this is a DIY LA2a that i built a while back, using the late, great, David Jahnsen's book, full props going out to this DIY pioneer, he was way ahead of us at the time,
anyway, i built an LA2a just like the book, then slowly tweaked it,
the last tweak was using 6SL7 fot the first preamp tube, a 6SN7 for the 12BH7a cathode follower, then later on to go full octal, a 6V6gt for the 6AQ5, and a 6SL7 to drive the 6V6 instead of the 12AX7a,
results were lowered gain, better sound, more control over the compression, and easier sockets to wire.
nowadays, 6SN7 and 6SL7 are kind of expensive, so a DIY cheapo trick is to order 12SN7's instead, i paid about 8 bucks compared to 25 at antique el3ctronics, and they sound about the same, you can buy us stock instead of russian at those prices,
ok, latest mod was forgetting about the old school tag strips on the back and wiring I/O to the front panel, but wait, that's not all, we also installed a bypass switch so you can hear the dry signal, then flick the switch and hear the line amp, this allows you to compare what the line amp is doing to the dry signal, and also allows level comparison between the mic pre and the compressor output level, same idea as total bypass on the Pultec,
you can use a single pole double throw, connect the output jack hot to the center of the switch, then one side gets the output transformer hot, and the other side gets the input jack hot, when the switch is set to bypass, you will still be connected across the input transformer primary, but this should not make much difference as if it were out of circuit.
i also am using a 600:600 input trans, did not think it would work going into a control grid, lost a little bass somehow, but we fixed that ( see below)
so we get to save money on the input, and we also get a much needed 10:1 gain reduction, and no high end loss.
in fact, with the added high end response, i added a 25uf across the second stage cathode resistor which makes the compressor sound more like the original signal as far as freq response,
do not add the cap to the first stage cathode as it has neg feedback, AC style, which will get shunted to ground and then your gain goes up and the reduction circuit can not keep up so you will not be able to compress as much.
that is a DIY T4b in the round can with the cool pull tab, and as always, no power trans allowed on the chassis, and DC for the heaters, Jensen 4:1 output, they only make one model, so easy to order,
anyway, i built an LA2a just like the book, then slowly tweaked it,
the last tweak was using 6SL7 fot the first preamp tube, a 6SN7 for the 12BH7a cathode follower, then later on to go full octal, a 6V6gt for the 6AQ5, and a 6SL7 to drive the 6V6 instead of the 12AX7a,
results were lowered gain, better sound, more control over the compression, and easier sockets to wire.
nowadays, 6SN7 and 6SL7 are kind of expensive, so a DIY cheapo trick is to order 12SN7's instead, i paid about 8 bucks compared to 25 at antique el3ctronics, and they sound about the same, you can buy us stock instead of russian at those prices,
ok, latest mod was forgetting about the old school tag strips on the back and wiring I/O to the front panel, but wait, that's not all, we also installed a bypass switch so you can hear the dry signal, then flick the switch and hear the line amp, this allows you to compare what the line amp is doing to the dry signal, and also allows level comparison between the mic pre and the compressor output level, same idea as total bypass on the Pultec,
you can use a single pole double throw, connect the output jack hot to the center of the switch, then one side gets the output transformer hot, and the other side gets the input jack hot, when the switch is set to bypass, you will still be connected across the input transformer primary, but this should not make much difference as if it were out of circuit.
i also am using a 600:600 input trans, did not think it would work going into a control grid, lost a little bass somehow, but we fixed that ( see below)
so we get to save money on the input, and we also get a much needed 10:1 gain reduction, and no high end loss.
in fact, with the added high end response, i added a 25uf across the second stage cathode resistor which makes the compressor sound more like the original signal as far as freq response,
do not add the cap to the first stage cathode as it has neg feedback, AC style, which will get shunted to ground and then your gain goes up and the reduction circuit can not keep up so you will not be able to compress as much.
that is a DIY T4b in the round can with the cool pull tab, and as always, no power trans allowed on the chassis, and DC for the heaters, Jensen 4:1 output, they only make one model, so easy to order,
