25 Watt Plexi with EL-34's???

[rdamato]

I was having a discussion with an amp builder expressing my desire for a ~25 Watt, 6V6 Plexi style amp. I really want the tone with less stage volume. He felt that he would prefer using EL-34's and lowering the output with a smaller OT, thereby maintaining the inherent tone of the original Plexi tubes.

I am a player, not a builder and want some opinions on this suggestion.

Please, feel free to comment.

Thanks,
Ron

[Cliff Schecht]

What he's going to build you is an amp that is loud but loses bass and treble response when pushed into distortion. A smaller output transformer will saturate earlier and limit your bandwidth. Tweed Fender amps have a certain sound because they use puny transformers that weren't meant to handle the increased current from distorted output tubes. If you bias the EL34's cold to cut down on the output volume they'll sound like crap when pushed (and when clean) and if you bias them normally, they will push as much current as the power transformer can handle. Then if you rely on the power transformer to limit current it will heat up badly and give you other issues (they spit out more noise when overloaded and can cause weird note ghosting and such). All in all it's not a great way to go about doing things.

If you build a Marshall circuit, choose an output transformer and set your voltages right then the amp will still sound like a Marshall even with 6V6's. Choosing a transformer with Marshall's turns ratio (i.e. 8k Ohm primary to 4/8/16 Ohm secondary) but more fit for 6V6's (something like 10k Ohm primary) will get you will get in the ballpark and trying out different output tubes will get you more than close enough. You'd be surprised in how much the output circuit topology effects the sound; tube choice is more for small tweaks to put icing on the cake.

[kevster]

I was having a discussion with an amp builder expressing my desire for a ~25 Watt, 6V6 Plexi style amp. I really want the tone with less stage volume. He felt that he would prefer using EL-34's and lowering the output with a smaller OT, thereby maintaining the inherent tone of the original Plexi tubes.

I am a player, not a builder and want some opinions on this suggestion.

Please, feel free to comment.

Thanks,
Ron

A gentleman by the name of Jeff Shook graciously sent me the plans (schematics, layout, and pics; and maybe even a BOM) for a 25 watt Plexi with Reverb a few years ago. I still have them, but I don't feel right passing them around the net. Search around for the guy on some of the popular boards and you might luck out... It may have been Metro Amps forum or something like that....

Here's a picture (or two) of his amp.

[IMG:150:113]http://img706.imageshack.us/img706/6645/exteriorfrontpanelwithr.th.jpg[/img]
[IMG:113:150]http://img194.imageshack.us/img194/5064/cabandheadsideview.th.jpg[/img]

[Reeltarded]

So, another way around the block.. as mentioned, smaller output is a bum deal.

A 6v6 Marshall sounds awesome. 25 watts? Are you sure? It's not very loud.. I mean it's not loud at all, and .. 25 watts? Really? I have heard a 4-6v6 that is wonderful, and wasn't quite able to keep pace with a 50wtr.

A normal Marshall variaced to the volume you want is also a fabulous sound. I have been doing this almost 30 years. It works. You don't get to back off the volume knob though. There is no clean sound in it, just less dirty.

[Jana]

As has been stated, going to a smaller OT is NOT the way to reduce power--you will wind up with an anemic sounding amp. Build a standard "50" watt plexi but use a 1202-164 power transformer from Marstan. This is will give you 370vdc B+. The amp will be ~35 watts. If you want to trim down the output a bit more just lower the voltage to the PI a bit. A pair of EL34's in an amp set up like this is very sweet sounding--I did this for years.

[rdamato]

So, another way around the block.. as mentioned, smaller output is a bum deal.

A 6v6 Marshall sounds awesome. 25 watts? Are you sure? It's not very loud.. I mean it's not loud at all, and .. 25 watts? Really? I have heard a 4-6v6 that is wonderful, and wasn't quite able to keep pace with a 50wtr.

