Tube problem, Fender TBP-1 (12AX7)

[AlexanderB]

I have a new old stock Fender TBP-1 that I bought recently. The regular, clean tone had a muffled and slightly distorted quality, never allowing for a really clean tone. Energy in the bottom octave would excite a buzzing noise in the mids, like damaged speaker.

Yesterday I took the lid off and swapped the two (GT labelled) 12AX7 tubes in it.
Result: now the OD channel sounds worse than before, and at high gain, it will oscillate and sends out a high pitched whining, even with no signal going through. That tube is definitely defective.

The clean channel now sounds better, but the problem that remains is that there is a slight clipping sound at the fundamentals in the lowest range, let's say below 50-60 Hz down to open B. I have to turn the gain VERY low (like 9 o'clock) to get rid of it, where as everything else sounds fine with gain up to 2 o'clock. Problem remains with different basses, but is most audible with my active fiver. It is not much affected byt the setting of the BASS knob on the amp.

Can it be that this low frequency clipping is totally tube related, and that both tubes are defective?

 

[staindbass]

are you positive you are not distorting the input ? turn down the volume pot 1/4 - 1/2 way on the bass, then turn up the amp and see if it is still there. usually tubes distort at all frequencies, not just the low end.

 

[AlexanderB]

That is exactly what is happening, even if I did not manage to convey it clearly. Distorting an input is not strange, but in this case, I can keep the gain at 2 o'clock or higher with clean sound, until I hit the low notes, mainly on the B string. Then it distorts, unless I turn the gain all the way down to 9 or 10 o'clock, which is a MAJOR gain reduction.
Hence my question if the frequency related clipping is indicating that this second tube is also defective - albeit in a different way.

 

[InsanityAmps]

It could just be the voicing of that amp - if the gain structure naturally has more low end then you will clip the low end first. Having the gain at 9-10 is nothing to be concerned about, especially if you have an active bass or one with a hot output. I would suggest just running the gain where it doesn't clip then use the master volume for overall volume. If you can still get full volume with the gain turned down then there is nothing wrong with the tubes or the amp.

 

[beans-on-toast]

The lower frequency notes require the most energy to reproduce. As such, they place the highest demand on the amp's power supply. One possibility is that the power supply caps are not performing to spec and can't keep up with the demand. You loose headroom and distortion set in. A tech can test your caps and determine if it is time to change them.

Another possibility is that the tube has an issue and needs to be replaced. If it slides into the socket too easily, it is possible that the tube socket needs to be retensioned. The tube is removed and the socket terminals are carefully pinched together to tighten the contact. If they are stretched out, the poor contact can result in a distortion. The tube pins and socket contacts should also be cleaned with Deoxit. This is normally done as part of a regular amp maintenance.

On the other hand, it could be how this amp works.

 

[AlexanderB]

Thanks!
There is no "voicing" at play here, as it seems to originate in the input stage before the passive eq.
The amp will not work properly with input gain set to 9 o'clock, especially with my medium output basses. It is lacking approx 15 dB to properly drive my Fender SPL 6000 power amp.
Power supply issue seems far fetched, as we look at a 15 or so dB gain reduction to mitigate the issue.
The unit is pristine. No corrosion or oxide on the pins.

I will put in some other tubes and report back.

 

[beans-on-toast]

There are many possibilities that could be causing this. They need to be investigated one at a time. It could be a component or a bad solder joint. Unfortunately, you might not resolve it without having a tech look at it.

Trying other tubes is your best bet at this point.