2 Caps in Series with Balancing Resistors

[PaGator]

A bit of a noob question, looking for some guidance. I am replacing a power supply capacitor 30/600 in an 1971 Ampeg B15 and I want to beef it up to 35/700. I can’t buy a capacitor in that value so I am going to build one. I will use two 70/350 capacitors in series for 35/700 and use two 150k, 2 watt resistors. My understanding is I need to install the resistors across the caps, so in my mind install each resistor to the positive and negative leads of each cap and wire the 2 caps in series (positive of cap 2 to negative of cap 1). Is this correct? I did this and tried to measure it with a multimeter and couldn’t get a reading? I was able to measure the caps in series WITHOUT the resistors and get 37uf, within the tolerances of the caps. I searched all over the internet for either a picture of this or a wiring diagram but couldn’t find anything (unfortunately I do not read schematics). I am familiar with amp safety, discharging caps, etc. Any help is much appreciated. Scott

[romberg]

Sounds like you are on the right track. The resistors are there to act as a voltage divider and make sure that each cap sees exactly 1/2 of the voltage applied across both of them. If they were not there, then the voltage at the junction of the caps would depend upon each caps equivalent series resistance (ESR). The ESR of the caps may not be matched very well. So, the resistors are added.

These resistors will, like you found out, play havoc with the capacitance measurements of a DMM. This is normal. As long as each measures close to 70uf individually, rest assured that two in series will be 1/2 the capacitance (or in your case 35uf).

Mike

[alnight]

Like this? It's how I like to do it.

https://ampgarage.com/forum/download/fi ... &mode=view

[PaGator]

Thank you for the quick reply’s. One quick follow up on the link to the other thread with the image of the drawing. I’m following the two 100/350 caps in series with the under board wire, however, the two 220k resistors appear to be going negative to negative and positive to positive on the caps, is that correct?

Thanks again, really appreciate the help!

[sluckey]

Thank you for the quick reply’s. One quick follow up on the link to the other thread with the image of the drawing. I’m following the two 100/350 caps in series with the under board wire, however, the two 220k resistors appear to be going negative to negative and positive to positive on the caps, is that correct?

yes. That's exactly what you asked for.

[romberg]

the two 220k resistors appear to be going negative to negative and positive to positive on the caps, is that correct?

They are also going + to - in series with each cap. The key is to pay attention to the red wire under the board. Look at it for a while and you will see that it is exactly the same. The layout is just different .

Mike

[Stevem]

If your using your Ampeg for Bass and since that model no longer used a tube rectifier I would stack two 100uf @ 300 volt filters to get a tad more punch on the E string, and especially at higher volumes and/ or if you play a 5 string.

[PaGator]

Thanks stevem I will consider as it is used for bass. Regardless of cap values I drew out what I’m thinking to make 100% sure I’m following, can you please confirm this is correct. Thanks again to everyone that has replied for your patience and assistance

[lonote]

Your drawing looks correct

Snip from BF Fender Twin Reverb schematic attached.

Series Reservoir Caps Fender Twin Reverb.jpg

[PaGator]

Thanks!

[solderhead]

If you're interested in reading material, there's a concise snippet on this subject in the Aluminum Electrolytic Application Guide by Cornell Dublier. What you're looking for is the section on Capacitor Bank configurations on pp. 10-11:

https://www.cde.com/resources/technical ... pGuide.pdf

This pub used to be a section in their hard copy cap catalog. Sadly, in the days of the internet everyone makes you perform parametric searches online and they link you to a PDF document instead of mailing you a hard copy of their documentation.

If you're a noob then you'll find a lot of useful information on caps in the pdf.

Have fun.

[PaGator]

That’s great, thanks for passing along solderhead. Always looking to improve my knowledge