Fender 100 Watt PA Head w Reverb

[TerryD]

OK, I got the California Blonde.   I've never worked on a 100 watt amp before.  Plenty of work on a 50 watt Bassman I had.

This is not putting out the power it should.  I'd say about a third.  Also, when I play it for 15 minutes, it starts sputtering and distorting a lot.  High pitched metally distortion.  I know from before I have to fix that first.

My plan of attack is this.  First clean all the pots.  Then check all the tubes.  Hope that takes care of the distortion and power loss.  The power loss is on all four channels so probably only a few tubes it could be. Any input on that??

Question 1:  I drain the power usually by two alligator clips V1 pin 1 to ground for a couple of minutes and then 6l6 pin 3 to ground for a minute.  I'll probably do all four 6l6s to ground.  Is that to much?  I just don't know if the 100 watt calls for other precautions.  Trust me I'll be as careful around this as around my mother -in -law's house.

Question 2: In the middle of my first picture is a brown ceramic cap that is definitely browned/burned. Is that shot completely?

Question 3.  After I find the reason for the power loss, what parts up here do I need to change?  Do those blue caps need to go?  Everything looks and even the tubes feel kind of "oily"?  Whats that? 

Question 4.  The power caps are starting to bubble, little pin prick type bubble.  I'll definitely change those.  But I've been told before here to get things working first.  That's right, right?

Question 5.  What kind of diodes are those metal can things?  What kind and size?  The values are not on my schematic.  Are they always good?  I do like FREDS but have only dealt with IN4007 replacements.

Question6.  Is there a layout for this amp?  As I'm enough of a novice that they help me a lot.  I have the schematic.

I definitely plan to mod this amp, as I think I can have my cake and eat it too.  Channel 1 &2 could stay original and 3 & 4 could go twin or something?  I'll look up old posts.

As always,
Thanks a lot,
Terry

The last thing you guys helped me with, bought me a new set of Rogers (my main instrument) the Bugera  and this amp. I'm thinking I'm liking this more than the Vibrolux.  I sure like the feel of those four 6l6s under the hood.

[HotBluePlates]

This is not putting out the power it should.  I'd say about a third. 

Do you have any means of applying a sine wave to the input jacks?

Before you do anything, you might want to go through and measure every d.c. voltage at every tube pin (a.c. for heaters) and for every filter cap. Post those here.

[eleventeen]

"Question 1:  I drain the power usually by two alligator clips V1 pin 1 to ground for a couple of minutes and then 6l6 pin 3 to ground for a minute.  I'll probably do all four 6l6s to ground.  Is that to much?  I just don't know if the 100 watt calls for other precautions.  Trust me I'll be as careful around this as around my mother -in -law's house."


The good practice is to have a resistor in there. 10K-20K. IF YOU START at V1, you are OK. IF YOU START at the 6L6 you gonna see big spark.  If you are (a) waiting a tad before shorting out the V1-1 (plate) and then doing the 6L6-pin 3 you are probably OK---the "node:" resistors are your bleeder current limiter. One thing we like to do is, when shutting off the amp, turn off the "POWER" switch while keeping the STBY switch up, or on. That way, the tubes stay connected to B+ and as B+ fades, the tubes conduct as much of it away as they can while they are still hot.


You do not need to drain every 6L6. Any one of them bled off to ground will bleed off the filter caps. Their plates are connected via the primary of the output tranny. I guess, safest would be to do the two inner or two outer 6L6.



I am not seeing your "burned cap". Caps typically do not "burn". OTOH, when you have a 30-40 cent part just change it out and remove the irritation.

You sound like you have a logical plan of attack.

[TerryD]

Thanks, I'll get those posted.  No sine wave though
Got it on the power drain, thanks.

[eleventeen]

Bringing this up *not* as a safety item.


After you do your cap discharge voodoo....remove your alligator clips.......and the voltage that got down to 2 or 4 could rise back up to 10 or 20 volts. This isn't a shock hazard but you *should* be aware that this residual voltage can sometimes play funny games with attempts to measure things in the amp with the power just-then shut off. Something to be aware of should go suspect a part, shut down the amp, drain the ecaps, than run to measure the part you think is bad.

