#1 · Apr 17, 2009 15:39 UTC
Seeing as it's on the web, now... and it was discussed, at one point in time.
Couple of things, here... the "Hot Plate" attenuator is built right into this schematic. It consists of TWO components: 1) C22 and 2) POT 5. That's IT! The OT output (hot) goes to the top of POT5/C22. The speaker "+" goes to the junction of C22 and the wiper of POT5.
So, what does THD do for 4, 8 and 16-ohm amps? The value of POT5 changes based on the amp. For 4-ohm loads it goes down to about 20 ohms. For 16-ohm outputs it jumps up to about 50 ohms. That's it.
Also, if you want THD's "noise reduction circuit" it's that little light-bulb (LB1) and SW4 (switch). You just put that across your output transformer. When the amp is at idle, the hiss/hum will "short" through the light bulb which is lower resistance than the speaker. When you play, the bulb turns on, heats up, and the filament becomes much higher impedance than the speaker and all the signal goes through the speaker.
Those are the two things that are really "interesting" in here. Oh... one more thing - This thing switches impedance (for different speaker loads) using SW5. You'll notice that THD, in this implementation, leaves the "Hot Plate" at the 8-ohm setting for 4, 8 and 16 ohm outputs. You can do that w/o causing any issues, as well.
If you have a 100-W amp, use a 100W rheostat - they make 'em. They're around $50 each, I think. The capacitor (2.2uF, 50V) is CHEAP! Just a pot and a capacitor and you've got the ENTIRE THD hot plate. How easy is that?
Here's the link for the schematic:
http://freestompboxes.org/members/bajaman/Baja/THD/UniValve_1.4_Schematic.pdf
Enjoy... lemmee know if you have any questions.
Dar
Couple of things, here... the "Hot Plate" attenuator is built right into this schematic. It consists of TWO components: 1) C22 and 2) POT 5. That's IT! The OT output (hot) goes to the top of POT5/C22. The speaker "+" goes to the junction of C22 and the wiper of POT5.
So, what does THD do for 4, 8 and 16-ohm amps? The value of POT5 changes based on the amp. For 4-ohm loads it goes down to about 20 ohms. For 16-ohm outputs it jumps up to about 50 ohms. That's it.
Also, if you want THD's "noise reduction circuit" it's that little light-bulb (LB1) and SW4 (switch). You just put that across your output transformer. When the amp is at idle, the hiss/hum will "short" through the light bulb which is lower resistance than the speaker. When you play, the bulb turns on, heats up, and the filament becomes much higher impedance than the speaker and all the signal goes through the speaker.
Those are the two things that are really "interesting" in here. Oh... one more thing - This thing switches impedance (for different speaker loads) using SW5. You'll notice that THD, in this implementation, leaves the "Hot Plate" at the 8-ohm setting for 4, 8 and 16 ohm outputs. You can do that w/o causing any issues, as well.
If you have a 100-W amp, use a 100W rheostat - they make 'em. They're around $50 each, I think. The capacitor (2.2uF, 50V) is CHEAP! Just a pot and a capacitor and you've got the ENTIRE THD hot plate. How easy is that?
Here's the link for the schematic:
http://freestompboxes.org/members/bajaman/Baja/THD/UniValve_1.4_Schematic.pdf
Enjoy... lemmee know if you have any questions.
Dar