6V6 1987 build sounds like a dying fuzz...

[alkuz1961]

Did you check the resistors with a multimeter before installing them? If I remember correctly, the five-ring marking is different from the standard four-ring marking.

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[MHProd]

Yes I did. It’s a 22K. I don’t remember what the 5th ring indicate (military??) but I know it doesn’t affect the value.

But, yes I double checked all resistors, caps, pots, continuity everywhere.

[pompeiisneaks]

Having those 8.2 and 9.1k resistors just tacked together like that leaves a lot of chance to bad connections. you should always connect them either at a turret/eyelet or at a minimum wind them together tightly to make a solid physical connection before soldering just to keep them in place. Solder tacking like that is very prone to noise/problems.

I'm not seeing anything else screaming out at me. Did you say you've chopsticked around the amp for any specific areas that change the noise?

~Phil

[sluckey]

I don’t remember what the 5th ring indicate (military??)

It's a reliability factor. I don't recall the specifics. I've never seen any color except yellow.

[pompeiisneaks]

My understanding (unless this is specific to older resistors) was that 5 band just gives you 3 digits of precision instead of two, and the last two bands still do the same thing, multiplier and tolerance.

So you may have had 2.2M in 4 bands is now 4.28k in 5 right? (or can have at any rate) (random values not meaning colors somehow changed, just showing the 4 vs 5 band stuff)

The gold should be 5%?

Or is this more of 'the older resistors used a different spec' question?
~Phil

[MHProd]

So I changed the 8k2/9k1 string to a single 10K. Nope...

Yes, chopsticked and also wiggled the preamp tubes while playing. Nothing...

So far I'm 99% sure:
-Not a component value error.
-Not something wired incorrectly.
-Not a continuity issue.
-Not DC leaking coupling cap.
-Not the tubes
-Not an external factor (guitar, speaker, cables).

Voltages seem OK.

The only "clue" i have so far is the presence pot between 8 and full taking away a lot of the noise/decay. I checked the PI/NFB (moved wires around, resoldered everything, changed coupling caps...) but it did not change anything.

The PI filter cap is a 16/16 can wired in parallel. Do you think the cap could be bad?
What i have for filtering is: Can 1: 50 mains/50 screens, Can 2: 16/16=32 PI, 32+32preamp.

The way the notes decay is not unlike when you turn the amp off standby and the note "disintegrates".

[MHProd]

My understanding (unless this is specific to older resistors) was that 5 band just gives you 3 digits of precision instead of two, and the last two bands still do the same thing, multiplier and tolerance.

So you may have had 2.2M in 4 bands is now 4.28k in 5 right? (or can have at any rate) (random values not meaning colors somehow changed, just showing the 4 vs 5 band stuff)

The gold should be 5%?

Or is this more of 'the older resistors used a different spec' question?
~Phil

I've measured them and they are all the right value. The PI tail measures 22.1K.

[alkuz1961]

If you look at the modern five-band marking of resistors with an accuracy of 1%, you will notice that the color of the multiplier differs by an order of magnitude, in a different direction. For example, red is 10K, orange is 100K, and so on.
And this shows an online calculator of resistor values

2020-03-30_22-50-12.jpg

[alkuz1961]

By the way, here's how it looks and sounds short circuit in the winding of the output transformer

https://youtu.be/H6AtM2wb0iY

[MHProd]

unfortunately I do not have a scope. Also, the amp doesn't feedback at all.

What would the Presence pot situation indicate?

[alkuz1961]

unfortunately I do not have a scope.

If I need to check the output transformer, I do the following - disconnect all loads from the secondary circuits and applying an AC voltage to the primary winding from the outlet. I am using a multimeter to measure the voltage between the side and the central leads on both halves of the primary windings. The voltages must be exactly the same. And the transformer should not be heated.

[Littlewyan]

One test we could do is to check voltages whilst playing. You say there isn't as much volume as there should be, have you tried monitoring the anode current of the 6V6s whilst playing? How are you checking the bias?

Edit: Reading back through, you say it's pretty much the same amount of fuzz between 2 and 6, have you tried the normal channel at low volume? This should be pretty clean. I'm wondering if there's a dodgy cap somewhere causing this.

[MHProd]

I have a bias probe which gives the plate voltage/current/wattage. As far as I can tell, it behaves fine when I play.

I haven’t made a 6V6 like this before, so I’m not sure how loud the amp should be. It’s certainly loud but I was expecting more but maybe that’s just me so let’s not make a symptom out of that for now.

I tried all 4 inputs and no, the behaviour is the same. Volume on 2, light strum, I still get that screechy noise. On all 4 inputs. All caps measure fine...

Ugh this is frustrating.

[alkuz1961]

I think you should try to disconnect this wire (white arrow), connect a temporary potentiometer 1 mohm, as a master, and connect to it any other preamp. This allows you to localize the area where the problem occurs.

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[cxx]

I had a failing 1987 that sounded like this and got progressively worse. Turned out to be an OT. If you have a spare might want to clip one in to see if it fixes it. I don't know of another way reliably tell if it's bad.