[HotBluePlates]

...  No sine wave though ...

Your original post stated there was lack of power. But unless you can apply a fixed, unchanging signal to the circuit, then you can't verify that impression. The schematic includes a.c. voltages all along the signal path given a 6mV input at the jack.

See, the resistors at the input jack alone are a severely-low impedance (heavy load) for a guitar's pickups and impose a lot of signal cut. The tone circuit is also a little bit lossy, compared to what it could be. So it sure would be nice to verify the amp is/isn't working like it should be rather than what you think it should be.

Question 3.  ...  Do those blue caps need to go?  ...

Not necessarily. No one loves them, but they're no necessarily bad, either.

Question 3.  ...  Everything looks and even the tubes feel kind of "oily"?  Whats that? ...

Ever heard of "tweed disease"? Sometimes Fender amps develop all sorts of crazy issues due to the black fiberboard. The board absorbs humidity over decades, making the black pigment in the board a little conductive, then all kinds of mayhem ensues. My opinion is Fender started realizing this in the silverface era and instituted a wax coating on their fiberboards.

Question 5.  What kind of diodes are those metal can things?  What kind and size?  The values are not on my schematic.  Are they always good?  ...

I'd leave them alone unless you know they're bad (which you'd likely find out because filters caps exploded with a bang). You could replace a string of three of those with just 1 or 2 1N4007's, but swapping parts unnecessarily early on just invites wiring errors and/or cold solder joints on your part (meaning you could introduce new faults that weren't there before).

Question6.  Is there a layout for this amp?  As I'm enough of a novice that they help me a lot.  I have the schematic.

Almost certainly no layout. Print out the both versions of the schematic (here and here) and figure out which variant you're looking at (probably by looking to see if there's a 220pF cap from pin 2 to pin 3 on the input tube for a channel). Actually, I'll guess you have the original version, not the revision, because I think I see a 3.3MΩ resistor in the middle of the board in the reverb circuit.

See if voltages are reasonable. Sure would be nice to verify normal operation of the amp by having a test signal you could trace through the amp to verify correct power output. Save noise-chasing for last. Make sure the bias balance control is set for middle of its range, and verify a reasonable bias voltage on pin 5 of the output tubes. If you think the amp is operating "normally" (for a PA 100), then consider doing what you said by changing resistors at the input jack, and changing the tone stack to match a Twin. You may consider changing the phase inverter to match the blackface Twin's inverter.

[eleventeen]

Excellent post, HBP.


A radio or an ipod or *anything* you can feed into the thing to provide a nominal signal....will help make the best decision as to what to do next.


I notice some jacks on the back panel that look they are switching jacks---suggestive of an effects loop. It's ridiculously common for age and corrosion to have applied themselves to those jacks in such a way that your signal is trashed going through the amp.


With even a radio piped into the amp, you could wiggle those switching reeds and/or plug in a 1/4" phone jack and work the switch and at least get some idea of condition. There's something that could weaken overall amp output, very easily. 

[shooter]

this is a pretty reasonable software tone gen that works for me, think about $25
anyway fwiw

https://secure.shareit.com/shareit/product.html?productid=300154061&languageid=1-1000

[eleventeen]

onlinetonegenerator.com

[TerryD]

Well, I already got my voltages. But before the tone generator posts etc.  So in any case here they are.  Hope it's clear.  I'll do anything I need.

v1. p1 226, p2 0, p3+8 001, p45 ac 3.4, p6 224, p7 0 ,p9 ac 2.9
v2. p1 214, p2 0, p3-8 001,p4-5 2.8, p6 191, p7 0, p9 2.9
v3. p1 435, p2 0, p3 003, p4-5 3.3, p6 438, p7 0, p8 003,p9 2.8
v4. p1 290, p2 0, p3 002, p4-5 2.8, p6 347, p7 0, p8 002, p9 3.3
v5. p1 230, p2 0, p3-8 001, p4-5 2.8, p6 226, p7 0, p9 3.2
v6. p1 320, p2 105, p3-8 138, p4-5 3.3, p6 323, p7 104, p9 2.8

