Tube Amps 101

[James_87_87]

I just scored a blackface fender bassman from a friends older brother. Overall the amp appears to be in pretty good condition and when I plugged it in it worked pretty well. However, the brother told me he hasn't used the amp much in the last 5 years or so and it's been a little longer than that since it has seen a tech and gotten some new tubes. While I have done some electrical work with guitar pedals and a few other oddities I'm pretty green when it comes to working on guitar amps. Hence I did a bit of reading, dug up some schematics and other info on the net and now was hoping you guys could help me put it all to good working order in my attempt to fix up this guy and turn it into a really steller amp. I have some pictures I took of the actual chasis which you can find below:

I got the schematics and layout from the Fender Field Guide: http://www.ampwares.com/ffg/bassman_bf.html
I think it is the AB165 circut but I'm not 100%.

Anyway, I was hoping to turn this thread into kinda a beginner's guide of what's what with a vintage tube amp and what type of repairs, maintaince and even mods can be done to improve the quality of the amp. My equipment is pretty basic, besides the assortment of screwdrivers and general tools all I have is a weller soldering iron and a DMM (which is capable of measuring ac/dc voltage, measuring ac/dc current, measuring resistance, checking continuity, checking diodes and checking batteries).

I want to be able to make sense of the schematic, in terms of identifying all the componets in the amp (possibly with the help of my DMM), replace any components that need to be replaced, and install new tubes and rebias the amp. I appologize for the length and vagueness of the post, I did a few searches and am having a hard time turning up anything concrete that will help me connect my theorectical knowledge with some practical experience. I wish I had a friend who could show me the ropes in person but usually people are coming to me to fix things so I turn to all of you for any assistance and guidance.
Thanks,

James

 

[CJ]

Right here, bro.

http://www.diyguitarist.com/Misc/J_Darr.htm

And welcome to the Lab!

cj

 

[James_87_87]

Thanks for the guide, I will surely take a look at it. To start with my repairs though I have a few questions which might get me started, seeing as I would like to order the necessary parts and get moving on this ASAP. Anyway, in the pics I posted in the chasis picture (4th from top) I know the component on the far left (the shiny light blue one) is an electrolyte cap but what are those 3 brownish red things (caps I presume but what type) and then what is the two black logs with red and yellow ends. They have two leads coming out of one end and 1 lead coming out of the other. They have + and - so I'm assuming they are polarized caps but I don't know for sure. Also how do I tell what type of resistor (or for that matter cap) I'm looking at. For instance, I can read the values (based on the stripes) or I'm assuming I could even use the resistance measuring fuction on my DMM (by connecting the red probe to one end and the black to other) to determine their value in ohms right?!? Is there anyway I can measure the voltage rating of a paticular component because I know I should use 1/2 or 1 watt resistors in many places but must use 5 watt components in some of the power sections. One last question, on the jack that you plug the main speaker cable into, 2 of the lugs are jumpered together, any reason for this, is this original or a mod and should I fix it? Thanks again for the help, any more ideas, guides, or assistance would be much appreciated.

 

[Butterylicious]

All the redish, blue, and black thangs are different types of caps. The redish one can be replaced with Vishay orange drops. If you are not certain of the voltage potential, just stick to 450V devices and you'll be OK. (This is probably a good time to remind you that tube circuits operate at lethal voltages so please be careful.)

Those black caps are 2 section electrolytics and will probably have to be replaced with 2 individual caps. The blue caps can probably be replaced with orange drops too.

Picture 6 shows the power supply electrolytics. Somebody changed those and they look like they're 350V devices. That's potentially really bad news. Those should be 525V devices according to the schematic. B+ is 425VDC so have a look-see at that. If nothing else, please put that cover back on B4 energizing the circuit.

The vast majority of those resistors are 1/2 watt unless otherwise noted. There's a couple of 1 watters by the output tubes and a couple o 4 more in the power supply. I wouldn't mess with the resistors.

