1st time build problems. Fender 5f8a

[josephbartolotta]

Hello All!

I'm new to a lot of things. This forum. Amp building. Somewhat new to Electronics...  Well I decided the best way to learn was to dive in.  I built up a weber kit for the Fender 5F8A twin...

Sparked it up and I was lucky.  Nothing blew up! I flipped off standby and the tubes weren't redplating, they looked good, the heaters were right at 6.3v.

plugged in a guitar and then i saw there was a problem.  Perma-distortion. I actually really like the choked, fuzzed distortion... Souds like a preamp tube being blown the hell out, kinda like the beginning of the first neutal milk hotel record... But this is a twin and should be sparkly clean most of the way up the dial... if i could flip the amp in and out of total shit mode i'd be happy, but i'd rather just have a good working amp.

 i've been going through everything with a meter checking it and i really just don't know where to go from here.

all the resistors seem to be spot on in value... i haven't checked all of the caps, but i've checked the filter caps and they seem fine...

Checking voltages it seems to me that every section of the amplifier (aside from the first preamp AX7) is running around 10% hotter than the voltages listed on the fender schematic/layout which it says would be within operating condition...   the first tube is as supplied and as drawn on the weber schematic, a 12ax7.  The fender layout says this should be a 12ay7 which is a much lower gain tube, could this be part of the issue in that stage?

I am wondering if due to the fact we're on 120v instead of 110 now, that this is why all of my values are high globally... the PT outputs to the rectifier (which is the solidstate recto supplied by weber in lieu of the gz tube of fender's design)  are feeding at 330V which is above the 300v on fender's schematic... the output of the rectifier on standby is 314, when the amp is fully switched on, it's almost 430... the A and B points of the B+ rail are riding at 430V and the C point is 385.  these are all 10% high to their respective schematic readings of 395, 395, 350... pretty much every value is almost exactly 10% high, which is about right for the change from 110 to 120... 10%... could this coupled with the higher gain ax7 be the source of my troubles? or in your experiences do you think this points to a different sort of fault.. resistor/cap/bad connection somewhere... i'm pretty certain that only the first preamp tube is really overloading... it's voltages are in the 220's instead of the listed 140-145 range...

Thanks so much for any advice,

Joseph L Bartolotta

[chocopower]

Voltages are ok. With those voltages, you´ll should have even a cleanest amp.
The highest voltages in the V1 are there for the use of a 12AX7 instead a 12AY7. If you put the 12AY7 shure you´ll be in that 10%.
What about bias voltage allong the whole circuit?

My first idea:
Bad tube, wrong value resistor (you checked, i know....)
if not:
Disconect the NFB loop (56k resistor going into the speakers jacks).
if that resolve the problem, change the primary widdings in the O.T. and put back the 56k resistor.

[stingray_65]

First, Welcome!

2nd, We'll get through this one step at a time. There is a wealth of very knowledgeable friendly technically minded people here.

You are about to embark on a new skill, Troubleshooting.

SO, you've built a known design that has worked for MANY people. (I'm assuming you followed Webers design and did not try to modify the circuit)

The amp does not work properly.

This means you've built something incorrectly. No shame in that.

Amp off, filter caps drained (measure with meter),Webers layout and schematic handy, and a highlighter in hand you're ready.

Carefully  start at the beginning on the layout sheet at the inputs, check the layout sheet, then the amp.

It's tedious, I know! but check every connection off on the layout, then the schematic.

Use a magnifying lens if you have to to make sure each solder joint is good.

OH! up load a gutshot of the amp if you can! lot of sharp eyed greybeards here might spot a wiring issue in a heartbeat. use the macro (flower) setting on the camera, break it down in sections and an overall shot to boot.

The 10% thing is expected, don't sweat it.

12AY7 or 12AX7 in V1 position doesn't matter either as far as functionality issues go, there is a tonal difference, but thats for tweaking after the amp's working good.


The 220V reading you say you are getting at pins 1&6 on V1 are high.

What voltage do you read at the junction of the 2 100k resistors that come off pin 1 & 6

Are those resistors 100k and not maybe 10k? silly, but more than 1 time this month, I've not had my meter range not set right.

That's enough for right now, Let us know your progress and we'll get through this together.

Ray

[Matty_V]

Hahaha, I don't have any advice, but props for the neutral milk reference. If you can make your guitar sound that awful you must be doing something right.

