Working on my first built and I need some input

[ejendres]

I'm building a Triode Vibrochamp clone. The schematic calls for two 8ohm speaker outputs in parallel. I want to wire it for an 8 and a 16ohm jack as I have both impedance cabs (I have an OT with the proper taps).

Theres a portion of the preamp wired to both output jacks, its the pink wire in the layout. I believe its a portion of the Vibrato circuit. My question is do I wire it to both jacks as shown in the layout, even though I'm running an 8 and 16 ohm jack, or do I wire it to just the 8ohm jack?

Right now my thinking is to wire it to both, but I just wanted to check with more knowledgeable people.


[IMG:800:725]http://i7.photobucket.com/albums/y297/ejendres/Capture_zpsdd3fb30e.png[/img]

[zeebee]

uhm that's the feedback line and you put it on 1 output only ! just check the original fender schematic and whether you put it on the 8 or 16 ohm tap i'd go for the 16 ohm tap since it's the highest output ohmage of you tranny! i hope you didn't really mean like soldering two wires to the different outputs and connect them at the other end did you?

[ejendres]

uhm that's the feedback line and you put it on 1 output only ! just check the original fender schematic and whether you put it on the 8 or 16 ohm tap i'd go for the 16 ohm tap since it's the highest output ohmage of you tranny! i hope you didn't really mean like soldering two wires to the different outputs and connect them at the other end did you?

Gotcha dude. Whats the feedback line do? Why would I put it on the highest ohm tap?

I'm still learning about all this stuff so any help would be appreciated.

[Phil_S]

Wikipedia gives a reasonably good description of NFB. http://en.wikipedia.org/wiki/Negative_f ... _amplifier
More technical here:
http://www.aikenamps.com/NegativeFeedback.htm
http://www.aikenamps.com/GlobalNegativeFeedback.htm

If you want to wire for both 8z and 16z there are two choices. One is to use an impedance selector switch and the other is to simple wire two jacks.

For this particular amp, I think I'd go with two jacks. Bridge the wire for the sleeve to both jacks. Put the 16z wire on one tip and the 8z wire on the other tip. Only use one at a time. If you plug in both at the same time, they are parallel and the jacks are now half of what they were. So, if you want to use both jacks, plug 8z speaker into the 16z jack and 4z speaker into the 8z jack.

I wouldn't be too concerned about the NFB resistor placement. Some people use a lift switch to turn it off.

[NickC]

An 8 ohm speaker in parallel with a 16 ohm speaker is approximately a 5 ohm load. Thus, I recommend not using both OT taps at the same time (with the proper loads respectively).

If there were also a 4 ohm tap on the OT, you could probably get away with paralleling an 8 ohm and 16 ohm and running them together off the 4 ohm OT tap. Otherwise, I advise against it.

[ejendres]

Wikipedia gives a reasonably good description of NFB. http://en.wikipedia.org/wiki/Negative_f ... _amplifier
More technical here:
http://www.aikenamps.com/NegativeFeedback.htm
http://www.aikenamps.com/GlobalNegativeFeedback.htm

If you want to wire for both 8z and 16z there are two choices. One is to use an impedance selector switch and the other is to simple wire two jacks.

For this particular amp, I think I'd go with two jacks. Bridge the wire for the sleeve to both jacks. Put the 16z wire on one tip and the 8z wire on the other tip. Only use one at a time. If you plug in both at the same time, they are parallel and the jacks are now half of what they were. So, if you want to use both jacks, plug 8z speaker into the 16z jack and 4z speaker into the 8z jack.

I wouldn't be too concerned about the NFB resistor placement. Some people use a lift switch to turn it off.

Sweet, I'll dig into those references.

As for the speakers, I have no intention of running 8 and 16 at the same time. I just want to option to run either. I don't want to bother with a switch.

Thats how I was originally planning on doing it until I talked with some other amateur builder and they thought I should just wire the NFB circuit to both jacks. They thought it was part of the vibrato circuit. Glad I asked here.

[Firestorm]

How much do you want it to sound like a stock Vibrochamp? Fender took the feedback signal from a 4-ohm tap. The 8 will provide more feedback and the 16 more still. You can achieve original performance by adjusting the feedback (2K7) or shunt (47R) resistors. More feedback will reduce output and "tighten" the sound. Less will be louder and looser.

If you use separate jacks for different impedances, you can't use the shorting type jacks so you have to be extra careful never to run the amp without a load.

[ejendres]

How much do you want it to sound like a stock Vibrochamp? Fender took the feedback signal from a 4-ohm tap. The 8 will provide more feedback and the 16 more still. You can achieve original performance by adjusting the feedback (2K7) or shunt (47R) resistors. More feedback will reduce output and "tighten" the sound. Less will be louder and looser.

If you use separate jacks for different impedances, you can't use the shorting type jacks so you have to be extra careful never to run the amp without a load.

I want it to be loose and really bloom. My VHT Pitbull does the tight/modern clean tone really well.

Could I wire the NFB circuit to the unused 4 ohm tap?

I also purchased Cliff jacks that do short. Why can I use them? Couldn't I just not wire the shorting portion and use them like normal jacks?

[Firestorm]

Yes you can use the unused 4-ohm tap. Yes you can simply not connect the shorting tab. These amps are a little funny because they were cheap-o "student" amps, had really small OTs and didn't do bass frequencies well. That's a blessing in disguise because SE amps do exhibit 60Hz and 120Hz hum at the output. Small OTs and feedback help tame it. If you go for wide frequency response, you may have hum to kill.

You might consider a lift switch at the bottom of the tone stack to go all "tweedy."

[ejendres]

Yes you can use the unused 4-ohm tap. Yes you can simply not connect the shorting tab. These amps are a little funny because they were cheap-o "student" amps, had really small OTs and didn't do bass frequencies well. That's a blessing in disguise because SE amps do exhibit 60Hz and 120Hz hum at the output. Small OTs and feedback help tame it. If you go for wide frequency response, you may have hum to kill.

You might consider a lift switch at the bottom of the tone stack to go all "tweedy."

I can play with the NFB circuit til I like it.

What would the lift switch do? Remove the tone stack?

[Phil_S]

The lift switch just breaks the contact of the NFB resistor to the speaker (or the jack), making it inactive.

[Firestorm]

The lift switch just breaks the contact of the NFB resistor to the speaker (or the jack), making it inactive.

I meant to lift the ground at the bottom of the stack making it inactive. For NFB, I think a pot in series with the feedback signal might be the ticket. I don't like how these sound with no feedback at all. YMMV.

[zeebee]

I think overall that this amp is a nice testamp to see what nfb does ,you can try all the several available types even with resonance sort of schematic (for instance a 220k pot with a 22n parrallel to it) but then you have to raise te 47ohm resistor as well and even a presence control on it

of course cutting the ground to the mid pot indeed sort of bypasses the whole tone circuit to go for a pure tone ...just experiment with it it won't hurt ..and is fun to do!

[zeebee]

i guess most of the tubetech's around started with experimenting on simple handwired amps like fenders and marshall at least i did (with a bassman top actually )

[M Fowler]

So if your OT has 4/8/16 ohm taps why not have three jacks all labelled or two jacks with an impedance selector? That would be the easiest and cleanest. You could still install the NFB switch or pot like Firestorm noted.

Mark