capacitor question + date question

[newperson]

Hi,
I am recapping an old 1st edition Ampeg Reverbrocket for a friend and it looks like someone at some point added two caps to the amp.  I have everything redone at this point and was wondering if someone could tell me if they are added and if I should replace them with new caps or just leave them out. 

I would also like to know if someone could tell me the possible date of the 2 additional capacitors.  They have printed on them:
ARCOLYTIC
BSS 4045
CTA 1445
40 MFD
450 WVDC
1128E36  85C

They don't look like they belong but they seem to be older since they have MDF on them.  They are 40uf 450v in line with the filter caps.  The can cap has 3 (20uf) for the filter and an additional second unit with 2 (20uf).  The schematic shows the additional unit should have 3 (20uf's) but it looks to be clear that there were only 2 ever there. 

My guess is that someone at some point felt the amp needed more power filtering or the main can cap was going weak and they just tacked in a couple more caps rather than doing it properly.  But that is just a guess.


I will post a copy of the closest schematic I could find for the amp.  The red is the two addition cap and the red circle is the cap that never seemed to be in the amp. 

Thanks for any thoughts on the matter,




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[amptramp]

You have two competing considerations when you increase the value of the cap at the rectifier cathode.  One is, you get less voltage drop between line alternations, so the buzz gets less.  The other is, the rectifier only conducts when its output voltage exceeds the capacitor voltage, so higher capacitance means higher currents conducted over a smaller conduction angle.  Peak plate current is 400 mA (GE tube manual) or 440 mA (RCA tube manual) for a 5Y3GT.  Since the larger capacitance holds the voltage up higher for a longer time, adding capacitance adds to the peak current and therefore to transformer heating.  This does not have as much effect on hum as you would think because the 6V6GT output tubes have a plate resistance in the range of 50,000 to 80,000 ohms, so the tubes act as a current source and the voltage variation does not create much hum.  If the output tubes are balanced, at zero output, you get zero hum because the stage cancels the effect of voltage variations on the output.

Everywhere other than the input capacitor, going larger makes things better.  The reat of the stages are based on triodes which have a low plate resistance and definitely pass the hum and buzz from the power supply along.  The earlier stages are also small-signal stages, so removal of power supply ripple helps.

There are people (like myself) who like to add voltage regulator tubes or zener diodes to amplifier power supplies.  These not only regulate the voltage, they absorb the turn-on voltage spike when the rectifier conducts but the other tubes have not started to conduct yet.  If you use voltage regulator tubes, they should not have capacitance across them or they will act like relaxation oscillators.

I would be concerned about caps being old - electrolytics have a finite service life and exhibit reduced capacitance and increased leakage as they age, particularly if they get hot.

[newperson]

Thank you for the reply.  I have all the electros replaced at this point and was just wondering why I would need to replace the last two "additions" to the amp with new ones.  It sounds like it would be best for me to leave them out, especially the first stage. 

Any guesses why a person would stick 80uf additional filtering in the amp?  I did look through all the revisions of the Reverbrocket and see each model past this one had more and more filtering.  This one looks to have about 100uf total for the power and the later models look like they go up to 150ish Uf, but the power tubes and rectifier change in each model. 

[amptramp]

Capacitors got cheaper and more compact as time went on, so it was economically not a problem to increase capacitance.  Also, manufacturers knew that the amplifiers may be in use for years, but maybe for a few hours per gig and not that many people had hundreds of gigs a year, so they didn't worry about rectifier replacement - the rectifiers could last for years and they were cheap.  The good side of more capacitance is less ripple and buzz and rectified AC has every harmonic of the orignal power line frequency, so it does get into the audio.

If you look at the 6V6GT, the Rp (plate resistance) is about ten times the load resistance, so if you have eleven volts of ripple, only one volt shows up across the transformer winding and since there are two windings that cancel each other out at zero signal, the earlier stages are more of a problem.  They are triode stages which normally run with load resistors nearly equal to the Rp, so two volts of ripple would become one volt of signal and there is no cancellation from push-pull stages.  Since the power supplies for these stages are isolated from the rectifier, the capacitance can be increased without any ill effect.  Higher capacitance also gives a buffer for the slow increase in effective series resistance and drop in capacitance over time.