Ultra Clean Push-Pull EL84 design?

[SerpentRuss]

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.

Grave robbing a dead Pro Jr.

Hey everyone - first post. A friend of mine is sending me an early, late 90s, fender pro jr. that was 220'd by a less than stellar electrician at an outdoor festival he was playing years ago. I'm not sure where the smoke came out yet, but I'd like to use the cabinet in a hand-wired, no PCB...

www.tdpri.com

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.

 

[dan40]

EL84's are so easy to drive in to distortion that you rarely see "high headroom" and "a pair of EL84's" in the same sentence. If you had more space in the chassis you could add two more but it seems you are quite limited on space.

 

[printer2]

How is it a 6V6 can make 18W, a EL84 can make 18W, but the EL84 has lower headroom?

 

[Phrygian77]

How is it a 6V6 can make 18W, a EL84 can make 18W, but the EL84 has lower headroom?

Amplification factor.

 

[RollingBender]

Spitfire clone might get you as close as you are going to get. Your tube compliment is there. Ditch the master volume and you have the correct controls.

 

[andrewRneumann]

This is a boring answer, but switching to a 12AY7 in V1 may be all it takes to clean this amp up. Never played one myself so I don't really have answer based on experience.

If you want to get more clean out of the EL84s with the existing PT, about the only thing I can think of is increase the screen voltage. How about reduce the screen supply resistor to 1K2 and double it's capacitor up to 47uF as possible mods? We're really nibbling around the edges though. I can't really tell the difference between 18W and 19W.

 

[Lowerleftcoast]

I would build a Pro Jr.

Change the Phase inverter to Blackface ab763 standard values. Reduce the filter resistor to provide more voltage to V1. Change the coupling caps before the EL84s to .047uF. 1k screen resistors on the EL84s. Change the NFB more like a Bassman. Maybe put in a NFB trim pot to find the sweet spot. Change the Filter caps to compliment the design. Probably use a trim pot for a presence as well.

Edit: I would add cathode caps to V1. Something in the 2.2uF to 4.7uF range. This would make more gain but reduce some sub bass. A reduction in gain (voltage divider) might be needed to counteract the increase. A 12AY7 might be considered but I would try to get there with a 12AX7 (Mullard/TungSol/Electro-harmonix/or Vintage. I wouldn't even try a JJ 12AX7.) I might increase the grid stoppers on the EL84 to 3.3k to 10k. Probably increase the grid stopper on V1 from 10k to 15k or so, not much. Get rid of the 470k resistor on the reservoir cap. Only use it if you turn it into a voltage divider to elevate the heater CT, but try to get rid of it. (The PT doesn't have much capability at 70mA and that resistor is robbing a little power.)

 

[Phrygian77]

The circuit has one gain stages too many. If you want to keep the topology and use 12AX7s, the simplest solution would be to use a split plate load on the second gain stage, going into the LTP.

 

[SerpentRuss]

Spitfire clone might get you as close as you are going to get. Your tube compliment is there. Ditch the master volume and you have the correct controls.

I do have a preference for a Cathode biased design, lazy that way I guess

 

[SerpentRuss]

EL84's are so easy to drive in to distortion that you rarely see "high headroom" and "a pair of EL84's" in the same sentence. If you had more space in the chassis you could add two more but it seems you are quite limited on space.

Amplification factor.

I said I didn't want to get too far off the beaten path... but I do have box of 6P1P-EVs and I'm really willing to buy a pair of anything for the output section that will work with the OT. That said, I don't think I can easily get octal tube sockets in the chassis, so I would like to stick with 9 pin tubes.

 

[SerpentRuss]

I would build a Pro Jr.

Change the Phase inverter to Blackface ab763 standard values. Reduce the filter resistor to provide more voltage to V1. Change the coupling caps before the EL84s to .047uF. 1k screen resistors on the EL84s. Change the NFB more like a Bassman. Maybe put in a NFB trim pot to find the sweet spot. Change the Filter caps to compliment the design. Probably use a trim pot for a presence as well.

Edit: I would add cathode caps to V1. Something in the 2.2uF to 4.7uF range. This would make more gain but reduce some sub bass. A reduction in gain (voltage divider) might be needed to counteract the increase. A 12AY7 might be considered but I would try to get there with a 12AX7 (Mullard/TungSol/Electro-harmonix/or Vintage. I wouldn't even try a JJ 12AX7.) I might increase the grid stoppers on the EL84 to 3.3k to 10k. Probably increase the grid stopper on V1 from 10k to 15k or so, not much. Get rid of the 470k resistor on the reservoir cap. Only use it if you turn it into a voltage divider to elevate the heater CT, but try to get rid of it. (The PT doesn't have much capability at 70mA and that resistor is robbing a little power.)

