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Old 13th February 2011, 05:16 PM   #1
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Hello,

I am new here and to tube amps, but seem to have taken on quite a project. My wife bought an old console record player that I have completely gutted. I bought a new record player and didn't care to try and fix the amp inside. I decided that I would just replace it, but then decided tube amps were expensive. So I figured I could just build one. I am excited about the project but climbing up a very steep learning curve. It's been a while since I learned about amps.

I d/l'ed schematics for a JA-30 EL34 PP type amp and will be running 2 mono channels.

I have ordered all the parts that I need except for the choke transformer. I am having trouble picking one. Then drawing calls for a 1.3H 1A xfrmer but this doesn't make much sense to me. Could somebody help be pick one that would suit the amp?

http://www.kitsparts.com/shop/downlo...c_pdf/JA30.pdf
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Old 13th February 2011, 06:08 PM   #2
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That diagram is incomplete. It shows two 12V power supplies that go nowhere, and some relay contacts with no coils.

The choke has to be rated for the maximum current it must handle but perhaps 1 Ampere is a little generous. A 1.3H 1A choke is a fairly big item, about 2/3 Joule energy storage.
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Old 13th February 2011, 06:14 PM   #3
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The 12V P/S goes to the heater filaments in the 12au7 and 12ax7 tubes. I haven't figured out the relays yet. I read somewhere it helps with high voltage delay, especially since I am using toroidal power xfmr's.
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Old 13th February 2011, 09:17 PM   #4
DF96 is offline DF96  England
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That circuit has some strange features. The 12AU7 does a partial phase split, which is then completed by the 12AX7 LTP. Is the idea to reduce LF phase shift, so more NFB can be used?

The AC part of the LTP tail is made smaller than it needs to be - perhaps to maintain DC balance? Otherwise, the circuit would require closely matched triodes in each envelope. Not sure what R13 and R14 do.

Always fascinating to look at other people's circuits!
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Old 13th February 2011, 09:43 PM   #5
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Holy cow! That is IMHO unnecessarily complex for a first build especially. The DC heaters are not necessary. Just use AC and tie the center tap of the heater winding to the cathode of an output tube to raise it up a bit.

I would have suggested a simple gain stage (say 12AX7 or 12AY7 common cathode) followed by a cathodyne (12AU7 would be fine here I think) and wire it either UL (ultra-linear) or pentode as you prefer with NFB taken back to the input cathode. (kind of like a Williamson without the extra driver stage). If you used UL output the feedback might not even be necessary.

For the choke it would seem that a Hammond of about 2H or so at about 300mA or so should be fine. I can't imagine a bias point that would need much more current than that.
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Old 13th February 2011, 09:46 PM   #6
chrish is offline chrish  Australia
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Why not rebuild the amp already inside? If the power and output transformers are ok you will have saved significant expense.
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Old 13th February 2011, 10:13 PM   #7
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I suppose I could have picked something easier for a first time but oh well.

I really didn't want to rebuilt the old amp, the thought of recapping it, and then troubleshooting it (the thing didn't do anything when I plugged it in which scared me since it was being stored outside), and then trying to troubleshoot a circuit with drifted resistors didn't sound like fun. My wife bought it more as a piece of furniture than anything else and then told me she wanted to listen to records on it.

I am cheating with a since I found a PCB for it. The whole PCB thing could go either way, it may be easier or harder.

That's enough of the backstory. I do have a couple more questions if somebody is willing to offer their time:

I see the relays in there and I read that they are to surpress an initial voltage spike from powering up the xfmrs. I have read that the toroidal xmfrs are really bad about this. Is this true or should I not worry about it?

Why are the there coupling capacitors in parallel (c8&c9)? Is this to get a better freq range?

Thanks for you time. I know I will have more questions. I will also try to get some pics up later since I know people like pics.
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Old 15th February 2011, 02:29 AM   #8
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Old 15th February 2011, 03:55 AM   #9
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That schematic looks okay, but it doesn't look suitable for a first project. Maybe something that is single channel, AC heaters with fewer tubes.
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Old 15th February 2011, 06:20 AM   #10
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Quote:
Originally Posted by chrish View Post
Why not rebuild the amp already inside? If the power and output transformers are ok you will have saved significant expense.
That's what I was thinking too, but judging from the photos it seems it's not a tube amp to begin with.

surfsanclem: having a PCB will be a tremendous help for a project like this. Definitely good to have.

The relay allows you to put the amp in hot standby and if you use a timer circuit it can provide a start-up delay allowing the tubes to preheat before applying high voltage (not that big an issue though). This also helps a little in reducing inrush current which can indeed especially for toroids be quite high and cause a fuse to trip.

Re: parallel coupling capacitors: larger value capacitors can have poor specs at higher frequencies (high ESR or parasitic inductance). This is especially true in electrolytic caps and much less so in modern film caps, e.g. MKP. A smaller parallel cap can sometimes improve the high frequency performance. As a matter of fact, people are divided as to whether this trick is needed or even desirable when using modern film caps.

Have fun with the build.

Kenneth
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