Hi everyone. XrayTonyB built a cool little tube amp kit from Douk Audio on YouTube about 4 years ago, and I want to try one myself. I haven't done anything with electronics in a while, so I want to check with someone who knows before I go messing with the circuit at all. It has two "magic eye" tubes in it that I really don't want. Since I plan to wire the kit point-to-point instead of using the included PC board for the audio portion, if I could eliminate those two tubes and the associated components, it would save me a whole lot of work.
Here is the schematic with the connections and components that I am considering eliminating marked with red. Can anyone confirm that I am not going to mess anything up in the other parts of the circuitry by doing this? Sometimes weird dependencies happen. It seems to me that they are completely isolated from everything else.
I would just ask XrayTonyB since he is a member here, but I don't seem to have the ability to send private messages to anyone. Link to his build videos:
With his few small mods, I think it would be a neat little amp for me to build. It tested good after he built it and put it through a series of tests on his bench.
Also, has anyone built this amp recently? Does anybody have one that they built recently, and if so how do you like it? It seems that current versions eliminate the loudness control that XrayTonyB cut out of his when he built it, but I haven't been able to confirm that yet.
Edit: updated schematic to show final values.
Here is the schematic with the connections and components that I am considering eliminating marked with red. Can anyone confirm that I am not going to mess anything up in the other parts of the circuitry by doing this? Sometimes weird dependencies happen. It seems to me that they are completely isolated from everything else.
I would just ask XrayTonyB since he is a member here, but I don't seem to have the ability to send private messages to anyone. Link to his build videos:
With his few small mods, I think it would be a neat little amp for me to build. It tested good after he built it and put it through a series of tests on his bench.
Also, has anyone built this amp recently? Does anybody have one that they built recently, and if so how do you like it? It seems that current versions eliminate the loudness control that XrayTonyB cut out of his when he built it, but I haven't been able to confirm that yet.
Edit: updated schematic to show final values.
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What appeals to you about that circuit? It would probably provoke a bit of discussion on this forum, mainly down to shared cathode bias and pentode mode with not a lot of regulation. There are some other designs here that are built in large numbers and well supported, e.g. Tubelab SPP (similar to that but separate cathode bias and can operate in different modes), El Cheapo and the Red Light Amplifier.
The circuit doesn't appeal to me necessarily. The kit price of $290 does though, as well as not having to search for and order a bunch of parts. It's all in one box ready to assemble. It performed fine on the test bench. Only one way to find out how it sounds. I'll look at the ones you mentioned if I can find them and see how much difficulty and expense is involved, but I suspect they will be considerably more difficult and expensive.
Break the connection to the magic eye circuit at the transformer and pull the tube. No other mods required.
I have a small Mingda 6P14P SE with magic eye tubes. I like them. The wife likes them! What is not to like about them?
Steve
I have a small Mingda 6P14P SE with magic eye tubes. I like them. The wife likes them! What is not to like about them?
Steve
It is a bog standard low power push pull circuit with common cathode resistor. Nothing wrong with that. But, $290 for chassis, transformers, parts, etc. is hard to knock.
I have acquired several different small Chinese amps for low prices. Within their limitations they provide useful results.
Pursuit of excellence may be a worthwhile goal. But, there is a point of diminishing returns wherein the cost of the improvement in the product does not translate to an improvement in the enjoyment of the device.
Steve
I have acquired several different small Chinese amps for low prices. Within their limitations they provide useful results.
Pursuit of excellence may be a worthwhile goal. But, there is a point of diminishing returns wherein the cost of the improvement in the product does not translate to an improvement in the enjoyment of the device.
Steve
I plan to wire point-to-point instead of using the PCB, and I don't want to wire all of the components for the magic eye tubes, which I'm not going to be able to see where the amp will be placed anyway.
Exactly! I haven't had a tube amp in many years, and I haven't built anything in many years. My budget is extremely low also. Since this little kit tested well for XrayTonyB after he built one, I'd like to try it. I just want to spare myself the time and effort of wiring the two magic eye tubes point-to-point, especially since I won't be able to see them anyway (amp sitting sideways on a shelf). XrayTonyB also built another equally inexpensive tube amp kit that did well after he modified the circuit a little bit. Very interesting videos if anyone hasn't seen them. I watched all the way through the "Electronics 101-1/2" tube amp build, which took three days for me to watch because it's about 10 hours long, but it was an excellent refresher course, and then he modified the circuit at the end to fix some distortion.
I misunderstood. When you said you were going to build just part of the circuit, and point-to-point instead of having a PCB, then I did not realise you were using a cheap kit of all the parts. Yes, you would struggle to build something in a smart case for that price.
Your schematic is correct, good luck.
Wire all the grid resistors directly to the grid pin with short leads.
