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It would work fine. The EZ81 is a very stiff rectifier, so it will be pretty close to the lead sound, but wtih some creamy tones too. You might want to add back some of the treble you took away though!

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

The EZ81 works great for EL84 and 6V6 (even both being run together), it does off a little bit of sag.

The 5AR4/GZ34 drops a little less voltage and is stiffer (no sag, less brown sound). The 5AR4/GZ34 is what the big 50W Marshalls used. In my 6V6 18W the rectifier is pretty "stiff".

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The last clip I did was more or less a dieing set of tubes so the bass response was pronounced (insert old amp tech joke make it sound like it was just before it blew up) . When I stuck new tubes in I then had a lot of treble and even more when I started playing with the screen supply and bias resistor and cap (to me this help stiffened the bottom end )so I don't think i'm gonna have to add anything back. At this point i'm trying to loose some treble . Which I thinking the 6V6's will help with.

Have been messing with the sudo mix resistor but I'm not happy either way with or without cap. With the cap nice mid grit I like, but too bright. Without cap loose the brightness (good) but also loose the mid grit (bad)

Interesting you mention playing with the screen supply.

This thread at Amp Garage has been going on for a while http://ampgarage.com/forum/viewtopic.php?t=18855 it talks about the affects of the screen voltage and tone. You can see my experimental findings later in the thread.

In my 18W build I used a choke in series with a 1K resistor to create my screen supply. This puts my screen voltage well below the plates (I cant remeber the exact voltage but it's around 20V I think) while keeping the supply tight and stiff. Putting the screens below the plates really reduces fizz and adds a nice bit of compression (fizz being common in 18Ws). The voltage divider mod mentioned in that thread further reduces the voltage and rolls off a bit more highs and adds compression. In an 18W amp I didn't think this mod worked well but in a Fender (where the voltages are high and the screens and plates run very close to each other) I could see this being an excellent mod.


Instead of bypassing that resistor with a cap play around with that resistor's value. The early Marshalls used 270K then later went to 470K. There is no reason you can't try other values, 47K-470K would be good. In my build I actually temporarily wires up a SPST to A/B between a 270K/470K mixer/grid stopper. Interesting tonnal qualities to both.

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Interesting thread. With my results being a little different but I did not use the 220ks to ground (will Have to try that)

From what I gather from you just run the 220k from pin 9 after the screen resistor to ground ?

-Started with removing the 100R and putting in putting in two 470R to each tube did not like that much
-Went to 1k where the 100R was and to two single 100r to the screens found better (fizzys also reduced)
-Then changed to dropping resistor on cap 2k2 (but in hind site this was two far)

But like anything you change one thing in this amp you can effect the other

What I have found at this point is that the change in screen is less effective for bass response on the TMB side (other than fizzy) than it is on the normal. Probably from all the gain and brightness and gain added to the TMB . I had been concentrating to much on TMB. When I went back to playing on the normal channel it had morphed to far towards Angus and much less Malcolm (All the bass response went way lower). I'm guessing similar to changing the bypass cap on the the normal channel's V1. So I had to go back and play with the dropping resistor across the cap. Settled on the 1k8 with the shared 1k screen and two 100r to the tubes. Left with a normal channel that some where close too middle of Malcolm and Angus.

On a side note did try a 180r and 1000uf on the cathodes of the el84's that lost too much of the brown but was great if you add the front end gain of dropping the cathode of v1 on the TMB (brightness not with standing)

As for the the mix resistor's voltage dividers I have all ready switched too 220ks and still have not found what I was looking for. Even went as far adding a bypass cap to the 220's. If I did my math right 680pf over the 220k would be a very similar frequency bypass as 330pf over 470k. But Think im gonna try to 1000pf or 1200 pf to shift the bypass freq lower.

Sorry to hijack your thread and thanks to Kurt (Chris M) and Coco.

Interesting!
You may want to try the voltage divider mod. It allows you to keep the screen dropping resistor the standard size (and not affect voltages downstream) while still lowering the screen voltages on the tubes.



I actually discovered last night I had my screens connected to the PI supply As a result my screens were way lower than they should have been (about 80V too low but the tone sounded great). I have sinced fixed this issue; voltage wise the screens and plates are about 1V apart with the screens connected to the actual screen supply. Tone wise the amp is kinda stiff, bright and harsh.

Cathode wise the voltage has changed now that the screens have been corrected. When the screens were connected to the PI supply the voltage was 18.5V now with the screens connected to the actual screen supply the cathode voltage is 24.5V.

Now regarding the somewhat stiff, bright, harsh tone I tried the voltage divider screen mod (seems like a good time). This mod totally fixed things up! Now the screens are 6V below the plates (344V and 350V respectively). The tone is smoother and livelier. After doing the voltage divider mod the cathode voltage came down a bit to 23.3V.

Next up I am going to add a 25V Zener in parallel to my cathode resistor and cap. This is part of the Paul Ruby fizz trick. I did the mod before with a 20V Zener when my cathode voltage was 18.5V. Though I have not experienced any fizz I think having the Zener mod will be good to have. I imagine with my ES-335 and the amp cranked this mod could be good insurance.

My 6V6's cathode is 271R with a 560uF (measured) bypass cap. Pretty stiff (fixed bias) cathode combo. The 25V Zener is 5W btw thought the cathode resistor is only dissipating ~2W.

