EL84 based PP amplifier: Baby Huey EL84

I just posted an update to my latest escapades in DIYA but seeing as this is the first mention here ( where I tend to hang out more ) I thought I would unravel a little more of the story.

After building the Pass F4, I decided that I would be done with building amps for a while - too many heavy boxes that are not being used already. So why did that conviction have such a fleeting existence? I will blame my spiritual guide .

A little back story ( which I have mentioned in the past ) , I started building amps a very long time ago , big beefy high power to "properly" drive speakers. But then, still early in my audio introduction, I was loaned a 1950's tube amp , a LEAK Stereo 20



It is a solid amp with big heavy transformers and changed my approach to audio , a meager 15Watts that delivered such exceptional sound from my little MIssion studio Monitors - 700s .

I managed to trade my dining table and chairs for a very sad example ( students have to use whatever means available ) . This forced me to learn about tubes and tube amp design as I slowly accumulated the parts needed to refurbish it.

A longer story involving small kids and exploring fingers led me to pass it along and dive into headphones as my primary listening experience which led down the better known path of me building far too many headphone amplifiers ... hmmm seems to be a recurring theme .. perhaps I am just cursed to build whatever it is that has my interest

Come full circle - my love of and "enchantment" with the EL84 power pentode has never left me, despite giving away a treasure trove of ( today silly expensive ) EL84 tubes. I happen to have ready about this unique circuit a while ago on DIYA and in the back of my mind I thought "perhaps one day when I retire" , no I am not retired but that day has come. Three things happened at roughly the same time a month or more ago = 1. I saw a hard to come by set of PCBs being offered for sale. 2. I saw a quad of EL84 on USAudiomart for not so silly money 3. I saw a pair of elektraprint OPTs with the ultralinear winding on sale ( not massive savings and still double the cost of new EDCORs but it seemed to speak to me )

So in fairly short order, I had boards, tubes and OPTs which meant that I had no choice but to source the rest of the parts

and now a duplicate post to that on DIYA as we have caught up with my story thus far.

I have been making slow progress and finally tested one channel last night - came up perfectly, a tribute to an excellent design

Putting the horse before the cart - playing with layout options before all the parts had arrived



I decided to go with the solid state power supply as buying a another transformer to accommodate the tubed regulator didn't seem the most sensible thing to do . The one thing I need to look into is the timer - it takes about 2 minutes to release. I seem to recall that it is just a capacitor value change.



Assembled and ready to be tested



and finally found some time to test one of the boards



The most challenging thing about about building this amp has been the inability to just sit down and build it

..dB

 

I love reading about your builds. Inspiring me to do something similar! I saw that set of Baby Huey pcbs for sale on diya. Tempted to build the EL34 version.

 

Well, some fun and games. The second channel did not behave nearly as well as the first. I couldn’t seem to get the anodes to balance out and initially thought it might be the tube but that didn’t make sense as I had measured some of my small tubes this weekend



Finally dropped the bias circuit from dual 80V to dual 40V ( 80V would have enabled trying other tubes but I am really building this for the el84 and 36V is spot on for it. Getting 2.5mA through the 12AX7 which is perfect . Finally just switched out the adjustment pot for a larger one and this allowed everything to be set.

Next step is to wire up both channels and allow it to cook after bias checked again. EL84 are running at 35mA.

..dB

 

If it still doesn't work...

https://www.canuckaudiomart.com/details/649719239-leak-stereo-20-and-leak-point-one-stereo-pre-amp/

Rip it up and start again!

 

god, I loved the 80's

 

Turns out I don't have to rip it up , only switch out a resistor



The 2.5mA bias through the 12AX7 was too high, I thought it needed 1.2mA per triode section, turns out it needs 1.2mA through both sections - adjusted and it all works ( electrically ) as it should - still a ways off from being able to plug music and speakers into it

..dB

 

Great project @dBel84! I've always been a fan of the EL84/6BQ5 pentode.

BTW, nice Weston tube tester! I had one in the early 90s.

 

LOL it was your mention of your blog post

hope you don't mind the duplication here that really set the niggle in the back of my mind again

JE's collection of PP EL84 Ultra Linear amps



Dyna ST35

This particular unit was so rusty I decided to repaint it. Very simple circuit, the 7247 is essentially a twin triode wherein the first section is like a 12Ax7 direct coupled to a 12AU7 cathodyne phase inverter. To take advantage of its wide bandwidth ability, I removed the input coupling cap. I used this a lot to drive the BBC LS3/5As and Quad ESLs.

Pilot SA232 - just like its big brother, beautiful midrange but compared to the other three, this sounds a bit soft at the frequency extremes. Tube rectified power supply.




Leak 20 - Mullard style circuit using 3 x 12AX7. A 12AU7 would have worked better in the long tail phase inverter position, but then the amp would not have enough sensitivity for the matching Troughline preamp which essentially has no line stage and just a 2 stage EF86 phono preamp. Tube rectified power supply using a GZ34 just like the SA232. I kept mine in stock form and used a voltage divider to lessen the input sensitivity.




Acrosound 20/20 - the Loftin-White PP amp, no caps in the signal path. If you get lucky enough to own one, make sure all the resistors have not drifted, otherwise the performance deteriorates. This is a design wherein the bandwidth is limited only by the output transformer.



From <http://jelabsarch.blogspot.com/2014/02/the-90s-angela-instruments-classic-audio.html>

 

LOL!

I just realized that this is schematic of that amp.

 

It has been a little while since i had time to work on the amp - needed time and ability to make LOTS of noise

let me say this again , as I am sure that I have many times before , I HATE casework - probably more to do with doing a lot by hand but I just cannot be fussed and it shows

I drilled and filed out the stainless reclaimed sheet I had opted to use and fitted some rubber grommets into the cut panel to protect the wires



Starting to assemble, binding posts and transformers fitted



then onto fitting the boards



and finally wiring it all up



And the not quite finished final product with electronics assembled (and working) = sounding very promising indeed



the little switches on top with the numeric display is for the bias - allows me to check and set the bias on the EL84 tubes.

Final thing to do it get some nice wood for the sides - am leaning towards Paduak. Plan is route out a small recess - will see how that goes

 

I was reminded recently that I never posted the end result / outcome for this project

Well the great news is it works and sounds really great - it has improved with time and I do not miss having massive Class A mosfets out of the system.

Here are the completed pics





and a recent fun listening event at a local audio geeks home.

 

I can't see the 3rd pic and...those nummeric displays, you said what they were for but can u explain to a bit more to a less knowledgeable member please?

 

hmm, 3rd pic was a copy paste = just a room full of speakers and amps

the displays are small voltmeters that were calibrated using my DMM and are used to check the bias on the power pentodes. the switch to the back of the amp gives the little meter power, the switch to the front of the amp toggles between the bias point for the front and rear tube. It is just an easy (and safe) way to check if the bias has drifted and the little holes in the top plate give access to the trimmers which enable you to reset the bias

 

Thanks for the answer, is it something you usually do or was this case special for any particular reason?

 

it just seemed to make sense as I was laying it out and figuring out how to make it as functionally easy as I could.