New 6G6-B build, blown OT???

[LarryLarry]

I have a looper pedal, so I can feed this into the input for continues sound.

Then using the probe as directed, I'll work my way back from the input listening for the looping guitar to lose all volume, correct?

Thank you all for taking the time to help!!

[LarryLarry]

Ok, I found out how to use it here:

https://tubeamparchive.com/viewtopic.ph ... 8f50f4b443

[sliberty]

Yes that's right. You can use your looper, or an ipod, cd player, or anything that will inject sound into the input of the amp.

The 1/4 jack of the probe needs to be plugged into something that will reproduce the sound. So, a known working amp, or even a set of computer speakers (that's what I use).

You will clip the alligator clip onto the chassis of the amp under test. Then, you will touch the exposed tip of the capacitor to key places in the amp. Start at the input jack. Then move to the grid of V1. Then the plate of V1, then the grid of V2, etc. I oversimplified the places to probe, but you get the point.

[John_P_WI]

That is a nice tool to use for debugging.

For a very quick test, the disturbance test is very quick and instantaneous. The sound should get louder as you move towards the input as the disturbance is progressively amplified.

[LarryLarry]

I did the disturbance test and got a faint pop on pin 4 of the 6L6s, but none on pin 5 of the 6L6s. I moved to the PI and got a noticable pop on pin 2 but nothing on pin 7...

[John_P_WI]

On the PI is pin 2 the feedback side or input side (sorry I did not take the time to look at your pics)? Check the voltages on the PI. Also with the volume turned up, the pops should get louder as you move towards the input of the amp.

[LarryLarry]

Thank you sliberty and John_P_WI - you guys ROCK!

I built the audio probe and started tracing the sound back from the input jack. When I got to pin 1 of the PI I still had audio, but beyond that I had nothing. It turned out to be the 100pf capacitor (not sure what it's called) that bypasses pin 1 and pin 6 on the PI (see photo). That capacitor was from an unknown source and was probably pretty old. I had a new replacement on hand so I'm back up and running!

Like 1 of you said, after I had the audio probe it took me 5 min. to find the problem and 5 min. to fix it!

[sliberty]

That is great news. Congratulations on t he SUCCESSFUL build
Now, we need photos and clips.

I bet that cap was essentially acting like a wire, allowing the majority of the signal from one side of your PI to cancel with the other side of the PI. It is supposed to cancel selective frequencies, but you were probably canceling the whole spectrum.

I love the Weber probe (as I call it), especially for diagnosing low or no sound. It helps you zero in on things very easily. And I love that is is a DIY tool too.

[Reeltarded]

Awesome! This place rocks.

I don't like probes.

[John_P_WI]

LarryLarry, Great! Rock on..... Glad you worked through it.

The cap that you asked about is a snubber cap. It can be used to control oscillations, tone shape (the larger the cap the lower the frequencies shunted) frequency shape etc. By the way, this cap is called a fizz cap in the Marshall circles as it is used to take away some of the hash or fizz when the PI is driven hard.

[jeff12]

Glad to see you found your problem. I have a 6g6 Bassman that had a problem that drove me crazy trying to find. Wish I had the forum back then. I would get an odd harmonic after a note was played. Not a musical harmonic at all. Tried everything I and a couple of friends could think of. Filter caps, shielded wire in the signal path and other tricks to fix the problem. No luck at all. Then one day looking at schematics I noticed that the 6G6B had a .0005uf cap coming off the 470k resistors of both channels to the phase inverter,and the 6G6 did not have one. Installed the cap in my amp, problem solved no more harmonic. Hard to think someone played that amp since the early sixties without fixing the problem. Drove me crazy every time I played it.