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Help on EL84 / ecc83 SE project -very low volume

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mcgaag

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Joined 2006
#1
I build this amp:

http://www.joddla.net/audio/6p14p/el84sexun_jpg.jpg

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.

regards,
Rob
 

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#2
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.
 
#4
http://cityradio.narod.ru/spr/tubes/russian/short/6n2p.html

http://tdsl.duncanamps.com/show.php?des=12AX7

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
 
#5
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.



.........................Blake
 
#9
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. 🙂
 

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#10
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?
 
#13
Yep, there is music now! Thanks Erik pointing me in the right direction. 🙂 Learned a lot building this amp.

Now the next challenge. Remove a nasty hum.

I will post a picture of my 1st diy tube amp!

Thanks for thoughts everyone.

Regards,
Rob
 
#16
mcgaag said:
It sounds nice except for that nasty hum. If i remove the tubes the hum is gone. Does this mean the tubes are broken?
Click to expand...

Not necessarily...

try elevating the heaters to +40-50 volts with refrence to chassis, Use simple two-resistor pot.divider from +b to chassis, and de-couple with summit like a 0.22uF cap....

That should remove the heater-bourne hum often found in gain-stages like that shown...
 
#18
mcgaag said:
I don't think the buzz comes from the heaters. If i remove the heater power when the amp is buzzing the buzz is still there.

By the way, i did not rectify the heater power so it is AC.
Click to expand...


I have spent many happy hours trying to fix hum problems.

Running heater wires near inputs is bad.
Good decoupling is vital or the hum on the rails will modulate the audio signal.
 
#20
mcgaag said:
I don't think the buzz comes from the heaters. If i remove the heater power when the amp is buzzing the buzz is still there.

By the way, i did not rectify the heater power so it is AC.
Click to expand...


Is it a buzz or mains hum ?

I had a problem with my pre amp with diode switching spikes.
I cured it with 100nF across the decoupling capacitors.
This sounded like a buzz rather than hum despite it being mains frequency.
 
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