dshortt9 said:Thanks for the lead. I'll try it and if I get some info I'll pass it along. Found a Mu of 5 g1 - g2 so that is in the ballpark. Might have to get one and light it up.
rcavictim said:The 4-125 is similar to the 4-65A. just twice the Pd.
I have given some thought to a SE amp using this tube, so will follow this thread with interest.
dshortt9 said:I had thought of using a 4E27A/5-125B but after hearing of your hum problems it might be harder than I thought. Someone posted triode curves of this tube showing an Rp of 1200. Perhaps using a HF switching supply would help?
316a said:
I don't know about using the 4-125A as a tetrode but as a triode it's probably no good unless run zero bias . It will have Ra over the 2.5k mark and would require running the G2 way over it's 600V limit . Also think about the filament rquirements . I had problems getting the 4-65A quiet with AC . Tried different variations of hum pots , CT filament transformers etc and gave up and went to LCL DC filaments . The 2 x [email protected] input chokes , filament transformer and 2 x [email protected] isolation chokes added 18kg to the amp's weight .
cheers
316a
dshortt9 said:Thanks to all for the help! You saved me many hours and much greif. I may still try one of these tubes but you have shortened my learning curve quite a bit.
astouffer said:316a: How are you driving the 4-65a, choke loading the T20?
dshortt9 said:I had thought of using a 4E27A/5-125B but after hearing of your hum problems it might be harder than I thought. Someone posted triode curves of this tube showing an Rp of 1200. Perhaps using a HF switching supply would help?
dshortt9 said:Hi Douglas.
Could not find the H-K 257B sheet. Tried Frank's and Vade-Mecuum.
PP should be quiet since the OT cancels the hum, I think? SE is harder obviously, and with efficient speakers becomes even tougher. I'm thinking of the Hammond 1629SEA in SEUL with CFB around 100ma and 700V. 6.5K load, 40% tap. 16ohm for CFB. Sound good?
Bandersnatch said:
Try hunting for the 4E27 then. I suspect that's what I searched for in the first place. I also got brass adapters turned so I could use 813 anode caps...seems like the perfect sub. All that's needed is to provide 10V instead of 5...
cheers,
Douglas
Tony said:
how about Philipps QE08/200? , i have a quad of thse tetrodes...
datasheet says that 750v plate and 3.6k a-a, a pair can do 300watts......