Build Log - #102 - So it Begins

[PicknStrum]

Some progress - got the valve sockets mostly wired up. Surprisingly didn't mind working around the heaters. Maybe I'm one of those bass ackward people!

ICaOjp0 (2).jpg

HS262JD (2).jpg

102_1.jpg

102_2.jpg

[Colossal]

Looks good!

[PicknStrum]

Thank you Colossal!

So haven't gotten too much further at this point but I did want to ask you guys questions. A couple of these are pretty theoretical but some are directly related to this amp. Hope you can help me out with these:

1. Hoping you can confirm this one: The only reason we are bringing 12v to the DIN switch is to power the LEDs, correct? There is nothing else being powered by that 12v off the DIN? Obviously I know the relays are also getting the rectified 12v but that's coming directly from the voltage regulator in my case.

2. Speaking of rectification. I'm wondering how it's possible that half-wave rectification can turn the power LED on solid. Wouldn't we get a strobe effect in theory? I guess I'm wondering why half-wave is okay to use for an LED. Why don't we need full-wave rectification?

3. Why use relays for switching instead of passive switches? What's the advantage of relays?

4. Specific to my build: On an updated layout diagram, I noticed that the input jack is grounded to the chassis through the jack itself and then also sent through a separate ground wire to ground to a different spot on the chassis. That would be a ground loop, correct? If so, I'm not going to send the extra wire to the separate grounding point.

5. Regarding the impedance selector: I see on the #102 schematic that the negative feedback resistor connects directly to the 4 ohm tap on the output transformer. Why is it only connecting to the 4 ohm tap? Shouldn't it be connected to common so that you're getting that resistor in any impedance setting?


Appreciate any info / advice. There are definitely more questions to come. I like to know the what's, why's, and how's of what's going.

[erwin_ve]

Thank you Colossal!

So haven't gotten too much further at this point but I did want to ask you guys questions. A couple of these are pretty theoretical but some are directly related to this amp. Hope you can help me out with these:

1. Hoping you can confirm this one: The only reason we are bringing 12v to the DIN switch is to power the LEDs, correct? There is nothing else being powered by that 12v off the DIN? Obviously I know the relays are also getting the rectified 12v but that's coming directly from the voltage regulator in my case.

2. Speaking of rectification. I'm wondering how it's possible that half-wave rectification can turn the power LED on solid. Wouldn't we get a strobe effect in theory? I guess I'm wondering why half-wave is okay to use for an LED. Why don't we need full-wave rectification?

3. Why use relays for switching instead of passive switches? What's the advantage of relays?

4. Specific to my build: On an updated layout diagram, I noticed that the input jack is grounded to the chassis through the jack itself and then also sent through a separate ground wire to ground to a different spot on the chassis. That would be a ground loop, correct? If so, I'm not going to send the extra wire to the separate grounding point.

5. Regarding the impedance selector: I see on the #102 schematic that the negative feedback resistor connects directly to the 4 ohm tap on the output transformer. Why is it only connecting to the 4 ohm tap? Shouldn't it be connected to common so that you're getting that resistor in any impedance setting?


Appreciate any info / advice. There are definitely more questions to come. I like to know the what's, why's, and how's of what's going.

1. Yes correct

2. The half wave is sufficient for the led not to go out on 50/60hZ.

3. With relays you can have SHORT connections for the signal. Passive switches require long cable runs for a footswitch and you dont want to ruin your signal that way.

4. Some jacks have isolation washers so they dont contact the chassis.

5. Impedance affects the frequency knee for the presence, it is a specified calculation , I guess you dont want unwanted effects of too much or too less GNFB.

[PicknStrum]

Thank you Colossal!

So haven't gotten too much further at this point but I did want to ask you guys questions. A couple of these are pretty theoretical but some are directly related to this amp. Hope you can help me out with these:

1. Hoping you can confirm this one: The only reason we are bringing 12v to the DIN switch is to power the LEDs, correct? There is nothing else being powered by that 12v off the DIN? Obviously I know the relays are also getting the rectified 12v but that's coming directly from the voltage regulator in my case.

2. Speaking of rectification. I'm wondering how it's possible that half-wave rectification can turn the power LED on solid. Wouldn't we get a strobe effect in theory? I guess I'm wondering why half-wave is okay to use for an LED. Why don't we need full-wave rectification?

