Tweet
This is my first thorough "conversion" project, and I thought I would share my end results with you guys. I bought this Univox U-75 2x10 combo a few months ago. It was manufactured in 1965, featuring 2 1965 Jensen C10S ceramic speakers, 2-6AQ5's at 12 watts, volume, tone and tremolo, 3 inputs, and an original circuit similar to a 5E3. I owned one a few years ago, and remember being a nice amp tonally. The original setup lended itself nicely towards a 5F11 circuit, which was my original motivation. However, I decided to go even more old school and experimenting with the 5D3 circuit at the expense of the tremolo.
Unfortuantely, this amp was a mess when I got it. Cosmetically it was ok, with the exception of a detached baffle and very loose grille cloth. However, it was hurtin' underneath the hood. As I was investigating, I noticed all components were original, which would explain some deterioration. It also seemed that someone had gotten to it before me and made matters worse, as it seemed some components had been rerouted and some wires misguided. Nearly all of the carbon comp resistors had drifted out of spec, coupling caps were leaky and cracking. The electrolytics were crusty and very leaky. There were frayed leads and some obvious patchwork. Tubes were original, but they were also very weak. The 6X4 blew immediately. The chassis was overall really dirty, and I realized I had a lot of work ahead of me.
After all the parts from Watts Tube Audio arrived, I went to work on the power supply. I immediately replaced the original 2 prong power cord with a 3 prong, safety being the primary reason. I then replaced all electrolytics in the power supply with 3x16uF Sprague Atom's, along with 2W metal oxide dropping resistors. The original cathode bias resistor was 5W/300 ohm, and I replaced with a 10W/270 ohm bypassed by a 25uF-50V Sprague Premium electroytic. At this point I also modified the grounding to a more conventional scheme.
Up next was the phase inverter. I was very excited to install the self-balancing phase inverter, as I had never worked on nor heard an amp with this type of circuit. I replaced the coupling caps with Sozos, and replaced all resistors with carbon film. One thing I appreciated was that the original turret board allowed for a nice layout, even though a little creativity was needed.
I then rewired the preamp. Again with carbon film resistors, but I used a Sprague 716P coupling cap instead. Volume/Tone circuit features CTS pots, Sozo and generic silver mica tone caps. I purchased a set of cream vintage pointer knobs for the pots. Shielded cable was used throughout the amp where needed.
At this point the amp was looking much better, but I had to do something with the tubes. The lineup features a 6X4, 2x6AQ5s, 2X12AX7. The original 5D3 schematic calls for a a 12AY7 in V1, but I had a new EH 12AX7 on hand that I opted for in the meantime. I replaced the original 6X4 with a NOS RCA, and I replaced the original 6AQ5s with a pair of NOS JAN Sylvanias in excellent condition. The preamp/PI tube is a new Mesa Boogie(JJ relabeld) 12AX7. Amp now has a fresh set.
I was able to salvage some original parts. The input jacks, tremolo jack and On/Off-Standby switch are Switchcraft, and they were in perfect working order. Plus they have aged naturally, giving it some character. I also kept the original indicator lamp. The transformers both checked out okay, and the OT actually sounds really nice. The tube sockets were also in great shape to my surprise. I also used a lot of the original wires. The original Jensen speakers were in perfect shape, and they both sound really nice, slightly dark tone. However, I replaced one with a 1957 Jensen 10J11 alnico speaker. VERY nice tone. Brighter than the C10S, with a little more presence and detail. The combination is a very nice match.
Overall I am very satisfied with the end result of this amp. I logged maybe 5-6 hours on this amp, as I was trying to be attention to detail. The tone from this amp is what you would expect from a great 50s tweed amp. Sweet mids, round low end and smooth top end. I love the transparency when its lightly overdriven. Also has great cleans and can rip when cranked. And I no longer have to worry about its stability. Hope you enjoy the pics
Note: I decided to keep the tremolo circuit within the amp for originality sake. It is disconnected, but would be easy to reinstall given a few changes were made
Unfortuantely, this amp was a mess when I got it. Cosmetically it was ok, with the exception of a detached baffle and very loose grille cloth. However, it was hurtin' underneath the hood. As I was investigating, I noticed all components were original, which would explain some deterioration. It also seemed that someone had gotten to it before me and made matters worse, as it seemed some components had been rerouted and some wires misguided. Nearly all of the carbon comp resistors had drifted out of spec, coupling caps were leaky and cracking. The electrolytics were crusty and very leaky. There were frayed leads and some obvious patchwork. Tubes were original, but they were also very weak. The 6X4 blew immediately. The chassis was overall really dirty, and I realized I had a lot of work ahead of me.
After all the parts from Watts Tube Audio arrived, I went to work on the power supply. I immediately replaced the original 2 prong power cord with a 3 prong, safety being the primary reason. I then replaced all electrolytics in the power supply with 3x16uF Sprague Atom's, along with 2W metal oxide dropping resistors. The original cathode bias resistor was 5W/300 ohm, and I replaced with a 10W/270 ohm bypassed by a 25uF-50V Sprague Premium electroytic. At this point I also modified the grounding to a more conventional scheme.
Up next was the phase inverter. I was very excited to install the self-balancing phase inverter, as I had never worked on nor heard an amp with this type of circuit. I replaced the coupling caps with Sozos, and replaced all resistors with carbon film. One thing I appreciated was that the original turret board allowed for a nice layout, even though a little creativity was needed.
I then rewired the preamp. Again with carbon film resistors, but I used a Sprague 716P coupling cap instead. Volume/Tone circuit features CTS pots, Sozo and generic silver mica tone caps. I purchased a set of cream vintage pointer knobs for the pots. Shielded cable was used throughout the amp where needed.
At this point the amp was looking much better, but I had to do something with the tubes. The lineup features a 6X4, 2x6AQ5s, 2X12AX7. The original 5D3 schematic calls for a a 12AY7 in V1, but I had a new EH 12AX7 on hand that I opted for in the meantime. I replaced the original 6X4 with a NOS RCA, and I replaced the original 6AQ5s with a pair of NOS JAN Sylvanias in excellent condition. The preamp/PI tube is a new Mesa Boogie(JJ relabeld) 12AX7. Amp now has a fresh set.
I was able to salvage some original parts. The input jacks, tremolo jack and On/Off-Standby switch are Switchcraft, and they were in perfect working order. Plus they have aged naturally, giving it some character. I also kept the original indicator lamp. The transformers both checked out okay, and the OT actually sounds really nice. The tube sockets were also in great shape to my surprise. I also used a lot of the original wires. The original Jensen speakers were in perfect shape, and they both sound really nice, slightly dark tone. However, I replaced one with a 1957 Jensen 10J11 alnico speaker. VERY nice tone. Brighter than the C10S, with a little more presence and detail. The combination is a very nice match.
Overall I am very satisfied with the end result of this amp. I logged maybe 5-6 hours on this amp, as I was trying to be attention to detail. The tone from this amp is what you would expect from a great 50s tweed amp. Sweet mids, round low end and smooth top end. I love the transparency when its lightly overdriven. Also has great cleans and can rip when cranked. And I no longer have to worry about its stability. Hope you enjoy the pics
Note: I decided to keep the tremolo circuit within the amp for originality sake. It is disconnected, but would be easy to reinstall given a few changes were made
Attached Files
Comment