June 21, 2012 at 8:55 pm #5114
This is my first post. I just finished my two-stroke amp yesterday (exactly from the book), but it’s not making any sound, not even a hum.
The pilot light is on and all the tubes light up, thus the 6.3V circuit should be fine. I double checked all the connections and the voltages from the book. The voltages between the output tubes (pin 3) and ground were 340VDC (compared to 340-360VDC in the book), the voltages between the gird (pin 4) and ground was 298VDC (300VDC in the book) but the voltage between the the 12AX7 (pin1 and 6) and ground was 107VDC (140-160VDC in the book).
Is the low voltage the problem and how could I fix this? The only difference from the schematic in the book was that I used a .0047uF mallory cap (as in the parts list) for the tone control instead of a .047uF. And a random .47uF cap
for the bypass cap (instead of a mallory series 150 one). But the parts list says that you should use a 47uF 400V axial cap. Can that cause the problem?
ThanksJune 21, 2012 at 11:35 pm #5653RobinParticipant
Assuming you mean the.47uf cap between pin 3 of V1 and the boost switch, that cap should be fine. The voltages do not sound like the issue.
1: Confirm the speaker connection is good to the OT.
2: Confirm that you have 6.3v on both sides of V1. The tubes heat up?
3: Double check the shielded cable from the input jack (the 68k resistor if you included one)and pin 2 of V1. Only the outer shield should go to ground (and then only be grounded on one end) and that the center lead does NOT go to ground (no matter what the illustration shows).
4: Double check the input jack connections and confirm they are correct. Switchcraft jacks have different lug assignments from some other jacks.
Post some photos of the chassis.June 22, 2012 at 1:16 am #5654
I appreciate your help.
1. The speaker connections seem to be solid. But the 68k resistor between the 1.5k resistor and the speaker connection shows only 1.5k on a ohmmeter, while the color code says it’s 68k.
2. All the tubes are glowing, and warm.
3. The shielded cable seems to be connected right as well. I checked with an ohmmeter that there is no connection between the shield and the center cable.
4. The input jack ground is connected to the boost switch and the 1.5k resistor. The “hot” end is connected to the 68k resistor.
here are some pics.
[attachment=51]DSCN1805.JPG[/attachment]June 22, 2012 at 1:17 am #5655
moreJune 22, 2012 at 1:23 am #5656
moreJune 22, 2012 at 1:25 am #5657
4June 22, 2012 at 1:26 am #5658
6June 22, 2012 at 1:27 am #5659
5June 22, 2012 at 3:41 am #5660RobinParticipant
Wow, you built the original design, with 2 6V6s! Did you get DH’s addendum and revisions? DH posted to the internet a couple of pages of updated info, corrections, revisions not too long after the book was originally published. Those are available here on the TAN site, go to the Wiki tab and you can get that info along with revised chassis layout and a real schematic. There is nothing wrong with the original 2 6V6 layout other than it has more parts. You can only use 6V6s if you want to use both sockets and one 6L6 = 2 6V6s (in terms of output watts, the tone is different).
Review the DH revisions and see if it helps resolve the problem.
Also, if you post your photos to a photo album under “my Photos” on your dashboard, the photos there can be downloaded and enlarged for better viewing. I’m still thinking it’s a misconnected lead or something going to ground that should not be. Note that on the amp building forums, you will read over and over again about the value of double and triple checking the build. It’s only after you have built a couple of these things that you appreciate the value of that advice. It’s natural to be convinced that it’s all built 100% correctly (after all, you built it and you know it all right)until you have that ah-ha moment and discover some simple flaw.
- You must be logged in to reply to this topic.