Sluckey Plexi 6V6 Build Questions

[Mason]

Hi, I'm looking to build Sluckey's version of the Plexi 6V6 that's shown here, http://sluckeyamps.com/6v6plexi/6v6plexi.htm.  I had a few questions for anyone that's built this version of the amp and the Hoffman version.

I've liked all the clips I've heard of these amps, but it's hard to get a really accurate idea of an amps character through just theses clips.  For those that have played these amps, especially Sluckey's version, would you say they sound more like a 50 Watt Plexi, JTM45, or an 18 watt?

Secondly, instead of a single input version, I'd like to build a traditional 4 input Plexi version.  Will I be able to use the layout of a 50 watt plexi for the preamp, or is there anything I'll need to change? I'll be using a standard 50 watt plexi chassis so I'm not worried about space.

Finally, I've got some components on hand leftover from previous builds that I'd like to try to use.  Would there be any problem with using 50/50uf caps for the filter caps instead of the 32/32uf that Sluckey uses? And would I be ok using 16uf bias filter caps instead of the 25uf listed? What differences, if any, would there be in tone?

Thanks for any help! I've built amps before, a 1987 Plexi and a JCM800 2203, but I've never done anything that doesn't have a clear layout so I need some help figuring this out.

[shooter]

can't comment on much since I don't play
I did build my version of Steves n Dougs n Marks, any values that were "different" I added them up n divided by that number, so 10k+15K/2 for my values

the 50/50 might be a bit big if you're using a tube rectifier, have to check the tube specs for cap input.
the thing I found best about the sound I got and real musicians got, came from the 1 jack and ability to blend the 2 halves into a sound that you could never get from just one

[sluckey]

50/50µF cap cans will be OK with a 5AR4 rectifier. I'd use them if I had them. I'd also probably use diodes if I were doing this amp again. 16µF bias caps would be fine too. My choice to use 32/32 caps, 25 bias caps and 5AR4 was based on the fact that I had those parts on hand and wanted to put them to use. No special mojo in my choice.

I'll be using a standard 50 watt plexi chassis so I'm not worried about space.

In that case I highly recommend you use a standard 50 watt plexi board also. My board layout was specially designed to fit into a 12" x 6.5" x 2.5" chassis. If you put my short board into a full size plexi chassis you will end up with some very long wires that will invite gremlins into your amp.

[Mason]

Thanks for the replies!  Sluckey did you like the hammond iron you used? I'm contemplating picking up an AO43 just for the iron.  My other option is going with a new set from Classic Tone.  Why do you think you'd use diodes for rectification if you did the amp again? A big reason I'm following your layout is because of the tube rectifier.  I'm looking to get some sag like that of a JTM.

[purpletele]

Thanks for the replies!  Sluckey did you like the hammond iron you used? I'm contemplating picking up an AO43 just for the iron.  My other option is going with a new set from Classic Tone.  Why do you think you'd use diodes for rectification if you did the amp again? A big reason I'm following your layout is because of the tube rectifier.  I'm looking to get some sag like that of a JTM.

Here is a ink to a thread on my 6 v 6 Plexi Combo.  There is a good link to a video as well.  I'm having an issue posting the video link this evening.

I used a solid state rectifier.  I have also used a rectifier in a similar circuit, the Stout TMB.  I changed it from el84's to 6 v 6's, I like it a lot because it's loose and forgiving.

My 18 Plexi is punchy with the SS rectifier, which I also like a lot.  My Plexi Combo is a favorite amongst my colleagues, it sits well and cuts well.

http://el34world.com/Forum/index.php?topic=21231.msg235210#msg235210

[sluckey]

Sluckey did you like the hammond iron you used? I'm contemplating picking up an AO43 just for the iron.

Yes. Those old organ amps used high quality iron. But I will not pay the crazy eBay prices today. I bought several different model Hammond amps several years ago on eBay. Never paid more than $40 shipped for any of them.

Why do you think you'd use diodes for rectification if you did the amp again?

Because I have a drawer full of 1N4007s and 1N5408s. I don't have any 5AR4s. I don't notice any sag with a 5AR4 in my Plexi 6V6.

A big reason I'm following your layout is because of the tube rectifier.

