Wirewound resistors are better as cathode resistors as they won't go high like metal film. Any of those you posted in the last post will do, but don't spend daft money on them. Make sure you get ones which are the same size as those you'll be replacing.
As for recommendations Welwyn WW resistors have a good reputation as being pretty bullet proof,sorry, can't remember the type, W22 maybe?
As for recommendations Welwyn WW resistors have a good reputation as being pretty bullet proof,sorry, can't remember the type, W22 maybe?
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First I want to thank everyone that contributed and replied to help me when I posted this. I left this alone for a year because other things like work are more important and take precedence. 🙂
Now that I have the replacement resistors, I forgot where the 560Ω resistors went!
I looked at the pic from the manufacturer, but that is the 240V version, mine is the 120V version. Here it has the resistor on adjacent pins on the KT88 sockets:
But on mine, as it sits, from looking at the KT88 sockets, it appears to go between 4 & 6 (6 has the yellow wire, and 4 is empty), based on where the white gunk was missing and the solder left on there:
The schematic that was found is the 240V version, so there is no 560Ω but it seems its a 270 (perhaps due to the different main voltage):
So i'm pretty sure I have it correct that it goes between 4 & 6, but I would like someone more knowledgeable to confirm please. Thanks for your time.
Now that I have the replacement resistors, I forgot where the 560Ω resistors went!
I looked at the pic from the manufacturer, but that is the 240V version, mine is the 120V version. Here it has the resistor on adjacent pins on the KT88 sockets:
But on mine, as it sits, from looking at the KT88 sockets, it appears to go between 4 & 6 (6 has the yellow wire, and 4 is empty), based on where the white gunk was missing and the solder left on there:
The schematic that was found is the 240V version, so there is no 560Ω but it seems its a 270 (perhaps due to the different main voltage):
So i'm pretty sure I have it correct that it goes between 4 & 6, but I would like someone more knowledgeable to confirm please. Thanks for your time.
KT88 connections:
Pin # 4 is the Screen
Pin # 6 is not connected to any internal KT88 elements. It is called a NC (No Connect).
That means if any wire or part is connected to the socket tab, Pin # 6, the tab is just being used as a standoff / support for that end of a part / wire.
Pin # 6 appears to have a Yellow wire to it.
That is probably the UL connection of the output transformer.
Look to see what part that Yellow wire goes to.
The schematic shows a 270 Ohm Ohm resistor connected from the output transformer UL tap to the KT88 Screen.
Connecting 270 Ohms from Pin # 4 to Pin # 6 with the yellow wire is probably correct.
I do not see any 560 Ohm resistor on the schematic.
Pin # 4 is the Screen
Pin # 6 is not connected to any internal KT88 elements. It is called a NC (No Connect).
That means if any wire or part is connected to the socket tab, Pin # 6, the tab is just being used as a standoff / support for that end of a part / wire.
Pin # 6 appears to have a Yellow wire to it.
That is probably the UL connection of the output transformer.
Look to see what part that Yellow wire goes to.
The schematic shows a 270 Ohm Ohm resistor connected from the output transformer UL tap to the KT88 Screen.
Connecting 270 Ohms from Pin # 4 to Pin # 6 with the yellow wire is probably correct.
I do not see any 560 Ohm resistor on the schematic.
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Correct there is no 560Ω on the schematic. The resistors I actually removed and saved from my actual unit are indeed 560Ω. I can only assume that the difference is because mine is the 120V version. So while the schematic only shows 270, I can assure you the ones I removed are 560Ω
The yellow wire indeed goes into a transformer.
The yellow wire indeed goes into a transformer.
There's no resistors on the mains side, and all on the other side of the transformer has the same voltage 120 or 240 (that's the point).
I see a 270r in the screen lead. This resistor is very uncritical. If it is zero, all is well until the tube randomly burns-up. If it is large, distortion may be a wee bit higher but if trouble strikes the resistor may smoke first. In EL34 guitar amps, 470r (or 560r) 2W is real common. In hi-fi, if I had a box of 270r I would use them up before ordering 470r. ----Ah, KT88. I would even use 1k 2W here, but a 560r in the hand is fine.
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I can't even respond since I know didly squat about tube amp electronics 😀
So just to re-emphasize, I personally removed the 560Ω resistors from the amp, so there should be no question about what came out of the amp, but stupid me didn't snap a pic thinking I would remember in a week when I had the new resistors in hand, LOL.
So then pin #4 & #6 on the KT88 socket, as shown in this pic I took today?
Where does that yellow wire go? To the output transformer primary?
It looks to me like the yellow wire is connected to KT88 pin 6, which is NC.
Pin 4 on a KT88 is g2 (the screen grid).
If the yellow wire is the OPT primary screen tap, then putting 560R from pin 6 to pin 4 will put that resistor in series with the primary winding screen tap, which would make sense.
