Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Diverted on August 28, 2019, 12:31:04 pm
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Hi,
A friend of mine has had a fender Princeton clone for a few years that I have a question about.
The amp uses a power transformer rated at 120ma on the high voltage secondary (ClassicTone 40-18016), a 4/8/16 tap output transformer (ClassicTone 40-18006) that has a 4K primary, and is running two 6L6GCs.
The transformer has a 3a 6.3v winding and currently the draw on it is 3.3a with the 6l6s, four 12xx tubes and pilot light.
The amp runs HOT, extremely hot, and I biased it cold when I had it (50 percent or so) so the tubes wouldn’t dissipate as much. It had been biased at 75 percent before. It still gets super hot (almost too hot to touch after only 10-15 minutes on) and we were thinking about changing around power supply as he is worried about it failing.
I suggested he may want to go with 6V6s as there’s be less strain on the power transformer. He doesn’t play it loud anyway.
My question:
Would we be able to keep the current output transformer by switching out the tap that is being used? Currently the amp is using the 8 ohm tap into an 8 ohmspeaker.
Or
Will we need to get an output transformer with a primary closer to 8k?
Thank you!
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That PT is kinda light weight for 6L6s, filament wise and HT wise. Putting 6V6s in it will help cool things down. Just relabel the OT secondaries, ie, 2 becomes 4, 4 becomes 8, 8 becomes 16, etc.
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Yeah, it definitely seems like it’s straining under the weight of those tubes. So go with 6V6s and just run the four ohm tap into the eight ohm load?
Thanks!
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I put 6L6's in an SF Princeton & the PT ran very hot. One possible solution is to remove the bell covers (if any) for more ventilation. My solution was a fan, see schematic.
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You can run JJ 6V6 that aren't really a true 6V6 but more like 5881 and I think you would be very happy with the result and not burning out your PT..
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Once I heard the 6L6, 6V6's were history for this amp. But, haven't tried the JJ's; maybe worth a try. (I sold my Princeton years ago)
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Once I heard KT 88's the 6's lost :laugh:
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Yes, but KT-88's in a Princeton. :sad2:
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Thanks, I've mostly always just used old tubes so have not heard of the JJs and the higher plate voltage they can take. I'll have to try a pair.
Anyway I swapped out the taps on the output transformer. Now at 4 ohm feeding 8 ohm speaker, with a pair of nice strong 1961 RCA 6V6GTs in there.
The plates are seeing about 405 volts at 57-60 percent dissipation. I'm hoping that'll be good and the tube life won't be shortened too much. Original Princeton spec is 415 on the plates at about the same dissipation, so I think they'll be OK. Time will tell.
Thanks!
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I've introduced Gibson n Fender pre-wee's to the 88, they both stayed :icon_biggrin:
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60% plate diss: I protest as a member of ATAGACB (Amp Techs & Guitarists Against Cold Bias).
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Ha! Thanks, I could up it to 70 and the plate voltage would drop but it sounds pretty good man!
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I have run JJ 6V6 up to 40 ma with no problems.. but not for an extended amount of time. They can not only take the higher plate V's but also higher current levels..
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http://www.allenamps.com/trans.html (http://www.allenamps.com/trans.html)
Allen amps TP25 is the one you want - supports 5AR4/GZ34/5Y3 rectifier with 5V @ 2A, 6.3V@ 4.5A and 620VCT @ 150mA. it's about 1" taller than the stock unit. 5.8Lb. fits the champ/princeton 2" x 2.5" mounting pattern.
--Pete