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05-05-2025, 12:51 PM
Hi Scott
The first thing to do is make sure the stock amplifier works and that the OT and PT are functioning properly, BUT...
The "really first thing" is to build a Power Limting Safety Socket for your test bench, unless you already have one. This uses incandescent lamps to restrict power to safe levels appropriate for each step of a test procedure and/or to accommodate different types of equipment to be tested.
Yes, a PA the circuit is designed to be used within its clean output range. We assume this is a 20W amp based on its model name? In any case, you need a load resistor that can briefly withstand the full output of the amp, but first, you do a test with an open load.
There are posts about how to do power tests in the Test and Measurement section of this forum, so I will not repeat the procedures here.
Assuming that the expensive parts of the amp are confirmed "good', the conversion to a guitar amp can begin.
The power amp is similar to many tweed-era PAs and will have a warm basic tone. As a guitar amp, you must install a 1k-5W screen-stop for each tube. If you want the tubes to survive continuous clipping and mismatched loads, this is mandatory as TUTs explain. The resistor should be elevated above the socket and tied to pin-4 and pin-6 with the wire from Vs tied to pin-6. You should add grid-stops, as well, right at the tube socket. These should be 1k5 to 10k 500mW hanging right off pin-5.
The next mod is to add cathode-current sense resistors and meter jacks for safe idle measurements. We offer this as BMK2 Bias Mod Kit for two tubes, or you can buy the parts from any broadline electronics distributor. TUT3 shows how this is wired.
To compliment the easy bias monitoring, you need easy bias adjustment. The stock bias supply has no adjustments, so you have to add them by replacing R38 (5k6) with a pair of parallel pots in series with a range resistor to ground, and also reduce the value of R21 (3k3). We typically use 25kB bias pots (panel pots mounted on the tube plane of the chassis) and a 4k7 or higher range resistor. R21 should be 1k or less.
Break the connection between the grid-stops R20,22 (100k) and feed each resistor from the wiper of its own bias pot.
The entirety of the above arrangement allows you to et the first tube by meter for safe operation and the second tube by ear for best hum-balance.
The plate filter cap is woefully undersized and should be 100uF minimum. It is the first point of filtering to reduce ripple and hum throughout the amplifier. To assure the lowest-noise and best note articulation, you should have TUT3 handy (The Ultimate Tone vol.3). This is the best guide to wiring and laying out amplifiers and every hobbyist, tech and amp builder should have it - most do and it has changed the nature of MI for the good
The preamp has three triodes available and this is enough gain to create any useful amount of distortion you may desire. However, this is the "Scott" amp and you are looking for your own tone. You need to define that goal and that will determine how the preamp is reconfigured. As it is now, there is only one gain stage between the input and the PA, and the PA has a low closed-loop gain.
You can open up the tone of the PA by increasing the values of R14 (1k) and R23 (5k6). The gain set by these is 6.6 overall, which is lower than even what fender uses. You could simply change R23 to some higher value, say 22k and have much more gain and an more dynamic tone from the PA. You could also simply lift one end of R23 to hear the open-loop sound and gain from the PA. If you like that sound, you could add a switch to open/close the loop, or add a pot to dial between 5k6 and nearly open-loop. The pot can be any value with higher values approaching the open-loop gain - anything over 100k will assure that.
If you change R15, you must also change R14 (100k) and these are the cathode-bias and plate load resistors, respectively.
These PA gain changes might let you assess how much gain you need from the preamp and are otherwise nice to explore.
The mic transformer sockets could be used as preamp tube sockets if you want to try out octal-based types. generally, they sound a bit mellower than the equivalent-mu subminiatures. As TUT suggested, you can achieve a similar sound using parallel triodes without gain loss, or use lower-mu tubes with the attendant gain reduction. If you plan to add tubes, you should add an auxiliary PT to support the extra tube heaters. You cannot count on the stock PT to support any more heater load than the stock tube set represents. If you ad this PT, you can bump its rating to accommodate the use of different power tubes, as well. Using alternate tube types will not change the power output but will change the tone and you can mix tube types.
