I bought a matched pair of JJ KT88 tubes to use in my SET amp. It has no bias adjustment. I was using a matched pair of reissue GL kt88, but wanted to try the JJ tubes as I was impressed with the look of the build quality and weight. The glass must be near on twice as think as other modern tubes I have seen. I have tried the GL and GE6550a tubes recently, they have always glowed a nice orange colour as regards to the grid filaments (if that is the correct term). The thing is these JJ filaments are glowing a bright yellow even after a few minuets. IE it is not start up flash. This concerns me, should I be worried?
Replace the resistor and if it is part of one channel "Left or Right" then change the same resistor in the other channel. If the resistor is in the cathode of the output tube then check the voltage across it to give you the current and wattage dissipation! You need to know what the voltage drop across the cathode resistors should be under "normal bias" condition and compare if you change output tubes!
All tubes tend to bias differently and should be checked. Do not assume that the same type of tube will run within correct limits.
Regards
M. Gregg
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What is that resistor connected to? Which pin on which tube (or whatever)? Got a schematic of your amp?
If you can do it safely, measure the voltage across that resistor (one meter probe on each of its leads) and the one in the opposite channel to see if there is a substantial difference. Do you know the value of that resistor?
..Todd
If you can do it safely, measure the voltage across that resistor (one meter probe on each of its leads) and the one in the opposite channel to see if there is a substantial difference. Do you know the value of that resistor?
..Todd
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Yes I have a schematic, but it is different from what is under the cover. The amp is a separo se88i. The mills are 12w 470r. The original resistors are 10w 470r. By the way I know less than a little about electronics. Just upgrading the coupling/decoupling and power caps has been a learning curve. I think M Gregg is correct from what I remember. The resistors are on the output tubes, thought to be sure if it is the cathode pin I will need to work out what pin does what. I am totally confused about what has caused this problem. I think it will be to do with tube rolling. I bought what was thought to be a matched pair of 6550a valves. I had them tested yesterday and they are not matched, same dates of production and test like new but not matched. 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. The values of the GL KT88 tubes and JJ tubes are very similar/close. Yet the JJ do not seem happy at all. So the JJ tubes must "run on" less current. 6550a are supposed to be a drop in replacement for a KT88 but how can they be when the PC and TC values are totally different from a KT88. I am confused!
Just for interest,
Link for kt88 pin 8 is cathode. Dissipation is shown in part2.
With Ohms law current is (Resistance of resistor divided into Volt drop across it = current).
KT88 and 6550 Tube Directory
Regards
M. Gregg
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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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!
Regards
M. Gregg
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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!
Regards
M. Gregg
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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
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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
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
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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.
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
richy
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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
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
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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