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4CX250B amp - have you schematic for that (London Power has)

 
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Kagliostro



Joined: 30 Dec 2009
Posts: 324
Location: Italy

PostPosted: Sun Oct 31, 2010 10:24 am    Post subject: 4CX250B amp - have you schematic for that (London Power has) Reply with quote

I'm interested in the 4CX250B tube

I've read about a London Power amp

http://archive.ampage.org/threads/0/ampgen2/014942/4CX250_tubes_in_London_Power_amps-1.html

does someone can give me the schematic ?

or any audio amp schematic that uses those tubes

the data sheet claims a pair of that at 1.000v can give more than 200w with a 3.500ohm OT

Many thanks

Kagliostro
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TheGimp



Joined: 08 Oct 2010
Posts: 199

PostPosted: Mon Nov 01, 2010 7:33 pm    Post subject: Reply with quote

Do be very careful with that level of HT supply. It does not shock you, it kills you. No second try.

I've got six of them (4CX250s) in the garage along with the HT transformer, bridge rectifiers and capacitors, and filament transformer. The set makes for a good size boat anchor.

They came out of a 250W wideband linear (150KHz-250MHz).
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FYL



Joined: 17 Feb 2006
Posts: 654

PostPosted: Mon Nov 01, 2010 10:04 pm    Post subject: Re: 4CX250B amp - have you schematic for that (London Power Reply with quote

Quote:
I'm interested in the 4CX250B tube


Urgh. You're into the KV range, where even a small mistake can be lethal. You'll need custom iron tested to 8 KV or more - heavy expensive. Furthermore, ceramic tubes such as the 4CX250B need forced air cooling - nasty, noisy, expensive.

Quote:
I've read about a London Power amp


The Spectrum. Out of production.

Quote:
the data sheet claims a pair of that at 1.000v can give more than 200w with a 3.500ohm OT


Use a PPP of KT88s or KT120s, four tubes, less dangerous voltages, an easier build.
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Kagliostro



Joined: 30 Dec 2009
Posts: 324
Location: Italy

PostPosted: Tue Nov 02, 2010 3:42 pm    Post subject: Reply with quote

Hi TheGimp & FYL

Yes, I know, B+ is "a bit little more" than the usual

I'm interested in a Bass amp with High power

(perhaps don't want to use the 2,000v B+)

so 200 / 240 w are plenty fine

the data sheets of the Eimac tubes give data for audio use at 1,000v

the idea comes because I've a very Huge PT

6,3v - 12v - 500v+100v+100v+100v

that was build for a tube linear amplifier for radio frequency

and also have

2 x NOS sockets 2 x NOS 4CX250B and only (at the moment) 1 x NOS chimney

this isn't a project to be build tomorrow

this will happen when I have all parts for cheap (those I have comes that way)

The big problems at the moments are an adequate schematic and the OT

But if London Power do that 4CX250B tube bass amp with 700W, I think I can build one with 200 / 240 W

Kagliostro

p.s.: Also to concretize a project in a schematic to be realized give me the way to learn a lot because for many years I was out from electronic constructions and I'm newly involved on it only this last few years
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TheGimp



Joined: 08 Oct 2010
Posts: 199

PostPosted: Tue Nov 02, 2010 4:43 pm    Post subject: Reply with quote

The tubes and transformers I have came out of a Scientific Instruments Linear amp that was used for RFI susceptance testing in our screen room.

There is a blower for forced air cooling as all ceramic RF output tubes use this type of cooling except a small number that are water cooled.

The blower is very loud, as it has to move a lot of air to keep the tubes cool!

I would have to check the transformer, but I believe it has one 500V windings that was bridge rectified for a B+ of around 700V. The tubes were run very conservativly with all six only putting out 250W, because it was tuned over a very broad range.
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Kagliostro



Joined: 30 Dec 2009
Posts: 324
Location: Italy

PostPosted: Tue Nov 02, 2010 6:21 pm    Post subject: Reply with quote

My transformer has a 500v+100v+100v+100v winding

so I can increase, by 100v step if desired, the AC till 800v AC

if I put a SS rectifier I assume to obtain

AC 500v x 1.4 = DC 700v

AC 600v x 1.4 = DC 840v

AC 700v x 1.4 = DC 980v

AC 800v x 1.4 = DC 1.120v

as it is to be used for a bass amp and to be used at his rated power 240W with ~1.000v B+ the noise of the fan is not a problem

also at this power I can have less noise because the fan must move less air than with the standard 2.000v B+

If you have the old chassis for 6 tubes may be an idea to run it at a lower B+ converting it to a bass amp

I've not enough tubes to do that


Kagliostro
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David Root



Joined: 04 Aug 2006
Posts: 3078
Location: Chilliwack BC

PostPosted: Wed Nov 03, 2010 3:21 am    Post subject: Reply with quote

I would re-read what FYL and The Gimp posted and think seriously about it. They are right.
1KV you will only screw up on once.
1KV can jump across all kinds of insulation, including ceramic tube sockets that are not specifically designed for it.