That jumper on the output jack is important. Leave it be. Tube amps are much happier with a short on the output vs. being completely open. Full power with no load can mean the end of your output transformer, so leave it be.

I'm nothing more than a hack but I've been inside many a Fender amp. Read through CJ's link and again be careful and use good judgement.

-Richard

 

[James_87_87]

[quote author="Butterylicious"]All the redish, blue, and black thangs are different types of caps. The redish one can be replaced with Vishay orange drops. If you are not certain of the voltage potential, just stick to 450V devices and you'll be OK. (This is probably a good time to remind you that tube circuits operate at lethal voltages so please be careful.)[/quote]

Vishay Orange Drops? Excuse my ignorance but are those a brand or type of cap? Also, is there a way using my DMM to measure the voltage of existing components in the amp. The more I look through it the more it seems like it is a mish-mash or original and replacement components and I want to determine if everything is the original values. Subsequently, can I use the resistance measurement setting on my DMM to determine the ohms of the resistors and is there a way to find out the value of the caps with my DMM?

[quote author="Butterylicious"]Those black caps are 2 section electrolytics and will probably have to be replaced with 2 individual caps. The blue caps can probably be replaced with orange drops too.[/quote]

Yeah, those black caps, what are they (I know electrolytics but I have never seen them before), I assume an obsolete type of cap? When replacing them, what should I use, 2 regular electrolytics and should their values be half of what the current cap is and then how should I wire them up? As for the blue caps, aren't they electrolytics, are orange drops electrolytics?

[quote author="Butterylicious"]Picture 6 shows the power supply electrolytics. Somebody changed those and they look like they're 350V devices. That's potentially really bad news. Those should be 525V devices according to the schematic. B+ is 425VDC so have a look-see at that. If nothing else, please put that cover back on B4 energizing the circuit.[/quote]

Admittly I don't really understand what your saying, in fact I am having a really difficult time comprehending the layout of the filter caps and can't find it on the schematic (which is kinda difficult to read anyway, if anyone knows where I can find one that is a bit more lediable I would appreciate it). I'm referring to the 2nd pic from the bottom. All those caps are the filter caps correct? The one furthers to the left has it's + side pointing up in the pic and the rest have theirs point to the bottom (I think). Since it has been almost atleast 6yrs since the amp has seen a tech and I don't know if the caps were even changed then, I want to swap them out but don't know exactly what their values are and how to configure them, you mention there is currently a problem with them.

As an aside don't worry I'm being safe, I used a 2watt 300ohm resistor and grouded it on one end and then attached it to the + lead of each filter cap. I then (with the amp unplugged) flipped it on and turned on the standby and then shorted pin 1 of one of the preamp tubes to the chasis. Just to be safe I then used an insulated screwdriver and touched both leads of every cap in the unit and lastly measured the DC volatage to make sure it is under 10. For some reason, the middle filter caps recharged a couple of times but I check them periodically.


[quote author="Butterylicious"]The vast majority of those resistors are 1/2 watt unless otherwise noted. There's a couple of 1 watters by the output tubes and a couple o 4 more in the power supply. I wouldn't mess with the resistors.[/quote]

I know what a resistor is and a cap and what they do but I don't have the experience of working with them in the real world so identitifing components and knowing the difference between a metal film resistor and a carbon film resistor is a challenge. How can I tell what type of resistor is in my amp, are there any physical features that distinguish carbon comp from carbon film, carbon film from metal film etc... And the reason I wanted to replace them is I was told I could get a cleaner sound with less noise if I replaced any carbon comps or carbon films with metal films, thus improving the sound of the amp.

[quote author="Butterylicious"]That jumper on the output jack is important. Leave it be. Tube amps are much happier with a short on the output vs. being completely open. Full power with no load can mean the end of your output transformer, so leave it be.

I'm nothing more than a hack but I've been inside many a Fender amp. Read through CJ's link and again be careful and use good judgement.