6l6 1. p1 48, p2 ac 3.1, p3 459, p4 458, p5 47, p6 457, p7 ac 2.8, 8 ground
6l6 2. p1 48, p2 2.8, p3 459, p4 458, p5 47, p6 457, p7 3.3, p8 gr
6l6 3. p1 48, p2 2.9, p3 457, p4 456, p5 43, p6 457, p7 3.3, p8 gr
6l6 4. p1 43, p2 2.9, p3 469, p4 456, p5 43, p6 471, p7 3.3, p8 gr

There is that, if it tells you anything.  So what exactly is next then??

Thanks, Terry

[TerryD]

What's the best replacements (for down the road) for the blue caps specifically for a fender like this??  The yellow Mallory?  How about the white mallorys in there?  Are they good?  Do they hold up? 

[HotBluePlates]

Well, I already got my voltages. ...

All of those look good, except:

...
6l6 1. p1 48, p2 ac 3.1, p3 459, p4 458, p5 47, p6 457, p7 ac 2.8, 8 ground
6l6 2. p1 48, p2 2.8, p3 459, p4 458, p5 47, p6 457, p7 3.3, p8 gr
6l6 3. p1 48, p2 2.9, p3 457, p4 456, p5 43, p6 457, p7 3.3, p8 gr
6l6 4. p1 43, p2 2.9, p3 469, p4 456, p5 43, p6 471, p7 3.3, p8 gr
...

Do you have another 6L6 handy? Or can you pull the #4 6L6 you measured (keep track of this one, mark it) and swap places with one of the other 6L6's? If you swap, re-measure voltages for pins 1 & 5 of those 2 tubes. See if the low grid voltage stays with the tube you marked.

[2deaf]

I notice some jacks on the back panel that look they are switching jacks---suggestive of an effects loop. It's ridiculously common for age and corrosion to have applied themselves to those jacks in such a way that your signal is trashed going through the amp.

Those are the speaker output jacks.  A couple of the schematics have a line missing, but both jacks have their hot (tip) wired to the hot side of the transformer.  The switches merely connect hot to ground if neither jack has a plug in it.

[2deaf]

Any chance that 6L6 #3, pin 1 has a bad measurement?  If it were -43V instead of -48V, everything would make more sense.

How is pin #3 (plate) different on 6L6 #3 and #4?  Aren't they wired together?

[TerryD]

It might take me a little while to get to this.  Thanks!   please keep posted

  I see I have a 43 on 6l6 3  p 5 as well.  As 2 deaf has said.  I'll recheck these.

[TerryD]

Ok I checked again p1 and p5 on the 6l6s again.  Here is what I got this time,  Maybe I'll check again tomorrow.

6l6 1.  p1 48, p5 48
6l6 2.  p1 48, p5 48
6l6 3.  p1 44, p5 44
6l6 4.  p1 44, p5 0  ( I checked that 3 times)

As an aside.  Why do 6l6s in these push pull things have to be so even steven.  Matched pairs and quads etc.  Would an unmatched set just be less power?  How about if I had my RCA 6l6s (matched) in 1 & 4 and my crappy sovtek 5881s in 2 & 3?  Would that be matched?? 

Thanks!

[eleventeen]

"Why do 6l6s in these push pull things have to be so even steven."

Well, they don't, but it is one of those things that is aimed for. A "best practice" if you will. For guitar, it hardly matters, but unmatched tubes are likely to produce some amount of distortion. Maybe you LIKE that distortion on guitar but it can't be said to be ideal or even desirable in a hi-fi situation.


If I (as a distributor) buy 100 tubes and set about developing matched pairs and you will pay me $20 more for a matched set, then...why not?

[2deaf]

Turn that amp off and discharge everything including the bias circuit.  Mark 6L6 #4 like HBP said and take all of the 6L6's out.  Diagnose the wiring on the sockets with an ohm meter.

