i want to change the caps in a motorola console amp and the electrolytics i see are GOOD ALL .02 600vdc what is the replacement for these [im learning slowly but surely] also what are the ceramic looking rectangular blocks ? they are marked 82ohm and the other is marked 330 ohm? PLEASE EXCUSE MY IGNORANCE

c.granger,

No apologies needed, ask all the questions you want-that's what the forum's for! I might've mis-read your post, but I suspect the .02(uf?)cap you asked about is a coupling cap. I think a Sprague Orange drop, .022uf 600v might be what you're looking for. These can be had at a lot of places; I ususally go to Antique Electronics for parts.

The ceramic rectangular blocks are wire-wound resistors. One of these is likely used in the power supply to drop voltage off the rectifier. The other may be used to bleed the power supply filter caps. These shouldn't need to be replaced-but you might check just to be sure.

Best,
mojo

those are indeed resistors - the 330 ohm drops a bit of voltage for the screen supply, and the 82 ohm is a cathode resistor shared by all 4 tubes. they should be rated for 10W if they are replaced. while you're at it, check all the resistors as they are carbon comps and can drift substantially with use.

the 0.02u caps are not electrolytics but are paper caps, and should be replaced with paper or film -- something like orange drops, as suggested by mojo. the electrolytics are in the can on top, and should be replaced; feel free to increase the capacitance (except on the first cap after the rectifier). i have one of these consoles (i assume this is model HS-696C) waiting to be fixed up, and i've traced the circuit - email me if you want the schematic.

Hi there Bundy,

I am trying to restore this amplifier. How do I get your e-mail so that you may send me the schematic?

Thank you ever so much,

Heather

Bunty,

why not increase the first cap after the rectifier? Is this amp specific, or is it just not necessary? Please elaborate as I have a different amp I'm circuit tracing right now.

Thanks

Jason

Gentlemen,

Hi, I'm Ed. I have a tube amp. It seems it is this very amp. I'm going to throw some caps at it and see if I can make it sing!

I'll need a bit of help, but I think I have the basics of what I'm up to. I'll start a thread, maybe late tonight or tomorrow evening. I've got a schematic and I think I'm about set to go. But I don't want to mess it up.

Big thanks to AKer Brainsmasher for thowing this thing in, along with a bunch of other stuff. I certainly got much, much more than my money's worth! Never had a tube amp before and I'm looking forward to it! Once I get it going, I'll spiff it up a bit.

Take care,

Ed

If you are having difficulty describing components you may want to read up a little before going any further. If you disconnect any bleeder resistors you may leave a cap with a lethal voltage that you end up discharging into your hand. By your description you have some high wattage resistors, so my inference is high voltage. By very careful. Since I read this thread I had to add.

Have fun.

Rogers

Bunty,

why not increase the first cap after the rectifier? Is this amp specific, or is it just not necessary? Please elaborate as I have a different amp I'm circuit tracing right now.

Thanks

Jason

Depends on what kind of rectifier the amp uses... some can tolerate bigger caps hung on their outputs, compared to others.

For example- a 5V4 rectifier can ONLY handle 10uf, directly connected to it (before it starts to go "arcs and sparks" inside, potentially!), while a 5U4 can handle 40uf, and a 5AR4 can handle 60uf without damage (over-current within the tube).

If you look up the rectifier in your amp (here's a great reference: http://www.nj7p.org/Tube.php - just type in your tube number, and the spec sheet will pop up), you can find out the maximum loading (cap) for any of them, under various working conditions (choke vs. resistor filters, etc)... it's a great way to make sure you're safe and that things will work as planned!

And as for replacement caps- yes, a .022 cap can generally be used to replace a .02 uf cap... as long as the voltage is as high or higher than the original rating. So, yes... the .022uf 630V cap mentioned above is perfectly suitable. These cap values (inside an amp- preamps are different!) aren't that critical... sometimes bumping them up a bit can actually help the sound (in the old days, caps were expensive, so the smallest cap that would do the job was used, in many cases)...

Just to add to Gordon's post, increasing cap values randomly is probably *not* a good idea. In certain cases, changing the values can give positive benifit to the sound, but in other cases it can make it worse. Be sure what the specific capacitor does so you can determine if changing the value will give you the results you want. Modern replacements will often have slightly different values, but as long as you're within 20% or so of the original, it won't really affect the sound. Most of the original capacitors had a marked tolerence of 10-20% when new, and they did drift over time.

Bunty,

why not increase the first cap after the rectifier? Is this amp specific, or is it just not necessary? Please elaborate as I have a different amp I'm circuit tracing right now.

Thanks

Jason

Gordon was indeed correct. The theory behind this maximum value has to do with the amount of current that will be drawn from the rectifier. Because of the way power supply filters work, the output of a filter tends to be a smooth, somewhat steady supply of current based upon the amps needs at the time. The input however is very different. Because the diodes produce pulsating DC, that is the voltage they produce drops to zero and then rises again to a max value, part of the time they are not feeding any current into the filter and the current supplied to the amp is drawn from the capacitors in the filter section. It is only when the voltage of that rising DC pulse exceeds the voltage on the first cap that current flows from the rectifier to the cap to recharge it. It has only a very short time to do this before the voltage again drops below the voltage on the cap and cutting off the flow of current. During that time the current draw from the rectifier tube is much higher then the amps draw. The maximum value of the first cap is set to keep that current rush down to a value that the tube can handle.

The NEETS pages can explain it much better with pictures and everything.

Hope this helps your understanding of the issue.

Shelly_D