18-watt rectifier considerations

[PharmRock]

Newly built 18-watt, cathode-biased 2xEL84.
Wall voltage fluctuates from 117 to 122VAC.  PT is 300-0-300, which ends up ranging from 293 to 315 on the secondary side.  The wall voltages moves around so much its a little tricky to know what voltage readings go with what sometimes.

Using a JJ EZ81, depending on the wall voltage, I get anywhere from 348VDC B+ (about the high end of "ideal") to 366VDC, which is a little more than I want to be putting on the EL84 plates....want to prolong the life of the tubes given the market we are in now.

At 117VAC wall voltage, EL84's read 340V on the plates, 331V on the screens, and 11.4V on the cathode.  Currently using a 150R/5W cathode resistor. 

I'm wondering if it would be better to install an octal socket and go with either a 5Y3GT (I have a couple of USA-made tubes) or something else, or a SS rectifier and use a sag resistor to bring the voltage down a little.

Schematic attached.  Rail voltages below refer to a wall voltage of of either 117VAC or 122VAC (in parentheses)

A: 348 (366)
B: 331 (348)
C: 299 (319)
D: 297 (316)

[bmccowan]

I like octal rectifiers because the options are numerous. You are pretty limited with noval base rectifiers, although Weber will make up various custom Copper Caps. Although I would not change a valuable vintage amp, I always go with octal rectifiers on builds.

[Bieworm]

You could always add some 5W zeners between the cap can and the b+ from the rectifier.  That's what I do...

[PharmRock]

You could always add some 5W zeners between the cap can and the b+ from the rectifier.  That's what I do...

I was just looking at this on the interweb.  Rectifier B+ to tag strip with Zener(s) then to the reservoir cap, right?  The cathode of the Zeners pointing to the rectifier. 

[PharmRock]

I like octal rectifiers because the options are numerous. You are pretty limited with noval base rectifiers, although Weber will make up various custom Copper Caps. Although I would not change a valuable vintage amp, I always go with octal rectifiers on builds.

I may try the Zener approach first, and if that doesn't get me where I want to be then I'll try to octal rectifier.  I'll need to order a socket, but other than that it would be easy to punch out a larger hole in the chassis and unsolder/resolder a few wires.  I've got some Zener diodes in the parts bin so that would be quick and easy.

[sluckey]

Does your PT have a 5VAC winding?

[SILVERGUN]

At 117VAC wall voltage, EL84's read 340V on the plates, 331V on the screens, and 11.4V on the cathode.  Currently using a 150R/5W cathode resistor. 

Voltage is not as much your enemy as current is. There are plenty of amps out there humming along with 360V+ plates on EL84s.
Your "current" situation has you running over dissipation because of over current.
You could lower the B+ a little but (if tube life is your biggest concern) you'd be better served by raising the cathode resistor value to cool things off.

In other words, you'd have to lower the voltage a lot to get dissipation under control enough to make a difference in tube life, whereas if you kept the voltage the same and raised the cathode resistor, there is a predictable effect of lowered current.

Picture voltage as a bunch of people crammed into the front of the stage. Everyone is ok until they start to mosh. Current is the mosh pit of electricity.

[PharmRock]

Does your PT have a 5VAC winding?

Yes

[PharmRock]

At 117VAC wall voltage, EL84's read 340V on the plates, 331V on the screens, and 11.4V on the cathode.  Currently using a 150R/5W cathode resistor. 

Voltage is not as much your enemy as current is. There are plenty of amps out there humming along with 360V+ plates on EL84s.
Your "current" situation has you running over dissipation because of over current.
You could lower the B+ a little but (if tube life is your biggest concern) you'd be better served by raising the cathode resistor value to cool things off.

In other words, you'd have to lower the voltage a lot to get dissipation under control enough to make a difference in tube life, whereas if you kept the voltage the same and raised the cathode resistor, there is a predictable effect of lowered current.

Picture voltage as a bunch of people crammed into the front of the stage. Everyone is ok until they start to mosh. Current is the mosh pit of electricity.

Thanks SG...good advice.  I calculated the bias and realized I was way up there at 100-105% plate dissipation.  I have a 200R/5W cement resistor to swap in there to see where that gets me. 
I have a 180R/5W like this: https://www.amazon.com/dp/B07RS4RBHN?psc=1&ref=ppx_yo2ov_dt_b_product_details
Any thoughts on using these? 
I may also increase the individual screen grid resistors from 100R to 470R, but that may be down the road. 

So the plan is for the following (reassess after each step):
1) increase cathode resistor to get into acceptable plate dissipation range
2) possibly increase screen grid resistors to 470R
3) Put a Zener string between rectifier and reservoir cap
4) change to octal rectifier

[pdf64]

Making such a significant increase in cathode resistor value is a classic recipe for a lot of bias shift at high signal levels, leading to a thin, reedy tone.

