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 Post subject: Want to build audio amp from vintage e-waste
PostPosted: Nov Sat 28, 2020 12:56 am 
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Joined: Nov Wed 11, 2020 10:59 pm
Posts: 42
Location: 78218 USA
I'm gathering parts and I guess I'm working from the ends towards the middle. I scrapped an old TV and I think I can make something out of the big iron, choke and output transformer. The specs are attached. I have other power/output transformers and chokes I just havent taken the time to measure them yet.

I am just not sure what to put in the middle! I have 5u4gb, 5y3gt, 5Z3 and more for rectifiers. 6aq5 6bq5 12a**7's 6550, 6v6, 6l6 and more. I guess I'm trying to find out what design and tubes I should use. Heck I have a few hundred loctals to make a rugged one if that's an option too. I have the giant 4,5,6 pin tubes too, so maybe a 77 to a 42 to 6A3's with a 5Z3?

I repair amps and radios all the time, and do quite well, but I am no designer. I can follow a schematic and troubleshoot and make changes to caps and resistors to get the correct voltage and current with relative ease.

RCA level input with a simple volume and tone control would be great.

I don't know, I'm kinda dumb when it comes to this part. I can appreciate art and critique it but you will never see me in front of a canvas with a brush.

Anyways I hope y'all had a great Thanksgiving here in the states and hope you also have a fantastic weekend.


Attachments:
File comment: from a 70's 20 tube TV
amp parts.png
amp parts.png [ 279.17 KiB | Viewed 873 times ]
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 Post subject: Re: Want to build audio amp from vintage e-waste
PostPosted: Nov Sat 28, 2020 2:01 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 26916
Location: Detroit, MI USA
Two problems right away.

That power transformer has no 5 volt filament winding, and the HV secondary has no center tap, so you aren't using any of the common 5 volt rectifier tubes with it. No doubt given the specs for the HV secondary, this TV used a solid state circuit for B+. You could use a full wave bridge and get a reasonable B+ voltage for a simple audio amplifier.

The output transformer is specified as a 12K primary under the conditions it was used in the TV. None of the commonly used output tubes you listed are looking for a load anywhere near that high. However, if you use a 4Ω speaker instead of the 8Ω shown in the specs, it will come closer to matching a 6AQ5, 6V6 or 42. But since TV's didn't usually have very much power output from the audio, it's probably only good for a few watts at most.

If you have other transformers available, you might find some which would be a better choice than these.

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 Post subject: Re: Want to build audio amp from vintage e-waste
PostPosted: Nov Sat 28, 2020 2:44 am 
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Joined: Nov Wed 11, 2020 10:59 pm
Posts: 42
Location: 78218 USA
A virtual center tap is not ideal but possible?

I have 6AL5, 6CA4 6X4 and a few others.

Here is the power and output parts


Attachments:
File comment: power and output
the power and output.png
the power and output.png [ 294.79 KiB | Viewed 852 times ]
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 Post subject: Re: Want to build audio amp from vintage e-waste
PostPosted: Nov Sat 28, 2020 2:57 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 26916
Location: Detroit, MI USA
Just use the power supply circuit it had originally, which is a solid state full wave bridge. You can eliminate the unnecessary parts associated with the instant on and the degaussing circuit and use a fuse instead of the circuit breaker. This thing is capable of providing way more current than you will need for an audio amplifier since it powered a color TV originally.

A 6AL5 wouldn't work for a B+ rectifier as it can only put out a few ma. To use a 6 volt filament type rectifier like 6X4 or 6CA4, you'd need an actual center tapped HV winding or else you'd still need to use two silicon rectifiers to create a full wave bridge circuit with the tube as the other half of it. I don't like that idea.

The Sams spec shown for the audio output transformer doesn't match what's shown in tube manuals for a 6AQ5, no idea what's going on there. 12K is more than 2X what the 6AQ5 wants to see as a load. It's easy to calculate the actual turns ratio of the transformer and determine what it really is.

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 Post subject: Re: Want to build audio amp from vintage e-waste
PostPosted: Nov Sun 29, 2020 1:32 am 
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Joined: Nov Wed 11, 2020 10:59 pm
Posts: 42
Location: 78218 USA
Oh well! On the shelf for some other project. Thanks for the input everyone, I'm still learning.

Cheers!


