• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Does 81ma sound about right?

This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
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?


  • Dynaco_A410-mod.JPG
    37.3 KB · Views: 174
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.

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
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.

This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.