3. Why use relays for switching instead of passive switches? What's the advantage of relays?

4. Specific to my build: On an updated layout diagram, I noticed that the input jack is grounded to the chassis through the jack itself and then also sent through a separate ground wire to ground to a different spot on the chassis. That would be a ground loop, correct? If so, I'm not going to send the extra wire to the separate grounding point.

5. Regarding the impedance selector: I see on the #102 schematic that the negative feedback resistor connects directly to the 4 ohm tap on the output transformer. Why is it only connecting to the 4 ohm tap? Shouldn't it be connected to common so that you're getting that resistor in any impedance setting?


Appreciate any info / advice. There are definitely more questions to come. I like to know the what's, why's, and how's of what's going.

1. Yes correct

2. The half wave is sufficient for the led not to go out on 50/60hZ.

3. With relays you can have SHORT connections for the signal. Passive switches require long cable runs for a footswitch and you dont want to ruin your signal that way.

4. Some jacks have isolation washers so they dont contact the chassis.

5. Impedance affects the frequency knee for the presence, it is a specified calculation , I guess you dont want unwanted effects of too much or too less GNFB.

Super helpful response - thanks! A couple of follow-ups:

1. So just for clarification on the jacks - if my jack is contacting ground (i.e. it doesn't have an isolation washer), then don't add that extra wire to a separate ground point because that makes a loop. If it has an isolation washer, I would need that extra wire to make sure ground is connected to the jack. Correct?

2. I'm still confused on the feedback being connected to the 4 ohm tap. As I write this out, I think it is becoming more clear. The feedback resistor and 4 ohm tap are part of a specific calculation that affects the frequency response going into the presence and then through the PI and power section. The feedback resistor is not actually passing signal to the 4 ohm tap (obviously it's not going through the power section at all) - so the frequencies that are being affected by the feedback resistor and 4 ohm tap continue through the power section and then into whatever impedance setting you have chosen. Is that correct? I guess what confused me about it is that the resistor is connected to the 4 ohm, so why would it work on any of the other impedance settings. If my preceding statements are true then I understand (to a point).

[erwin_ve]

Thank you Colossal!

So haven't gotten too much further at this point but I did want to ask you guys questions. A couple of these are pretty theoretical but some are directly related to this amp. Hope you can help me out with these:

1. Hoping you can confirm this one: The only reason we are bringing 12v to the DIN switch is to power the LEDs, correct? There is nothing else being powered by that 12v off the DIN? Obviously I know the relays are also getting the rectified 12v but that's coming directly from the voltage regulator in my case.

2. Speaking of rectification. I'm wondering how it's possible that half-wave rectification can turn the power LED on solid. Wouldn't we get a strobe effect in theory? I guess I'm wondering why half-wave is okay to use for an LED. Why don't we need full-wave rectification?

3. Why use relays for switching instead of passive switches? What's the advantage of relays?

4. Specific to my build: On an updated layout diagram, I noticed that the input jack is grounded to the chassis through the jack itself and then also sent through a separate ground wire to ground to a different spot on the chassis. That would be a ground loop, correct? If so, I'm not going to send the extra wire to the separate grounding point.

5. Regarding the impedance selector: I see on the #102 schematic that the negative feedback resistor connects directly to the 4 ohm tap on the output transformer. Why is it only connecting to the 4 ohm tap? Shouldn't it be connected to common so that you're getting that resistor in any impedance setting?


Appreciate any info / advice. There are definitely more questions to come. I like to know the what's, why's, and how's of what's going.

1. Yes correct

2. The half wave is sufficient for the led not to go out on 50/60hZ.

3. With relays you can have SHORT connections for the signal. Passive switches require long cable runs for a footswitch and you dont want to ruin your signal that way.

4. Some jacks have isolation washers so they dont contact the chassis.

5. Impedance affects the frequency knee for the presence, it is a specified calculation , I guess you dont want unwanted effects of too much or too less GNFB.

Super helpful response - thanks! A couple of follow-ups:

1. So just for clarification on the jacks - if my jack is contacting ground (i.e. it doesn't have an isolation washer), then don't add that extra wire to a separate ground point because that makes a loop. If it has an isolation washer, I would need that extra wire to make sure ground is connected to the jack. Correct?