I also have my plate dissipation hovering right around 100% so right where I want it. 107% actually

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Well the two noval covers came in the mail today and I installed them in the amp. The amp is now done!

I am calling this amp a 20W Plexi.

Here is the interesting stuff





















Beautiful head cab built by Trinity. Using NOS JAN 6V6GTs, a NOS Slyvania 12AX7 in V1, EHX 12AX7 in V2 and a NOS RCA 12AX7 in V3. The cabinet on the bottom is my 18W sIII, cab again built by Trinity.

Added a VRM to scale the entire amp (limiting resistor is 220K). The 1M pot is linear taper.

The circuit of the amp is a mix of a JTM45 and a 1959. The voltages of the amp are all within 5V of that of a JTM45 except for the plates of the 6V6's which run 40V less.

The toggle switch closet to the input selects the bypass cap on V2. The switch allows you to select between no cap, 0.68uF cap and 150uF cap. This essentially allows you to go from no boost, to high/mid boost to fat boost. The jack beside the switch allows you to connect a simple SPST footswitch to do the boosting remotely.

The other toggle switch reconfigures the tonestack slope resistor and V2 grid stopper simultaneously. This lets you switch between a JMT45 setup or a later Plexi (1959) preamp. This switch along with the boost switch can make the preamp either a JMT45 preamp or Plexi preamp. The Plexi's ran high voltages than the JMT45s so when in JTM45 mode it is best to turn down the VRM just a touch to dial in a true JMT45 tone.

Going back to the voltages the output stage is setup very carefully. With a 560uF bypass cap and 25V 5W (Paul Ruby mod) Zener the output section is very tight and mimics a fixed bias output section very well. I spent a lot of time getting the dropping resistors and voltages just right, as a result the output tubes are biased at 96% plate dissipation with a 271R 5W cathode resistor. Wouldn't want to go higher than 96% with the large bypass cap (it negates the usual cathode bias NFB).

Added a 10K trim pot on the PI plates to dial in the right amount of "bloom" and balance in the PI.

With all the overspeced parts, elevated heater CT and hum pot, lead dress and design the amp has very little white/floor noise!

Since I already have an sIII I tried to make this amp different and unique from it. This amp is not really an 18W I'd say, it's more of a 20W Plexi. I spent a lot of time trying mods and dialing in the sound (amp has been done a while). Very thick lively tone on this amp. Surprisingly good clean tones (seems all the Plexi's are coming out like this). The amp sounds great with the EQ set flat which is something I don't do much with the sIII. 50uF-50uF filtering along with a inductive/resistive screen supply keeps the amp tight yet very dynamic and rich.

I'll try and post a schematic sometime but circuitry wise it's a real mix of the early and later Marshalls.

Hoping to record some clips of my 18W sIII with Tone Tubby San Rafeal vs. the 20W Plexi with Tone Tuby AlNiCo. Waiting to get my ES-335 back from the shop.

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We'll get this into the studio some time as well.

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Actually, a choke has the direct opposite effect on screen voltage. It makes the screen voltage move up and down more, as it attempts to regulate the screen current flow. I know this is contrary to common belief, but it happens to be true. A big cap on the screen supply will make it stiff, as with Hiwatts and Triwatts.

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Yes you're correct, my bad!

I even hooked up my DMM to the screens while playing to watch how they changed with regard to input signal.

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Heh, NP. This is a very common misconception that has gotten propagated around pretty widely.
So in reality, adding a choke into the screen supply of an amp will give it a looser and more compressed feel, with a less dynamic attack. I think this is one of the key differences between the dynamics of a Marshall 18W and a typical Vox/Matchless style amp.

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Schematic (large image)
http://i1.wp.com/pdfelectronics.com/wp- ... ematic.jpg?


Recorded the amp this week with my tele but waiting till I get my ES-335 back from the shop before posting clips.

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Did you find the hum balance pot useful or not? Did you hav hum you wanted to eliminate or did you just want to dial out as much as possible?

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Wanted to just try it and see how much hum I could get rid of. Hard to say how much the balance pot did as I also elevated the heater CT (pot's wiper goes to output tubes 25V cathode). That said the amp is very quiet at idle.

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I might try and see if I can make it zero - I will report back. I have used this on amps with no heater centre tap but never had reason to do it on an 18 yet, so wanted your feedback.

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

I'd try the heater elevation first. It's only moving one wire. Might be worth a try

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Finally got my guitar back from the shop. Going to make some clips of the amp this week.

In the couple hours of playing the amp with the ES-335 I am enjoying it a lot but it's a different experience EQing the sound. After only using single coils for my electrics a big semi-hollowbody with humbuckers is quite the change.

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Got some clips recorded. They turned out ok, curious to hear what others think. I am no recording whiz so I did the best with what I had/know.

http://pdfelectronics.com/18w-plexi/
Clips at bottom of the page

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Like I said before, we have to get this into the studio for a taste test. The tone is very thick and saturated on OD, but still tight. Sounds good. I'd like to hear someone take a rip at it.

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Man still looks great but sometimes pictures and relative size can be deceiving. Picked up 194a choke today for my next build and man it seems small physically in size.

And was wondering if you ever considered the 155j choke for this sorta closer to the 20 h in the original jtm45 ?

And what did you end up with for your B+

And yes do like the clips would hope for some with more crunching beatin' the crap out of some power cords (sorry but that's my thang)