chrispbass
Well-known member
--Todays episode is bought to you in conjunction with UTM Industry--
You are live in the lab with Chris P Bass (well kind of...just go with it!)
So the output transformer has arrived, and very nicely made it is. Weighty with a good quality feel about it...thank you UTM
So last time, I was calculating plate currents, and this was really in regard to cross referencing the schematic and for me to calculate dropper resistors as I've gone off piste with regards to the PSU.
V3 = 1.725mA
V4 = 8.65mA
V1/V2 = 3.95mA
So adding those up and doubling them we get about 28.6mA
The meter will draw a further 1mA plus a smidge for the detector circuit giving say 29.6 or about 30mA.
Looking at the schematic for the PSU, Z measured a voltage drop of (259.8 - 245) across the 500 ohm resistor giving 14.8/500 = 29.6mA which is pretty darn good!!
I decided to build the psu on a separate board just so I'd got plenty of room to allow for testing etc.
Ive got 3 stages of filtering on my PSU and running regulated DC heaters. The block connectors bought off the pcb were handy to help dial in the voltages.
Initially, i just connected up the heaters to test these first, and one of the 6SS7 didn't seem to be getting warm. Inserting a 0.1 ohm resistor in the supply and measuring the voltage drop confirmed this. We have a duffer! I've got a LM338 regulator and with the heat sink, its barely getting warm. The bridge rectifier is getting a bit hot. Its rated for 4A, but may use something beefier when it comes to building the unit properly.
Initial B supply of 245V is on the pcb going to the block connector on the left to get the B supply for the 6SS7 and also the 24V for the detector. I used a trimmer rather than the 22K resistor, as I didnt have one to hand, but also helps with dialing that in.
I'm a smidge off, but more than close enough with my hotch potch of dropper resisitors, and happy to report that my measured voltages pretty much tally with Z's measured ones, so anyone planning on building this can be confident and go with what Z has on the schematic as your reference.
Before the output Tx arrived, I'd used a couple of 150 ohm resistors to compensate for the primary resistance to V4.
This was so I could check voltages etc and tbh, I'm glad I did as although I try to be very careful, I did slip with a probe causing a tiny short. This took out one of the 150 ohm resistors (not with a bang, just open circuit), but it could have been the primary of the transformer had it been connected!! Lucky!!
So with voltages correct and everything connected up, one last check to make sure there was no DC where there shouldn't be, and connected the output transformer.
All good, so time to fire up Mr signal generator and see if its amplifing. Using the multimeter, shows its amplifing. Using the scope verifies this although the output waveform shows its crapping out when upping the input level!! Also, doesnt seem to be compressing!!.
After a bit of head scratching I decided to check the detector output which is the -ve DC control voltage to the 6SS7. Measuring at the centre tap of the input Tx showed no voltage. Disconnecting it, bought it back so must be a short to ground! I checked resistance from the centre tap to ground and there was indeed a short circuit. Although I couldnt actually see a physical connection, I re-soldered and now all good. Re-connecting the control voltage to the centre tap confirms this. Unit now compressing and with a nice clean waveform straight through
Very happy and more testing to do of course. Also, there will be a case of matching V1 and V2.
Going back to the PSU, I'm thinking that the 150V shown on the schematic is probably a reference for the 15dB point. I will try to confirm this.
Happy Sunday all
You are live in the lab with Chris P Bass (well kind of...just go with it!)
So the output transformer has arrived, and very nicely made it is. Weighty with a good quality feel about it...thank you UTM
So last time, I was calculating plate currents, and this was really in regard to cross referencing the schematic and for me to calculate dropper resistors as I've gone off piste with regards to the PSU.
V3 = 1.725mA
V4 = 8.65mA
V1/V2 = 3.95mA
So adding those up and doubling them we get about 28.6mA
The meter will draw a further 1mA plus a smidge for the detector circuit giving say 29.6 or about 30mA.
Looking at the schematic for the PSU, Z measured a voltage drop of (259.8 - 245) across the 500 ohm resistor giving 14.8/500 = 29.6mA which is pretty darn good!!
I decided to build the psu on a separate board just so I'd got plenty of room to allow for testing etc.
Ive got 3 stages of filtering on my PSU and running regulated DC heaters. The block connectors bought off the pcb were handy to help dial in the voltages.
Initially, i just connected up the heaters to test these first, and one of the 6SS7 didn't seem to be getting warm. Inserting a 0.1 ohm resistor in the supply and measuring the voltage drop confirmed this. We have a duffer! I've got a LM338 regulator and with the heat sink, its barely getting warm. The bridge rectifier is getting a bit hot. Its rated for 4A, but may use something beefier when it comes to building the unit properly.
Initial B supply of 245V is on the pcb going to the block connector on the left to get the B supply for the 6SS7 and also the 24V for the detector. I used a trimmer rather than the 22K resistor, as I didnt have one to hand, but also helps with dialing that in.
I'm a smidge off, but more than close enough with my hotch potch of dropper resisitors, and happy to report that my measured voltages pretty much tally with Z's measured ones, so anyone planning on building this can be confident and go with what Z has on the schematic as your reference.
Before the output Tx arrived, I'd used a couple of 150 ohm resistors to compensate for the primary resistance to V4.
This was so I could check voltages etc and tbh, I'm glad I did as although I try to be very careful, I did slip with a probe causing a tiny short. This took out one of the 150 ohm resistors (not with a bang, just open circuit), but it could have been the primary of the transformer had it been connected!! Lucky!!
So with voltages correct and everything connected up, one last check to make sure there was no DC where there shouldn't be, and connected the output transformer.
All good, so time to fire up Mr signal generator and see if its amplifing. Using the multimeter, shows its amplifing. Using the scope verifies this although the output waveform shows its crapping out when upping the input level!! Also, doesnt seem to be compressing!!.
After a bit of head scratching I decided to check the detector output which is the -ve DC control voltage to the 6SS7. Measuring at the centre tap of the input Tx showed no voltage. Disconnecting it, bought it back so must be a short to ground! I checked resistance from the centre tap to ground and there was indeed a short circuit. Although I couldnt actually see a physical connection, I re-soldered and now all good. Re-connecting the control voltage to the centre tap confirms this. Unit now compressing and with a nice clean waveform straight through
Very happy and more testing to do of course. Also, there will be a case of matching V1 and V2.
Going back to the PSU, I'm thinking that the 150V shown on the schematic is probably a reference for the 15dB point. I will try to confirm this.
Happy Sunday all
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