Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Bieworm on February 10, 2021, 02:37:54 pm
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Hi
Anybody ever used a configuration as this?
Normal biased with bypass cap stage 1
Voltage divider A1M
Cold clipper stage 2
Tonestack marshall single tone knob stage 3, cathode biased
Is the cold clipper going to cost a lot of OD?
And how about a cathode follower on stage 3, with the single knob tone stack?
Thanks
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I only know a bit but that’s not too far off from a JCM800. Stage 1, stage 2 cold clipper, then a warm bias stage 3 into cathode follower. Classic Marshall.
I don’t know the repercussions of a cold stage into a CF though. Wonder if it’s be better to do cold clipper first, then warm stage into CF.
With three stages though you’re leaving half a triode out. May as well just go full Marshall with it? Or ditch the CF? Just a single tone control doesn’t really seem to need the buffer that CF provides? The CF is only there because of the loss of the full tone stack, no?
Are you intentionally only wanting 3 triodes? That leaves 1 on the table, unused, unless you’re doing something else with it already.
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Are you intentionally only wanting 3 triodes? That leaves 1 on the table, unused, unless you’re doing something else with it already.
Use the 1 on the table for the CF. :wink:
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Are you intentionally only wanting 3 triodes? That leaves 1 on the table, unused, unless you’re doing something else with it already.
Use the 1 on the table for the CF. :wink:
He’s suggesting stage 3 is the CF, no? Cold clipper into the CF/tonestack.
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I'm so confused. :icon_biggrin:
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Yes I only have 3 triodes to spend. So that's why I asked. Is a cold clipper stage between the 1st stage and the tonestackdriver a good option for nice OD/distortion? Or is there a downside to this?
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The cold clipper stage has very little gain, so there’s a risk a single stage can’t push it enough to get a lot of clipping.
Another option would be to skip the CF and use three gain stages instead.
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That's what I have right now. I just wondered what the audible spectrum the cold clipper would add.
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Try it out and let us know the results!
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If I understand this correctly, you would have a cold-clipper direct coupled to a cathode follower. This isn't a very good idea because the CF would have significant grid current at idle.
The cold-clipper stage has low current at idle resulting in a small voltage drop across the plate resistor. The cathode voltage on the CF is close to the plate voltage on the cold clipper so that the anode-to-cathode voltage of the CF is small - - like 30V with a 10K cathode resistor on the cold-clipper. If you plot a load line for the CF, 30V anode-to-cathode puts you way to the left of the zero volt grid curve and firmly into grid current territory.
Fender put an 820 ohm resistor on the stage that is direct coupled to the CF. This idles that stage at a higher current than the standard Fender 1.5K so that there is a larger voltage drop across the plate resistor and the resulting CF anode-to-cathode voltage puts the idle to the right of the zero volt grid curve.
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Thanks guys. Well I have tried the cold clipper likr this:
V1A input triode: Ra 100k / RC 820R - 1uf
V2A 2nd stage: from plate V1 .02uf to A1M voltage divider + 10k grid stopper / Ra 100k / Rc 22k
V1B 3rd stage tonestack driver: grid parallel to Grid V2A / Ra 100k / RC 1.5k - 2uf
First findings: less overdrive but creamier, really nice actually.
Next I'm going to try cascading the cold clipper stage into the 3rd triode, again with an A1M voltage divider. And see where that takes things.
I was also thinking - if this turns out good - of putting in a dual ganged pot for the 2 voltage dividers, so stage 2 will pump up stage 3 while stage 3 pumps up... making that a B1M dual pot should drive it progressively?
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Ok. I tried it... though it sounded awesome , but there was some interaction with my clean channel which I didn't like. I will finish the schematic 'as built' because I think the amp is finally finished and really awesome. Much better to my ears compared to the tone king imperial clone on which it started out...
On the clean channel I enhanced the mids so that BF scooped mids is off the field. Changed the slope resistor to 56k and the 10k mid resistor was upped to 27k. But the most significant change was the 250pf treble cap to 500pf. The cleans really opened up...
The OD channel was nice at first, but 2 things bothered me:
1. It was insane loud from the kick-in of the OD and..
2. the midbite knob took over the entire tone setting when pushed.
The paraphase PI was changed to a LPT PI , which made the overall volume more controlable.
This is now a real A$$kickin' mean machine with very nice cleans and a lite version of a 2204 on the OD side.