It would be easy to pick up a couple of volts on V1, I think that last filter resistor is 22K and V1 sits at about 230V.

 

[VintageSG]

If you want a really clean EL84 amp, build half a Mullard 5-10. It's an old hi-fi design that delivers 10 Watts RMS. Use an overdrive to get an overdriven sound and the amp to do the lifting.
The 6p1p-EV can be subbed in with only a few wiring changes and a change of cathode resistor, but take care with the screen resistor choice. I found through trial and smoke that they prefer a higher value.

 

[SerpentRuss]

If you want a really clean EL84 amp, build half a Mullard 5-10. It's an old hi-fi design that delivers 10 Watts RMS. Use an overdrive to get an overdriven sound and the amp to do the lifting.
The 6p1p-EV can be subbed in with only a few wiring changes and a change of cathode resistor, but take care with the screen resistor choice. I found through trial and smoke that they prefer a higher value.

I'd be interested to know what the initial (too low) value of screen resistor was on your 6P1P build. I have an SE project I'm tuning and tweaking now and if I recall, I'm using 1.8K, 2 watt.

I just looked at the Mullard 5-10, it's interesting. I don't think a EF86 pre-amp would be all that pedal friendly, although I have no experience, in this case, it has to be running at very low V+ since it's power is downstream of a 150K resistor.

 

[Lowerleftcoast]

I do have a preference for a Cathode biased design, lazy that way I guess

Cathode biased does not equal *a lot of clean headroom*.

 

[SerpentRuss]

Cathode biased does not equal *a lot of clean headroom*.

Yes, I may have to break down on this point, LOL.

 

[VintageSG]

I'd be interested to know what the initial (too low) value of screen resistor was on your 6P1P build. I have an SE project I'm tuning and tweaking now and if I recall, I'm using 1.8K, 2 watt.

I just looked at the Mullard 5-10, it's interesting. I don't think a EF86 pre-amp would be all that pedal friendly, although I have no experience, in this case, it has to be running at very low V+ since it's power is downstream of a 150K resistor.


View attachment 944263

Re the 6p1. Ignore me. I've gone back over what I did and it's on me... I got confused and I apologise if I've mislead.

Re the EF86. Vox used it in the past, and it's fun. Pentode front ends do sound different to triodes. Perhaps a pentode/triode such as the 6f1p-EV?. The way Musikding implement it in the Madamp G3 Blues is unlike anything you currently have ( probably ) Changing the screen voltage via P1 alters both gain and compression. You've then got a triode to play with.
The 6k4n-EV, 6j1 and 5654 ( and equivalents ) could possibly sub in for the EF86.

 

[schmee]

EL84's are so easy to drive in to distortion that you rarely see "high headroom" and "a pair of EL84's" in the same sentence. If you had more space in the chassis you could add two more but it seems you are quite limited on space.

I agree with this. Although at very low volume they can be a bit sparkly clean. But anything above that, or playing strong chords, they do tend to have a grainy smoothness to the sound.

Cathode biased does not equal *a lot of clean headroom*.

Yep, To me Cathode bias is just the opposite of clean headroom compared with push pull.

 

[schmee]

For clean headroom I'd probably build a BF Princeton non reverb into that chassis, or a Tweed Harvard. 2 x 6V6. They are noted for being clean. You could combine the tone as just a 'tone' instead of 'bass, treble' or use a stacked pot.

 

[SerpentRuss]

For clean headroom I'd probably build a BF Princeton non reverb into that chassis, or a Tweed Harvard. 2 x 6V6. They are noted for being clean. You could combine the tone as just a 'tone' instead of 'bass, treble' or use a stacked pot.

I wanted to maintain the PT. Is there really enough juice for a Harvard or Princeton? The schematic shows ~315 B+

 

[schmee]

I wanted to maintain the PT. Is there really enough juice for a Harvard or Princeton? The schematic shows ~315 B+

Hmmm, probably not for a Princeton. I think the Harvard asks for something in the low 300V range though doesn't it?
The Brown Princeton is down there too! 315 v I think