This amp has been modified as can be seen in the picture with the lamp cord.

[TerryD]

I plan on moving the tubes around. I just havn't got to it yet.

[2deaf]

I plan on moving the tubes around. I just havn't got to it yet.

If 6L6 socket pin 5 has zero volts on it, a good tube will red-plate (if pin 3 and 4 have HT).   

[TerryD]

Whoops!  My alligator clips were coming undone and I had to solder it.  that's the reason for the "0".

I moved the tubes around.  Here are the results

6l6 1.  p1 47, p5 47 with suspect tube
6l6 2.  p1 47, p5 47 with second maybe questionable tube
6l6 3.  p1 43, p5 43 with tube from 6l6 1
6l64.  p1 43, p5 44  with tube from 6l6 2

[Willabe]

47, 45, 43, 44, what? AC, DC, mA?

[2deaf]

So all the 6L6's have bias voltage on their grids.  You can make all the pin 1 voltages the same by adjusting the pot. with the screwdriver slot over by the diodes.  Now I would like to see the voltages on the other pins of the 6L6's.

[HotBluePlates]

Any chance that 6L6 #3, pin 1 has a bad measurement?  If it were -43V instead of -48V, everything would make more sense.

Sharp eye! Thanks for the input, as I think you're right and the output tubes are probably okay (discrepancies just a matter of measurement issues).

[TerryD]

OK, I'll get on that.  This is so different from any Fender I've messed with.  Where are the 25 mfd electrolytic bypass caps.  Whats that metal wall for?  What are the basic changes to make this more guitar friendly?  Thanks  What will changing the phase inverter to Blackface twin do soundwise?

[HotBluePlates]

Where are the 25 mfd electrolytic bypass caps.  ...

See that white Mallory in the preamp @ 750uF? That's your "25uF cap" except it's one cap for all input stages of the 4 channels. Because all 4 channels share a single cathode resistor, its value is much lower than typical, so the cap's value must be correspondingly higher than typical.

Whats that metal wall for? 

It's a shield.

On one side are volume & tone controls; these are right after the 1st stage of the relevant channels, and have very low signal levels. On the other side is reverb, negative feedback and phase inverter circuitry, some of which are the highest signal levels in the amp apart from the output tube plates. I presume Fender had some trouble with this layout before the shield was installed between those two circuit points.

What are the basic changes to make this more guitar friendly?

Leave the input stages themselves alone for now, but look to the input jacks (I already mentioned this before, just not step-by-step). The resistors are a pair of 22kΩ and a 47kΩ. Install 68kΩ in place of each 22kΩ, and install 1MΩ in place of the 47kΩ. I suggest doing this on 1 channel only for now so you can directly hear the effect by plugging into a modded & un-modded channel. Changing this alone might return the "missing power" you noted earlier, though I really wanted you to confirm proper operation of the amp before changing anything.

Look at the tone controls for that channel whose input jack resistors you've changed. The Treble control is just like a 1-knob tweed tone control (in a Princeton or Deluxe) except the pot is 250kΩ instead of 1MΩ. The Bass control is added to that simple tone control; it rolls off highs through the resistor & cap to ground, but when you turn up the Bass control, it also allows lows to bypass the 500pF cap that is part of the Treble control.

The downside for modding is all of the tone circuit parts are mounted at the pots, and I think there is a terminal strip mounted on the back of the Bass pot which holds the 270kΩ channel mix resistor for that channel and provides a mounting point for the wiring connecting those together (ad for the shielded wire running to V5a). Some of all that will likely be in the way while you try to adjust the parts. You'll need 250pF, 0.047uF and 0.1uF caps and at least a 6.8kΩ 1/2w resistor. Normally Fender tone stack caps are high voltage rated, but there's a coupling cap before you even get to the tone circuit in this amp; you may want to look into getting caps rated no more than 200-250vdc so that they are physically small and can be mounted at the pots.

I imagine you'll want a layout of the positions of the new parts; maybe someone can help with that as I'm not set up at the moment to draw layouts.

