Sears Solid State Guitar Amp LA-500
- Thread starter CJ
- Start date
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Didn't the name "bread board" come from re-purposing old bread cutting boards into prototype circuits with nails and fahnestock clips etc.
JR
JR
yes on the breadboard name,
here is a prelim schematic for the LA-500 which was drawn by tracing the circuit board,
looks like someone did not realize that the output transistors were supposed to both be NPN, so they stuck in a comp. pair, NPN-PNP which is why some resistors fried,
both the drivers and the pwr transistor part numbers are not known at this time,
looks like a diff pair feeds a pre driver which feeds the drivers, also a looks like there is some neg feedback involved>
here is a prelim schematic for the LA-500 which was drawn by tracing the circuit board,
looks like someone did not realize that the output transistors were supposed to both be NPN, so they stuck in a comp. pair, NPN-PNP which is why some resistors fried,
both the drivers and the pwr transistor part numbers are not known at this time,
looks like a diff pair feeds a pre driver which feeds the drivers, also a looks like there is some neg feedback involved>
Attachments
CJ said:
It looks like you could convert that from quasi-complemenatry to full complimentary by just connecting the base of the PNP power device to the emitter of the driver transistor instead of the collector. The full complementary output stage is faster so the quasi-comp design may be over compensated. For symmetry I would be tempted to change a few other part values.
What's the worst that could happen?
JR
thanks John!
this thing has a TIP41c and TIP42c as drivers, what would be a good complimentary pair of power transistors that would work in this circuit?
cj
this thing has a TIP41c and TIP42c as drivers, what would be a good complimentary pair of power transistors that would work in this circuit?
cj
Whatever are already in there...CJ said:
JR
> you could convert that from quasi-complemenatry to full complimentary
Sure... but WHY? It's a SilverTone gee-tar amp. It isn't supposed to be perfect.
2N3773, 2N5302, and MJ802 are beefy yet era-correct NPN parts. TO-3 type cases, 150W-200W nominal dissipation, and SOA lines that "can" survive this application with no protection circuit.
Sure... but WHY? It's a SilverTone gee-tar amp. It isn't supposed to be perfect.
2N3773, 2N5302, and MJ802 are beefy yet era-correct NPN parts. TO-3 type cases, 150W-200W nominal dissipation, and SOA lines that "can" survive this application with no protection circuit.
PRR said:
I find it hard to be nostalgic for quasi comp, but agree the old to-3 NPNs were pretty robust. My still working living room amp that I designed back in the early '70s is quasi-comp (2N3773).
My bad taste about quasi-comp topology was when Crown repackaged the old DC-300 into modern SMT but didn't bother to take advantage of modern power devices that are capable of both zigging and zagging.
JR
L´Andratté
Well-known member
Good learning thread thanks!
(EDIT) Deleted stupid question (;
(EDIT) Deleted stupid question (;
5K was the Bukowski constant for most preamps back then, 
hey i got these drivers in for the Acoustic 120 project, have an extra pair, could i use these in place of the errant TIP41c and 42c drivers team them up with the Acoustic pwr transistor part numbers, MJ15024?
40410 and 40409 are the RCA drivers, they have a really nice heat sink pre-attached,
and i said the input to this thing was a diff input, looks more like a current mirror,
hey i got these drivers in for the Acoustic 120 project, have an extra pair, could i use these in place of the errant TIP41c and 42c drivers team them up with the Acoustic pwr transistor part numbers, MJ15024?
40410 and 40409 are the RCA drivers, they have a really nice heat sink pre-attached,
and i said the input to this thing was a diff input, looks more like a current mirror,
Attachments
> i said the input to this thing was a diff input, looks more like a current mirror
Nevertheless, Q4 Q5 form a differential pair input.
R15 Q6 R19 form a "current mirror" with significant current gain (a magnifying mirror).
The whole thing IS an op-amp. See Self's pages/book on basic op-amp guts. Just has a monster output stage, "poor" input specs, and isn't compensated for low gains.
Nevertheless, Q4 Q5 form a differential pair input.
R15 Q6 R19 form a "current mirror" with significant current gain (a magnifying mirror).
The whole thing IS an op-amp. See Self's pages/book on basic op-amp guts. Just has a monster output stage, "poor" input specs, and isn't compensated for low gains.
thanks for that, if it ain't cathode bias then i'm lost,
those parts came in from China, earlier than expected,
they look pretty good, i hope they work, so if i get this old board going we can do an A/B test,
have some 3773 parts from ON semi coming,
there were some Toshiba and RCA 3773's on evilbay but sometimes you get leaky pulls, so i went with digikey,
those parts came in from China, earlier than expected,
they look pretty good, i hope they work, so if i get this old board going we can do an A/B test,
have some 3773 parts from ON semi coming,
there were some Toshiba and RCA 3773's on evilbay but sometimes you get leaky pulls, so i went with digikey,
Attachments
It doesn't look like a current source because it isn't,,,
Q6 is a common emitter gain stage, Q4 and Q5 are an input stage long tail pair.
