First D-style (102) build opinions solicited & build thread:

[martin manning]

...if anything were to get short-circuited from that point on out -- i.e., towards the footswitch...

I don't think this is a problem. LM78XX regulators have internal current and thermal protection which will cause them to shut down in the event of a short at the output. The down side of a placing a current limiting resistor inside the amp, as opposed to individual resistors in the footswitch, is that the LED's will vary in brightness when multiple functions are engaged.

[ayan]

...if anything were to get short-circuited from that point on out -- i.e., towards the footswitch...

I don't think this is a problem. LM78XX regulators have internal current and thermal protection which will cause them to shut down in the event of a short at the output. The down side of a placing a current limiting resistor inside the amp, as opposed to individual resistors in the footswitch, is that the LED's will vary in brightness when multiple functions are engaged.

Let's make sure we're talking about the same thing:

1. Dumble never mixed the connection from the voltage regulator to the relays with the connection form the relay power supply to the channel switching and PAB LEDs. I suspect the reason was reliability of the relay switching function.
2. Making the connection to the LEDs directly from the board with the limiting resistors in the footswitch allows for the possibility of a potential failure. If there were a short circuit in this configuration -- which we can refer to as the Dumble configuration -- it would take place at the input of the voltage regulator, not the output.
3. Using one limiter resistor per feed, i.e., one for the channel switching LED and one for the PAB LED, results in parallel connections. Each LED sees a constant voltage and its brightness is not affected by whether or not the other LED is on.

G.

[martin manning]

Yes, with one jack pin per relay coil and one pin per LED (Dumble configuration), the limit resistors can be placed inside the amp, with equal brightness on the LEDs. All good, and that is what is shown on the 124 schematic.
But suppose you want three functions on a 5-pin jack? A common 12V feed to the footswitch solves that, but relies on the regulator's internal protection if the 12V pin is somehow shorted to ground.

2. Making the connection to the LEDs directly from the board with the limiting resistors in the footswitch allows for the possibility of a potential failure. If there were a short circuit in this configuration -- which we can refer to as the Dumble configuration -- it would take place at the input of the voltage regulator, not the output.

Either way, I don't see how shorting the footswitch jack pins could produce a short at the regulator's input.

[ayan]

Yes, with one jack pin per relay coil and one pin per LED (Dumble configuration), the limit resistors can be placed inside the amp, with equal brightness on the LEDs. All good, that is what is shown on the 124 schematic.
But suppose you want three functions on a 5-pin jack? A common 12V feed to the footswitch solves that, but relies on the regulator's internal protection if the 12V pin is somehow shorted to ground.

2. Making the connection to the LEDs directly from the board with the limiting resistors in the footswitch allows for the possibility of a potential failure. If there were a short circuit in this configuration -- which we can refer to as the Dumble configuration -- it would take place at the input of the voltage regulator, not the output.

Either way, I don't see how shorting the footswitch jack pins could produce a short at the regulator's input.

Once again referring to the "Dumble model," the output of the relay PS board, which is what would get shorted in the absence of limiting resistors, is what connects to the regulator's input.
G.

[martin manning]

Sorry, I don't follow. Here is the schematic based on 124. How do you short the regulator input (pin 1) by shorting the output (pin 3)?

[ayan]

Sorry, I don't follow. Here is the schematic based on 124. How do you short the regulator input (pin 1) by shorting the output (pin 3)?

Touche!I stand corrected, Martin -- shouldn't have typed without having a look at the schematic!
G.

[Raoul Duke]

Gents,
Thanks for the lesson. I think I learned a few more things reading these posts and appreciate the discussion.
Spent my free time today exploring the power transformer wiring in a few layouts and build threads to try and understand the intricacies better. I know there are different ways to do many things and have created my own “problems” by opting for the 50w version of a 100w design - but with a little help I’ll figure it out.

Also got the relay/LED board wired up as well:

[Raoul Duke]

Printed out the specs for my transformers today in my attempt at figuring out how to wire the power section. Probably should have done that awhile ago; might have changed the way I did a couple of things (like relays and 12v system). I should mention that I got nearly all the parts I’ve been working with in a big box in a trade for an 8k generator and did my best to understand what I have before starting. As the saying goes: “you don’t know what you don’t know”. Oh well, more authentic this way I guess lol.

Here’s today’s progress at the back of the chassis. Hopefully I did THAT correctly, lol.