What will changing the phase inverter to Blackface twin do soundwise?

The 2 biggest changes in the phase inverter (although there are 4-5 changes total) are increasing the plate resistors from 47kΩ each to the old values of 100kΩ and 82kΩ, and increasing the coupling caps to the output tubes to 0.1uF. The changed plate resistors will lower the phase inverter plate voltage and increase the inverter's gain somewhat (perhaps making the output stage seem more sensitive), while increasing the coupling caps would allow more lows through. That said, the smaller caps and plate load resistors were probably installed to allow quicker recovery from grid current blocking ("blocking distortion").

[TerryD]

Thank you

I think the amp might be running pretty good after cleaning all the pots and the tube pins.  I changed 1 suspect tube and the reverb tube.  It sounds pretty stable but I'd like to play it for an hour before I say for sure.  The real nasty distortion kicked in after 15 minutes of play.

I adjusted the bias as you said.  Here is what we have now.

6l6 1.  p1 47, p2 3.3 ac, p3 459, p4 457, p5 47, p6 460, p7 2.9 ac
6l62.  p1 47, p2 2.9, p3 459, p4 458, 'p5 47, p6 459, p7 3.4
6l63.  p1 47, p2 2.9, p3 455, p4 455, p5 47, p6 456, p7 2.9
6l64.  p1 47, p2 2.9, p3 457, p4 455, p5 47, p7 3.4

That looks pretty good doesn't it?

Thanks for all of the above info!

Hopefully forward.

parts list type of questions:
The power caps are starting to bubble little tiny pin prick type of bubbles, not a lot, but they are the cardboard mallorys, so I ought to change them, right?  I probably will.

Should I get some more appropriate pots?  I want to have a midrange pot, which I'll stick in one of the  input holes.

So "sonically" those blue caps are fine?  Debatable?  I know sometimes upgrades are just a different flavor.

I've seen some talk about I channel being like a plexi...but that may be too complex.

I like the idea of blackface twin phase inverter.  This sure doesn't lack for any bass though.

[HotBluePlates]

I adjusted the bias as you said.  ... That looks pretty good doesn't it?

Good to go!

The power caps are starting to bubble little tiny pin prick type of bubbles, not a lot, but they are the cardboard mallorys, so I ought to change them, right?  I probably will.

When in doubt, change 'em out. 40-year old electrolytics are probably ready to be replaced, so call it preventive maintenance if you like.

Should I get some more appropriate pots?  I want to have a midrange pot, which I'll stick in one of the  input holes.

Treble and Bass pots are already the correct value for a blackface-type tone circuit (250kΩ). If you wanted to keep a tweed-style single tone control, the Treble pot could be 1MΩ and would make the circuit a little less lossy.

Obviously, if you want midrange then you'll need a 10kΩ (stock blackface-type value) up to 50-250kΩ (if you want a "Raw control" type feature that reduces tonestack signal loss).

So "sonically" those blue caps are fine?  Debatable?  I know sometimes upgrades are just a different flavor.

I really don't know.

I know I've ripped them out of silverface amps when doing a "blackface conversion" because that's what the customer wanted. I also know I heard a gigging musician's silverface Twin Reverb for years in the 80's & 90's, which sure sounded Fendery enough. While he was a skilled electronics tech, this was also before orange drops or other cap-swaps became a fad, and I suspect he wouldn't have traded them out unless they were leaking. I had a different friend in Junior High who had a silverface Champ (and probably a late-70's or early-80's model) which sounded Fendery enough as well.

You had a Vibrolux... I think you'll know after you switch the input jack resistors and configure a tone circuit whether the amp is sounding like it should...

I've seen some talk about I channel being like a plexi...but that may be too complex.

There's plenty of space to build whatever... as long as you rip out the existing board and put in a completely new board to facilitate the new circuit. You'll probably want to leave that shield in place as it's good insurance regardless.

