Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Leevi on March 08, 2012, 04:31:35 am
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Marshall JTM45/100 is a some kind of reissue model with KT66 power tubes.
It has a dual OT power amp. Has that been implemented with two separate OTs or is it only one big OT.
Schematic:
http://forum.metroamp.com/download/file.php?id=4349&sid=afaaaa3889bd89bbef5bb0151715f9cc (http://forum.metroamp.com/download/file.php?id=4349&sid=afaaaa3889bd89bbef5bb0151715f9cc)
Marshall specs:
http://marshallamps.com/product.asp?productCode=JTM45-100&pageType=SPECS (http://marshallamps.com/product.asp?productCode=JTM45-100&pageType=SPECS)
/Leevi
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on the Marshall website you can see that it is 2 OT's with 2 impedance selectors and 2 jack outputs.
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Gotta love the clean layout!
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Yes, thanks.
I suppose the primary impedance of the OTs is based on two power tubes.
So if 4 x KT66 require 4k plate-to-plate primary impedance the two OTs here must have
8k plate-to-plate each?
/Leevi
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Looks like two KT66s drive one OT and one speaker output. Then the other two KT66s drive another OT and speaker output. So the impedance of each OT would be matched to the plate impedance of each P/P pair of KT66s, whatever that is.
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In the schematic there are two different circuits drawn for the output.
In the chassis picture there are two outputs clearly visible with their output selectors.
In the schematic the first alternative shows just one 100W output where both OT secondaries are connected together.
Is that an option in this amp?
/Leevi
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The pics I posted are not the same amp as shown in the schematic.
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> 4 x KT66 require 4k plate-to-plate
Much more likely to be 4K per pair, 2K per quad.
They had 2-tube 45W amps and OTs. They didn't want to special-order a 100W OT... they were not going to sell many amps that big. So they used two of the OTs from the 2-tube 45W in a 4-tube 2-OT setup.
It was apparently wired two ways: both parallel with single output, or two outputs labeled for two speakers. Since few speakers could handle 50W, much less 100W, the 2-jack option makes a lot of sense for the time.
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Much more likely to be 4K per pair, 2K per quad
Yes, it seems to be 5k with fixed bias and on 475v anode voltage.
I took the values from the auto bias part where the anode-to-anode load resistance
is 8k on 450v.
http://tdsl.duncanamps.com/pdf/kt66.pdf (http://tdsl.duncanamps.com/pdf/kt66.pdf)
/Leevi
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> I took the values from the auto bias part
Power output is 30 Watts.... we need 50W.
I suggest they took the fixed-bias suggestion, but let plates and screen both be 400V-425V, allowing similar output in 4Kpp (per pair).