[martin manning]

PT looks fine.
- You don't need the 5V rectifier winding.
- HV winding with bias supply tap is good to go
- Heater winding has no CT, so you will need to find a place for a couple of 100R resistors to make a faux CT.

The 12V relay supply is completely separate.

On the chassis photo, I would shorten the leads on the snubber caps mounted on the V2 socket, but keep them well separated. I would consider removing them and reinstalling after the other wiring on that socket is in place.

[GAStan]

Gents,
Thanks for the lesson. I think I learned a few more things reading these posts and appreciate the discussion.
Spent my free time today exploring the power transformer wiring in a few layouts and build threads to try and understand the intricacies better. I know there are different ways to do many things and have created my own “problems” by opting for the 50w version of a 100w design - but with a little help I’ll figure it out.

Also got the relay/LED board wired up as well:

Looks good so far, but a couple of things I want to bring attention to so the more experienced members will hopefully comment on.

1. Is you LM7812 regulator elevated off the chassis? I bolted mine directly in contact with the chassis to let the chassis "sink" the heat from it.

2. The solder joints on top of the orange capacitors and RN65D resistor, are those completed? If not I normally fill the eyelet completely with solder with enough heat to insure its not a cold joint. But that's my way of soldering, again I'm hoping some more experienced members will comment.

You are making fine progress, this portion of actually assembling an amp is my favorite.

[martin manning]

1. Is you LM7812 regulator elevated off the chassis? I bolted mine directly in contact with the chassis to let the chassis "sink" the heat from it.

On the chassis surface is better. It will probably be ok as it is, or a U-shaped heat sink could be slipped under it.

[Raoul Duke]

Thanks Martin!
Will do on the snubber caps. I have a couple of 100R, would those be pin 1 to ground on both octal sockets? Will that necessitate running the 5k1s in line?

Thanks also Glenn!
The 7812 is bolted on the 6-32 screw between two keps nuts for slight elevation. I did it that way because of the lead space I needed to cover and to allow some air to get under the VR. If my thinking isn’t correct, I can bolt it right down and figure out the rest differently.

I’ll definitely recheck all eyelets as well. My first time working with such deep ones - used to Fender cardboard, lol.

Just did the FET board bias check as well. For clean boost, the closest I could get to half of the 18k was 8.7 which measured 4.02 on the pot. Would that equate to 400r for Rs? Tried to follow the directions and read a bunch of others’ questions also - so I have my fingers crossed I did it correctly.

Thanks again for all the feedback guys! I really appreciate it!

[martin manning]

I have a couple of 100R, would those be pin 1 to ground on both octal sockets? Will that necessitate running the 5k1s in line?

No, you need ~100R from a pin 2 to ground and ~100R from a pin 7 to ground. Those are actually shown on the 102 layout, on the power tube socket closest to the preamp, 2x 120 ohms.

Just did the FET board bias check as well. For clean boost, the closest I could get to half of the 18k was 8.7 which measured 4.02 on the pot. Would that equate to 400r for Rs? Tried to follow the directions and read a bunch of others’ questions also - so I have my fingers crossed I did it correctly.

I don't quite understand that, but see below. You need to find a value for the resistor I marked "Adj" that gets 9V on the FET Drain with 18V across the big cap. You have a 22k where I marked 18k.

[Raoul Duke]

Understood on the 100R. I see it now.

As for the FET board - I think I have an 18k I can put in there and re-attempt. If not; I’ll get one, give it another shot, and report back. Thanks for taking the time to create the diagram from my picture. Very helpful; and I appreciate your patience with my probably very rudimentary questions. I learn something new on this board every time I look - usually a dozen times a day lately, lol.

Really appreciate your expertise Martin (and everyone)! This has been a fantastic learning experience and I’m grateful for the guidance!

[Raoul Duke]

Been comparing the power sections of the 102 layout and the 3rd gen ODS layout trying to figure out what to mix and match to get the 50w 102. Commonalities I see are the artificial CT, PIN 8 grounded, the 470r screen resistor (although they are mounted differently - one to pin 6, one to a terminal strip) and different value grid stoppers from pin 5 to either pin 6 or pin 1.

Uncommon parts are the aforementioned “2nd pins” above and tying pins 6 and/or pins 3 - which both look like 100w specific connections to sync pairs and tie the OT primaries (as a guess).

I understand that they are two different designs, but figured I’d start there. My aim is to wire-up the power section to test as much as possible before diving into the main board (read about Marcus doing that on his first build as recommended by Martin). I prefer to test things as I go vs when going through power up if at all possible - so any guidance to that end is appreciated!