[josephbartolotta]

thanks for the replies guys...

so yeah, I am slightly new to electronics troubleshooting... I work as a mobile broadcast engineer building/configuring/maintaing temporary television studios... so I have a lot of experience tracing signal flows and finding airgaps and failed bits of kit... but at the component level not as much... I worked my way up to here from being a wireman building television trucks and my year in the maintenance shop was spent mostly checking out the rental kit to see if it still worked... so many thousands of monitors turned on and verified... the volume of equipment coming in and out meant I didn't get a whole ton of bench time with truly faulty equipment because I was the guy in the shop concerned about the 20 palletts of equipment which needed to be checked in..  and I was also the guy in the shop with a strong computer background and very little electronics... useful guy for that shop. the guy still learning in this shop..  And there's certainly no shame here... but yes.. understanding and diagnosing problems in amplifiers? can't tick off the "highly experienced" bubble on the resume yet.

I will go through with a highlighter tomorrow... will try to get pics too... some of the work i find a little embarassing right now though... i started swapping best guess faulty parts with some spares clipped out of an old circuit.. I just clipped the leads and tack soldered the replacement component in place... though it sounds a little dumb to me i just want to get it working correctly and then order some parts with full leads on them so I can go back and make everything prettier (hopefully not opening a new can of worms in the process) i also have some wires to the tube sockets which are a little too long... it looked really nice when i first "finished" it though... in this more than any other work i've done i know neatness counts/do the job right or don't do it, and the look inside the amp is just messy enough to really bother me now. but anyways...

the 100k junction reads 324V.. and the bias voltage is reading -51 instead of -41... i have a trimmer pot i've been meaning to put in for adjustable bias which i'm probably going to get around to come morning..

i think i have to get some shuteye.

[josephbartolotta]

so i've rechecked all resistors. All good.

I used continuity on my meter to check all connections from the outlet of the component to the next place in circuit, be it ground or a tube pin (checked at the front of socket), or at the head of the next component, etc... seems to be all good.. eyeballing the joints looks good to me for the most part, although i used the supplied cloth wire and it does a relatively good job of fraying and obscuring your vision, hence the component head to component head buzzing out method...

i pulled the caps from the V1 section and they all tested good. checked the caps in the bias supply... they're good.

I haven't put in that bias adjust pot, but i'm pretty sure the issue isn't bias... i'm reading -44 instead of -41, still in voltage tolerance...

The issue i'm rather certain is at V1, with those too high plate voltages... I actually just noticed as well that the cathode voltage is a little low, 1.7V instead of 2.1...

I redid my filter caps this morning, i had replaced them earlier in my troubleshooting when i had a busted meter.. same rating different style, but now that i'm back on the supplied caps my readings are closer to the 5-6% over mark than 10%... except that lousy V1... i'm starting to consider upping the plate resistor values, but obviously there's an underlying problem which needs to be dealt with...

[chocopower]

The issue i'm rather certain is at V1, with those too high plate voltages... I actually just noticed as well that the cathode voltage is a little low, 1.7V instead of 2.1...

As i said. That´s for the use of a 12AX7 instead a 12AY7. Not worry about that.

If voltajes are ok, and resistor/cap values are ok too... Check grounds.

[stingray_65]

While 220V is not in excess for the 12AX7 in position V1, it still seems high for what the circuit should be.

Correct me if I'm wrong, Are you measuring the 220v at pins 1&6 ?

If so, what is it measuring at the junction of the 2 100K resistors that are attached to pins 1 & 6 ?

Hold on, lets do this another way.

If you look at Webers schematic for the 5F8A, what are the voltages at points :A, B, C, D on the B+rail and what are the voltages of Pin 1 and pin 6.

Not being a smart-alec, just covering all the bases here, but measure with amp on, pots at "0", black lead of your meter to the chassis, red to test point, meter set to DC.

they should read about:

A 400v, B  3-5v less than A, C 350v, D 300v, pin 1 150v pin 2 150v

IF A, B, C and D are close, AND those 2 100K resistors measure 100K THEN I would say the tube itself is not drawing current enough to draw the voltages down at pins 1 and 6

IF A, B, C, are close, D is high (like maybe 350v) I would check pins 1 & 6 on the PI (V2). pin 1 (V2) should read about 150V and pin 6 will = D

Do be safe when checking

A live amp has some seriously scary voltages lying around.

Again if you can, upload a couple pics.

