Andre
Well-known member
Hey Everyone,
After some time I've finally wrapped up one channel of a tube preamp based on the 1567a schematic. Here are some pics and notes about the build...
So... What is it?
It will be a 2 channel tube preamp, with each channel closely resembling a complete 1567a mixer circuit, except that only 1 mic input is included for each of the parallel circuits. The other mic input channels have been replaced by a resistor which creates a voltage divider similar to the parallel resistance of the mixing resistors when all other channels are turned all the way down.
About the transformers:
-The input and output transformers are Cinemags. The input is wired 150R:50k and the output is 10k:600R. The output differs from the original Altec which is 15k on the primary.
-The power transformer is a custom Edcor # XPWR018 - Please feel free to use this design without having to pay a customization fee. This tranny was designed specifically for tube mic preamps. It has 120V primary and secondaries at 230-0-230 (50mA), 0-40 (100mA) for phantom, and 6.3-0-6.3 (2.5A) for use with 12.6 V heaters.
Power supplies:
There are 4 power supplies:
-A regulated phantom power supply provides 44VDC. It's breadboarded and mounted on the right side of the chassis.
-The main B+ power supply is next from the right and built on turret boards and resistors hanging off of the multi-can caps. This starts with 230-0-230 VAC through a full wave rectifier. It's filtered and then split down parallel paths to provide separate power for each of the channels. It's output is about 10V less than what's listed on the schematic.
- The filament power supply is a made from the Slow Start Heater circuit sold for the Poorman's 660 build. This is fed from a 12.6 VAC going through a bridge rectifier. After much trial, errors and more errors, I found that I had to run the heaters at 12V, which means the 6CG7 filaments are in series. As expected the voltage drops across the 6CG7s are not perfectly even, but this shouldn't cause an issue.
D.I.
Direct injection is done with a switched phone jack that cuts out the mic input transformer and feeds the instrument straight to the tube. There is no 600ohm line input.
Sound.
I haven't been able to put it through much more than a couple of quick scratch tracks, but I'd describe it as glassy and effortlessly full.
Noise and hum:
There is a -84 dbfs (using Reaper's analyzer) hum that appears at 60 Hz and odd multiples of 60. The hum moves to even multiples if I remove the second tube, which makes me believe the hum is due to power supply ripple. If it bugs me enough I might trace it out and increase B+ filtering. The unit doesn't have any issues with buzz or noise.
Gain:
Too much. The original unit spec sheet indicates 92 dB gain. I'm thinking about changing the second gain pot to a resistor and pot in series so that I can use more of the pots rotation.
That's enough for now. Hope you enjoy it.
- Andre
After some time I've finally wrapped up one channel of a tube preamp based on the 1567a schematic. Here are some pics and notes about the build...
So... What is it?
It will be a 2 channel tube preamp, with each channel closely resembling a complete 1567a mixer circuit, except that only 1 mic input is included for each of the parallel circuits. The other mic input channels have been replaced by a resistor which creates a voltage divider similar to the parallel resistance of the mixing resistors when all other channels are turned all the way down.
About the transformers:
-The input and output transformers are Cinemags. The input is wired 150R:50k and the output is 10k:600R. The output differs from the original Altec which is 15k on the primary.
-The power transformer is a custom Edcor # XPWR018 - Please feel free to use this design without having to pay a customization fee. This tranny was designed specifically for tube mic preamps. It has 120V primary and secondaries at 230-0-230 (50mA), 0-40 (100mA) for phantom, and 6.3-0-6.3 (2.5A) for use with 12.6 V heaters.
Power supplies:
There are 4 power supplies:
-A regulated phantom power supply provides 44VDC. It's breadboarded and mounted on the right side of the chassis.
-The main B+ power supply is next from the right and built on turret boards and resistors hanging off of the multi-can caps. This starts with 230-0-230 VAC through a full wave rectifier. It's filtered and then split down parallel paths to provide separate power for each of the channels. It's output is about 10V less than what's listed on the schematic.
- The filament power supply is a made from the Slow Start Heater circuit sold for the Poorman's 660 build. This is fed from a 12.6 VAC going through a bridge rectifier. After much trial, errors and more errors, I found that I had to run the heaters at 12V, which means the 6CG7 filaments are in series. As expected the voltage drops across the 6CG7s are not perfectly even, but this shouldn't cause an issue.
D.I.
Direct injection is done with a switched phone jack that cuts out the mic input transformer and feeds the instrument straight to the tube. There is no 600ohm line input.
Sound.
I haven't been able to put it through much more than a couple of quick scratch tracks, but I'd describe it as glassy and effortlessly full.
Noise and hum:
There is a -84 dbfs (using Reaper's analyzer) hum that appears at 60 Hz and odd multiples of 60. The hum moves to even multiples if I remove the second tube, which makes me believe the hum is due to power supply ripple. If it bugs me enough I might trace it out and increase B+ filtering. The unit doesn't have any issues with buzz or noise.
Gain:
Too much. The original unit spec sheet indicates 92 dB gain. I'm thinking about changing the second gain pot to a resistor and pot in series so that I can use more of the pots rotation.
That's enough for now. Hope you enjoy it.
- Andre