help troubleshooting an issue with my preamp: Tube Works Blue Tube RT-904

[merrthad1]

Hello there! I am getting pre-amp distortion/volume abnormalities (maybe some phasing issues) still after replacing the tube twice with two different new tubes to make sure that's not the problem (one tube in this preamp)..it starts out sounding fine but after running for a little while it starts to sound ugly..sometimes it goes back to sounding okay..sometimes you can tap on the pre amp chassis and it will go away for a while..its a Tube Works Blue Tube RT-904 preamp by B.K. Butler and runs 117 VAC and 6 Watts...I have looked inside and don't see anything obvious like blown capacitors and open wires but I haven't made any measurements yet as I brush up how to measure caps with a multimeter etc...AND when the pre-amp is bypassed and sound is only going through the power amp, everything is fine, fyi..any suggestions? And thanks ahead of time for looking and sharing your brain!

[Audiotexan]

...after replacing the tube twice with two different new tubes to make sure that's not the problem (one tube in this preamp)..it starts out sounding fine but after running for a little while it starts to sound ugly..sometimes it goes back to sounding okay..sometimes you can tap on the pre amp chassis and it will go away for a while.

Welcome to the forum!
If you can help us out a bit by letting us know how much technical experience, and what diagnostic tools you have available, it would be helpful.

Also, I have a schem for the Blue Tube RT-903 available, but nothing for a 904. Not sure how close or far the revision deviates, but I'd be happy to post what I have if anyone thinks it'd help!
-AT

Oh! Forgot to add, if it clears up when you tap it, it may be something as simple as broken solder joints (as noted in the following pic):

[eschertron]

Hi! Welcome to the forum!

..sometimes it goes back to sounding okay...I have looked inside and don't see anything obvious like blown capacitors and open wires

Since it sometimes works OK then the culprit is hiding in portion of the circuit that hasn't failed completely yet, and so may well look just fine. If the issue is temperature-related, be advised that it may not be a component like a capacitor, it could easily be a solder joint between the cap and the board (I'm using the cap as an example, any soldered component is just as likely). A close inspection - without power, preferably unplugged - and a 'wiggle test' may reveal a weak solder joint.

The joy of troubleshooting, however, lies in the fact that problems very often aren't visually revealed. A knowledge of the circuit, it's signal flow, and expected behavior are all important tools in your arsenal. Speaking of tools, do you have a DMM, a scope, or any other diagnostic gear?

Also if you could rustle up a schematic for the unit, it will bring more educated guesses to the table.

edit: simulpost... audiotexan has some answers already

[Alan0354]

It really depends whether you are provision with basic electronics and more important knowing all the safety precautions. Don't be brave, know your limits and you live long. Don't think just because you turn off the power and unplugged the amp, that it's safe to stick your fingers in.

Sounds like you have a loose connection or bad solder joint. Look at the schematic if you have one, use an insulated stick ( like a wooden chop stick) to tap the wires and components following the signal path and hopefully you can find the sore spot. It happened to me twice today on my build because the amp has been worked on far too many times.

Tap and eye ball check is the quickest and easiest way. Using a stick is the safest way as the first step. Report back of your experience and post the schematic.

[mozwell]

as far as i know (and i stand to be corrected) the RT-904 is just a rack mount version of the RT903

[Audiotexan]

as far as i know (and i stand to be corrected) the RT-904 is just a rack mount version of the RT903

Ahh cool (didn't bother to google it). I'd bet your right.

BTW, those are the same schems I had for the 901/903, so that saves that! *chuckles*

[teemuk]

as far as i know (and i stand to be corrected) the RT-904 is just a rack mount version of the RT903

They are all similar being based on a similar design.

But not the same. Question is, with how much similarity can you live with? RT-901 and RT-903 designs for sure ARE NOT the same as RT-904 design, and as a matter of fact, they do not even have certain features found from the said unit.

Pedals like the RT-903 or RT-901, for instance, do not have "controur" or "bias" controls, which on the other hand are found from the rack units like the RT-904 or RT-902 (Real Tube Rack). So, RT-904 is probably much closer to RT-902 than what it is to RT-901 or RT-903.

[J M Fahey]

I am getting pre-amp distortion/volume abnormalities ... it starts out sounding fine but after running for a little while it starts to sound ugly ... sometimes you can tap on the pre amp chassis and it will go away for a while ... I have looked inside and don't see anything obvious like blown capacitors and open wires but I haven't made any measurements yet

Hints at bad solder/connections, maybe some bad pot or jack , but just looking won't help much.

What you really need is to scope it.

Inject 100 mV 1KHz tone to the line input , set controls to:
Contour: 0
Boost: off
Bias: highest value, don't know if that means 0 or 10
Hi/Lo/Mid/Master: 10
Drive: start on 0

Set your scope to 2 or 2.5 V/Div and scope the output, pin 7 of the scond TL072.

Start raising gain s l o w l y until it starts to clip.
I expect some 10VPP or better beforeb that.

When the pedal starts misbehaving we'll trace signal backwards towards the input, until we find at what stage that happens.

Then we can start measuring around it checking for abnormalities.

Before that, it's just Poker, wild betting on unseen cards .
Scop