2. I'm still confused on the feedback being connected to the 4 ohm tap. As I write this out, I think it is becoming more clear. The feedback resistor and 4 ohm tap are part of a specific calculation that affects the frequency response going into the presence and then through the PI and power section. The feedback resistor is not actually passing signal to the 4 ohm tap (obviously it's not going through the power section at all) - so the frequencies that are being affected by the feedback resistor and 4 ohm tap continue through the power section and then into whatever impedance setting you have chosen. Is that correct? I guess what confused me about it is that the resistor is connected to the 4 ohm, so why would it work on any of the other impedance settings. If my preceding statements are true then I understand (to a point).

1.You can do both as a failsafe. If there's a hum caused by a loop, just use one.

2. The signal from OT is going from the 4 Ohm tap to the GNFB resistor to the PI section, in a certain way you can see it as phase cancellation to the PI. In theory this would result in a more linear response of the power section.
If you choose another tap, let's say the 8 Ohm, the calculation is different to get the same result.( other resistor combinations and possible presence cap.)

[erwin_ve]

And one other thing: you may have noticed the 2,2uF presence cap for #102 and a 1uF presence cap for #124.

Essentially the bigger the cap , the lower the frequency( that is affected; fe. 1uF doing top treble, 2,2uF doing from top treble to high mids.

[PicknStrum]

And one other thing: you may have noticed the 2,2uF presence cap for #102 and a 1uF presence cap for #124.

Essentially the bigger the cap , the lower the frequency( that is affected; fe. 1uF doing top treble, 2,2uF doing from top treble to high mids.

Got it - so the larger cap is allowing the presence knob to control a larger frequency range. Makes sense.

Thought of additional questions:

1. In looking at a couple different layouts of the #102, there are small differences. In regards to the send/return jacks of the effects loop, I'm seeing that one layout is showing 500pf caps between the ground and switch in addition to a 220k attaching the switch lugs of both jacks. Wondering what the purpose is and why some layouts exclude them?

2. Similar to question 1: On some layouts, I'm seeing an extra 0.1uF capacitor going from common to one side of PAB and OD switch, respectively. Presumably, this is to minimize a popping sound when switching? If not, what is the purpose?

[erwin_ve]

And one other thing: you may have noticed the 2,2uF presence cap for #102 and a 1uF presence cap for #124.

Essentially the bigger the cap , the lower the frequency( that is affected; fe. 1uF doing top treble, 2,2uF doing from top treble to high mids.

Got it - so the larger cap is allowing the presence knob to control a larger frequency range. Makes sense.

Thought of additional questions:

1. In looking at a couple different layouts of the #102, there are small differences. In regards to the send/return jacks of the effects loop, I'm seeing that one layout is showing 500pf caps between the ground and switch in addition to a 220k attaching the switch lugs of both jacks. Wondering what the purpose is and why some layouts exclude them?

2. Similar to question 1: On some layouts, I'm seeing an extra 0.1uF capacitor going from common to one side of PAB and OD switch, respectively. Presumably, this is to minimize a popping sound when switching? If not, what is the purpose?

1. The master volume has a 68pF cap, depending on the setting, this will add a fair amount of high. I think originally this was to compensate the high loss for Robben's use of the Dumblelator. Long cable run in the fx loop are prone to lose high content in signal.The 500pF are to cut some high when nothing is in the fx loop.
Just to be sure: #102 in the files section with layout made by Talbany is the right one. Others are spin offs.

[PicknStrum]

And one other thing: you may have noticed the 2,2uF presence cap for #102 and a 1uF presence cap for #124.

Essentially the bigger the cap , the lower the frequency( that is affected; fe. 1uF doing top treble, 2,2uF doing from top treble to high mids.

Got it - so the larger cap is allowing the presence knob to control a larger frequency range. Makes sense.

Thought of additional questions:

1. In looking at a couple different layouts of the #102, there are small differences. In regards to the send/return jacks of the effects loop, I'm seeing that one layout is showing 500pf caps between the ground and switch in addition to a 220k attaching the switch lugs of both jacks. Wondering what the purpose is and why some layouts exclude them?

2. Similar to question 1: On some layouts, I'm seeing an extra 0.1uF capacitor going from common to one side of PAB and OD switch, respectively. Presumably, this is to minimize a popping sound when switching? If not, what is the purpose?

