Dynaco MKIII Full Rebuild + VTA 6SN7 Driver Board Upgrade & Testing

Video Description

Follow along as I rebuild a pair of ailing Dynaco MKIII KT88 tube amplifiers, upgrading them with 6SN7 driver boards from VTA. https://patreon.com/novaluxstereophonic Become a Novalux Stereophonic Member Today! https://www.youtube.com/novaluxstereophonic/join Products Featured in This Video: https://amzn.to/4mN3LqY In this video, I take on a full rebuild of two classic Dynaco MKIII monoblock tube amps. This set arrived on my bench in rough shape, with an aftermarket 12AX7 based driver board that could barely deliver 1 watt of clean output before severe distortion. To restore proper performance, I completely strip and rebuild both amps: * I rebuild the power supply with fresh parts * Install VTA 6SN7 octal driver boards from Tubes4HiFi.com * Add an NTC thermistor to reduce filament inrush current * Replace RCA inputs, switches * Add bias test points Once rebuilt, both MKIII’s were bench tested for distortion and max clipping power into an 8-ohm dummy load. While the performance didn’t fully meet original Dynaco spec (similar to the VTA 12AU7 ST-70 I tested previously), the listening tests proved that they still sound excellent in real-world use. Chapters: 00:00 Introduction 00:51 Initial Inspection 04:01 Initial Bench Testing 08:58 Testing the Output Stage w/ an External Balanced Signal 10:23 Tubes4Hifi.com & Dynakitparts.com Replacement Parts 12:02 Thoughts on Replacing the Original Multi-Section Capacitors w/ an Authenticap 15:27 Gutting the Chassis 16:22 Installing the Authenticap & Yellow Sheet Mod 20:14 Output Tube Socket Wiring & Cathode Current Sense Resistors 20:37 Front Panel Parts Installation & Negative Bias Supply 22:33 Populating the VTA 6SN7 Driver Boards 42:14 Addressing Some Mistakes - Multi-Section Cap Ground, Bias Sense, Filament Wiring 44:35 Mounting the 6SN7 Driver Board 46:09 Wiring in the 6SN7 Driver Board 48:22 Wiring in the RCA Jack & Bias Test Points 49:03 Grid Stopper Resistors 49:44 First Look & Initial Power Supply Verification 53:49 Loading in the Tubes 54:21 First Test with Tubes Installed 58:17 Diagnosing Issues with the Output Signal 1:01:29 Testing the MKIII After Fixing the Wiring Mistake 1:02:48 Investigating the 12AX7 Board One More Time 1:03:37 NTC Considerations for Inrush Current Limitation 1:04:49 Second Chassis Gut & Rebuilt Time Lapse 1:05:42 Adjusting the Placement of the NTC Thermistor 1:07:00 Testing the Second Amplifier 1:07:31 Testing Both Amplifiers Simultaneously 1:09:43 Bias Sense Wiring Issues 1:12:10 70mA vs at 50mA Bias Distortion Differences 1:13:00 Correcting the Bias Sense Wiring, Uncovering a New Issue 1:14:12 Final Bias Adjustment Before Listening Tests 1:15:11 Listening Test Results 1:16:22 Transformer Heat Concerns Due to Increased Filament Current 1:17:13 Project Takeaways... 1:19:25 Outro