fender blackface amp

[henry]

hi everybody.i am new to this forum,so hello.i am attemping to bring a 1965 blackface fender amp back to life. i have a couple of questions that you experienced techs can help me.i am going to replace all the paper caps with sprague atoms,can you tell me what value that the resistors that are mounted along side the first two are?they are kind of hidden in between,and also the two that run along the bottom of where all the caps are mounted.as u can tell schematics confuses me.also if i get a 2x12 cab should the speakers be 4ohms each?these capacitors are the ones mounted to the top of the chassis where the tubes go.there are five.thank u very much for your consideration.

[Chuck H]

First off... If you are not experienced with electronics you have no business at all inside a tube
amp. Even shut off and unplugged tube amps can harness LETHAL voltages unless you know how to properly discharge them. Now...

I have used ATOM caps for many years (decades). I don't plan to use them anymore because they have not performed acceptibly for me of late and there are better options available.

If you are having trouble grasping speaker impedance issues I really think you should take your amp to a qualified tech. Your amp has historic and cultural value (as well as revered tone) and should be cared for by someone who not only loves (like you) but also understands these amps.

The price a reputable shop will charge to bring your BF back into everyday service is mice nuts compared to the trouble a bunch of bad solder joints and uninformed component coices will cause. I think you should either find an amp tech that is familiar with these amps or commit to a long process of learning for yourself. It's your call at this point.

JM2C

Chuck

[henry]

chuck thank u for your concern regarding my safety.i am new to this forum but not new to electronics. i have good soldering skills and can read a schematic somewhat. i know the resistors in question look like 220k 1/2 watt w 10% tolerence,i just wanted some input and to know if i should use that or something else.i made sure all the caps were drained before istarted this project.i just need somebody to verify certain things.can u tell me what resistors i can use in that top cap series,i measured the ones in between the two 70/350 caps and they seem within specs.should i just leave them,or do they go bad ?thanks again for your concern,i also have replaced the two prong plug with a new three prong wire.thank you

[Gibsonman63]

I would leave them if they are in spec. I haven't had to replace them any any of my Fender amps. The 470 Ohm resistors on the power tube sockets are subjected to a lot more heat and are a more common source of crackling noises and drift. Save the old parts in case you ever sell the amp, sounds silly, but people are silly.

Here are a few more suggestions:

While you are in there change out the rest of the electrolytic capacitors. Most will be 25uF/25V or 10uF/25V. The byass supply capacitor should be at least 50V.

Clean the pots and input jacks. The input jacks have contacts that open when you plug in a guitar cable. Make sure they make a good connection when the cable is unplugged or the amp will hum like a guitar cord is plugged in without a guitar.

Retension the tube sockets.

Look at the board with a magnifying glass and a lot of light and reflow any suspect solder joints.

Check the fuse to make sure it is the correct size. A lot of people will throw anything in there, especially if it failed during a gig.

Change the power cord to a grounded cord. It won't hurt the value of the amp and will make it much safer.

If the amp does not currently power up and play, than build a light-bulb current limiter. It will cost less than $10 and easily pay for itself in time and fuses.

All of these things should already be covered in the forum if you search. If you have any doubts, research and ask questions.

[Chuck H]

Cool. Be sure to discharge "bleed" the power supply before working on the amp. Look into the new Kemet caps that Mouser is selling. They look pretty good but I think they're on a long lead time because they're a new product. If you can find a way to use radial caps (though I've never tried this under a Fender "doghouse") the choices are better. I like the Nichicon PW series caps and Mouser just started carrying some Panasonic products that look really good too. Atoms have been giving me some ripple ghosting lately.

The 220k 1/2watters under the cap are there to balance voltage across the series caps. Originally I believe they were carbon composition. CC resistors drift with heat and age and probably weren't ever rated for the voltage in that circuit. Use metal film resistors instead if you want to replace them. The other resistors in there are power resistors for seperating the amplifier stages and also drop some voltage. I haven't had any trouble with either wire wound or metal oxide types. Because they share space with the caps and create heat I would use over reated watt values for these to reduce heat under the cover. Aluminum caps hate heat.

To give you more information on speaker impedance or component values I'll need to know the amp model, Bassman, Twin Reverb, etc.

