JTM45 circuit diagram question

[TheRedDevil]

Hello.

I looked at these circuit diagrams of the JTM45:






plus attachment.

Some questions about the found differences:

1. Why are often two 10uF capacitors are used instead of 8uF like in the marstran.com schematic ?

2. Why I can find a 25K/22K resistor under the 56K resistor or only the 56K resistor next to the both 10/8 uF capacitors ? Which combination is right ?

3. I also found a 1K2W and a 8K2 2W resistor instead the 10K resistor next to the both 10/8 uF capacitors ? Which combination is right ?

4. I found a 220uF or 250uF resistor near 820 ohms resistor. Which combination is right ?

5. Why is there sometimes a 180K or a 150K resistor under the semiconductor. Which combination is right ?

I really hope that you can say which combination is original for a JTM45 out of the '60s with Angus Young AC/DC-Powerage sound ?

Regards,

Johannes

[wadeberglund]

Hey Johannes-

1. Almost no difference in circuit performance between these two values. I'm sure whoever assembled the amp used whichever part was closest at the time.

2. That 25K resistor is actually a 25K potentiometer used to set the bias accurately. With older tubes, they could get away with just using a 56K resistor here since tube construction is better. My favorite thing to do is to have a 33K resistor in series with a 50K pot. This covers a huge range of bias conditions.

3. The 10K is stock I believe.

4. See #1. You'll have an easier time finding a 250pF cap though.

5. The 180K resistor is used if the bias voltage is taken directly off of the HT windings. This acts as a voltage divider to supply the correct voltage to the bias circuit. If there is a separate bias winding, you don't need the resistor as the correct voltage is made available off the transformer.

Hope this helps!

[TheRedDevil]

Hey Johannes-

1. Almost no difference in circuit performance between these two values. I'm sure whoever assembled the amp used whichever part was closest at the time.

2. That 25K resistor is actually a 25K potentiometer used to set the bias accurately. With older tubes, they could get away with just using a 56K resistor here since tube construction is better. My favorite thing to do is to have a 33K resistor in series with a 50K pot. This covers a huge range of bias conditions.

3. The 10K is stock I believe.

4. See #1. You'll have an easier time finding a 250pF cap though.

5. The 180K resistor is used if the bias voltage is taken directly off of the HT windings. This acts as a voltage divider to supply the correct voltage to the bias circuit. If there is a separate bias winding, you don't need the resistor as the correct voltage is made available off the transformer.

Hope this helps!

Hi. Thanx for your detailed answers !!!
But I also found the schematic in the attachment some minutes before. I think that Metroamp schematic is more actual. I found some different changes, too:

3. Now it's also 8.2K.

4. Here it's a 330uF cap. What do you think ?

5. Here it's a 220K resistor. No, in my reissue there is also a resistor with 220K. Which resistor do you use ? In Germany we have got 230-240 volts. Does it make a difference for my PTP layout ?

Thanx again alot !!!

Regards,
Johannes

[wadeberglund]

3. 8.2K isn't too much different from 10K. You'll find several instances where whoever was assembling the amp will substitute parts that are 'close enough' if he/she happened to run out of the correct part that day. That's part of the reason that there are so many differences between Marshalls and debate over what is correct. My advice is to not to get too hung up on these things since it in all likelihood isn't going to make more than a minutely perceptible difference.

4. Same as #3. You won't be able to here a difference here.

5. The European wall voltage won't make a difference since this is handled by the number of windings on the PT primary. As for the correct value of the resistor, it will entirely depend on your PT. If you aren't replacing the PT on your RI, the 220K resistor is what you should use. If you are having a hard time getting the bias voltage to get in range, this would be a good place to experiment with different values. I bet you don't have a problem though.