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Old 26th October 2011, 11:31 AM   #1
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Default Biasing a Jadis Orchestra

Hello all. Iíve been lurking on here for a while now, but this is my first posting. Iím a fairly recent convert to valve/tube amps, and Iím trying to get to grips with the characteristics of different valve types and the mysteries of biasing etc. Although I covered basic electronics as part of my maths and physics studies, that was over forty years ago, so please forgive me if I seem a bit clueless!

My current amp is a Jadis Orchestra. Itís a simple integrated push-pull amp using ECC83s and EL34s. According to the spec it operates in Class A/B and is rated at 40 watts output. The amp has manual (fixed) bias and according to Jadis can also be used with KT88s, KT90s, 6550s, and 6CA7s.

Jadis have advised me that the correct bias, using matched tubes, is set by measuring the voltage drop across the R5.62 resistor between the anode/plate and the output transformer and adjusting the bias pots to achieve a voltage drop of between 90 and 120 mV, with 110mV (=19.6mA) being ideal. See attached image.
Click the image to open in full size.

According to Jadis, the same figures apply whether you are using KT88s, 6550s or 6CA7s. With KT90s they recommend going up to 120mV.

Iíve used these figures when re-valving with EL34s with no problems, and the amp sounds fine. However, Iím curious to know how this method of biasing relates to the figures Iíve seen for EL34s which suggest that the cathode current should be between 50mA and 30mA, depending on plate voltage.

Also, as different cathode figures apply to other valve types, how can a single figure apply to the anode measurements, pretty much regardless of valve types?

Any advice, explanations would be gratefully received.
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Old 26th October 2011, 11:40 AM   #2
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Hi JADISMAN, I'm not a tube guy so can't help with the questions (plenty of others who can though) but I can give you a warm welcome to diyAudio

Tony.
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Old 26th October 2011, 02:12 PM   #3
M Gregg is offline M Gregg  England
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Hi,

You have to remember that all tubes will be slightly different in gain etc.
Even a matched set will be very slightly off. So the amount of negative voltage applied to the grid to turn the tube "off/down" will be slightly different with each EL34 to get the same standing " idle" current. The idea that the "current<<not bias voltage" would be the same for different tubes is because if the load resistance is the same (only the tube has changed) then with simple ohms law the voltage drop across it or load will be the same with the same current the tube type is irrelevant. However the amount of bias required to turn "off/down" the tube may be higher or lower. In some cases (wrong tube type) you may not have enough negative voltage to turn down the idle current enough and the tube /op Tx will (over heat).

Hope this helps

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Old 26th October 2011, 03:45 PM   #4
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Jadisman: Can you confirm the value of the R that you are measuring across? 20ma seems very light for idle current for any of the tubes listed. What is the 3rd color band on the R? The green and blue (for 5,6) are pretty clear. Are you sure that those resistors are between the plate and the transformer and not on the cathode? If the resistors are on the plate side they are at high voltage so be careful when measuring. If they are on the cathode they are at much lower voltage.

If you can carefully measure the voltage from either side of those resistors to ground that would tell you what side of the tube they are on, and if they are on the plates, it will give you an indication of the B+ voltage on the tubes.

Running KT88's at the same B+ and idle current as EL34's would have them loafing along.....

Many amps have the measuring resistors shared between pairs of tubes (on the cathodes). Since this appears to be a PP amp, do you have 4 sets of R's and pots or two sets of R's and pots? If you only have two sets, they are most likely on the cathodes.

On other (maybe obvious) thing........ EL34's and 6CA7's are pretty much the same tube as far as power dissipation and idle current, etc, and 6550's are light versions of KT88's.
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Old 26th October 2011, 04:39 PM   #5
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Thanks for the replies "M Gegg" and "boywonder"

Boywonder: See attached image, which I hope is a bit clearer . This shows one pair of tube sockets. The other pair are identical. The resistor I'm talking about is the one between the yellow wire on pin 3 and the output tranformer. The blue wires connect to external test probe sockets which are used to measure the voltage drop. The bias pots are adjusted with a plastic screw driver inserted through the ventilation hole directly above.

Click the image to open in full size.
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Old 26th October 2011, 04:40 PM   #6
DF96 is offline DF96  England
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Does the Jadis run them deeper into Class B? If so, the quiescent current will be smaller and the average current will increase more when signal is present. 40W from a pair of EL34 is consistent with running nearer to Class B.
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Old 26th October 2011, 04:50 PM   #7
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by DF96 View Post
Does the Jadis run them deeper into Class B? If so, the quiescent current will be smaller and the average current will increase more when signal is present. 40W from a pair of EL34 is consistent with running nearer to Class B.
I agree, and further the measurement resistors appear to be in the plate circuit, so high voltages on the order of 450V or more are certainly present. Use extreme caution when making these measurements!

Can't say I am the least bit thrilled to see how Jadis put this amp together.. Make sure when you have been in there that you have not disturbed any components so that an arc to another part or the chassis is possible.

Also welcome to the forum!
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Old 26th October 2011, 05:13 PM   #8
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Quote:
Originally Posted by kevinkr View Post
Can't say I am the least bit thrilled to see how Jadis put this amp together..
I totally agree. High voltage buss line left uninsulated, and resistors left suspended in the air on wires. Output tube filaments in series? When will those fools learn?
Anyway, welcome to the forum!
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Last edited by HollowState; 26th October 2011 at 05:28 PM.
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Old 26th October 2011, 09:23 PM   #9
bigwill is offline bigwill  United Kingdom
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I would be ashamed of that even as a prototype!
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Old 27th October 2011, 10:37 AM   #10
DF96 is offline DF96  England
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Is that a chassis-mounting fuseholder being held up in the air by a solder joint at one end onto a bus? Eek! I wouldn't even do that in DIY. Never, ever, rely on a solder joint for mechanical strength. It will fail; the only question is when.
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