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Old 13th January 2011, 05:58 PM   #11
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I have checked the resistor location and against the JJ spce sheet. The resistor is on pin 8 of the output tubes. I have taken a couple of images of the tubes but cannot figure how to upload them any pointer? Looking through the 3 holes in the tubes, I can see a plate wrapped in wire. Kind of looks like an old bar heater. The glow from this on both types of tube JJ and GL are the same in colour. It is the tube tops that are glowing a different colour. On the JJ tubes I can also see a blue line of "corona"? in each tube. The GL do not have this but those unmatched 6550a tube had this blue line effect inside the metal structure.
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Old 13th January 2011, 06:13 PM   #12
M Gregg is offline M Gregg  United Kingdom
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Click post reply scroll down to manage attachments!

You need to reduce the size with windows picture manager or paint see sizes in the manage attachment quoted sizes.

Pin 8 is cathode. I don't know if you have discharge resistors on this amp watch for HT power still "up" after power off!

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Last edited by M Gregg; 13th January 2011 at 06:17 PM.
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Old 13th January 2011, 06:58 PM   #13
taj is offline taj
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Quote:
Originally Posted by bottletop View Post
My amp has no bias pots and what is even more confusing for me is that in the owner's manual it says there is no need to bias this design of amp, just used matched pairs of output tubes.
That's a pretty common scenario. There are two really common methods to provide bias to the output tubes. One way is to provide a separate voltage from the power supply, and that usually comes with bias adjustments pots.

The second way is to generate an appropriate bias voltage (a difference between cathode and grid) by adding a resistor between the cathodes and ground. This method is usually called self-bias or auto-bias, and that's what you have.

..Todd

Last edited by taj; 13th January 2011 at 07:09 PM.
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Old 13th January 2011, 07:15 PM   #14
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So in that case then; I guess I do not need to worry about bais just have to make sure outputs tubes are a matched pair? What would happen if I use tubes that are not a matched pair? Like what would be the result? Could this have caused the resistor to swell?
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Old 13th January 2011, 07:19 PM   #15
taj is offline taj
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well, you said it's a single-ended circuit, not push-pull. So you only have one output tube per channel. So, the worst (well maybe not the WORST)
that will happen is that your left and right channels could sound different and/or be a slightly different volumes due to the electrical differences that their circuits will be operating under with mismatched tubes.

In a push-pull circuit (2+ output tubes per channel) you also have to worry about the interdependency between the push and the pull tubes, and mismatches there causing all sort of problems. That's why you can (and probably should) buy matched quads for push-pull amps.

I don't have enough experience to know what conditions would cause that resistor to swell. Unless it was just a manufacturing fault of the resistor itself.

..Todd

Last edited by taj; 13th January 2011 at 07:38 PM.
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Old 13th January 2011, 08:09 PM   #16
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Taj, do you know what the score is on using GE Jan 6550a tubes in place of kt88? My gold lions test at Ip:71, Gm:8,300. The JJs test at PC:70, TC8. The 6550a are both different at PC: 49.8, TC: 5.9 and PC:42.6, TC:5.6. The chap at the shop with the tester said the 6550a's test as new but are not matched. So how can 6550a tubes be a direct drop in replacement for KT88 with such different PC and TC ratings? I did have a bad 12au7 tube that had a short on the same channel as the puffed resistor. Heard a buzz/hum through the speaker as well. Perhaps it was that.
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Old 13th January 2011, 08:28 PM   #17
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From my dusty files, I found JJ KT88's required a lower bias voltage for the same quiescent current compared to other KT88's: that would imply that the JJ88's plugged in would be somewhat underrun but would still work. The problem has to be elsewhere, oscillation or another defective component.

richy
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Old 13th January 2011, 08:39 PM   #18
tomchr is offline tomchr  United States
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To manage attachments, click the "Go Advanced" button and use the attachment function there.

It sounds like you're seeing the end of the filament poking through the cathode sleeve. Nothing wrong with that. If you look through the holes/slots in the anode you should see the cathode sleeve glowing red. The grids are the "chicken wire" wrapped around the sleeve. Actually there's a space between the sleeve and the chicken wire.

Now if the anode is glowing yellow or even a dull red, you've got problems.

~Tom
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Old 13th January 2011, 08:40 PM   #19
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Hi Richy, Taj has pointed out to me that my single ended triode amp is self/auto biasing. Can oscillation be heard? The other possible defective components then must need to be tested. I am not too keen on doing this on a live circuit. Once again showing my ignorance; can I test resistors and caps on a dead circuit?
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Old 13th January 2011, 08:49 PM   #20
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Tom, yes you have described exactly what I am seeing. The grey anode plates are not changing colour at all. It is the part in the top top of the tube that is in the middle of the mica. Do you have any light to throw on the tube values above?
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