Have fun
The first thing to do is make sure the stock amplifier works and that the OT and PT are functioning properly, BUT...
The "really first thing" is to build a Power Limting Safety Socket for your test bench, unless you already have one. This uses incandescent lamps to restrict power to safe levels appropriate for each step of a test procedure and/or to accommodate different types of equipment to be tested.
Yes, a PA the circuit is designed to be used within its clean output range. We assume this is a 20W amp based on its model name? In any case, you need a load resistor that can briefly withstand the full output of the amp, but first, you do a test with an open load.
There are posts about how to do power tests in the Test and Measurement section of this forum, so I will not repeat the procedures here.
Assuming that the expensive parts of the amp are confirmed "good', the conversion to a guitar amp can begin.
The power amp is similar to many tweed-era PAs and will have a warm basic tone. As a guitar amp, you must install a 1k-5W screen-stop for each tube. If you want the tubes to survive continuous clipping and mismatched loads, this is mandatory as TUTs explain. The resistor should be elevated above the socket and tied to pin-4 and pin-6 with the wire from Vs tied to pin-6. You should add grid-stops, as well, right at the tube socket. These should be 1k5 to 10k 500mW hanging right off pin-5.
The next mod is to add cathode-current sense resistors and meter jacks for safe idle measurements. We offer this as BMK2 Bias Mod Kit for two tubes, or you can buy the parts from any broadline electronics distributor. TUT3 shows how this is wired.
To compliment the easy bias monitoring, you need easy bias adjustment. The stock bias supply has no adjustments, so you have to add them by replacing R38 (5k6) with a pair of parallel pots in series with a range resistor to ground, and also reduce the value of R21 (3k3). We typically use 25kB bias pots (panel pots mounted on the tube plane of the chassis) and a 4k7 or higher range resistor. R21 should be 1k or less.
Break the connection between the grid-stops R20,22 (100k) and feed each resistor from the wiper of its own bias pot.
The entirety of the above arrangement allows you to et the first tube by meter for safe operation and the second tube by ear for best hum-balance.
The plate filter cap is woefully undersized and should be 100uF minimum. It is the first point of filtering to reduce ripple and hum throughout the amplifier. To assure the lowest-noise and best note articulation, you should have TUT3 handy (The Ultimate Tone vol.3). This is the best guide to wiring and laying out amplifiers and every hobbyist, tech and amp builder should have it - most do and it has changed the nature of MI for the good
The preamp has three triodes available and this is enough gain to create any useful amount of distortion you may desire. However, this is the "Scott" amp and you are looking for your own tone. You need to define that goal and that will determine how the preamp is reconfigured. As it is now, there is only one gain stage between the input and the PA, and the PA has a low closed-loop gain.
You can open up the tone of the PA by increasing the values of R14 (1k) and R23 (5k6). The gain set by these is 6.6 overall, which is lower than even what fender uses. You could simply change R23 to some higher value, say 22k and have much more gain and an more dynamic tone from the PA. You could also simply lift one end of R23 to hear the open-loop sound and gain from the PA. If you like that sound, you could add a switch to open/close the loop, or add a pot to dial between 5k6 and nearly open-loop. The pot can be any value with higher values approaching the open-loop gain - anything over 100k will assure that.
If you change R15, you must also change R14 (100k) and these are the cathode-bias and plate load resistors, respectively.
These PA gain changes might let you assess how much gain you need from the preamp and are otherwise nice to explore.
The mic transformer sockets could be used as preamp tube sockets if you want to try out octal-based types. generally, they sound a bit mellower than the equivalent-mu subminiatures. As TUT suggested, you can achieve a similar sound using parallel triodes without gain loss, or use lower-mu tubes with the attendant gain reduction. If you plan to add tubes, you should add an auxiliary PT to support the extra tube heaters. You cannot count on the stock PT to support any more heater load than the stock tube set represents. If you ad this PT, you can bump its rating to accommodate the use of different power tubes, as well. Using alternate tube types will not change the power output but will change the tone and you can mix tube types.
Have fun
