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Topic: Modifying Standel Custom & Imperial hybrid amps to use 6L6's |
Michael Brebes
From:
Northridge CA
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Posted 1 Apr 2019 4:18 pm |
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As some of you know, all of the Standel Custom and Imperial hybrid amps use either 6GW6 or 6GT5 power tubes. The 6GW6 tubes have the same octal pinout as a 6L6 except that the plate is not on pin 3 but is connected via a cap with a wire going to the plate cap on the top of the tube. Lucky for us that the cap wire is attached to pin 3, the same pin used for the plate on the 6L6. The 6GT5 is an odd-duck tube, which is approximately as big around as the 6L6 but does not use an octal style tube socket but a large 9 pin socket similar to preamp tube sockets, which looks to be approximately the same size as the standard octal tube socket. It just so happens that the 6GT5 tube has the same characteristics as the 6GW6, which made it so Standel did not change the power section values at all when moving to the newer tube.
In the coming entries I will put in a new output transformer in Chassis #3 (which is spec'ed out to match the characteristics of the original output transformers), then get Normal channel operational. Upon verification/repair of the operation of the signal path to a speaker, I will then test my theory of how to modify the chassis to run off a pair of 6L6 power tubes. Because of the power supply and other designs, the amp should still put out its original 30-40 watts, but will now have the option of matched tubes and more readily accessible power tubes.
So, here we go!!
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Michael Brebes
Instrument/amp/ pickup repair
MSA D10 Classic/Rickenbacher B6/
Dickerson MOTS/Dobro D32 Hawaiian/
Goldtone Paul Beard Reso
Mesa Boogie Studio Pre/Hafler 3000
RP1/MPX100 |
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Jerry Roller
From:
Van Buren, Arkansas USA
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Michael Brebes
From:
Northridge CA
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Posted 9 Apr 2019 8:40 am
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Installed a 40 watt rated output transformer that I originally purchased to repair an Imperial hybrid (only to find that the transformer was too big for the Imperial chassis). The transformer with the same specs but in a size that would fit inside the chassis, and matched the size of the original, was only rated for 30 watts. That's when I first realized that these amps didn't put out as much as I originally thought. Because of the chassis construction of the Custom hybrid, it was easy to accommodate the larger size transformer.
As previously noted, there was one Blue module still attached but floating. No fuse or fuse cap so borrowed from the other repaired Custom for testing. As can be seen from the attached photo, the Blue module is actually passing a signal. This is the first working blue module I have encountered in the three Custom chassis. Now I need to make another green summing module and wire it in. The circuitry for the power amp section visually looks okay for the most part, except for possible wires in the phase inverter circuit.
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Michael Brebes
Instrument/amp/ pickup repair
MSA D10 Classic/Rickenbacher B6/
Dickerson MOTS/Dobro D32 Hawaiian/
Goldtone Paul Beard Reso
Mesa Boogie Studio Pre/Hafler 3000
RP1/MPX100 |
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Michael Brebes
From:
Northridge CA
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Posted 15 Apr 2019 2:56 pm
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The green summing module has been built and installed. The tone circuit capacitors have been installed. Even though it's too high of gain, I installed a 12AX7 to verify the operation of the phase inverter circuit. As can be seen in the picture, I am now getting the normal and inverted signals through the phase inverter. Next, I need to verify the wiring of the power tube circuits and installed the 6GW6 tubes, to test and make sure that everything is working all the way to the output of the output transformer.
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Michael Brebes
Instrument/amp/ pickup repair
MSA D10 Classic/Rickenbacher B6/
Dickerson MOTS/Dobro D32 Hawaiian/
Goldtone Paul Beard Reso
Mesa Boogie Studio Pre/Hafler 3000
RP1/MPX100 |
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Michael Brebes
From:
Northridge CA
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Posted 18 Apr 2019 3:13 pm
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Got the power amp section working. Scope is reading the output of the output transformer. With the 6GW6 tubes it measures about 30 watts RMS at approximately 1KHz into an 8 ohm resistor. Next is to modify it to run 6L6 tubes and see what happens.
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Michael Brebes
Instrument/amp/ pickup repair
MSA D10 Classic/Rickenbacher B6/
Dickerson MOTS/Dobro D32 Hawaiian/
Goldtone Paul Beard Reso
Mesa Boogie Studio Pre/Hafler 3000
RP1/MPX100 |
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Michael Brebes
From:
Northridge CA
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Posted 16 May 2019 10:08 am
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It's been a while. Got sidetracked with other things for a while. Installed a set of 6L6GC tubes and hot-wired the grid voltage to deliver what was needed to drive the tubes. Powered up and they are working in the amp!
The amp could be wired so that it could easily be switched between the 6GW6 and 6L6 tubes but I don't see any advantage to that. If the newer chassis, with the 6GT5 tubes, is converted over there is no easy conversion back, unless you wanted to go to the older 6GW6 tubes, since the sockets have to be swapped out to accommodate the standard octal base of the 6L6.
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Michael Brebes
Instrument/amp/ pickup repair
MSA D10 Classic/Rickenbacher B6/
Dickerson MOTS/Dobro D32 Hawaiian/
Goldtone Paul Beard Reso
Mesa Boogie Studio Pre/Hafler 3000
RP1/MPX100 |
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