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 Post subject: Re: Want to build audio amp from vintage e-waste
PostPosted: Nov Sun 29, 2020 7:05 am 
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Posts: 7160
Location: Vancouver Island, Canada
Don't give up on it, just collect some other parts.

A basic amp just needs one or two 12AX7 dual triodes in front of the power tube(s). Coupling between stages is simply a capacitor. Either one or two triode stages can be phase inverter if you use two output tubes and a tapped output transformer.

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 Post subject: Re: Want to build audio amp from vintage e-waste
PostPosted: Nov Sun 29, 2020 7:24 am 
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Joined: Nov Wed 11, 2020 10:59 pm
Posts: 42
Location: 78218 USA
Never said giving up, I just gotta go through the rest of my iron! That isn't gonna run quad 6550 or even a SE PP 6L6GC
that 25 pounder on the right came from a 1000 watt 2u rack mount karaoke unit from the philippines 240/50 that they fried here in the states, free on craigslist, just finished pulling a a few turns off each lead until i got back to enamel coat, same for the one on the left.

Output transformers I'm short on and i just need to repair the power ones I do have.


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20201113_224812.jpg
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 Post subject: Re: Want to build audio amp from vintage e-waste
PostPosted: Nov Mon 30, 2020 4:05 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 5031
Location: Rochester NY USA
I used a similar TV power transformer for a push-pull stereo 6AQ5 amplifier. Bigger than necessary for 10 watts per channel, but voltage was on the low side for the bigger power tubes. I'd offer a schematic but haven't written it down yet :(

If you have that Halldorson transformer, it's ideal for a push-pull 6L6 or EL34 amp, 20-30watts. A pair would be even better, but good luck with that...

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 Post subject: Re: Want to build audio amp from vintage e-waste
PostPosted: Dec Thu 03, 2020 10:24 am 
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Joined: Nov Wed 11, 2020 10:59 pm
Posts: 42
Location: 78218 USA
Tested a few more transformers, I'm pretty sure I found two more that are output. Here are the numbers I got.
both have 3 wires on input side so they are PP?
both are with 9VAC input.

Turns ratio 54.5:1 ish
Z=2970 ish
11.9K @4ohm
23.8K @8ohm

Turns ratio 28.125:1 ish
Z=79.00 ish
3164 @4ohm
6328 @8ohm

Digging through my handy RCA Receiving Tube Manual RC-27 and this chart as a guide.
https://www.angelfire.com/electronic/fu ... power.html

Thoughts?


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 Post subject: Re: Want to build audio amp from vintage e-waste
PostPosted: Dec Wed 09, 2020 9:28 pm 
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Joined: Nov Wed 27, 2013 5:59 am
Posts: 748
Location: Metzger Oregon
I’ll give my advice, for what it’s worth, as I’ve done something similar to what you’re doing. I started by looking at a box with all the parts in it that I had stashed, knowing it would be more than I ever needed for fixing old radios. Here’s the process I went through, your mileage may vary:

First, decide what you want to build; Hi-Fi amp, Low-Fi amp, or guitar amp? (I’m sure there are other options but these were the ones I came up with). I use "Low-Fi" to mean something with output transformer(s) that are smaller and can’t produce as much bass – though they can still sound good and be a lot of fun.

Next I decided if I wanted stereo or mono. I didn’t see much point in a mono amp build, aside from a guitar amp, as I could just as easily add in the parts for a second channel, so why not. That’s personal preference though.

Finally, I had to decide on my choice of components. I decided that any output transformer that was larger in size (likely higher wattage and better bass) would be set aside for something Hi-Fi, and smaller transformers would be for budget builds. I saw no point in using a more expensive tube, like a 6BQ5 or 6L6, in something that didn’t use a nicer output transformer. When building from scratch there is just no reason not to design in something cheaper and more common. I found that the 6AQ5 is super common and cheap, and as long as you don’t exceed the max voltages, it’s identical to the 6V6. Likewise, 6AV6/6SQ7 triodes are much cheaper than their dual triode counterparts. As long as you can provide enough current for the extra heater, might as well use them. There are lots of other small signal triode options as well, but they might require more circuit design experience in order to use them. Finally, I found that most of my output transformers have a high impedance ratio, as they were intended for use with 3.2 ohm speakers. Both the 6BQ5 and 6K6 list a 7000 ohm load, higher than most other pentodes, unless you start looking at low watt battery tubes. The 6K6 is much cheaper, as is the 7A5 loktal version.