[PharmRock]

Swapped out the 150R for 200R
Wall: 122VAC

Plates: 367
Screens: 355
Cathode: 13.3

Calculated plate dissipation = 11.1W per tube (92%)

[PharmRock]

Alright...I ran two 8.7V/5W Zeners between the EZ81 to the 32uF reservoir cap.  With a wall voltage of 122VAC, I ended up getting:
plates: 352
screens: 344
cathode: 12.95
calculated plate dissipation = 10.4 watts...this is using the 200R cathode resistor.

Is a 180R the next move?  I still haven't cranked it yet but the cleans are sounding really really good. the EF86 is definitely a winner.

[dude]

You’re there

[Bieworm]

Alright...I ran two 8.7V/5W Zeners between the EZ81 to the 32uF reservoir cap.  With a wall voltage of 122VAC, I ended up getting:
plates: 352
screens: 344
cathode: 12.95
calculated plate dissipation = 10.4 watts...this is using the 200R cathode resistor.

Is a 180R the next move?  I still haven't cranked it yet but the cleans are sounding really really good. the EF86 is definitely a winner.

at 18watt.com we end up using 180R cathode resistor, parallelled with a 2200uf 63V cap

[PharmRock]

at 18watt.com we end up using 180R cathode resistor, parallelled with a 2200uf 63V cap

That may end up what use as far as the resistor goes.  Eventually I will split the cathodes of the EL84s so I can run that attenuation circuit, at which point I will need to double the resistor value.
Regarding the 2200uF...where do you source these?  I was able to get a couple of 1000uF/50V caps but they took a while and were on backorder.  Is there much difference between a 1000uF and 2200uF cap as far as the behavior and tone of the tubes?

[shooter]

there may be an extra zero in that 2200UF cap.  even 220uf is probably over-kill for Ck

[PharmRock]

there may be an extra zero in that 2200UF cap.  even 220uf is probably over-kill for Ck

that's what I thought when first looking into all this but that is actually correct.  The 18-watt forum guys have been using huge bypass caps on the EL84 cathodes for a while. 
https://www.18watt.com/viewtopic.php?t=17468&start=15
They swear that using the larger cap makes the bass response much better and gives the amp a more fixed-bias feel.  This is my first time doing this amp so I can't speak to it directly.  Bieworm may have something to say about it since he's built a number of these.

[SILVERGUN]

The 18-watt forum guys

Guitar player to reality conversion is only about 12%,
Most of what we think we hear is a result of so many other factors. Fresh ears being a big one. Context being another.

But, for the cost of a cap you can convince yourself one way or another.

[dude]

SG, very true. But from experience the 1000uf bias cap does “seem to my ears” to tighten up the bass, 2200uf IMO, you ain’t going to hear the difference.  Shooter’s 500uf cap vs. 1000, probably no difference either. .
You’re at 10.4 dissipation, tube max is
11 watts, if you put the 180 in you’ll be at 11.4.
“Don’t fix it if it ain’t broke” 

[PRR]

If you treated the EL84s nice, Rk hardly matters.

If you are going to BEAT it into class AB, then yes a huge Ck delays the "bias shift at high signal levels, leading to a thin, reedy tone." (Great way to say it.)

[PharmRock]

Pardon my lack of understanding on this, but I'm getting a little confused here. 
It seems that I have my tubes properly biased using the 200R cathode resistor, since I am right at 10.4 watts, even though I started out with the traditional 1974 schematic resistor of 150R. 

You’re at 10.4 dissipation, tube max is
11 watts, if you put the 180 in you’ll be at 11.4.
“Don’t fix it if it ain’t broke” 

However, PDF's comment (referenced by PRR below) about thin-reedy tone arising from too much of an increase in the Rk made me think I was going in the wrong direction....or maybe in the right direction but I overshot the goal a little bit.  Should this amp be better biased near 100% dissipation on the EL84's (using 12W per tube as reference)?

A lot of the 18-watt guys recommend using a large cathode cap is a great change to the circuit, but it may just masking/delaying some bias shifting symptoms?  Looking at the posted schematic in the Library, it seems these amps shipped with Cks ranging from 50-500uF.  I have a 500uF in there at the moment because that's the largest I had on hand when I built it (the 1000uF's were backordered).
 
I'm not quite sure how I'll be using the amp....the clean tones on both channels sound great, but I would like to get that totally cranked 18-watt gnarly tone as well (haven't had a chance to crank the amp quite yet). 

