Billfort
Addicted Member
A lot has changed with my DIY 300B SET amps so I thought I would document things before I fundamentally change the layout & circuit.
Driver Stage
A friend who built this same amp has tried endless variations of single tube driver stages and the one I liked best was his 6SN7 version so I went in this direction. All the Solen plastic caps are gone now (accept on the 300B humpot), I used a Blackgate on the 2nd stage cathode and all Tantalum resistors. I eliminated the first stage cathode bypass cap to reduce gain, making for a better (lower apparent hum) match with my tube pre-amp. A balanced heater ground was also added in the quest to further reduced hum.
So far I’ve tried various 6SN7 (GE side getters, Sylvania green & yellow label chrome domes), a CBS-Hytron 5692 and Brimar CV1988. The 5692 was very nice but my favorite by far is the 1988.
300B
Changed out the ceramic wafer socket with fork pin clips for a NOS Amphenol piece with tube style clips. I probably just got some bad parts but the fork clips where never real tight and the tensioning clips started braking and falling off leading to loose connections and intermittent hum; the Amphenols work much better.
Have only tried a few 300B’s and I keep going back to the Western Electrics. Maybe not “flavor of the month” but I like em and I’m done looking. The driver changes & even coupling caps seem to make big differences in this amp’s balance and tone so I’ve concentrated on those areas to flavor to taste – a hell of a lot cheaper than messing around with 300B’s.
DC heat on the 300B. This is one thing I wanted to avoid but I’m resigned to the fact that I will never be happy with 5v AC heat in this circuit and layout with my 100db speakers. I am probably over-reacting, I hear way more hum in many other tube systems, but it bothers me every once in a while and that is once too often. I built a DC board consisting of Schottky diodes, C-R-C filtering (10,000uF caps) and adjusted R to get close to 5v on the heaters. My next change will be to add a choke and try C-L-C filtering. The hum bastard is finally dead but I think this change made the amps sound a touch drier which seems to have been nicely resolved by re-flavoring to-suit with driver tubes and coupling caps.
Coupling Caps
Before these changes, I liked MultiCap RTX or Auricaps best overall, even though the midrange with the Vitamin-Q PIO’s was stunning. I now prefer the Angela/AudioNote copper foil in oil caps by far, and don’t care for any of the plastic caps I have tried.
The Future
These amps are my first DIY electronics undertaking and seem to never be “done” as I keep changing them to learn and hopefully, improve. My next experiment with them will be a complete redesign of the circuit and chassis top plate. I want to try a 2 stage driver arrangement using a 76 indirectly heated triode first and ½ a 6SN7 (or a 6J5) second stage. The power supply will be beefier and use Cerafine caps and an additional choke. I’m hoping to get even better dynamics and bass control with these changes. I’m going to keep all the 120v AC and the rectifier in the back corner of the chassis away from the amp section and try different grounding layouts. I’ll again try to get AC heating to work on the 300B but I’ll prep the chassis for simple bolt-in of the DC board if (when) the hum gets to me again. The chassis top will again be stainless but the tubes will mount in rubber isolated sub assembly plates.
This is a pretty drastic change but it’s easy to redesign the layout with CAD and I have access to a sheet metal shop with an NC turret press. I’m going to wait till my 45 SET is done before embarking on this – my wife won’t let me “wreck” the amps anymore unless I have something else to listen to .
Billfort
Driver Stage
A friend who built this same amp has tried endless variations of single tube driver stages and the one I liked best was his 6SN7 version so I went in this direction. All the Solen plastic caps are gone now (accept on the 300B humpot), I used a Blackgate on the 2nd stage cathode and all Tantalum resistors. I eliminated the first stage cathode bypass cap to reduce gain, making for a better (lower apparent hum) match with my tube pre-amp. A balanced heater ground was also added in the quest to further reduced hum.
So far I’ve tried various 6SN7 (GE side getters, Sylvania green & yellow label chrome domes), a CBS-Hytron 5692 and Brimar CV1988. The 5692 was very nice but my favorite by far is the 1988.
300B
Changed out the ceramic wafer socket with fork pin clips for a NOS Amphenol piece with tube style clips. I probably just got some bad parts but the fork clips where never real tight and the tensioning clips started braking and falling off leading to loose connections and intermittent hum; the Amphenols work much better.
Have only tried a few 300B’s and I keep going back to the Western Electrics. Maybe not “flavor of the month” but I like em and I’m done looking. The driver changes & even coupling caps seem to make big differences in this amp’s balance and tone so I’ve concentrated on those areas to flavor to taste – a hell of a lot cheaper than messing around with 300B’s.
DC heat on the 300B. This is one thing I wanted to avoid but I’m resigned to the fact that I will never be happy with 5v AC heat in this circuit and layout with my 100db speakers. I am probably over-reacting, I hear way more hum in many other tube systems, but it bothers me every once in a while and that is once too often. I built a DC board consisting of Schottky diodes, C-R-C filtering (10,000uF caps) and adjusted R to get close to 5v on the heaters. My next change will be to add a choke and try C-L-C filtering. The hum bastard is finally dead but I think this change made the amps sound a touch drier which seems to have been nicely resolved by re-flavoring to-suit with driver tubes and coupling caps.
Coupling Caps
Before these changes, I liked MultiCap RTX or Auricaps best overall, even though the midrange with the Vitamin-Q PIO’s was stunning. I now prefer the Angela/AudioNote copper foil in oil caps by far, and don’t care for any of the plastic caps I have tried.
The Future
These amps are my first DIY electronics undertaking and seem to never be “done” as I keep changing them to learn and hopefully, improve. My next experiment with them will be a complete redesign of the circuit and chassis top plate. I want to try a 2 stage driver arrangement using a 76 indirectly heated triode first and ½ a 6SN7 (or a 6J5) second stage. The power supply will be beefier and use Cerafine caps and an additional choke. I’m hoping to get even better dynamics and bass control with these changes. I’m going to keep all the 120v AC and the rectifier in the back corner of the chassis away from the amp section and try different grounding layouts. I’ll again try to get AC heating to work on the 300B but I’ll prep the chassis for simple bolt-in of the DC board if (when) the hum gets to me again. The chassis top will again be stainless but the tubes will mount in rubber isolated sub assembly plates.
This is a pretty drastic change but it’s easy to redesign the layout with CAD and I have access to a sheet metal shop with an NC turret press. I’m going to wait till my 45 SET is done before embarking on this – my wife won’t let me “wreck” the amps anymore unless I have something else to listen to .
Billfort
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