One last word of advice. I spent a lot of time looking for used 4 ohm speakers and trying to match output transformers to my power tubes, in order to have things well matched. Then one day, I connected a 15” 8 ohm Stephens speaker to a PP 6V6 console amp, intended to drive a 3.2 ohm speaker, just for the heck of it. I was blown away, it sounded great – that put an end to hunting for 4 ohm speakers and possible plans to build my own speaker cabinets. In other words, don’t get too hung up on impedance matching unless you’re investing lots of $$ in a Hi-Fi amp build.


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 Post subject: Re: Want to build audio amp from vintage e-waste
PostPosted: Dec Wed 09, 2020 11:42 pm 
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Joined: Nov Wed 11, 2020 10:59 pm
Posts: 42
Location: 78218 USA
classicelectronicsguy wrote:
I’ll give my advice, for what it’s worth, as I’ve done something similar to what you’re doing. I started by looking at a box with all the parts in it that I had stashed, knowing it would be more than I ever needed for fixing old radios. Here’s the process I went through, your mileage may vary:


Wow, thank you. Certainly valuable advice and I appreciate your time and sharing
I'd like to have nothing but nice little monoblocks from mid grade to the best ear porn I've ever had.

As for this OP, I shoulda probably mentioned that at that time it was all I had to work with without destroying innocent tabletops I rescued in the first place. My best friend birthday is now in 13 days, my wife in 14 days and christmas is 15 days away and I don't have a single amp to give to anyone with what I have. :/ ?


Let Let me give a little history/backstory.

I cant build, design, code a darn thing.
I can copy, paste, edit, paste again........"there is no more original ideas"...THATS ME. And I cant look at a pile of parts and say oh....ROCKETSHIP!! and build a freaking rocket ship. Although I read RCA tube manuals for fun. It's slowly sticking.

It was 40 years ago I got whupped pretty good when dad caught me with my new magnets from christmas playing with his aa-100.i was sent to to my room to use my new frankiln ace1000. I've lived around musicians and tradesmen. Given enough time I can fix most things electrical, mechanical and things that control/compute. Venue LD, Burninman interactive art, Sr. Net Eng. for ISP.HostedPBX, computer certs up the you know The list goes on/
:oops:
Last count I had over 40 12AX7 and probably a few thousand in the 12**x family that are in the 80-100 outta
0 to 135, om my B&K 606 and 700 then there is the "let the kids play" and a test only boxes.
4x807 , 7x6550, 11x2a5, Gallon plastic milk jug 6bq5, half one 6bq6 5 gallon bucket of 6l6** 6v6** 6k6 etc 50c5 a few gallon ziplocks of em.
Some what I call exotics like 4 and 5 pins, mercury vapor and such. the old NE2 in my 30's ish supreme box. OOOH PRETTY COLORS, as well as a few piles of salvaged vfd, nixie and a other indicators, decent pile of rectifiers. My resistor and pots/switch game is solid mostly. Apple boxes full of new pc power cords and stuff like that.

Everything I have for power and output weighs less than 31 pounds

I have enough now for a PP 6*6**. I'm gonna need to see which will be the lowest THD whith what I have as of midnight tonight! The 2 other gifts? just going to be repurposed radio chassis that I will stuff into custom cabinets with a cheap BT thing I have a small qty box of.

So with deadline looming I'm just gonna do some bad things to get the gifts off my plate. I'll list the leftovers like the pink rca dialface and clock. I'm just gonna take the speaker amd transformer. There's nothing else of the radio, no back or board or anything. It was "post modern art" I bought it for cheap AF.


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 Post subject: Re: Want to build audio amp from vintage e-waste
PostPosted: Dec Fri 11, 2020 1:46 am 
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Joined: Nov Wed 27, 2013 5:59 am
Posts: 748
Location: Metzger Oregon
Based on what tubes you say you have, I would tend to look at using the 6BQ5s for use with a higher impedance transformer, and the 6L6 or 6V6 family for lower impedance, depending on power needs. I have found that the internet is full of schematics, as is this forum, so no need to have much design knowledge. Are you using these as mono blocks attached to a separate preamp? If so, then a low gain triode like a 6CG7, 6SN7, or something along those lines will work great as a first amp stage and a phase inverter. Sounds like you have push pull transformers so I am assuming that is the direction you are going.


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