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 Post subject: GE 5751-5814 tube amp/pre-amp from vintage military gear
PostPosted: Jan Fri 25, 2019 9:34 pm 
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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.

viewtopic.php?f=6&t=349458

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.

Circuit diagram:
Attachment:
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 Post subject: Re: GE 5751-5814 tube amp/pre-amp from vintage military gear
PostPosted: Jan Fri 25, 2019 11:03 pm 
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Quote:
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?
I don't fully understand your setup perhaps you can clarify. I think you are saying that phone-->tuner-->computerspeaker has good volume, but phone-->preamp-->computerspeaker has low volume. Is that right?

Typical audio inputs to amplifiers and pre-amplifiers (including your computer speakers) respond primarily to signal voltage changes, stronger is louder. Headphones and speakers, however, respond primarily to current, not voltage. Typical speaker and headphone output drivers are able to supply high enough current to drive speakers and headphones loudly but they do so with fairly low voltages, particularly true for headphones, lower voltages in fact than many amplifier inputs are happy with. Perhaps it is the very low voltage of your phone's headphone output jack is not enough for your computer speakers to amplify enough to be satisfactory. In that case, an extra stage of pre-amplifier gain will help.

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 Post subject: Re: GE 5751-5814 tube amp/pre-amp from vintage military gear
PostPosted: Jan Sat 26, 2019 6:06 am 
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@richfair Thanks for the reply. I connected the following way Phone -> Tuner(Aux mode) -> Pre-amp -> Computer Speaker.

In the first case, Phone -> Tuner(Aux mode) -> Computer Speaker.
I ran without the pre amp in the above setup and the music played fine(kinda liked the warm tone it put out) and the volume was good just like any normal sound from amplifier. I could also boost the sound if needed with the Level Set control present at the back of the tuner if needed.

In the second case, Phone -> Tuner(Aux mode) -> Pre-amp -> Computer Speaker. Here to get any sound at all I have to drive the Level set pot to all the way to max. And still the sound is just like very low but audible.

What am I missing here. Or it’s that I should put the preamp between the phone and the tuner?

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 Post subject: Re: GE 5751-5814 tube amp/pre-amp from vintage military gear
PostPosted: Jan Sat 26, 2019 12:21 pm 
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Update: I managed to fix the low volume part. One of the resistors didn’t get soldered properly. Now the volume is working fine. But the issue I noticed is that there is a loud hum present while the audio hours through the pre-amp.

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 Post subject: Re: GE 5751-5814 tube amp/pre-amp from vintage military gear
PostPosted: Jan Sat 26, 2019 4:39 pm 
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Glad the preamp is working now! Hum can come from many things such as unshielded high impedence audio paths, poor power supply regulation, and most commonly from unintended ground currents, a.k.a. "ground loops". First troubleshoot step is to disconnect the phone+tuner, and short the preamp's input (jumper grid to ground). Is the hum still present?

Assuming the hum is still present, take a look at audio grounds and AC safety grounds. It is very common to have hum simply because safety grounds are properly connected to each other at the power outlet, and audio grounds are properly connected through their cables, yet hum exists because currents existing in each pathway become mingled within the audio devices themselves. There is usually a way to solve the problem safely (keeping safety grounds connected) once the pathway(s) are identified.

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 Post subject: Re: GE 5751-5814 tube amp/pre-amp from vintage military gear
PostPosted: Jan Mon 28, 2019 5:52 am 
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Hi Richard,

I had the ground issue in the beginning when I was using only there 5751 i.e before I added the 5814 stage. Whenever I used to touch my hand on the tube tuner and the body of the pre-amp the hum would disappear. But now even doing that doesn't seem to solve the issue. I suspect the electrolytic cap too. It's a Philips 385V 22uF capacitor from an expensive temperature controller that I had salvaged few years ago. I checked it on my capacitor leakage tester and it worked fine. So may be I should try swapping that.

Also I had mentioned in the original post that the +300V power supply was those tiny transformer already present in the amplifier. I fed a 14VAC from a 7 or 9A transformer which got stepped unto 355V in that small transformer. The Voltage dropped to 304V when I was using only the 5751 but after adding the 5814 it has dropped drastically to below 200V. I'll need to get a power transformer and see. Don't know though this is a reason for the hum.

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 Post subject: Re: GE 5751-5814 tube amp/pre-amp from vintage military gear
PostPosted: Jan Tue 29, 2019 4:27 am 
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Your +B is from a fullwave rectifier. Is the hum fundamentally 120Hz or 60Hz?