1. The master volume has a 68pF cap, depending on the setting, this will add a fair amount of high. I think originally this was to compensate the high loss for Robben's use of the Dumblelator. Long cable run in the fx loop are prone to lose high content in signal.The 500pF are to cut some high when nothing is in the fx loop.
Just to be sure: #102 in the files section with layout made by Talbany is the right one. Others are spin offs.

Okay - I'm not using a Dumbleator so I'll wire the effects loop jacks according to talbany's layout.

What's the word on the capacitors going across the PAB and OD switch in talbany's layout? What are they for?

[erwin_ve]

Got it - so the larger cap is allowing the presence knob to control a larger frequency range. Makes sense.

Thought of additional questions:

1. In looking at a couple different layouts of the #102, there are small differences. In regards to the send/return jacks of the effects loop, I'm seeing that one layout is showing 500pf caps between the ground and switch in addition to a 220k attaching the switch lugs of both jacks. Wondering what the purpose is and why some layouts exclude them?

2. Similar to question 1: On some layouts, I'm seeing an extra 0.1uF capacitor going from common to one side of PAB and OD switch, respectively. Presumably, this is to minimize a popping sound when switching? If not, what is the purpose?

1. The master volume has a 68pF cap, depending on the setting, this will add a fair amount of high. I think originally this was to compensate the high loss for Robben's use of the Dumblelator. Long cable run in the fx loop are prone to lose high content in signal.The 500pF are to cut some high when nothing is in the fx loop.
Just to be sure: #102 in the files section with layout made by Talbany is the right one. Others are spin offs.

Okay - I'm not using a Dumbleator so I'll wire the effects loop jacks according to talbany's layout.

What's the word on the capacitors going across the PAB and OD switch in talbany's layout? What are they for?

If you have nothing in the loop you can experiment to omit the 68pF on the master, YMMV.

You mean on the back panel: they are to prevent arcing when flipped.

[Vertigo]

2. Speaking of rectification. I'm wondering how it's possible that half-wave rectification can turn the power LED on solid. Wouldn't we get a strobe effect in theory? I guess I'm wondering why half-wave is okay to use for an LED. Why don't we need full-wave rectification?

FWIW, if you're using the 12V LED that I linked earlier, it doesn't flicker.

[PicknStrum]

So I had a heckuva time trying to get my FET biased. Mainly due to my inability to follow directions and not making sure I had new batteries to begin with.

After a ton of screwing around with different combinations of resistors, including the recommended 22k leading to the drain resistor and the recommended 2k source resistor, I finally just threw a couple of potentiometers in. I put a 50k pot in place of the 22k resistor and a 10k pot in place of the 2k resistor. The closest I could get to 18v on top of the drain and 9v on the bottom was with about 47k in place of the 22k and 1.8k at the source. Does that sound accurate?

I'm going to order fixed resistors for those spots and also order a few resistances on either side of them to make sure I'm covered.

EDIT: I guess I'm just concerned b/c in the "FET biasing" thread, it seems that most people actually went down in value from 22k in order to get 18v at the top. For me, I had to raise the 22k to get close to 18v. I'm not as concerned with the 1.8k as that is close to Martin's recommendation. I'm mainly concerned that I had to go up to 47k to get the voltage divider at 18v. Let me know if that sounds okay.

[martin manning]

You should be setting the source resistor value with just the 18V battery supply connected at the top of the drain resistor. Once you get that dialed in, disconnect the battery and connect the B+5 voltage (with all tubes installed, and the bias set correctly) and adjust the 150k/22k voltage divider to get 18-20V Vdd.

[PicknStrum]

You should be setting the source resistor value with just the 18V battery supply connected at the top of the drain resistor. Once you get that dialed in, disconnect the battery and connect the B+5 voltage (all tubes installed, and bias set correctly) and adjust the 150k/22k voltage divider to get 18-20V Vdd.

Understood - so don't mess with the 22k resistor at all with the 18v battery source. Only adjust source resistor to get 9v. Then adjust the voltage divider once B+ and tubes are installed to get 18-20v.

I should've asked that first - I got myself all confused. Is it okay that when I hookup my 18v supply to each side of the 100uF cap (as shown in your diagram in FET bias instructions) that I'm not getting 18v at the drain? Don't worry about that? I'm actually getting a bit more; a bit above 19V with newer 9v batteries.