[dude]

Put the 180 R in, you want 12 watts dissipation if that’s what the tube data says, l thought it was 11.
At the time l build my 18 watt, l was getting my info from 18watt.com, tried the 1000uf and liked it, seemed to tighten up the amp to me. Tone is different to everyone, you won’t hurt your tubes with the 180kr, try a 1000uf, let your ears tell you, my 1974 model has a 150/1000 and sounds great to my ears, at 2 and 10.

[dude]

Put the 180 R in, you want 12 watts dissipation if that’s what the tube data says, l thought it was 11.
At the time l build my 18 watt, l was getting my info from 18watt.com, tried the 1000uf and liked it, seemed to tighten up the amp to me. Tone is different to everyone, you won’t hurt your tubes with the 180kr, try a 1000uf, let your ears tell you, my 1974 model has a 150/1000 and sounds great to my ears, at 2 and 10. You can parallel two 500ufs for 1000ufs.

[SILVERGUN]

...(if tube life is your biggest concern) ...

...is like saying if engine longevity is your biggest concern, use less RPMs (not fun)
If getting fat is your biggest concern, eat less (not fun)

I doubt that you or I would actually hear a difference between 150 and 180 ohms. The tube would run slightly cooler on 180 Rk and last slightly longer.

My suggestion was to change the way you were using the appliance to achieve your goal of making it last longer.....TONE be damned.
Says the guy who is running 6K6GTs at 120%.
I am not afraid of the The Tube Apocalypse and I beat them like I own them.

One thing that may have confused you:
pdf64 made his comment in relation to increasing Rk whilst PRR referenced increasing Ck in relation to pdf's comment

[PRR]

..haven't had a chance to crank the amp quite yet

So do that. With a hot iron. Won't take long to try 50, 500, 5000.

[shooter]

Says the guy who is running 6K6GTs at 120%.I am not afraid of The Tube Apocalypse and I beat them like I own them.

 
you've learn't well the lessons of self biased amplification!

[PharmRock]

Thanks everyone for all the replies/feedback.  As I am still learning, I suppose my approach to tweaking the power amp was based on several things...the first being a realistic plate voltage.  With a 300-0-300 PT (vs 290), and a wall voltage that sometimes hits the 123 or so range, I figured that bringing the voltage down a little would be more appropriate for what is typically spec'd in the 18-watt schematics and layouts.  I definitely understand about having fun with the "high RPMs", but I also want the proper "idle" RPM which would be true whether I'm driving a 2015 Camry or a '66 GTO.  A while back I built a sort of clone of a stripped down VoxAC15 that sounded absolutely phenomenal, but ate my pricey Gold Lion EL84s pretty quick.  Granted, I was still very much the novice at the time and really didn't make any adjustments to the voltages, but man did it sound good! 
Along the same lines as the plate voltage is making sure I stay within the proper plate dissipation, which obviously brings me to the Ck Rk changes.  Having built a few fixed bias amps recently, I appreciate getting the plate dissipation to (ideally) no greater than 70% max is the way to go.  Cathode-biased, on the other hand ( as I am learning) has a lot more wiggle room to really push the limits of the tube.

I was able to crank it to "11" last night, if only for a little bit.  Nice overdrive in both the EF86 and Trem channels, with no ratty/crappy tones coming out of the amp at all.  I wouldn't call it "smooth" by any means, but that's not what I was expecting anyways.  I am pretty pleased with it right now, but have some tweaking to do related to the EF86 voltages and the tone control I have on that.  I'll probably start another thread related to that to see what ideas you guys may have. 

As for the EL84's, I may play around with Ck and Rk a little more when I split the cathodes to install the attenuation control originally developed by Geezer et al.  I have some 330R and 360R resistors on the way that may put me in the perfect spot as far as separated cathodes go.

Thanks again everyone for the comments and suggestions...keep 'em coming!

[PharmRock]

Made a few tweaks today.  Paralleled two 330R resistors on the EL84 cathodes. 
Plates: 341
Screens: 333
Cathode: 12.1

This puts me right at 95% max dissipation (was around ~81% with Rk=200R).  WOW...what a difference!  Silvergun...you are right about the fun factor.  This really opened up the amp and definitely has more character and responsiveness. 

Then I dropped in a 1000uF/50V cap for Ck in place of the 500uF.  I couldn't tell that much difference, and if it was a blind taste test I doubt I would know which is which. In any case the bottom end stays together really well even when on "10".

I also replaced the 1uF cathode cap on the EF86 and put in 10uF.  That woke up the EF86 a little bit...a little gainier than before, not quite as "crisp" with the 1uF but still clean and chimey.  I may see if I can shoot the middle with a 5uF cap, but that may be splitting hairs. 

[thetragichero]

A while back I built a sort of clone of a stripped down VoxAC15 that sounded absolutely phenomenal, but ate my pricey Gold Lion EL84s pretty quick.

sounds like you did a good job and built it just like vox does!