My take on a 2nd Gen ODS

[Matt J]

Hey everyone!
2nd Gen/70s builds seem to be the trend around here so I thought I would add in the latest one I have been working on, a 50 watt model that will be going in a 1x12 combo cab.
- Chassis, boards, and 1x12 suede cabinet by Taylor at Amplified Nation. I cannot praise enough the quality of these parts for the DIY crowd.
- Most of the signal resistors are Dralorics I sourced from Jelle at Welagen Amps.
- Plates and cathodes are Dale RN65s, miscellaneous resistors in the bias/relay boards are Vishay PR02 with metal oxide dropping resistors in the B+ dropping chain.
- PT is a Mojotone 12597D, OT is a 125A9A, Choke is a Mojotone 777, Relays are powered by a Hammond 166G6B

This is first time I’ve been an ODS in quite a while that features the Deep Switch instead of the Mid switch like on later models, so it took a little time to get things straightened out converting it from a slider switch seen on most of the layouts to a toggle switch like this. I just had to follow the flow of the schematic. If anyone sees anything awry, please let me know!

Matt J.

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[Matt J]

I have built the footswitch (forgot to grab a picture of it this morning), went ahead and tested the relay wiring to check for shorts. I applied a 120VAC source to the relay PT primaries, footswitch LEDs all work as they should when the relay switches are in the pedal mode.

No FET on this one. I thought about building it and making it footswitchable, but I've never been that all enthralled by the internal FET in the past so I may look at some FET pedals later and just use one of them.

You will see some points not fully soldered in or just tack-soldered, but that because other connections are going in there later.

More pics to come as I finish up some of the last bits of chassis prep.

Matt J

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[martin manning]

Matt, you have a 6.3VAC transformer feeding a FWB, which will get you about 7VDC. You need to use a voltage doubler for 12V relays. I suggest you convert your rectifier to a doubler like this: [edit: revised flying lead locations]

[Matt J]

Thank you very much, Martin! I was wondering why I was only getting 7VDC out to the relays.
Instead of rebuilding the whole board. Is there an alternate transformer you would recommend?
Matt J.

[martin manning]

Hammond 166F12B would be fine, you just need something with 12V secondary at a couple of hundred mA. Picking one with the same mounting centers would make things easy. Converting the board you have just requires one more cap, and two less diodes. No new holes to drill. This works out nicely, same relay PS board pattern for FWB or voltage doubler rectifier.

[Dr d]

Looks great so far. I would strongly recommend that you have an FET as it is very useful in this circuit. The secret, IMHO, is to bias the FET with low headroom using a 8k2 or 10K voltage divider (R1). Martin Manning has some great instructions to do this.

[martin manning]

Instead of rebuilding the whole board. Is there an alternate transformer you would recommend?

PS If you don't have another 1000u cap to convert your FWB to a Doubler as shown, you could move the 1000u at the left over to the far right (where the bridge is now), and use something else for the far left spot. 22u at 25V would be fine. Easier and cheaper than ordering a new 12VAC transformer, IMO.

[Matt J]

Hey everyone! Thank you for your input and suggestions, and sorry on the late response. My wife fell sick so I have been looking after her.

I decided to go with swapping the PT out with a Hammond 166F12B. I wasn't 100% happy with how I had wired up and mounted the 166G6B, and I needed to order parts for future builds down the line (I plan to build a low plate classic and a high plate skyliner later this Spring) so I added that to the list I'm just waiting for them to arrive now. I will post picks once I get it installed.

As for the FET, I'm going to omit it for now, but I do have the parts to build one in the future. It's something I may try once I get the amp finalized and dialed in 'as-is'.

Also, Martin, I can't thank you enough for writing on one of Raoul Duke's previous threads your method for the checking the polarity of the secondaries of an OT by applying a small voltage across the primary when hooked up to a proper load. I've had a build before in the past where the secondaries were revered from the spec sheet and space was tight so it was a major pain to diagnose, disassemble, and fix. I had been meaning to build a new load box with an 8ohm, 100 watt, resistor for testing/burn in, so I added jacks to insert a low AC voltages across it. I modded an old power adapter for a beard trimmer so you can easily hook it up to it and it puts out 4.5 VAC on the dot. I look forward to using it on this and future builds.

Matt J.

[martin manning]

I can't thank you enough for writing on one of Raoul Duke's previous threads your method for the checking the polarity of the secondaries of an OT by applying a small voltage across the primary when hooked up to a proper load.

Sure, IDK why checking that in advance isn't more common. You don't really need a load on the secondary to do it, but it's a convenient way to open the speaker shorting ground. Plugging one end of a cable in, or just a 1/4" plug in would work.

[Matt J]

I can't thank you enough for writing on one of Raoul Duke's previous threads your method for the checking the polarity of the secondaries of an OT by applying a small voltage across the primary when hooked up to a proper load.

Sure, IDK why checking that in advance isn't more common. You don't really need a load on the secondary to do it, but it's a convenient way to open the speaker shorting ground. Plugging one end of a cable in, or just a 1/4" plug in would work.

I'm thinking back on all the books I've read in the past on amp building and can't think of this type of phase testing being mentioned in any of them. I can recall a few forum posts on other pages years ago about the issue, but they just said you were out of luck if a mismatch happened after you've wired everything up so you would just have to start pulling wires and resoldering things.

[martin manning]

I've also heard people putting the odds at 50-50 for years. At least some manufacturers provide phase info on their data sheets (Hammond, for one), but there is always the possibility of a factory lead color error.

[Matt J]

Had to deal with another kidney stone attack so things got set aside for a while. Finally had time to swap out the relay PT with a Hammond 166F12B today. Used some jumper wires to test it with the footswitch plugged in.
With 120 VAC running from my variac into the power socket"
12.11 DC running from the rectifier board to the relays.
12.10 DC on the relay/footswitch junction when not engaged (PAB shown for example)
0.007 DC when the footswitch is engaged.

Matt J.

[Matt J]

Interior of the footswitch.
These are one of the things I always have the most troubles with when building Dumble style amps because I always manage to wire something backwards.

Matt J.

[Raoul Duke]

I hear ya Matt, me too, lol.

Glad you’re doing better and back to this great looking build!

[Matt J]

I hear ya Matt, me too, lol.

Glad you’re doing better and back to this great looking build!

Thanks!
Been trying to pass a 3mm kidney stone for the past three weeks now. Tomorrow morning I'm having shockwave lithotripsy done to try to smash up the 7mm stone that's still up in my left kidney. I've been dealing with stones for about four years now. I've changed my diet but they run in the family so I'm sort of stuck with them. 0/10 would not recommend!
Matt J.