Simulate KT88's in a Plexi

Darrin66

Inspired
My friend plays in an awesome Zeppelin tribute band that focuses on TSRTS live era, and the rumor is Page had KT88's in his plexi at the time. I installed a quad in his Metroamp 68 12000 head and we both thought it sounded great, more open and clearer than the EL34's. I changed the bias resistor, rebiased, and that was it, same as Royal amps (determined from pics).

I have a pretty good Page preset started but I want to simulate this in the AF3. From reading the Tech Notes, it's my understanding that if the OT in the plexi has a primary impedance of 1.75k, and the KT88's are usually installed in a circuit that has an OT with a 2.4-2.6k primary impedance (not 100% certain of these KT88 OT numbers, got them from a quick internet search), then I should set the Transformer Match level to about 0.7? Determined this by 1.75k/2.5k = 0.7. Going to try it tonight to see how it sounds. Anybody play around with this to see how it sounds?
 
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I mod plexis with different tubes and transformer settings but just by ear.
Gets interesting results and characteristics i love it.
 
Aren't KT88's and 6550's the same thing ?
Pretty much, although I think the 88’s have a higher voltage rating. But I could be wrong.
Yes, they are the same architecturally speaking. But the KT-88s are more robust and have some higher voltage ratings and also draw a bit more heater current. So you have to be careful that the PT will also deliver that current if you swap them in a real amp.

Keep in mind also that modern reproductions of the 6550 are often closer to KT88 specs in many ways than they were when the Tung Sol 6550s were first introduced.
 
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My friend plays in an awesome Zeppelin tribute band that focuses of TSRTS live era, and the rumor is Page had KT88's in his plexi at the time. I installed a quad in his Metroamp 68 12000 head and we both thought it sounded great, mope open and clearer than the EL34's. I changed the bias resistor, rebiased, and that was it, same as Royal amps (determined from pics).

I have a pretty good Page preset started but I want to simulate this in the AF3. From reading the Tech Notes, it's my understanding that if the OT in the plexi has a primary impedance of 1.75k, and the KT88's are usually installed in a circuit that has an OT with a 2.4-2.6k primary impedance (not 100% certain of these KT88 OT numbers, got them from a quick internet search), then I should set the Transformer Match level to about 0.7? Determined this by 1.75k/2.5k = 0.7. Going to try it tonight to see how it sounds. Anybody play around with this to see how it sounds?
A quad of KT88s would usually have about 2.2K primary Z. A Plexi typically has around 1.75K so Transformer Match would be 1.75/2.2 = 0.8.

As always use your ears. The reason the amp sounded more "open" is precisely because the transformer became undermatched.
 
A quad of KT88s would usually have about 2.2K primary Z. A Plexi typically has around 1.75K so Transformer Match would be 1.75/2.2 = 0.8.

As always use your ears. The reason the amp sounded more "open" is precisely because the transformer became undermatched.
Thanks Cliff, will do!
 
Aren't KT88's and 6550's the same thing ?
I built a 150W SSS for my other friend (SRV Tribute) and when we swapped between new production EH 6550's and JJ KT88's, the KT88's won, better defined bass/low mids, or at least that's what we perceived at the time. The low end thump and clarity coming from that head into the Dumble 4x12 clone cab with EVM-12L's is unlike anything else I've ever heard, in person. I'm guessing that this corresponds to Cliff's Tech Notes explanation, the perceived response differences between the 6550's and KT88's would be due to the OT mismatch for one of the quads and surrounding circuit, but that was the real life situation we were in, we only had one OT, and I didn't change anything else, Negative feedback, etc. I wasn't looking to redesign the amp, we just had a set of each so we tried them. So as close as those tubes might be on paper, the sound really did change when the tubes were swapped and rebiased. If I'm wrong, hopefully someone will chime in and explain.
 
You know there's an option to change tube type to KT88 in the Axe right?
From my understanding, when you select KT88's, the corresponding OT match to KT88's is corrected as well, that's why you don't hear much of a change. I'm looking for the effect that putting KT88's in the Plexi with the OT specified for EL34's gave us. Same as always, If I'm wrong someone please explain.
 
From my understanding, when you select KT88's, the corresponding OT match to KT88's is corrected as well, that's why you don't hear much of a change. I'm looking for the effect that putting KT88's in the Plexi with the OT specified for EL34's gave us. Same as always, If I'm wrong someone please explain.

I hear a big change when I switch from the EL34 Mullard (default on almost every Marshall model) to almost anything else, especially the KTs. There was a time when changing power tube types didn't do much in the Axe III (to my ear), but at some point changing power tube type and preamp tube type became very noticeable.
 
This is one of the great beauties of modeling... the opportunity to mess with the interaction between the tubes and the OT in virtual space! In a real amp, mismatching impedance may well change the sound of course. But it also affects the potential power transfer through the OT to the speaker load which may matter to you for a variety of reasons in a real amp. But the modeled amp doesn't care when your real world power may actually be being developed by some real (and possibly transparent) amp external to the model. Coolness.
 
Yeah, you're not limited to the real world durability of components as well. You can mismatch transformers, crank the tube bias and dime the volumes all day long and not worry about burning through a set of tubes in a few hours or smoking out a transformer. You get the sound effects without the negative impacts on the components themselves.
 
Yeah, you're not limited to the real world durability of components as well. You can mismatch transformers, crank the tube bias and dime the volumes all day long and not worry about burning through a set of tubes in a few hours or smoking out a transformer. You get the sound effects without the negative impacts on the components themselves.
Sure but then the "analog is better" crowd will say modellers just dont have that 'something special' you hear when playing thru an amp with speakers and components that are actually on fire, smoking, or covered in beer (or other spilled liquids). 🙂
 
A quad of KT88s would usually have about 2.2K primary Z. A Plexi typically has around 1.75K so Transformer Match would be 1.75/2.2 = 0.8.

As always use your ears. The reason the amp sounded more "open" is precisely because the transformer became undermatched.
It's nice to have a ballpark to start in, tho. Saves time....
 
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