I'm currently looking at a pair of mid 80s EL34 PP monoblocks on the bench (these beasts have 8 EL34 per block arranged in 4-4 parallel push pull). These had a few badly swollen capacitors, which were obviously in need of replacement and I've done that job. I wanted to do a check on bias etc to make sure all is OK, and that they are within reasonable operating parameters. Would someone mind just throwing their eye on the figures below to check all is OK (I think it is, but its near max dissipation):
Anode voltage 410V
Screen voltage 380V
Plate to Cathode 384V
Bias voltage -25V
Cathode resistor 470R (one per valve) - and running 27-29V or so over this.
That seems to work out at ~60ma, 23W and about 90% of rated maximum. Would that sound right to you?
Fran
Anode voltage 410V
Screen voltage 380V
Plate to Cathode 384V
Bias voltage -25V
Cathode resistor 470R (one per valve) - and running 27-29V or so over this.
That seems to work out at ~60ma, 23W and about 90% of rated maximum. Would that sound right to you?
Fran
Your values seem to be very close to the values on the top right hand side of page 3 of the Mullard datasheet for the EL34. So they look OK to me.
Yes, its all reassuring to see the numbers seem to add up.
Need to figure out the front end of it now - looks like a 12au7/12ax7 pair with CCS for both.
Need to figure out the front end of it now - looks like a 12au7/12ax7 pair with CCS for both.
Bias is a tad high, my parallel PP monoblocks were biased at 43mA, but my HT was higher 450v UL. Unless your rich and can afford new valves easily it might be better to reduce bias voltage so your valves are at 75% which is about normal for most amps.
Whilst in my case altering bias was easy as they worked in fixed bias it's mean changing a lot of resistors for you, that or adding small value resistors.
Whilst in my case altering bias was easy as they worked in fixed bias it's mean changing a lot of resistors for you, that or adding small value resistors.
With cathode resistor bias you are "forced" to use higher bias/current than for fixed bias , as it will drop with the power output .
You have 4 EL34 in parallel push pull for each channel, Right?
I would not worry about a little less power output from self biasing, unless you have very in-efficient loudspeakers, or are trying to fill an auditorium.
I prefer to always use individual self biasing, and matched tubes.
If you want a little more life out of your EL34 tubes, then try increasing the individual 470 Ohm self bias resistors to 560 Ohms.
Try it and see how it sounds (loudness, and distortion).
Unless you really want to put the amplifier on a test bench, then trust your ears.
When you use the 560 Ohm resistors, the B+ will go up slightly. Make sure the B+ filter caps are rated for more voltage than the now increased B+ voltage.
Or just use the amplifier as it was designed, with the 470 Ohm resistors.
Happy Listening.
I would not worry about a little less power output from self biasing, unless you have very in-efficient loudspeakers, or are trying to fill an auditorium.
I prefer to always use individual self biasing, and matched tubes.
If you want a little more life out of your EL34 tubes, then try increasing the individual 470 Ohm self bias resistors to 560 Ohms.
Try it and see how it sounds (loudness, and distortion).
Unless you really want to put the amplifier on a test bench, then trust your ears.
When you use the 560 Ohm resistors, the B+ will go up slightly. Make sure the B+ filter caps are rated for more voltage than the now increased B+ voltage.
Or just use the amplifier as it was designed, with the 470 Ohm resistors.
Happy Listening.
Thanks all - very helpful indeed to have a second set of eyes. I will leave as is unless it is eating tubes - I suspect it was designed that way as it sounded best to the maker.
FYI: You measured cathode current not plate current. Plate and Screen dissipation combined is 33 watts. Therefore you are running the tubes at approximately 70% dissipation.
Steve
Steve
OK thanks
I put in my numbers to an online calculator (I think from a guitar site) and it gave me the 90% figure.... but even better for valve life if its back at 70%
I put in my numbers to an online calculator (I think from a guitar site) and it gave me the 90% figure.... but even better for valve life if its back at 70%