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?
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?
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.
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.
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?
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?
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...
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...
The feedback is what makes this amp...
It would change the circuit into something else...
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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When using a triode as the driver for the RH amplifiers, the cathode is left un-bypassed to increase the driver's plate resistance, this is one of the design features specified by Mr. Kitic.
I thought that the intended use of the amp was with an mp3 player on the input. Oh well...
The voltages were measured through a Vector socket on top of the chassis. I will have to remeasure, as they don't quite add up. 340v is a bit strong on the power tubes, so I will have to add some sand.
I will try adding a small cathode bypass cap. If that doesn't do it, I will have to add another triode stage. I am sure a couple of 6C4 could fit in my chassis with a pot. A gain of 2 would likely be all I need. Hopefully it doesn't come to that.
The voltages were measured through a Vector socket on top of the chassis. I will have to remeasure, as they don't quite add up. 340v is a bit strong on the power tubes, so I will have to add some sand.
I will try adding a small cathode bypass cap. If that doesn't do it, I will have to add another triode stage. I am sure a couple of 6C4 could fit in my chassis with a pot. A gain of 2 would likely be all I need. Hopefully it doesn't come to that.
It's Schade Feedback. There's nothing magic about it... I can't comment on AK's design choices, only say that it works well either way.
~Tom
2bamp-
If you think it's definitely the mp3 player output, you could give this nifty little trick a shot:
Antique Radio Forums • View topic - Passive Amplifier for your iPod 2 radio jack
I've always wanted to try it, but I haven't really had a need (perhaps when I finish my RH84 build...)
Part in question:
RadioShack Audio Output Transformer : Transformers | RadioShack.com
$2.99 a pop.
If you think it's definitely the mp3 player output, you could give this nifty little trick a shot:
Antique Radio Forums • View topic - Passive Amplifier for your iPod 2 radio jack
I've always wanted to try it, but I haven't really had a need (perhaps when I finish my RH84 build...)
Part in question:
RadioShack Audio Output Transformer : Transformers | RadioShack.com
$2.99 a pop.
Hopefully you were right about the lack of oomph from the driver. Would make sense that it's just not seeing enough V's from a headphone out. A 2V out from a CD player (or a mp3 player with that little transformer trick) might be all you need. Will be interested to hear results as I've just ordered some Transcendar iron for my build...
I know what you are saying, I was merely pointing out that it was a key design choice on the RH amplifiers - it was left out on purpose.
I did some A/B testing with my consoles that use similar tubes. I can honestly say that I am happy with the RH84 as built. I just needed to listen more carefully.
It doesn't have quite as much power, but the sound quality much better. I won't bore you with "audiophile speak," but the frequency response and voicing are on a whole different level. The improved design built with NOS PIO caps instead of ceramic probably has a lot to do with it. My solution to the reduced power: move the amplifier to a smaller room.
For my next build, I am going to try making an integrated amp optimized for the lower line level of my digital sources. I am thinking of a single triode stage on the front end into a volume pot. I have lots of TV tubes, so perhaps a 6SN7, 6CG7 or a pair of 6C4.
Anybody done this already? I have scoured the interwebz, but haven't found any examples.
It doesn't have quite as much power, but the sound quality much better. I won't bore you with "audiophile speak," but the frequency response and voicing are on a whole different level. The improved design built with NOS PIO caps instead of ceramic probably has a lot to do with it. My solution to the reduced power: move the amplifier to a smaller room.
For my next build, I am going to try making an integrated amp optimized for the lower line level of my digital sources. I am thinking of a single triode stage on the front end into a volume pot. I have lots of TV tubes, so perhaps a 6SN7, 6CG7 or a pair of 6C4.
Anybody done this already? I have scoured the interwebz, but haven't found any examples.
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