[Ritchie200]

I may be stating the obvious so pardon my intrusion if so...  Please bear with me as the story does eventually have relevance!    Due to the scarcity and collectable cost of the Marshall Major, fans of the amp have been able to score the PA version of the Major for considerably less.  This is an advantage for anyone adept with a soldering iron and an imagination.  The stock circuit is very flat sounding due to it's intended job of reproduction and not very well suited for guitar or bass (I am assuming your Fender PA is likewise?  I have not looked at the schemo).  I have seen conversions of the Major PA with separate preamp designs ranging from original Lead or bass Major to Champ to JCM800 - in the same amp.  Bottom line, you have the opportunity to make this the swiss amy fender!  This could be a really cool project!


+1 on the supply caps.  Make sure you have a solid base (so to speak) to build on.


Jim

[TerryD]

(Question 5. from above)  "What kind of diodes are those metal can things?  What kind and size?  The values are not on my schematic.  Are they always good?  ..."

HBP quote "I'd leave them alone unless you know they're bad (which you'd likely find out because filters caps exploded with a bang). You could replace a string of three of those with just 1 or 2 1N4007's, but swapping parts unnecessarily early on just invites wiring errors and/or cold solder joints on your part (meaning you could introduce new faults that weren't there before)."

I still need to know if the present diodes in the PA 100 are a big metal can version of the In4007.  I do like to replace the In4007 with freds and I know the exact equivalent for the  IN4007.  It doesn't say anything about these diodes in my shematics, blackface or PA 100.

To my ears the FREDS soften the hit to your guitar strings just a little.  Kind of half way between the sound of the diode rectifier and a tube.  And once done gets rid of a little more funk the In4007 introduces that I don't miss.  A little itsy bitsy high pitched grit, I'll call it. 

Swiss Army Fender would be great.   any ideas would be appreciated.

Thanks

[HotBluePlates]

I still need to know if the present diodes in the PA 100 are a big metal can version of the In4007.

Those diodes have a number on them; they are a 1N-whatever-they-are. Maybe a datasheet still exists online, maybe not.

Your PT is 340v-0-340v (though maybe higher with today's wall voltage). 340v * 1.414 = ~481v peak, and 481v * 2 = 962v. If the PT voltage is not higher than ~350vac, a single 1N4007 is sufficient to connect each high voltage winding-end to the 1st filter cap.

What I calculated was the peak voltage of the a.c. applied to the rectifier diode, as that's the highest voltage the filter cap will see (and charge to when there's no current drawn from the power supply). If the 1st filter is charged to that value, when a winding end swings negative, the maximum reverse voltage (aka, "peak inverse voltage" or "PIV") seen by the diode is the filter cap's charge plus the peak of the negative a.c. (calculated as 962v for 340vac input). A 1N4007 is rated for 1kV reverse voltage, and is sufficient by a slim margin.

Assuming you play it safe and count on the wall voltage to go high, pushing high voltage winding higher-than-spec, you use 2x 1N4007's (or UF4007's) per leg. That's 2kV of composite peak inverse voltage rating, and plenty by an almost-absurd margin.

So you see why layouts & builds with 3x 1N4007's (or their UF/FRED counterparts) per leg only reveals the builder doesn't know why the circuit is the way it is. Apparently the 1N4007 was available as early as 1966, but the diodes Fender use in their amps had a lower PIV rating (perhaps 400v? I've never looked up the actual part to be sure).

[Willabe]

Ultra Fast diodes (UF1N_ _ _ _)  have a much shorter switching time (turn on/turn off) than regular diodes. So they have less of that "little itsy bitsy high pitched grit" as you called it.

You should try them, might not need the FRED's, easy to find, inexpensive, and much smaller size then Fred's.   

[TerryD]

Ok the can diodes on this thing read like this;

             RCA
     
            35604

     FN            4C

As best as I can make out. 

Can I get FREDS to replace those.  Are they 3 amp diodes?  I looked for Ultra fast diodes and have not found them yet.  Could anyone have mercy on me and give me a part number.  I mean besides the mercy already shown.

What are those White Mallory caps made out of??  How about those blue buggers (that I will leave alone)?

