Looking at the schematic it appears that the two input capacitors (2mu2 and 22n) are completely superfluous.
The idea behind input capacitors is blocking dc from whatever source. In over 40 yrs of repairing amps I've never come across a pre amp, cd player or tuner putting out dc . I would take them out because they only degrade the sound of an otherwise very good amplifier.
The idea behind input capacitors is blocking dc from whatever source. In over 40 yrs of repairing amps I've never come across a pre amp, cd player or tuner putting out dc . I would take them out because they only degrade the sound of an otherwise very good amplifier.
A friend recently had an amp that had a DC coupled input and DC protection circuits trigger from a small amount of DC leakage from a preamp. It isn't an improbably scenario.
From what I've been reading, it seem that removal of the input cap, and pot are common modifications for the older MK1 like mine. The bias pot seems to also be a source of trouble, and worthy of modification.
I inspected the other monoblock and it actually looked OK. No gushing capacitor guts. So I've got it on the variac and I'm easing it up to operational voltage with two 8ohm speakers on the 4ohm output. Hilarious thing is the speakers don't need more than a watt to be absolutely blazing, so the EAR 509 is a bit absurd for it's claimed power output. LOL.
Pretty quiet amp. Miles D. is sounding pretty good on the full-rangers!
I've been staring at the schematic trying to figure out how to bias the amp. I see the two trimmers, and the two + points on the output tube board. Nothing is labled though. The schematic I'm looking at suggests 600mw across the test points. What?
According to my schematics, there are resistors of 15 ohms between the cathode and the OPT connections. 600 mV across 15 ohms means 40 mA cathode current, which seems to be reasonable.
Best regards!
Best regards!
Eurotubes sells JJ EL509S which electrically is identical but using an octal socket.
This is new production and is available in matched set's. You will however need to
replace the sockets and move the plate connector to the octal socket.
They're across both 15R resistors, the positive one at the cathode, the negative one at the cathode winding terminal.
Best regards!
Ok Ok. I must be doing something wrong because I'm getting voltages that seem too high. I need to figure out the PCB layout. Once I figure it out I'm going to add better test points!
Well, that shouldn't be too difficult. Cathode pin is #9, which is also connected to the beam plate pins #2 and #7. Easy to find at the PCB, the trace to these three pins is clearly shown at your pic. The cathode resistor most probably is a big
wirewound one, due to it's 4 W rating.
I'd do the following: First pull both PL509's and power the unit. Measure the control grid voltages (pins #1 and #8) and adjust the bias trimpots to their maximum values. Power off. Insert the PL509's (don' forget the plate caps!), turn the unit on again and adjust the bias potentiometers for 600 mV across the cathode resistors. Repeat the last step at least twice. Done.
Best regards!
wirewound one, due to it's 4 W rating.
I'd do the following: First pull both PL509's and power the unit. Measure the control grid voltages (pins #1 and #8) and adjust the bias trimpots to their maximum values. Power off. Insert the PL509's (don' forget the plate caps!), turn the unit on again and adjust the bias potentiometers for 600 mV across the cathode resistors. Repeat the last step at least twice. Done.
At it's best the 2nd best idea, I think, as the power transformer's 6.3 V heater winding surely won't provide the additional 4 A heater current for two EL509S's and as those EL509S's share the pinout with 6L6, KT88 etc., with the plate pin #3 adjacent to the heater pin #2. Last but not least the Magnoval tube holders are soldered to PCB's. What a mess!
Best regards!