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Help with stereo tube amp design from parts
Help with stereo tube amp design from parts
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Old 9th January 2011, 05:57 AM   #1
CivicProtection is offline CivicProtection  Spain
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Default Help with stereo tube amp design from parts

Hello all! I'm a beginner, bit off a little too much. I removed the amplifier guts from an old 1961 Phillips reel-to-reel tape recorder. I hacked the wires and now I have three sub assemblies.
Long story short: I have no use for an old tape machine, I am on my way home in a month, can't take the heavy ***** with me, and all I want is to have a tubey headphone amplifier for my notebook with perhaps output for small speakers. And not see an antique go completely in the rubish bin.

Now after disengaging the amp circuits from the tape mechanism, I now have a bunch of hacked wires, daughterboards with big hairy iron transformers, tube sockets, old caps, and audio pots and connectors.

I just made a pile of crap. Some of the circuitry is now not needed, IE circuit to controll volume level tube, junk for the recording/ playback head, selector between PA input/mono/stereo radio. Many tape functions now obsolite.

I have no idea how to make all the sub-asemblies fit back together
and eliminate the unessesary circuits.

I do know I have a surplus of good valuable components to design a new amplifier (and better) stereo amp.

What I want is a tube stereo amp for headphones with jacks for speakers.
I have the headphone jacks that swich off sound to speakers.

I will document everything better for you folks later, but for now
I will give a list of what I can see:

-big power transformer (110-220)

-two big (why so big?) output transformers. Not sure IF they are outputs but what else could they be?

-tube sockets

-Mono audio sockets, stereo audio socket.

-pots dual and single

-nuts bolts

-old wiring still (felxable) power cord

-rotten old caps, and resistors

2x ecc83 12ax7
2x ef86 6267
2x el84 6bq5

Thankyou so much for any kind of designs or help of any kind, I'm excited to get started. I am hoping I can make something with what I just pulled apart.
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Old 9th January 2011, 06:22 AM   #2
singa is offline singa  Singapore
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Save those resistors! They are probably carbon compostion type
like allen bradleys. Good sounding.You can use those pcbs with the tubes
but you have to check what tubes they are by their numbers or letters
printed on the glass.Then knowing what tubes, you can find a headphone amp circuit that use them or similar tubes and start from there but most likely
you may have to change some of the connections on the pcb in order to use them in a amplifier circuit.Should not be too difficult.As for the transformers
if there are markings as to their voltages so much the better if not you will
have to power them and check with a multimeter but be careful as high voltages (DANGER)
are involved.Safety first!


Last edited by singa; 9th January 2011 at 06:25 AM.
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Old 9th January 2011, 11:05 AM   #3
mastodon is offline mastodon  Netherlands
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I'd recommend tracing the schematic from the boards you have there. The tube complements and circuits themselves will show what each part's function was/is.

Then, you could consider just using the original output topology (which is single ended, assuming it was a stereo amplifier). The original probably uses the EL84's in pentode mode, which you could consider changing to triode mode. As you're aiming for a headphone amp, the power loss is not really a problem. Impedance mismatching, however, may be an issue, but again, this will mostly result in further power loss. Suppose you end up with a power output of say 500mW (sort of a worst case scenario), you'll still have ample power to drive any mid-range set of headphones.

Preamp stage topology will probably be a bit different from the original Philips stuff, since you're going to get rid of some functionality. You might only need one 12ax7 for both channels if you don't need any tone control or mixing capability.

There's dozens of schematics out there describing single ended EL84/other pentode amps, either or not in triode mode, and thousands of little triode preamp stage circuits. You will have to match them, which will involve some experimentation. Do you have the time for that? If not, then opt for something ready-made. Google turns out this one for example: Free Schematic Diagram at www.circuitdiagram.net
But there's lots more out there.
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Old 9th January 2011, 10:06 PM   #4
tomchr is offline tomchr  Canada
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EL84 single-ended output stage driven by either the EF86 or ECC83. That would be my starting point with that tube collection. Something along the lines of the Tubelab Simple SE.

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Old 9th January 2011, 11:37 PM   #5
CivicProtection is offline CivicProtection  Spain
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Sorry it was super late when I wrote all this.

