Rocktron Velocity Issues

[J M Fahey]

maybe next time I'll just keep to myself .

No, please , donīt think that way.
Itīs only that , unfortunately, you took a *very* complex and temperamental amp as your first project.
Even those *quite* experienced call it a "dog" (although using polite words).
Not even the original project on which itīs based is much trusted, go figure.

Iīm sure if you had a more "normal" amp (think midrange Peavey/Laney/Crate) things would have been quite different.
Good luck

[Steve Conner]

Rant: I see way too many posts by wannabees that are darn lucky they do not kill themselves.
I have nothing against learning how to repair equipment.
It is the willy nilly, lackadaisical (I had to look that one up), cavalier attitudes that just frost me.
(Oh, this is burnt, what do I do next)
End Rant.

I think you touch on quite a big issue here. The forum isn't an exclusive club, anyone can use it, and the vast majority of people don't have the temperament needed to do complicated electronic troubleshooting. (You need a dose of Asperger's or OCD. )

If you do have "the knack" then it will seem as if most of your time will be spent giving troubleshooting lessons to complete beginners who will ignore your advice and give wrong data at random. Assuming you accept that mission and don't just ignore them.

I admit that the Alexander amp I built was a real pain in the ass to get working, and in the end it didn't seem to sound as good as the traditional circuit.

I can't believe Rocktron used the circuit for a guitar amp. I think they were also a bit optimistic using one pair of output devices with 50V rails, and then putting "300" in the name, and that might be why it blew up.

Yup, I'd be building a Peavey Bandit clone into the case about now

[Chuck H]

I think you touch on quite a big issue here. The forum isn't an exclusive club, anyone can use it, and the vast majority of people don't have the temperament needed to do complicated electronic troubleshooting. (You need a dose of Asperger's or OCD. )

Hey now. I take great resemblance to that remark.

it will seem as if most of your time will be spent giving troubleshooting lessons to complete beginners who will ignore your advice and give wrong data at random.

LOL

I'd be building a Peavey Bandit clone into the case about now

The Bandit was my first amp. I used it for many years before I discovered tubes. At the time most gear was rack mount preamps and crap. I went through A LOT of "tube" crap before I finally discovered real amps like old Marshalls and Fenders were the way to go. In that transitional period when I was experimenting with rack stuff I actually missed my Bandit. The Bandit also got my vote on the "best ss amp" thread.

[Tasos]

Hello,
i wish you the best of luck with your repair!
Wish i could help...
Anyway,could you please post some pictures of the inside of this amplifier?
It would be really appreciated!
Thanks

[MarkusBass]

I need some help with the amp. There were some semiconductors shorted in the amp but this is solved now. I think that the supply circuit for the fan was modified. Can someone tell me whether there should be a thermal switch for the fan? In the amp that I have the fan is supplied with a 220 Ohms resistors (and there is no switch). In the heatsink you can see a place where most probably a thermal switch could be fitted. Has anyone got pictures of the original amp (of the inside)?

BTW, Tasos I can post some pictures of the amp. What exactly do you need to see?

Mark

[J M Fahey]

I think you are right.
Besides, the Thermal switch footprint is unmistakeable.
Not sure about its function though, it might be a N.O. type to turn fan on above, say, 50 to 60šC *or* a N.C. one which opens 220V power or speaker out.

[Jazz P Bass]

Well, if it is a Velocity 300, there is indeed a snap thermal switch. 130 degrees.
CA 130 is the part#.
N/O contacts.

[MarkusBass]

Thanks Jazz P Bass. On the schematic I see CA-140. SO this is something like 60 degrees Celsius. I was going to buy this one: 2455R-100-79 - HONEYWELL S&C - PRZE??CZNIK TERMICZNY, NO, 45 | Farnell Polska
The original one is from Selco but I see that they do not have distributor in Europe (and the minimum order is $50).

Mark

[Jazz P Bass]

Thanks Jazz P Bass. On the schematic I see CA-140. SO this is something like 60 degrees Celsius.

Yup.
It does say CA-140.
Optometrist time.

[MarkusBass]

Finally, I ordered the thermal switch from Farnell. Last question: does anyone know how the quiescent current should be set up in this amp? The ballast resistors are 0.05 Ohm and I cannot measure any voltage on the resistors. But there are trimmers to set it so there must be a way to do it. By turning them I can change the voltage between driver's bases - the circuit works. I can't see any hint on the schematic.

Mark

[Enzo]

The point of the bias adjustment is to tune out the crossover notch but not cause excess current. When a schematic says so and so current through a ballast resistor, that just means that in their estimation, that amount of current results in proper operation. So in the absence of some short cut such as that current note, you adjust the amp for crossover notch.

[MarkusBass]

Everything is fixed now. Thanks everyone for help.

Mark