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I had a really cool experience repairing this amp in which the customer put me in touch with Pat Quilter (QSC founder/owner) himself. I wanted to share his thoughts on this amp in case anyone ever comes across one. I pieced together a bunch of emails between us with his thoughts and suggestions on the amp. I ended up keeping the original transistors and changing a bunch of caps and carbon comps. After re-biasing the transistors to get the 12V he mentions, the gain came right back and sounds pretty nice for a good rock/bluesy tone. I read that this may be the first amp to ever have a master volume, he called it "output power". Enjoy and by the way this one ended up being serial number 133.
This was our first “reasonably nice” amp head, with a proper low noise, 1 meg input impedance, reasonably active EQ, and “fairly warm sounding for solid state”. Although my schematic notes a nominal 12V as the idle voltage on the drain or collector, there is some argument for a somewhat lower voltage for maximum voltage swing when driving the loading of the next stage. A pull-up resistor can only pull up so far into a load, whereas the drain/collector can pull down pretty close to zero. You could scope the voltage swing and dial in for symmetrical pos and neg peaks at clipping. You’re probably pretty close. The replacement FET (J113) presumably has a slightly different gate threshold voltage so it’s not surprising that re-biasing might be called for, although it may end up closer to 1.8K after conducting the above test. Of course, an un-bypassed source resistor provides negative feedback which reduces gain; hence the 10uF bypass cap. I would personally replace any drifted carbon comp resistors with suitable 1/4W-1/2W metal film (small signal parts) and 2W metal oxide (3.9 ohm bias resistors). Carbon comp resistors are “dreadful things” that are noisy and drift prone but it is what we had back then. The two preamp NPN transistors 40407 should be readily replaceable with general purpose 2N3906 or 2N2222. (be sure to watch for altered lead pattern). They are used in feedback circuits so the gains will remain equal or maybe higher, and bias points should self-regulate. If they are metal-case they are probably still OK. Hopefully the metal-case power transistors (TO-66 driver, TO-3 outputs) are OK since the amp passes signal. The two FETs are the most likely to deteriorate. You need depletion mode FET’s that use “cathode bias” (much like a triode tube). The part I found is this type (LND150) , and probably has more gain than originally. The input section should accept the better part of a 1Vpk signal before clipping. If necessary to trim gain, add some series resistance with the 10uF bypass cap to reduce gain to about 10X. The second FET after the EQ section needs enough “make up gain” to compensate for EQ attenuation without premature clipping. Extra gain here should not be an issue per se (the amp will just have higher gain). This overall EQ section has a fairly active cut-boost action on half-Gain, but as the gain pot maxes out, the action will tend to be cut-only. Some amps were customized to voice the EQ to customer preference, or for bass playing. Normally, there should be plenty of overdrive with the gain on full. If you set up some mild clipping in the preamp, increasing the “Output Power” control (Master Volume) should raise this signal up to the full output of the power amp, with just a little extra clipping, on full “Power”. The amp should not do excessive “squirming” or “choking” when played on high gain; it should have well behaved, somewhat soft clipping.
I find it fairly typical that our 50-year-old amps don’t quite reach their 120W rating at two ohms, although with speakers, the impedance peaks will allow for extra voltage swing. The four-ohm output should clip solidly at about 65-70W (with Power on full). Looking at this objectively, the original amp had pretty smooth overdrive, decent EQ, and a fair amount of warmth for solid-state, but is not nearly as vibrant as our current Quilter products. It looks like the AC cord is now a 3-wire type, which would negate the need for the big orange-drop “ground reverse” cap; which I copied from vintage Fender practice; although 600V provides a reasonable safety margin, current UL requirements call for at least 1500V rating for line-to-ground capacitors (so called “Y-caps”). Snip it out. I think we started serializing these models at 100, so you can estimate where this falls in the roughly 120 that we made. It’s nice that this one has the rarely-preserved original brick-red grille cloth we used in an attempt to be somewhat distinctive. There should be a black-painted perf-metal screen over the back to protect the heat sinks from external shorts, since they are electrically “live”. Peak voltage is 55V so it’s not really a shock hazard but a short to the upper heat sinks could damage the transistors. Hopefully you also have the hand-formed aluminum shield covering the preamp board.
--The 1 watt carbon comp bias resistors in the power section (3.9 ohms?, located on terminal strips on the power transistor panel) often drift over the years and cause the amp to either run hot or operate somewhat off-center. They may be replaced by modern metal-oxide 1W or 2W resistors. Match the value shown by the resistor stripes (either 3.9 or 4.7 ohms if memory serves).
--The power supply is a nominal 55V and therefore, the feed to the speaker DC blocking capacitor should be half of that give or take a volt or so. (27.5V). If you see this voltage and the heat sinks don’t get very warm at idle, it’s probably OK.
--The filter caps seem to hold up OK so far. (I hade to replace the physically smaller caps)
--If the power amp is doing its job, it should produce about 70W into 4 ohms, and 100-120W into 2 ohms, at about 5% clipping (slight visible flat-topping). This is of course at full Master, and clipping will occur at lower powers as you reduce Master (which we also called “Power”).