[Willabe]

Can I get FREDS to replace those.  Are they 3 amp diodes? 

Your PT is 340v-0-340v (though maybe higher with today's wall voltage). 340v * 1.414 = ~481v peak, and 481v * 2 = 962v. If the PT voltage is not higher than ~350vac, a single 1N4007 is sufficient to connect each high voltage winding-end to the 1st filter cap.

What I calculated was the peak voltage of the a.c. applied to the rectifier diode, as that's the highest voltage the filter cap will see (and charge to when there's no current drawn from the power supply). If the 1st filter is charged to that value, when a winding end swings negative, the maximum reverse voltage (aka, "peak inverse voltage" or "PIV") seen by the diode is the filter cap's charge plus the peak of the negative a.c. (calculated as 962v for 340vac input). A 1N4007 is rated for 1kV reverse voltage, and is sufficient by a slim margin.

Assuming you play it safe and count on the wall voltage to go high, pushing high voltage winding higher-than-spec, you use 2x 1N4007's (or UF4007's) per leg. That's 2kV of composite peak inverse voltage rating, and plenty by an almost-absurd margin.

So you see why layouts & builds with 3x 1N4007's (or their UF/FRED counterparts) per leg only reveals the builder doesn't know why the circuit is the way it is. Apparently the 1N4007 was available as early as 1966, but the diodes Fender use in their amps had a lower PIV rating (perhaps 400v? I've never looked up the actual part to be sure).

HBP has explained this to you. Slow down and Please read it again. 

Leo Fender at the time he switched to SS diodes in his hi power amps instead of a tube rectifier, HAD to use 3 SS diodes in series to achieve the necessary PIV voltage rating. (HBP explained this to you.) Fender stayed with the 3 SS diodes in series EVEN after much higher PIV SS diodes were readily, inexpensively and reliably available.     

He also gave you the voltage numbers needed to decide if you only need 1 or need 2 SS diodes in series for your amp.

 

Can I get FREDS to replace those.  Are they 3 amp diodes? 

No, 1N4007's are 1 amp.

Can I get FREDS to replace those.

Yes, sure you can. BUT do you really NEED Fred's? Try the WAY cheaper UF's 1st. Your gonna be in this amp for a while, why not try the UF's 1st and see/hear what they sound like??????   

Even if you don't think they sound as good as the FRED's in the end, you'll be way safer while your 'tweaking' the amp, so if you make a mistake you won't blow up the pricey FRED's.     

[Willabe]

I looked for Ultra fast diodes and have not found them yet.

Go to Mouser.com and do a search in their search/part number box, top of page, right side, for; UF4007, it will come right up. 

[Willabe]

Could anyone have mercy on me and give me a part number.  I mean besides the mercy already shown.

Mercy, in my understanding, is for someone who has done something wrong and is guilty and so they or their family ask/beg for mercy.

You haven't done anything wrong. Geez, stop it already Terry!       

No need for that. You are among good friends here.   

Just SLOW down a bit and you'll get the responses to some at 1st, then most and even all, over time, to the questions that your asking.     You will have to do your own home work also as things go forward. 

[sluckey]

Just leave the diodes alone.

[PRR]

> Just leave the diodes alone.

Agreed. A good hammer does not need micro-polishing.

AFAICT-- NTE Number NTE5804 claims to be "35604 NTE Equivalent NTE5804 400V 3A diode..."

So a 3-string is close to 1200V 3A.

HBP explained why a 1000V part is too close to rating, and Leo listened (or vice versa).

I agree that a 100W tube amp *may* be right near the limit for a 1A part. I know it has been done, but it's awful close.

Our host has 1N5408 (1000V 3A) in the link at the bottom of this page, "Lamps/Diodes/Channel Switching".

But if they aint broke, don't fix them.

[TerryD]

OK.  I read and reread what HBP said on the diodes.  To me it's kind of like listening to a good string quartet.  I certainly like it but am not sure I completely understand it.  Other people on this board have warned me that working with a 100 watt amp could expand my mind to the point of having to clean it off my bedroom walls.  So I have to be "certain" as other have told me on this board of what I'm doing.  At least the physical aspects of what I'm doing.

