My JTM45/100 and 20 watt 2204 builds

[fusionbear]

Hey guys, I haven't been around here much, but here is a clip of my JTM45/100 and 20 watt 2204 in a A/B set up. Was shooting for vintage Jimi/Trower tones.

https://www.youtube.com/watch?v=e8q2yLsPBfU

Thanks for looking....

[danman]

Nice job...they both sound great! I may have to put together a 45/100 myself here soon!

[Haze13]

What are the Outputs for the 2204?

[Haze13]

By the way, sounds very good!

[fusionbear]

What are the Outputs for the 2204?

They are two JJ 6V6 tubes into a Classictone 6.6k Primary TW Express OT

[Haze13]

Any mods? I see 4 preamp tubes... Have you changed the feedback? 2 bridges - one for DC for led I see 4 output jacks... Cathode biased outputs? No buss wire on the backs of the pots! What is the circuit with a 2 x 39nF caps?

Clean build! Looks nice!

[fusionbear]

Any mods? I see 4 preamp tubes... Have you changed the feedback? 2 bridges - one for DC for led I see 4 output jacks... Cathode biased outputs? No buss wire on the backs of the pots! What is the circuit with a 2 x 39nF caps?

Clean build! Looks nice!

The tube with blue base and shield is the loop recovery tube. It is a 12AT7. The loop comes from the MV pot to the send, it has a 1M for attenuation before it hits the send jack. After the return jack it hits a 47k grid stop with a 1 Meg to ground. The output of the 12AT7 is super clean with lots of headroom so it doesn't change the tone. It feeds the PI circuit. I used .039's instead of .022's because they were the only affordable mustards I could find. In that part of the circuit, it barely affects the tone. The bridge by the filter caps is the high voltage bridge. The other is coming from the extra 5 volt tap of the PT and feeds dc to the bright green indicator LED only. Standard fixed bias bias circuit, not cathode biased. Two output jacks only, the other two are for the loop. I can't stand the buss bar being soldered to the backs of the pots, it makes the ground sketchy IMO.The preamp is all stock values, including the stock 10k cathode resistor on V1b. I wanted as close to stock 2204 types as I could. The amp rocks IMO...

[bill bokey]

What power tubes are you using on the 45/100 and what's the B+ ?

[fusionbear]

What power tubes are you using on the 45/100 and what's the B+ ?

a pair of KT88's @ 560B+

Filtering is B1+ 32uf, B2+ 23.5uf, B3+ 47uf, B4+ 16uf

V1 is wired in parallel to "Brilliant" spec, no "Normal" channel

[bill bokey]

Thanks !

Is this the OT you're using ? http://www.classictone.net/40-18067.html
Ever tried the same setup with KT66s ?

[fusionbear]

Thanks !

Is this the OT you're using ? http://www.classictone.net/40-18067.html
Ever tried the same setup with KT66s ?

Yes, that is correct. I have built other JTM45/100's with the same OT with 4 -KT66's and the amps sounded really great. I prefer the pair of KT88's. The Primary impedance is perfect for a pair of KT88's

The problem with KT66's is that modern ones don't handle the higher voltages too well and using lower voltage takes away from the tone IMO. The KT88's can handle the voltage and still sound beautiful IMO... Many guys make the mistake of using a quad of KT88''s in the JTM45/100 but I think is sounds like a horrible, mushy mess that way.... YMMV...

[bill bokey]

So you use a pair of KT88 instead of 4 x KT66 ? With exactly the same setup, B+ etc... ?

What KT66s did you try ? I'm currently building a 45/100 and was wondering what KT66s could handle the high B+. I'll try 2k2 screen resistors with the Shuguang quad of KT66 I have.

[Haze13]

Have you tried the triode operation of these tubes... 4k is exactly what they need for this thing, the voltage is high though... I'am thinking a lot of this...
There are many amps that have a switch that can change pentode to triode operation, but the problem is that the amp that was designed for pentode operation is not going to sound good in triode mode because of the different primaries, bias, voltages are not the same, and the currents are different, so basically it's just for attenuation or some sort of "Why not" feature.

About 2 kt88... The max power dissipation for that tube is 42 watts... running 2 of them for 100w operation is going to kill them very quickly. Ampeg SVT 300w suffers from this because there are 3 pairs of 6550 and each pair has to give about 100w to get 300w and this kills power tubes very very quickly. I work as a tech and I saw couple of these... New production tubes are not holding this power very good. There are good tubes like Sovtek 6550ew and if they are matched very tight, they will work about 2-3 years.

The reason I wrote all of this, is that you can try kt120 in place of kt88. They are rated for 60 watts anode dissipation, there is no reason to mod the amp, because they can replace 6550/kt88 without any mods. BUT! Their filaments rated for 2A, but since you have an amp that can accept 4 kt66 (1.6A each tube) there should be no problem with that. I've read that people who have build 100w with two 6550/kt88 prefer the kt120 sound wise and that's the reason why there are no 100w audio amps that use 6550/kt88 pairs. Running el84 beyond their maximum is one thing, but 6550/kt88 is another...