New build: Supro 1624T in a Marshall Lead 12 donor amp

[klooon]

First quick sound bites:
Channel 1: https://youtu.be/AceX1MlJgso

Channel 2: https://youtu.be/GqD6d6BQEdg

Please notice also the difference in volume....

[copperheadroads]

First quick sound bites:
Channel 1: https://youtu.be/AceX1MlJgso

Channel 2: https://youtu.be/GqD6d6BQEdg

Please notice also the difference in volume....

Is that cranked ?

[klooon]

Is that cranked ?

In both clips the volume is at 12 o'clock (audio pots), so it can be turned up a lot more. The transition to more overdrive that you hear is due to picking harder. So a very touch sensitive design.

[copperheadroads]

In both clips the volume is at 12 o'clock (audio pots), so it can be turned up a lot more. The transition to more overdrive that you hear is due to picking harder. So a very touch sensitive design.

I was curious how the Sag resistors would simulate the sag & kind of like my 5E3 on bust you can really hear it when it's breaking up .

[klooon]

I was curious how the Sag resistors would simulate the sag & kind of like my 5E3 on bust you can really hear it when it's breaking up .

I can't make a comparison with a tube rectified version, but FWIW I don't feel "I miss something" when playing the amp in terms of sag.

I even changed the amp (for now) bypassing the sag resistor, to establish a larger B+ and even then I feel the amp is responding 'right', but I must admit I haven't A/B-ed it directly with the same settings.

The B+ without sag resistor is now about 315VDC and the bias of the tubes is now around 94% and the amps sounds mighty!!

[klooon]

Tremolo issue solved!!

I found the solution for the tremolo issue!

After looking at the schematics again. I saw that the 270k to ground in the plate supply circuit which was indicated as 'optional', was not present in the later version of the 1624T, which has the tremolo circuit which I implemented. I removed this 270k and bingo! Enormous increase in gain and a perfect working tremolo.

[klooon]

In the Marshall Lead 12 donor amp I am using, there is a 5-pin power switch. The neon lamp supposedly requires 120VAC (originally from the voltage selector 'tap').

Is there a way I could 'just insert' it from e.g. one of the taps of the 250VAC secondary from the PT? So run a wire from the bridge rectifier input to the 5th pin on the power switch (around 125VAC)????

[mozz]

Neons need a resistor according to what the input voltage is, they fire at 80-90 volts. I would run it off mains voltage if possible.

[Enzo]

I would duplicate the circuit and wiring pattern of the MArshall Lead 12.

[loudthud]

Usually, one side of the neon bulb is connected to one side of the Mains inside the switch. Your best option is to connect a resistor from the 5th terminal to the other side of the mains. Something like 220K 1/2W should work. The switch in my 3005 USA Lead 12 only has four terminals. Switch is made by Russen Berger in France.

[klooon]

Usually, one side of the neon bulb is connected to one side of the Mains inside the switch. Your best option is to connect a resistor from the 5th terminal to the other side of the mains. Something like 220K 1/2W should work. The switch in my 3005 USA Lead 12 only has four terminals. Switch is made by Russen Berger in France.

Great! I put the 270k that I just removed in and works like a charm!!

[klooon]

Is there a reason why in this amp I don't get a B+ closer to the estimation: 1.4 x V_sec? So roughly 1.4 x 250 would give 350 VDC and I measure around only 300VDC...

[Justin Thomas]

That 250x1.4 is for an unloaded amp, IE, no tubes in. I bet if you pull the power tubes, you'll get flower to the expected voltage. Since the power tubes are drawing the most current from the power supply, the voltage will drop. It'd be even worse with a tube rectifier.

Justin

[catalin gramada]

You measure an relative RMS with an average DMM. 1.4142 X means peak voltage. With a regular DMM shows more x1,3-1,35 maybe slightly better with improved large caps filtering network (you need a true RMS meter to determine RMS voltage). It is a difference between Vaverage, Vpeak and Vrms. But do atention: the network elements (caps voltage rating) always should be designed by Vpeak
Edit: on a power supply you also have to substract the voltage droped by each diode from rectifier string
And yes . you have to measure the voltages on you tranny and PS (AC and DC) in the same conditions, loaded or unloaded. You get nothing usefull if you mix

[klooon]

Thanks for the explanations! That makes sense. I was triggered towards the loaded values due the amp which I made before this amp: an ecc99 powered amp, for which the 1.4 rule was pretty okay using the loaded B+.
Good that I initially thought about a large dropping resistor when choosing the PT....