I got this CUTE Ampex 620 suitcase from eBay. Didn't have near enough gain for guitar, so I replaced V1 EF86 with 12AX7 (gain stage, tweed tone control, gain stage), same as my other Black Buttes. Now we're ripping! Just too much hum. I'd like to try gNFB, but since this isn't a standard LTPI, not sure how to implement. Suggestions, please? Attached schematic 1 is before mods, schematic 2 is after (V1 not shown).
2023 Monkeymatic Black Butte (from Ampex 620)
Moderators: pompeiisneaks, Colossal
2023 Monkeymatic Black Butte (from Ampex 620)
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I build and repair tube amps. http://amps.monkeymatic.com
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Re: 2023 Monkeymatic Black Butte (from Ampex 620)
Nice! These projector amp/film amp conversions never disappoint. I dig all the bumblebee caps in there too. Some fools will gladly part with $$ to buy those for Les Pauls it seems
"It's like what Lenin said... you look for the person who will benefit, and, uh, uh..."
Re: 2023 Monkeymatic Black Butte (from Ampex 620)
I ran a gNFB line from the OT secondary, thru 15K resistor, to V1b cathode. Much better, lower noise, still enough gain. Putting out 3-4 watts into 8 ohms, a little better into 16R.
I build and repair tube amps. http://amps.monkeymatic.com
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Re: 2023 Monkeymatic Black Butte (from Ampex 620)
That monkey is always busy and does great work.
Signatures have a 255 character limit that I could abuse, but I am not Cecil B. DeMille.
Re: 2023 Monkeymatic Black Butte (from Ampex 620)
Looks nice! I rebuilt one of those a couple years ago but never got the noise floor quite low enough for my taste. Input right next to the mains is odd, but maybe that was just a lazy excuse.
Re: 2023 Monkeymatic Black Butte (from Ampex 620)
It's a super cute package!
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I build and repair tube amps. http://amps.monkeymatic.com