Ray

OH and try swapping around the 2 12AX7's or replace them 1 at a time with another if you have one.

Don't sweat the bias at this point, one mountain to climb at a time.

Ray

[tubeswell]

Go right through the amp and (carefully) measure all the DC (idle) voltages both in the B+ supply (at each filter cap node) and at the plates, screens, grids and cathodes, and then report them all back here.

[josephbartolotta]

So, I went back through the grounds checked out.. cleaned up some of the wires to tube sockets which were a little too long...

those readings i gave were from pins 1 & 6...

here's a list of readings:
A-410
B-410
C-370
D-320

V1-p1  220
v1-p2  0
V1-p3  1.65
V1-p4  3.18 AC heater
V1-p5  3.18 AC heater
V1-p6  225
V1-p7  0
V1-p8  1.65
V1-p9  3.18 AC heater


V2-p1  185
v2-p2  0
V2-p3  1.2
V2-p4  3.18 AC heater
V2-p5  3.18 AC heater
V2-p6  330
V2-p7  184
V2-p8  185
V2-p9  3.18 AC heater


V3-p1  250
v3-p2  27
V3-p3  43
V3-p4  3.18 AC heater
V3-p5  3.18 AC heater
V3-p6  245v
V3-p7  28v
V3-p8  43v
V3-p9  3.18 AC heater
 
i had purchased a spare set of tubes at the guitar store figuring i would explode something in this process... I have swapped all tubes, that was one of the first things I tried...

thanks for the help guys.

i'll try to get some pictures up tomorrow.

Joe

[tubeswell]

So, I went back through the grounds checked out.. cleaned up some of the wires to tube sockets which were a little too long...

those readings i gave were from pins 1 & 6...

here's a list of readings:
A-410
B-410
C-370
D-320

V1-p1  220
v1-p2  0
V1-p3  1.65
V1-p4  3.18 AC heater
V1-p5  3.18 AC heater
V1-p6  225
V1-p7  0
V1-p8  1.65
V1-p9  3.18 AC heater


V2-p1  185
v2-p2  0
V2-p3  1.2
V2-p4  3.18 AC heater
V2-p5  3.18 AC heater
V2-p6  330
V2-p7  184
V2-p8  185
V2-p9  3.18 AC heater

Nothing much wrong here, suffice to say- you are probably running 12AX7s in the pre-amp (and not 12AY7s) right?

V3-p1  250
v3-p2  27
V3-p3  43
V3-p4  3.18 AC heater
V3-p5  3.18 AC heater
V3-p6  245v
V3-p7  28v
V3-p8  43v
V3-p9  3.18 AC heater

Pin 2 (grid of driver stage), should be 0 AFAICT. and Pin 3 (the cathode) should be around 1.5V. Check your Rk value again.


Pin 7 (grid of cathodyne stage), does seem a little odd at 28V - in my mind it should be closer to the cathode voltage (around 48 or so). Maybe the coupling cap driving that grid is leaky?

[HotBluePlates]

V3-p1  250
v3-p2  27
V3-p3  43
V3-p4  3.18 AC heater
V3-p5  3.18 AC heater
V3-p6  245v
V3-p7  28v
V3-p8  43v
V3-p9  3.18 AC heater

Pin 2 (grid of driver stage), should be 0 AFAICT. and Pin 3 (the cathode) should be around 1.5V. Check your Rk value again.


Pin 7 (grid of cathodyne stage), does seem a little odd at 28V - in my mind it should be closer to the cathode voltage (around 48 or so). Maybe the coupling cap driving that grid is leaky?

This is the long-tail phase inverter of the 5F8A Twin. I think you had a split-load inverter in mind.

Further, the bootstrapping of the phase inverter grids means the effective input impedance is very high. Enough that even a 10M impedance from a typical meter will load down the voltage, and cause it to read low (as in this case). A better way is to measure from cathode to ground (as already done); for the grid voltage, measure from the junction of the 470 ohm bias resistor and 10k tail resistor to ground. The measured voltage should be closer to expected grid voltage.

Regardless, all voltages look reasonable to me.

Do the speaker jack and input jacks have good mechanical ground connections? Does the feedback loop/presence pot have a good ground connection?

I'd be looking hard at all grounds, and evaluating solder joints/mechanical connections. After those are ruled good, look for a 47k where a 470 ohm should be (like the long-tail bias resistor), and other build-error (oops!) moments.

