Recent Posts

71

Tube Amp Building - Tweaks - Repairs / Re: Princeton Reverb circuit refinements

« Last post by ac427v on April 16, 2026, 06:42:37 am »

After spending many many hours troubleshooting hum on my build, I replaced the reverb recovery cathode bypass cap with a 220u. C9 on your schematic. Dead silent! (Thanks Stratomaster). Now I do that mod in all reverb circuits. The part is dirt cheap. Just add it to your order "in case"    ;-)

72

Tube Amp Building - Tweaks - Repairs / Re: Princeton Reverb circuit refinements

« Last post by tristanc on April 16, 2026, 04:48:53 am »

So after many hours trying to clever and shoehorning the circuit changes on to the existing turret board I gave up and laid out a PCB:

https://github.com/tristancollins/PrincetonReverbUpdate/tree/fab

They should arrive next week.

I have gone a bit overboard and used multiple ground plane zones to emulate multiple star grounding points. Just tried to keep things logical and compact - and all on one board.

Attached are an updated schematic, a layout (borrowing Rob Robinette's source file) and a quick render of the board. This should be fairly quick to assemble. All the sources are on GitHub in KiCAD 10. I'll be updating things as the build progresses (I'm expecting something to go wrong with the board - I did it very quickly) so at the end should be a verified PCB should others wish to use it.

73

Tube Amp Building - Tweaks - Repairs / Re: Help with Vibrolux Reverb

« Last post by pdf64 on April 15, 2026, 01:58:04 pm »

Your best next step would be to equip yourself out with a light bulb limiter.

74

Tube Amp Building - Tweaks - Repairs / Re: Help with Vibrolux Reverb

« Last post by acheld on April 15, 2026, 12:32:23 pm »

Printing out your schematic or your layout and then going back to your build -- checking each connection (where the the wire go, what does it connect to) -- seems so mundane that many folks with less experience ignore this step in the startup sequence.

But really, it's not much different from a pilot's "before engine start" checklist -- also a mundane checklist, but actually very important to do routinely.

Best to print your layout/schematic, and go through it all highlighting each connection with a marker.   You will know . . .

Even though I'm now an experienced builder, I find mistakes from time to time.   These are complex systems, despite what the internet leads one to believe.   Really worth the effort. 

75

Tube Amp Building - Tweaks - Repairs / Re: Help with Vibrolux Reverb

« Last post by passaloutre on April 15, 2026, 10:46:07 am »

I said a week ago: "I’d start by making sure your layout matches what you think it is. Print it out and use a highlighter as you check each component."

Did you do that yet?

76

Tube Amp Building - Tweaks - Repairs / Re: Zener Bias "Trick" for Cathode Biased Amps

« Last post by kagliostro on April 15, 2026, 10:31:12 am »

To me it seems an alternative way to obtain a Mixed Bias

----

1.  Fixed Bias

Characteristic: Fixed negative voltage applied to the grid, cathode connected to ground.

Advantages: - Maximum power and efficiency - Greater headroom - Better control of the operating point

Disadvantages: - Requires precise adjustment - Risk of thermal runaway - More sensitive to tube variations

2.  Cathode Bias (self-bias)

Characteristic: A resistor on the cathode automatically generates the bias voltage.

Advantages: - Self-regulating - Increased safety - No adjustment required

Disadvantages: - Lower power - Reduced efficiency - Greater signal compression

3.  Mixed Bias

Characteristic: Combines fixed negative voltage with a cathode resistor.

Advantages: - Good stability - Partial self-compensation - Balance between performance and safety

Disadvantages: - Intermediate efficiency - More complex circuit - Does not reach the performance of pure fixed bias

---

Sound Characteristics Summary

  System         Sound Character
  -------------- ----------------------------------------------------
  Fixed Bias Cleaner, more defined, dynamic, greater headroom


  Cathode Bias   Warmer, softer, more compressed, “spongy” response


  Mixed Bias     Balanced, slightly softer but still well-defined


Franco

77

Other Topics / Re: Meme of the day

« Last post by shooter on April 15, 2026, 10:11:37 am »

...

78

Tube Amp Building - Tweaks - Repairs / Re: Zener Bias "Trick" for Cathode Biased Amps

« Last post by jbrrrrr on April 15, 2026, 09:09:10 am »

I remembered where the source of this came from - it was from the AX84 archive forum - since it's kind of intermittently accessible and not in a state of maintenance, here's a PDF of the original thread from 2005 - not a whole lot more info here but it does describe using it.  Maybe someone here remembers this or has tried it out?

79

Tube Amp Building - Tweaks - Repairs / Re: minor Major 7 combo

« Last post by tubenit on April 15, 2026, 06:03:53 am »

Thanks for sharing your experience and success with this build!!  Hope you enjoy the amp for yrs to come.


With respect, Tubenit

80

Tube Amp Building - Tweaks - Repairs / Re: Groove Tubes Soul-o 45 Rev B 1997 8 18 blowing main fuse

« Last post by wsscott on April 14, 2026, 01:11:49 pm »

Ha Ha--I was testing you.  The Voltage drop is 140VDC, so with the formula, and a R=10K, that's Power of 1.96 watts.  So I guess a 3W would be fine.  And I think that's what this is, given its size.  Maybe even a 5W.

I checked the voltages and they are the same for A+ and B+, which is ok because of the caps being in series giving a higher Voltage.

C+ dropped to 356VDC
D+ dropped to 306VDC

Much better than before, ie. 492 and 420VDC

Plus the Grids stayed at the same -50.1VDC
and the Bias stayed at the same 43.1/39.8ma

So I think this feels much better to me, and obviously it looks like the colors on the resistor had faded.  A 10K/3W seems to be the one to use.

Thanks again for your help.