You can add a tube rectifier to a standard size Plexi chassis. But you should also use a standard Plexi board for the reason I already said.

[Mason]

I will be using a standard plexi board, though I may experiment with some component values.  It doesn't look like I'll have room to put the choke next to the PT like in your build, would there be any problems with placing it on the other side of the OT down near the preamp tubes?

Also, in your opinion which resistors can I get away with using carbon comp and which should I definitely use metal film? I know the differences are fairly minuscule but I've like the sound of carbon comp in my other builds, I just want to cut down on noise.

[sluckey]

It doesn't look like I'll have room to put the choke next to the PT like in your build, would there be any problems with placing it on the other side of the OT down near the preamp tubes?

That's fine.

I know the differences are fairly minuscule but I've like the sound of carbon comp in my other builds, I just want to cut down on noise.

If you like the sound of CC resistors then use them. If you want to cut down on noise then use metal film. I prefer metal film for three reasons... Low noise. Tight value tolerance. Skinny leads (I can put four leads into a single turret).

[Mason]

Had one more quick question.  The choke I'm going to use is this one, https://s3.amazonaws.com/tubedepot-com-production/spree/attached_files/40-18040.pdf?1487779944  It has some different values to yours.  Is there anything I will need to change about the circuit in order to use this choke?

[Ed_Chambley]

Choke is fine.  You can actually use a bit more if you want.  Plexi amps are well filtered and have more henries traditionally.  This creates a tighter response, but keep in mind you are using 6V6 tubes.


I have tried JJ 6v6S tubes in a 4 hole Marshall.  Put the El34's back since the amp is original.  The main difference I have noticed between EL84, 18 watt marshall and 6V6 is the onset of power tube distortion is not as gradual.  Seems like EL84's have a longer onset of distortion.  Sort of like you are playing with some all the time.  Closer to EL34's, just not as loud.


With the 6V6 I ended up changing a couple preamp values to add some more gain and added a load of touch sensitivity.  I used SS rectification and all 50uf/450 Electrolytics, just like a 1987 Plexi.  Then I took it apart.  I liked it ok, but it is not what I think of as a Marshall tone.


It sounds like a Plexi preamp plugged into a Fender Deluxe which is cool.

[Mason]

I finally built the amp this weekend and it sounds great! There are a few issues I’m having that I want to try to straighten out though.

First, on standby theres still a small amount of signal going through.  I can faintly hear my guitar if I strum.  No other amp I’ve played has been like this.  I used the standby from a JTM45, so I have it wired between the rectifier tube and one of the filter caps.

Second, there’s no hum at low volumes, but when I push the volumes up past 8 I get some hum, not terribly loud and from what I can tell it goes away when I start to play but its there when I stop playing.

Third, when I unplug my guitar with the amp still on theres a loud hum.  I haven’t noticed this with any other of my amps.  Is this a grounding issue?

Finally, I have the 6V6’s biased at about 24mv, is that a good bias point? I went with 70% dissipation based on B+.  For reference all my voltage readings are within about 5% of what Sluckeys are on his layout.

Thanks for the help!

[shooter]

Is this a grounding issue?

most likely, are you using a grounding jack?, If so verify the tip is grounded when your guitar cable is unplugged.  might also be related to the stby "passing" signal

[Mason]

They’re the traditional cliff style jacks used in most Marshalls. They’re wired just like in this layout. http://site.triodestore.com/JTM45LAYOUT.pdf

[sluckey]

First, on standby theres still a small amount of signal going through.  I can faintly hear my guitar if I strum.  No other amp I’ve played has been like this.  I used the standby from a JTM45, so I have it wired between the rectifier tube and one of the filter caps.

The stby switch on a standard JTM-45 removes B+ from the entire amp so it would be impossible to get a faint sound through the amp. Maybe your switch is defective or it's not wired like the JTM-45? Do you measure any plate or screen voltage when in standby?

Third, when I unplug my guitar with the amp still on theres a loud hum.

Unplugging the guitar usually results in loud hum on any of my amps. Unplug the cable from the amp jack. Still loud hum? If so, the jacks are not wired like that JTM45 layout.

[Mason]

I’ll check the standby switch wiring when I get home. The hum goes away if I unplug from the amp, unless I turn it up to past 7-8. The hum I’m getting when I unplug my guitar is at almost all volume levels.

