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Old 20th April 2012, 01:42 AM   #1
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Default 300w Model T Clone

Howdy,

I am looking into building a custom Model T clone for a gigging pal of mine. I mentioned Verellen's new Meatsmoke amp, a 300 watt monster using KT120's and he is very interested in that kind of power (inexplicably, but money forces us to disregard our own logic sometimes) and would like it to sound like a Model T. My current (literally!) dilemma is finding the appropriate transformers and necessary power supply mods for such a build.

The Model T schematic dictates 400ma at 375 volts, so I'd be looking at somewhere around 800ma(?) at the same voltage. I've already relegated myself to using a separate filament transformer due to the absurd heater draw of the KT120's, but neither Hammond nor Weber offers a plate or output transformer like that.

Thoughts?
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Old 20th April 2012, 02:07 AM   #2
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This belongs in the Instruments and Amps forum, One of the Mods will move it.

Which Model T? The 70's first or second generation, or the Sunn by Fender 300W model?
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Old 20th April 2012, 02:15 AM   #3
Pano is offline Pano  United States
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Old 20th April 2012, 03:39 AM   #4
Enzo is offline Enzo  United States
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You are looking in the generic parts market for transformers that are pretty specialized.

If you are cloning the Fender/Sunn Model T, it is rated at 120 watts. But Fender also makes the 300T and Bassman 300, which are rated 300 watts output. The power and output transformers are the same on the two models. Fender part numbers:

050853 for power
050854 for output

Fender adds a couple extra zeroes on both ends, but they will know those numbers. Call Fender and ask if they still stock them. If so, then you can order them from any Fender dealer or parts seller. That woould get you a set of transformers for a 300 watt amp.


Maybe I am an old softy, but it seems unfair to specify the huge tubes, then turn around and call their heater needs absurd. That is like Hummer owners griping about the cost of gas.

There are other companies making amps in that range. You could probably do with a set of transformers for the Ampeg SVT Classic head.
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Old 20th April 2012, 03:40 PM   #5
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Quote:
The Model T schematic dictates 400ma at 375 volts, so I'd be looking at somewhere around 800ma(?) at the same voltage. I've already relegated myself to using a separate filament transformer due to the absurd heater draw of the KT120's, but neither Hammond nor Weber offers a plate or output transformer like that.
The Sunn Model T ran plate voltages over 500V. So no, you don't need a goofy transformer. Unless the low B+ is a requirement of the KT120. In which case I would seriously consider using some other tube.

If I received such a commission and I do indeed deliver bespoke builds of custom amps and effects and sub assemblies, I would back up and simplify the requirements. The Model T preamp section, yes, keep it. Since you want more output power (why, we will never know) just build it out with more 6550s. I'm pretty sure six of them should be enough. But hell, use eight and run them easier.

If KT120s need weird low-volt high-current supplies then they will need unusual power and output iron. KISS. I would consider the Hammond P-T1650W output tranny and shop a plate transformer for power. An SVT tranny gives something closer to 700 for a B+, regulate a screen supply and you'll be fine. Or find something that gives the 500 ish volts of the Model T with sufficient current. Military and radio surplus should supply one.

Count on needing a separate heater supply. You'll want to preheat tubes anyway. A separate bias supply tranny isn't a bad idea either.

At these voltages and current supplies this stuff starts getting a bit scary. And by scary I mean this thing could kill the f*** out of you. Yeah, yeah, that's the standard warning yada yah. But this time IT. IS. FOR. REAL.. The 450 - 500 VDC of a typical guitar or hifi amp with the usual current limitation doesn't hurt any worse than the occasional oopsy with getting a finger on a power plug. This thing OTOH will lock you on and cook you like a Ronco Weenie Roaster. You need to start thinking like industrial / military gear instead of happy little guitar amp / hifi mode.
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Old 21st April 2012, 09:36 AM   #6
es345 is online now es345  Germany
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Just to share my experience with the KT120.

I have built a power stage with 2 KT120 PP, 200W output which I think is somewhat the max you will get out of them. The configuration was 750V at plate, 570V at G2 ultralinear at full output power. I have separate G2 winding at the OT (Here in Germany you can get customized PT and OT at a reasonable price level).

The stage is running stable and with good frequency response, but there are some issues to be mentioned:

- the tube is somewhat sensitive to mechanical vibration, you can hear that through the speaker
- there is no marging to max cathode current: If you overshoot only a bit over time, you risk that that output power will go down after some hours. So avoid powerstage "overdrive"

As a result I would use them for HIFI, but not for music amplifiers. For 300W I would use either 6 KT88 or 6 GU50.
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Old 24th April 2012, 05:19 AM   #7
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I'm definitely starting to come to terms with this project... I've spent a while dealing with soft starts, separate bias transformer, etc. On top of that I wasn't particularly pleased with the OD I got out of a Verellen KT120 based amp (it was going through a hot plate)... they were too fuzzy for Doom (which is itself pretty dependent on low order harmonics via power tube OD. Thus I think I will tone this down to a relatively easy 200 watt config with 6 6550s. I'm still gonna have a separate heater transformer, mostly because I like to run amps on a variac without dropping the heater and stripping the hell out of the cathodes.

Any recommendations on filter chokes? Sunn doesn't mention their inductance, power rating, or resistance. I would guess ~600mA at about 1H, but I'm not sure.
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Old 24th April 2012, 07:55 AM   #8
Enzo is offline Enzo  United States
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Oh, don't make me look up the schematics every time, how about some part numbers?

The Sunn Model T with 120 watts probably won;t be the "correct" size parts for your 300 or 200 watt power stage anyway.

The Fender part number for the Sunn Model T amp is 0036486000. There is no description other than choke in the swervice literature, but you might contact Fender and ask what the specs for it are. Worst can happen is they say no.
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