-Richard[/quote]

Thank you very much for the help. In the 3rd pic from top there is a smaller circut with a cap, 2 1 resistor and a bunch of diodes. The leads on some of the diodes look to be a bit corroded, should I replace them and if so what do you recommend, any improvements or better components I could get. Also does the amp look to be properly grounded, I didn't get any shocks when I played through it so I'm assuming it's ok but I don't want to take it to a gig and get a nice suprise. Also I read somewhere I could remove the resistors on each of the inputs because they were installed to allow for 2 things to be plugged into the same channel and I will only be using this amp for 1 instrument at a time and the resistors have a negative inpact on the frequency response or should I just leave them? I can't thank everyone enough, I'm going to turn this into a picture guide on the web like the one's they have over at tonepad.com so fellow newbies like myself can see how to work on their own amps. Thanks again and keep the suggestions coming.

 

[James_87_87]

I have been reading through the guide CJ posted and I have a few more questions. I know the bassman has a solid state rectifier, where is that, how is that implimented?

Upon closer examination of the PS and grounding scheme, it looks ok, but there appears to be a cap missing which I think is good but I would prefer some support as I especially don't want to wing this.

One last thing, I read somewhere else on the net about a mod one can do to the bass channel to make it more conducive to guitar, but I can't find where it's described. I'm sure it has something to do with swapping out some of the caps in that channel with different values to change the frequency response but I'm not sure, if anyone can point me in the right direction would be much appreciated.

 

[pstamler]

[quote author="James_87_87"][quote author="Butterylicious"]All the redish, blue, and black thangs are different types of caps. The redish one can be replaced with Vishay orange drops. If you are not certain of the voltage potential, just stick to 450V devices and you'll be OK. (This is probably a good time to remind you that tube circuits operate at lethal voltages so please be careful.)[/quote]

Vishay Orange Drops? Excuse my ignorance but are those a brand or type of cap?[/quote]

Yes. They were formerly made by Sprague.

Also, is there a way using my DMM to measure the voltage of existing components in the amp.

No. One way to figure out what voltage a cap is rated at is to take it out of circuit and put increasing voltage across it while measuring the voltage. When it explodes, you know you exceeded the rated voltage.

A better way is to read the package; almost all electrolytics have the "working voltage" rating printed on the outside.

The more I look through it the more it seems like it is a mish-mash or original and replacement components and I want to determine if everything is the original values. Subsequently, can I use the resistance measurement setting on my DMM to determine the ohms of the resistors

Yes, if you desolder one end. Testing them in-circuit can lead to misleading results.

and is there a way to find out the value of the caps with my DMM?

Probably not; most DMMs don't have a capacitance-measuring function. A few do, though; check your DMM's manual. Or look for markings on the caps.

[quote author="Butterylicious"]Those black caps are 2 section electrolytics and will probably have to be replaced with 2 individual caps. The blue caps can probably be replaced with orange drops too.[/quote]

Yeah, those black caps, what are they (I know electrolytics but I have never seen them before), I assume an obsolete type of cap? When replacing them, what should I use, 2 regular electrolytics and should their values be half of what the current cap is and then how should I wire them up?

Typically the two wires come from the + end of the cap and go to different places in the amp; the single wire comes from the - end and goes to ground. It will be marked something like "30-30 mfd", meaning two caps of 30uF each. So find two 30uF caps of sufficient voltage rating and connect their positive leads to where the positive leads of the old caps were, and connect both their negative leads to where the negative lead of the old cap was.

As for the blue caps, aren't they electrolytics, are orange drops electrolytics?

Orange drops are usually Mylar (polyester), except for a few that are polypropylene.

[quote author="Butterylicious"]The vast majority of those resistors are 1/2 watt unless otherwise noted. There's a couple of 1 watters by the output tubes and a couple o 4 more in the power supply. I wouldn't mess with the resistors.[/quote]

I know what a resistor is and a cap and what they do but I don't have the experience of working with them in the real world so identitifing components and knowing the difference between a metal film resistor and a carbon film resistor is a challenge. How can I tell what type of resistor is in my amp, are there any physical features that distinguish carbon comp from carbon film, carbon film from metal film etc...

