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vermeero 12th January 2012 10:27 PM

RH84 amplifier build, but very weak distorted output
Hi all,

Haven't posted much yet, been here occasionally to get some inspiration!
Last week I build the RH84 amplifier (SE EL84) and since this is my first real build I closely followed the original schematics...which means I didn't change any values. I even ordered a powertransformer wound to spec. according to the schematics. Original to be found here: RH 84 - Tube Audio ...... RH DESIGN

When I power it on and wait for a while having a preamp connected with music playing at a fairly loud level, only a very weak sound is heard from the loudspeaker, mostly from the left channel. When I really cranck up the volume it doesn't get much louder but it turns out distorted.
Really searched and searched, checked my wiring and placement of components but I can't find anything wrong. Swapped the EL84's but no change.
Measured some voltages (always with the negative pin of my meter connected to the amplifier's ground) and they appear like this:

B+ : 353

Right channel EL84:
pin 3: 11,9
pin 7: 332
pin 9: 343

Left channel EL84:
pin 3: 12,3
pin 7: 330
pin 9: 342

pin 1: 341
pin 6: 317

Filament voltage is like it should be.
After about a minute or two I switched it off... thinking this can't be good.
No hum actually.

DF96 12th January 2012 10:39 PM

Where is DC isolation? The RH84 does not appear to have an input coupling capacitor and your ECC81 triodes don't appear to be taking any anode current.

vermeero 12th January 2012 10:55 PM

1 Attachment(s)
To clarify myself more... an added photo from the wiring inside the cabinet I build.
About the picture... the AC is grounded (before I started testing the amplifier), so now a wire from AC ground runs to the central ground of the amplifier itself.
I'm really new to this... trying to learn through reading etc.
So... what do you mean by isolating the DC?

What struck me is the high voltage on the primary of the output transformer... how is it supposed to make a voltage swing when it's this high for an EL84?

DF96 12th January 2012 11:06 PM

How are your heaters wired? I suspect you may be supplying 6.3V to ECC81 pins 4 and 5, but no connection to pin 9. You either need 12.6V to 4 and 5, or join 4 and 5 together and put 6.3V between there and pin 9. This would explain why the ECC81 is taking no anode currrent - the cathode is not hot enough.

vanofmonks 12th January 2012 11:36 PM

I agree with DF96. The ecc81 heaters not wired correctly.

vermeero 13th January 2012 12:32 PM

3 Attachment(s)
Thanks a lot! I knew it had to be something simple... just didn't look closely enough at the ECC81's pinout. :o
The amplifier is now working as it should! No hum and goes plenty loud on a pair of Yamaha NS1000.
DF96's instruction was very clear, thanks again.
Added a picture of how it's wired now.

GloBug 13th January 2012 12:34 PM

Right on!

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