70s build - some questions (new - added some clips)

[wjdunham]

I did some searching and didn't find what I was looking for - sorry if this has been discussed before:
I'm building (converting) my latest build to a #40 after all the great discussion of Lindley tone - I started with the PI. My problem is that I have no gain in the power amp now. I ran a signal from another amp into the FX return, and I have to turn the donor amp all the way up to get any volume. I checked all the resistor values and I built exactly as shown in #40 schematic in the files section. Things were working fine with the 80s style PI before I changed things over, so I know all the power tube/OT stuff is OK. Is this to be expected? I disconnected the NFB and tried a couple of different tail resistors (100R, 1K), no significant change. I have the correct (at least per the schematic) 51K/47K plates and 470R/22K cathode setup. Just shy of 300V on the plates.

I haven't finished converting the preamp, and it looks like the gain structure is pretty different from the 80s style amps, which is what I have built previously.
Thanks as always.
Bill

[Structo]

Is this the schematic you are going by?

[wjdunham]

Yes, except that I think the feedback resistor in that schematic is wrong (93R is too low, I have 820r there, but I disconnected anyway until I get it working. Also, I'm only building a 50W version. Take a look in the files section for the #40 schematic.

[wjdunham]

I should say, I THINK the FB resistor in that schematic is wrong, I'm no expert here.

[Structo]

OK, yeah I have that schematic too, the one with the orange lines and pink text.

I see the 820R and the 100R tail.

I'm sure you checked but are you sure the tube you have in the PI is good?

Stranger things have happened.

[Bob-I]

Modding the resistor values shouldn't make that big of a difference. I'll bet something else was touched, like a ground connection or a bad tube.

I'd put things back and see what happens.

In cases like this a signal generator and scope are invaluable. I'd inject a sine wave at 1KHz and see where the signal is getting dropped by looking at the gain through each stage.

[wjdunham]

I tried a few different tubes, so something else must be off - I was hoping to not have to put it back the was it was, but maybe that's the best approach. I have a scope and waveform generator which I haven't applied to the problem yet. Not sure what voltages I should be looking for at the PI input (20-30V?).

[Bob-I]

I tried a few different tubes, so something else must be off - I was hoping to not have to put it back the was it was, but maybe that's the best approach. I have a scope and waveform generator which I haven't applied to the problem yet. Not sure what voltages I should be looking for at the PI input (20-30V?).

No, I don't think you want to push that much into the PI input. I'm looking at the Fender Deluxe test voltages which I use as a reference, and they show 5-6V P-P. You should expect about 20-30V P-P at the out of the PI. http://www.ampwares.com/schematics/65_D ... erb_RI.pdf

But the approach I'd take would be to put about 30-50mv at the input with the amp on a dummy load. Read the voltages at each stage and look for a gain increase. Of course the tone stack will drop gain.

[talbany]

Can you post some pics


Tony VVT

[LPSGME]

If it's just resistors that have been changed, any chance you just ordered the parts and someone sent the wrong values and you're just not noticing the full color coding is wrong?

[wjdunham]

I checked all the resistor values - I'll snap a few pics this morning if my probing with the scope doesn't yield anything.

[talbany]

tried a couple of different tail resistors (100R, 1K), no significant change

wjdunham
Thought I would mention is make sure the other side of the 100R has a good ground..Maybe when changing the resistor out could have lost it or forgot to hook it back.. I've done this a few times easily overlooked.. That would give you a rather large reduction in gain.. Thought I'd mention..Waiting for the pics..Good Luck!!

Tony VVT

[Bob-I]

I checked all the resistor values - I'll snap a few pics this morning if my probing with the scope doesn't yield anything.

It really has to yeild something. If you're not getting the output, at the very least it has to be able to measure that. Then you can trace back and find where the gain is lost.

[talbany]

Just for grins..I ran the Numbers on the 2 PI's 70's and 80's like 124... Here is what I found..

80's 12AX7..........

inverted output voltage gain=-26.104580610775
In-phase output voltage gain=27.79284653131

70's w/7025

Inverted output voltage gain =-16.936418364051

In phase OPT voltage gain=18.225167978761

A pretty noticeable difference in gain here...The 70's style is almost half that of the 80's...If you are using a 12AX7...

Now a 70's PI w/12AT7 here is what you get

Inverted side=-23.856659778189

In Phase Side=25.97588174606

More gain..Looks to me like the 70's style is I would say better suited around the AT...

Numbers were derived from my Tube Cad Modeling software for the LTP PI>>

Tony VVT

[wjdunham]

Sure enough, dumb problem - the return jack was not making good contact - emphasis on "good", it was making SOME contact. 5 minutes with the scope and problem was solved. I also had a weak solder joint affecting the OD channel.
Once I got it set up with the 802R/100R feedback ratio I'm back to everything working. I do notice the lower gain in the PI relative to an amp with the later style PI into the same speaker, but not huge. The OD channel with the early tone stack on this thing rocks! Especially with the strat, so my experience seems to be similar to what I've read here - early stack good with single coils, later stacks good with humbuckers. I'm adding the "funky" OD entrance (still have the 220k/100k trim/68K). Then finish the FET input. 10 more years of practicing and we'll see if I get that David Lindley sound out if it