If it's in a blackfront Fender amp, it's carbon comp.

And the reason I wanted to replace them is I was told I could get a cleaner sound with less noise if I replaced any carbon comps or carbon films with metal films, thus improving the sound of the amp.

Well, the sound will indeed be clearer and quieter. Be advised, though, that not everyone prefers that on a musical instrument amp. I do, and have replaced the carbon comps on a blackfront Deluxe (non-reverb) with metal films, thereby causing heart-attacks to all the vintage-amp collectors, but I'm weird. You'll have to decide whether you want to do that. Oh, probably the collector's value of the amp is already shot by the capacitor replacement, but it'll go down to nothing if you put in metal films. On the other hand, my Deluxe sure as hell sounds nice to me.

Thank you very much for the help. In the 3rd pic from top there is a smaller circut with a cap, 2 1 resistor and a bunch of diodes. The leads on some of the diodes look to be a bit corroded, should I replace them and if so what do you recommend, any improvements or better components I could get.

I'd leave those alone; the corrosion on the diode lead probably isn't hurting anything. In fact, aside from caps, I'd leave things be unless they're busted.

Also does the amp look to be properly grounded, I didn't get any shocks when I played through it so I'm assuming it's ok but I don't want to take it to a gig and get a nice suprise.

If the amp has a non-grounding 2-prong plug, take it to a tech and have a grounding 3-prong plug installed before you do anything else. This isn't something to do yourself at this stage. Really. This could kill you DEAD.

Also I read somewhere I could remove the resistors on each of the inputs because they were installed to allow for 2 things to be plugged into the same channel and I will only be using this amp for 1 instrument at a time and the resistors have a negative inpact on the frequency response or should I just leave them? I can't thank everyone enough, I'm going to turn this into a picture guide on the web like the one's they have over at tonepad.com so fellow newbies like myself can see how to work on their own amps. Thanks again and keep the suggestions coming.

Leave 'em in; they also help keep radio stations from appearing on your amp.

Peace,
Paul

 

[peterc]

Picture 6 shows the power supply electrolytics. Somebody changed those and they look like they're 350V devices. That's potentially really bad news.

I've worked on Fenders before that have had 2 x 350v electrolytics in series in the power supply.

If you look at the schems
http://www.ampwares.com/ffg/schem/bassman_ab165_schem.gif
http://www.ampwares.com/ffg/schem/bassman_aa864_schem.gif

you'll see in the bottom RH corner the 2 100uF caps with 220k resistors in parallel which are visible in photo 6.

Regards
Peter

 

[Butterylicious]

Vishay is a brand and Orange Drop is a style/name. They are available at http://www.mouser.com I would suggest going ahead and buying all the different uF values (ie .01uF etc) at 450V. The existing caps should be marked.

Those black caps are effectively 2 caps in 1 package with the negs tied together. Replace them with the same value.

Those are the filter caps shown in the 2nd to last picture. I guarantee you they are not the original caps. I would double check these just because.

I wouldn't mess with the resistors. I always use carbon comp resistors in tube circuits. As far as making it better than it originally sounded...... I don't know. Seems to me Fender has been manipulating that circuit for years and the older ones are still more popular. This paragraph is purely my humble opinion. As I noted before, I am just a hack and there are plenty of people here who are N to the x power smarter and well versed than I.

The 3rd picture is a bit blurry. I'd bet the corrosion is probably just flux. Just clean it for now. If the amp fires up and works, then those diodes are OK. Those diodes are your solid state rectifier. Don't mess with the input jack resistor, unless you want to listen to AM radio while you play your bass.

As for grounding, the power cord is grounded unless some fool has removed the ground pin from the cord cap. If this is the case fix it first! Grounding is the single most important aspect of electrical work period.

As for tone control mods, it's merely a case of messing around with the R-C tone circuits. The popular Fender mod is commonly called "Boogieing". This is where you add a switch and a coupling cap so that you can break off the output of one channel and push it through the other creating a kick butt distortion. I might be wrong but I think this mod is shown for a Twin at the Duncan amp site.

