My new project, 2xKT88 60W amp
Hi!!!!!
I need to share the emotion...... I just boutgh the tubes for my amp project... it will be 2xKT88+12AU7 for output stage and 1x12ax7 diff amp (with solid state current source). I didnt finish the schematic... I have to make some questions to a teacher of my college... as soon as I had it, i will post it
meanwhile... some pics!!
look at the tube sockets, they're ceramic - gold! they look really nice...
Hi!!!!!
I need to share the emotion...... I just boutgh the tubes for my amp project... it will be 2xKT88+12AU7 for output stage and 1x12ax7 diff amp (with solid state current source). I didnt finish the schematic... I have to make some questions to a teacher of my college... as soon as I had it, i will post it
meanwhile... some pics!!
An externally hosted image should be here but it was not working when we last tested it.
look at the tube sockets, they're ceramic - gold!
they look really nice...An externally hosted image should be here but it was not working when we last tested it.
Some news!!!!
My output power transformer will be:
input: 5kohm (400H ) with 43% ultralinear taps, output: 6ohm
4,5kg M5 iron (dont know how is in english "grano orientado")
PSU choke: 10H 250mA
PSU transf: 370+370 V
For the tube heaters I will use 6.3v DC regulated... I need to find a good circuit, maybe with a LM317 / 338.
the schematic will be something like this:
but with a diffamp with 12AX7.
My output power transformer will be:
input: 5kohm (400H ) with 43% ultralinear taps, output: 6ohm
4,5kg M5 iron (dont know how is in english "grano orientado")
PSU choke: 10H 250mA
PSU transf: 370+370 V
For the tube heaters I will use 6.3v DC regulated... I need to find a good circuit, maybe with a LM317 / 338.
the schematic will be something like this:
An externally hosted image should be here but it was not working when we last tested it.
but with a diffamp with 12AX7.
I am builing the exact circuit you are refering to, with EF86, ecc83 and EL34.
I cannot answer these questions to their full extent, as I in my project depend on guiding from more competent persons. But:
Are you sure 370 V B+ will give you 60 watt out with kt88?
Are you sure a choke rated at 250 mA can deal with the current needed for 60W out?
In my project I am aiming at 30 - 40 watt out. My B+ is 460 volt and my 5H choke is rated at 300mA.
I cannot answer these questions to their full extent, as I in my project depend on guiding from more competent persons. But:
Are you sure 370 V B+ will give you 60 watt out with kt88?
Are you sure a choke rated at 250 mA can deal with the current needed for 60W out?
In my project I am aiming at 30 - 40 watt out. My B+ is 460 volt and my 5H choke is rated at 300mA.
Are you sure 370 V B+ will give you 60 watt out with kt88?
nono, I will use 370+370 transformer so the B+ will be somethin like +470V
Are you sure a choke rated at 250 mA can deal with the current needed for 60W out?
mmmm... I'm not sure, the guy who builds my output trans told me that I have to use that 10H 250mA choke. I will take note of that you said, thanks!!!!
nono, I will use 370+370 transformer so the B+ will be somethin like +470V
Are you sure a choke rated at 250 mA can deal with the current needed for 60W out?
mmmm... I'm not sure, the guy who builds my output trans told me that I have to use that 10H 250mA choke. I will take note of that you said, thanks!!!!
Funny.
This is from the Byrith book right ?
I'm collecting parts for the project he describe in his book right now.
I don't think you need regulated power for the heating in an power amp, only a pre amp is wort that much trouble.
I will stick with a regulated power supplyfor the el34 model.
And here is the link to the book
http://www.lundahl.se/claus_b.html
This is from the Byrith book right ?
I'm collecting parts for the project he describe in his book right now.
I don't think you need regulated power for the heating in an power amp, only a pre amp is wort that much trouble.
I will stick with a regulated power supplyfor the el34 model.
And here is the link to the book
http://www.lundahl.se/claus_b.html
hehe, this seems yo be a popular project, I myself have just collected most parts for the same amp. I will use 6550 instead of EL34 thou.
As stated earlier, just keep it comming!
EDIT:
have you guys noticed the differences between the schematic in the appendix and the one in the article?
Different values on caps and some that are present in one isn't there on the other.
As stated earlier, just keep it comming!
EDIT:
have you guys noticed the differences between the schematic in the appendix and the one in the article?
Different values on caps and some that are present in one isn't there on the other.
This is a variation of the Mullard circuit. There's a very nice deconstruction of the design in "Valve Amplifiers."
May I simplify things a bit?
First, if you pay some attention to grounding and layout, there's no reason you can't use unregulated AC on the heaters. None of my power amps use DC heaters, and they all are dead quiet.
Second, if you're using a current sink in the second stage cathodes, the plate balance adjust is unnecessary. Just use matched plate resistors and matched grid resistors for the output stage (they're part of the diff amp's load) and the CCS will force things to be balanced.
May I simplify things a bit?
First, if you pay some attention to grounding and layout, there's no reason you can't use unregulated AC on the heaters. None of my power amps use DC heaters, and they all are dead quiet.
Second, if you're using a current sink in the second stage cathodes, the plate balance adjust is unnecessary. Just use matched plate resistors and matched grid resistors for the output stage (they're part of the diff amp's load) and the CCS will force things to be balanced.
400H for o/p tranny is alot of L.......Is it a toroid ? I
noticed 390K output stage grid leaks......this is way high for KT88 and especially for 6550's in fixed bias and no HF phase correction in 1st stage anode.
Those KT88's in the packets..wait before you use them...I say get design going with EL34's and study the data sheets of the KT88 and 6550C.
Svetlana spec for 6550C suggests 200k absolute max for grid leak and I'm using not even half that in my fixed bias designs.
Variants of KT88's are also sim.
Anyone else flogging ECC83's as phaseplitter drivers for KT88's ?
richj
noticed 390K output stage grid leaks......this is way high for KT88 and especially for 6550's in fixed bias and no HF phase correction in 1st stage anode.
Those KT88's in the packets..wait before you use them...I say get design going with EL34's and study the data sheets of the KT88 and 6550C.
Svetlana spec for 6550C suggests 200k absolute max for grid leak and I'm using not even half that in my fixed bias designs.
Variants of KT88's are also sim.
Anyone else flogging ECC83's as phaseplitter drivers for KT88's ?
richj
Can you help me with this values? I'm not sure about the 90ma, the value of I, V and V2.
thanks!!!!
An externally hosted image should be here but it was not working when we last tested it.
thanks!!!!
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
richj
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
richj
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
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
zafira1981 said:
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
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
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
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)
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)
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
richwalters said:
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...
peritus307 said:
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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