New Amp Project SET EL84, 6BQ5, 6P15P

[henry4841]

Got in a few hours of shop time this morning after cutting big limb that fell in my GF's back yard this morning. I believe I am about ready now to start some assemble and using my soldering iron. 

A good friend of mine does not care too much for all the holes I drill in my top plate. Personally I like the look and they do serve a service of letting the heat underneath the amp escape which means long life for the amp. I now build amplifiers that look like they are built like a tank, meant to last decades and sound good with good parts. There is always talk of capacitors needing replacing in crossovers on this forum. Nelson Pass stated on another forum about seeing his 40 year old amplifiers and the caps still test good. Using capacitors rated above what is needed, where they are not working close to their limit, and keeping everything cool is how to achieve long life in a product. 

 

By the way the pictures show the back side of the plates which have some scratches that are not going to be seen. I still have the protective covers on the plates but I thought one could get a better idea on how the chassis is going to look showing the back shiny aluminum. 

 

 

[henry4841]

Coming together. No surprises so far which is a good thing. The IEC connector is a little crooked, so what. It is more than likely going to stay with me and I do not have a habit of looking at the back of an amplifier that often. The front pot has to be wired before it is installed. I think it would look better if I took those ugly blue covers off the transformer and painted them back but I am going to go with what I now have. If it were for sale it would be done. 

[henry4841]

A little more work done this morning. I should be ready to build a circuit come Monday. It will be the power supply circuit first and then if time permits wiring up the tube filaments. That is my plan at least. 

 

 

[henry4841]

For those interested, the orange wires are the heater ones, red ones B+. The red and black stripe along with the white and brown stripe are center taps that are going to a ground point I made right close to the power inlet. The B+ red wires have diodes attached to them and then the diodes are tied together attached to one point on the standoff. The diodes have turned the AC from the transformer to DC but not pure DC yet. There is a good bit of AC ripple still with the DC needing smoothing in the power supply section before it can be used by the amp. Main AC from the IEC connector goes first to the fuse, then switch and finally to the transformer. The other lead from the transformer goes to the common connection on the IEC connector. On the schematic one may notice on the heater section Steve used a transformer without a center tap making a virtual ground with two 100 ohm resistors. My PS transformer has a center tap so I do not have to make a virtual ground for the heater section on my build. It is easier to understand what is going on if one just takes it one section at a time using the schematic. I stopped on the schematic Saturday at the 4007 diodes. Actually I used a larger diode 1N5408 I believe is the number with a 1kV 3amp rating vs the 4007 1K 1 amp rating. I have plenty of them along with the 1N4007 diodes in stock using the larger ones because of their stronger leads. The 1N4007 is plenty good enough for this little amplifier. 

 

[henry4841]

The above schematic is a later one from the one I am I will use. I just got my scanner working again so here is the main schematic I am working from. Being there are some changes in the latest published schematics by Steve that do not show the values of part I will deviate from this schematic and implement some of those updates. Figuring out the value is not that hard if I choose to do so. I am using SS rectification instead of tube for this build. I just do not worry too much of the type of rectification as others. I am sure though that rectifier tubes do color the sound but having built both types I really do not hear that much difference. I have about as many tube rectifier amps as I do SS rectified ones to say this. 

 

This is an older published schematic once circulating on the web. 

 

 

[henry4841]

Got what I was planning on finishing this morning done. The power supply section and the tube heaters are wired. First I tested my capacitors with my Peak meter just to be sure nothing was damaged during removal. All test within tolerance. These caps are close to 8 or 10 years old but with less than 50 hours on them. Probably more like 20 hours being with so many amps none of my tube amps have their tubes broke in yet with me constantly taking one out and putting another in. Most consider 100 hours necessary before tubes sound their best. The power supply section went well with no hiccups. Steve's chose of cap values is somewhat unusual because the second cap is more like 100uf to 150uf on most of the modern designs I have built. The first cap is a normal value when used in a tube rectifier circuit, 33uf. My build is with SS rectification so I could probably get by with more uf in that first cap if I chose to. The final two caps in the PS are in series with a working value of 10uf when in series. Steve is using 2 caps in series of 250V's increasing the amount of voltage the caps can take to 500V's. Overkill in that position where the working voltage is going to be much less. If I were designing this amp I would use one 150uf 350V cap in that spot. When I originally built the amp I used two 33uf 450 caps in series, probably back then not knowing exactly why the two resistors were in series. I was just following the directions, schematic, building without understanding circuits as well as I do now. Being my caps are 33uf 450V's ones I hooked the two I had in parallel creating 66uf of capacitance with a 450V rating. Not going to make a difference audio wise but may show better numbers during testing using more capacitance. 

