I bought this amp as a repair project. Found it had blown outputs and inadequate outputs and resistors in one channel. I have the amp working now both channels. I am not sure what the bias should be. I have it set currently @ 120mv and have about a quarter turn to lower it if need be. I think the bias is to be a bit high for this amp but does anyone know what it should be?
Also upon power up and power down (about 8 second delay) there's a thump through the speakers. Most notably on power up. I have the dc offset in single digits so is this normal for an amp like this and is it safe for my speakers?
Also upon power up and power down (about 8 second delay) there's a thump through the speakers. Most notably on power up. I have the dc offset in single digits so is this normal for an amp like this and is it safe for my speakers?
Attachments
I do have this service manual but is for a range of ab systems amps. does have schematic for my 410 but nothing on bias specifically
info for my amp is on page 25 & 28 for schematic
page 11 has bias adjustment for 900 model
the amp has (2) 5.6ohm emitter resistors and (7) 0.43 ohm resistors @ outputs. bias is measured across the 5.6ohm resistors individually. If I was to try and set @ 55vdc I would need to turn the pots all the way down and still will be a bit higher than that after warmed up. I did install new trimmer pots but are the same value replacements. thank you
info for my amp is on page 25 & 28 for schematic
page 11 has bias adjustment for 900 model
the amp has (2) 5.6ohm emitter resistors and (7) 0.43 ohm resistors @ outputs. bias is measured across the 5.6ohm resistors individually. If I was to try and set @ 55vdc I would need to turn the pots all the way down and still will be a bit higher than that after warmed up. I did install new trimmer pots but are the same value replacements. thank you
Last edited:
55 mV? More like 550 mV. Not a good spot for judging bias anyway. Could you try checking between the bases of Q10/Q11, i.e. the 100R/3k3 junctions? Theory says optimum bias is obtained when you're getting about 26 mV there, which would equate to 30 mA per pair of outputs. That's assuming thermals will allow, so do have an eye on temperature.
thank you sgrossklass I will try that. I currently have it @ 120mv and was not getting hot, I would guess around 105F. but I will post back with some real data. This board layout is a real pita, can only do the bottom one first.
At that point you're only running 20-odd mA through Q14/15, with Q16-21 doing nothing. Possibly it was even intended to be like that, so that the outputs would only start conducting when needed. You can do that, of course, but output gm variation (and resulting distortion) won't be pretty. May have been deemed acceptable for PA, I guess. At 550 mV you'd be at almost 100 mA for Q14/15, seems a bit much. For "proper" bias one may have to up the resistors to something like 15-18 ohms.
I'm not sure what the diodes in the input LTP are supposed to be good for. That part is weird anyway. They've got a current source in one leg and a zener (for voltage / dissipation reduction?) in the other. Why not a current mirror?
The VAS is interesting in that it seems to be sort of a distortion canceling type, a bit like a Kuartlotron except not a buffer.
I'm not sure what the diodes in the input LTP are supposed to be good for. That part is weird anyway. They've got a current source in one leg and a zener (for voltage / dissipation reduction?) in the other. Why not a current mirror?
The VAS is interesting in that it seems to be sort of a distortion canceling type, a bit like a Kuartlotron except not a buffer.
Most of those old amps were intended to be run class AB+B, with nothing in the outputs. Those QC triples were often unstable with the outputs biased, especially with the transistor types of the day. Low Rbe values were used to force the driver transistor to a high bias to keep crossover (and gm variation) distortion "acceptable", and to provide as low an output impedance at low levels as possibe given the other constraints. If refurbing with 15024's, you may be able to raise Rbe on the output stage, and give the outputs some bias.
The current source load on the input stage was likely to enforce differential balance without having to resort to matching transistors. If you use a current mirror they need to be matched, as well as the input pair needing to be matched. It was a bigger deal to do back then than it is now.
The current source load on the input stage was likely to enforce differential balance without having to resort to matching transistors. If you use a current mirror they need to be matched, as well as the input pair needing to be matched. It was a bigger deal to do back then than it is now.
That makes sense.
Also note how the first stage is run entirely off of base current of the second stage, another pecularity seen in 1970s-vintage amps. Simulated performance is not exciting but it kept power dissipation in check in the days when drivers were TO-18 at best.
I was suspecting it could be something like that. Getting these old power transistors fast enough would have required so much idle current that you'd have had a bit of a thermal / SOA problem.
Also note how the first stage is run entirely off of base current of the second stage, another pecularity seen in 1970s-vintage amps. Simulated performance is not exciting but it kept power dissipation in check in the days when drivers were TO-18 at best.
Thank you for all your help. I am just a novice learning what I can. I will be using MJ15024 in both channels. Currently just installed in one channel but I have them on hand to install. I will monitor the temps and see how it does upping the bias around 550mv/ 30ma
Set bias to 550mv @ 5.6ohm resistors/ 82ma. @ idle was getting 12.3mv Q10/11 base to base.
I don't see cooling being a problem at all. I've been playing it loud under 8ohm load and reading 100F/38C on the internal heat sink plate. not even breaking a sweat.
stock main filter caps are 9800uf 100v. If I was to change them 12000uf be a safe upgrade?
thanks everyone for your input. This is a cool amp. I'd like to clean it up and keep it.
I don't see cooling being a problem at all. I've been playing it loud under 8ohm load and reading 100F/38C on the internal heat sink plate. not even breaking a sweat.
stock main filter caps are 9800uf 100v. If I was to change them 12000uf be a safe upgrade?
thanks everyone for your input. This is a cool amp. I'd like to clean it up and keep it.
Last edited:
- Status
- Not open for further replies.