Low Watter Amp - 7 pin output

[RJ Guitars]

For years I've been on the quest for the ultimate tone in the smallest package. I mostly play at church and I've come to believe the answer must be an amp that plays clean until pushed hard - somewhere between 3 and 8 watts RMS. I've had some limited success but still looking for the holy grail.

Most of what I know has come from building Trainwreck circuits and this next project is again based on a Trainwreck Rocket but at this point is a pretty far deviation from the standard. A few years back I put together a bunch of parts packages for subminiature tube Rocket amps and several of us built them... but they never really took off so to speak. http://ampgarage.com/forum/viewtopic.php?t=9816

After a few years of sitting on the shelves I've decided to look for an alternative use for the leftover pieces. Subminiature tubes are a bit of a pain so I've decided to drill out the PCB boards and put in chassis mounted tube sockets and use more standard tubes.

The current plan is build it for EL-95 seven pin output tubes but I have a few other candidates in mind as well. It will be 12AX7's in the preamp holes and will closely follow the Rocket schematic. I am hearing good things about the EL-91 output tube so I've got a few of those on order and a host of other 7 pin output tubes that I already have here. For anybody interested in building one of these I am going to clean the shelves and drawers in the shop and I expect these to be ~$100 for the entire parts package. Here is an early look at it...

[Stevem]

I would think that your needs will be reached better with a cathode biased output stage if that is not what your plan is.

[10thTx]

RJ,

I built this some years ago using ECL84 tubes. Might be worth taking a look at it?

http://ampgarage.com/forum/viewtopic.ph ... ight=ecl84

http://ampgarage.com/forum/viewtopic.ph ... ight=ecl84

Maybe you can use some of the ideas?

I've also attached this schematic which is an amp I have NOT built. So, consider it totally a DRAFT idea.

The thing that amazed me about the ECL84 is the bass response of that tiny tube!

With respect, 10thtx

[billyoung]

just curious how you made your pcbs ?

thx in advance

bill

here is 7 & 8 pin amp i been playing with

designed for el91 and 6k6 s

[johnnyreece]

Color me interested...

[RJ Guitars]

I would think that your needs will be reached better with a cathode biased output stage if that is not what your plan is.

I agree, this isn't intended to be an amp with a hard edge, so cathode biasing will probably work out best. The Rocket amps are typically Cathode biased and I'll post the schematic as soon as I get it workrd up. I also find cathode biasing to be especially handy when I do a lot of output tube swapping.

RJ,

I built this some years ago using ECL84 tubes. Might be worth taking a look at it?... The thing that amazed me about the ECL84 is the bass response of that tiny tube!

With respect, 10thtx

Bass response is a big part of the trick in small amps. Sounds like a tube I should look into a bit more. Thanks!

just curious how you made your pcbs ?
thx in advance
bill ...

This was the first PCB board design work I did and I actually did it in AutoCAD then wrestled with a fab house until we got it converted into something they could work with. Not a recommended procedure for sure. I've since gone to Express PCB for everything and although it's expensive it is relatively painless otherwise.

There were several mistakes I learned from on this first effort - mostly making holes too small for all the stuff that needed to fit in them. I've since changed my technique to creating a separate pad for each lead and they work much nicer especially when you have to swap out a part. On these boards you can see where I drilled out the undersized pads and put in an eyelet. One real positive with the PCB boards is that I've found them to not only be very tidy but I don't notice as much hum and hash as I have with the equivalent amp using traditional wiring.

I know PCB's can be personna non grata to many purists but I have grown to like them when I apply a few basic guidelines in implementing them. I never solder anything that heats or moves directly to them... such as tube sockets or potentiometers. The greatest weakness in a PCB seems to be breaking the traces when they get flexed. Using this technique they have been trouble free for me.

Color me interested...

another amp to add to "your problem"

[Phil_S]

+1 on EL95's. Best low wattage tube I've come across. A bonus is that they have a very low current draw, <30mA for plate + screen. This allows you to use a much lighter PT. Another bonus is that you can buy them cheap. If you haunt eBait for a little bit, you might get a pair of Telefunken or Mullard for next to nothing. Example: http://www.ebay.com/itm/10-X-EL95-MULLA ... 58c878fc93

[RJ Guitars]

Thanks Phil, Everything I could read about these indicated that the EL-95 tubes were designed for radio amps and the idea was to keep the power usage minimal... would lend itself to lighter iron.

I've created the schematic for my build. In the ultimate iteration I'll use a little more B+ but for now I have a handful of leftover PT's that I want to see if they can handle the job. This will also make the amp friendly to a host of the lightweight 7-pin options out there.

I was thinking about having a special OT wound that would have both 12K & 24K primary taps so this could accommodate the EL95's and the EL91 tubes. In my case I'll actually be using an 8K tranny with a thought that I'll use a 16 ohm speaker with the EL91 tubes.

[Phil_S]

You can pretty much do whatever you want in terms of mix and match. From time to time, I troll eBay for a derelict chassis and get lucky -- my price point is rather low. I got one a while back that included rather small iron that seemed to be just right for a 5F2A variant. I stripped off the PT and OT and put on another derelict chassis. I don't use the amp much, but the I liked the result very much. http://home.comcast.net/~psymonds/5F2A+EL95PP.html

[johnnyreece]

Color me interested...

another amp to add to "your problem"

Problem? What problem? I can quit anytime I want!

[RJ Guitars]

Update - Did a little soldering and trying to stick to my plan to use only what I have here... thus the 100uF filter cap. I only had to mildly thrill myself once before I decided to add that 1 Meg filter cap bleeder... even 120V DC can bring out some flavored vocabulary. I need to add another 1 Meg on the other side of the Standby switch just to make it more idiot proof.

[Stevem]

There was a simple 3 watt one tube build floating around a few years ago that used a 6BM8 which is a triode and pentode in one bottle .
I have a few UB45 Univox amps that use these in PP and they sound good even with the cheap Jap OT they come with!

[RJ Guitars]

I had a Univox Bass amp with four 6L6GC's and a pair of 12's in a sealed cabinet that I played guitar through. That was one of my best sounding amps ever... blew the OT and had a Carvin OT installed. Great sounding amp!

[RJ Guitars]

For those brave enough to get past the malware attack site warnings (which seems like a pretty small crowd)... I'll add another progress update. I am down to the control pots and associated wiring and I'll be ready to light her up. If it sounds half way decent I'll do the right thing and get a proper set of filter caps for her... in the meantime I have stuck with my effort to use up what I have here instead of ordering more new stuff.

[Stevem]

Use the Amps standby switch to drain the power supply, and keep that switch in the on position while working on it.
For my own comfort and safty this is why I like standby switches to this day, even if Cathode stripping is not a issue!