NAD: CJ's Cavalcade of New Smallish Cheapish Amps - #2

[culturejam]

As promised, I am becoming the JimiLee of small cheapish amps. 

I have more new amps than time to post about them, but I'll do this one tonight. First entry was the MonoPrice 5W 6V6 10" combo for $99.

Tonight is the SET5 5W EL84 super-tiny head. It is $99 including shipping. Got it from here.

It is a LOT smaller than I thought it would be. See first pic below for size reference (never miss a chance for product placement!). Seems quite well made overall. OT is a little small, but I can't say I expected big iron in a 5W $99 amp.

Here's the specs:
• 5 watts via 1x EL84 and 1x 12AX7 (Chinese tubes, of course)
• Treble, Bass, and Volume controls
• 1W and 0.1W attenuation modes (slide switch on rear)
• 8 and 16 ohm outputs

Frankly, it sounds pretty good. It doesn't have a ton of umph, but it does have a decent tone at full volume. Kinda sounds like a Valve Jr, but with a touch more gain and a little smoother. There's clean headroom with humbuckers up to about 3 or about 4 with single coils. Bass and Treble are actually useful. The 1W setting sounds pretty good with the amp cooking; the 0.1W has too much high cut and frankly isn't loud enough to do anything useful other than rock out next to a sleeping baby. Haven't tested any other tubes in there yet, but I have some in the mail. I don't expect massive improvement with different tubes, but swapping is half the fun of tube amps.

Internal build quality was nicer than I expected. And nothing is surface mount. Modding should be relatively easy. I have emailed the distributor and asked for a schematic, so we'll see what happens.

I noticed there are 8 or 10 1/4-watt resistors (and they are carbon film). I assume these guys did the math and feel okay with that low of wattage in certain spots. Also, the "anchor" tabs on the power tube socket were not soldered, so the socket moves more than I like when removing a tube. I will fix that shortly. Some of this appears to be wave soldered, while other components are definitely done by hand. They didn't do any flux cleanup, which isn't the end of the world, but obviously a little on the sloppy side. A few components are little crooked, but I'm being picky.

The cage is actually three pieces, and the front/back grill are removable. I'm totally going to 3D print a new badge for the front.

Overall, it's a nice little amp. Great platform for modding or doing a gut-n-rebuild. But it's also okay just as it is as a tiny amp head for practice or recording. Not a damn thing wrong with it out of the box.

PICS!

[ahiddentableau]

That's cool.  Hundred bucks for that seems like really good value.  Transformers look way better than I expected, and as you say, you could mod it like a madman.

Would be interested to see the schematic, particularly for how they dealt with the power attentuation.  From the pick it looks like they're just spilling heat with those metal oxide resistors.  Have you poked around the innards at all?  Is there more under the board?  Am curious.

[culturejam]

Would be interested to see the schematic, particularly for how they dealt with the power attentuation.  From the pick it looks like they're just spilling heat with those metal oxide resistors.

Yeah, I think that's the extent of the attenuation. Maybe an added treble bleed cap would improve things? Not sure, but I can definitely give it a try and see what happens.

Have you poked around the innards at all?  Is there more under the board?  Am curious.

Not yet. Just barely had time to give it a 20-minute test and do a writeup. I'll keep all progress pics and info in this thread.

I think it might be possible to add an additional preamp tube for more gain. But failing that, I think there could be easy mods for negative feedback and probably a pentode/triode switch. And of course, there will be some easy mods for increaseing or decreasing gain via resistors and tightening bass via cap swaps.

[287m]

im sorry, but im curious
how the sound in 0.1W mode
hey man, dont laugh  i never play tube in low watt like that

[blearyeyes]

Makes me want to buy one for my bench amp...

[stringsthings]

Makes me want to buy one for my bench amp...

+1

[culturejam]

how the sound in 0.1W mode

It's very quiet. Like, almost unusably quiet. Guitar string noise can be heard over the amp at the 0.1W setting.

[ahiddentableau]

I think it might be possible to add an additional preamp tube for more gain. But failing that, I think there could be easy mods for negative feedback and probably a pentode/triode switch. And of course, there will be some easy mods for increaseing or decreasing gain via resistors and tightening bass via cap swaps.

Looking forward to this.

One of the things that caught my eye was that the power transformer label says it can put out 2.5A on the filament winding.  That's surprisingly generous.  If memory serves, an EL84 requires 0.75A, and 12AX7s are 0.3A a piece, I think, so it should be able to power several more tubes.  That's cool.  If you don't like the amp, it's gonna be really modable.

[culturejam]

The distributor provided the schematic!

Interesting setup. The second triode and EL84 look exactly the same as a Valve Jr. (down to the values). Front half is its own thing, as far as I can tell. Not sure about that tone stack setup. It almost looks like a Bassman / Marshall TMB at first, but then I see how Treble is setup differently.

I think it should be easy to implement half a dozen minor mods to this thing without a lot of trouble. Biggest issue is that there just isn't much room to add a lot of hardware. Squeezing another tube in there is going to be tricky, so I might just leave that one alone. But man, adding an EF86 up front would be sweeeeet.

