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Old 26th October 2006, 03:51 PM   #11
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I have just finished this project with EL34 and all A/C heating.

****Dead Silent
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Old 26th October 2006, 04:11 PM   #12
SY is offline SY  United States
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Quote:
Anyone else flogging ECC83's as phasesplitter drivers for KT88's ?
Not optimum for sure, but I don't feel like doing a full redesign...
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Old 26th October 2006, 06:19 PM   #13
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Can you help me with this values? I'm not sure about the 90ma, the value of I, V and V2.

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thanks!!!!
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Old 26th October 2006, 08:40 PM   #14
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That o/p stage schm is the GEC 88-50 business end. The anode currents are correct around 87mA. Hint, the tubes are run at 2x37W quiescent , so anodes will glow reddish..and circuits of this type are best put to work which reduces anode heating effect. Generally I don't like pushing tubes with this VA (shorter life) unless there is a good reason.. KT88's aren't cheap...and 500V is dangerous and requires respect for first time builders.
Earlier NOS 88's could take 10,000hrs with such voltages and higher, but I doubt newer manuf'd ones would last that long.
G2 current in UL should be around 5-10mA depending on tube and signal. SO g2 volts is sim to anode.

The orig transformer for the 88-50 has a primary res of app 75 ohms each half with g2 around 36 ohms each half/outer to centre tap. Ohms law can work out the rest.

Why not have a peep at the 30W GEC amp it's better sounding (near class A) and one can use the same o/p tranny for the 50W operating at 375V with lower anode dissip. Circuit is in Duncan amps.

More anon
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Old 30th October 2006, 04:13 PM   #15
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For some days I was reading, and reading......... about theory and that stuff.

I'm kind of lost. I dont know where to start to modify the first schematic that I posted to use it with KT88 instead of EL34, and to have 60W instead of 30.

How do I know the voltage swing that I need in the primary of the OPT if I know this??

output speakers= 6 ohm
secondary impedance of OPT: 6 ohm
primary impedance: 4K
UL taps: 43%

what else do I have to check? the "mu" of the EL34 & KT88?

thanks
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Old 30th October 2006, 09:50 PM   #16
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Quote:
Originally posted by zafira1981
For some days I was reading, and reading......... about theory and that stuff. the "mu" of the EL34 & KT88?

thanks

Reading about tube stuff is good for tube amp design entry.
The Radiotron handbook 4th ed is a MUST for serious tubers and every bit of it is relevant in todays world. It makes one eschew semiconductors and chuck'em away.

For first tube design... to get the feel I would stay with EL34's with low B+. The 5-20 design is pretty reliable.
-> The mu for both EL34's and KT88's is about the same in tet/pent config, although the screen grid (g2) resistors are different with UL. KT88=270R; EL34 = 1K.
The 6550 series in Ultra Linear (UL) is restricted to 470V+.
The 5-20 EL34 circuit requires an A-A Z of 6K. There are plenty of trannies about.

richj
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Old 31st October 2006, 07:13 PM   #17
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Very usefull information, thanks!

I have more questions.

One day, a teacher told me that the most important part of an amplifier is the power supply.

In the kt88 datasheets says that for UL connection and 70W, the stand by current is 2x50ma and for max signal, 2x140ma.

So, if the amp will be stereo, I will need a power supply capable of delivering 4x140mA, that is 0.56A x 460V

Am I right?

thanksssss
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Old 17th November 2006, 12:36 AM   #18
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Default Time to update!!

GOOD NEWS!!!! I have new stuff.

- Power supply transformer
- OPT's
- choke

I've been building the chassis with 2mm tick aluminium.

Some pics of the irons, the chassis (in progrress) and some components:

8x220uF 400V caps for the power supply,
BY228 fast diodes (1.5KV / 2A)

Metal-film 3W resistors,
10x120R 3W resistor for the phantom load (my speakers impedance are 6 ohms, so the OPT's)

Click the image to open in full size.
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Old 17th November 2006, 02:30 AM   #19
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Quote:
Originally posted by richwalters



Reading about tube stuff is good for tube amp design entry.
The Radiotron handbook 4th ed is a MUST for serious tubers and every bit of it is relevant in todays world. It makes one eschew semiconductors and chuck'em away.


Thanks for the tip.... I found the manual you mention online in pdf form.. Check it out...

http://geek.scorpiorising.ca/RDH4.html

haha.. The TOC is 32 pages long...
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Old 17th November 2006, 01:48 PM   #20
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Quote:
Originally posted by peritus307



Thanks for the tip.... I found the manual you mention online in pdf form.. Check it out...

http://geek.scorpiorising.ca/RDH4.html

haha.. The TOC is 32 pages long...
woow! that book is amazing!!!! I'm printing the audio chapters right now

thanks!!!
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