DIY Audio Projects Forum

Hello everyone, it has been a while since I have made any posts on the forum, so here is a project that I have been working on since christmas, yea it took a while to get this beast up and running due to the fact that the shape it was in was so bad that I had to strip the chassis down and soak it with cleaning solutions as it had rat and mouse droppings all over it. I will post some pictures.... I think anyone would agree that it really looked like a worthless piece of junk that likely had one of the two transformers fried on it.

The amplifier is a classic old EL6431 Philips amp, it uses 4XEL34 with an insane supply rail of 825V!

It has a funny output transformer, 70 volts all the way down to 10 volt switchable impedance, this being said, 10V = the 8 ohm tap.

In the end this thing turned into a really cool looking purple chassis instrument mono block amplifier, AKA "The purple Amp" or "The Crushinator" due to the very dense mass of the thing.

I will post some pictures, so here are some added features....

- Selenium be gone, better ventilation, much cooler running and better power efficiency due to the use of Silicon rectifiers.
- High voltage relay added to give the output tubes more time to warm up.
- Bias supply is mosfet regulated for added stability and reliability, it is also voltage adjustable with the use of a pot.
- power supply uses two 330MFD 450V high temp capacitors in series to gain better noise rejection and regulation in the 825 volt supply .
- One mic input converted to high impedance guitar input with 1/4" jack.

As for sound wise, this thing sounds great, it has a very "fat" bottom end, not "soft" like when a tube rectifier is used. perfect for an electric bass, which sounds way better than the amp that came with my bass. The best part is that I can hook up any audio source and gig along with what I play as each input has it's own volume control.

Music sounds smooth in the highs, a very unique but natural sound, and there is that warm sound of tube distortion, and the bottom end reveals each bass line that is played in a song very well, like I said before, the amplifier sounds quite "fat" in terms of bass.

I'm not sure how common this amplifier is, but If anyone was to get a hold of one let me know as I will have schematics and procedures at which to take when it comes time to fire the amplifier up as it is not safe just to plug it in and hope for the best, especially if with the state I found this one in.

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My Mini 12AU7 Tube Preamp, Pioneer SX-D7000, JVC XL-V221BK, JBL L80T, DCM TF700

• Project Page - Mini 12AU7 Tube Preamplifier
• Support/Questions - Mini 12AU7 Tube Preamp thread

Nice. Looking forward to pics. What is the output impedance of the amp?
Did a comparison with the P3A?

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Kind Regards,
Dinesh

Some of my diy audio projects:

• Elliot Sound Products - P3A 100W Amplifier
• TMM 2.5 Way Loudspeaker with Scanspeak 18W8546
• 2-Way MTM Loudspeaker with Scanspeak 21W8554 and D2904-7100

Well Dinesh, the problem with comparing the two is that the P3A is a stereo amp and is also solid state versus tube, comparing a tube amp and a transistor amp is like comparing a spoon to a fork.

If I had two of the amplifiers it would be easier to compare, but because it is summer, and the tube amp takes a good half hour to fully warm up, I much prefer the P3A for it's cooler running efficiency with a clearer quicker sound sound by comparison, especially the bottom end for example, the P3A isn't as "fat" sounding in that area as the Philips amp.

The other difference is that this amplifier is not just a strait power amp like my P3A, where I have no tone controls and other audio mixing in the signal path. I also don't have the P3A running at it's best, I only have 20 volt rails, which I have been tempted to bias the P3A into a bit of class A for this reason.

Well, here are some pictures for now, I might make some more thorough comparisons with the P3A to see how the sound really compares by using the same audio source and speakers.



The output impedance is calculated in terms of the output voltage settings. There is 70V, 50V, 35V, 25V, and 10V taps with a single 4.4V tap on the output transformer. For an 8 ohm speaker I have it set at 10V, I can't recall how I worked this out but it is in the manual some where, and the amplifier runs happily at this setting.

Here is a site with the manual; http://frank.pocnet.net/instruments/Phi ... L6431.html

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My Mini 12AU7 Tube Preamp, Pioneer SX-D7000, JVC XL-V221BK, JBL L80T, DCM TF700

• Project Page - Mini 12AU7 Tube Preamplifier
• Support/Questions - Mini 12AU7 Tube Preamp thread

And some more pictures, well, the after picture....

I will add the new schematic for the amplifier at some point when I get around to drawing it up.

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My Mini 12AU7 Tube Preamp, Pioneer SX-D7000, JVC XL-V221BK, JBL L80T, DCM TF700

• Project Page - Mini 12AU7 Tube Preamplifier
• Support/Questions - Mini 12AU7 Tube Preamp thread

Wow that is very impressive!

