18Watt.com

The earliest JTM45 1962 lead schematic shows a 20H choke,the specs for the choke are 20 Henries@200mA and DCR of 475ohms.
The schematic shows 450V before and 440V after the choke which is a voltage drop of 10 volts. When I was experimenting with sag resistors in the 18W a 100 ohm resistor dropped the B+ by 10 volts so how can a choke with a resistance of 475 ohms only drop 10 volts?

That's a good question for the folks on ppwatt.com, where they have a forum dedicated to the JTM45 and its F***er Bassman predecessor.

geoff 1965 wrote: ↑

Fri 02/18/22 11:43 am

The earliest JTM45 1962 lead schematic shows a 20H choke,the specs for the choke are 20 Henries@200mA and DCR of 475ohms.
The schematic shows 450V before and 440V after the choke which is a voltage drop of 10 volts. When I was experimenting with sag resistors in the 18W a 100 ohm resistor dropped the B+ by 10 volts so how can a choke with a resistance of 475 ohms only drop 10 volts?

Geoff.. could this be some relative voltage drop per amount of B+ and the amount of current draw involved? If you put a 200R resistor instead of the 100R it isn't going to drop another 10V I think..
Just a thought...

Don’t know Bieworm,i’ll try a 475 ohm same as choke resistance and see,I ’m Reworking my plexi 6V6 into a new chassis & cab and going to have switchable gz34/diode rectifier.the power section is the lite6V6 in the downloads but the pre amp is lite2/ plexi TMB so I was just thinking of simulating the choke sag of the JTM on the diode rectifier side.

There are schematics floating around (I have the "JTM 45 Basic Schematic for Marshall Trem Amps Types 1961, 1962, 1987/T") that show the choke before the OT CT with 450/440V on either side. But other schematics and plenty of standard layouts show the choke after the OT CT with only power screens, PI and preamp current flowing through it.

I've always wondered if the schematics showing it before the OT CT are just wrong, or if some were actually built that way. But if you take it that the standard arrangement in a JTM 45 is choke after the OT CT, then it's very different to playing with a sag resistor after the recto, and the 10V makes more sense.

I noticed that as well Katopan,also the later amps including the re-issue dropped the choke value down to about 3 to 5H so those first JTMs would be quite saggy in comparison, if the schematics are correct.

geoff 1965 wrote: ↑

Fri 02/18/22 11:43 am

The earliest JTM45 1962 lead schematic shows a 20H choke,the specs for the choke are 20 Henries@200mA and DCR of 475ohms.
The schematic shows 450V before and 440V after the choke which is a voltage drop of 10 volts. When I was experimenting with sag resistors in the 18W a 100 ohm resistor dropped the B+ by 10 volts so how can a choke with a resistance of 475 ohms only drop 10 volts?

Is that the same schematic that shows voltages hand-printed with a hand-drawn circle and arrow? If so, it also indicates bias on the output tubes of -105V. They would be completely cut off under these conditions. The only current to speak of through the choke would be the preamp and phase inverter. I'm thinking there might be something wrong with this schematic.

https://www.thetubestore.com/lib/thetub ... ematic.pdf

Jack

That looks similar Jack,I just googled earliest jtm45 schematic.

The schematic shows a 150K resistor, a bias diode and a 16K (?) resistor in series. At the end of that string there's a 56K to ground. If any of those resistors drifted, bias voltage could change significantly. It sure looks like all those voltage readings were made in an amp that wasn't working. Having that much bias applied to the grids of the outputs would definitely explain the small 10V drop across that choke!

Jack