Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Johntb on May 06, 2020, 03:39:41 pm
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Been a while since posting.
A while ago I got a Unicord 1221. It was in rough shape and a lot of bad caps although I did get sound out of it PT voltages were good and because I got sound I assume the OT was good. That being said...I want to use from the 6AN8 to the 6l6's not the pre-amp. I am thinking of the "clean pre amp" from AX84.
Can I just use the BIAS circuit minus the Tremolo? Or?
Thanks
John
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I cut out most of the "unwanted"
not sure where to cut on the bias, if the 150k needs to be there :dontknow:
660vdc for plates is a bit risky :w2:
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Thanks
I actually measure just over 600. I did not want to go that high. I was going to just use the 300 +/- from B+ 1. I am sure that will change all kinds of power output B+ and possibly the bias.
This is supposedly a 100 watt amp. I really don't think so from what i have read and seen on other forums. On this amp there was a sticker on the back that said "100 watt" but that sticker was right next to the electrical plug. I think it consumed 100 watt of power but what do I know.
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> 660vdc for plates is a bit risky
Safe for all 1970s 6L6GC. Probably safe today.
This is a 60 Watt amp.
If you use only the 300V feed it will be a 15 Watt amp the weight of a 60 Watt amp.
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> 660vdc for plates is a bit risky
Safe for all 1970s 6L6GC. Probably safe today.
This is a 60 Watt amp.
If you use only the 300V feed it will be a 15 Watt amp the weight of a 60 Watt amp.
Thanks.
So actually not using 600 may have and affect on the BIAS then?
Do you anything about the BIAS circuit?
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I don't see anything wrong with 660V on the plates when there is only 320V on the screens. The 10K primary is well matched to this combination for sixty some odd watts, or maybe even closer to seventy watts.
I would discourage you from running 6L6GC's at 360V or whatever you'd get with a bridge rectifier across RED and BRN on the PT. The 10K primary would be a serious mismatch and it might even put you in the land of excessive screen current.
6V6's at 360V with a 10K primary looks interesting to me. 21W or so with the weight of a 69W amp so it won't blow over in the wind.
I have three bias circuits for your consideration. The first one is for removing the tremolo and leaving the power amp stock. As long as you're modifying the thing you may as well make the stock bias adjustable.
The second one is for an all new bias circuit for the stock power amp.
The third one is an all new bias circuit for 6V6's at 350V.
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See attached. No need to revamp bias ckt: Mods I outline are minor and can be easily reversed. Recommend that you replace the 150V 5uF and 10uF 50V electrolytics in the bias ckt.
Use Groove Tubes 6L6GC-GE tubes as they seem to hold up to high (>600V) B+, otherwise hunt for a pair of Sylvania STR 6L6GC - They were preferred tube in MusicMan amps with 700V B+.
Pull the unused preamp tubes.
Interesting active load? for the trem ckt. (12AU7). Schematic missing a grid to grid connection for 12AU7 pin 2 to pin 7? Why would Unicord leave one grid unconnected?
--pete
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Thank you for the info.
This helps out alot and gives me a little bit more to think about.
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> Interesting active load? for the trem ckt. (12AU7).
It is a variable resistance. Both grids were connected together. Not all 12AU7 worked properly.