bugera scem

[Valvehead]

Need a schematic for a Bugera 333XL head. anyone have one ?

my email is 493dart at gmail

[Jazz P Bass]

Here is a zipped folder/ 333XL.

[Valvehead]

thanks !!

looking at the psu scem, looks like the usa version needs 120 ohms worth of resistance in line with the ac hot ?

or are those 60 ohms each ??

[Jazz P Bass]

USA: 60 ohms x 6.

[Valvehead]

ok, never seen this before. whats the point ? to lower voltage ??

[Jazz P Bass]

They are thermistors.
CL-60 - GE MEASUREMENT & CONTROL/GE SENSING - NTC THERMISTOR, 10 OHM, -15% TO | Newark
They are being used as inrush current limiters.
http://www.ametherm.com/inrush-current/calculators

[Enzo]

They start out at that resistance, and once powered up, as they warm, their resistance drops to a low figure.

[Valvehead]

I dont see these in most amps..... are they necessary for this model ? If so, why?

[flyingdutchman]

are they necessary for this model ? If so, why?

In fact you don't usually see inrush limiters in guitar amps. Some designers of high-end amps might want to use them in order to make the amp more reliable. Valve heaters for example have considerable inrush current, which may lead to a dead valve at the most inconvenient moments(*) - or nuisance blowing of a heater circuit fuse if the amp has one. Also, if one inadvertently switches the amp on with the standby switch set to "ON", the HT will come up more slowly, this avoids damage due to full HT on cold valves. Last but not least, inrush limiters will of course also reduce the chance of nuisance blowing of the mains fuse.

The downside of NTC inrush limiters is that these devices operate at temperatures significantly above ambient so they create a hot spot inside the amp - the designer must take care to allow for this.

(*) It is well known that valve heaters tend to die at turn on. When the first valve computers were designed, people had doubts about reliability of such complex machines with thousands of valves. The solution was to avoid any power cycles unless strictly necessary, which in practice meant the machines were left powered up 24/7.

Edit: The only other amp I know of that uses an inrush limiter is the original version of the Vox AC50. It had a so-called "brimistor" between the rectifier and the first HT reservoir cap.

[Leo_Gnardo]

In fact you don't usually see inrush limiters in guitar amps. Some designers of high-end amps might want to use them in order to make the amp more reliable.

Inrush limiters in lots of current (and last 20+ years) Fender amps, Hot Rod series, RI Twin, etc.

Peavey also. I forget which model but one amp I had to fix had an entire extra circuit board paved with about 10 of 'em in series. Of course, one was open-circuit. Cheap fix.

McIntosh hi reliability hi fi tube amps - yup, in there too, models since the late 50's thru early 70's. Don't know if they're in the current crop of tube Macs.

[flyingdutchman]

lots of current (and last 20+ years) Fender amps

In fact you're right. Never even noticed 'em. Which means that, at least in the HR series, they seldom cause trouble

[Enzo]

Leo, I think the ten of them in series amp was not a Peavey, I believe it was a Bugera, the topic of this thread.

As others have said, they are in a lot of amps.

[Leo_Gnardo]

Leo, I think the ten of them in series amp was not a Peavey, I believe it was a Bugera, the topic of this thread.

Nope, I'd swear on a stack of RCA tube manuals, Peavey. I've only worked on 2 Buggery amps and I'd like to keep it that way. Maybe Boogera borrowed the idea - "soften up" the AC supply for that squashy response. Kind of like having a light bulb in series only in reverse.

BTW I'm delighted to see you back on the case Enzo. Just read your account in the "ACA coverage" discussion. Crikeys! Please stay well, the world need you!

[Enzo]

I've been a Peavey repair center for a LONG time, and i can;t think of an example of that. I remain open of mind to contrary evidence.