A normal Marshall variaced to the volume you want is also a fabulous sound. I have been doing this almost 30 years. It works. You don't get to back off the volume knob though. There is no clean sound in it, just less dirty.

4- 6V6's?? Never even considered it. Great idea!!

Anyone care to comment??

Thanks

[CHIP]

I built 6V6 Plexi, designed by Mark Huss, a few years ago. http://mhuss.com/Plexi6V6/index.html
Recently, I changed a few components and made it into a (1986) model.
It sounds great. Duane Allman like tone.
Strat sounds great with it also.

[Phil_S]

I was having a discussion with an amp builder expressing my desire for a ~25 Watt, 6V6 Plexi style amp. I really want the tone with less stage volume. He felt that he would prefer using EL-34's and lowering the output with a smaller OT, thereby maintaining the inherent tone of the original Plexi tubes.

IMO, this is a no brainer.

To get a 25W out of a pair of 6V6 is just silly, IMHO. 22W is about the practical upper limit. If you want plexi tone at 25W, I don't see how it will happen with the power tubes stretched beyond what's reasonable.

The reasonable thing is to build the JTM45 with a pair of EL34's. It will do the plexi tone as it should be done and properly biased it will put out about 30W. Forget about the undersized transformer. Get something right sized and let the circuit do the work. Roll back the volume on your guitar if you need to do it.

4x 6V6 will give you about 38-40W. Nothing wrong with that, but it will be really loud. 30W is also really loud and plenty to cut through the mix over a drummer in an reasonable sized venue. You're not playing any stadiums, sheds, or theaters, are you?

[diagrammatiks]

maybe try 4 6aq5s.

they are beam tetrodes similar to 6v6s but have less dissipation.

[Cliff Schecht]

6AQ5's sound great. Used 'em in a Rocket build and I love how that amp rings out.

It's probably important to note that the sound difference between 22 and 25W is not that much. The difference between 30W and 22W isn't that much. A pair of 6V6's coupled with a 2x12 loaded with high sensitivity speakers is loud enough for any situation.

The point Jana made about using EL34's with a lower plate voltage is also spot on. This will change the tone a bit, lower current means a higher plate resistance and a lower bandwidth but this can be compensated for in the circuit and with speaker choice if need be.

[diagrammatiks]

6AQ5's sound great. Used 'em in a Rocket build and I love how that amp rings out.

It's probably important to note that the sound difference between 22 and 25W is not that much. The difference between 30W and 22W isn't that much. A pair of 6V6's coupled with a 2x12 loaded with high sensitivity speakers is loud enough for any situation.

The point Jana made about using EL34's with a lower plate voltage is also spot on. This will change the tone a bit, lower current means a higher plate resistance and a lower bandwidth but this can be compensated for in the circuit and with speaker choice if need be.

Ya I really like 6aq5s. 4 of them gives about 16 watts. The reason why I recommended them is that you'd be running them hot at 300 b+ as opposed to running el34s under spec at a lower current.

[Tone Lover]

You can find Jeff at Hoffman forum his ID is (Tubnit) fine gentlman extremely helpfull, ask him He will give you the schematic if he still has it.
You gentlman have forgot one other option KT66 use a jtm 45 transformer at 8k or 6k6 I forget, sounds awsome at about 30watts .
use a what they call ppimv master volume. or others call it a larr/mar master, It will rock at lower volumes .
Thanks Bill

I like the 6aq5,s x4 but its hard to find a 180Ma transformer that will come in below 300 they like 270 b+ unless you have the 6005 or 6005W tubes.
Cliff how is your amp holding up with 6aq5,s are you running them at 300 B+ I really like those tubes but always worried about going through them to quick, although there still reasonable on the internet

[KT66]

I agree, smaller OT is not the way to go, you will lose low end and you can only go so small anyway until you will have core saturation issues and regardless of what you may have read that sounds like ass.

