New Amp Project SET EL84, 6BQ5, 6P15P

[henry4841]

I just realized that I have not shown some better pictures of the finished amp. Probably look better if I painted the awful blue PS transformer black but I was too lazy to take the time to do so. Can always be done in the future if I decide to. 

[Shakeydeal]

Maybe I missed it, but what is the knob on the front panel for?

[henry4841]

That is the volume pot. This amp has enough gain not needing a pre. You could always turn the pot wide open and use a pre though. In fact that is the way my system is but not an active pre but rather a Firstwatt B1 buffer. I probably have a better looking knob but at the time I just put on what was handy. This amp will not get much use, not that it does not sound good but because have other SET's I like better. It is nice though when this amp becomes a topic on the forum I can pull it out and hear the sound it makes again. 

[Shakeydeal]

It’s good to have options.

[capo72]

I like adding the volume pot when possible because I mess around with bi-amping a lot and it helps with level matching. It can also be useful on mono blocks in a case where your tubes aren't perfectly matched.

[henry4841]

Steve adds two switches on the Zen he sells. One for changing the resistance of the cathode on the input tube changing the texture of the sound and another for two source inputs. This amp is for my personal use and I have no need for either. I consider the switch on the input tube as a gimmick, some may like it though, but the other most will appreciate having the option of two inputs to switch between. I do that with my B1 buffer not needing two inputs.  

[henry4841]

I would like to comment on the accessary mods which are extra on the Decware Zen. Steve adds some small value film caps across some of the electrolytic capacitors. This is a common addition for many designers thinking adding a .1uf across a large electrolytic cap is going to improve the sound. It could very well change the texture of the sound to some degree. Some may well call it an improvement. They are very inexpensive except for the time it takes adding them in the circuit. Personally I thing it a waste of time adding a small value film cap across an electrolytic. I've seen designs where the main electrolytic cap can be 1Kuf and designers add a .1uf film cap. Just how can 1/10 of a uf can improve the sound of a 1,000uf one no matter the type.

 

The thing most do not consider is there is nothing sonically wrong with the sound of an electrolytic capacitor. Nelson Pass of Passlabs recently used a large 10Kuf cap on the output of one of his projects. This was common in the early seventies in amplifiers and receivers, a large cap between the amplifier and the speakers blocking the DC. I first heard of an electronic sounding fine from Roger Modjeski in one of his lectures at the Burning Amp Festival in San Francisco. He even went on to say if it were not for the slight amount of leakage all electrolytics have he would use one for a coupling cap, in the signal path in a tube amplifier. He went on to say call me a heretic for saying this. Just use a good Nichicon cap and forgo all the trouble installing film caps across an already great sounding electrolytic cap. There are electrolytic caps called audio grade for those wanting the best which I will assume are better made ones. If one wants to build a SET EL84 such as this on a budget I say use an electrolytic in place of the 3.3uf film caps on the input tube. I do not believe it will hurt the sound in the least. In fact one could use a larger value electrolytic cap which may improve the sound a touch over the small 3.3uf film cap. 

[CWelsh]

I will be interested to hear how you think this amp compares to The Little Sweetie versions you've built.

[Scott Grammer]

It's good to see folks still building their own amps. I used to do more of that, 20+ years ago, now I just don't have time. I did recently get hold of one of the cheap Chinese amps they sell on Amazon and eBay, and after stripping it to bare metal, I built it back the way I wanted it. I'm quite happy with it. Single-ended, class-A, EL34 outputs, 6SN7's as driver and gain stage. It does about 7WPC, and I ran my JBL L80t's with it for several months before getting my '68 Cornwalls out of storage. 

 

I still have only one of the many amps I built back in the day. A tiny, 2.5 WPC job with just three tubes: a 5AR4, a 6SL7GT, and a 6AS7G. Built it back in 1995. I like it a lot, but it has very little gain and must be driven with a preamp. 

[Scott Grammer]

By the way, I kind of like the blue Edcor transformer....

