18 watt plexi

[MrPlexi]

Hi there guys, I am new at this forum. I have built two single ended amps so far, but now I am planing to build 18 watt push pull plexi. I have few questions about this:
1. Can I build this amp with jmp 50 preamp instead of 18 watt preamp?
2. Can fender deluxe reverb output transformer be used for this build (primary 8500R CT, secondary 8R 25 watts)?

[shooter]

I'd think you're find on both counts,  if you're using 6V6 or EL84

[sluckey]

Welcome to the forum.

1. Can I build this amp with jmp 50 preamp instead of 18 watt preamp?
2. Can fender deluxe reverb output transformer be used for this build (primary 8500R CT, secondary 8R 25 watts)?

Yes to both questions.

Have you decided on which power amp to build? There are several popular ways you can go with this project. And you can combine most any of the Marshall preamps with several different power amps. The Plexi (1987) preamp is my favorite. Then you can drive an EL84 cathode biased power amp (think Marshall 1974 18 Watt). Or use a fixed bias EL84 amp like the November from AX84.com. And Mark Huss's 6V6 Plexi is popular too.

Our host, Doug Hoffman has plans for a TMB Stout which is basically a Marshall preamp driving a cathode biased EL84. He also offers a 6V6 Plexi that has a fixed bias 6V6 power amp.

     http://el34world.com/Hoffman/files/Hoffman_Stout.pdf
     http://el34world.com/Hoffman/files/Hoffman_Plexi_6V6-V2.pdf

And here are my versions of the November and 6V6 Plexi. (schematics and layout at the bottom of these pages.)

     http://sluckeyamps.com/november/november.htm
     http://sluckeyamps.com/6v6plexi/6v6plexi.htm

Lots of other ideas out there too. These should give you something to think about. This will be a fun project. We would love to see what you come up with.

[MrPlexi]

Thank you very much!
Your november plexi is just what i was looking for. I think I am going to split the chanels, and use 4 inputs i will also use solid state instead of tube rectifier and fixed bias. What choke should be used?

[sluckey]

I just used the small Fender choke like Hoffman sells.

[MrPlexi]

Can I use this http://www.banzaimusic.com/Fender-Choke-50mA-DC.html ?

[sluckey]

Can I use this http://www.banzaimusic.com/Fender-Choke-50mA-DC.html ?

Yes. That's exactly what I used.

[Vlada]

I'm planing to build 18 watt amp, and I've just found schematic from this topic which looks very interesting to me. Is 15k value for resistors in phase inverter corect? I can see that you changed some values in that part of the schematic

[sluckey]

15K is correct. You can see the original project at ax84.com. Revision 11 is the latest version and schematic link is at the bottom of this page...

     http://www.ax84.com/index.php/oldprojects.html?project_id=november

[labb]

Anyone on here belong to the 18watt.com forum? I did years ago before it crashed. I think it is back up and running. Anyone interested in the original Marshall 18 watt should check it out.

[Vlada]

Thanks sluckey
I will order parts now, and I'm going to post photos of my build in near future

[Vlada]

Gathered all parts except tubes and power transformer. I cant decide what tubes should I use for build, 6v6 or el84. Sluckey, I saw on your site that you have built both 6v6 an el84 plexi type amps. How would you compare them soundwise?

[tubenit]

I personally am NOT keen on EL84 tubes.  I've gotten rid of every amp I've owned or built that had them. They typically sound harsh to me and I've not done well getting as smooth of a tone from them as I have with 6V6's.

There used to be an 18watt forum where there were constant posts of people trying to get rid of the "fizziness" of the EL84's.  I don't ever remember that degree of concern about harshness/fizziness with 6V6's on any forum.

Having said that,  many many people seem to love them & I have heard some YouTubes of EL84 amps that I really liked. It may be either an issue of personal taste or personal experience, but I have never bonded with them.

I have built two Plexi inspired 6V6 amps that were cathode biased both with reverb and liked them a lot.  My son still has one of them. I think it's a great sounding amp.

With respect, Tubenit

[dennyg]

I would highly recommend using a Post Phase Inverter Master Volume for this amp - it gives you the option of running clean or overdriving the PI ever so slightly to get what I've found to be a very satisfying crunchy tone - whether you run the plexi or standard 18watt pre-amp or not

[Vlada]

After some research I decieded to build huss 6v6 plexi. There is a 5k resistor in series with choke. I was wondering what is the purpose of that resistor? Voltage drop or something else?

[purpletele]

After some research I decieded to build huss 6v6 plexi. There is a 5k resistor in series with choke. I was wondering what is the purpose of that resistor? Voltage drop or something else?

Here is a link to the Hoffman 6 v 6 Plexi that I built a few years back.  I think it sounds fantastic

Let's see if this works

[Vlada]

I thing they are identical except little difference at output stage and power supply. I have 325-0-325 power transformer that puts 440v B+ and I’m assuming that 5k resistor is there for voltage drop. Made some calculations and the voltage drop after resistor and choke should be 100 volts, on Marks schematic it’s 40 volts. I’m so confused

[tubenit]

I think it sounds fantastic

Both your amp and playing sound fantastic!   Thanks for sharing that!