There are a number of ways you could go for a lower output plexi tone and/or to use the circuit with 6V6s. First of all, the 6V6 that you want in this circuit is the JJ6V6S - it sounds good in a plexi, is a good value for the money, they are rated to take 500V on the plates and are very durable/reliable and have a low failure rate. Also, keep in mind that running 6V6s at a high plate voltage typical for a Plexi such as 500V, you are going to get somewhere between 25-30 watts per pair out of them. So, that being said, here are some ideas for your build :

- Just build it to use EL34s and then try it with the JJs, if you do this the impedence selector of 4,8,16 would become 8,16,32 for 6V6s

- Build it as a 50W but with four output tube sockets, that way you could run it as 2 X EL34 or 4 X 6V6, you could even put in a half power switch to take 2 of the 6V6s out of the circuit. Many people also feel that there is an added harmonic complexity that exists in amps run with quad output sections compared to that of those that using a duet.

- put in a LARMAR PPIMV

- Use one of Dana's VVR kits to scale the output level.

- get a good attenuater, I like my plexi builds through my THD hotplate

If you just build it to only take 6V6 with a high plate voltage, I would recommend using an OT that is rated for at least 30W, like the Allen TO26 : http://allenamps.com/parts.php#transformers


I have built this circuit a number of times in a number of different ways, but it always sounds good - you just jumper the mud channel and the icepick channel together and that's rock 'n roll.

[Darkbluemurder]

Definitely don't skimp on the output transformer. A pair of EL34 tubes is perfectly fine for the project. I have built two amps each with a cathode biased pair of them (one started out as a Top Boost circuit, the other is a Chieftain clone). Both have 50W transformers and can be clean and dirty when I want them to.

[10thTx]

When I built "Tubenit's" Plexi 25, I also had gutted a JCM900 and rebuilt it into a Plexi 50w during the same time frame. I noticed several things.

I used a Hammond 272HX PT and a 1650F 25W OT. There was very little difference between the stock Marshall 50W OT and the Hammond 1650F OT. Look at the picture that was posted ealier in the thread and those are the same model Hammond PT and OT. The 1650F is a good sized OT. You can compare the dimension specs of the stock Marshall 50w OT with the Hammond 1650F & see they are not very far apart.

I built the Plexi 25w as cathode biased & when I measured across the OT while playing, it was 23 watts using 5881's. I think I used a 360R/10w cathode resistor or a 400R. I was able to switch from 5881's to 6V6's without having to rebias using that method. The Deluxe Reverb is 22w & has been plenty loud when I've heard them with bands in local clubs numerous times.

3rd thing both the owner of the Plexi 50w & I noticed was that the 25W Plexi reverb could EASILY match the volume that club owners would allow him to play his 50w amp at. In other words, the Plexi 25w and the 50w had equal "useable practical" volume. Except the Plexi 25w had better natural tube overdrive still maintaining a clear tone. Both chords and single notes never got muddy and maintained a clear tone even when overdriven somewhat.

We played both amps A/B thru the same Marshall 2X12" cab of his. Neither of us felt the 25w had less of anything. And his amp was not twice as loud at all.

I later did another version of the Plexi 25w using 6V6's and called it the Texas Raptor. The editable ExpressSCH layout and schematic are here:
http://www.el34world.com/Forum/index.php?topic=2442.0

The original Plexi 25W SCH schematic and layout are here:
http://www.el34world.com/Forum/index.php?topic=567.0

I liked the insertion point of the reverb better on the Texas Raptor so that is why I mention it. Essentially it is the same 25W Plexi idea but with different PT & OT using 6V6's instead of 5881's. The reverb is a more subtle reverb emphasizing mids more than the fender. I think it has a warmer sound but is not surf music intense.

I think you could use the Texas Raptor schematic and layout and a 272HX with a 1650F Hammond AND use EL34's and have a super nice amp that should be loud enough to hang in with a band in almost any small local club, IMO.

With respect, 10thtx