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Old 2nd March 2014, 11:05 PM   #1
2bamp is offline 2bamp  Canada
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Default 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?
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Old 2nd March 2014, 11:20 PM   #2
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More power or the RH84 needs more input grunt? Output power should be broadly similar to 6V6.

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Old 3rd March 2014, 12:19 AM   #3
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measurements would help...
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Old 3rd March 2014, 12:50 AM   #4
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2bamp, what kind of PSU are you using? We talked about one I proposed in that other thread.
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Old 3rd March 2014, 12:59 AM   #5
2bamp is offline 2bamp  Canada
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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.
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Old 3rd March 2014, 01:34 AM   #6
2bamp is offline 2bamp  Canada
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Sodacose, I have 2 on the go. This one with a conventional PS and EL84s, and another with 6V6s and a voltage doubler PS.
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Old 3rd March 2014, 05:00 AM   #7
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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?
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Old 3rd March 2014, 06:07 AM   #8
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Quote:
Originally Posted by 2bamp View Post
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...
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Originally Posted by 2bamp View Post
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 View Post
Perhaps the feedback is sapping the power?
The feedback is what makes this amp...
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Originally Posted by 2bamp View Post
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...
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Old 3rd March 2014, 06:44 AM   #9
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Quote:
Originally Posted by 2bamp View Post
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.
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Last edited by aardvarkash10; 3rd March 2014 at 06:50 AM.
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Old 3rd March 2014, 06:45 AM   #10
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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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