RCA BC-2B

[mjrippe]

Hi Doug, is the 285v B+ in this schematic correct?  Using a regulated B+ supply so it won't change with line voltage, just don't want to cook OPTs.

 

[mjrippe]

I also have this ancient ASCII drawing of the circuit which says "B+ is 363 Volts W/Load" !?!

 

[emrr]

285 official.  I see no real reason to run over 250. 

 

[mjrippe]

285 official.  I see no real reason to run over 250.

Thanks Doug, I figured something in the 250-300v range would be safe but you have far more experience than me with these circuits.

 

[Tubetec]

I saw a reference by Emrr on the bc-2b ,mentioning that he felt there was too much capacitance in the psu ,or that maybe it might sound better with less , any chance you could pick up on that point again Doug and explain how larger smoothing caps effect the sound ?

The engineering on those things is unbelieveable , bomb proof more or less , simplicity and super high grade transformers its a nice way to go if you can find the iron. Ive been through CJ's winding sheets for the cream of the crop of signal tx's ,its  like a layercake of neatly interwound sections often with screening foils ,each layer with  precise number of turns and spacings .typically  large cores for output transformers too .

 

[buildafriend]

I saw a reference by Emrr on the bc-2b ,mentioning that he felt there was too much capacitance in the psu ,or that maybe it might sound better with less , any chance you could pick up on that point again Doug and explain how larger smoothing caps effect the sound ?

  interesting question. I know we can easily go overkill here but too much I’m curious about

 

[Tubetec]

I had read of people who upgraded guitar amps with bigger caps in the ht ,but didnt like the result .
Seems to be some suggestion that a very low impedance  ht supply along with bias supply or cathode decoupling cap thats doesnt cope with very low frequencies might be a bad combo . Your ht is rock solid with brand new 470 uf caps ,if you kept the original value of cathode bias cap  the bias  hasnt the abillity to supply the current on the peaks and starts to fall away .

Buddy of mine got a Fender Bassbreaker , Its my guess they have used traditional 22uf caps ,theres a lot of hum even with the volume down ,its a single ended amp so no great expectations in the bass end,  the mids and highs are lovely ,the bass end is the sonic equivalent of an over weight belly dancer ,it wobbulates  and not in a musical way .

If we go large on ht smoothing would we not also then need to adjust the cathode bias capacitor so that the time constants are of similar proportions  , again I suspect the traditional value of 25-100 uf is used at the cathode of the Fender .
Maybe on account of the extra capacity in your power supply  adjusting the coupling cap responsible for the dominant lf cut off up a bit be a good plan ,just to prevent any extra subsonic frequencies  from hitting the transformers .

 

[emrr]

You’d have to show the context for me to have any idea what I was talking about. 

 

[kosi]

« Reply #36 on: May 22, 2012, 11:23:19 AM »
Quote
I wouldn't bother there.  If you were after obvious PSU effects, I think there's already way too much capacitance in a BA-2.

 

[emrr]

Oh.  Meaning if you want obvious power supply effects you probably need to be in the class of ancient amps with 4-12 mfd in the PSU and 0.5-1 mfd filters in the preamp modules.  Nothing to do with sounding better, but to do with overload and sag characteristics. 

 

[ruffrecords]

It is often forgotten forget that there is very little HT sag with a decoupled class A amplifier because the average current is constant. The local decoupling capacitor acts as a local energy store and supplies output current on positive peaks and is replenished from the HT supply on negative peaks. In a class B amplifier the supply current is directly proportional to signal level and there is current draw on every half cycle.

Cheers

Ian

 

[emrr]

If you overload them enough with tiny filter caps, they will sag and sound different in overload than if they have modern filtering reservoirs that can keep up.  Two very different sorts of 'limiters'.

 

[Tubetec]

Maybe for a simple context if we used the example of the fender champ .
8uf caps x3 and 25 uf on the cathode  and some feedback from transformer secondary .

I can see you need to keep a close as possible to the original with the RCA's , theres no shortage of chassis realestate so presumably if the designer had wished he could have had bigger filter caps . There as they are for a reason .

So even though were just talking about a straight preamp , the LF time constants we put in the powersupply control the attack /release time and onset of distortion of the output stage , very much like a limiter.

So would I be right in saying ,
If we have huge caps the amount of sag will be small ,but the time it takes the voltage to rise back up would be longer , where with smaller caps sag will be greater but the return to normal voltages will be quicker .

I think we all know instinctively the buttery sound of the RCA's , Sinatra records from the 50's had it all over , the peaks are gobbled up only they have a slight harmonic sparkle around the edges now .

 

[ruffrecords]

So would I be right in saying ,
If we have huge caps the amount of sag will be small ,but the time it takes the voltage to rise back up would be longer , where with smaller caps sag will be greater but the return to normal voltages will be quicker .

In a class A amp bigger caps should simply lead to less sag. The return to normal voltage will be the same as it always was because with bigger caps it has a smaller voltage to recover.

Cheers

Ian