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Kind Regards,
Dinesh

Some of my diy audio projects:

• Elliot Sound Products - P3A 100W Amplifier
• TMM 2.5 Way Loudspeaker with Scanspeak 18W8546
• 2-Way MTM Loudspeaker with Scanspeak 21W8554 and D2904-7100

Fantastic work Alan. The rebuild looks superb!
Cheers

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[ DIY Mains AC Power Cable Cord ] - [ Gobo LM1875 Amp Kit ] - [ Tang Band D4-1 Horn Speaker Kit ] - [ Monoblock Push-Pull KT88 Tube Amp Kit ]

A few updates, I added the shields to the 12AX7 tubes (which I decided to be creative and paint them green) and added the RF shield that goes behind the control panel, did a quick paint job on that as well and looks surprisingly good. I also put some newer 6AU6 tubes in, the ones I had in there were very noisy, rattled and rang as I played my guitar through the amp. They also made an arcing sound every now and then, very microphonic indeed.

I do have a rather annoying "buzz" that I think is coming from the power transformer, something I still have to look into but I have found this to be a very typical problem on these old PA amplifiers as there grounding schemes are very poor, likely doesn't help with the noise any.

Thanks for the encouragement guys, there should be some better pictures up in the gallery, but I still have yet to repaint the cover that goes with this amplifier.

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My Mini 12AU7 Tube Preamp, Pioneer SX-D7000, JVC XL-V221BK, JBL L80T, DCM TF700

• Project Page - Mini 12AU7 Tube Preamplifier
• Support/Questions - Mini 12AU7 Tube Preamp thread

Hello,
This is my first post here. I saw your nice work and because I also have one of these beasts, decided to exchange some ideas.
I'm planning to convert mine into a guitar amp. My idea is to use a standard fullwave CT rectifier (not a voltage doubler) so to decrease the anode voltage and use other operating point close to that of the typical guitar amps in class AB. If the voltage in this connection is too low for 100W amp I may convert it to a 50W.

The OT has many taps so I think it won't be a problem to connect it for 3.4 Ohm (for 50W) or 1.7Ohm (for 100W) anode load.
I have to test the transformers first and then I will see what can be done of this amp.

Best regards,
Peter

Hello Peter, welcome to the forum, I am very glad to here that I am not the only one with the same intent for this amplifier. For starters I recommend changing as little to the amplifier design as possible, it is quite unique and in doing so you can be sure the amplifier will work properly. Also, be VERY careful with this one, it is running over 800 volts in the power supply at a decent current, more than enough to be fatal.

Because you seem to have the same idea as I do, I would recommend you do the same upgrades as I did, it means you are almost guaranteed to have success with converting the amplifier, and really all I had to do was change one of the mic inputs to take a 1/4" jack and have a higher input impedance, it works fine for me with a bass guitar, although I am going to change the 6AU6 to a 6AV6 or 12AV6 with a FET input to get better noise rejection and hopefully eliminate the microphonic "rattle" when I am plying the amplifier really loud.

The first most important part to change in the power supply is that stinky selenium rectifier block, get 8x 1N5406 rectifiers, 8x 0.01uF 1KV ceramic capacitors, and 8x 390K 1 watt resistors. This and the supply filter cap upgrade is the best improvement. I don't really recommend the fullwave CT setup because I'm not sure how well it will actually work without redesigning the power supply completely.

As for the loading, 3.4 ohms is a bit low, but will work on the 4.4 volt tap on the output transformer, and I'm not sure how well 1.7 ohms will do. I have an 8 ohm speaker on the 10 volt tap, not sure of the output but there is a chart in the manual for this amplifier.

I would love to see some pictures of the amplifier you have, it would give me an idea of what you need to replace if I can see what kind of shape it is in. Do you know if the amplifier works? If you havn't tried it though, don't plug it in, there is a chance you will get a very nasty cloud of garlic smoke coming from the amplifier you do.

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My Mini 12AU7 Tube Preamp, Pioneer SX-D7000, JVC XL-V221BK, JBL L80T, DCM TF700

• Project Page - Mini 12AU7 Tube Preamplifier
• Support/Questions - Mini 12AU7 Tube Preamp thread

Thank you for the info.
My amp looks similar to yours (it has a letter C in the serial number) only the connectors on the back are different.
Here are some pictures taken when I received it:









You can see that one of the cap cans was exploded and has disappeared. There was paper from it all over the top of the amp.

I started with removing the selenium rectifiers and some cleaning. Also measured the resistance of the transformers' windings.
I have some experience with tube amps (have builded one 18Watt from scratch and repaired several amps including one Fender Rivera Concert II).

My plan is to lower the B+ (I expect to get half than original - about 415V) and to use only two power tubes. I have made a typo in the plate loads - I meant kiloohms, not ohms...
In the manual there is a lot of info about the transformers - number of turns , diameters of the wires (for PT only), so I don't think it will be a problem to convert the amp into 50W.

How does yours sound? I expect it to be very loud and clean, which is good for bass maybe but I would like to get more overdriven sounds from it. I'm also afraid that the current production tubes will not handle such a high anode voltage. You are using Mullards so that will not be a problem for you. I also have Mullards (some of them came with this amp) and RFT's.

Here is a picture from another Philips that was sold recently on ebay with melted anode on one of the Ei EL34s:



I will post info and pictures when there is some progress with my work.

Peter