Ultra Clean Push-Pull EL84 design?

printer2

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1.1

qxhQpBj.png
 

printer2

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AB2 6AQ5? 20 watts maybe?
Should be able to. Know of someone that maxed out at 30W but they would not last at that level for guitar work. Mind you, for those clean headroom moments...
 

2L man

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How many guitar amps have you seen lately running AB2? I doubt too many people here would want to go that route. And you can only pull out of the amp what the power supply will give. In this case there is not a lot more to draw out of it.
Hardly any! Thats why I wrote that EL84 is better tube for less distortion than 6V6. I like 6V6 more when its distortion comes smoother when wave tips "squish" when g1 drive signal approach zero and Screens begin to pull more current.

EL84 "clip" more sudden and it happen on about 20% higher power.
 

2L man

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Have you or anyone test this circuit? I have few 6N6GT which I'd like to use but example drive stage schematics use low winding ratio interstage transformer which are difficult to get without high cost. But I have IFR820s :)
 

printer2

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I have used the splitter and the source follower. The driver for the grids are just a source follower. The person that started the use of Mosfets in the hifi world has been blowing up tubes (in the interest of science) for a long time.



 

SerpentRuss

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Parts from Hoffman have been ordered. The Die is cast, LOL.

Current pre-amp tubes are JJ ECC803s. I've thought about down-grading them because they're thought to be more microphonic and in a small, loud combo, that's probably not the best match. I will probably wait and see on that swap and start up with those tubes. The matched set of JJ EL84s have less than 100 hours of play on them, so they should be fine.

I'll take lots of pics.
 

SerpentRuss

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I ordered my parts from Hoffman on Friday and they were in my mailbox Monday night. I didn't realize that we lived in the same state. I must say that the quality of the turret board is really fantastic. $30 dollars well spent.

The Hoffman conversion is pretty faithful to the original Pro Jr. schematic. I am thinking about some grounding changes and maybe elevating the heater ground reference. I'd like some feedback.

Essentially, there is a 470K bleed resistor on the Power Supply. I would add one more turret and make this a divider with a 100K resistor. That should give me about 50 volts DC as a ground reference. This is about the same voltage as the cathode of the PI.

I would also unground the power supply negative from the chassis so the only chassis reference (other than the safety ground from the power cord) would be at the input jack. I would omit the ground wire from the pots to the board and make two separate ground runs to the power supply "Z" capacitor negative. One from the PI section, one from the pre-amp section. I'm including a mark-up of what I'm thinking about. Omitted grounds are crossed out, new grounds connections are green.

I'm also thinking about putting the NFB on a On-Off-On switch with two values, the stock 100K and then something smaller with more cancelation.

HoffmanChanges.jpg
 

SerpentRuss

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Well everyone - We have lift-off. The Pro Jr. Conversion is finished.

As you can see I decided to go back with the Pro Jr. circuit using the Hoffman turret board. I also ordered the parts from him as well. I added a couple of resistors that he had really good prices on to my order, but I think the cost of the Pro Jr. board and parts was about $100. Money, very well spent.

I'm not the quickest builder and I'm far from the neatest. I guess it took me about 10 to 12 hours of work, total.

Mods to the original design.

1. IEC power cord.

2. Heater elevation ~46 Volt DC. You can see that suntan 10 UF cap under the reservoir cap in the photo, that's the filter for the heater voltage reference.

3. Three-position NFB switch (next to the power tube) 100K (stock), Off, and 43K. I picked the last value without testing thinking it would provide a noticeable decrease in gain and distortion, but it was really subtle. If I ever get it back from my son for any length of time, I may experiment more with different values.

4. Tube saver power resistor on the speaker jack switched connection.

5. Switched pot that becomes the power switch and Trainwreck (type 3) master volume. This was a 1MA pot that allows the grid signals to mix. I think a linear pot would do a better job. but it does dial down the volume nicely say between full on and half on. After that it quickly makes the amp silent. Still, I think it's a great addition to the amp.

6. Changed the grounding scheme from Fender as noted in the picture of the Hoffman board above. It seems to be much quieter but part of that could be the tube swapping I did.

7. Changed V1 to JJ 5751. I had my son play the amp both ways, the 5751 warms up the amp and takes some of that nasty edge out of the sound. It also allowed more gain from his pedal board without getting harsh.

8. Swapped in a JJ ECC81 into the PI location. This was a subtle change but definitely positive You have to crank the amp to hear it, but the ECC81 broadens the clean to dirty transition. The amp seems more touch responsive. We did not try the ECC81 with a 12AX7 in V1.

9. Lastly, variable bias of the Hoffman design allowed us to set the bias down to -12.6 volts from the stock -10.4 volts. That put the bias current at 29 mA on one tube, 28ma on the other. Interestingly, this set of power tubes were more closely matched when they were first put in a few months ago. The red JJ paint is almost gone, I don't know how quickly that normally happens, but I think these tubes are a bit baked already. They probably will last a lot longer now.


Incidentally, B+ and screen voltage are 325V and 306V respectively, and the pre-amp voltage with the tube swap is a bit lower than factory spec at 229V

Generally speaking, the goal for more clean sound was achieved. In the condition the Pro Jr. was in before the re-wire, it sounded surly early with the occasional spastic excursion that could only be attributed to PC board / tube socket issues.


20220211_190052.jpg
 
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SerpentRuss

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Yes, that's the green wire. It attaches via a toothed tab to the left hand screw that's just visible peaking over the chassis flange. It was more convenient to take the ground to that point than use the original location to the left of the transformer



IEC.JPG
 

SRBMusic

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Thanks for a great build/mod thread. I’d appreciate it if you could talk a little bit more about the sound, now that it’s done.
 

PCollen

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My son's Pro Jr. coughed up another hair ball. Short story, it got fried and was stored in a shed for many years. Here is a thread talking about when I first started working on it. It has acted up twice since I got it working, mainly circuit board issues.


So, he is happily playing my latest project and we have a few more tweaks for that, but I've promised him I would rebuild the Jr. as my next thing. He's requested something that has a lot of clean headroom and likes pedals.

As near as I can tell, the tubes are good (new), the OT and PT are fine, and the speaker is fine. I'd like to work off of a tried and true design and use as many of those parts as possible. As you're aware, there aren't many holes in the Pro Jr. faceplate either; just a tone and single volume.

Anyway, I'm open to suggestions. This thread will become the re-build thread and work will start very soon.
You might want to look into an Ultra-linear EL-84 power amp design.
 

PCollen

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Hey! It is a guitar amp after all.
Dr. Z made his Rt-66 amp with an Ultra-linear KT66 power amp stage. I had one for a few years. Great amp. No reason not to do the same with EL84's. it would be interesting, and unique.
 

printer2

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Dr. Z made his Rt-66 amp with an Ultra-linear KT66 power amp stage. I had one for a few years. Great amp. No reason not to do the same with EL84's. it would be interesting, and unique.
I was just kidding about it. The trouble is most of the transformers people use do not have taps for ultralinear operation. The Pro Jr would not have taps for UL.
 




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