I'll try a few things and put together a quick guide on what I did.

Side note: I put a set of JJs in there tonight. Doesn't per se sound *better*, but it definitely has more breakup now and longer sustain.

[Aristatertotle]

I've had the idea to use something like an ECF82 which has both a pentode and triode in it to do like a variation of a Matchless Clubman preamp. Could be a cool application for a little amp like that.

I'm pretty sure on the AX84 forums I saw where someone posted something similar to what I described actually. Triode -> pentode -> power pentode.

[ahiddentableau]

I've had the idea to use something like an ECF82 which has both a pentode and triode in it to do like a variation of a Matchless Clubman preamp. Could be a cool application for a little amp like that.

I'm pretty sure on the AX84 forums I saw where someone posted something similar to what I described actually. Triode -> pentode -> power pentode.

Wow, talk about timing.  I just bumped into something like this the other day on Merlin Blencowe's page.  He offers a set of PCB boards for amp prototyping, and just released a board for the ECL86--almost precisely what you're talking about, a triode/SC pentode combo tube.  If you're interested in checking it out, here's the link:

http://www.valvewizard.co.uk/ecl86pcb.html

[Aristatertotle]

I've had the idea to use something like an ECF82 which has both a pentode and triode in it to do like a variation of a Matchless Clubman preamp. Could be a cool application for a little amp like that.

I'm pretty sure on the AX84 forums I saw where someone posted something similar to what I described actually. Triode -> pentode -> power pentode.

Wow, talk about timing.  I just bumped into something like this the other day on Merlin Blencowe's page.  He offers a set of PCB boards for amp prototyping, and just released a board for the ECL86--almost precisely what you're talking about, a triode/SC pentode combo tube.  If you're interested in checking it out, here's the link:

http://www.valvewizard.co.uk/ecl86pcb.html

Merlin has some cool stuff, I've got two his books.

The ECL86 is a combo triode with a power tube in one. I meant more like the combo of a triode and a small signal pentode. I know there are some that are essentially a 12ax7 and an EF86 in one tube.

Those ECL86's are super cool though. One of those with a 12ax7 in front could give you a nice range of dirt options. Just that extra bit of oompf over a valve Jr.

[ahiddentableau]

The distributor provided the schematic!

Interesting setup. The second triode and EL84 look exactly the same as a Valve Jr. (down to the values). Front half is its own thing, as far as I can tell. Not sure about that tone stack setup. It almost looks like a Bassman / Marshall TMB at first, but then I see how Treble is setup differently.

I think it should be easy to implement half a dozen minor mods to this thing without a lot of trouble. Biggest issue is that there just isn't much room to add a lot of hardware. Squeezing another tube in there is going to be tricky, so I might just leave that one alone. But man, adding an EF86 up front would be sweeeeet.

I'll try a few things and put together a quick guide on what I did.

Side note: I put a set of JJs in there tonight. Doesn't per se sound *better*, but it definitely has more breakup now and longer sustain.

It's pretty cool of them to send the schematic.  Seems like a lot of companies are refuseniks on that account.  Anyway, thanks for posting it.

As you said, it's a pretty standard design (I mean that in a good way - very modable).  As far as I can see, the tone stack is basically your standard passive Baxandall with a bright cap and a couple cap changes off the treble control.  They removed the cap on the wiper and added an extra cap to ground.  How do you find it?  IMO, at low volume amps generally need a really heavy-handed approach to tone control in order to compensate for the whole Fletcher-Munson effect thing.  But it doesn't look like they've done too much here.

[thesmokingman]

the hammond ao-44 amplifier carried a pair of ecl86 and by grafting a preamp tube instead of a transistor you get a nice little guitar amp with a tiny footprint ... fun tubes to play with. in a single ended amp you'd be pretty short on watts unless that was your design goal

[culturejam]

Those ECL86's are super cool though. One of those with a 12ax7 in front could give you a nice range of dirt options. Just that extra bit of oompf over a valve Jr.

I've dreamed of this for years! ECL86 + one 12ax7 and you could have a single-ended JCM800!  Definitely going to check out the Merlin info / boards on this.

Also, I bought a 6AF11, which is two triodes and a power pentode in one bottle. I've seen a few projects that are billed as "Champ in a bottle". I've also read it can be really noisy from the close proximity of the triodes, but these guys seem to make it work out: https://zeppelindesignlabs.com/product/percolator-2w-amp-kit/  Probably the metal shields they put between the transformers and the tube that do the trick.

How do you find it?  IMO, at low volume amps generally need a really heavy-handed approach to tone control in order to compensate for the whole Fletcher-Munson effect thing.  But it doesn't look like they've done too much here.

It's quite effective, in my opinion. At full volume, there's more treble and bass than most people would need, so dialing back is nice. But at low volume, you gotta crank those back up to keep it sounding decent.

Tone stack bypass switch might be a neat thing to add. Seriously, I'm thinking about buying a second one. It sounds pretty damn good as is, and it would be nice to have a stock unit to do a real comparison and not just have to rely on listening memory (which is anything but reliable). Or I could do a high-gain mod on one and clean headroom mod on the other. So cheap!