Why Do Outputs Blow?
- Thread starter Sriskie
- Start date
Binkman
Lunatic Member
I dug up a couple of threads about this 450 including Sriskie's earlier april thread. and couple of others where 'drift' of the bias adjustment is 'troublesome'. I certainly can't give a description of how components may react but the 440 manual does describe how the outputs are taken out. As noted 'king' couldn't get to the second level adjustment. One thread posted using a variac to observe the p.s. but this was just a one line reply dead end.
I couldn't find anything on a 450 sm and people say they're identical aside from cosmetics. Cosmetics don't usually warrant a new model number..
Wonder if the 8ohm load is erroneous in that considering the amperage they get hit with maybe needs to be pretty beefy. if all the other components aside from the transistors check out. I'd take a stab at cutting down the initial bias setup to perhaps half the mv. maybe let the unit cool down before removing the 8ohm load and attempting the second adjustment. Perhaps let cool down and leave the 8 ohm load connected and do a second boot up.
edit.. maybe bring it up on a variac and check the sm out?
--- my 2 cents considering the transistor replacements and hfe and the way the amp switches up. I don't have enough experience to tell you why that base opens. any relation to the DC servo circuit? ...just a question.
I couldn't find anything on a 450 sm and people say they're identical aside from cosmetics. Cosmetics don't usually warrant a new model number..
Wonder if the 8ohm load is erroneous in that considering the amperage they get hit with maybe needs to be pretty beefy. if all the other components aside from the transistors check out. I'd take a stab at cutting down the initial bias setup to perhaps half the mv. maybe let the unit cool down before removing the 8ohm load and attempting the second adjustment. Perhaps let cool down and leave the 8 ohm load connected and do a second boot up.
edit.. maybe bring it up on a variac and check the sm out?
--- my 2 cents considering the transistor replacements and hfe and the way the amp switches up. I don't have enough experience to tell you why that base opens. any relation to the DC servo circuit? ...just a question.
In my case it was an early 60's 6V6 push/pull guitar amp. My drummer who was also an excellent blues guitarist was jammin' on the Les Paul and me and the Bassists were commenting how unusally good it was sounding when all of a sudden the volume dropped to zero followed by a puff of smoke. Turned out one of the power tubes came loose so it was only feeding off one tube.
bktheking
Gitter Done!
Spoke to a local tech and described what was going on with this unit- replace the op amps he said so I did- ran the unit up to 41v 1khz sine wave and it didn't blow up, been listening to it for the last 2 hrs. Seems the left channel op amp was causing the unit to oscillate and blown all 6 outputs and the 8 amp fuses.
Sriskie
Well-Known Member
Well, the bitch is back!
Special thanks to Brad for getting it sorting it out. It's working well and without issues now.
Back to the question of why outputs blow, I think I know what happened in this case. Mind you, there's much I don't know but my theory seems to fit. I believe it was a grievous error on my part that caused it. Aside from that I think there were some problems going on that I wasn't aware of, out of spec thermistors, and the above mentioned op-amps. The amp wasn't right to begin with even though it was working fine.
This amp is very sensitive to temperature changes. There's bias transistors mounted on the outputs as well as thermistors an the main board. That board is facing down and under the amp. When I set the bias I had the amp upside down with no way to trap the heat from surrounding TRs and resistors. I was having trouble getting it to stabilize, The bias kept creeping up and down depending on which way I adjusted the pots. I noted previously that the amp ran hot, that's not really the case. The thermistors kept dragging the bias down and I kept cranking it up. With the amp sitting upside down without the covers, It didn't have proper thermal management for the thermistors, After two hours of tweaking I had it set pretty close to the 18mV spec and buttoned it up. Worked fine for a while but when I turned it on out on the sun porch on a +2 Celsius (35 Fahrenheit) morning, the bias must have gone through the roof!
When I got it back I noticed the bias went from 10mV shortly after I brought it in from outside (hot) to 30mV once it was acclimated to my cool basement. There doesn't appear to be anything there to limit how far the thermistors can swing the bias. Heat doesn't seem to be a problem, I've run it for hours pushing two pair of speakers and the heat sinks stay cool to the touch. The cold on the other hand...:thumbsdn:
Thoughts?
Special thanks to Brad for getting it sorting it out. It's working well and without issues now.
Back to the question of why outputs blow, I think I know what happened in this case. Mind you, there's much I don't know but my theory seems to fit. I believe it was a grievous error on my part that caused it. Aside from that I think there were some problems going on that I wasn't aware of, out of spec thermistors, and the above mentioned op-amps. The amp wasn't right to begin with even though it was working fine.
This amp is very sensitive to temperature changes. There's bias transistors mounted on the outputs as well as thermistors an the main board. That board is facing down and under the amp. When I set the bias I had the amp upside down with no way to trap the heat from surrounding TRs and resistors. I was having trouble getting it to stabilize, The bias kept creeping up and down depending on which way I adjusted the pots. I noted previously that the amp ran hot, that's not really the case. The thermistors kept dragging the bias down and I kept cranking it up. With the amp sitting upside down without the covers, It didn't have proper thermal management for the thermistors, After two hours of tweaking I had it set pretty close to the 18mV spec and buttoned it up. Worked fine for a while but when I turned it on out on the sun porch on a +2 Celsius (35 Fahrenheit) morning, the bias must have gone through the roof!
When I got it back I noticed the bias went from 10mV shortly after I brought it in from outside (hot) to 30mV once it was acclimated to my cool basement. There doesn't appear to be anything there to limit how far the thermistors can swing the bias. Heat doesn't seem to be a problem, I've run it for hours pushing two pair of speakers and the heat sinks stay cool to the touch. The cold on the other hand...:thumbsdn:
Thoughts?