 

The heater circuits are pretty much self explanatory. There a number of ways to wire the heaters and this is the way I settled on some years back. Later this week, hopefully tomorrow morning, I will start on the first audible circuit, the 6P1 tube. I may make some changes in the circuits when I start wiring them.  

 

Using old parts with leads cut short I had to add wire to the leads to make the connections I wanted to use. Not as neat and clean looking as when using new parts but this is after all a rebuild using the old parts and the parts I have on hand. I do not believe I am going to have to make an order with Mouser to complete this amp. 

[Marvel]

Nice progress, Henry. I don't have a workspace, which means I have to pull stuff out of a utility closet, do some work and clean up. That's my excuse and I'm sticking to it.

[henry4841]

I finished the ground connections this morning to start with. Then I started wiring up the two sections of the input tube. The 6P1 tube has two triodes in one envelope enabling the use of one tube for both channels. Steve elected to use two resistors in parallel on the cathode of each of the 6P1 triodes. What the old schematic I used years ago does not show is a switch where you can use both resistors in parallel for a resistance of 964ohms on each cathode or switch to where you are only using the 1.5K ohm on the cathodes changing the sound to some degree. Could be something to play with if one chooses but I honestly do not like playing with an amplifier I am listening with so I elected to just use one 1K ohm resistor for each cathode. I listened to this amp this way for many years and enjoyed the sound so why change. Roger Modjeski was the first person who pointed it out to me how one could change the sound of your amp by substituting a variable resistor on the cathode of the input tubes tailoring the sound to ones taste. You may consider this if building this design yourself. 

 

So far things are going well and I am almost finished with this build. I estimate 4 or 5 more hours and it should be finished. This is with me being slow and old. A young talented person could finish it in much less time I am sure. 

[Marvel]

20 hours ago, henry4841 said:

A young talented person could finish it in much less time I am sure. 

That would leave me out on both counts!

[henry4841]

It's alive. More pictures and discussion of amp build later. 

[henry4841]

Did some testing this morning on this new build along with taking some pictures of work underneath the chassis. 

[henry4841]

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.  

[henry4841]

Below is the square wave signal at 1khz 1/3 watt, respectable. 

 

Some comments on this design and build. Anyone is welcome to join in. Going through the parts I removed I found the two .1uf film caps that I assume were attached to the screen grids to K of the first build called by Steve the hazen mod. The hazen mod was not shown on the first schematic I built from but evidently I knew about the hazen mod and installed the caps. Taking measurements this morning I found I am running the 6P15P tubes at 10.95 watts. A touch more than I like being this is a 12 watt tube which possible could shorten the life of the tube. But since this amp is for my personal use and knowing with so many amps I will never put that many hours on it I think I will let it be. It possible could sound and work better working a little harder. I do have a 200ohm 5 watt resistor left over from the first build I assume was installed in the power supply section in series with the 1Kohm 6 watt resistor lower the voltage some which in turn lowers the dissipation of the tube probably more close to the 10 watt range I was shooting for. For now I am just going to let it run as is. Running a 12watt tube at 11watts is not critical and I see no red spots on the plates. 

 

One thing that caught my attention is the overkill on some of the resistors wattage rating. I measure only .66watts dissipation on the cathode resistor of the 6P15P tubes. Steve's schematic shows a 150ohm 5 watt resistor. IMHO a 3 watt resistor would be plenty good enough. I must say now so not as to cause confusion this is not the Decware Zen amplifier currently being sold by Decware nor one of the old ones either. Different components are being used from different manufacturers along with me not following the schematic part for part either. I just call it a SET EL-84, 6BQ5, 6P15P amplifier. I've used the 6P15P tube in some of my other builds such as my PP class A amplifier by another designer. Rock solid tube from when Russia was making some fine tubes. I was short with only one part, the 1K 2watt suppressor grid resistor. I had a 800ohm 2 watt one and a 1watt 200ohm one in my stock so I tied them in series for the 1Kohm. I was concerned if the 1 watt 200ohm was being pushed hard so I took some measurements and found only .32 watts in the suppressor grid circuit. Another case where a 1watt 1K resistor would work just fine. 

