I just built a 5F1 and powered it up and it kind of amplifies but mostly pops and sounds like static on an old shortwave radio. The B+ voltages seem to be low (336 vdc), but pop back up (395 vdc) when I take the tubes out. I originally suspected leaky caps and so have replaced all of the electrolytics, have swapped out the 25/25 bias cap. I have replaced the 10K and 220K voltage divider resistors with 1W (not in this picture). I pulled up an end of each of the .022 ufd caps to see if they were pulling it down. I have checked all of my grounds (ohmed them out with a VOM) I have gone back through and double checked all connections. I installed a replacement new OT ( i had a second one as I'm building 2). Now I am completely stumped!! I have swapped out both the 6V6 and the 7025. I have added the missing ground on the pot. I have replaced the pot. The only thing that hasnt been swapped is the 5Y3 and the PT.
Any help much appreciated -
I attached a short video clip where you can hear it doing it's shortwave radio impression. If you plug a guitar into you can hear audio but not very loud.
My voltages are:
B+ 395
after 10K R - 321
after 220K - 230 vdc (without tubes installed)
with tubes installed:
B+ 336
after 10K 252
after 220KR 162 vdc
This gives me the following tube voltages: (without tubes installed)
The image link doesn't link and I can't the .mov on my cell. But I sort of know this amp and if you get pics up we could get it sorted. Simple amp; hard to screw up in a major way unless something fried.
What's the voltage on Pin 8 of the 6V6 with a tube installed. It's pointless to measure without. That .4 is your meter. The B+ side of a power supply sucks away every electron it can find. No tubes means no electron flow (unless you shorted something). The meter lies to you because it completes the circuit (at a very high impedance).
Would I be correct to assume the 0.4V you measured on the cathode of the 6V6 was taken with the 6V6 out of the socket? If so, what is the cathode voltage with the 6V6 installed?
yes - that .4 v was out of circuit - in circuit the 6V6 cathode voltage is 25.7 vdc. I meant to include as the measured voltage.
And yes - if I pull the 7025, the B+ stays up at 395...
I screwed up in the image URL - here is the thing...I populated the board with parts I purchased...Yes I thought this would be a simple amp, but then again that's what I get for thinking...
Interestingly, when I plug in a guitar into the "loud" side, it does produce amplified guitar, just not clean (still has a hiss and pop) I will record a video of that and post it on YouTube to get the effect...
I noticed that the B+ volts age drops from 395ish to 330 as the tubes warm up and the B+ stays high when I pull the preamp tube...so it seems like it's somewhere in there - I did swap out a 7025 but will do it again... It does amplify to some extent but just isn't loud as I think it should be and there is this hiss...
Assuming you used a 470R cathode resistor (that's what the schematic I found on-line calls for), then you've got 54.6mA through the cathode. Even if the screen grid is pulling 5mA, you've still got 49.6mA of plate current. With the oreamp tube removed, you've got 395 - 25.7 = 369.3V across the tube, times 49.6mA = 18.3 of plate dissipation at idle, which is pretty high. Is the 6V6 red-plating?
I would check your grounds again, make sure they are in the right places as well. I know that sounds odd, but some of those Tweed circuits like to be grounded where they are in the schematic/layout.
Check your lead dress, look at a picture of a tweed champ, see how Fender ran the wires, any extra should be removed. Good luck with her!!!
Chris
What Martin says, let's see a new and clearly stated set of voltage readings with tubes in sockets.
This amp has a very low parts count. It should be relatively easy to verify the build and this is how I would do it.
Print two copies of the schematic. Verify it two times. The first time, work from the input jack to the speaker, using a yellow (or color of choice) highlighter to mark each part and each wire as you verify that it conforms to the schematic. Then, do it a second time, starting at the speaker and working back to the input jack. As I said, this amp has a low parts count and you should be able to do it in about 20 minutes. Take your time.
While you are looking around, use a magnifying glass to search for stray wire hairs and other unintended contacts.
You might be surprised what turns up. The worst case is that you spend 20 minutes and find nothing. Little to lose, much to gain.
I will go back and do new tube measurements. And I did get rid of the "death cap".
The 6V6 appears to be operating normally - not red-plating. and all-in all it sounds like it's trying to work...I did find a wire that came loose on the bottom on the first inspection and will re-check the grounds.
Here are some fresh measurements with tubes installed. Now that I've caught on to the fact that the voltages go south as it warms up it is quite pronounced so I include two measurements. Quiescent, or at power-up and as it warms up and starts delivering static. Then the tubes tart operating and something seems to be sucking current, causing the static and popping noise (like a cap leaking?)
-
2379]