With respect, Tubenit

[sluckey]

I thing they are identical except little difference at output stage and power supply. I have 325-0-325 power transformer that puts 440v B+ and I’m assuming that 5k resistor is there for voltage drop. Made some calculations and the voltage drop after resistor and choke should be 100 volts, on Marks schematic it’s 40 volts. I’m so confused

How did you calculate a 100V drop?

Hoffman's 6V6Plexi uses a 307-0-307 PT and he does not use that 5K resistor. My 6V6Plexi uses a 330-0-330 PT with a 5AR4 rectifier and I don't use that 5K resistor either. Both Hoffman and I have 400VDC out of the rectifier in our amps. Hoffman's schematic is on this website. Here's my schematic...

     http://sluckeyamps.com/6v6plexi/6v6plexi.pdf

Huss probably put that 5K resistor there to lower the voltage to the screens. Did you notice this quote on Huss's page?

"Builder's Note: In many cases, I used whatever parts I had lying around to build this. So, some of the parts are atypical or a bit different. There is no need to slavishly copy every part value and voltage reading. One of these days I'll build a proper 6V6Plexi using commonly available parts. :)"

[Vlada]

I’m using ss rectifier so:
325*1.4=455 volts minus diodes voltage drop it’s about 450 volts, after goes resistor and choke ecc83 tubes dram 6mA and typical screen curent for two 6v6 is 13mA:
5000*0.019=95. I know I made mistake somewhere but cant figure it out

[sluckey]

Your calculations look fine to me. Have you actually built this amp? If not, the B+ from your rectifier is likely to decrease due to loading when all tubes are installed and the amp is biased at about 70%. If your PT will support a 5AR4/GZ34 rectifier tube, that's another way to lower the B+. I used a 5AR4 specifically to lower my B+.

[Vlada]

I'm still waiting for some components to arive. Some of them are backordered. Power supply is bothering me now, maybe B+ is too high. Capacitor rating needs to be more than 500 volts. Should I put resistor after rectifier?

[SILVERGUN]

The 5K is there for "artificial" sag- which is intentional voltage drop at max. current, probably to partially simulate a tube rect. since his was SS

I'm guessing Mark took his V reading at idle and your calculation is for max

[Vlada]

I'm very grateful for your help. If I raised voltage on ecc83 (use lower value resistors in power supply filters) would it effect sound?

[SILVERGUN]

I'm very grateful for your help. If I raised voltage on ecc83 (use lower value resistors in power supply filters) would it effect sound?

Theoretically, yes
Raising the plate volts to the preamp will make the preamp slightly cleaner

Your biggest concern with a higher than spec'd B+ is the 6V6's ability to handle it. They will also stay cleaner with a higher B+.
Some guys have reported that the new JJ 6V6S will take the heat. It somewhat depends on how much you need to depend on this amp and/or how often you'll mind changing power tubes.

Keeping that 5K on the screen supply will help the screens cope.
 

[Vlada]

Yes, the tube choice is limited to JJ 6v6s

[SILVERGUN]

Since your PT matches the spec on MH's schematic, I would just go with his schematic, but don't miss the build guide and board layout on sluckey's site:
http://www.sluckeyamps.com/6v6plexi/6v6plexi.htm

Start your own thread at some point, 'cause we all love to see a new build

Don't worry too much because there are many amps out there running 6V6 over 400V

[Vlada]

I have just one more question. Ordered hammond choke which is rated 400vdc max. Can I use that choke? Because voltage on screens is slightly higher

[sluckey]

Hammond part number?

[Vlada]

I’m sorry I didnt write that. Hammond 158L

[sluckey]

15H, 75mA, 411Ω DCR, 400V

If I had it on hand, I'd use it, but if I had to buy one I would not pick that one. Why did you choose the 158L?

[Vlada]

I saw that huss used something very similar. What is better choice? Why you wouldnt use that choke?

[sluckey]

I don't like the high DCR or the low operating voltage.

[Vlada]

What should I use than?

[pdf64]

I had a spare 158 choke that was handy for a 2xKT88 amp with a 600V HT I was tinkering with.
I emailed Hammond and they said it should be fine, they're tested at 1kV.
It's been in use for a few years and is fine.

With pentodes especially (not sure if you're totally settled on 6V6 yet), plenty of resistance between the plate and screen grid HT nodes is beneficial, as it helps to limit screen grid dissipation under overdrive.
I like to add resistance to low resistance chokes, to get them up around 500 ohms; I think it helps to give power tubes an easier life and makes them more resilient to heavy overdrive, which is how I use them

[MFowler]

The Hammond choke 158L is what us Trainwreck Rocket builders use, a very good choke and it is same spec as the Stancor C-1002 choke Ken Fischer used.


Mark