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mcgaag 5th June 2009 08:00 PM

Help on EL84 / ecc83 SE project -very low volume
2 Attachment(s)
I build this amp:

The amp works but the volume is very low. I mean really low, i have to put my ear next to the speaker to hear something (speakers are a pair of Fostex 94db). What could be wrong, i double checkt every connection and that seems to be in order. Is the ecc83 broken? Is this design ok? The tubes came from a working radio but are old, 20+ years minmimum.

Some advice please.


ErikdeBest 5th June 2009 09:12 PM

Hi Rob

I think that the best first pass in this troubleshooting is to measure all DC voltages in your amp, to check biasing of the stages. You have got a nice schematic there, so you can open it in paint and write down all DC values, and post back here. With all DC voltages I mean the voltages at the terminals of the tubes (plate, grid 2, grid 1, cathode) of each tube.

Panicos K 5th June 2009 10:24 PM

Your thread speaks of 12AX7,your schematic says 6N2P.Maybe you have to rewire the tube sockets for 12AX7 or use 6N2P tubes?

Nihilist 5th June 2009 10:44 PM

Pinout looks the same. Double check you have proper voltages, make sure that you haven't accidentally shorted/touched any wires together.

I recently wired some new pickups into my nephews electric guitar, we hooked it up and NOTHING ! ! ! I was pulling my hair out. Then I realised that when I heated up the wire for soldering, it had melted through the twisted pair insulation and bonded to the other wire , causing a dead short.

Accidents happen. Double check your work.

Are your driver tubes (6N2P, or 12AX7) heaters lighting up ? Check your heater wiring ?

What did you use for a source ? CD , turntable, tuner ?

Good luck........................Blake

Nihilist 5th June 2009 11:00 PM

I just looked at your schematic again. Perhaps I'm crazy, but your grid stopper resistors on your input tubes (6n2p/12ax7) are only 82k. That seems like it would be dumping alot of your input signal to ground.

Try a 500k or 1meg resistor, or just for a quick test, unhook one side of the 82k resistor (effectively taking it out of the circuit). I bet this cures your low volume issue.

Let me know how it works out.


Panicos K 5th June 2009 11:09 PM

2 Attachment(s)
Maybe this can help.

chrish 6th June 2009 12:01 AM

I would also check that your input and speaker terminals are isolated from the chassis, ie they are not shorting out to ground.


planet10 6th June 2009 03:07 AM


Originally posted by Nihilist
in. Perhaps I'm crazy, but your grid stopper resistors on your input tubes (6n2p/12ax7) are only 82k.
Grid Leak resistor (no grid stopper in this circuit). It is quite common to use 100k pot in this position, so 82k fixed shouldn't be a hardship.


mcgaag 6th June 2009 11:38 AM

2 Attachment(s)
I checked the heaters and those are all ok. 6,3 Volt and they heat up nicely. Though the heater on the 12AX7 glows up bright when i turn on the amp and then glows down again to normal. I don't know if this is normal. I did pay attention to the different way the heater is connected though, so that should not be a problem, (4-5 and 9).

I also checked that input and speaker terminals are isolated from the chassis. Thats all ok to. I am sure the wiring is ok. Triple checked everything.
Here is the schematic with the pin voltage. The B+ is 252 Volt.

Thanks so far everyone. :)

ErikdeBest 6th June 2009 12:25 PM

Cathode voltages appear to be good: 1,4V on the input and 7V on the output tube. The only voltage that worries me is that 210V at the plate of the driver... that one just can't be right.

The input tube is drawing a bit more than 1,1mA (1,4V / 1k2), which is fed through the 220k resistor. The voltage drop on this resistor would therefore be about 250V, which can't be right.

I think you measured 210V at the junction of the 33k resistor and the 47uF capacitor... (1,1mA * 33k = ~40V, which subtracted from 250V would give the 210V). Furthermore, I think that your plate resistor is much lower in value than the indicated 220k... maybe you put a 22k unit there instead?

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