New EL84 Push-Pull Monoblock

[Kashmire]

I finished the first (serious) revision of my transformer-coupled EL84 push-pull monoblock. Yea!

The amplifier uses a Lundahl LL1540 input transformer for the phase splitter, a 12A*7 driver tube (12AX7 currently installed), and Edcor transformers.

Thus far, the performance has been excellent with my Triangle Acoustics, although not so good with my Definitive Technology, and horrible with Fostex (although I've never had the Fostex sound good, but I'll save that for another thread).

It was primary designed for my Triangle Acoustics, so I consider this a success. This is the first amp that has excelled with the Triangles. They seem to prefer low (poor) amplifier damping factors, high output impedance, and no amplifier global feedback. It is currently configured in UL mode, and I'll be switching it to Pentode mode for further listening.

I'm very pleased with the Edcor transformers. They have solid construction, nice and heavy, and good fit-n-finish.

Pictures attached. Click for full schematic and more description.

[tinitus]

My, you have a very impressive site, and skills, from advanced engineering to making artwork pens

Nice looking tubeamp
You mention hum on sensitive speakers
Could it be partly due to having RCA connectors and wires too close to transformers
To my knowkedge it should be avoided
And its usually recommended to twist supply wires

oh, I see now input RCA is on print board
Well, I hate that, not solid fore tight connecting plugs
Not sure about such small choke either
I have experienced one making a transformer hum

[nicoch58]

very nice

can help a copper plate beetween the two pbc ?

put a better caps, like ampohm can help ,the alu/oil and alu wax sound a lot better then auricap .... My Capacitor comparisons: Mundorfs, VCap, Sonicap Platinum, Auricap, etc

[Kashmire]

Righto.

Thanks Tinitus for the comments. I have screw terminals on the power supply PCB for an external choke. I installed a Hammond 159T with 2.5H (300mA) for testing. It reduced the B+ ripple to 2.5mV. Without the choke, the ripple is 3.2mV. I could not hear any improvement with the Fostex speaker. All of the hum is from the filaments, not the B+ power supply.

The tiny choke on the B+ power supply is for high-frequency rejection. High frequency noise crosses big chokes (such as the 159T) easily because of wire-to-wire capacitive transfer. Therefore, small chokes are better at stopping high frequency.

The MOSFET capacitor multiplier takes care of the 120Hz ripple. It has excellent ripple rejection, much better than a big choke.

The filament power supply uses a high power toroidal inductor. My next design will use two high power toroidal common mode inductors.

I put the RCA jack on the PCB for easy testing. I also used a 2.2uF coupling capacitor from the RCA to the LL1540 transformer, because the LL1540 can not have DC offset. I will remove the coupling capacitor and the RCA jack when testing is over.

Nicoch46, a copper plate between is a good idea, but I think the hum is caused by the filaments - the power supply needs improvement (see comment above about using two common mode chokes). I will try the copper plate idea next week.

For capacitors, I used relatively cheap capacitors for now. It is very easy to upgrade capacitors in this amplifier. My testing used Mundorf MKP, Jansen Blue, Solen, and Dayton Audio. I'm trying to optimize some performance issues before worrying about the capacitors - such as tube bias current and driver tube plate resistance.

Nice post on capacitor comparisons! I will be reading it!

[bigjppop]

I really enjoyed reading your site as well! I enjoy seeing people with a wide range of hobbies; especially when those hobbies are not things like "I like watching TV or playing video games in my spare time."

So, the amp looks great, but more importantly... good for you for getting the creative juices flowing!

[Kashmire]

I went back to the 12AT7EH/ECC81. The 12AX7 had too much gain, and I was operating my volume knob at the 9:00 position. New circuit values pictured below.

I also cranked up some values in the power supply, and lost a few volts of B+. Therefore, the newly designed power supply operates at 250V and the total noise+ripple is about 2mV. Therefore, the EL84's cathode resistor has been decreased to 75 Ohms (shared) to operate with the lower voltage.

A future upgrade will be increasing the transformer voltage to Edcor XPWR074 (245-0-245@150mA) instead of the XPWR057 (210-0-210@120mA) to regain some lost voltage.

I'm operating the 12AT7 with 130 Ohm (shared) at 4mA with 135V at the plate, 1V at the cathode, and 29k plate resistance.

At the same time, I converted the EL84 to operate in Pentode mode. My Triangle Acoustics seem to favor low (poor) damping factors, and Pentode mode has the highest output impedance.

Exchanging the 12AX7 for a 12AT7 lowered overall gain, but changing EL84 to Pentode mode increased gain.

Now I still have too much gain! Next week, I'll try a 12AU7. This also opens the opportunity of using a 6CG7, which is my favorite.

An interesting experiment will be using an LED for the 12A*7 bias (schematic below). Any comments?

[Tyimo]

Hi Kashmire!

Could you tell me how much inductance has the LL1540's windings?

Tyimo

[mashaffer]

Maybe try 12AY7.