The #124 build

[Turret]

Sure. It'll be interesting to see what you come up with.

It's not going to be a lot, to be fair. I plan to keep the magic numbers intact for now. So, the preamp is what it is. I will definitely use your idea of an On/Off/On switch for the PAB in the middle position. There won't be any added parts or changes to the core preamp wiring or arrangement, so there's no reason not to try it.

My main focus will be on making the amp safer and quieter, so I'll concentrate on the power supply while keeping the preamp as it is. It's incredible how many ideas I've borrowed from your schematic; I went from the "beta forever" stage to "pretty much there" in just one day.

However, I do have a few questions. I'm curious about the reason for removing the R/C network on the send in your schematic?

I was talking with a friend who works on amps, and he suggested adding a 470k resistor in parallel to each of the mains rectifier diodes. I've seen people using 10nF capacitors there, but I can't recall ever seeing the resistors. Apparently, this helps with even voltage distribution across the rectifier and provides some noise suppression. I would love to hear your opinion on this. Wouldn't a bridge rectifier achieve a similar effect? I’m not saying I'm considering adding it; I just haven't seen it in any schematics before, so it's hardly essential. I have seen series resistors, but those are typically used from the transformer secondaries to the tube rectifier, not in solid-state rectifiers. I'm simply asking to learn something new, I guess.

Would you say adding a resistor in series with a treble cap on the master volume could help the amp sound less harsh at low volumes? I did that modification to a JVM I had some time ago, and it made the master control sound more natural across the entire spectrum.

I've also noticed that you changed the OD trimmer network values in your schematic. Could you provide some theory behind that, or point me to a thread where it's discussed? A friend of mine with a #183 clone relocated his trimmer to the back panel and adjusts the value depending on the volume he is playing at. Do you think that’s a good idea?

[martin manning]

I will definitely use your idea of an On/Off/On switch for the PAB in the middle position.

Not my idea, that's how the original was set up. That is clear from the gut shots, as no solder had ever been applied to the upper lugs on the PAB pedal/manual switch on the back panel.

I'm curious about the reason for removing the R/C network on the send in your schematic?

I don't think there is a definitive answer as to why that network was in 124 (likely to help with treble loss in cables), but the other ODS documented here (102, 183) don't have it, and most people building 124's delete it.

I was talking with a friend who works on amps, and he suggested adding a 470k resistor in parallel to each of the mains rectifier diodes. I've seen people using 10nF capacitors there, but I can't recall ever seeing the resistors. Apparently, this helps with even voltage distribution across the rectifier and provides some noise suppression.

Caps will also help equalize voltages. You can do either or both (RC in series), or place an RC snubber across the HV secondary. There would be some experimentation required to find optimum values for the capacitances around that part of the power supply. The easiest way is to use UF4007 fast recovery rectifiers and delete the snubber caps.

Would you say adding a resistor in series with a treble cap on the master volume could help the amp sound less harsh at low volumes? I did that modification to a JVM I had some time ago, and it made the master control sound more natural across the entire spectrum.

I don't know. A mod that sounded good in another circuit may or may not improve this one.

I've also noticed that you changed the OD trimmer network values in your schematic. Could you provide some theory behind that, or point me to a thread where it's discussed? A friend of mine with a #183 clone relocated his trimmer to the back panel and adjusts the value depending on the volume he is playing at. Do you think that’s a good idea?

124's OD entrance with the ~345k trimmer is different from the other ODS documented here. I suggest building it like 183 and 102 (220k into a 100k trimmer), and experimenting with the series resistor value to try different AC loads on the clean section. Ayan Gil states that the 124 trimmer was set at ~100k to ground with the 220k. That adjustment is more of a find-the-setting-you-like-and-leave-it control, IMO, but IIRC at least one HAD ODS had this "Trigger" control on the panel. If you want to start with the OD entrance of 124, use a 470k series resistor and a 100k trimmer turned all the way up. Parallel another 470k for 235k and turn the trimmer down to about 25% rotation to see what that sounds like.

In general the advice given to people who want to try different mods is build it to the original as a baseline and then play around from there.

PS I made a few tweaks to the TAG 124 Schematic to make it more like recent builds and to improve the explanatory notes

[Turret]

In general the advice given to people who want to try different mods is build it to the original as a baseline and then play around from there.

That's a great advice

[Turret]

...is there an error on TAG schematic? the Presence capacitor should go negative to ground, right?

[martin manning]

Correct. The author of that scheme was a bit loose with the symbol. I'll add a + to it.

[Turret]

Correct. The author of that scheme was a bit loose with the symbol. I'll add a + to it.

legend!

[Turret]

Martin, would you use shielded coaxial wire for the Master wiring? That's a lot of exposed wire to pass through the chassis.

[Turret]

Making slow progress, this is the first version of my layout. I still have to double-check the connections and clean up the wiring, ensuring that AC, audio and high current are not too close to each other.

EDIT: files delated, see updated files later in the thread

[martin manning]

Martin, would you use shielded coaxial wire for the Master wiring? That's a lot of exposed wire to pass through the chassis.

The signal level is high enough that it isn't necessary.

[pottedplant]

Making slow progress, this is the first version of my layout. I still have to double-check the connections and clean up the wiring, ensuring that AC, audio and high current are not too close to each other.

This surprised me, nice layout I was not expecting to see that!

[Turret]

Making slow progress, this is the first version of my layout. I still have to double-check the connections and clean up the wiring, ensuring that AC, audio and high current are not too close to each other.

This surprised me, nice layout I was not expecting to see that!

Thank you... I think.

Early stage, things most likely get moved later

[martin manning]

Consider using shielded cable for the V2a grid (OD entrance). That is a sensitive lead.

[Turret]

Consider using shielded cable for the V2a grid (OD entrance). That is a sensitive lead.

Will do that. How about FET input?

[martin manning]

Consider using shielded cable for the V2a grid (OD entrance). That is a sensitive lead.

Will do that. How about FET input?

That's a really short lead, probably ok.

[Turret]

Consider using shielded cable for the V2a grid (OD entrance). That is a sensitive lead.

Will do that. How about FET input?

That's a really short lead, probably ok.

Yeah, I noticed this after I posted. Thank you

While we there, Switchcraft 12A good for the fx loop and speaker outs? Whey aren't the stereo type, right? I will be using 13E for the FET