VT100 low power / high distortion

[tbonuss]

Hello J M Fahey!

1. The schematic for the VOX VT50 is here:
http://bmamps.com/Schematics/vox/Vox_vt50_sm.pdf

2. I am working on the VOX VT100 and the VT50 is the closest schematic we can find. I also have an actual VT50 amp available for reference as necessary.

3. R28 from the pic above (VT100) is the same as R17 on the schematic (VT50). This is the one that is open (OL res measure). It can be found in the upper right quadrant of the scat, on pin 13 of the STK404-100S Audio Power Amp IC.

4. R24 from the pic above is the same as R18 on the schematic. It can be found on pin 12 of that same IC. We think its okay.

Thank you in advance for any help at all!

[tbonuss]

Replaced both resistors and put in a couple of refurbished Celstion Seventy 80's I got from Sweetwater ($60.00 for the pair!). Seems to be fully operational now. Sincerest thanks to DrGonz78 and everyone for the help. If anyone ever needs any help with one of these things, let me know. I'm still a little rusty, but I'm happy to help in any way I can.

I'll keep looking for the VT100 schematic and post if I ever come up with it.

Rock on!

[tbonuss]

So after this initial Valvetronix fix, I've fixed four more of different age and size. People basically give them away when they break. Of those, four of the five had fried speakers due to DC out from the audio amp. I've put fuses on the speaker lines and don't understand why they don't come this way. Not sure what the downside is...cost in manufacturing?

[J M Fahey]

So after this initial Valvetronix fix, I've fixed four more of different age and size. People basically give them away when they break. Of those, four of the five had fried speakers due to DC out from the audio amp. I've put fuses on the speaker lines and don't understand why they don't come this way. Not sure what the downside is...cost in manufacturing?

1) glad you solved it.
2) no, itīs not a cost problem, but that fuses donīt know DC from AC.

And in general, a fuse which will blow with DC, will nuisance blow all the time when you play loud or even worse, overdriven, and drive you crazy.
Will blow in the middle of the best solo of your life.

A fuse which lets you play, will not blow with DC or best case take a LONG time.

Real speaker protectors check DC all the time and disconnect speaker with as low 4 or 5 Volts DC, within 10 seconds, or a few milliseconds if you have, say, 30 or 40V DC, while they will let 20 or 30V of audio pass free all day long.

[tbonuss]

1) glad you solved it.
2) no, itīs not a cost problem, but that fuses donīt know DC from AC.

And in general, a fuse which will blow with DC, will nuisance blow all the time when you play loud or even worse, overdriven, and drive you crazy.
Will blow in the middle of the best solo of your life.

A fuse which lets you play, will not blow with DC or best case take a LONG time.

Real speaker protectors check DC all the time and disconnect speaker with as low 4 or 5 Volts DC, within 10 seconds, or a few milliseconds if you have, say, 30 or 40V DC, while they will let 20 or 30V of audio pass free all day long.

Thanks Juan. I understand that what you are saying is that in practice it can be a nuisance, especially if the fuse is too low. In this case, however, I think it will work just fine, and it has stood up to max saturation and gain tests. The problem is that the max DC p/s is put onto the speaker, whereas, the AC audio just doesn't produce this much current / power. I'm not sure what you mean that the fuse doesn't know DC from AC, but the serial inductive circuit sure does--which is what the fuse conducts. I mean, that's why the speaker blows in the first place. Not disagreeing with your points that it can be a nuisance, but in this case its moderating against a known common destructive fault, and for my purposes seems to be prudent. Also: yes, I'd rather lose a guitar solo than a speaker, but thats just me YMMV.

Thanks! Now that I posted this they'll probably pop every time I go to half volume.