How to connect stereo preamp to Schedule 40 GA-5 amp?

[accmeint]

Hello,

Sometime back I posted that I built the stereo preamp project:

http://www.el34world.com/projects/images/stereopreampschematic.gif

And I was trying to hook it up to the Schedule40 GA-5 amp that I build a while back:

http://www.el34world.com/projects/images/Schedule40GA-5.jpg

I got some recommendations where to introduce the preamp input into the GA-5 amp.  (See the attachment "pi_2.gif").  I finally got around to trying them out without success.  I am wondering if perhaps I did the hook up incorrectly.  One problem was that the circuit in the attachment was a little different than the Schedule 40 GA-5 project schematic.  That was my fault because when I first asked this question I put in a link to the incorrect schematic.  I believe I found the corresponding connection points on the Schedule40 GA-5 schematic however.

To insert the preamp into the GA-5 circuit I plugged a 1/4 inch jack into one of the preamps output jacks and connected the ground wire of the 1/4 inch jack to the GA-5 amp's ground strip.  Then I disconnected the 5.6k resistor from pin 2 (grid), of the EL84 tube and connected the other wire, (the non-ground wire), of the 1/4 inch jack to pin 2 of the EL84 tube.   It seemed that the preamp was overdriving the GA-5 amp with various  oscillations.  I then removed this modification and reconnected the 5.6K resistor to pin 2 of the EL84 tube to restore the GA-5 amp to its original state.

Next I removed the wire connecting pin 7 of the 12AX7 tube to the wiper of the volume pot at the volume pot.  I  connected the non ground wire of the 1/4 inch jack from the preamp output to this wire.  This put the output of the preamp into pin 7 of the 12AX7 tube.  The GA-5 amp immediately began to sound like a motor boat.

I know I am doing something wrong here,  I hope my explanation of what I tried makes some sense because it is somewhat difficult to explain verbally.
Could it be that I am incorrectly introducing the preamp into the GA-5 circuit?   Thank you for reading this and any suggestions on how to connect one channel of the stereo preamp project into my Schedule 40 GA-5 amp would be appreciated.

[HotBluePlates]

I think you're right about being wrong.  Sorta.

From the attached diagram showing where to insert the preamp signal to the GA-5, we see 2 suggested points. The first point is at the location of a ground reference resistor (shown as 220k on that diagram, but really a 470k on the Schedule 40).

You did not insert your signal there, although it might appear to you as if you had. Instead, you unhooked the 5.6k grid stopper resistor, and attached the cable hot directly to the EL84 grid.

That's no bueno. The function of the 5.6k grid stopper is to prevent oscillation in the EL84 due to long grid wires. In a practical build, you keep this wire as short and direct as practical/possible, and then add the grid stopper to knock down any oscillation that may occur despite your efforts. Long grid wires act as an antenna for RF, and possibly other signals. When you disconnected the 5.6k resistor (and, consequently the 470k grid reference), you made your EL84's grid wiring extremely long. That all but guaranteed oscillation.

Next up... I know the stereo preamp will present an impedance to ground, so you don't technically need a ground reference resistor (the 470k in the Schedule 40). However, had you back up your connection point from the actual EL84 grid to the junction of the (likely wire to the) 5.6k resistor and the 470k resistor, you would have kept a local ground reference (the 470k) and utilize the grid stopper to effectively shorten the length of the grid wiring.

In fact, the real way to wire this up would be to use a switching jack (as used for most input jacks). The tip would connect to the junction of the 470k resistor and the 5.6k resistor (or wire leading to the 5.6k), while the switched lug would connect to the output of the stock Schedule 40 preamp (the wire that normally connects where you're inserting the stereo preamp). Naturally, the ground goes to the ground buss.

You might solve some of the overdrive issue (if there was one aside from the oscillation) simply by fixing how you insert the preamp signal. The 470k ground reference resistor will be in parallel with a part of the output pot from the stereo preamp, and will likely reduce signal level slightly.

[EL34]

I use a Stereo Power amp after my Stereo Pre amp.

You need two Schedule 40's to run Stereo

[jjasilli]

2-Channel vs. Stereo:  Per Doug, the word "stereo" implies 2 full, independent channels, including 2 power amps driving separate speakers.   With one (mono) power amp, your preamp might be more clearly described as 2-channel, rather then stereo. 

Another suggestion is to input the preamp signal into the vol pot (lug 3) of the GA-5. Then you can set the GA-5's vol pot to attenuate incoming signal so it doesn't overwhelm the GA-5.  If necessary you could also:  add a small series resistor after the coupling cap to the vol pot to isolate the the outboard preamp signal from the prior internal stage of the GA-5; or b) per Hotbue's post, use a switching jack to plug-in the preamp, and simultaneously disconnect the vol pot from the prior GA-5 preamp stage.

[accmeint]

Gentlemen,

Thank you for your responses to my question.  The image below is my interpretation of HotBluPlates' connection recommendation.  The red circle on the schedule40 GA-5 schematic represents what is shown at the top of the image.  Please let me know if this input jack hook up looks correct.

Thank you

[sluckey]

No, that's wrong. Looking at the schematic...

Disconnect the right side lead of the .02 cap from the 470K and 5.6K resistor and connect that cap lead to terminal J2 of your jack.

Now connect terminal J1 to the junction of the 5.6K and 470K resistor.

That 5.6K and 470K resistor MUST ALWAYS remain connected to the EL84 grid.

[jojokeo]

I've got a feeling that even the switch could be miswired when discussing this with those of less experience. Connect the 470k right to the 5.6k resistor so there's always a grid leak resistor present. I know what you mean sluckey but other's learning may not exactly. $.02

[accmeint]

Thanks Guys,  How's this?

[sluckey]

Now you're cookin'.

However, before I went to the trouble of disconnecting any components or wiring, I'd just run a wire from the jack tip (J1) straight to the junction of the cap and the two resistors. Very easy to do and undo, especially if you have a gator jumper. If it works like that, then I'd consider wiring the jack switch as you've drawn.

[EL34]

I think I am missing the whole point of running a stereo pre amp into a single GA-5 amp? 

[HotBluePlates]

No, you're not missing it.

But the real problem was whether the connection was right, rather than if he made connection x2.

If he fixes the connection issue, then going stereo is just a matter of doing the same thing for a second time.

[accmeint]

Thank you gentlemen for the infomation.  I will make the mods this weekend and let you know how I make out.