A very long time coming, but I recently finished my Stereo Guitar Amp build. It will be evaluated at rehearsals for a while before using it on show dates.
The chassis is .080 aluminum with welded seams. 17 by 8 inch surface and 2 inch deep, plus bottom plate. No apparent flexing in spite of considerable weight of the iron.
All valve sockets are ceramic with gold plated pins. The layout was a challenge and calculated with a mock-up of the major components. Considerations made of transformers relative to one another and attempted to keep the first stage preamp as far away as possible.
All input and output jacks are isolated from the chassis. A star ground scheme was employed throughout. The main circulating currents of the power supply are kept within that loop and one tie to the star. It is a "common" power supply to each channel with appropriate decoupling.
A separate small transformer underneath, circuits isolated from chassis, feeds TO-220 case regulators for 12V fans and the 5V LED supplies.
Each channel is identical with three ECC83/12AX7 and PP EL34. The first stages are older Mullard, the others are older Sylvania. Outputs are Winged C SED. Currently I have "used tires" SED output valves for initial testing purposes. Saving the new (expensive) ones after I am satisfied with the amp's operation.
All iron is from Magnetic Components purchased from Triode. I had consulted with an engineer from Magnetic Components before ordering. His recommendation of the "upgrade" power transformer was appreciated since I was at the limits on heater current of the regular "100 watt" initial selection I made. Winged C EL34 has a slightly higher heater current requirement...
Each dual triode has a separate star connection, plus each output section likewise. The common of the output transformers has the negative feedback to the long tail and is star connected at that dual triode each side. I have separate output jacks for the 4, 8 and 16 ohm connections.
I initially tested the amp after finishing the power supply and one channel. I had the dreaded screech, and quickly realized I had the output plate leads inadvertently reversed. After reversing the leads all was well.
I was very pleased to find almost no gain hiss, no hum, noises or oscillations even with both gains wide open, no input jack connections.
Power out for each channel is right at 50 watts at threshold of clip into 8 ohm load resistors. Output plates at 490 volts idle, and according to the one ohm cathode resistors, each valve is currently running at about 30 or so mA, K current. O'Scope waveform looks nice, minimal crossover distortion at this bias setting.
The circuit is similar to the “not well liked” Marshall 4140, and since I will be feeding the inputs from a pedal board into some vintage rack mount devices, then to the amp - I left out the first stage “boost” cathode cap.
Overall, I am pretty happy how this turned out and looking forward to burn-in testing at rehearsals and eventual live shows.
Deric
The chassis is .080 aluminum with welded seams. 17 by 8 inch surface and 2 inch deep, plus bottom plate. No apparent flexing in spite of considerable weight of the iron.
All valve sockets are ceramic with gold plated pins. The layout was a challenge and calculated with a mock-up of the major components. Considerations made of transformers relative to one another and attempted to keep the first stage preamp as far away as possible.
All input and output jacks are isolated from the chassis. A star ground scheme was employed throughout. The main circulating currents of the power supply are kept within that loop and one tie to the star. It is a "common" power supply to each channel with appropriate decoupling.
A separate small transformer underneath, circuits isolated from chassis, feeds TO-220 case regulators for 12V fans and the 5V LED supplies.
Each channel is identical with three ECC83/12AX7 and PP EL34. The first stages are older Mullard, the others are older Sylvania. Outputs are Winged C SED. Currently I have "used tires" SED output valves for initial testing purposes. Saving the new (expensive) ones after I am satisfied with the amp's operation.
All iron is from Magnetic Components purchased from Triode. I had consulted with an engineer from Magnetic Components before ordering. His recommendation of the "upgrade" power transformer was appreciated since I was at the limits on heater current of the regular "100 watt" initial selection I made. Winged C EL34 has a slightly higher heater current requirement...
Each dual triode has a separate star connection, plus each output section likewise. The common of the output transformers has the negative feedback to the long tail and is star connected at that dual triode each side. I have separate output jacks for the 4, 8 and 16 ohm connections.
I initially tested the amp after finishing the power supply and one channel. I had the dreaded screech, and quickly realized I had the output plate leads inadvertently reversed. After reversing the leads all was well.
I was very pleased to find almost no gain hiss, no hum, noises or oscillations even with both gains wide open, no input jack connections.
Power out for each channel is right at 50 watts at threshold of clip into 8 ohm load resistors. Output plates at 490 volts idle, and according to the one ohm cathode resistors, each valve is currently running at about 30 or so mA, K current. O'Scope waveform looks nice, minimal crossover distortion at this bias setting.
The circuit is similar to the “not well liked” Marshall 4140, and since I will be feeding the inputs from a pedal board into some vintage rack mount devices, then to the amp - I left out the first stage “boost” cathode cap.
Overall, I am pretty happy how this turned out and looking forward to burn-in testing at rehearsals and eventual live shows.
Deric
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