Eimac 4CX250B Tube - Is possible to have a bass amp with less than 1000v B+ ??

[kagliostro]

From a friend I've the set you can see

2 x 4CX250B - 2 x sockets - 1 x chimney -- (at the moment I miss 1 chimney)

here is the tube data sheet

http://www.g8wrb.org/data/Eimac/4CX250B.pdf

on the data sheet is reported that those (very fascinating tubes) can be used for audio purpose

with a 1,000v B+ and 3,500ohm OT to obtain 240W audio

(this power will be fine for me to obtain a bass amp)

(London Power do one, the SPECTRUM a 700W amp - I think they used a full 2,000v B+)

I've a HUGE PT that was build for a RF Linear Amplifier

primary

220v

secondaries

6.3v

12v

500v+100v+100v+100v

so I can have

500v AC or 600v AC or 700v AC or 800v AC

the 6.3v is a bit redundant for those tubes which are rated for 6.00v +/- 5% and in many forum (about Linear Amplifier construction) is often told to use 6.00v or better less, because the tube don't like higher filament voltages

1,000v is a very large B+

may be building an audio amplifier in VHF Linear Amplifier style will give good result

also my be that the craftsman I know can build an adequate 3,500ohm OT (adequate for the 1,000v B+)

But many people told me to look if it will be possible to build the amp with a lower B+ in the order of max 700v DC or near

Please can someone give me help, I'm not able to read Data Sheet curves and to establish a work point by myself

With a lower B+ (lower than the 1,000v on the data sheet) is possible to obtain a good power ?

Which will be a good compromise ?

My PT with its voltage is adequate to this project ? (about current I think it can supply a 300/500W amp)

Unfortunately I wasn't able to find 4CX250B modulators or audio amp schematics

I only find the T 195 / GRC 19 modulator schematics that uses 4x150A tubes and is not very useful for me

Crazy idea ???

THANKS for any indication

Kagliostro

[sluckey]

Wow! That's pretty ambitious. Tell your wife to take out a big insurance policy BEFORE you begin. 

Seriously, that's gonna cost a lot of money and time just to see if it will work. Wouldn't you really rather have a good ss amp that you could carry by yourself?

[DummyLoad]

you're gonna need a lot of money & time. the OT will need to be custom wound - likely it'll need to be wound with intricate interleaving to keep the leakage inductances within tolerable limits, not to mention it'll have to be a *very* large core just to get the windings in a window large enough to get enough wire on the core to gain enough inductance to have a decent f-3dB roll-off for bass work at the power levels you're looking at.

then there's the PT. you don't spec current ratings on the unit you have and for 250 watts, you're probably going to to be needing something with the VA ratings of a RF amp which is going to put you in high volts, unless of course you have deep pockets for custom iron. ...then there's the G2 supply...

then there's the cooling issue, those tubes *require* forced air cooling to make the power levels you state, not to mention what happens when a cheap chinese fan goes kaput and the valve(s) melts down. you'd likely want some fail-safe circuitry for g1 bias V, g2 V, some temperature telemetry, and cooling fan status.

best to just leave the esoteric stuff be and use (n)pairs x 6L6/6550/KT88/KT90 etc., in parallel - if you have a need for such an esoteric amp, go to plitron and buy some custom toroids from them - they have 300W and 400W output iron and will certainly wind something in the way of a PT for you as well. this plan get you in the 300-400 watt region without the insane B+, however, you'll still need some telemetry for an amp that hefty.

to answer the question; no, i don't believe a workable plan at less then 1KV would be a good plan for those tubes, others much more in the know than i will hopefully chime in and set us all straight. slucky has commented, is definitely one those in the know and he's not biting.

again, YOU SHOULD CONSIDER ANOTHER PLAN. such as tube front end with nice clean transparent sand state amp... i like the crown DC300A and family. buttery and i have a very nice bass-pre that we can sell you for not_a_lot-of_dough + used DC300A or the like for small investment, you'll likely not get killed, and you can unload the 4CX250 on a HAM that can use them.

respectfully,

--ISO



   

[Willabe]

Hi kaglisostro,

   Just an idea, since you noted London Power (KOC) have you checked out the "Super Scaler"  amps? Loose Change could tell you more about them (maybe?). I thought he was going to build one. Any way its in TUT 4 & TUT 5 (KOCs books). The best part as I understand it is you can take  ---ANY AMP ---  and x it! But now not only is the input amp/source amp tube (read tone amp), if you want, * but * the "Power Scaler" amp is tube also! Now its set up clean but it still tubes still adding some harmonics (?) You can also stack them one feeding one to the next for more power. That way you can just take with you what you need to a GIG!  Hey, a lot lower B + (and thats way good!) You drive the "Super Scaler" amp from the output trans output to a fliped secondary to primary. Then you drive the input of the "Power Scaler" amp from/to the screen grid (grid 2)     More than one way to skin a cat ! Just my thoughts on a safer way to get the same power/volume!

I hope PPR weighs in on screen drive multis!

       Brad     

[PRR]

This tube is a true TETRODE, with the tetrode's flaw: plate voltage can not swing below G2 voltage.

Therefore to get any decent Power, the plate idle voltage must be much higher than G2 voltage. Preferably 5 or 10 times higher, though 3 times works "OK" with considerable waste.

Reduce G2 voltage? That reduces maximum plate current (before G1 current), which is also bad.

Use the 1,000V condition.

OR run it Triode. Rp is near 250 ohms. G2 and Plate should be limited to 400V. Looks like 65 Watts output in 2KCT load.

Big blow.

No glow.