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Old 25th April 2004, 08:13 PM   #101
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Hi Reinout,

Many thanks for the info. that helps tie things down. I think I will need to look at raising the static Va to 1000V. in an attempt to reduce the Ia requirement. The OPT primary impedance info is a useful guide too.

For a driver stage I have been looking at a couple of options - both push pull into step down interstages. The first is a pair of PL802s in triode mode through a 2.5:1 interstage that should give me up to +/- 70mA of drive current. The alternative is a pair of PCL82 in push pull partial feedback configuration through 4:1 step down. This should give +/- 120mA of drive. I'll probably build both and see which I prefer.

The power supply is a little bit daunting! I thought I would use a bipolar supply at +/- 500V to give me the 1000V but let me feel a little bit safer! Of course this means doubling up on the number of chokes in the psu...

I have to say I am very excited about finishing this amp and very interested in how it will compare with the 845 amps I have built before.

I should also mention that I am building a pair of 211 cathode follower push pull pair of amps of about the same output power. I have been asked to develop a pair of valve amplifiers to drive a pair of Apogee speakers - hence the need for a lot of power and good current delivery as the speakers are 3 ohm nominal impedance with a low of just under 2 ohms... should be interesting to see which amplifiers manage this the best.

ciao

James
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Old 26th April 2004, 05:42 AM   #102
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Here is my latest amp.

It's a stereo 35W SE with Eimac 250TL tubes. The output, power transformer, and chokes are Tamura. The circuit is identical to the 833A amp, but the power supply use vacuum tube rectifier for both driver and output stage, double chokes and oil caps.
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Old 26th April 2004, 05:53 AM   #103
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Default 250TL tubes glowung in the dark

Here is the picture of the Eimac 250TL glowing in the dark.
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Old 26th April 2004, 04:54 PM   #104
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Default Very Pretty!!!

Almost feminine compared to the 833 beast

How does the sound compare? Is the bass performance better?

James
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Old 26th April 2004, 05:56 PM   #105
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Thanks James,

I hesitated quite a few time when I started building this 35W SE amplifier using the Eimac 250TL tube which I have never seen anybody used them in audio amplifier. The parts I gathered for this amp was originall designed for an 845SE amplifier. The glow of the 250TL has started it all.

This amp sounded much better than my 833A amp and also a pair of 845SE that I build (using identical Tamura output). In the pass, it is just as strong as the 845, the 833A lacked the bass, but It might be due to the output transformer that I used and the higher bias current. On the mids the 250TL is warm, the 845 is darker and the 833 is lighter. The highs on the 250TL is more open than the 845, the 833 has a slight touch of brightness.

I would not say the 250TL tube is better than the 833 since both amp are constructed with different parts quality, they do share same circuit though. The 250TL used tube rectifier for driver/output stage while the 833 used solid state rectifier.
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Old 26th April 2004, 07:15 PM   #106
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Default More transmitting tubes !

Hi Unison845,

it's good to see that there are more transmitting tube users. There is so much out there. 75TL / 100TL / T-55 / HY-75 / 250TL / 800 / 838 / 833 or go for the 450TL if you're really a daredevil......
Personally i would not chose for the 152TL/304TL. Those tubes are simply more 75TL's stacked together in a single glass envelope. Parallelling tubes outside would be the same....go for parallel 833's ?

There are options ! Don't stick to the 211/845. not that these are bad.....but there is a whole new world out there (ever seen a UV-204 bases amp.....also known as a VT-8....check Ebay now under "uv-204-a")

The commercial tubebuilders are just stepping into the transmitting tubes. For example Electronluve with their stylish 75TL (if you like the style).

Why did i pick the 833 ?
- nice specs. 5 KOhm opt is a nice ratio so you don't have to overcome huge ratio-differences. The higher the impendance the more difficult to get more wire on to the bobbin... So your opt will be even more large or the wire will be to thin which is not advisable with these mA's rushing through....
- still in production. And NOS reasonable priced.
- there are commercial amps using the 833 available......which means it is possible. For instance the Wavac HE-833 received good comments.

Yes, i did consider the 250TL but the impendance of the opt would be around 10K-ish ?

Of course that is al theory. I never had the luxury to listen to both a 833 and a 250TL amp using the same components and schematics. To be honest: who has ??

The most important thing: you've build a great looking amp and i bet it sounds fantastic. With 2 nice features:
1. DIY....you should be proud of your acomplishment
2. not everyone has a 250TL amp

I think you're not the first 250TL builder. Ed Billeci (audio bizarro) made one a couple of years ago and of course those Japanese are sitting still and do nothing.

Included is a picture of the the Billeci amp. I'll try to find others and in any case the 450TL-amp. And that is seriously way out of my leage !

Again: congratulations with the 250TL-amp. Wish we could listen to each others projects........

Reinout
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Old 26th April 2004, 07:27 PM   #107
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Default transmitting tubes 2

Hi Unison845,

nope......only found Japanese 808-based amplifiers.

And Ed Billeci has abandoned his WE437/WE437/250TL amp.

So that means that you have the only functional 250TL audio amp. Very nice ! Now that is something special.
I know already a handfull diy 833-amps and there even available commercial form Wavac, Lancroft, Lumley and Legend Audio. That makes the 833 "common" ??

Nevertheless my "statement" in the last post remains the same: go out there and reach for these funny tubes.

Included is a picture of a Japanese 808 amp. And i've got here some 808's and 826's lying around. They simply beg to be used....

Reinout
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Old 26th April 2004, 07:30 PM   #108
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WOW!!
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Old 26th April 2004, 07:30 PM   #109
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Default transmitting tubes 3

Hi Unison845,

found it !

This is a 450TL based amp. Certainly not as smooth as your wooden 250TL-chassis. Which will have burnmarks whith this tube (4500 Volts.....)

I guess the constructor dooes not have cats.....

Reinout
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Old 26th April 2004, 07:36 PM   #110
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This had turned into a very nice thread indeed. In more than one way...

Slowly but surely the BIG valve builders are crawling out of the woodwork

And I thought a 845 was a big project!! Was I ever wrong.

It is refreshing to see something different to the 45/2A3/300B/EL84/EL34 crowd!!!

Keep em coming...I can handle it!!!!

But please don't let it deteriorate into a mine is bigger than yours contest!!!

Cheers,
Bas
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