Touching the tuner would ground your body, somewhat, so if that in turn reduced hum when you touched the preamp chassis, it indicates a shielding problem somewhere (even if that no longer seems to work). Is the chassis fully shielded?

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 Post subject: Re: GE 5751-5814 tube amp/pre-amp from vintage military gear
PostPosted: Jan Tue 29, 2019 4:50 am 
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The tiny transformer could well be a 400 cycle type, which would be typical for aircraft use, owing to its lesser weight. It may put out some voltage and current with a 60 cycle input but not as much as it would if feed with 400 cycle power.

inter-chassis hum problems can sometimes be mitigated by using a twisted pair, shielded cable. If the twisted pair's conductors were colored black and red, use red for the signal hot, black for the grounded (formerly shield) conductor. Connect the overall shield of the cable together with the black wire, but only at the end that the signal is going to. This might help.

This photo is an amplifier that drove one of the motors in a navigation display. (ID-499/ASA-13) The tubes are one 6AU6 and two 6AQ5. Note size of the power transformer, which has a primary of 115 VAC, 400 CPS, and an output of 500 volts, center tapped.
It is also a "tiny transformer".


Attachments:
IMG_5419.JPG
IMG_5419.JPG [ 39.15 KiB | Viewed 976 times ]

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 Post subject: Re: GE 5751-5814 tube amp/pre-amp from vintage military gear
PostPosted: Jan Thu 31, 2019 11:44 pm 
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Hi Richard,

It could be 60Hz from the supply. There was a grounding issue since the tuner is only two prong. Tried flipping the cord and the hum reduced.

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 Post subject: Re: GE 5751-5814 tube amp/pre-amp from vintage military gear
PostPosted: Jan Thu 31, 2019 11:54 pm 
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@COM-NAV-ECM The is what I was also thinking. The frequency is 400Hz but I wasn’t sure about the input voltage that was fed.

Also I’ll try with some twisted cable and see. I saw on Mr Carlson’s lab on youtube where Paul explains in a restoration of a tube equipment how AC wires need to the twists and kept close to chassis and other important stuffs.

Yes, my transformer is also very tiny similar to that! I think its my clumsy wiring that has to do with it. Because to day I removed the 5814 and 5751 tubes and used the 6x4W power supply and fed it to a pre-amp that I obtained from an EMUD 713ST radio on eBay for like $10. It was a tiny pre-amp using a 5751 since the original ECC83 was missing as the seller on eBay send a EF89 instead. It was working perfectly fine with that supply. No hum!! and sounds perfect. Strange!


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 Post subject: Re: GE 5751-5814 tube amp/pre-amp from vintage military gear
PostPosted: Feb Fri 01, 2019 3:12 pm 
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It's hard to comment without having more detail, but perhaps there is unwanted leakage in that 400Hz transformer you used for voltage step up, between windings or from a winding to the chassis. Perhaps it was removed from service because of the problem.

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 Post subject: Re: GE 5751-5814 tube amp/pre-amp from vintage military gear
PostPosted: Feb Fri 01, 2019 10:57 pm 
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Hi Richard,

Sorry for the confusion. So I'll explain in a brief manner what I did. I had acquired a pair of "Cylinder Head Temperature Amplifier" from a military aircraft. I opened the amplifier to find that it has three transformers and makes use of a 5814 & 5751 tubes with a 6X4W tube as rectifier.

So, I stripped down the circuit boards present in to just the bare bone tube sockets and only one power transformer. To get the required B+ supply of 300V I fed the input of the tiny power transformer from the aircraft amplifier with a 13VAC from another big center tapped transformer which is then fed to the 6X4W to get +300VDC. I had made a simple single stage pre-amp first only with the 5751 and then later on added the 5814 also to make a two stage pre-amp. But that didn't improve the hum situation I had.

Now the second experiment I decided to try was to use the German ECC83 pre-amp that I got from a working EMUD 713ST ( https://www.radiomuseum.org/r/delmonico_emud_713st.html ) phono/tuner as shown in the above picture. So, now I took the tiny power transformer and the big transformer along with the 6X4W to make a standalone B+ supply. I've attached a picture below of the supply I made. Since, the main transformer was center tapped I can get 6.5VAC for the filaments. I powered up the ECC83 pre-amp with the supply I made and now I noticed that the hum was no longer present and the audio was pretty pleasant to hear.