[henry4841]

10 hours ago, CWelsh said:

I will be interested to hear how you think this amp compares to The Little Sweetie versions you've built.

Different but same sound. Contradictory statement but both are SET's with the Little Sweetie being more lush I guess is a good word. Little Sweetie has more personality that many would like. I do at least. That 6Y6 tube in the Sweetie has personality. This 6P15P amp is more what one is used to hearing from a SET amp. 

[henry4841]

On 7/20/2023 at 10:51 AM, henry4841 said:

This was a rebuild of an old amp I built years ago and I tried not to replace or change anything from the original build other than appearance. Below is some distortion pic's and numbers to look at. I tested this little 2 watt amp at 1/3 watt. It is only fair not to test a 1 watt to show but I did take a look myself yesterday and even at one watt it was respectable. Last night while listening to it I measure the average wattage number I was using with the music at my normal level. Your wife would call it loud, and 1/3watt was my average level so that is all the power my Klipsch speakers need to really sing loud. One thing that is very important to know is when I originally built this amp I was not expecting a lot from 2 watts so I just purchased some really cheap output transformers. Good quality mind you but lower on Edcor offerings. This OPT's sell for just $32 right now and work great. At least I think so. At 1 watt output I did noticed some distortion of the signal at the extreme high and low frequencies but never heard any of it while listening last night. Reason being my average usage is just 1/3watt. Testing at 1/3watt this morning pushed the performance with a clean signal to 40hz and 18khz. These little OPT's exceeded my expectations. They sound really good for not a lot of money.  https://edcorusa.com/collections/tube-amplifier-single-ended-output-transformer/products/xse10-10k-10w-10k-ohms-single-ended-tube-output-transformer

 

Just for the record I have two sets of new Hammond 125dse OPT's I could have used but they are just not needed for this build. I think one of the Hammond 125CSE's or the Edcor GXSE10-8K- 10 watt ones will work perfectly but for those wanting to build on a budget I find nothing wrong with these $32 OPT's. 

 

In the screen shots the distortion numbers are predominantly 2nd harmonic, the kind many want in their music. Touch of 3rd as well. What you expect to see with a SET with no feedback. 

 

I will post more test results later today if I have time or first thing in the morning. I do want to discuss this build more and what I think of it.  

Those that have been keeping up with this build may notice an improvement in distortion in this build vs the original one I also checked before taking apart on page one. No parts were change other than new resistors and better layout and technics learned from years of diy. Much cleaner signal.

 

Notice a 60db difference between primary signal and PS noise. I may have misspoken and said 50db is quite on another thread, subject, but 60db is what I know to be quite enough on my 103db LaScala's. If I put my hear inside my bass cabinet I do hear a tiny amount of hiss but nothing a ft. or two away. 

[grindstone]

Just popped-in to say I'm with Scott -- glad people are still building and posting.  There must be a pile of threads out there for these type things by now if people want to do variations on the basic theme (ask 5 people, get 6 opinions).  I think what you did is important in keeping it all accessible w/o deep-diving, Henry.  There's not much of that around anymore and there are plenty of places for the other thing.  This thread has a good vibe carrying enthusiasm and encouragement so you've succeeded.  i've not met a "tube diyer" that hadn't at least once hacked on a console carcass to get some ear-time on something of the ilk. 

[henry4841]

I just want to add about the comparison of this amp and tubefanatic's (Maynard) Little Sweetie. As the suggest the Sweetie's sound is sweet. The Sweetie uses a tube the 6Y6 tube which predates the 6V6 tube and from the research I have done on the web was introduced around 1937. If you want to hear the sound I am sure some of our ancestors heard way back then well the Sweetie is the one to build to hear that sound.  

[henry4841]

I mentioned Roger Modjeski at least once on this thread. For those that do not know who he is he taught electronics at one of the Ivey League schools before starting is own company. Unfortunately for us tube lovers he passed away in Dec 2019. Two of his presentations are on youtube at the BAF conference. Below is a link for those interested in his accomplishments. 

 

https://www.ramtubes.com/about