I'm restoring one of these that needs everything...cap job, tubes, bias circuit rework, etc.
Owner wants me to mod Bass channel also.
I'm going to ditch the UL screen taps, and am wondering the best way to arrange the screens .
I'm planning on tapping the B+ rail at the 2k7 10W and 1K junction( and adding screen resistors), but wondering if I should ditch the 20W 30K resistor to ground.
What's the purpose of that anyways? To simulate a choke?
Also entertaining ideas for the Bass channel. He says he wants it Marshally, and I explained I'd need to rob the shared 3rd stage to create a cathode follower, and he wants to keep the normal channel stock.
So I'm thinking 2 stages w marshall values then plate driven tone stack, into shared gain stage, but I've found that a gain stage after any tone stack in a marshall topology doesn't really add anything good.
Bassman 70 UL
Moderators: pompeiisneaks, Colossal
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Smokebreak3
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Bassman 70 UL
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Re: Bassman 70 UL
I've become somewhat fond of UL operation. I usually wire it to a switch in amps with an OT that has UL taps, just to keep it as an option.
I think your idea of the plate driven stack is a good one. An additional gain stage after the stack shouldn't hurt anything and it'll help recover some gain lost from the tonestack. (especially with it not being CF driven)
You also have the option of adding an LND150 or similar FET stage to the front end of the bass channel. This would leave you with an extra triode if you wanted to do something with it. The input stage normally isn't going to clip, so this will give you a high signal to noise ratio, that if voiced right, will be virtually unperceivable from an all tube setup. You could also run the FET as a source follower, but then you won't get the harmonic distortion that you'd get from a DC coupled cathode follower.
I think your idea of the plate driven stack is a good one. An additional gain stage after the stack shouldn't hurt anything and it'll help recover some gain lost from the tonestack. (especially with it not being CF driven)
You also have the option of adding an LND150 or similar FET stage to the front end of the bass channel. This would leave you with an extra triode if you wanted to do something with it. The input stage normally isn't going to clip, so this will give you a high signal to noise ratio, that if voiced right, will be virtually unperceivable from an all tube setup. You could also run the FET as a source follower, but then you won't get the harmonic distortion that you'd get from a DC coupled cathode follower.
It's true i've lost my marbles and i cant remember where i put them
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Stevem
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Re: Bassman 70 UL
Another consideration with these amps is what kind, and how many speakers it will be used with.
The black face ones with all the electronics right and a fresh set of good power tubes could pump out 45 watts RMS and was a loud amp thru 2-12" when the output stage started to kick in adding it's nice grit into the sound.
These 70 watters are even noticeably louder yet even with tight bottom end hanging in there maybe too much due to the UL.
The black face ones with all the electronics right and a fresh set of good power tubes could pump out 45 watts RMS and was a loud amp thru 2-12" when the output stage started to kick in adding it's nice grit into the sound.
These 70 watters are even noticeably louder yet even with tight bottom end hanging in there maybe too much due to the UL.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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Smokebreak3
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- Joined: Wed Dec 28, 2022 12:45 am
Re: Bassman 70 UL
Thanks y'all. The mod channel came out great, 1st stage 200K Rp, 2k7/.68 Rk, 22n couple to 470K divider>> 2nd stage 100K Rp, 1K5/1u Rk to standard Marshall tone stack , then 470K mixers to shared 3rd stage.
I just clipped the NFB and left the UL taps. Jeez this amp is loud.
I just clipped the NFB and left the UL taps. Jeez this amp is loud.
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Stevem
- Posts: 4610
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: Bassman 70 UL
To answer your other question. You can not as you say Ditch that 20 watt resistor.
CBS, there engineers and penny counters who purchased Fender found they could save 3 bucks by using that 15 cent resistor in place of a Choke they did so.
I guess they may have traded the cost of that choke and sunk it into the more costly UL OT.
Having a choke in there is part of having a true blackface response out of the amp if your playing it up at 4 or better on the volume knob, not to mention the better power supply 120 hz hum rejection it also provides.
CBS, there engineers and penny counters who purchased Fender found they could save 3 bucks by using that 15 cent resistor in place of a Choke they did so.
I guess they may have traded the cost of that choke and sunk it into the more costly UL OT.
Having a choke in there is part of having a true blackface response out of the amp if your playing it up at 4 or better on the volume knob, not to mention the better power supply 120 hz hum rejection it also provides.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Bassman 70 UL
Isn't half of V2 not used ? I only see 5 triodes used plus the PI.
Link to schematic: https://el34world.com/charts/Schematics ... sman70.pdf
Link to schematic: https://el34world.com/charts/Schematics ... sman70.pdf