Fender BF filter caps

Karl Oberlander · From: Austin, Texas, USA

What is the proper orientation of the 5 filter caps found in many of the Black Faced Fender amps? I have a Pro that has three 20mF x 550VDC caps that are oriented the same as to positive and negative. Then there are two 80mF x 450VDC caps one of which is oriented the same as the three 20mF x 550VDC's and then the one on the end, the other 80mF x 450VDC cap is turned 180 form the other four. In my Super Reverb the two 80mF caps are oriented both the same direction which is opposite the Pro. Both circuits seem very similar most ways.

Also, the schematic for the Pro shows that all 5 should be 20mF x 550VDC. What does upping the value of the two attached to the standby switch to 80mF from 20mF accomplish?

I'd like to get this amp sounding a bit better if I can. So I await the Fender guru's answers. Thanks in advance for any information guys!

Kobe

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Gibson D-8 Console Grande - Stringmaster T-8 - Alkire EHarp D-10
Fender Super Reverb
kobe@austin.rr.com
http://home.austin.rr.com/kobeco

Ken Fox · From: Nashville GA USA

The two 80's that are oriented 180 degrees are in series (total of 40uf). The two twenties that are in in the same direction are in parallel (total of 40uf).

I just looked at the Blackface Pro, Blackface Pro Reverb (non reverb) and Super Reverb. All show 70uf in series.

Five 20uf caps in the same direction is typical of a Deluxe Reverb amp. Five 16uf in the same direction is typical of a Deluxe (non-reverb).

Not sure what is going on in your amp there!

The Brownface Pro (6G5A) has 7 at 20uf. The other Brown era Pro (6G5) has 5 at 20uf.

[This message was edited by Ken Fox on 28 February 2004 at 04:07 PM.]

[This message was edited by Ken Fox on 28 February 2004 at 04:11 PM.]

Karl Oberlander · From: Austin, Texas, USA

Ken,

Thanks that is what I was thinking might be the case. So it looks like the orientation is not a problem if the values are such that they add up to the same total. The Pro AA-763 schematic I have shows the two 20uF's in parallel so the two 80's in series would work out correctly. On my Super's the schematic does show a pair of 70uF's and they are shown in series. I wonder what the value increase does for the tone if anything?

KObe

------------------
Gibson D-8 Console Grande - Stringmaster T-8 - Alkire EHarp D-10
Fender Super Reverb
kobe@austin.rr.com
http://home.austin.rr.com/kobeco

C Dixon · From: Duluth, GA USA

These are what we call in electronics, B+ filter capacitors. And that means they have little if anything to do with the tone of an amp.

Their most common failure; causes hum when they lose capacity or open up. In fact that is their purpose, IE, to filter out hum in the power supply busses. Also to eliminate low level "crosstalk" between circuits sharing a common power (B+) bus.

They are electrolytic capacitors, and rarely if ever short, because of their inherrent design. Outside hum you may, I repeat MAY, notice some power loss, but most would not notice it; unless you were running wide open; and compared it to an amp where the filters were ok.

Tone in an amp is almost exclusive to the preamp ciruits, output transformers and speakers.

carl

Bill Hatcher · From: Atlanta Ga. USA

Karl. If you want to increase the bass response in the amp, bump up the value of the 80mfd caps to 100mfd. There are several websites on the net that sell these and can give you an explanation of why this occurs with a little higher value filtering. I use these on all my Fenders that I have rebuilt and the low end is increased and feels a little tighter, not so much mushyness when you really hit into the amp with some serious level.

Karl Oberlander · From: Austin, Texas, USA

Thanks, guys. I really appreciate all the information.

Kobe

------------------
Gibson D-8 Console Grande - Stringmaster T-8 - Alkire EHarp D-10
Fender Super Reverb
kobe@austin.rr.com
http://home.austin.rr.com/kobeco

C Dixon · From: Duluth, GA USA

Dear friend Bill,

I have heard similar claims on this forum. With much sincere respect, I doubt these claims could ever pass a triple blind test.

