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PostPosted: 20 Feb 2020, 21:13 
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Hello,
I did my best to search for other threads on the topic and it seems similar questions come around from time to time. But I couldn't find a satisfactory answer so here I am.

I am completely new to tubes but I have done a number of garden variety solid state projects. I am excited to try building some kind of stereo tube amp project and I was hoping to make use of some 50c5 and 12av6 tubes I happen to have.

Its an attractive concept to me because:
1. I already have the tubes and their 7-pin sockets.
2. It seems I could get away with just an isolation transformer, which I also have (saving money and keeping the voltage low for my first go at this {Of course, I will still treat the power as potentially lethal and take all due precautions}).

But, even though I am mostly hoping to learn from this, I would rather not build something that sounds like garbage or is hopelessly under powered. I would actually be even more excited about a headphone amp or phono preamp.

But, I have only found one schematic out there for anything like this (attached). I am a complete novice but a few things about this give me pause...two chokes in the power supply... and output transformers of that spec seem hard to come by.

If anyone out there has advice or can steer me towards a drawing for a decent project using these tubes, I will be very grateful.

Cheers


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PostPosted: 21 Feb 2020, 08:25 
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Joined: 19 May 2011, 05:38
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tie pins 4 and 5 to pin 3 on the 12av6's to make them inoperative (prevent any stuff)
This amp will make 1.5 to 2 watts


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PostPosted: 22 Feb 2020, 13:25 
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Thanks for the advice. I feel like the pin numbers are all wrong, too. Pins 5 and 6 are diodes according to this datasheet. 3 and 4 are the heater... Doesn't give one much confidence in the schematic. But if no one has a better suggestion for what I can do with these components, I'm gonna give it a try.


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PostPosted: 22 Feb 2020, 14:33 
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Minarai wrote:
I feel like the pin numbers are all wrong, too. Pins 5 and 6 are diodes according to this datasheet. 3 and 4 are the heater... Doesn't give one much confidence in the schematic.
That's because you're looking at using a 12AV6 and the pin numbers are for a 12SQ7. You will have to translate for the 12AV6.

The schematic looks ok for the 12AV6 and 50C5 if you use the correct pinouts. I would however bypass the cathode on the 50C5 otherwise the low end will likely suffer due to excessive Rp in the power stage. The feedback will correct some of that but likely not all.

I would stick with the 2.5kΩ output transformer as you'll get better performance overall. And watch your plate dissipation. The ratings on the 50C5 are all over the map on voltage but they are all max 5.5W on the plate. You likely be getting about 150V on the B+ meaning that you may need to increase the power stage cathode resistor to control your bias point and keep the plate dissipation within specification.

If you have a bunch of 12AV6s and 50C5s this is a reasonable schematic to try out. Just remember that the tubes are not the most expensive part of the amp by a long shot. There are better "first amps" out there if you are willing to spend a little on some other tubes. Just a thought.

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PostPosted: 22 Feb 2020, 17:20 
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Thanks for the input!
You are certainly right. My parts list for this is already over $100, even though I thought I had a lot of the components. Might be smarter to build something else but I guess I am emotionally invested in making use of this junk that I have kept around for the last year+ so I am going ahead.

Suggestion on the 50c5 bypass noted. That would mean adding a cap parallel to the cathode bias resistor? Will read up and try to figure out a value for that myself. Reading about measuring plate dissipation now, lol.

And last question (for now anyway)...
I am a little confused about what is going on at the speaker side of the output transformer. Is that supposed to show one leg is grounded and the other connects to the cathode of the 12av6 (via a 39k resistor)?


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PostPosted: 22 Feb 2020, 17:47 
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Minarai wrote:
That would mean adding a cap parallel to the cathode bias resistor? Will read up and try to figure out a value for that myself.
Yes. Quick back of the envelope says you'll need something at least about 270µf for power stage cathode bypass. Pentodes require a lot of bypass capacitance.

Minarai wrote:
Is that supposed to show one leg is grounded and the other connects to the cathode of the 12av6 (via a 39k resistor)?
That's exactly what it is supposed to show. Be sure to get the winding polarity correct. If not the feedback will be positive with horrible results.

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PostPosted: 23 Feb 2020, 15:13 
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Hi, It should work, like Matt said watch the pins on the driver and connect the diodes to the cathode to prevent it from "detecting" emi and noise. I also recommend the cathode capacitor. It is not a "hi end" amp but still is worthy of a project...since you already have the tubes. The 50C5s have recently gone up in price and are not as common as they used to be. With an isolation transformer it should be safe to build and use. I would add an EMI filter on the AC input to reduce crud from the mains contaminating the sound. An IEC power inlet connector with built in filter and often fuse is ideal. If you shop around they show up for under $5. Then just use a "PC" power cord for the amp. New ones of those are inexpensive as well. Watch the phase on the output transformer. One way will be fine the other will cause the negative feedback to be positive and sound horrible or even screech. I would isolate the signal ground from the chassis. This is now common practice. It allows the chassis to be a safety barrier and EMI shield. I like to use a type X2 capacitor (0.1uf about) and a resistor in parallel of between 120 and 150 ohms (1 watt size is fine) between the signal ground and the chassis. If you check out some of my various projects you will see how it is done. (picture is worth a 1000 words). True all this stuff adds to the cost, but IMO makes the project better and is worth it.

Good listening
Bruce

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PostPosted: 23 Feb 2020, 23:03 
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Thank you all for your input! I will give this a try and let you know when I have more questions, as I almost certainly will.


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PostPosted: 07 Mar 2020, 17:22 
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Ok, I have probably jumped the gun on almost everything here but I have made a start. Cobbled together a housing. Realized its a terrible design... made some changes. Learned a lot.
I am now making progress on building the power supply section and that is going smoothly. I think.

But, I am posting because I realized I'm not certain I can use the output transformers I bought or if so, exactly how to wire them. They are Hammond 125J (datasheet attached).

And if these are not usable for this project (as I now suspect), any recommendations on where I can get appropriate 2.5k:8 OTs would be great.

Thanks,


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PostPosted: 08 Mar 2020, 00:16 
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Location: australia
Hammond international distributor link attached. https://www.hammfg.com/electronics/dist ... assic-intl
If you have a mouser near you They tend to have all things Hammond. Just double check your opt choice. To me and I’m no expert it looks like your circuit is single ended and may require the relevant opt for that. The tech dept at Hammond are very helpful if you want to call.

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