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Discussion in 'Shock Brother's DIY Amps' started by TATomlinson, Feb 17, 2020.
Thank you Cheesy Cucumber... words I never thought I would type.
I aim to please let us know how it goes!
You should see voltages that are more
in line with what's expected.
So, you were right CC, I had those resistors switched. I fixed it and not much changed. The amp got slightly louder but everything metered the same.
I’m metering around 280v where the expected values are 165v.
280V on the plate of the 2nd triode? Means little or no conduction through that side of the 12ax7. No / low current -> no / low voltage drop. Maybe you've got an incorrect ground somewhere..?
First, can you check if the voltage on pin 8 is ~1.5V wrt ground?
If it isn't, do the following with the amp turned off:
check if you've got proper continuity to ground on the v1b cathode resistor (the 1.5k you just moved)
also check the resistance to ground on the leg of the 1.5k resistor facing the top of the chassis.
check if the v1b cathode to ground resistance reading is 1.5k (i.e. pin 8 of the 12ax7 to ground)
If there's no continuity or the resistance value is far outside the range of 1.5k +/- 20%:
check if there's continuity between the v1b cathode resistor and your b+3 node ground
then check if the ground on your b+3 node (the rightmost 8uF cap) is connected to the chassis / ground
From your pictures, it looks like the b+3 node is indeed connected to your preamp ground point on the input jacks but we should check anyway in case there's a broken connection.
Another possibility is that the heater connection for that triode is bad. Check that you have ~3.15VAC to ground on both pin 4 and 5 of the 12ax7 and ~6.3VAC between pins 4 /5 and pin 9. Check the pins, not the wires as you may have a bad solder connection.
While you are at it, check the 220k carbon film resistor on the board near the input jack. I can't tell from the picture but if it is a 5 band color code that would be 22.4k.
Thanks. I’ll check it out. I’ve resoldered nearly every connection on the filaments and the board and am still having the same issues.
Hmm, if you've done the checks in my last post and everything is in the right ballpark and also resoldered the filaments, maybe you've got a bad 12ax7... Have you tried another?
Yep, I have plenty of preamp tubes. Same result. I think it's the volume pot. It's scratchy between 1-3, no real sound until around 8 or 9. Either that or my preamp ground bus. That's under the fiber board.
Did you do the ground checks in #25?
You could troubleshoot by adding a temporary jumper from the chassis to any of the grounds connected to 2nd triode
You could try a jumper across the two terminals (yellow wires) on the vol pot. That would be full "on" as long as the other terminal (green) goes to ground.
So I replaced the 1 meg pot. The sound at 12 is now great. Sounds about how I expected. Now there's all sorts of static from 1-12. Once I'm at 12, it sounds cool. Is it possible I got two bad 1 meg pots from Mojotone?
What would that tell me? Not being a jerk, I really don't know.
I tried that. At least I think that's what I was doing.
It would eliminate the pot as a suspect for loss of signal. If the wiper in the pot was not making good contact, connecting the two yellow wires would bypass the wiper. The bypass would be the same as turning the pot all the way to "12".
It is possible. There may be some oxidation on the wiper of the pot. Try turning the pot back and forth several times. Maybe apply some deoxit if you have some.
A classic "tell" for capacitor leaking DC is a scratchy pot. Coupling capacitors are suppose to allow the AC signal through but keep the DC voltage from passing to the next stage of the circuit. The wiper contact with the carbon resistance element of a pot does not handle DC well. Hence the scratchy sound.
The cap in question would be the one furthest toward the input side of the board. Use your meter to see if there is DCV where it connects to the pot. Only a volt or two can be problematic. Another way to check is to replace the cap.
Good to know you are getting volume. Some progress has been made.
Alright, Lower Left Coast you may be on to something. Getting around 3 volts where the coupling Cap connects to the volume pot. I took out the cap, cleaned up the eyelet and reinstalled, same issue. I’ve ordered a couple caps. I’ll let you know how that goes.
Have you checked this? Do this on pin 7 and pin 8. While you are checking, sweep the volume pot and note any changes. It is an easy and common mistake to mix up the grid and cathode wires at v2b.
Rooting for you here TATomlinson. You're getting great advice -- hang in there.
Did you say your preamp ground bus is under the board? That worries me, not because this seems like a ground problem, just Murphy's Underboard Law -- if something can go wrong under there, it will. But let's work on things we can see and test and reach for now.
Hmm, or could it be a ground problem??? Two bad pots isn't likely. But what if your pot ground to that preamp bus is flaky, and you're bypassing that ground when you're on 12? I hope I'm wrong. And you may have already tested this above.
If not, that's a simple ground bus. Can you disconnect the pot from it and alligator a temporary ground from the pot to the chassis? We can worry about ground loops later.
Ok everyone. I don’t know which change it was specifically but the monster is alive! After resoldering just about everything and switching the resistors Cheesy Cucumber pointed out, I started to suspect the the primary wires off the OT weren’t the problem in the first place (i had some feedback in the beginning). So I switched the wires back again and boom, it came to life. There’s a little noise but I will take it. I want to thank everyone who commented on this thread. I learned a ton and can’t wait to build another one.