PV amp, short circuit protection?
- Thread starter CJ
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It does and it doesn't. It does, in the sense that Q32/33 monitor (only) the current going through Q29/34 and their associated emitter resistors.
When there's more than 1.8A or so (or perhaps more, i see a resistive divider there too) going through them, Q32/33 start draining off some of the base drive - in essence, acting as limiters.
If one (or several) of the other four output transistors turned into a wire, though, that's an "SOL" situation.
I invite more knowledgeable minds to correct me, but i think that could be remedied with additional 2.2k resistors into the bases of Q32/33, from the other four output transistor emitters.
When there's more than 1.8A or so (or perhaps more, i see a resistive divider there too) going through them, Q32/33 start draining off some of the base drive - in essence, acting as limiters.
If one (or several) of the other four output transistors turned into a wire, though, that's an "SOL" situation.
I invite more knowledgeable minds to correct me, but i think that could be remedied with additional 2.2k resistors into the bases of Q32/33, from the other four output transistor emitters.
Gene Pink
Well-known member
CJ said:
It has current limiting.
Gene
Edit: whoops, Khron snuck a much better reply in.
q32 and q33 provide adequate current limiting... It is standard practice to sense across just one output pair. The 0.33 ohm emitter degeneration resistors force all the power device to share current equally.
@ Kron, adding resistors from the other emitter resistors would not automatically protect against one outlier since the four resistors form a divider at the base of current limit transistor.. Using diodes to extract a "greater of" would result in an additional diode drop messing up the current calculus. So you would have to add not only more resistors but more current limit transistors that could be parallelled and work. Of course this is impractical. If a power devices fail as short circuit it could take out the circuit breaker, two opposing devices failed certainly would trip breaker/fuse.
I recall an old BGW amp design that could not effectively current limit power stage so used a crowbar shunt across the power supply to trip the breaker/fuse when it detected a fault. Some Peavey amps use output relays and probably some used crowbar, but apparently not this one.
Is that a thermal breaker in series with the speaker near the bottom of the schematic?
JR
@ Kron, adding resistors from the other emitter resistors would not automatically protect against one outlier since the four resistors form a divider at the base of current limit transistor.. Using diodes to extract a "greater of" would result in an additional diode drop messing up the current calculus. So you would have to add not only more resistors but more current limit transistors that could be parallelled and work. Of course this is impractical. If a power devices fail as short circuit it could take out the circuit breaker, two opposing devices failed certainly would trip breaker/fuse.
I recall an old BGW amp design that could not effectively current limit power stage so used a crowbar shunt across the power supply to trip the breaker/fuse when it detected a fault. Some Peavey amps use output relays and probably some used crowbar, but apparently not this one.
Is that a thermal breaker in series with the speaker near the bottom of the schematic?
JR
That'll do it.... I recall one unfortunate incident where a certain Peavey speaker crossover would make a certain Peavey topbox oscillate. The speaker then not only released its magic smoke, but flamesCJ said:
.... That problem was found and corrected pretty damn quickly (by the speaker guys, who screwed up the crossover design ). Peavey amps are designed to be stable driving almost any load, but stuff happens. I have also heard of breakers failing closed or stuck on, back when we first started using resettable breakers instead of fuses (on transformer primaries)... but that should be pretty rare these days.
JR
somebody snipped the ground prong om the pwr cord, in a high gain situation like this it proved to be fatal to those poor voice coils,
can not localize the trouble, as soon as we clip the gnd lead from scope to chassis, problem goes away,
time for an iso xfmr for the scope,
can not localize the trouble, as soon as we clip the gnd lead from scope to chassis, problem goes away,
time for an iso xfmr for the scope,
> current limiting.
I believe it is VA limiting. It allows more I if the transistor V is low. This puts full power in rated loads and doesn't try as hard when the amp is shorted. This was fairly well understood from early days; PV's Jack probably knew it better than most. This one is a bit too complicated for me to work out in my head.
I believe it is VA limiting. It allows more I if the transistor V is low. This puts full power in rated loads and doesn't try as hard when the amp is shorted. This was fairly well understood from early days; PV's Jack probably knew it better than most. This one is a bit too complicated for me to work out in my head.
Hi Gus! This amp is a Peavey Special 212 Transtube Series.
going to see if we can tame that Lead circuit in case someone uses a gnd lift adapter thus saving a Shefield cookoff contest.
https://www.electronica-pt.com/esquema/audio/amplificadores-audio/peavey/peavey-transtube-special-212-orig-1995-schematic-28100/
going to see if we can tame that Lead circuit in case someone uses a gnd lift adapter thus saving a Shefield cookoff contest.
https://www.electronica-pt.com/esquema/audio/amplificadores-audio/peavey/peavey-transtube-special-212-orig-1995-schematic-28100/
Yes Jack (RIP) was the man for preventing secondary breakdown using VI limiting (kind of like foldback current limiting).PRR said:
JR
OK if that is transtube there is more to think about... some transtube models used positive feedback to manipulate the amplifier output impedance (to be more tube like) and promote speaker/cabinet impedance interactions. If that non-typical NF is wonky oscillation could be a result.CJ said:Hi Gus! This amp is a Peavey Special 212 Transtube Series.
going to see if we can tame that Lead circuit in case someone uses a gnd lift adapter thus saving a Shefield cookoff contest.
https://www.electronica-pt.com/esquema/audio/amplificadores-audio/peavey/peavey-transtube-special-212-orig-1995-schematic-28100/
In mexico if the ground pin was not cut off yet, the amp is considered box new... :
JR
Not a service tech but Grant Brown ran the entire service department probably that long... If he isn't retired now he is due..CJ said:
Grant Brown is good people. 8) When I was running my mixer engineering group, I would meet with his service guys once a month. Like walking into the lion's den, if one of my products had a design/build problem, they knew from multiple customer calls and let me have it with both barrels. :
The service guys were pretty good about handling the customer's complaints without involving the design guys who need to be free to focus on new stuff, but occasionally they would kick problems up to me.
