Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Duncan on September 16, 2024, 11:36:04 am
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I've got a Fender Bassman AB165 (that has been rebuilt, so there's no vintage/sentimental value left), that I've been looking at turning into a firebreather (more or less a Jose mod to the normal channel). Most of the schematics I've researched about this sort of build have a CF before the master volume and tone stack. I'd like to avoid drilling any new holes in the chassis (if possible), so I've essentially got what the Bassman's normal channel has for preamp tubes.
Anyone care to comment on this? I plan on using as many existing holes for potentiometers as possible (but if some have to go in the back, that's not the end of the world).
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This looks like the perfect use for the FET source follower.
No new tubes and dead easy to retrofit.
I tried it in a cheap Origin 50 and I'm a believer now.
The loses through the Jose mod are immense plus the low value slope resistor will definitely load down the plate. I think you'll find a buffer (FET or CF) very useful in making this work.
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There seems a lack of gain for a fire breathing overdrive preamp?
The AB165 should have a spare triode in V2 which could be used to provide some push?
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There seems a lack of gain for a fire breathing overdrive preamp?
The AB165 should have a spare triode in V2 which could be used to provide some push?
Updated with using that unused Triode and with a FET follower.
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You've shown a FET gain stage into the follower. You just want the follower.
See the output of the 2nd stage for how I shoe horned mine in.
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Gotcha. So more like this?
(Sorry, I've never used a FET before, so this is new to me).
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You want to tap off the other side of the plate resistor (HT, not plate voltage). Where it is in the signal chain determines where the impedance is transformed. I think where you have it makes sense to drive just the tonestack and Jose clippers. You could also put it directly after the plate of the previous stage and buffer before attenuation/bright caps. The beauty of this simple circuit is you can really stick it anywhere the voltages work. You can experiment as you see fit. To me, the biggest drag is the 33k slope resistor.
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That MOSFET is not biased properly
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Revised attached.
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Hi Duncan,
For reliability you need to consider that Zener (Gate to Source) and probably a small resistor in series with Gate (before Zener)
Regards
Mirek
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Duncan
Just for clarity, example of Tubenit's use of IRF820 with Zener and Gate resistor
Regards
Mirek
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The HT supply won't work, the winding CT needs to connect to common.
If the winding has a bias tap, then yes, that tap can feed to bias supply. But the CT still must be connected to common.
The feed to the screen grid HT node is weird, there's an additional RC smoothing stage with a 10k HT dropper resistor that the screen grid current must pass through. That's going to cripple the power amp because with signal, the screen grid current draw increases a lot.
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Duncan
Have a look at Sluckey's Power supply (Very good working example and thoroughly tested on this forum :laugh:). This will solve pdf's concerns with your latest version of schematic,
Obviously you are using SS Diodes instead of Tube rectifier, resulting in higher +HT for this example. Also you have chosen higher values for PS electrolytics, no problem as long as Mains Tx can support this. HT (all nodes) and Bias are correctly wired.
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Duncan
Have a look at Sluckey's Power supply (Very good working example and thoroughly tested on this forum :laugh:). This will solve pdf's concerns with your latest version of schematic,
Obviously you are using SS Diodes instead of Tube rectifier, resulting in higher +HT for this example. Also you have chosen higher values for PS electrolytics, no problem as long as Mains Tx can support this. HT (all nodes) and Bias are correctly wired.
Amazing. Thank you! I'll make some revisions to my schematic!