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2bamp 2nd March 2014 10:05 PM

Disappointing results from RH84
 
I just completed an RH84. It uses the schematic unaltered, except for using a 5Y4 rectifier and 150uf as the final filter cap. I also omitted the 220 ohm resistor connected to the input grids, as none of Alex Kitic's other designs use it.
The power supply is giving a robust 340v on the anodes of the EL84. The output transformers are salvaged Loewe Opta. I had one of them in a guitar amp, and with the 6CM6 it had a very healthy output. I used the same connections to the OT, so it should be in the ballpark.
The amp runs dead quiet and sounds fantastic, but the output is really weak. I am using a pair of Noresco speakers with 6.5" Isophons and 4" tweeters. I also tried some alnico RSC Jensens that I pulled from a defunct console.
I am comparing this to a couple of consoles that I use that are SE EL84 and 6V6. I have connected my Playbook right into the phono input on all of them, and they have WAY more power than this amp.
I tried a couple of 6201 and even a 12AX7. It seems like the driver just isn't pushing enough into the power tubes. Has anyone else run into this?

planet10 2nd March 2014 10:20 PM

More power or the RH84 needs more input grunt? Output power should be broadly similar to 6V6.

dave

aardvarkash10 2nd March 2014 11:19 PM

measurements would help...

Sodacose 2nd March 2014 11:50 PM

2bamp, what kind of PSU are you using? We talked about one I proposed in that other thread.

2bamp 2nd March 2014 11:59 PM

EL-84 has 340v on plates, 330v on screens, 11.8v on cathode, 314v from plate to cathode.

6201 has 158v on the plates, 1.94v on cathode.

These voltages seem exactly where the should be, right?

I tested the tubes, and tried different ones. Perhaps the feedback is sapping the power? It sounds like a 1w amp, about half as much power as my console amps using the same tubes. I know I have the OTs connected properly. It has lots of power into an 8 ohm load with the 6CM6, so EL84s into 6 ohms should be pretty close.

2bamp 3rd March 2014 12:34 AM

Sodacose, I have 2 on the go. This one with a conventional PS and EL84s, and another with 6V6s and a voltage doubler PS.

2bamp 3rd March 2014 04:00 AM

It seems like a matter of insufficient drive. My mp3 player isn't strong enough to give a decent volume, but I was able to set up my Playbook with "headphone audio boost." It is now much better, but still not quite right. It isn't anywhere near clipping at max volume.

How would adding a cathode bypass capacitor to the driver interact with the plate-to-plate feedback loop? If I could add just a bit more gain, I'd have a winner. I notice the 807 version has 350v B+ and 33k as the plate load. Perhaps that would help?

fullrangeSR 3rd March 2014 05:07 AM

Quote:

Originally Posted by 2bamp (Post 3839966)
I also tried some alnico RSC Jensens

I too have those - 12'', with 3'' tweeters in the middle. Volume pot turned past twelve and it's really loud, or should I say too loud...
Quote:

Originally Posted by 2bamp (Post 3839966)
I have connected my Playbook right into the phono input

If it's a phono with crystal or ceramic cartridge then it's no wonder that your Playbook is sufficient. But for RH84 *power amp* you either need a line pre or at least a volume pot...
Quote:

Originally Posted by 2bamp (Post 3839966)
Perhaps the feedback is sapping the power?

The feedback is what makes this amp...
Quote:

Originally Posted by 2bamp (Post 3839966)
How would adding a cathode bypass capacitor to the driver interact with the plate-to-plate feedback loop?

It would change the circuit into something else...

aardvarkash10 3rd March 2014 05:44 AM

Quote:

Originally Posted by 2bamp (Post 3840042)
EL-84 has 340v on plates, 330v on screens, 11.8v on cathode, 314v from plate to cathode.

6201 has 158v on the plates, 1.94v on cathode.

These voltages seem exactly where the should be, right?

I tested the tubes, and tried different ones. Perhaps the feedback is sapping the power? It sounds like a 1w amp, about half as much power as my console amps using the same tubes. I know I have the OTs connected properly. It has lots of power into an 8 ohm load with the 6CM6, so EL84s into 6 ohms should be pretty close.

340 - 11.8 =/= 314

Are those as measured, or calculated on your schematic?

Additionally, what ^ he said. Your playbook output is around half what the RH84 is expecting an input signal.

tomchr 3rd March 2014 05:45 AM

It shouldn't be a problem to bypass the cathode resistor on the driver with a cap. That's what I do in my Novar Spud.

~Tom


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