B+ DIY Power Supply w/ 6X4W:
Attachment:
B+ power supply.jpg
B+ power supply.jpg [ 88.77 KiB | Viewed 914 times ]


Fully working Pre-amp:
Attachment:
ECC83 pre-qmp.jpg
ECC83 pre-qmp.jpg [ 89.39 KiB | Viewed 914 times ]

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 Post subject: Re: GE 5751-5814 tube amp/pre-amp from vintage military gear
PostPosted: Feb Fri 01, 2019 11:13 pm 
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Quote:
I powered up the ECC83 pre-amp with the supply I made and now I noticed that the hum was no longer present and the audio was pretty pleasant to hear.
Ah, okay, now I understand. Presumably the 400Hz transformer is okay. So you have a clean power supply but do you still have a hum problem with the preamp you built, or is that no longer an issue?

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 Post subject: Re: GE 5751-5814 tube amp/pre-amp from vintage military gear
PostPosted: Feb Sat 02, 2019 3:38 pm 
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richfair wrote:
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So you have a clean power supply but do you still have a hum problem with the preamp you built, or is that no longer an issue?


Yes, when I use it with the pre-amp I built the hum exists. But using the same supply on the ECC83 pre-amp works perfectly fine. I suspect the messy wiring or a bad tube here.

To be absolutely sure I might need to power up the pre-amp I built from my HV bench PSU and see. Waiting for my Heathkit IP-17 to arrive.

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 Post subject: Re: GE 5751-5814 tube amp/pre-amp from vintage military gear
PostPosted: Feb Sun 03, 2019 6:24 pm 
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Vintage Collector wrote:
richfair wrote:
Quote:
So you have a clean power supply but do you still have a hum problem with the preamp you built, or is that no longer an issue?


Yes, when I use it with the pre-amp I built the hum exists. But using the same supply on the ECC83 pre-amp works perfectly fine. I suspect the messy wiring or a bad tube here.

To be absolutely sure I might need to power up the pre-amp I built from my HV bench PSU and see. Waiting for my Heathkit IP-17 to arrive.

The "Construction" chapter in any RCA Receiving Tube Handbook has a lot of good information about the proper physical layout
and wiring of a chassis. I followed those guidelines when building my first big (to me) vacuum tube project. It was a Dyna MK III amplifier relocated in the former chassis of a Gibson Lab Series musical instrument amplifier that had gone to Blue Smoke Mode. I also used in the project the RCA Handbook's 5879 based "Low Noise Preamplier", and the "Bass and Treble Tone Control Amplifier" (Baxandall using a 6EU7).
The resultant Frankenfier sounded squeaky clean with no significant hum or noise. There's a lot of good information about tube engineering in those handbooks (duh!) and also in the ARRL Annuals

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 Post subject: Re: GE 5751-5814 tube amp/pre-amp from vintage military gear
PostPosted: Feb Tue 05, 2019 8:16 am 
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I need to get the RCA handbook and need to do more depth reading. Now I really get why they are so neat with wirings in port to point wiring. Especially in amps and signal generating devices!

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 Post subject: Re: GE 5751-5814 tube amp/pre-amp from vintage military gear
PostPosted: Feb Sat 09, 2019 8:54 pm 
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richfair wrote:
Quote:
I powered up the ECC83 pre-amp with the supply I made and now I noticed that the hum was no longer present and the audio was pretty pleasant to hear.
Ah, okay, now I understand. Presumably the 400Hz transformer is okay. So you have a clean power supply but do you still have a hum problem with the preamp you built, or is that no longer an issue?


The hum is very very subtle when played through a speaker, but using a headphones yes it's much more hearable. but if I stop the song aka pause it, I can clearly hear a hum. But if I play the music the hum just goes away.

So, I tried an experiment by connecting the stereo audio output from the pre-amp( The German ECC83 EMUD prebuilt pre-amp, not the one I built) on an oscilloscope with the music paused which is when the hum is present and snapped a picture of the waveform. Here is what I got. Is it normal?


Attachments:
rsz_img_0436.jpg
rsz_img_0436.jpg [ 90.31 KiB | Viewed 782 times ]

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 Post subject: Re: GE 5751-5814 tube amp/pre-amp from vintage military gear
PostPosted: Feb Sun 10, 2019 2:17 am 
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Your pic doesn't say what the vertical timing is but that certainly looks like ground-based power supply noise, either 60Hz+120Hz or 50Hz+100Hz depending on your country. Look into ground loops, "star grounds". Others here have far more tube experience than I and will hopefully weigh in but I know that placing AC filaments in close proximity to a high impedance signal path is an invitation to hum problems that, on a 'scope, looks just like that! Also, when power ground currents intermingle with signal ground currents, any ripple in the supply can be imposed onto the signal, which also looks just like that! So, layout and where things are connected together...

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