Hello. I'm not sure to understand exactly what you mean, but I had the suspect that the OT may be involved. I pull the power tubes and read the resistance between CT and pin 3 on both sockets (the two windings): they are different.
After that I try to apply a 100Vac to each winding and measure the secondary: 12.4Vac in one case and 9.2Vac in the other. It seems that there is an internal shortage between windings in one of the primary. In your opinion this can confirm the behavior of the amp?
Well they're multiple problems, you have to deal separately. I mentioned before you have mismatched OT or tubes.
My OT 3.5K has turn ratio of 11, both are very close. It's necessary that they're close. If you've shorted turn, you can probable feel the heat esp. if you've the load connected. You can check comparing loaded and unload sec. voltages. Not sure if you can measure the AC current in primary through a resistor in series, then you can check if extra current drawn or not when sec. is unloaded. If the current drawn is about the same, then there is no shorted turns, but defective.
My OT 3.5K has turn ratio of 11, both are very close. It's necessary that they're close. If you've shorted turn, you can probable feel the heat esp. if you've the load connected. You can check comparing loaded and unload sec. voltages. Not sure if you can measure the AC current in primary through a resistor in series, then you can check if extra current drawn or not when sec. is unloaded. If the current drawn is about the same, then there is no shorted turns, but defective.
My OT has a turn ratio of 14.3:1 (in attachment a picture and the datasheet).
View attachment OT Hammand Electronics 1750N.pdf
If I'm not wrong I have two different problems:
1) the two secondaries have different impedencies and this is proven by the different resistance I've measured (24.7 and 45.9 Ohm) and by different Vac (12.4 and 9.2 Vac) I measures at their end (CT and pin 3 of Power tubes sockets - with tube removed) applying 100Vac to the primary.
2) both secondaries haven't the expected voltage and this change the impedence that the tubes see:
v4 impedence = (100/12.4)²*16 Ohm = 1024 Ohm
v5 impedence = (100/9.2)² * 16 Ohm= 1890 Ohm
Is it correct? If yes it seems that OT is defective and need to be replaced or rewired
View attachment OT Hammand Electronics 1750N.pdf
If I'm not wrong I have two different problems:
1) the two secondaries have different impedencies and this is proven by the different resistance I've measured (24.7 and 45.9 Ohm) and by different Vac (12.4 and 9.2 Vac) I measures at their end (CT and pin 3 of Power tubes sockets - with tube removed) applying 100Vac to the primary.
2) both secondaries haven't the expected voltage and this change the impedence that the tubes see:
v4 impedence = (100/12.4)²*16 Ohm = 1024 Ohm
v5 impedence = (100/9.2)² * 16 Ohm= 1890 Ohm
Is it correct? If yes it seems that OT is defective and need to be replaced or rewired
Applying 9Vac to the secondary I've:
- 25.3 on one primary (CT to pin 3)
- 14.8 on the other primary (CT to other pin 3)
- 40.1 from pin 3 to pin 3
Disconnected but the resistence are the same: 24.7 and 45.9 ohm.
Quite strange to me that when apply a voltage to the secondary I see the results of my previous post, otherwise applying 39.5Vac to the primary, between:
- v4 pin 3 and CT --> secondary 3.5vac
- v5 pin 3 and CT --> secondary 5.0vac
- pin3 and pin 3 --> secondary 2.6vac
That is because when applied a voltage to sec, it needs to step up the voltage but the VA is not the same as when applied from Pri, so the loss is (much) greater if from sec -> pri. It appears both pri winding is defective. As you reduced the pri voltage you probably also reduced the loss hence the diff.
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Hello. Installed the new OT = no more redplating and full output power again
I'm in doubt, now, about trying however to reduce B+ (switchable?) to complete the transformation to a old style ('80) JCM800 2204. Your opinion?
In any case thanks again for your patience and your suggestions: I learnt a lot!
and full output power again I'm in doubt, now, about trying however to reduce B+ (switchable?) to complete the transformation to a old style ('80) JCM800 2204. Your opinion?
In any case thanks again for your patience and your suggestions: I learnt a lot!
Congratulation! But you should protect the OT from shorting happened again in future:
1) Install a 100 ohms 2 watts resistor across the secondary, this acts as dummy when the speaker/load is accidentally pull from output jack.
2) Install diode protection in Primary as in newer Marshall amp
3) HT can be reduced by adding series resistor in each secondary of power transformer before the rectifier bridge. If max. AC current drawn is 200mA, then 100 Ohms resistor will drop 100*0.2=20V on each secondary AC tap. So 20*0.2=4 Watts, you can rate up to 10 Watts resistor. Alternately you can reduce the screen voltage by simply adding series resistor after choke T3. (Remember reducing screen voltage is more effective than reduce plate voltage in term of plate current reduction hence possible red plating)
1) Install a 100 ohms 2 watts resistor across the secondary, this acts as dummy when the speaker/load is accidentally pull from output jack.
2) Install diode protection in Primary as in newer Marshall amp
3) HT can be reduced by adding series resistor in each secondary of power transformer before the rectifier bridge. If max. AC current drawn is 200mA, then 100 Ohms resistor will drop 100*0.2=20V on each secondary AC tap. So 20*0.2=4 Watts, you can rate up to 10 Watts resistor. Alternately you can reduce the screen voltage by simply adding series resistor after choke T3. (Remember reducing screen voltage is more effective than reduce plate voltage in term of plate current reduction hence possible red plating)
time to celebrate!Continuity
Just wondering. When you speak of leaking to center conductive area, can't you get to that area and measure for possible continuity between center area and any turrets.
Hello Spurlte,
a lot of time has been passed from that issues, but I remenber I tried to do so, but without success. The leakehe seems to appear only when HV was applied. The low voltages that a meter apply to measure the resistance probably don't generate the leakage itself.
Have happy Christams holidays.
Ivan
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