Firestorm wrote:It may not be necessary and there are better ways to do it.
Better ways, heck yeah I'd leave out that balance pot entirely and just go straight to ground with those cathodes. Or, if you want to get fancy, a pair of 1 ohm 1% WW resistors to sense bias current.
Here is a semi-revised schematic with the red highlighting changes. Is it necessary to have the pilot lamps? I did a Supro 1624t build without them and the amp has been working fine.
dawsonaudio wrote:Here is a semi-revised schematic with the red highlighting changes. Is it necessary to have the pilot lamps? I did a Supro 1624t build without them and the amp has been working fine.
Horrors! No pilot lamps??? Welllll, I guess you could do without. << Just a little windup, because I DO love old Fender style pilot lamps, all those jewel colors, makes my day. Besides signaling "the amp is on" they have no other function, perfectly OK to not have any.
Schemo looks good, keep us up on how it's working out.
The field coil would be connected in place of the 1K 10W resistor. You might want to check the DC of the coil to be sure it isn't crazy high (like 5Kohms). If you will ever drive the amp full out, you might want a larger first filter cap. SB-12s ghost something fierce with the 30uF cap.
Everything before the transformer wasn't used. I did eliminate the first 10k resistor and just have the field coil in place. I'm assuming the voltage drop would be too much with both in place...?
What do you mean by "ghost" something fierce...? Sorry, not hip on all the lingo here. And what would be a better suited value?
Ghost noting is a type of intermodulation distortion where the 120Hz power supply ripple modulates the note(s) being played to produce additional notes that are the sum and difference of the frequencies present. Since one of the frequencies is always 120Hz, the interval will be dissonant with pretty much every note you can play except "E." The lower frequencies produced are most noticeable, so are sometimes called "subharmonics" except they are unrelated to the notes being played. This isn't usually obvious in a band situation, but if you are playing loud at home it probably will be. Beefing up the first filter in the power supply will help. This will make the amp feel a bit stiffer, which may be undesirable with guitars but not so much with bass. Some amps (mid to late '60s Marshalls) have this phenomenon as part of their characteristic sound.
I would think 50 or 60uF would work. As with any tube rectifier feeding a semi-large cap, do not "hot switch" the amp by turning it off and then on again while the tubes are still warmed up.
If you wire the field coil off the B+ as in your proposed schematic (as opposed to the M3 where the coil is connected to the center tap), with the 1K resistor in place the current will divide not quite in half and the effective resistance of the coil and resistor in parallel will be roughly 412 ohms. Not sure that's what you want. Often field coils take the place of a choke.
I actually didn't place the speaker coil in the right place in my revised schematic...I do plan on connecting it off the center tap of the transformer as in the M3 schematic. I would imagine that I will now keep the resistor in there, correct?
dawsonaudio wrote:I actually didn't place the speaker coil in the right place in my revised schematic...I do plan on connecting it off the center tap of the transformer as in the M3 schematic. I would imagine that I will now keep the resistor in there, correct?
The speaker field coil can stay where it is, but the negative terminal of the 33uF reservoir capacitor should be connected to the CT, and that junction should not be grounded. The 1k resistor will also stay where it is.
I have another question here while I'm wiring up the amp...the output of the transformer has green/yellow/black wires. In my previous amp build(supro 1624t), the green and yellow wires went to the Jensen F12N. For this schematic, the green wire is taken back to a 100k resistor and the yellow and black are going to the speaker. Could I get an explanation here on the difference between the two speaker hookups?
I should say that the output transformer is obviously not the same as in the ampeg schematic. It's the transformer that came with the speaker.
After looking at the schematic a little closer, the green wire from the output transformer has a label of 16 which I'm assuming is 16 ohms. My output transformers green and yellow wires are the speaker taps at 8ohms and the black wire is going to ground.
Should I ignore the connection of the green/16 wire to the 100k resistor off of the 12ax7 pin #8? As well as ignore the 100k resistor?
Another question...what would be a good grounding scheme? How many different grounding points? I plan on bringing everything back to a star ground at the transformer secondary grounding point...which is also where my electrolytic caps are tied in.
My main a/c ground is tight it at the chassis once it comes into the box.
Any thoughts on the field coil speaker connections from the transformer? The transformer I have is 8ohms only...just wanted to find out if I need to disregard the 16ohm connection on the schematic since my transformer isn't the same.
Also, at the bass input jack, is that a switched leg that connects the 100k resistor to ground when nothing is inserted or is that resistor hard wired directly to the tip?
Looks like I'm almost done with the amp. I may need some help adjusting the bias as well as some other issues that may arise. I'll post some images of the finished project over the weekend.