Can you confirm where the yellow wire goes?
It looks to me like the yellow wire is connected to KT88 pin 6, which is NC.
Pin 4 on a KT88 is g2 (the screen grid).
If the yellow wire is the OPT primary screen tap, then putting 560R from pin 6 to pin 4 will put that resistor in series with the primary winding screen tap, which would make sense.
Can you confirm where the yellow wire goes?
I don't know enough to answer your question in such detail, sorry about that.
I do know that it goes to one of the transformers.
There is no grid stopper on the KT88 which might have lead to the problem in the first place. Remove the wire from pin 5 on the KT88 and add a 220 ohm resistor in line with it. It can be a half or one watt. I’m surprised they don’t have one in there to keep the amp stable. Some people like carbon comp resistors here but it doesn’t matter. It won’t affect the sound, just the amps stability.
looks like they use the unused pin on the socket as a wire terminal.
yellow transformer wire goes to unused pin, then circuit is completed to screen by resistor.
looking at the pictures the socket is numbered.
stability issue is good point, looks like they raised the screen resistor value.
to possibly fix known issue.
grid stopper could help
yellow transformer wire goes to unused pin, then circuit is completed to screen by resistor.
looking at the pictures the socket is numbered.
stability issue is good point, looks like they raised the screen resistor value.
to possibly fix known issue.
grid stopper could help
1. Many manufacturers used the KT88 Pin # 6 as a standoff / insulated terminal to connect a resistor to.
Otherwise, they had to mount a separate standoff or insulated terminal to mount the resistor to.
This is the way it was wired on the original poster's amplifier; it is a perfectly safe and good technique.
Be sure to keep the yellow wire also connected to Pin # 6.
2. The picture and resistor drawing in Post # 27 is perfectly correct.
3. The original resistor in the schematic was 270 Ohms, should be OK.
Many schematics and production runs have changes.
A 560 Ohm resistor will work very well there, may very slightly increase tube life.
As was mentioned by WhiteDragon, the 560 Ohm resistor might increase the stability slightly.
And because there is global negative feedback, the worst that can happen with 560 Ohms, is distortion might go up very slightly.
Try the 560 Ohm resistor; I bet the amplifier will sound great.
Happy Listening!
Otherwise, they had to mount a separate standoff or insulated terminal to mount the resistor to.
This is the way it was wired on the original poster's amplifier; it is a perfectly safe and good technique.
Be sure to keep the yellow wire also connected to Pin # 6.
2. The picture and resistor drawing in Post # 27 is perfectly correct.
3. The original resistor in the schematic was 270 Ohms, should be OK.
Many schematics and production runs have changes.
A 560 Ohm resistor will work very well there, may very slightly increase tube life.
As was mentioned by WhiteDragon, the 560 Ohm resistor might increase the stability slightly.
And because there is global negative feedback, the worst that can happen with 560 Ohms, is distortion might go up very slightly.
Try the 560 Ohm resistor; I bet the amplifier will sound great.
Happy Listening!
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The situation in your unit doesn't coincide at all with the manufacturer's picture. The manufacturer's show resistors betweens pins 3 (the plate) and 4 (screen grid, which indicates triode strapped finals. But where do these yellow wires go to here? A lowish resistor between the OT's plate terminal and the possible primary tap??
In addition, can anyone tell the purpose of relating the 2nd filter caps C16, C17 to the finals' cathodes instead of ground?
Best regards!
In addition, can anyone tell the purpose of relating the 2nd filter caps C16, C17 to the finals' cathodes instead of ground?
Best regards!
Be sure to check the cathode bypass cap(s) C9 and C14 (100uF 100 volts) before proceeding / switching on. A shorted output valve, the original failure, will put full B+ across the cathode bypass cap and damage it.
Replacements would be a wise choice. They are polarity sensitive so make sure the positive +ve goes to pin 8 and the negative -ve to the GND (ground) line.
Replacements would be a wise choice. They are polarity sensitive so make sure the positive +ve goes to pin 8 and the negative -ve to the GND (ground) line.
Ok will do. Thanks for the feedback.
OK, I getcha as to why they routed it like that.
Will do!
In the picture I posted a couple posts back, you can see it leading to one of the transformers.
Thanks I will look into this. Should I also get a higher voltage value in the replacements? That way if it happens again, the full B+ voltage will less likely damage a more robust cap?
Stay with the same values, 100uF and 100 volts.
A simple 500mA T fuse in the B+ (HT+) supply (between R4 and C1) will stop the loss of the resistors and caps should another KT88 fail...
A simple 500mA T fuse in the B+ (HT+) supply (between R4 and C1) will stop the loss of the resistors and caps should another KT88 fail...
Also check capacitors C8 and C12 coupling capacitors. If shorted will result in run away of the KT88's