My safest bet right now seems to get the UF4007s and replace each diode three in a row etc. 

With the FREDs on my 50 watt amp the three diodes could be replaced with "1" 1 amp 1000v fred (or was that 1200v?)  I'm not thinking I can jerk around like that with this 100 watt.

I have so enjoyed the FREDS upgrades that this is one of the few times I'd disagree with the naysayers.  But trust me, I have, to date,done literally 100s of dollars worth of useless upgrades on various amps.  I get the jist of what you are saying.  I'm now with you heart and soul i.e. the blue gumballs will probably stay and etc.

I use Dan Torres book to help me a long as well.

[TerryD]

I changed the input resisters.  What a difference!  I think I'm getting the polishing a hammer idea.

Plus those two parts of the blackface phase inverter.  Better yet.

Any ideas for the other inputs?  There is not a lot of "info" on the PA 100 mods that's reliable

[HotBluePlates]

I changed the input resisters.  What a difference!  I think I'm getting the polishing a hammer idea.

I'm guessing your "missing power" showed up?

Do you see why we advocate a circuit-change first to get big differences rather than a parts-type-change, which is often subtle at best?

[TerryD]

HBP  Could you give me the other steps for the phase inverter conversion to Blackface twin.  My PA is similar to a post CBS twin but not exact.  I have a blackface layout as well.  I did the .1 caps and the 82 & 100 resisters.  Thanks

[PA100]

HBP  Could you give me the other steps for the phase inverter conversion to Blackface twin.  My PA is similar to a post CBS twin but not exact.  I have a blackface layout as well.  I did the .1 caps and the 82 & 100 resisters.  Thanks

Hey TerryD....how are you going on your PA100 changes? I have been playing around with one for a while.
 I blackfaced the PI as above (I also changed the 330K resistors to 1M which is I think what you were after above).
I have used 0.1 coupling caps throughout, changed the input resistors to 68K and 1M (I have actually further changed 3 inputs to 8M and 5.6M )
I've been venturing into reducing the preamp voltages to have a different voltage and different breakup for each channel.
I have converted it into a squat combo amp as well...1x12 and 1 x 10 speakers.
Have you tried altering the negative feedback circuit yet? I have been toying with the idea of having a switch to be able to remove it altogether and maybe have an altered value resistor also....maybe the clever guys can tell me what this would do or best way to attack it...I am not sure what rating the switch for it should be?

[TerryD]

I like the negative feedback idea.  Mines been sitting on the bench waiting for the final touches.  I did the blackface PI and that was great.  I put FREDS in for the diodes.  I always like that. I ended up with (for my purposes) 1 channel stock as I liked that ultra clean sound. channel 2 is like a blackface reverb preamp and I just jerked around with the caps right on the pots.  I did do the individual cathode by pass 25 mfd caps 1500 resisters to ground.  I like that .  Then I chained the last two channels for some low level gain.  I have the bypass cap off on both of those channels to reduce the gain.  first channel of that thing is just a volume control stock 250 then the black face preamp on the second channel of that chain.  I left the stock inputs on that again to reduce the gain.  Beautiful grit for my taste on that.

[PA100]

Thanks TerryD , that's given me some more to think about! I also changed the screen resistors to 2w 1.5K and am running 6n6c (6v6 type) valves...reduced to 48w but more breakup. Biased really cold at 6mA they sound great . I added a trimpot also so I can adjust bias as well as balance.

[TerryD]

THAT...is changing that amp around!

[PA100]

I call it my fender frankenstein....part Bassman , part super reverb, part twin...and a bit of PA100! Its all exact same dimensions so the amp can be removed and put back in the head cab in about 10mins.
I like the idea of your mod "individual cathode by pass 25 mfd caps 1500 resisters to ground" - is that detailed anywhere?

[TerryD]

No, not really, but I did get the idea from others.  There is plenty of real estate to put the 4 in the basic same spot as the big capacitor was.