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Old 16th December 2010, 02:16 AM   #1
Pez is offline Pez  United States
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Default SSE bias question

Hi all, it's been almost a year since I've built and assembled 2 SSE amps. I love them to death! they are the finest amps I have heard regardless of design or price. I haven't heard nor built the TSE because I am so happy with the SSE.

Any way I believe I am running my tubes too hot, the silvering is starting to recede quite a bit on both amps on the KT88s which are about a year old. When I was building the amp I was steeped in the design where I probably could have figured this all out on my own, but now 1 year later I've pretty much forgotten all details of how to measure the bias, what resistors to change out to adjust it etc.

I was hoping if someone could give me a quick and easy answer as to what pins on the tube socket I need to measure to see my bias voltage AND what resistors I need to swap out to lower the bias if necessary.

I'm using KT88s, 5AR4 rect, and my PT is the Allied (hammond) 6K7VG.

This amp is so hands off and easy that it has lead me into a false sense of security.

Thanks for any assistance!
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Old 16th December 2010, 02:54 AM   #2
rknize is offline rknize  United States
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When the amp is idling (with tubes), check your plate voltage (pin 3) and cathode voltage (pin 8) of the the power tubes. The bias is setup by the cathode resistors (R17 and R27).
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Old 16th December 2010, 03:12 AM   #3
Pez is offline Pez  United States
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Ok thank you. Plate voltage is 43 and .3 for cathode voltage... Not sure if that's right...
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Old 16th December 2010, 03:42 AM   #4
rknize is offline rknize  United States
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The plate voltage should be 100s of volts. Be sure to hook up the meter with the amp powered off and then apply power. You don't want to be holding meter leads on such high voltages. A better place to grab the plate voltage is at the terminal where the blue wire from the OPT connects. The cathode voltage can be taken right off of the cathode resistor lead that faces the back of the PCB. They are the big, white resistors in this picture:

Simple SE Manual
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Old 16th December 2010, 04:35 AM   #5
Pez is offline Pez  United States
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I'll have to do it that way, it's very hard to get a reading on this amp. I'll have to pull it apart the case to get to the board. Thanks for the help.
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Old 16th December 2010, 01:22 PM   #6
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Quote:
Originally Posted by Pez View Post
It's very hard to get a reading on this amp. I'll have to pull it apart the case to get to the board.
What size cathode resistors did you use? The cathode resistor is the big white cemented rectangles. I'd guess that a 560 ohm resistor is probably about right for KT88, assuming you are using one of the popular power transformers (Allied 6K7VG). What brand KT88 are you running? I've got a set of New Sensor "Gold Lion" reissues, and I'm seeing the same kind of fading of the getter. It's particularly noticeable on the side getters.

You could put three extra holes in your chassis, and mount a few test jacks. Then just run wires from the test jacks to the appropriate places on the circuit board.

72913-2 Pomona Electronics Test Plugs & Test Jacks
72913-0 Pomona Electronics Test Plugs & Test Jacks

You want to take your voltage measurement across the cathode resistor. In the photo below, it's the big white rectangular cemented resistor. You really only need three test points. The ends of the cathode resistor that are closest to the big orange coupling capacitors are tied to the ground plane, so it is common on both.

Click the image to open in full size.
http://i69.photobucket.com/albums/i4...E/P1100567.jpg
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Old 16th December 2010, 02:31 PM   #7
ixe13 is offline ixe13  Canada
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Hi,

I've used Gold Lion kt-88 also, well, 3 actually, one started to be noisy, I replaced it, then the bias on the other one wouldn't stay put, kept rising slowly. Didn't want to wait till it "exploded".

I should send it to George

And guess what? the getter on one side was fading away quite a bit.

If you look at it through some light you would see throught it easyly...
and was getting smaller also.


The bias on the SSE was allright. The tube were at 25 watts or so...

Got rid of the third one and bought a pair of Tung Sol 6550 and the sound is as good as the kt-88 and I think the highs are "sweeter"

I say "I think" because sometime, we hear what we wanna hear...
But they sound as good that I'm sure of...

A bad batch maybe? I bought them last year around Chrismass.

Yves
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Old 16th December 2010, 03:05 PM   #8
Pez is offline Pez  United States
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Quote:
hat size cathode resistors did you use? The cathode resistor is the big white cemented rectangles. I'd guess that a 560 ohm resistor is probably about right for KT88, assuming you are using one of the popular power transformers (Allied 6K7VG). What brand KT88 are you running? I've got a set of New Sensor "Gold Lion" reissues, and I'm seeing the same kind of fading of the getter. It's particularly noticeable on the side getters.
Well luckily I had enough sense to take hi rez pics of both sides of the board pre assembly! I am using Yageo 5 watt 560 ohm resistors in both amps. So yes, it does appear to be "correct", but the fact remains these tubes are merely a year old and they are starting to show age normally associated with a tube twice it's age. Then again I have been listening to this amp A LOT! I realize that Class A circuitry is generally tougher on tubes and this is probably part of the problem.

