Help With Tube Preamp

[jsefer]

hmmmm. o.k.  I had to ask as I had a similar problem with an amp I had on the bench and it turned out the bridge rectifier failed. but you have ac heaters so not a problem.  Do you have access to an o-scope and a tone generator? Might be time to start  to scope it out. Also double check all the solder joints and such.

No access.... im SOL in that department...

 

[jsefer]

I mesured a few this morning before I left to work, and some seem out of spec, reading at about 240k...

That is around 5% which is perfectly acceptable.  It is when they go excessively high (towards open) you should suspect a bad one.  A 250K that reads say 2M or more.

this would bypass the output fader correct?

correct.

So to you this seems more like a Pot issue or cap, more than a bad resistor.... Now just thinking about it, I cant seem to think it might be a resistor acting strange under load...

Can you guess why that strange behavior going upto 340v then back down to operating could attribute??

 

[pucho812]

I would redo all the solder joints and then continue.

 

[lassoharp]

So to you this seems more like a Pot issue or cap, more than a bad resistor.... Now just thinking about it, I cant seem to think it might be a resistor acting strange under load...

I can't tell for sure other than it seems to be localized around last 12ax7 stage. Taking signal off the plate will hopefully rule out  or point to cap connections, polarity or a bad pot.

As soon as I power it up, the Gain wich is VR1 500k on the 12ax7 stage, does not seem to have effect when turned, and the output volume has little effect.

Verify that both of these vol pots have solid ground connections.  Those M8s have a grounding scheme that is kind of spread out.  I remember inadvertently losing a ground connection when pulling out all the non essentials during a rebuild.

 

[jsefer]

So to you this seems more like a Pot issue or cap, more than a bad resistor.... Now just thinking about it, I cant seem to think it might be a resistor acting strange under load...

I can't tell for sure other than it seems to be localized around last 12ax7 stage. Taking signal off the plate will hopefully rule out  or point to cap connections, polarity or a bad pot.

As soon as I power it up, the Gain wich is VR1 500k on the 12ax7 stage, does not seem to have effect when turned, and the output volume has little effect.

Verify that both of these vol pots have solid ground connections.  Those M8s have a grounding scheme that is kind of spread out.  I remember inadvertently losing a ground connection when pulling out all the non essentials during a rebuild.

Just got home from work, no time for food just troubleshoot...!

Man I feel dumb so i fire it up and sound, everything seems right, nothing fades, all voltages still the same, how ever gain nob doesnt seem to have effect but volume pot seems to work right, I start looking at my ground traces and found a broken ground solder joint!

Solder gun heating up now, but seems like we might have solved this mastery my friends thanks a million.


Edit: Problem SOLVED!!! Note to self: Check grounds 1ST.

 

[buildafriend]

Do you mind posting a gut shot of your work? I am gutting an M8 right now.