So for a little op amp 101, typical op amps have a differential input stage that converts two input's difference voltage into a current.
Then the typical op amp has a middle gain stage that converts the input current signal into a large signal voltage swing.
Finally the output stage is followed by a simple buffer (voltage follower) that allows the high impedance high voltage signal to drive lower impedances.
The compensation cap is c11 that rolls off the gain of Q6 at very high frequency to insure stability.
JR
Q6 is a common emitter gain stage, Q4 and Q5 are an input stage long tail pair.
So for a little op amp 101, typical op amps have a differential input stage that converts two input's difference voltage into a current.
Then the typical op amp has a middle gain stage that converts the input current signal into a large signal voltage swing.
Finally the output stage is followed by a simple buffer (voltage follower) that allows the high impedance high voltage signal to drive lower impedances.
The compensation cap is c11 that rolls off the gain of Q6 at very high frequency to insure stability.
JR
> walk me thru the part of the circuit
When in doubt, consult Self.
http://www.douglas-self.com/ampins/dipa/dipa.htm#4
Fig 1a is a basic generic power amp.
TR2 TR3 are a differential input.
TR4 is the main voltage-gain stage.
In your amp, the functions of TR1 TR5 are replaced by resistors. Works OK, and saved quite a few pennies back in the day.
AND for extra fun, the whole thing has been designed *flipped* + to -. So TR2 TR3 are NPNs, TR4 is PNP. Works the same either way. No general preference one or another.
Self's TR7 TR9 are complements, you got both the same. Significant difference in wiring. Minor but "significant" differences between Darlington and what Self calls "CFB", differences that matter to angel-on-pin counters. Nevertheless really excellent audio amps have been made with "asymmetrical" output stages.
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In your sick amp, suspect (replace) Q9 Q10 because without protection these parts eventually get killed. When that happens, Q7 q8 try to pull the load, but obviously can't, they die. Then Q6 tries to pull the load through the now-shorted output stage. Either Q6 or your R20 (or both) will die. If R20 is bigger than R19, R19 could die first.
R27 (twice!), and R28 could be stressed when Q9 Q10 quit.
It would be quite odd for an output fault to stress Q4 Q5.
A prolonged fault could blow C13.
SilverTone was lowest-bid so *any* part could be bad by now.
This amp will run (weakly) without Q9 Q10. It won't pull 8 Ohms, but no-load it should pull its output to less than a half-Volt DC, and show tenth-Volts at Q9 Q10 B-E pins. (Not half-Volt! check R24 and the two diodes if that happens, or just short the R24-D1-D2 string to get a cold-but-safe bias for smoke-testing.)
When in doubt, consult Self.
http://www.douglas-self.com/ampins/dipa/dipa.htm#4
Fig 1a is a basic generic power amp.
TR2 TR3 are a differential input.
TR4 is the main voltage-gain stage.
In your amp, the functions of TR1 TR5 are replaced by resistors. Works OK, and saved quite a few pennies back in the day.
AND for extra fun, the whole thing has been designed *flipped* + to -. So TR2 TR3 are NPNs, TR4 is PNP. Works the same either way. No general preference one or another.
Self's TR7 TR9 are complements, you got both the same. Significant difference in wiring. Minor but "significant" differences between Darlington and what Self calls "CFB", differences that matter to angel-on-pin counters. Nevertheless really excellent audio amps have been made with "asymmetrical" output stages.
_______________________________________
In your sick amp, suspect (replace) Q9 Q10 because without protection these parts eventually get killed. When that happens, Q7 q8 try to pull the load, but obviously can't, they die. Then Q6 tries to pull the load through the now-shorted output stage. Either Q6 or your R20 (or both) will die. If R20 is bigger than R19, R19 could die first.
R27 (twice!), and R28 could be stressed when Q9 Q10 quit.
It would be quite odd for an output fault to stress Q4 Q5.
A prolonged fault could blow C13.
SilverTone was lowest-bid so *any* part could be bad by now.
This amp will run (weakly) without Q9 Q10. It won't pull 8 Ohms, but no-load it should pull its output to less than a half-Volt DC, and show tenth-Volts at Q9 Q10 B-E pins. (Not half-Volt! check R24 and the two diodes if that happens, or just short the R24-D1-D2 string to get a cold-but-safe bias for smoke-testing.)
