GE 5751-5814 tube Audio amp/pre-amp from vintage military tube gear

Thread Starter

Rahulk70

Joined Dec 16, 2016
445
Hi folks,

A little while ago I had acquired two military aircraft cylinder head temperature amplifier from eBay. After some reverse engineering I retraced the circuit and decided to convert it to an audio amplifier. Below is the link to the previous question.

https://forum.allaboutcircuits.com/threads/schematics-or-manual-for-single-channel-cylinder-head-temperature-tube-amplifier.153998/#post-1324668

So, searched all over the internet and nothing came up for this amplifier. So, upon going through the circuit I noticed that it uses a 6x4W rectifier, 5814( similar to 12AU7) & 5751(similar to 12AX7 but with on 70 gain). I decided to make a pre-amp with this.

I've to admit that I'm fairly new to the tube world and until now have only done some regular restoration like capacitor & resistor restoration and minor mods on some tube gears with help from this wonderful website. But this is my first go at building something from ground level.

So, I trimmed all the wires and capacitors etc just leaving two transformers and the three tubes. I am planning to make a mono pre-amp for my Pilot FA-540 tuner. I want to feed the output from the cathode follower (12AU7) to the pre-amp. The audio from the tuner to a computer speaker sounded okay. But originally the tuner included a pre-amp and a power amp. But the later two are very expensive so I didn't buy those.

Now coming to my project. I built a simple pre-amp using the circuit as shown in the attachment below. I re-purposed on of the transformers already in the amplifier before to make the B+ supply. I fed input to the transformer from a Xentec 14VAC 6A transformer to the transformer input and was able to get 355VAC at the output & the center tap I grounded. I followed this website for making the 6x4W supply with just replacing the resistor for a 3.3M ohm to get a 304VAC output. The capacitor is a 385V 22uF Philips capacitor. I will try to get a Thordarson/stancor power transformer once I get the pre-amp properly working and a new chassis too.

http://w5dxs.tripod.com/6x4-power.htm

The audio after feeding from my phone to the tuner and then the output to a computer speaker was working fine and the audio volume was also good. But after using the pre-amp through the phone the audio volume went quite low. Is this normal? Like is it because of the lack of a power amplifier I'm seeing this issue? Also if I were to use the 5814 as an extra stage before the 5751 will it make any difference to the volume strength?

At present the circuit only uses the 5751 tube for amp. I've left the 5814 tube for now.
 

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Thread Starter

Rahulk70

Joined Dec 16, 2016
445
Hello,

If you are interested in tube based electronics, you could have a look here:
http://www.tubebooks.org/

Bertus
Hi Bertus,

Thank you very much for the link. I managed to fix the low volume issue. Turns out that a resistor didn't get soldered properly.Also now I added the 5814 as an extra stage. Now the amp works as its expected. Now the only issue is that there seems to be a hum in the background of the music.
 
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