[PRR]

> Pin 7 (grid of cathodyne stage), does seem a little odd at 28V - in my mind it should be closer to the cathode voltage (around 48 or so).

That's normal. (Check your cathodynes.)

When he is not looking at it, it IS up at 46V.

When he puts a meter from grid to ground the meter loads the ~~1Meg grid resistor. Not just the obvious 1Meg/10Meg error, but much more. "Half" is typical.

Existance of a happy cathode voltage and equivalent drop in plate resistor is fair proof the cathodyne (OR Fender long-tail) is happy.

[tubeswell]

V3-p1  250
v3-p2  27
V3-p3  43
V3-p4  3.18 AC heater
V3-p5  3.18 AC heater
V3-p6  245v
V3-p7  28v
V3-p8  43v
V3-p9  3.18 AC heater

Pin 2 (grid of driver stage), should be 0 AFAICT. and Pin 3 (the cathode) should be around 1.5V. Check your Rk value again.


Pin 7 (grid of cathodyne stage), does seem a little odd at 28V - in my mind it should be closer to the cathode voltage (around 48 or so). Maybe the coupling cap driving that grid is leaky?

This is the long-tail phase inverter of the 5F8A Twin. I think you had a split-load inverter in mind.

Oops yes I was looking at the (5E8A) - wrong schematic sorry

[josephbartolotta]

So, between holdiays and traveling for work I just got back around to working on the amp...

Took it apart all the way and started over, just because the thoughts of bad solder joints were plaguing me... Once I got to the part where I was putting the components back in the eyelet board I had a realization. The capacitor connected to the anodes,pins 1&6 of the phase inverter... turns out i had put the 47uF cap from the standby switch there, and the 47pF cap for there on the standby switch...

I'm not sure I want to venture into this territory... but what exactly do you think was happening with that mistake? would it be anywhere near safe to put the correct and incorrect capacitors on a switch so that I could put the amp into "totally destroyed" mode?  I wouldn't want to do it often, but I could see it being useful for some of my client's projects in my recording studio...

And also, thanks so much for the help, poking around with what I thought was the issue and then going through the build process again helped me learn a lot. definitely better with a meter, and definitely triple quadruple checking things out now.

Joe

[ajeffcote]

Josephbartolotta said "The capacitor connected to the anodes,pins 1&6 of the phase inverter... turns out i had put the 47uF cap from the standby switch there, and the 47pF cap for there on the standby switch..."

I'm just almost as new to this as you are, so I don't have much to offer in the way of help. But I am currently reworking a 5E8A for a friend, and it was his first build and it has some problems. So there is a little similarity here.
My question is about the cap to ground from the standby switch. Why? Isn't the amp grounded with a 3-conductor cord? Does the cap actually serve a purpose?

[HotBluePlates]

The capacitor connected to the anodes,pins 1&6 of the phase inverter... turns out i had put the 47uF cap from the standby switch there, and the 47pF cap for there on the standby switch...

I'm not sure I want to venture into this territory... but what exactly do you think was happening with that mistake? would it be anywhere near safe to put the correct and incorrect capacitors on a switch so that I could put the amp into "totally destroyed" mode?  I wouldn't want to do it often, but I could see it being useful for some of my client's projects in my recording studio...

Well, you mean 47nano-F or 0.047uF cap, but we know what you're saying.

What happened in the amp is this:
The intended 47pF cap allows very high frequencies in the output of the phase inverter to bridge from one side of the output to the other. Then, because they're of opposite polarity (and presumably the same size), those frequencies cancel out. This reduces output at that range of frequency.

When you used the 0.047uF cap, almost the entire range of guitar frequencies were bridged. That should cancel all output from the amp, but I think you found out that the outputs are not perfectly "equal and opposite".

Further, that cap would now look to the phase inverter like a near-short-circuit at a.c., which would be a heavy load on the phase inverter, and cause it to lose gain and crap out (heavily distort the signal). I'm not sure exactly how the long-tail would cope with that; might be interesting to slap a scope on it and see. By the way, the terms "heavy load" and "near-short" as I'm using them apply to signal conditions only; the stage is still happy as far as d.c. is concerned, so there should be no fear that damage might ensue.

What output signal was left over probably barely drove one side of the push-pull output stage... which it sounds like made for a usefully trashy sound. And yes, you should be able to switch that cap in and out (I'd personally do it in Standby mode) to get the garbage at will.