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Question About Capacitor Upgrade in Tube Amp

Hi,

I am preparing to do a coupling capacitor upgrade on a recently purchased tube integrated amp. The two 0.22uF on the preamp tubes are fairly straight forward. But I noticed another similar model 0.33uF cap on the large filter capacitor for the B+ supply that is installed across the hot lead to ground.

Does this cap on the B+ just block high frequency noise from the power supply or does it have any effect on the amp tone? Is there any reason to "upgrade" this cap?

I know it may be hard to tell exactly what is going on without a schematic.

Also any recommendations on a good cap to use in the upgrade of the coupling caps? I was looking at Mundorf SilverGoldOil for the quality at not too crazy a price. The amp already sounds good but lacks a little clarity that I think a coupling cap swap will help with. It is SET 300B amp.

Thank you!

 

calieng
fisher_400
71 posts
 

A simple answer. Yes, the small cap across the large el4ctrolytic cap reduces high frequency noise.

ieales's avatar
ieales
1,201 posts
 

Prove it.

HF bypass caps are common in IC circuits as opamps can have multi-MHz bandwidths coupled with 100dB gain.

Tubes can also be multi MHz oscillators, but willy-nilly hanging a film cap across the B+ filter maybe just the ticket to convert an amplifier into an oscillator.

jaytor's avatar
jaytor
763 posts
 

@ieales - I'd be real interested to understand how adding some film decoupling caps cross the B+ supply could cause a tube circuit to oscillate. 

ieales's avatar
ieales
1,201 posts
 

Tubes are capable of several MHz operation.

Oscillators are made of LCR and a device to drive them. The combination of the PSU, wiring - printed or P2P, transformer LCR and a tube to drive them can create a self-resonant circuit.

If one searches the DIY sites there are innumerable posts of capacitor changes turning tube amplifiers into oscillators.

jaytor's avatar
jaytor
763 posts
 

I can see how adding too much capacitance to a regulator could cause the regulator to oscillate, but oscillation in tube amplifiers is rarely due to the power supply, and when it is, it's because the power supply has inadequate regulation (too high an output impedance), not the other way around. 

Yes, obviously changing capacitor values in the amp itself, and particularly in the feedback circuit, can cause an amplifier to oscillate. I spend a fair amount of time reading DIY audio sites (and building my own amps and preamps), and I don't think I have seen a case where adding a small amount of additional capacitance to a coupling cap or power supply cap ever caused an amp to oscillate. 

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