Here is a revised parts list of the components I seperated this morning, and baggied up:


2x EL84 6BQ5

3x ECC83 12AX7

2x EF86 6267


1x 200k (double ganged)

3x 500k (double ganged)

1x 200k (double but seperate knobs on one shaft)


2x output transformers: 291 WT 511 22

1x Power transformer: 301 WT 511 121

Rectifier: solid state 2 amp bridge rectifier SR-250. (small flat metal box)


Mono audio input sockets, stereo headphone jack with internal insert cutoff switch.

Resistors, lots of bad caps. Still good 1961 era power wire, audio coax, low voltage wire, wall supplu wire and plug.

8x tube sockets. Yes you counted 7 tubes in my list, but there is a magic eye tube, but I won't be using it.

I'd like to use up all the tubes and components if possible, I want to make the most of it.

Unfortunatly the transformers are "un capped" and all connections are exposed. So I guess I won't be bolting to the top of a box and showing it off along with the tubes.

Need a nice stirdy box to house all this in, possibly house it all in a metal project box with a vent to cool and show off the glow. Interestingly I also have the small electric moroe that was just for blowing air arounf the larger tape player case. Use it here too? Man that is one nice quality motor.

Last edited by CivicProtection; 9th January 2011 at 11:44 PM.
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Old 10th January 2011, 12:15 AM   #6
tomchr is offline tomchr  Canada
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I wouldn't use a fan in an audio amp. The fan itself makes noise (both acoustic and electronic) and moving air is not a quiet process.

You may not be able to safely expose the trafos on the top of the chassis, but you could still have the tubes topside. You could also build a small enclosure for the trafos. Along these lines.

Keep the magic eye tube. Those things are expensive...

Google "EL84 amplifier". I'm sure you'll find a schematic. Or search this forum. I linked to a schematic in my earlier post...

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Old 10th January 2011, 12:20 AM   #7
tomchr is offline tomchr  Canada
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RH84 is a common EL84 design. I've seen several threads on this forum that mention it. It uses ECC81 (not ECC83) to drive the EL84 so you'd either have to redesign a bit to use the ECC83 or EF86 in place of the ECC81. Or just buy an ECC81 tube... They're not that spendy.

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Old 10th January 2011, 01:10 AM   #8
CivicProtection is offline CivicProtection  Spain
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Thank you all for the help and advice. I certainly appreciate it all!
This is still a live project so I'm still open to anything you have to suggest/advise. My time limit is Feb 1. I am settling on the design suggested earlier:
Free Schematic Diagram at www.circuitdiagram.net

I had hoped to fill this baby with all the tubes I got but four (2 per channel) should be good.

I am exploring the possibility of using an ATX power supply,not the standard one, one of the longer ATX kind, a dead (power user type). Initially I was hoping to use one of the ugly grey breaker box or junction boxes from home depot, and spraying it orange. Nounting everything inside, tubes, everything. The 12v fan is brushless and run at 6.3v rectified from the transformer. It wouldn't produce the noise as explained to me earlier with the AC motor type fan. Containing all this in the box will save it from getting smashed to hell on my way back home. But I will entertain other box construction options. ATX power supply is one thing, but I think I might spend the dollars and get a convention grey metal project box.

As I said; I am settling on the circuit mentined before, but what should it sound like with my nice heavy, good quality transformers and tubes? What is this single ended thing?

At the moment I am trying to figure what all the posts mean on the audio output transformers. I know which wire on the power transformer are the heater wires.
But I don't want to break nothing.
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Old 10th January 2011, 03:43 PM   #9
boywonder is offline boywonder  United States
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Originally Posted by CivicProtection View Post
What is this single ended thing?
It's the topology of the output stage, either single ended (one output tube per channel) or push pull (two output tubes per channel).

Single ended output requires different output transformers than push pull, and from the info you have provided so far, the assumption is that your transformers are single ended.

Since you harvested two EL84s, There are two possibilities for the tape machine; it was either a push-pull mono topology, or a single ended stereo topology.

How many wires were connected to the output transformers? Was the tape recorder stereo or mono?
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Old 10th January 2011, 05:03 PM   #10
DF96 is offline DF96  England
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Is this another homework assignment?
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