I will concede that if one played an electric bass thru the amp and played the lowest notes on that bass; with the amp wide open; there might be a very small increase in the level; IF one changed the 80mfd filter caps to 100mfd, but even here, I doubt seriously again if it is anything more than subjectivity outside of scientific testing.

Also, the additional "surge" of current thru the power rectifiers when the amp is intitially turned on would negate by far any perceived improvement in low low bass response. Remember the higher the value of a power supply filter the higher their initial current when they are first being charged; and this current flows directly thru the rectifiers.

This is why powers supply designers (since the advent of solid state devices in tube amps) have had a real problem with filter caps. The lower their value, the lower current they exude during initial charge, and vice versa. The higher their value, the closer one comes to pure DC on the B+ lines, which means the ideal no hum and no crosstalk.

I feel the same way about "blanket" changeout of all the caps as a routine service procedure. Having taught amplifier and power supply theory; and serviced amps for many years, I have NEVER encountered a situation where all the filter caps needed to be changed out at one time.

If there is NO sign of hum or minor "crosstalk", there is little if ANY gain in this practice. Not to mention the unnecessary cost to the customer or "do-it-yourself" mechanic.

But as always, if it is true, I respectfully stand corrected,

Take care friend,

carl

Bill Hatcher · From: Atlanta Ga. USA

Mr. Carl,

Hope you had a good Sunday service.

First of all, the imeasurable amount of experience and knowledge you bring to discussions on this forum are well documented and appreciated. Everything you said about the filter cap world is correct, but may I give you some things to consider and you let us know what you think.

The general environment in which Fender amps live today is totally different than the 50s and early 60s when many of these were designed and sold. I remember watching Buck Owens live on TV in the early 60s. Buck and Don Rich played through little Delux Reverbs, Brumley had a Fender Bandmaster, and the bassist had a Bassman Amp. All these amps totaled about 160 watts RMS and things seemed to be just fine volume wise. Today you probably have a steel amp in your living room that has more power than the whole Buck Owens band had!!

These days the majority of players use the Fender amps pretty much cranked for all kinds of distorto sounds whether sounding not too different than a chainsaw or just plenty loud for a little "edge" and saturation right at that point where you start to feel the natural compression that comes about when you drive the tube into clipping. Keep in mind the demands that this style of playing puts on a 40+ year old amplifier. It is a true testament to the design genius of Leo Fender that these amps are still with us both in their original form and countless variations. I am certain when Leo designed these in the 40s and 50s and early 60s that he never envisioned his amp being turned on "stun" and a Les Paul guitar blasting through it. The sound pressure levels today are monstrously louder compared to then.

So in regards to the Fender amp, the added value in the first stage of the filter section does indeed serve a function of "tightening" up the low end at pretty high volume levels. You are very correct in that at a lower more "sane" volume the player probably would not perceive the change in low end. Today's players have just as much "persnickityness" when it comes to distortion in amps as do the opposite group of audiophiles have the same attitude towards the cleanliness in an amp sound. Some of these guys can really hear the difference in capacitor values and mod the amps accordingly.

I used to do a lot of Fender repair. I found that the best repair for the most reliable amp service under the most grueling conditions was to go ahead and replace ALL the filter caps if they were the original units. You are talking caps that are 40 years old! When is the last time you watched a 40 year old TV or listened to a 40 year old radio. The Fender schematics say that the values of the parts can be within 10% or so. Well yes the amp will still play with the parts out of wack 30% but it will not sound anything like the amp did when new and all the resistors and caps within tolerance. This brings up a whole discussion as to what we really think is the "vintage" Fender sound. Is it the sound of the amp with 40 year old parts which surely cannot sound anything at all like it did when it was new, or is it the sound of the amp with new parts functioning and sounding as it did when it was just made. I have had those caps blow out on me while working on amps and had amps that would run for about an hour and then pop fuses because of old filter caps that appeared to function fine. Also the strange low sub tones that accompany old filter caps are reason enough to get rid of them.
One thing for sure is that the quality of today's caps and resistors is vastly better than the old stuff that came in the amp. It is always a treat for me to take an old ragged out Fender amp and "blueprint" it like the mechanics would do to a car engine. Take the schematic and put everything to EXACTLY the specs that Fender designed the amp to run at. They sound so good when they are running at specs!!

