Unknown SE EL84 amp schematic...

[abram]

howdy folks,
i'm curious if anyone out there has come across or has any experience with this schematic. i've been attempting to build a little mock version of this which doesn't seem all that hard, but this schematic is a little vague on how to wire up the heater elements to the 12AX7 and the EL84. i'm also wondering the purpose of the the connection between the 6.3v AC tap and the 3 pin on the EL84. should that not run to ground along with the 100uf bypass cap? experience tells me that it's definitely not advisable at it is drawn here.



anyhow, thanks for the help.

[Albert Kreuzer]

experience tells me that it's definitely not advisable at it is drawn here.

Why not? What's your experience with this?
It is a simple way of elevating the heaters to reduce hum (search "elevated heaters"). The connections are pins 4+5 / 9 on the 12AX7 and 4 / 5 on the EL84.
The schematic doesn't look bad at all. For my taste the EL84 cathode resistor is too low, I'd go with a 270 or at least 220. Screen resistor is a bit high (safety?), most designs I know use 100 Ohms.

[52 Bill]

The heaters for the 12AX7 are at either 4 & 9 or 5 & 9 depending upon which half of the tube you use. The EL 84 heaters are at pins 4 & 5.

The return of the center tap resistors to the cathode of the output tube will lift the heaters from ground by whatever the bias voltage is. This is done to reduce hum.

[abram]

hah. by "my experience," i mean that the fuse would pop until i disconnected the heater taps from the circuit. this is the first amp i've attempted to build from scratch, so i wanted to get an outside opinion on the schematic. i'm sure i just messed up something in the wiring. the heater elements would heat up fine without running them "elevated" so to speak. i didn't have to time to test the signal through. it was late and i was trying to get to a stopping point. thanks for the recommendations. i'll definitely try out those values with the cathode resistor.

[abram]

good to know. i'm correct in the understanding that connecting the 4/5 pin on the 12AX7 will result in 6.3v operation?

[52 Bill]

good to know. i'm correct in the understanding that connecting the 4/5 pin on the 12AX7 will result in 6.3v operation?

No.

There are 2 complete triodes in the 12AX7. The heaters for the two tubes are connected from pin 4 to pin 9 and from pin 5 to pin 9. If you connect to pins 4 & 5 the two heater sections will be wired in series and will need 12.6 volts to work correctly. If you use the first tube section (pins 1, 2, 3) wire the heater to pins 4 & 9. If you use the second half of the tube (pins 6, 7, 8) then wire the heater to pins 5 & 9.

You should look up the tube diagram online and this should all be very clear to you.

[abram]

much obliged, good sir.

[Dr. Tube]

Umm... can someone tell me is this a guitar amp or something "bad HIFI" in between... And yes, why is EL84 grid resistor so high, 5.6 kilo ohms, wow what will damage that sweet distortion, i saw designs up to 2kOhms... but this is too high resistance...

[eschertron]

I'd like to hear this amp when you're running with it!

The 'extra' resistors in the screen supply for pentode may lower the voltage to the screens and help equalize the volume differences between triode and pentode mode?
With only 1 gain stage in the preamp, I'd guess this is mostly 'clean', and so the EL84 won't be pushed to draw grid current. So I think the 5k6 stopper will be inaudible, except maybe to make triode mode a bit darker.

Abram, this is great little amp to learn design chops on. Get a book (huh, showing my age!) and do some research on the intended functions and limitations of various components. For example, you can make the EL84's grid stopper about any value you like just to see what you like. AFTER the amp is running perfectly as built, of course Other components will also change the character of the amp, but as Albert mentions, some component values can be bad for the amp if the operating window is broken. That's the reason for the research. Have fun, and report back often!

[Dave H]

i mean that the fuse would pop until i disconnected the heater taps from the circuit.

It shouldn't do that. Does the heater winding also have a grounded centre tap?

[Enzo]

Dave, abram's issue was 7 years ago.

[Dave H]

Dave, abram's issue was 7 years ago.

Ha! I didn't notice that with the thread being exhumed today.

[eschertron]

Ha! I didn't notice that with the thread being exhumed today.

!!! Slipped by me, too I may never hear those sound clips now.

[J M Fahey]

And as far as grid stoppers, I have seen as high as 220k in *good* amps. (think Soldano/Mesa Boogie and most high gain ones).

[abram]

sure you will! hah. but yes, your guess was right. the single stage was mostly clean. unfortunately, the power transformer i wound up getting at the time was horribly under powered (cutting teeth), but it sounded pretty good as drawn here even with low plate voltage. i agree with your statement about the grid stopper resistor though. since we're dealing with audio frequencies in this amp, there is practically no current flow into the grid so the voltage there should be the same on the grid regardless of the resistor value. i imagine that the value of the resistor would affect the frequency response of the amp. i doubt 2k vs 5.6k would be much of a noticeable difference except maybe with the volume knob cranked, but i haven't tried it myself. i may have to crack it back open and swap 'em out just to see what the effects are.

anyhow, here's a link with the very first sound clip. i wound up putting a mojotone oil and paper cap for the 22 nF DC blocking cap between the preamp and power stage. i'd definitely recommend it at a first build for sure.

No. 5 speaks. | notes to self