I have a carslbro pa 100 r for repair , one of the ecc83 valve heaters had blown . replace the valve with a good second hand one , and the amp works fine , however checked the voltages etc and found the ht 520v , heater voltage 7 v .The mains selector on the rear is connected and wired correctly . The only schematic i have found online says the ht is 460 v .The transformers are all original . just wondering if anyone has experience of this model ? and are there different versions or should the ht be 460 v . My initial thought on measuring the ht , was the mains input voltage selector had been wired wrong at some point in its life , but this appears to be fine . I do not have the use of a variac anymore , so cannot run it or a long periods whilst i check the circuit .
Mains voltage is higher these days than when these amps were manufactured. Use a variac. Slowly turn up the variac till your filament voltage measures 6.3 volts. Now measure HT voltage; I bet it's lower too. Now measure voltage going into the amp from variac; you can use a buck/boost transformer in "buck" mode to lower input voltage to your amp.
I'm not familiar with the carslbro pa 100 r; does it have a tube rectifier? this may be another reason, but chances are that the mains voltage is higher now than what it used to be. This is where the discrepancy is coming from.
Ooops! Sorry, didn't see that you don't have access to a variac.
I'm not familiar with the carslbro pa 100 r; does it have a tube rectifier? this may be another reason, but chances are that the mains voltage is higher now than what it used to be. This is where the discrepancy is coming from.
Ooops! Sorry, didn't see that you don't have access to a variac.
7V on 6.3V suggests you want a 12V CT transformer wired auto-transformer to give 5% and 10% reduction of modern line voltage. This can be generally useful if you work on old 110V-117V stuff in a 120V-125V world, and can be built-in on any old gear which will do enough hours to justify it. The 12V/6V winding current should be => the estimated current at 115V (so if this is a 100 Watt amp, think 2A rating).
Don't worry about the HT. There is no issue but if you are concerned about the heater voltage and you are sure your test meter is accurate, place a pair of diodes back to back in series with the heaters. That will drop the voltage by 0.7V or so And ... no, noise from the diodes is not an issue, before the uneducated get involved.
thanks for all your contributions , fitted the back to back diodes in the heater supply , and down to 6.3 v . The ht is 517 v , second ht after 22k drop resistor is 407 v . Happy with that . Just done some resoldering of some very poor joints and tidied up some sloppy repairs done by someone else . A great sounding amp !
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