As noted, the collector value of this amp has already gone down since it's had the filter caps changed. Collector value vs a working amp, I'd choose a working amp. The vast majority of the time, all these amps need in the TLC dept. is fresh tubes and capacitors, unless it smoked.

 

[louder]

alo james
to put it short:
replace all electros with sprague atoms.
replace those two 100k resistors in v shape near first preamp tubes,because they all make noise after some time,and drift in value.
replace the two 470 ohm`s resistors in the output tube socket`s,and while you`re there do the same to the two 1500ohm`s resistors in those socket`s.
hard part:the bias circuit has been changed.there`s lot`s of resistors around the bias pot.ithink,from memory that only the 15k shold go from a outer leg to the case of that pot.
another sorce of problems are those 68k resistors in the input jack`s,that with use tend to break;replace them.
the blue cap`s in the board are malory`s.unless they opened,keep them.
the other redish square tipe,replace with orange drop(600v).
the small ones,are ceramic,and unless they opened,keep them,or replace with the same tipe.don`t need to go with silver mica.
remember one thing:HIGH VOLTAGE IN THERE.BE CAREFULL,VERY VERY CAREFULL.
tell us how thing`s are going.
best regards
pedro

 

[Gus]

Xicon and IC brand caps, YUK! IMO some of the worst caps for the power supply of a guitar tube amp. I have measured and tried different caps. There is science to Al electros that matches to the "sound".

Atoms often work very well in fender amps.

With Al electros you often get what you pay for.

Go to geofex.com and read about the sound of CC resistors and there are other good tube amp writeups.

 

[James_87_87]

[quote author="Gus"]Xicon and IC brand caps, YUK! IMO some of the worst caps for the power supply of a guitar tube amp. I have measured and tried different caps. There is science to Al electros that matches to the "sound".

Atoms often work very well in fender amps.

With Al electros you often get what you pay for.

Go to geofex.com and read about the sound of CC resistors and there are other good tube amp writeups.[/quote]

Gus, do I have Xicon and IC brand caps in my amp or did someone suggest them and I missed it, I'm confused?

Got a couple more questions. I have been reading non-stop on all of this and am feeling very overwhelmed. Anyway, 2 terms I have come across which I don't understand are grid resistors and decoupling, specifically when referring to decoupling caps.

Also I'm still a bit confused about some of the components. First of all I know I need to replace the big brownish/red caps with orange drops but what are those big ugly brown things called (what type of cap) so I know what to call them next time I see them. One thing about replacing them though, the amp does work, it's a bit noisy and dull sounding but I figured that could be fixed with new tubes and some new filter caps, maybe swapping out the rest of the electrolytes but do I really need to replace all of these too, will it make my amp sound better. I'm not concerned about collector's value, I do want to have some resale value on the amp but I plan on using this guy not putting it in a musuem so I just want it to sound it's best.

Also I attached a picture below with color circles and numbers. Cap #1, the shiny blue one with the red circle around it, it appears to be a replacement cap because it looks quite new, isn't that an electrolyte and subsequently shouldn't I replace that with another electrolyte not an orange drop. For Cap #'s 2-4 (the one's circled in green) are those the blue mallory's I should leave alone.

According to the schematic, it does look like it lists the 350v caps but I know sometimes what fender did and what they should have done are 2 different stories, what should I replace the filter caps with?

2 last questions, as far as safety, once I have discharged the filter caps and the remaining electrolytes (ie: using a resistor that is grounded to chasis and touching each cap and then using a screwdriver to touch pin 1 of preamp tube to chasis and then going back and touching every cap with a screwdriver) shouldn't the amp be pretty much safe to work on, assuming I don't plug it in and turn it on again? or is their something else I have to do or something else I have to worry about?

Last thing, to clean the pots and some of the dirt and corrosion around the amp what should I use caig deoxit or something else? Once again I can't tell you all how much I appreciate the assistance.

