Hello there,
I am tackling my first point-to-point DIY amp project with the JE Simple 45/2a3: JE Labs Arkiv (up to 2008): JE Labs Simple 45/2A3
The author kindly helped me with my choice of transformers, however I still have an open question.
My choice is now between a full set of Edcor (XPWR271 power, CXC125 choke, GXSE15-5K OPT) or a full set of One Electron (BFT-1B power, PRC-2 choke, UBT-2 OPT) iron. (There is a $300 difference between the setups but if it's worth it I might still go with One Electron.)
The question I have is that the schematics call for a 600V secondary, while the BFT-1B has 770V and 520V terminations. The author said I could use the 520V taps for the rectifier. Even though tube designs allow for a 5-10% error, that still seems quite far from the specs (13% less voltage).
What would the consequences of an under-fed rectifier be? Could I get the desired B+ by modifying the resistor values downstream?
Alternatively, is it advisable to use the 770V terminations with drop-voltage resistors? I think I have read about side effects of this approach, but I'm not sure.
Thanks.
gm
I am tackling my first point-to-point DIY amp project with the JE Simple 45/2a3: JE Labs Arkiv (up to 2008): JE Labs Simple 45/2A3
The author kindly helped me with my choice of transformers, however I still have an open question.
My choice is now between a full set of Edcor (XPWR271 power, CXC125 choke, GXSE15-5K OPT) or a full set of One Electron (BFT-1B power, PRC-2 choke, UBT-2 OPT) iron. (There is a $300 difference between the setups but if it's worth it I might still go with One Electron.)
The question I have is that the schematics call for a 600V secondary, while the BFT-1B has 770V and 520V terminations. The author said I could use the 520V taps for the rectifier. Even though tube designs allow for a 5-10% error, that still seems quite far from the specs (13% less voltage).
What would the consequences of an under-fed rectifier be? Could I get the desired B+ by modifying the resistor values downstream?
Alternatively, is it advisable to use the 770V terminations with drop-voltage resistors? I think I have read about side effects of this approach, but I'm not sure.
Thanks.
gm
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Remember these are 300 volt secondaries used in a full wave configuration.
520 volt (so 260-0-260) will give a lower B+ voltage. Reducing resistor values such as the 50 ohm will increase the voltage a little but will also increase peak rectifier current which might be unacceptable.
The 770 volt option (so 385-0-385) is quite a jump up. Increasing R values will lower B+ back to where you want but at the expense of higher dissipation in the resistors. So more heat.
Also unloaded (which includes warm up of the amplifier section) will see a big rise in B+ for a short while and so cap voltage is a consideration. You would also have to check the rectifier can withstand the higher PIV (peak inverse voltage) that it will see in use. 385 volts rms is 545 volts peak or 1090 volts peak to peak. The rectifier has to withstand that plus a good safety margin.
520 volt (so 260-0-260) will give a lower B+ voltage. Reducing resistor values such as the 50 ohm will increase the voltage a little but will also increase peak rectifier current which might be unacceptable.
The 770 volt option (so 385-0-385) is quite a jump up. Increasing R values will lower B+ back to where you want but at the expense of higher dissipation in the resistors. So more heat.
Also unloaded (which includes warm up of the amplifier section) will see a big rise in B+ for a short while and so cap voltage is a consideration. You would also have to check the rectifier can withstand the higher PIV (peak inverse voltage) that it will see in use. 385 volts rms is 545 volts peak or 1090 volts peak to peak. The rectifier has to withstand that plus a good safety margin.
Use the Edcor power iron (right voltage) and the One-Electron output transformers (built by an audio-fan).
Thanks for your suggestions. All things considered (including additional shipping cost + 10 week delay for just the Edcor power tx) I might as well get a cheaper Hammond exactly to spec from the same supplier, which maybe I can replace with a better quality one later.
I noticed that the same high voltage resistor is also connected to the filament secondary which complicates things further. It would be too much for a newbie to deal with compensation on top of all the other first-time troubleshooting I'll surely have to make...
gm
I noticed that the same high voltage resistor is also connected to the filament secondary which complicates things further. It would be too much for a newbie to deal with compensation on top of all the other first-time troubleshooting I'll surely have to make...
gm
There's a reason for being cheap. Their transformers are often bad or flaky, due to "floating the scope".
You'd have to explain that one to me, im referring to the mains transformers from the ancient 500 series mainframes like the 545
As far as im aware, only the 502 suffers from insulation failures.
As far as im aware, only the 502 suffers from insulation failures.