Hope this provides you some sonic food for thought.

C Dixon · From: Duluth, GA USA

Thank you Bill,

You do indeed give much "food for thought"

May Jesus bless you always,

carl

Ken Fox · From: Nashville GA USA

I must say Carl and Bill that I enjoy both of your inputs on the Forum so very much! I repair and restore a lot of Fender amps and really look forward to your comments on the Forum.
I can see both of your opinions on the filter caps. I think it is a preventative maintenance item to replace really old caps. I have probably replaced hundreds in the last few years. I am sure some had a lot of life left, but I was selling amps that I did not want to see back in my shop for a long time!
I fired up a 1954 Tweed Deluxe that had sat for 30 years, using a Variac. After a slow start up it was fine and now the amp is running on original caps. I fired up a 1966 Deluxe that had sat a long time, without a Variac, and it promptly blew out every filter cap!
I recently replaced some in a Peavey Session 400 and was surprised at my findings and results. The amp displayed a light buzzing as a note that was played died away. I looked at the waveform and saw spikes in the saw tooth, from the DC rails, that were better than 25% of the waveform height. According to Peavey techs, that is a sign to replace them. I did so and the spikes were gone. The buzzing was also gone. The amp did not have any hum or signs of cap leakage. From what I have gathered the older caps become a high resistance conductor to DC (leakage).

If you guys could shed any light the above, I would appreciate the education.

[This message was edited by Ken Fox on 29 February 2004 at 06:25 PM.]

C Dixon · From: Duluth, GA USA

Ken,

This is explained by where you saw the spikes. Follow along. The filter caps closest to the rectifiers will exhibit loud hum and noise IF open or partially capacitive. As you get further down the line, you will find they do not cause audible hum when there is NO signal since the ones closest to the rectifiers are primarily responsible for getting rid of hum.

What you experienced is what I mean when I say crosstalk of a sort. NOT crosstalk like on a telephone line. Rather signals that use the B+ buss for transmission which should never be.

In other words, as long as there was no signal, hum would be very low or inaudible, but any signal would be fed "around the Loop" from the stage that did not have adaquate filtering when there was signal.

Then even tiny (inaudible) amounts of ripple would ride around on top of the signal and be REamplified by being fed back around by the buss from the latter stage. This then is what you heard ONLY when notes were played.

You were correct in scoping the B+ buss (rail as you say). For a scope is the quickest way to isolate which filter is bad when B+ anamolies occur. This provided you KNOW in advance what is; and what is not normal.

When I used to teach electronics, I had the techs put a scope every where; and then record what it should look like at each place. In this way, they had a "norm" to compare the next time.

A scope in the hands of the best tech in the world is worthless, IF the tech does NOT know what is normal. And all manufactures are almost worthless in providing enough (if any) normal scope shots.

By the same token, a scope in the hands of a sharp tech who KNOWS what a waveform SHOULD look like at any given place in an amplifier (or any other electronic device) can find problems other techs would search for, for days. And in some cases never find.

This is one of the most common scenarios I discovered in training techs all over the country for many years.

I will sight you just one of thousands of example of this. I had a tech once call me from another city 200 miles north in Fla; and he was desparate. Bless his heart he was at his wits end on a brand new color TV set. The set had NO color. NONE. I could tell how frustrated he was. He literally was trembling over the phone.

