EL 84 Single Ended Integrated

[ivegotmono]

As I said in a previous post, I picked up an AMD EL 84 Single ended integrated amplifier from a friend. It has too many problems with bad components to bother with and I can't even find a schematic or parts list anywhere. What I'd like to do is just rewire it completely. The power and output transformers are good and the case and front is perfect. It has a EZ81 rectifier, a 12a?7 phono stage, two 12a?7 preamp tubes and the EL 84 outputs. Can anyone recomend a simple single ended integrated design that I could clone using this setup.
I plan to use it for my computer so I don't even really need the phono stage. ( It would be nice to have though)
Thanks
Steve

[dhaen]

Well it doesn't take long to draw one from scratch. Just download the tube pinouts. If you can measure some voltages too, that'll make it easier. Post a rough drawn scheme here and we'll pick it to bits... no, no, we'll make any corrections

If you do decide to rebuild, make sure you know the all the transformer interconnects. It can save a LOT of time!

[Sherman]

Originally posted by ivegotmono
... Can anyone recomend a simple single ended integrated design that I could clone using this setup.

I plan to use it for my computer so I don't even really need the phono stage. ( It would be nice to have though)
Thanks
Steve

Here is a SE EL84 I built. I put in a DPDT switch so it can be switched from triode mode to ultralinear. Triode mode gives about 2WPC and ultralinear is about 4WPC. You need pretty efficient speakers to get the most out of it.

You could also put in a three way rotary switch and switch between triode, ultralinear and pentode. Pentode will give you about 5.5WPC but it doesn't sound nearly as good as triode in this amp.

A note about the switches- unless you can find switches rated for 350V (or whatever your B+ voltage is) don't switch between modes with the power on.

I'm using it with a "passive pre" (basically a 100K Alps pot and a source selector) but you could wire in a 50K or 100K volume pot between the input and first resistor.

B+ of 350 is a little hot but seems to work fine. You could do this with a 250V B+ if change the value of the plate resistor on the 12AU7.

[rdf]

Hi Sherman,

How's the top end? I've played with near every bias point south of your B+ and never been satisfied with the reproduction of sibilance. JJ tubes and Thorsten's recommendation of fixed bias with secondary winding as EL84 cathode feedback helped substantially but still not where I'd want to build it out permamently. The OPTs arethe excellent James 6123, which I know you've used and liked.
Appreciate your opinion.

[Eli Duttman]

As was previously indicated you are going to need EFFICIENT speakers: 95 dB. for pentode and 98 dB. for triode. I strongly suspect that the O/P trafos don't have ultralinear taps.

Those SE EL84 amps were made on the cheap. While the EZ81/6CA4 is an excellent tube, its use is a cost cutting measure. A single filament winding in the power trafo takes care of both the signal tubes and the rectifier.

I wonder if the unit ever had a RIAA phono section in it. A DC heater supply is needed and that is an expense. I suspect the phono I/P is for a piezoelectric cartridge.

There are enough sockets available to build both the power and phono sections into the chassis. You are going to need a 15 VDC/1 A. "wall wart" for the phono section heaters. The 600 mA. offloaded from the internal filament winding will come in handy.

Frankly, I'd prefer triode wiring the EL84s. No loop NFB will be needed. Loop NFB will get messy, as there are multiple caps. in the circuit. Also, phase compensation is TRICKY. If triode mode is acceptable, check out the DECWARE SE84's topology. DECWARE The gain of a 6n1p or 6922 could be a little low. Using the EXCELLENT 5965 twin triode instead of a mu 33 tube resolves any insufficient gain problem.

The "classic" RCA phono stage circuit that takes a 12AX7 in each channel deals with that need. Some tweaking is needed. If you are interested, we'll talk about the tweaks.

[icebear]

Hi Eli

Can I change the 6922 in my SE84SC directly with a 5965? No changes? And sound improves?

Cheers
bjorn

[Eli Duttman]

Originally posted by icebear
Hi Eli

Can I change the 6922 in my SE84SC directly with a 5965? No changes? And sound improves?

Cheers
bjorn

No. At a minimum, the heater power connections on the socket have to be changed. Also, for solid performance, the cathode and plate resistor values need to be reworked. The cathode resistor value is matter of CONCERN.

[archtopas]

I new in this deall(diy stuff)..I'm just wondering(thinkt 2 build smth like Sherman showed) and looking for a parts, which i have or could get in while. Just wondering, is there a possibility to change the el84 to 6bq5. If there is some chance, so for some time the 6bq5 could stand instead.

Is there any difference in sound of these???

tom

[Eli Duttman]

Originally posted by archtopas
I new in this deall(diy stuff)..I'm just wondering(thinkt 2 build smth like Sherman showed) and looking for a parts, which i have or could get in while. Just wondering, is there a possibility to change the el84 to 6bq5. If there is some chance, so for some time the 6bq5 could stand instead.

Is there any difference in sound of these???

tom

6BQ5 = EL84 Same type, different naming systems. The Russian 6p14p-ev, AKA EL84M, is a decent tube. Non-high military grade 6p14p stock is not so good. The lower case Latin characters in the Russian types are phonetic transliterations for Cyrillic characters.

Different production runs from a single manufacturer can sound different. The same type from different manufacturers will have different voices.

[Sherman]

Originally posted by rdf
Hi Sherman,

How's the top end? I've played with near every bias point south of your B+ and never been satisfied with the reproduction of sibilance. JJ tubes and Thorsten's recommendation of fixed bias with secondary winding as EL84 cathode feedback helped substantially but still not where I'd want to build it out permamently. The OPTs arethe excellent James 6123, which I know you've used and liked.
Appreciate your opinion.

I don't profess to have golden ears and I can't hear anything above 15K (too many years of playing music too loud I guess) but the top end sounds pretty good. Cymbals sound clean and higher notes on guitars and violins sound "detailed". The bass isn't house-shaking, how could it be with 2 watts? But it gives a nice full sound. My most efficient speakers are only about 92dB/W so I can't get the best from a triode strapped EL84.

I'm using Hammond 125ESE OPTs on this unit which was built as my first foray into "designing" a tube circuit myself. The PS is very good, with a 5U4G rectifier, 40uF motor run cap, 1.5H choke, 100uF motor run, 1.5H choke and a final 100uF motor run. No visible ripple with my scope set at 5mV/Div.

The first set of tubes I tried were EH. They were drawing 40 to 41 mA. I plugged in a set of Sovteks which drew 42 mA, pretty darn close. It is hard to put into words how something sounds but I think the Electro-Harmonix tubes sounded slightly "crisper" in the treble, midrange was surprisingly similar between the brands and the Sovteks may have had an edge in defining bass. Overall though I felt it was pretty much a toss-up.