Hi All,
I'm helping a friend Refurbish and re-bias his Forte model 6 amp. The amp measured about +/- 67 VDC at the filter caps and the voltage drop at most emitter resistors I could reach was around 50 mV. The amp sounded pretty good and the heatsinks got warm but not too hot at all. Nice, detailed sound, but a little weak in the bass and a little crispy/crunchy in the highs.
A common path with these amps seems to be lowering the supply voltage and increasing the bias while remaining within the heat dissipation capabilities of the heat sinks to operate further into class A. This sounded enticing to my friend to try, so let's go for it!
Poking around the amp I discovered that the power transformer had some specs on the side that I could sort of make out. The red colored secondaries (the ones that were initially connected) were 48-0-48 VAC and I saw that there was also a set of unused green colored secondaries at 29.5-0.29.5 VAC. This seemed like an obvious choice so I hooked up the other pair of secondaries the same way as the original configuration and now I had about 41 VDC unloaded at the filter caps. Great, I think, so I go about setting the bias and set it up to about 75 mV on each channel at first to be conservative. I found that after the voltage changes, I now had some significant DC offset at the outputs so I used the other set of trimmers to zero this out (I assumed these trimmers were for DC offset and they did change it but I'm wondering if they are changing something else). Everything looked good and just letting the amp idle for some time, the heatsinks got hotter than the original configuration but not so hot that I couldn't permanently leave my hands on them.
I figured I was ready to fire the amp up on my current limiter and then listen to some music. No problems or shorts and now onto the music. I noticed something different right away in that I had to crank my preamp much higher to get the same amount of volume. Also the sound was Ok but not great or necessarily better than before. This is confusing because most people rag on the model 6 and praise the model 4. I've also heard my friend's model 6 converted to model 4 specs and thought it sounded incredible. On this amp, the sound was now less dynamic and more laid back than the original configuration and a touch smoother but it didn't grab me as that great in any way. Of course this was a different time, speakers, room, etc...so I'm aware of the limitations of psychoacoustic memory but I do trust my ears.
So, at this point, I'm a bit confused as everything seems to check out upon a cursory inspection as the voltage and bias are where I would expect them to be with the changes I made. Maybe there is a new issue with my preamp but that seems highly unlikely. By lowering the voltages by a little over 50%, is it expected that the input sensitivity would drop significantly? This doesn't make sense to me. Of course headroom and power output before clipping would drop but the volume that I was playing music at should have been nowhere near that. Is it possible i made some mistake I'm not aware of? Should I try with an even higher bias?
Lastly, I don't understand how these amps supposedly go from being 200 watt AB amps to 50 watt Class A amps. Where I'm at currently, I think I could maybe go up to at most 100 mV across the 0.47 ohm emitter resistors before the heatsinks were too hot so this should be about 200 mA per device, correct? If we assume that class A requires quiescent current and output current into the load to be equal, this bias level appears to be nowhere close to 50 WPC class A.
Thanks for any help!
I'm helping a friend Refurbish and re-bias his Forte model 6 amp. The amp measured about +/- 67 VDC at the filter caps and the voltage drop at most emitter resistors I could reach was around 50 mV. The amp sounded pretty good and the heatsinks got warm but not too hot at all. Nice, detailed sound, but a little weak in the bass and a little crispy/crunchy in the highs.
A common path with these amps seems to be lowering the supply voltage and increasing the bias while remaining within the heat dissipation capabilities of the heat sinks to operate further into class A. This sounded enticing to my friend to try, so let's go for it!
Poking around the amp I discovered that the power transformer had some specs on the side that I could sort of make out. The red colored secondaries (the ones that were initially connected) were 48-0-48 VAC and I saw that there was also a set of unused green colored secondaries at 29.5-0.29.5 VAC. This seemed like an obvious choice so I hooked up the other pair of secondaries the same way as the original configuration and now I had about 41 VDC unloaded at the filter caps. Great, I think, so I go about setting the bias and set it up to about 75 mV on each channel at first to be conservative. I found that after the voltage changes, I now had some significant DC offset at the outputs so I used the other set of trimmers to zero this out (I assumed these trimmers were for DC offset and they did change it but I'm wondering if they are changing something else). Everything looked good and just letting the amp idle for some time, the heatsinks got hotter than the original configuration but not so hot that I couldn't permanently leave my hands on them.
I figured I was ready to fire the amp up on my current limiter and then listen to some music. No problems or shorts and now onto the music. I noticed something different right away in that I had to crank my preamp much higher to get the same amount of volume. Also the sound was Ok but not great or necessarily better than before. This is confusing because most people rag on the model 6 and praise the model 4. I've also heard my friend's model 6 converted to model 4 specs and thought it sounded incredible. On this amp, the sound was now less dynamic and more laid back than the original configuration and a touch smoother but it didn't grab me as that great in any way. Of course this was a different time, speakers, room, etc...so I'm aware of the limitations of psychoacoustic memory but I do trust my ears.
So, at this point, I'm a bit confused as everything seems to check out upon a cursory inspection as the voltage and bias are where I would expect them to be with the changes I made. Maybe there is a new issue with my preamp but that seems highly unlikely. By lowering the voltages by a little over 50%, is it expected that the input sensitivity would drop significantly? This doesn't make sense to me. Of course headroom and power output before clipping would drop but the volume that I was playing music at should have been nowhere near that. Is it possible i made some mistake I'm not aware of? Should I try with an even higher bias?
Lastly, I don't understand how these amps supposedly go from being 200 watt AB amps to 50 watt Class A amps. Where I'm at currently, I think I could maybe go up to at most 100 mV across the 0.47 ohm emitter resistors before the heatsinks were too hot so this should be about 200 mA per device, correct? If we assume that class A requires quiescent current and output current into the load to be equal, this bias level appears to be nowhere close to 50 WPC class A.
Thanks for any help!
Since you're changing the available rail swing, total available output power would change; i can't see why the gain/sensitivity would change.... I "think" I remember seeing elsewhere on the site that Nelson Pass makes a comment or two about the bias settings regarding what you're doing. Did you search?
Thanks! Yes, I've done extensive searching on this. The change in input sensitivity is the big mystery. The amp biases as expected with +/- 41 VDC from the exchanged secondaries.
I will be further disassembling the amp to change the rectifiers, filter caps, etc...There is more writing on the PT about the different taps that I can't make out right now so maybe that will unlock some clues.
I can always return the amp to stock form and retest. The only wiring I changed was on the rectifier. I'm wondering if swapping something by mistake here could be the culprit. It seems if I made a mistake on this, more obvious problems would result.
Oh, I did undo all the grounds to the aluminum plate attached to the filter caps to get better access so I'll revisit this area as well.
I will be further disassembling the amp to change the rectifiers, filter caps, etc...There is more writing on the PT about the different taps that I can't make out right now so maybe that will unlock some clues.
I can always return the amp to stock form and retest. The only wiring I changed was on the rectifier. I'm wondering if swapping something by mistake here could be the culprit. It seems if I made a mistake on this, more obvious problems would result.
Oh, I did undo all the grounds to the aluminum plate attached to the filter caps to get better access so I'll revisit this area as well.