700V should give you what you want in power and is less risky and very safely available from your PT.
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Kagliostro



Joined: 30 Dec 2009
Posts: 324
Location: Italy

PostPosted: Wed Nov 03, 2010 11:36 am    Post subject: Reply with quote

With 700v DC (B+) I think I can obtain a ~170W amp

not my intention or I must use (and have) 4 tube / 4 socket / 4 chimney

about voltage (1.000v) must remember that VHF linear amplifiers are build with 2.000v B+

So, I think that using VHF Linear Amplifiers standards for construction will give a reasonable safety using a 1.000v B+

but last word isn't say and can consider also to use a 700v DC if I'm able to find help for planning the schematic with this voltage for which I haven't data reference (Eimac give only for 1.000v B+)

Kagliostro
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Cliff Schecht



Joined: 30 Dec 2009
Posts: 2616
Location: Austin

PostPosted: Wed Nov 03, 2010 4:28 pm    Post subject: Reply with quote

If you aren't comfortable with designing the amplifier around 700V yourself then I would venture to say that you don't know what you are doing enough to safely build that amp. If you don't understand how to design around 700V instead of 1000V then you must go figure out how to properly bias the output stage. You can make the amp put out 200W at 700V and at 1000V if you know how to adjust the bias accordingly (lower B+ means higher current for the same power).
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Kagliostro



Joined: 30 Dec 2009
Posts: 324
Location: Italy

PostPosted: Wed Nov 03, 2010 6:29 pm    Post subject: Reply with quote

As you say I'm not able to figure how to build the amp with 700v

there are problems (for me) to read the data sheet curves and to establish a correct value for bias voltage, OT impedance and others

also for the construction with a 1.000v B+ I have some problems because I haven't a schematic

the reason of this post was I'm looking to schematics to have a base from which to start

say that

To build an amp is different from draw a correct schematic

about draw a correct schematic I'm here to say I'm not able


about build materially the amp I think I've the necessary ability

to build it with unusual high voltage involve the use of particular materials also for the simple wires and also the disposition of components will be adequate

for that reason I told to build the amp in VHF Linear Amplifier style

one thing is to ask to someone that is usual to install High Voltage neon illuminated signs to draw the schematic of a guitar amp

other thing is if it is able to give a good assemblage to the parts to obtain one

Hope someone can find a schematic and give the link

Kagliostro

Also hope this will clarify
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TheGimp



Joined: 08 Oct 2010
Posts: 199

PostPosted: Wed Nov 03, 2010 6:37 pm    Post subject: Reply with quote

I've been looking at the tube data sheet and My God! They actually call out design specs for "Audio Frequency Power Amplifier or Modulator, Class AB". No distortion specs given.

So, I guess you can get there from here at 1KV, but probably not below that.

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

Recommended conditions at 1KV anode is 200mA anode current. They call for 240W out pp.

The linear amp I stripped used 6 of the 4CX250s to get 250W out. I suspect tube life is the reason they used 6 tubes to do the job that 2 would normally do.

And this is the easy part.

Now find a custom OPT that will handle 1KV at steady state at 200mA and 3.5KV peak voltage and 500mA peak current at the primary and 3500 ohms p-p. They will have to wind it for 6KV isolation to be safe just to cover their butt.

Those kind of voltages don't require touching to be lethal, all you have to do is have part of your body in close proximity and an arc will jump to it.

I'll probably take my stuff to the next Ham-fest and see if I can find someone interested in spare tubes for their linear, or interested in building a linear IF they have enough experience. Otherwise they will set on the shelf. I'm not about to tackle a project like this.


Last edited by TheGimp on Thu Nov 04, 2010 3:13 am; edited 1 time in total
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TheGimp



Joined: 08 Oct 2010
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PostPosted: Thu Nov 04, 2010 3:13 am    Post subject: Reply with quote

OK update, here ya go, notice the transformer is 7" x 3.4" across the top and 6" tall and weighs 39.6#. Your OPT will be just as massive if not more so.

The filament/Heater was supplied from a second transformer. Why a second transformer for heater/filament? Because it provided additional isolation maybe. Each transformer has isolation to frame and the winding isolation becomes the sum of the individual isolation.

I would try to make a small amp out of one or two 4CX250s running 500V, but if you look at the area below 300V on the plate curves you will see typical (Non-KT) pentode kinks that are responsible for massive amounts of distortion. This is another reason why you need a 1KV supply or greater, to allow you to operate above the 300V distortion zone. Unless, that is distortion is what you are after.
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Kagliostro



Joined: 30 Dec 2009
Posts: 324
Location: Italy

PostPosted: Thu Nov 04, 2010 9:37 am    Post subject: Reply with quote

Yes OT is a problem if use the 1.000v B+

Also if I'm not so confident with data sheet I have already noticed the problem at low voltage and ..... no, If I build a 200W or more amp I think to a bass amp and distortion is not wanted

(for guitar amp I think a 15 / 30 / max 60 W is plenty fine)

a 600v 700v B+ based amp can solve the OT problems about voltage ratings of the same for safety, but don't think a 3,500ohm OT will be fine with this low voltage

Kagliostro

p.s.: Your chassis with the tubes looks VERY nice, the power supply ........ not so much Smile
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Kagliostro



Joined: 30 Dec 2009
Posts: 324
Location: Italy

PostPosted: Sat Nov 06, 2010 6:15 am    Post subject: Reply with quote

Reading some documentation about 4CX250B based Linear Amplifier I've something read that there is a little more problem

My PT has a 6.3v winding and I've read that this tubes are not rated to 6.3v for filaments

they are rated to 6.00v +/- 5%

so they are 6.00v +/- 0.3v

which is the problem ?

in many thread about Linear Amplifier I've also read to be very careful with filament voltage and to underrate it a bit for safety

this can be a reason for which in your PS you have separated filament PT

Kagliostro
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