 

A few of our electronic geek members have chose to move on since the big purge which is a shame. Perhaps we can generate some more members on this forum to try diy. This is the reason for my posting builds on this forum. Join in guys. Your thoughts, opinions welcome. 

[pnort]

Henry, what speakers are you using that 1/3 watt is sufficient?

[henry4841]

1 hour ago, pnort said:

Henry, what speakers are you using that 1/3 watt is sufficient?

LaScala's  It is easy to check your speakers yourself if you have a AC voltmeter and use ohms law to find watts. Most are astonished just how small power is needed with Klipsch speakers. The 1/3 watt figure is average with peaks being more. At the level I listen at the signal never tops 1 watt. This is a 2 watt rated amplifier at 10% distortion. It will do more with more distortion but the thing is it will be on peaks and not noticeable. A tube amp can clip and you will never know it. Two watts of tube amplification is all we need with our speakers. 

 

Nelson Pass has oscilloscope hooked up to his system set for a 1 watt window and the signal stays inside the 1 watt range when it is playing loud with less efficient speakers than ours.  

[henry4841]

12 hours ago, henry4841 said:

LaScala's  It is easy to check your speakers yourself if you have a AC voltmeter and use ohms law to find watts. 

To make it simple, if you see 1.62V average with your digital voltmeter you have reached 1/3watt assuming you have 8 ohm speakers. As stated in most all speaker spec's it takes 2.83V's into 8 ohms for 1 watt. When listening with my new amp checking usage the other night I never saw a full 1V of use. In the neighborhood average as loud as I normally use I saw an average of less than .5V usage. Most are shocked just how little power our speakers actually need. 

 

While checking this amp I noticed Steve is just running around 2ma of current through the input tube 6P1, not much. Looking back at Maynards Little Sweetie I saw much the same from his input tube. I reached out to Maynard and confirmed just a few milliamps is all that is needed for this tube as well as his build. Learn something everyday playing with electronics trying to learn more about circuits. 

[KT88]

On 7/12/2023 at 9:52 PM, henry4841 said:

I have been busy with chores at 3 houses the last few days. I have a GF that lives on my street a few houses down that I have been seeing for 14 years. This year her single daughter moved on the very same street between her and myself so now I have 3 houses to take care of. I consider myself blessed to still be needed at 74 but it does take away from my shop time. Monday was installing ceiling fan in GF's daughter house, yesterday cutting grass at my house and GF's. This morning fixing leaking sink at GF's daughters house. But later this morning I did manage to sit down at my bench at start cleaning up the parts I am going to be using on this project. Not a glamorous job and time consuming but necessary. Also started layout of pieces on top plate. Of late I like to put all the AC lines, PS components and PS transformer on one side and the audio section on the other side keeping them apart as much as possible. For appearance sake though I am going to install the PS trans in the center of the amp with the wires going to the PS part. Being on top with the audio OPT's underneath will provide adequate distance to not be a problem. 

 

 

It's always nice to still be needed by friends and to have your amp self-made ones that gives the feeling of freedom and peace. And of course fantastic results.

[Shakeydeal]

Nice build. The EL84 is one of my favorite output tubes.

 

 

[KT88]

4 hours ago, Shakeydeal said:

Nice build. The EL84 is one of my favorite output tubes.

 

 

I have two old Leak Stereo 20 with EL84 and a Matchless Spitfire guitar amp which also uses EL84, the most simple Matchless circuit. I love them both.

[henry4841]

I want to say after rebuilding this amp and more closely looking at the schematics Steve Deckert has published I like what I see. Steve has overbuilt this amp to the extreme if he is still building as per the previously published schematics with the parts list. For instance the EL84 cathode resistor. It is shown as a 5 watt one where a 2 watt one would be overbuilding being the resistor is only dissipating .66 watts on my amp. Another example is the choice of 450V power supply capacitors where a 400V or possibly 350V one would work satisfactory. If built like the schematic I can see how he can give a lifetime warranty. At a price of $1200 and hand built with quality parts I can honestly say it is a good deal for someone wanting a quality built amplifier you can pass on to your kids when the time comes. For diy'ers it is a easy relatively easy amp to build and if on a budget can use good satisfactory parts and still get amazing SET sound. I highly recommend this amp even with the long waiting period.