I did have to read Grant the riot act, when his service guys were telling old AMR console customers how to find me, years after I quit Peavey. One of the sundry tricks in the transamp series was to emulate a tube amp running at lower voltage (like Eddie's old variac trick), so you can just dial back the power while getting the nice (?) tube saturation sound without making your ears bleed.. Not about how the waveform looks but about how it sounds. ;D
I am not a (guitar) player. but i recall when these were under development and peavey engineers would carry transtube prototypes to their bar band gigs (most Peavey guitar amp engineers were also active bar band players).. I am not a tube guy, but they definitely sounded a ton better than the old solid state amps they replaced (IMO). ;D
JR
PS: The PA rigs and backline for my house parties were often liberally populated by engineering prototypes... How better to test the gear?
holy cow who played at your house parties?
yeah Grant was very nice on the phone, talks kind of slow like mid-western types, sounds like he wears a beard and goes "noodling" for blue cats on the weekends,
here is the preamp, the Ultra and Lead channels look almost identical, so now we have a way to compare signals side by side and see what differs. lots of discrete Darlingtons, eh?
yeah Grant was very nice on the phone, talks kind of slow like mid-western types, sounds like he wears a beard and goes "noodling" for blue cats on the weekends,
here is the preamp, the Ultra and Lead channels look almost identical, so now we have a way to compare signals side by side and see what differs. lots of discrete Darlingtons, eh?
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my primary house band was Robodrum https://soundcloud.com/robodrum/10-robodrum-beer?in=robodrum/sets/robodrum keep listening more than one song, but the homebrew Bob mentions in Beer song is Johnny Beer. If you stay with the songs ... I watched them get their power cut off and kicked out of the bar when they played the country version of "Highway to hell", no sense of humor even in MS. ;D (I can't vouch for the sound quality of that cloud poop..we tracked on AMR 4T cassette and mixed down to 2T didn't totally suck, but you know how memory works.)CJ said:
Besides robodrum any local bands that weren't working would show up and jam sessions often ensued later at night as players wanted to show off. I had one friend bring his drum kit, and we cobbled together a couple SOS (speakers on sticks) powered by a few hundred watt topbox, and a couple live vocal mics. No need to mic anything else in my living room. :
Backline was often bring your prototype guitar amp from work. ;D
The local musicians who weren't working that night (my parties were always on Saturdays) would all show up. And the musicians who were working would show up early for beer and chili... I started serving my chili at noon and at least one time ran out of chili before the night party even got going. ( I also served barbecued chicken and tater tots to the evening crew...). I never ran out of beer.
Yes, Grant is a good old boy and I've seen him with and without beard. I can't say what he does on weekends (i don't want to know).yeah Grant was very nice on the phone, talks kind of slow like mid-western types, sounds like he wears a beard and goes "noodling" for blue cats on the weekends,
Back in the days of hand insertion darlingtons were nice because it was two devices for one hand pop (IIRC I paid something like $0.09 per hand pop, not to mention drilling less holes.). After machine insertion (a nice panasonic vertical insertion machine), then later SMD, the Darlingtons became less attractive. One time I stuck a darlington in place of a normal junction device to fix some unexpected problem in production... I don't recall the details, but it was easier (cheaper) than halting production and redesigning the circuit board.
JR
holy cow those guys are smokin hot! sounds like a strat/humbucker thru a classic 30 with a tube screamer in front,
who is that guitar player?
took out the 'death caps" when this amp came in, turns out a .01 from neutral to chassis kills the microwave oven noise,
presence control would control the amplitude of the osc.
upped the voltage to 500 with a ceramic, time to call it good and move on, Mr. Brown told me not to change any parts but it was too late, luckily we have a PV bandit sitting out i n the garage that has offered to donate a 12 inch and there is stuff hanging from the ceiling so hopefully we will never see this amp again.
HF osc and the cap that fixed it along with the 3 lead cord being reinstated.
who is that guitar player?
took out the 'death caps" when this amp came in, turns out a .01 from neutral to chassis kills the microwave oven noise,
presence control would control the amplitude of the osc.
upped the voltage to 500 with a ceramic, time to call it good and move on, Mr. Brown told me not to change any parts but it was too late, luckily we have a PV bandit sitting out i n the garage that has offered to donate a 12 inch and there is stuff hanging from the ceiling so hopefully we will never see this amp again.
HF osc and the cap that fixed it along with the 3 lead cord being reinstated.
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That is Bob Hopkins on lead guitar, Jim Albert on bass, Brian Hughley drums. Now the three are spread around the country.CJ said:
Bob won best guitar player in MS contest at least once but yawn... Has day job as industrial designer (NHT speakers), but still gigs and records with local bands around SF/Napa.
https://www.youtube.com/channel/UCKu37UVpIr5zcGxaORNQtng
Of course i like the old Robodrum stuff better...
Back when he first started writing music it was just him and a bass player, and he used a drum machine for the drum parts. He had a man sized cardboard cutout of Mr Robo on stage for gigs... I added LEDs to the eyes of Mr Robo that would flash in time to the music (added a mic input to a blinking circuit to sync with the music).
I like 3 wire line cords on guitar amps... good job.
JR