He had tried every thing imaginable. Nothing helped. He even changed the entire chroma board. Still NO color. I told him to bring it to me. He said he had almost a hundred hours on it. NO way was he ever going to make any money on this one!

He put it on the bench, I took the scope and in less than 20 seconds found the problem as he was in total amazement. He thought I walked on water from that day on.

I didn't. And I don't. I am not worthy to call Jesus' name. I just had something he didn't. I knew how to use that scope and I knew what one should find at every viable point in the set because I had taken the time long before to record all the norms.

So the first point I checked I did not have a very necessary pulse from the HV circuit. I traced it back and found a wire that was never soldered at the factory.

A hot soldering iron and color appeared for the first time. Now lest anyone think carl is searching for brownie points or acolades let me tell you now, I do not want them or need them. I am only trying to show what happens and why.

My blessings come from Jesus, praise his holy name.

carl

Bill Hatcher · From: Atlanta Ga. USA

Ken. Carl is the electronics wizard here. I would not open up a TV set if my life depended on it!! I only work on Fender amps from about 1975 and back and that is all!

Back when I was fixing a lot of old Fender amps, I decided to quit charging money to track down problems, only fix the problems and charge for the time spent tracking them down. What the customer ended up with was a repaired amp that was destined to come right back with another failure.

I sat down and figured out what the price would be to go through the entire amp and refurbish it and found that I could charge just about the same amount of money to the customer and hand him back a reliable amp that would not go out on him at the gig or at a recording session. I never had one come back.

First thing I did was to replace EVERY electrolytic in the amp. Bring up the voltage with a variac very slowly to "form" the caps. Then I replaced with precision resistors the resistor on the bias card and the 470 ohm grid resistors on the power amp tubes (flameproof here). Next I redo the phase inverter section with two new sprague orange drops and matching 220K resistors in the bias circuit and the 100k and 82K resistors going to the phase inverter tube and the two 1500 ohm resistors on the power tube sockets with identical matching resistors. The negative bias voltage must be identical on both sides. I never change out the caps in the tone control circuits unless one tests bad. Next thing is to change out all the 100k resistors in the preamp circuits (6 of them) with precision parts. You can do the 100ks in the vibrato section too if you like. This is where about 99% of the egg frying sounds come from when the amp has been running awhile and is hot. Next thing is to take out the two screws that hold the two pieces of fiber circuit card together and then separate the cards and spray them out real good with the cheapest grade of contact cleaner you can buy. Don't use your best stuff here as you are just cleaning out mold, oxidation, airborn junk, cig smoke residue etc. You will be amazed how much more crackling and such is repaired by this simple task. Clean the pots, check the input jacks and make sure the shorting jack works when the amp has no cord plugged in. MAKE SURE THE CORRECT FUSE VALUE IS IN THE AMP. I have seem plenty of 20 amp car fuses in Fender amps. Put in a good matched set of tubes. I just shunt the output over to pin 3 on the output tubes with my meter on DCMA and look for 30MA current draw through each 6l6. Then listen to the amp and see if tweaking the bias by ear gets you the tone you like. Check the filiment voltages and make sure they are balanced in order to minimize hum. Take a good look at all the little eyelets on the circuit card and just remelt some solder in the ones that look suspect. Also address the wires that run from the board to the 9 pin tube sockets. You can change a significant amount of hum in the amp just from bending these wires around a little. Don't forget to clean out the tube sockets and tighten up the sockets by bending the metal pieces with a sharp object. Inspect the tube sockets for any traces of arcing between pin 3 and the adjoining pins. If you see a burned spot, just dig out the carbon from the burned hole and paint on some enamel paint. This will stop the arcing.

IF you do this repair to Fenders, they will run like brand new and you can feel good about giving the customer a good reliable amp to gig with.

Your mileage will vary.

The only other problems you run into with these amps are transformer problems that sometimes are tricky to find. I just keep a few spares and alligator clip them in to see if a tformer is acting up.

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