was reaching to change out something in the Phase Inverter and WAMO !!! got thrown like 8 feet and on my back ...with my Les Paul still hanging around my neck.. Big blue spark with smoke..My band-mates yelled at me to stop clowning around and get up and start the next song..somehow I got up and everything was OK..
So sometimes it's best to just leave amp be and play some music...
And yes JTM45's made 450V when everything was all lined up...

[J M Fahey]

Ouch!!!!!
Alcohol and High Voltage don´t mix, period.
Same as in my Shooting Club: they DO have a bar on the premises, and there is a door from the shooting area leading to it, it is a revolving door type : you can cross it one way but not go back, only exit is outsiden to the street, so you go home.
And they have a BIG sign: Alcohol and Gunpowder do not mix.

Back to the amps, I would not worry about the 400V vs. 450V "problem", and it definitely does not need or warrant changing OT impedance just for a little "optimization".

If standard OTs are meant for 450V supply amps, and you build a 400V one ... just leave it , amp will sound exactly the same as before , it will just put out a little less power than before, no big deal.

We are talking a meager 10/12% variation!!!!

As said above, there is way more tube to tube variation than that.

No need to be messing with (printed) tube curves for such a small difference, specially because you have no guarantee your tubes follow them at all.

In fact I suspect modern tube makers just copy old datasheets so people think modern tubes are same as old ones ... I very much doubt that.

If anything, and it worries you, get an "universal" OT like the original RS one (MM will love your cash input) or one ofn the Hammond ones or if you are really committed, a Turner Australia one , try different loads with *your* Tubes and pick the best by ear.

If you are really comitted to experimenting, these Turner Audio transformers are the real deal: primaries split in 10 sections, secondaries split in 9 , all fully interleaved, by series-parallel combining sections on each side you can get a mind boggling set of combinations, such as anything from 560 ohm to 8k , all in the same OT.
Basically he winds one layer and brings its ends out, so , say, 12 layers=12 separate windings, which you mix and match as you wish. Mind boggling possibilities and the Experimenter´s dream.
I bet there is at least one model which you buy with a 100W core and allows you to build practically any Guitar Amp ever sold, go figure.
for-sale-2-output-transformers



Funny thing is, he gives all winding parameters in the descriptions, so these transformers can be cloned by a local winder .... I probably will build one or 2, a project for long cold boring Winters or endless raining seasons.

[mikepukmel]

Ouch!!!!!
Alcohol and High Voltage don´t mix, period.
Same as in my Shooting Club: they DO have a bar on the premises, and there is a door from the shooting area leading to it, it is a revolving door type : you can cross it one way but not go back, only exit is outsiden to the street, so you go home.
And they have a BIG sign: Alcohol and Gunpowder do not mix.

Back to the amps, I would not worry about the 400V vs. 450V "problem", and it definitely does not need or warrant changing OT impedance just for a little "optimization".

If standard OTs are meant for 450V supply amps, and you build a 400V one ... just leave it , amp will sound exactly the same as before , it will just put out a little less power than before, no big deal.

We are talking a meager 10/12% variation!!!!

As said above, there is way more tube to tube variation than that.

No need to be messing with (printed) tube curves for such a small difference, specially because you have no guarantee your tubes follow them at all.

In fact I suspect modern tube makers just copy old datasheets so people think modern tubes are same as old ones ... I very much doubt that.

If anything, and it worries you, get an "universal" OT like the original RS one (MM will love your cash input) or one ofn the Hammond ones or if you are really committed, a Turner Australia one , try different loads with *your* Tubes and pick the best by ear.

If you are really comitted to experimenting, these Turner Audio transformers are the real deal: primaries split in 10 sections, secondaries split in 9 , all fully interleaved, by series-parallel combining sections on each side you can get a mind boggling set of combinations, such as anything from 560 ohm to 8k , all in the same OT.
Basically he winds one layer and brings its ends out, so , say, 12 layers=12 separate windings, which you mix and match as you wish. Mind boggling possibilities and the Experimenter´s dream.
I bet there is at least one model which you buy with a 100W core and allows you to build practically any Guitar Amp ever sold, go figure.
for-sale-2-output-transformers



Funny thing is, he gives all winding parameters in the descriptions, so these transformers can be cloned by a local winder .... I probably will build one or 2, a project for long cold boring Winters or endless raining seasons.

wow that is the most photo detail about transformers ive ever seen. Some places won't even give out drawings of the case. I guess his business is good enough he doesn't care if someone copies stuff.