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Old 16th February 2011, 09:23 PM   #31
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by ManUtd View Post
What is the watt tolerance for Grid resistor (220K) for this specific amplifier
according to this scematic?
I have at hand 150K 2Watt. Will they be fine?
Two watt resistors will work just fine, all you would really need is a 1/2W type. Don't use metal oxide or carbon film types as they aren't the most linear types out there.
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Old 17th February 2011, 07:35 AM   #32
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Originally Posted by richwalters View Post
I believe your amp runs at 500V ???.......at 100mA bias for KT88's is dangerously high, the tubes will be glowin' orange .....enough to grill a chicken. Increase neg bias volts to give around 50-60mA per tube and no higher. Once you set the quiescent current, multiply that by the anode voltage and one roughly gets the tube dissipation. 30-40W for new build KT88's is enough otherwise they will have a very short life.

V/R=I ...Ohms law. You know what R is..It's as simple as that.

richy
Rich, the GEC 88-50 cct has 50V on its 525 ohm cathode resistors which gives, 95.2mA idle current (cathode bias). does this indicate a problem with high current?

cheers
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Old 17th February 2011, 09:53 AM   #33
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Originally Posted by puginfo405 View Post
Rich, the GEC 88-50 cct has 50V on its 525 ohm cathode resistors which gives, 95.2mA idle current (cathode bias). does this indicate a problem with high current?

cheers
At that plate voltage( 500V ), yes it does. At 400V of B+ it would be tolerable if the tube can make the ratings usually published for KT88's.
cheers,
Douglas
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Old 17th February 2011, 02:39 PM   #34
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Enclosed in pic are the ratings for the original KT88-50 amp. This amp used the MOV generic KT88 which was ultimately more longer lasting than todays variants.
Using JJ'KT88's also bias with less and will run hotter.

I know the cathode resistors are as you stated, but todays tubes aren't really replicas of the "way they were made" reliability etc. For autobias I'm tempted to use a 560R and with fixed bias go around 50-60mA quiescent on each.

Give'em a chance.

richy
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Old 12th March 2011, 11:18 AM   #35
ManUtd is offline ManUtd  Norway
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Hi Again,
Now amp is adjusted (BIAS) but i`m not satisfied with result.
The tubes i changed is KT90 to a matched pairs of KT88.
These have a value stamped on them: 47 on one pair and 52
on the other pair.I putted the pair with 47 on Left side and
the 52`s on Right Channel.I adjusted then Bias with this cables :
Tube amp bias test probe 6l6 el34 kt88 or 6bq5 el84 - eBay (item 290531020309 end time Apr-05-11 20:34:06 PDT)

Left side measures 0,66V and Right side measures 0,62V.
After around 10-15minutes of settling time.

But the right side is not sounding ok, it is disturbed in midrange/bass.
I`m not sure if i can turn Bias further more up to eg. 0,75V ?
I also recognice sound turns ok for around 1-2 seconds when turning power Off! Sound goes higher and without distortion during this shut off time..Does it mean Bias is still to low and needs another adjustment?




Quote:
Originally Posted by kevinkr View Post
I don't know what your plate voltage is but if it is anything over 400V, 100mA of plate current is going to melt most current production KT88 before your very eyes.

I would recommend something like 75mA up to about 450V, and somewhat lower at 500V for best longevity.

Yes you can measure the bias across a resistor in the cathode circuit, this of course is combined plate and screen grid current which is fine, and will give you an accurate picture of the combined plate and screen dissipation as long as you know the plate voltage.
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Old 12th March 2011, 06:57 PM   #36
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We need to know what power levels you are measuring at. In fixed bias I've used tubes with large gm variations without effecting sound quality. Remember setting Iquies sets the position on loadline and is nearly always approximate, the AC gain (gm of each tube) will determine the rated anode+ screen current at signal power in an AB stage. As Kevinkr mentions set 75mA, the full signal power will be roughly 110-130mA depending on the degree of gm matching. I can never detect the difference in quality as global nfb does an excellent job of correction.
You mention suddenly sounds good when turning amp off......I wonder if the output stage triode/pentode switch is switching properly on both halves. Those Jap mini toggle swiches are notorious for poor contacts.

richy
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Old 12th March 2011, 07:52 PM   #37
ManUtd is offline ManUtd  Norway
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Thanks Richy!

I will check the switch.
Is there any other "parts" i should replace in this chinese amp
It`s an Ming Da amplifier so might i should change caps too?

Thanks Again.

Quote:
Originally Posted by richwalters View Post
We need to know what power levels you are measuring at. In fixed bias I've used tubes with large gm variations without effecting sound quality. Remember setting Iquies sets the position on loadline and is nearly always approximate, the AC gain (gm of each tube) will determine the rated anode+ screen current at signal power in an AB stage. As Kevinkr mentions set 75mA, the full signal power will be roughly 110-130mA depending on the degree of gm matching. I can never detect the difference in quality as global nfb does an excellent job of correction.
You mention suddenly sounds good when turning amp off......I wonder if the output stage triode/pentode switch is switching properly on both halves. Those Jap mini toggle swiches are notorious for poor contacts.

richy
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