And +++ to Gibsonmans post. There's a lot of things that can be done to restore the reliability of the old amps. But once it's done you may not ever need to do it again.

Chuck

[henry]

gibsonman 63,thank you for all the useful info,i have already replaced the power cord,should i disconnect that so called death cap on the ground switch,or just leave it?i will check out all other issues and keep u posted on how things turn out.by the way my cousin found this in a house he was demolishing and gave it to me,nice cousin right......thanks again...........

[henry]

thanks chuck,its a bandmaster,from mid 1965,i think it has a 4ohm tranny,so i guess 4ohms speakers,or maybe two 8ohms wired the right way maybe? i think they used jenson back then,what about the eminence? thanks again.this is my winter project so i,m not in any hurry,i'm here to learn from you guys with the smarts and experience......thanks again..........

[Chuck H]

Two 8 ohm speakers in parallel (both + speaker terminals to the hot lead and both - speaker terminals to the black lead)

IIRC up until early 65 it was Jensen. Then it could have been Oxford or Jensen. By late 65 it was mostly Oxford speakers. But this info is from memory of other posts. I wasn't there (almost though)

The first power resistor (connected to the choke tranny) is 1k. wattage isn't specified but a 3 watt part is fine. The one in tandem with that is a 4.7k. Three watt is fine for that one too.

Oh, and the 220k resistors are spec'd as 1 watt.

DO check those screen grid resistors as suggested.

Also, since many of the parts you may be removing still "work" in the technical sense you will want to save them in a little bag in case you ever want to put all the original parts back in (for a museum piece I guess).

You should ideally bypass the "ground" switch with the new power supply AC hookup. It's not doing anything now anyway with the chassis grounded.

Chuck

[Gibsonman63]

I always clip it out. To me they no longer serve a useful purpose and they can fry under the right conditions. If you are really worried about it, put a varistor across the line voltage coming in.

The Bandmaster is a cool, underrated amp. Very Fender-ey, just a different flavor of Fender. I have the 2 x 12 cabinet with mine that has the tiltback legs. I found mine in a pawn shop holding up part of a shelf in the early 80's. $200 well spent. Life before the internet had it's advantages.

[henry]

thanks chuck,i installed the caps well there spragues i ordered them before i got your info,i could always change them again if the give me trouble,i also replaced the 50/50 bias cap.it seems the other two caps 25/25 have one wire on the neg side but 2 on the pos side,can install two seperate ones maybe to make up the difference,in other words use two lower value caps to make 25/25.also i disconnected the death cap and wire nutted the end,just in case anyone would ever like to reinstall the two wire power cord,i couldn't imagine why,but you never know.thank you and everyone for your help...........

[sgelectric]

Henry, The three-legged capacitor you described is a dual 25mf/25v capacitor.They share a common negative lead.Replace it with two 25mf single caps.As Gibsonman63 advised,it is generally a good idea to increase the voltage rating on these bypass caps to 50 volts as they will last longer.This will not affect the tone of your amp.Changing the mf values will. Hope this helps. Steven

[überfuzz]

About that ground switch. I've seen some amps with the from 2 to 3 prong upgrade mounting a fan. The ground switch or the hole is populated by the fan switch... I actually consider doing this my self. There are some really nice, silent, fans made for computers.

Lets say the caps are old and you don't replace them just because you heard they can be replaced. If so, you're gonna love that amp after this... resurection.

Good luck with that amp!

[henry]

first of all let me thank god and the internet and you experienced techsfor info i could never have attained in years of research,thanks guys.now for the more mundane questions.in replacing the bias cap w/50/50 should i have used a 100/100 and if so why? and last what are the differences between ceramic,metal etc resistors,are one type preferred over the other or do they all serve a different purpose? thanks again to all you fine gentlemen................

[greekie]

The bias cap should in my view be rated at 100V or more, as the bias circuit can put 60-70-80 volts across it!

[henry]

ok guys i replaced all the major caps and resistors,left on standby for about 20 minutes, hooked it up to the speaker in my blues jr first.turned it in,low and behold amp heaven,never heard my asat classic sound so pure.thanks for all the help u guys gave me.one questionn though,when i plug into the vibrato channel it just comes out regular,no effect with the speed and intensisty pots.do i need the foot switch for this to work?thanks again......