Preamp Tube Difficuluties

[Mikeb]

I put a brand new 12AT7 in my 5E3 preamp (V1) and it is causing a lot of weird oscillations and noise. I tried another with the same result.

I moved the 12AT7 to the PI slot and it works OK with a 12AX7 driving it. It sounds kinda ratty but it works.

In circuit voltage at pins 1 & 6 with the tubes in the preamp are as follows...

12AT7 - 1 = 138VDC, 6 = 145VDC
12AX7 - 1 = 186VDC, 6 = 188VDC

Any ideas???

Thanks, Mike

[Brad166]

I never had any luck with the 12at7. Mine kit came with a Electro-harmonix
12AT7 but it was microphonic. I could tap on the amp almost enywhere and hear it threw the amp. I then tried a bunch of diffrent 12ax7. Much better in my opinion. There is a little more gain and a little less clear headroom, but thats okay with me. I built this amp for the overdrive anyway. I tried a NOS 12ax7 JAN-phillips, a NOS 12at7 JAN-GE, New 12ax7 electro-harmonix, new 12ax7 Tung-sol, and new JJ/Telsa 12ax7. The Tung-sol and JJ where the best. I like the JJ the best in this amp. Sound great with tons of tweed overdrive.

[stokes]

Why would you want to use a 12AT7?I have always found them to be very flat and dull sounding used anywhere but in certain PI positions,go with the 12AY7 and AX7 the amp was designed for.

[Mikeb]

Hey Stokes... I know we covered this 12AT7 issue, but remember that when you advised me, I had already purchased them. So, having bought them I was merely trying to lower my gain while I wait on my AY7's to arrive. And happ'd upon this horrible oscillation problem.

I am going to find some spec sheets this morning to see if there is a clue there.

It sounds like a motor boat and as much as I love to fish, I would prefer it if my amp didn't sound like my boat.

Beside poking fun at my poor choice in tubes, any ideas on what might cause that?

Thanks fella...

[stokes]

Mike,I didnt mean to be poking fun at your choice of tubes,didnt remember our past exchange at first until you mentioned it,the mind is showing its age.As for what is causing the motorboating or low frequency oscillation,the amp doesnt like the AT7 in that position,it probably has a less than optimum bias point in the circuit there,or too big a voltage drop across the plate resistor,cant say for sure,but you know its not the amp or you would get it with the AX7 as well.I am sure someone else could give you a more precise answer with formulas,etc.,but for me knowing it is the tube and not the amp is the bottom line.If you were really bent on using the AT7,I am sure you could tweak the plate and cathode resistors and get rid of the oscillation.Maybe change the value of the bypass cap or get rid of it all together.

[Mikeb]

I did a little research today and found that the internal resistance (Ra, need to further clarify the exact definition) of the AT7 is much lower than that of an AX7, hence the significant difference in voltage drop.

I have some Ay7's on order that will be in by the weeks end. We'll see.

I tend to agree with your theory that if the amp was hosed, the AX7's wouldn't work either. Very good point. Sometimes I get so caught in trying to understand theory, I lose sight of the simple things.

I don't mind you guys having a little fun with my tube ignorance. Ignorance is temporary. At least you're telling me rather than letting sweat it out.

Thanks again...

[Mikeb]

To hell with the 12AT7 problem. It still dosen't work right but who cares... The AY7 in the pre driving an AX7 in the PI is kicking ass!!!

Thanks guys for all the help!