Winston O Boogie was nice enough to send me a original choke for a V72. It needed a breakout lead soldered but other than that, it was fine. I had built a DIY V72 already with a homemade choke, so I decided it would be cool to see how good a job I did.
This is the choke on the second stage, EF804S, that feeds the cap which feeds the output transformer.
I am using a Triad HS-52 output.
Original choke: DCR = 11.76 K Ohms 800 Henries on a Gen Rad 1650-B set on the Ls setting
DIY Choke: DCR - 1.9 K Ohms 150 Henries on Gen Rad, set to Lp.
177 Henries on B-K precision.
Original choke had too many henries to measure on the B-K.
Here are the two chokes. The DIY version is wound on a Ni core, gapped 0.001 625EI Lams from Mag Met. The stock version is on a one piece M6 self gapped lam.
http://vacuumbrain.com/The_Lab/TA/Telefunken/V-72/v72_choke_final.jpg
http://vacuumbrain.com/The_Lab/TA/Telefunken/V-72/v72choke_complete.jpg
Here is a freq plot of the two chokes. I used a 50k resistor in series as a dummy load, and measured the ac volts across the choke at different frequencies. Not exactly the most scientific approach, but I was going to do a listening test also, so who cares.
Here is the graph. Original choke is green, DIY red:
http://vacuumbrain.com/The_Lab/TA/Telefunken/V-72/v72_choke_shootout.jpg
The original had 4 times the henries, which meant better bass rolloff, as expected. The DIY choke had a little better highend, but nothing to write home about. This could be expected from having less turns which means less capacitance.
Next was the listening test. I used a SM57 and an Audio Technica 4033 on the input and drove a headphone amp with some Sony Pros hooked up.
It was very close. Nothing bad about either of the chokes. I could not hear any difference withh my stock headphone amp, so I used my other headphone amp that has zero headroom, and zero output, but an extremely detailed sound.
With the second headphone amp and carefully listening, I noticed the guitar coming thru slightly better with the DIY choke and a little less boom on the bottom. The bass rolloff on the freq plot did not really show up that much on the listening tests. This is probably because the choke sits on the other side of the first mic stage, and thus is being fed a stiff enough signal to where the less henries really did not drag it down much.
A little more bass rolloff might not be a bad thing on a vocal mic. If I was going to mic the next door neighbor's sub wolfer in his 63 Impala, I might want to use the original choke. LEss low end also helps to minimize what I call "condenser pop", that sound you get when you jar a mic, which is probably the diaphram hitting the backplate.
So all in all, I am very happy with the DIY choke, which is good because it would be a pain in the rear to swap it out. The V-72 amp sounds absolutely unbelievable on even a Shure SM-57, perhaps better than any of my other pre's, which includes a Langevin, Melcor, VAlley People and Telefunken V76.
I would highly recomend the V-72 as a biginner's mic pre, as it is fairly insensitive to component values and types, has only two tubes in a two stage design, and will sound good no matter how bad you screw up.
Here is a diagram showing my V72-S (Beatles/EMI version) and the voltages using a slightly higher B+ supply. The two plate voltages on the second stage are shown, using both chokes. The lesser DCR on the DIY is responsible for the higher reading.
This is the choke on the second stage, EF804S, that feeds the cap which feeds the output transformer.
I am using a Triad HS-52 output.
Original choke: DCR = 11.76 K Ohms 800 Henries on a Gen Rad 1650-B set on the Ls setting
DIY Choke: DCR - 1.9 K Ohms 150 Henries on Gen Rad, set to Lp.
177 Henries on B-K precision.
Original choke had too many henries to measure on the B-K.
Here are the two chokes. The DIY version is wound on a Ni core, gapped 0.001 625EI Lams from Mag Met. The stock version is on a one piece M6 self gapped lam.
http://vacuumbrain.com/The_Lab/TA/Telefunken/V-72/v72_choke_final.jpg
http://vacuumbrain.com/The_Lab/TA/Telefunken/V-72/v72choke_complete.jpg
Here is a freq plot of the two chokes. I used a 50k resistor in series as a dummy load, and measured the ac volts across the choke at different frequencies. Not exactly the most scientific approach, but I was going to do a listening test also, so who cares.
Here is the graph. Original choke is green, DIY red:
http://vacuumbrain.com/The_Lab/TA/Telefunken/V-72/v72_choke_shootout.jpg
The original had 4 times the henries, which meant better bass rolloff, as expected. The DIY choke had a little better highend, but nothing to write home about. This could be expected from having less turns which means less capacitance.
Next was the listening test. I used a SM57 and an Audio Technica 4033 on the input and drove a headphone amp with some Sony Pros hooked up.
It was very close. Nothing bad about either of the chokes. I could not hear any difference withh my stock headphone amp, so I used my other headphone amp that has zero headroom, and zero output, but an extremely detailed sound.
With the second headphone amp and carefully listening, I noticed the guitar coming thru slightly better with the DIY choke and a little less boom on the bottom. The bass rolloff on the freq plot did not really show up that much on the listening tests. This is probably because the choke sits on the other side of the first mic stage, and thus is being fed a stiff enough signal to where the less henries really did not drag it down much.
A little more bass rolloff might not be a bad thing on a vocal mic. If I was going to mic the next door neighbor's sub wolfer in his 63 Impala, I might want to use the original choke. LEss low end also helps to minimize what I call "condenser pop", that sound you get when you jar a mic, which is probably the diaphram hitting the backplate.
So all in all, I am very happy with the DIY choke, which is good because it would be a pain in the rear to swap it out. The V-72 amp sounds absolutely unbelievable on even a Shure SM-57, perhaps better than any of my other pre's, which includes a Langevin, Melcor, VAlley People and Telefunken V76.
I would highly recomend the V-72 as a biginner's mic pre, as it is fairly insensitive to component values and types, has only two tubes in a two stage design, and will sound good no matter how bad you screw up.
Here is a diagram showing my V72-S (Beatles/EMI version) and the voltages using a slightly higher B+ supply. The two plate voltages on the second stage are shown, using both chokes. The lesser DCR on the DIY is responsible for the higher reading.