Any way The tubes I'm using are the Shugaung Treasures from Grant Fidelity. They are WAY better than the gold lions in my experience and the gold lions aren't bad by any measure. Yes they are uber expensive, but this amp deserves it and so do my ears.
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Old 16th December 2010, 03:09 PM   #9
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The comments on the GL KT-88 are interesting. While I have used them in my Primaluna 7 monoblocks for over 2000 hrs with no problems, for me, they are too expensive to "experiment" with in my DIY projects. The GL tubes have a rep for decent, if not stellar durability. Ask Jim McShane.

Using 560R cathode resistors, and the same xfmr as Pez, my SSE has so far killed (2) Chinese KT-88's. Though the sound is marvelous, I think something in this circuit may be affecting tube life. I'm still learning about operating points and such. I'm sure some experts will chime in to fill in the blanks.

Edit: I should have mentioned I'm currently using Russian 6P3S-E in the SSE. Ty Bower turned me on to these a while ago. They're connected with 820R resistors. Output a bit less but sound good. We'll see how they stand up over the long haul, but they're way cheaper than any KT-88.
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Last edited by neonthecat; 16th December 2010 at 03:39 PM.
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Old 16th December 2010, 05:12 PM   #10
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Quote:
Using 560R cathode resistors, and the same xfmr as Pez, my SSE has so far killed (2) Chinese KT-88's.
I have Electro Harmonix KT88's because I got them used for $10 each. I am running then HOT with 430 ohm resistors and the tubes have been in the amp for 2 years. It was used daily for the first year, and randomly since I built the SPP. 430 ohms is a bit too low since the tubes run at 100 mA each. This is sucking about 220 mA out of the power transformer which makes the already warm Hammond run HOT! The diodes blew early in the life of the amp si I removed them. The amp has been flawless ever since.

560 ohms should be OK for any KT88 that meets its specs. Some of the early Chinese stuff (10 years ago) would red plate or just blow up when operated within their ratings. There was rumor of a bad batch or two of KT88's from Shuguang about a year ago. The pair I have work OK but haven't been used much. I don't have any first hand information on the high priced tubes, since I tend towards low priced tubes.

Quote:
I was hoping if someone could give me a quick and easy answer as to what pins on the tube socket I need to measure to see my bias voltage AND what resistors I need to swap out to lower the bias if necessary.
You need to know what value resistors are in the amp for R17 and R27 (you have 560 ohm). You need to measure the voltage across these resistors with the amp on but the volume turned down. The voltage can be carefully probed on the back of the board at pin 8 of the tube socket (black meter lead grounded). You also need to know the plate voltage. It can be probed at pin 3 of the tube socket. With your tubes and transformers the plate voltage should be about 420 to 450 volts and the cathode voltage should be 35 to 45 volts.

Divide the cathode voltage by the resistor value to get the tube current. Subtract the cathode voltage from the plate voltage to get the tube voltage. Finally multiply the tube current by the tube voltage to get the total tube dissipation. Vcath/Res * (Vplate - Vcath) = Pdiss. Example:
Measure 40 volts across R17 and 430 volts on plate. 40/560 *(430 - 40) = 27.857 watts. The KT88 is rated for 40 watts. It should be fine at 28 watts. Most experts say a class A amp should run 70 to 80% of maximum dissipation. 70% gives longer life, 80% gives better sound. That is 28 to 32 watts.

Extreme example: (mine) 44 volts cathode, 425 volts plate, 430 ohm resistors.
44/430 * (425 - 44) = 38.98 watts. Above the 70% to 80% recomendations, but still within the specs.

To change the bias on the output tubes the cathode resistors must be changed to a different value. A higher resistor value lowers the tube current. Several users have installed a large value resistor (820 ohms) and use a switch to parallel another resistor to raise the current. This allows multiple bias settings and is useful if you like to change tubes. One of my amps uses 750 ohm resistors and I have several other resistors with aligator clips on the ends for boosting the current when needed.

Quote:
Though the sound is marvelous, I think something in this circuit may be affecting tube life.....I realize that Class A circuitry is generally tougher on tubes and this is probably part of the problem.
It is true that a class A amp is hard on the tubes. Any class A amp. Why? The tubes are running at 70% - 80% of full power ALL OF THE TIME! A class AB push pull amp usually idles at about 30% to 50% of full power. Some guitar amp guys run hotter than this, and guess what.....they eat tubes. The average power in most music is far less than the maximum, so even if the push pull amp is played loud, on the edge of clipping, the average dissipation in the tubes will be 50% of maximum or less.
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