Wire all the grid resistors directly to the grid pin with short leads.
So, in other words, for all of that magic eye tube circuitry in the diagram that I posted, it will be fine to leave all of it out of a point-to-point build without messing anything else up. I suspected it would be ok, but I haven't built anything in a long time, so I wanted someone to have another look just to make sure.
Of course, I could do it the "easy" way and just use the included PCB, but the thing is so simple - if I leave out the magic eye tube circuitry - that I might as well just do it point-to-point.
The EL84s will be biased quite hot with 350V B+ and 150ohm on the cathodes. The Soviet 6P14P or modern derivatives should be fine.
Xraytonyb bench tested it with the stock Chinese 6P14 tubes that come with it. I don't recall what he said about the possibility of using EL84 tubes. I'll have to go back and watch the 3 videos again, which I need to do anyway. Would you expect a problem with the EL84? I figure I'll just put the old military 6P14P tubes in it when needed. They are supposed to be more durable.
I have the kit in my shopping cart, but I haven't checked out yet.
I have the kit in my shopping cart, but I haven't checked out yet.
The original EL84 is rated for 12W plate dissipation. I believe 350V B+ and the cathode bias resistor will result in higher dissipation at idle. The Soviet 6P14P (and, for example, Sovtek El84) is rated at 14W. I'd also increase R21/22 to 160ohm, reducing a bit the idle current.
There will be no immediate problems, just the EL84 may have a shorter life, that's it.
PS: I mentioned the EL84 because that's what was shown in the schematic.
There will be no immediate problems, just the EL84 may have a shorter life, that's it.
PS: I mentioned the EL84 because that's what was shown in the schematic.
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Thanks, I'll jot that down in my notes. I'm not sure B+ is correct per xraytonyb's measurements. The diagram isn't 100% accurate. As I watch his videos again, I'll update the diagram with what he actually measured on the bench and post an updated version in case anyone reads this in the future.
If this little thing works and sounds ok, I'll probably just stick the old 6P14P in it when the stock Chinese tubes die. One of the nicest things about building it yourself is knowing what to change and where to change it if/when needed because you built it yourself. I haven't fiddled with a tube amp since 1993 when I gutted my then-new Dynaco ST-70 version ii (the 1990's reissue by Panor Corp) and upgraded all of the components and wired it point-to-point. I still have it, and it sounds great since I upgraded it internally, but I put it on the shelf 10 years ago due to lack of space (downsizing) and I sold the CJ preamp that I was using.
A big part of the appeal of this little amp is its very small size and the fact that it won't heat up the entire room. With 92 dB efficient speakers and a small room, I think 10 watts/channel will be fine. Of course it's very easy on the budget too. 🙂
If this little thing works and sounds ok, I'll probably just stick the old 6P14P in it when the stock Chinese tubes die. One of the nicest things about building it yourself is knowing what to change and where to change it if/when needed because you built it yourself. I haven't fiddled with a tube amp since 1993 when I gutted my then-new Dynaco ST-70 version ii (the 1990's reissue by Panor Corp) and upgraded all of the components and wired it point-to-point. I still have it, and it sounds great since I upgraded it internally, but I put it on the shelf 10 years ago due to lack of space (downsizing) and I sold the CJ preamp that I was using.
A big part of the appeal of this little amp is its very small size and the fact that it won't heat up the entire room. With 92 dB efficient speakers and a small room, I think 10 watts/channel will be fine. Of course it's very easy on the budget too. 🙂
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After doing some reading tonight, the EL84 really isn't designed for these voltages, correct? Why are they telling people that it can use the EL84? Marketing? It seems like the designers intended to use the old Soviet 6P14P, but to save a few bucks they are shipping it with the Chinese clone 6P14. Unless I am missing something, EL84 would be a poor choice to replace the 6P14P? I don't want to be replacing tubes constantly.
After some research and trying to sort out good information vs. bad, it does look like ECC85 is the correct "upgrade" for the Chinese 6N1 that comes in the kit. Anyone disagree? It does say ECC85 both on the PCB and the schematic.
Edit: I updated the attachment to my original post to reflect the tubes that are included in the kit (but they are Chinese clones), and I added the power supply too.
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It would be good to have separate cathode resistor (and capacitor) for each output tube. This will compensate possible un-balance of idle current of the output tubes.
Common cathode resistor actually emphasize the difference of idle currents.
Common cathode resistor actually emphasize the difference of idle currents.
Would that affect sound quality or tube life or both? How large could the impact be? I would like to know more. Please elaborate when you have a few moments.
Will the PCB be included with this kit? Are there any objections against using it? Why would PTP wiring be superior? At least assembling the kit by populating the PCB will be much (!) easier. Just leave out the indicator related components, according to your schematics modification.
Best regards!
Best regards!