James

PS. sorry just remember one last thing, since the old 2 prong cable has been removed and a new 3 prong cable has been properly installed (from what I can tell) what exactly does the ground switch do now anything.

P.S.S Also, I'm going to order a few more fuses for backups, the schem says 2 amp slo-blo anything else I need to know or is that all I have to look for?

 

[louder]

alo james
your number 1,it`s electro:replace.
your 2,3,4,are malory,keep them if good.
those black ones are electro and are 2 in one(2*25mfd 25v):replace those.if you don`t find in double mode,stick two electros of 25mfd 25v,each positive leg to the right place.
something new:your amp it`s model ab165,and the bias it`s diferent of the others.go to the schematic,and see if everithing it`s in place.
the big electros outside the amp,must be replaced with new ones(sprage atoms).replace the resistors in there too.
replace the 100k resistorsin the preamp as they make scrathy noises with time.
carbon film resistors tend to be quieter than those carbon comp you have in there.
best regards
pedro

 

[James_87_87]

Thanks to everyone for their assistance, I think I have a pretty good handle on what needs to be done but still have a few questions from my last post lingering which would be a hugh help if someone could address. Basically one of my biggest problems since I'm so new to all of this is I know what a carbon film resistor is and what a metal film resistor is, I know the differences between the 2 but I have no idea what each actually looks like. Same goes for caps, I can usually spot an electrolyte because of the polarity and I have seen a bunch of different ceramic disks so I feel pretty confident identifying those but all to often I look at a schematic and I realize I need x component of a certain value but if a voltage isn't specified I don't know which one to select and then I never know if I need a ceramic disk, orange drop, wire wound etc... Any online sites that clearly explain these or books I could find at the library or purchase?

Also once I get everything working I'm going to retube the amp. Not sure what I'm going to go with yet but when I have to rebias the amp usually I have done it by having a friend play thru the amp and then I use a screwdriver and adjust the bias pot so the tubes are just before they start to glow, is that an acceptable method or do I really need to consider one of the more advanced/involved approaches? Thanks again I have already learned so much from this board and all the helpful people here.

 

[louder]

alo james
the resistors in your amp,are carbon comp,the most part being 1/2 watt.
metal films are smaller,and usualy have a blue body.see them in our diy.
in guitar amps,i use carbon film 1watt,unless otherwise noted.this is the most comon resistor you find at the stores.the brand i use,it is of a light brown body color.
if you look at your power tube sockets,the 470 ohm resistors,are bigger than the others;these are 1 watt,and are in all fender amps with 6l6 tubes.
note that modern resistors are smaller than the ones you have in there,so if you ask for 1 watt resistor don`t be scared of the smaller size.
as for caps:in the schematic sometimes you`ll see a electro cap with 25/50:means:25 mfd/50volts.
for the other kind of caps,in tube amps,400v and up,it`s safe.
orange drops are orange in color,and rated 600v.good to use in fenders.
if you are in the usa,mouser sells them;use the model 715,polypropilene.
for ceramics,use disc ceramics 500v.
for biasing the amps,i use a bias king,that i got at tube amp doctor(germany).they have almost everything for amps.
try to get aspen pittman book of groove tubes.it`s a good start.
best regards
pedro

 

[louder]

hi james
try this:
metroamp.com
look at the forum.
best regards
pedro

 

[louder]

alo james
just a note:
in a schematic of this kind of amp,when you see a cap in pf range,usualy it`s a ceramic disk,or silver mica.
best regards
pedro

 

[CJ]

Hey James.


1) cold bias (more negative volts) -which equals more power, better for death metal crunch, longer tube life, but more zero crossing artifacts and distortion

2) medium bias- which equals a good all arounfd bias point, jack of all trades, master of none, medium tube life, probably the way the unit was shipped

3) hot bias-less negative volts (what you will get with your method described on page 1) , shorter tube life, not many zero crossing artifacts, more mellow "brown sound", shorter tube life than either of the above.