[sluckey]

The hum goes away if I unplug from the amp, unless I turn it up to past 7-8.

That's normal.

The hum I’m getting when I unplug my guitar is at almost all volume levels.

Try another guitar cable. Some are better than others, but none will eliminate hum when there is an open circuit at the end of the cable. The tip of your plug becomes an antenna for everything electrical in your environment. If you live close to an Interstate highway you may even hear an occasional "Breaker, breaker!" 

[Mason]

Ok Sluckey, if it’s normal then i won’t worry too much about it. I’ll double check my wiring just to make sure.

Does my bias rating look right to you?

[sluckey]

Looks good to me.

I'm still curious about that standby switch thang. Can we see some gut shots?

[Mason]

I can’t take pictures at the moment. In reference to your layout I wired it inbetween pin 8 of the rectifier tube and point A on the filter cap. Is that the right spot?

[sluckey]

I can’t take pictures at the moment. In reference to your layout I wired it inbetween pin 8 of the rectifier tube and point A on the filter cap. Is that the right spot?

No. That would only disconnect B+ from the output tube plates. You would still have B+ to everything else in the amp.

There are two wires connected to pin 8 in my layout. If you simply disconnect the choke wire from pin 8 and reconnect it to point A on the cap it will work correctly.

The reason I connected my choke directly to the tube socket rather than the cap can was simply because the blue lead on my choke was too short to reach the cap can in my layout. Remember, my iron was robbed from an AO-43 amp. The choke had one looong lead and one short lead, but it works well in my layout.

[Mason]

Thanks! I’ll try that when I get home. It’ll shorten up that lead for me too which is great.

[Mason]

That worked Sluckey thanks!

I doubled checked my plate voltage and its actually a lot higher than I thought it was, its 454.  Must not have been reading the meter right when I first checked.  My PT is 330-0-330 so I'm confused as to why the plate voltage is so high.  I checked and I'm getting almost 123V from the wall outlet.  Could that be why my plate voltage is higher? I'm using a GZ34 rectifier tube.  Should I be concerned about the plate voltage?

[sluckey]

454v! Guess you're not using AO-43 iron. Which PT are you using? What is the current rating for that 330-0-330 secondary? Is you're bias point still at 24mV? Are you using a tube rectifier? If so, which one?

[Mason]

I changed my bias to 21mv. I’m using a GZ34 rectifier tube. The transformer is a classictone 40-18016. You can see the spec sheet here. http://www.classictone.net/40-18016.pdf

[sluckey]

That should all be fine.

[Mason]

Ok. I’m using JJ 6v6S tubes so I know they can take the voltage. Though I may use another rectifier tube to drop the voltage a bit.

[sluckey]

Ok. I’m using JJ 6v6S tubes so I know they can take the voltage. Though I may use another rectifier tube to drop the voltage a bit.

An alternative would be to use a 250Ω/10W resistor to drop B+ a bit. Easy to try. Just temporarily solder the resistor across your standby switch. Now the switch will select between the full B+ and the lower B+. (Note, you will lose the standby function while doing this test.) You can easily see how much voltage difference it makes and also get a good feel for how it affects the sound. (Use different value resistors for different voltage drops).

If you try this and decide you like the option of switching between different B+ voltages but still want to have a standby position, just take a look at the Standby Plus mod in this pdf...

     http://sluckeyamps.com/VAC15/Vox_AC15_mods.pdf

Hoffman sells this special three position toggle switch. He calls it "Carling DPST Switch".

[Mason]

I think I’ll try that Sluckey, thanks! What differences do expect I’ll hear from lowering the B+? Is there room to experiment with that resistor value, or should i just stick with the 250 ohm?

[shooter]

Is there room to experiment with that resistor value

you can experiment, and "how" it sounds is subjective, my last build I started at 250 and settled at 470, but went out to 1k

[sluckey]

I didn't hear much if any difference on my AC-15. I removed that mod after about a week. Your meter will definitely see a difference. You can play around with any value from zero up to about 500Ω. A 500Ω/10W resistor "can" safely drop about 50V without overheating the resistor. Your actual voltage drop will depend on the current flowing through your amp.