4-65A Triode curves?

[dshortt9]

Anyone ever try a 4-65A in triode? 65 watts Pd and 10 for G2 could give a 75 W dissipation for 15 or 20 watts SE. 600 volts allowable on G2. Always looking for the poor man's 300B.

[astouffer]

http://www.curtcass.com/OTS/2003VSAC.html#vsac

Take a look at the 4th picture. I emailed the Oregon Triode Society asking if they could put me in contact with the builder. Sadly they never got back to me. Maybe you will have better luck.

The 4-65a is useless in tetrode mode so I always wondered about it triode strapped or even screen driven

[dshortt9]

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.

[316a]

Originally posted by dshortt9
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.

I am using the 4-65A at present and have traced the triode curves on the tester up to 400V . 845 curves are close enough , Ra is around the 1.6k mark , mu of 5 . Currently running mine with James 6123HS at 530V/50mA , really these need to be run at around 650V / 55mA to get some decent power . Here's a pic of the Mk1 breadboard with T20 driver , it's totally different now and has LCL DC filament supplies , I would recommend implementing DC fils with all the big Eimacs with spiral filaments such as 100TH , VT127 and 4-65A . Also don't forget the heat dissipating caps !!!!!!

cheers

316A

[rcavictim]

I have a bunch of 4-125A`s, sockets and associated parts pulled from some 500 watt transmitters (FRT-501). A bank of four in parallel ended up a PWM modulator to kick a magnetron for a radar astronomy telescope. These are an attractive radiation cooled tube. When the plates are glowing orange (permitted) they look very serious.

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.

[316a]

Originally posted by rcavictim
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.

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 8mH@10A input chokes , filament transformer and 2 x 30mH@5A isolation chokes added 18kg to the amp's weight .

cheers

316a

[dshortt9]

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]

Originally posted by dshortt9
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?

4E27 will have Ra around 2k as a triode . The Eimac datasheet specifies mu of 5 , gm of 2.15mA/V @ 50mA and I would expect similar as a triode at 500V . The 750V 20 watt G2 rating looks nice though I have no idea what switching power supplies sound like . Iron in the filament supply sounds good . I have plenty of iron but no SMPS so I'll stick to that for the time being Maybe I did something silly when AC filaments were tried . Try AC anyway but make sure you use an SG240 'surgeguard' on the transformer's primary

cheers

316a

[rcavictim]

Originally posted by 316a


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 8mH@10A input chokes , filament transformer and 2 x 30mH@5A isolation chokes added 18kg to the amp's weight .

cheers

316a

You are probably right that the 4-125A represens a challenge to fit into a practical audio amplifier. Of course this challenge, the journey as it were and the resulting esthetic beauty of transmitting bottles in your Hi-Fi setup are rewards some of us enjoy.

I have a pair of 450-TH`s here that just beg to be used in a monoblock subwoofer amp project!

[dshortt9]

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.