--Your lack of gain (while still passing signal) is most likely due to aging of the FET input transistors due to imperfect sealing of the early type plastic cases, which I have noticed over the years. These were JFET type parts, rather hard to get these days. I enclose a link and image below of a possible replacement but I have not tried it (LND150).
You should be able to get good solid overdrive with a Gibson type guitar, and still pretty good with Fender type pickups.
455 Schem 1970.pdf
This was our first “reasonably nice” amp head, with a proper low noise, 1 meg input impedance, reasonably active EQ, and “fairly warm sounding for solid state”. Although my schematic notes a nominal 12V as the idle voltage on the drain or collector, there is some argument for a somewhat lower voltage for maximum voltage swing when driving the loading of the next stage. A pull-up resistor can only pull up so far into a load, whereas the drain/collector can pull down pretty close to zero. You could scope the voltage swing and dial in for symmetrical pos and neg peaks at clipping. You’re probably pretty close. The replacement FET (J113) presumably has a slightly different gate threshold voltage so it’s not surprising that re-biasing might be called for, although it may end up closer to 1.8K after conducting the above test. Of course, an un-bypassed source resistor provides negative feedback which reduces gain; hence the 10uF bypass cap. I would personally replace any drifted carbon comp resistors with suitable 1/4W-1/2W metal film (small signal parts) and 2W metal oxide (3.9 ohm bias resistors). Carbon comp resistors are “dreadful things” that are noisy and drift prone but it is what we had back then. The two preamp NPN transistors 40407 should be readily replaceable with general purpose 2N3906 or 2N2222. (be sure to watch for altered lead pattern). They are used in feedback circuits so the gains will remain equal or maybe higher, and bias points should self-regulate. If they are metal-case they are probably still OK. Hopefully the metal-case power transistors (TO-66 driver, TO-3 outputs) are OK since the amp passes signal. The two FETs are the most likely to deteriorate. You need depletion mode FET’s that use “cathode bias” (much like a triode tube). The part I found is this type (LND150) , and probably has more gain than originally. The input section should accept the better part of a 1Vpk signal before clipping. If necessary to trim gain, add some series resistance with the 10uF bypass cap to reduce gain to about 10X. The second FET after the EQ section needs enough “make up gain” to compensate for EQ attenuation without premature clipping. Extra gain here should not be an issue per se (the amp will just have higher gain). This overall EQ section has a fairly active cut-boost action on half-Gain, but as the gain pot maxes out, the action will tend to be cut-only. Some amps were customized to voice the EQ to customer preference, or for bass playing. Normally, there should be plenty of overdrive with the gain on full. If you set up some mild clipping in the preamp, increasing the “Output Power” control (Master Volume) should raise this signal up to the full output of the power amp, with just a little extra clipping, on full “Power”. The amp should not do excessive “squirming” or “choking” when played on high gain; it should have well behaved, somewhat soft clipping.
I find it fairly typical that our 50-year-old amps don’t quite reach their 120W rating at two ohms, although with speakers, the impedance peaks will allow for extra voltage swing. The four-ohm output should clip solidly at about 65-70W (with Power on full). Looking at this objectively, the original amp had pretty smooth overdrive, decent EQ, and a fair amount of warmth for solid-state, but is not nearly as vibrant as our current Quilter products. It looks like the AC cord is now a 3-wire type, which would negate the need for the big orange-drop “ground reverse” cap; which I copied from vintage Fender practice; although 600V provides a reasonable safety margin, current UL requirements call for at least 1500V rating for line-to-ground capacitors (so called “Y-caps”). Snip it out. I think we started serializing these models at 100, so you can estimate where this falls in the roughly 120 that we made. It’s nice that this one has the rarely-preserved original brick-red grille cloth we used in an attempt to be somewhat distinctive. There should be a black-painted perf-metal screen over the back to protect the heat sinks from external shorts, since they are electrically “live”. Peak voltage is 55V so it’s not really a shock hazard but a short to the upper heat sinks could damage the transistors. Hopefully you also have the hand-formed aluminum shield covering the preamp board.
--The 1 watt carbon comp bias resistors in the power section (3.9 ohms?, located on terminal strips on the power transistor panel) often drift over the years and cause the amp to either run hot or operate somewhat off-center. They may be replaced by modern metal-oxide 1W or 2W resistors. Match the value shown by the resistor stripes (either 3.9 or 4.7 ohms if memory serves).
--The power supply is a nominal 55V and therefore, the feed to the speaker DC blocking capacitor should be half of that give or take a volt or so. (27.5V). If you see this voltage and the heat sinks don’t get very warm at idle, it’s probably OK.
--The filter caps seem to hold up OK so far. (I hade to replace the physically smaller caps)
--If the power amp is doing its job, it should produce about 70W into 4 ohms, and 100-120W into 2 ohms, at about 5% clipping (slight visible flat-topping). This is of course at full Master, and clipping will occur at lower powers as you reduce Master (which we also called “Power”).
--Your lack of gain (while still passing signal) is most likely due to aging of the FET input transistors due to imperfect sealing of the early type plastic cases, which I have noticed over the years. These were JFET type parts, rather hard to get these days. I enclose a link and image below of a possible replacement but I have not tried it (LND150).
You should be able to get good solid overdrive with a Gibson type guitar, and still pretty good with Fender type pickups.
455 Schem 1970.pdf
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