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Old 13th December 2010, 11:50 PM   #1
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Default Does 81ma sound about right?

I just finished a scratch build 6BQ5 PP stereo amp. I copied the Dynaco A-410
Schematic below. It sounds very good to me. I check the current flow to see if my power transformer had enough power to handle the two channels. I measured 81ma between pin 3 of the EZ81 and the 220 ohm resistor with my DMM. That seems low to me for a two channel amp. Then I checked the voltage drop across the 220 ohm 25 watt and measured 16 to 19 volts drop and that works out to about 80 or so millamps. My power transformer puts out 180 ma so I'm good if my readings are good. What do you think?
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Old 14th December 2010, 01:08 AM   #2
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With 12.1 vdc at the EL84 cathodes across a 150 ohm cathode resistor, that equals about 81 ma which is right on for a pair of these tubes at the B+ voltage you are running. The only thing I would question is running two channels off of one 6CA4 rectifier tube. It can do it, but it will be very stressed in doing so, and short lived. If the power transformer will allow it, you could either parallel another 6CA4 with the original one to double the rectifier capacity, or convert (if possible) to a single 5AR4, which could handle two channels with ease.

Dave
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Old 14th December 2010, 01:19 AM   #3
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Are you sure the schematic does not have an error, resistor between cathodes, and shunting cap to the ground from the cathode of the bottom tube?
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Old 14th December 2010, 01:41 AM   #4
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Hi All, thanks for the reply.

Wavebourn.....all four cathodes are tied together and go to ground thru the one (1) resistor.

dcgillespie....I have a new PT on order (200ma). I will use a 5U4 rectifier with it. BUT I dont know if I really need to go that high. The whole amp is only pulling 81ma. I have it configured as two channels with a common power supply. The EZ81 is rated for 150ma. I will put the new PT and 5U4 in when I get it.

Thanks for the reply
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Old 14th December 2010, 01:48 AM   #5
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Wavebourn.....I just reread your reply. you were talking about the 12ax7. The schematic is correct. google dynaco a410 or lookup DIY audio projects and you will find the schematic.

Thanks

Bruce
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Old 14th December 2010, 02:43 AM   #6
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Bruce -- are you saying all four output tube cathodes are tied together and use one common cathode resistor for all four tubes? What value are you using then for the cathode resistor? If you are still using 150 ohms, that is the reason that the amp is not pulling enough current. With all four tubes connected together, the cathode resistor should be about 100 ohms in that scenario.

A 5U4 would be a bit of overkill. It's also a relatively high drop rectifier tube, and would not provide a slow warm up characteristic. A 5AR4 would be perfect and also provide a slow warm up time.

Dave
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Old 14th December 2010, 10:13 AM   #7
DF96 is offline DF96  England
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Why deliberately introduce second-harmonic crosstalk between the channels at low frequencies? This seems a rather perverse thing to do. Keep the output cathodes separate i.e. share a cathode resistor within a channel if you wish, but don't share between channels.
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Old 14th December 2010, 03:02 PM   #8
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Most of the old Magnavox console amps that were push pull el84 amps also used ONE shared cathode resistor for all 4 tubes. I presumed it was to save a few cents on production costs and add some warmth to the output stage.
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Old 14th December 2010, 03:09 PM   #9
jez is offline jez  United Kingdom
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Sounds about right for the description of the amp
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Old 14th December 2010, 03:20 PM   #10
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Take a look at SY's Red Light District PP el84 amp and do away with the cathode resistor(s) and cap(s) altogether.
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