5f3 Build Nubie

[Ed_Chambley]

I hafe been working on  5f4 build and have gotten to where I can hear faint guitar when played.  I have checked and double checked resistor values, found many mistakes.  Also found some wiring mistakes.  I am using a Mercury Magnetic Output transformer SUP-TH-O.  It is for a Supro Thunderbolt. Has an 8 ohm tap and on the primary it has a brown, Red, Blue in that order in the schematics.  I assume the red is the center and the blue and brown is the pin 3 connections on the 6L6 and the red is 4.  I have a negative bias of 52, but plan to install adjustable as soon as i figure out what is going on.  The power trahsformer is from hoffman and it is for the super reverb Part number 022756.  I am getting 318 acv on the  red wires to the rectifier.  The dcv at the filter caps seem to be in line with the fender schematic. There is a green and common wire from the OT.  I have the green on the tip and the black to ground on the jack then continues to a star ground.

With a 12ax7 in v1 I get slight sound, if I put a tube in v2 I can only hear hum.  I have tested and changed the 6l6's using 2 different sets, both will work in my Super Reverb and I am using a weber copper rectifier.  I have also tried other rectifier tubes.  I have the input jacks grounded to a star ground and all pots soldered to the baks then go to the star ground.  All center taps from the PT are grounded at the PT bolt.

I am at a loss as where to look next.  Is it possible the OT is not compatible.  I am thinking the OT is fine, I just don't know where to check next.  Any help would be appreciated.

Thanks

[HotBluePlates]

...  Is it possible the OT is not compatible.  ...

Almost certainly not. While there have been one or two cases of bad, new OT's experienced by forum members, it is so extraordinarily rare that you'd want to rule everything else out first (unless you have a spare OT on hand).

With a 12ax7 in v1 I get slight sound, if I put a tube in v2 I can only hear hum. 

I'm not sure what you mean here.

Is that, with tubes in both the V1 and V2 positions, you get sound, but with V1 pulled, you get only hum? If so, this is normal, except for the hum. A properly working amp might have a little hiss, but shouldn't have any obvious hum, especially if the input tube is pulled.

A first normal step would be to measure B+ voltages at all the tubes, while taking into account the difference between your B+ at the output of the rectifier and the voltage listed for this point on the original schematic. Let's say for a sec they are close. You might then expect a variation of a few 10's of volts in either direction on the plate of any tube, and maybe a few 1/10th's of a volt to a volt on the cathode of most tubes (more on a phase inverter or cathode-follower cathode). If you're off by 100's of volts on a plate, or several-to-10's of volts on a cathode, you have a problem with the wiring in that stage, or in the power supply node feeding that stage.

A possible next step would be to clip your meter's ground to the chassis, and carefully probe tube plates and grids with one hand, with the meter set to measure voltage (set it to the highest scale you have). You'll hear a pop through the speaker that might startle you the first time or two you do this.
-   Start at the output tube grids. The pop will be smallish. If you hear a pop through the speaker, everything is good from that point to the speaker (most likely). Move back to the phase inverter plate. The pop should be essentially the same, as there is only a coupling cap between them.
-   Move to the phase inverter grid(s). The pop should be louder, due to the gain provided by the phase inverter. If you hear no pop, or its not louder, investigate that stage.
-   Continue working backwards towards the input jack. Each earlier stage should create a louder pop compared to a later stage (earlier stage plate roughly equiv to later stage grid), except when the stage in question is a cathode follower, or you have volume or possibly tone controls between stages.

I have checked and double checked resistor values, found many mistakes.  Also found some wiring mistakes.

Sad to say, there are probably more. Or poor grounding, or poor soldering, etc. Hoffman's Law states, "If it was wired properly, it would be working right now." As much as I'd like to say that's a jerk thing to say when my own amps aren't working, it's been true every time. Ultimately, the error turned out to be a wire to a wrong eyelet, a 470k where a 470 ohm should be, etc.

Consider printing a copy of the schematic and layout, and checking your amp against them. Mark off each part/wire as you systematically check each one. Be critical of your own sloppiness (we all get in a hurry in the excitement to complete an amp). You will likely find a build error that might be difficult to troubleshoot otherwise.

If you're stumped still, post your voltages at every tube pin.

[Ed_Chambley]

Thanks for the input.  I have traced many of the problems and now have the amp playing.  Running down all the voltages pointed me where the problems were.  They are now repaired.  I have some tweaking to do and some questions about them.  My voltages are not as high as the fender schematic suggests.

My filter caps are not in the 400 range, they are in the 330 to 300.  I am using a 5u4gb tube.  My rect readings on 4 and 6 are 338 acv on both.

Aldo the reading at the bias cap is -21. Fender schematic says -40.  I am using the Bias adjustment setup from Ceriatone 5F4 layout and I have a 6k8 resistor, 1n4007 diode and a 47k resistor back to the positive side of the cap.  I am reading 49 acv on the bias tap.  I don't know if the pot should change the negative reading on the negative side of the 100/100 cap, but it does not.  Should I reduce the resistors, if so which one?  also I did get the thing hot from a wiring problem.  Not a short as my current bulb was dim.  The problem was from the wire coming from the intersection of the 2, 220k that feed the diode has fallen off. Not sure what this is called.  Also in checking the diode it reads 1.87 and I am wondering if it out of spec.

I also have some noise.  Should I use shielded wire on the inputs going to v1, pin 2 and 7?  Is there anywhere else where shielded wire helps reduce noise.  I know some of the noise is from wires that are too long, but I did this intentionally as I plan to some rewiring.  Since this was my first build, there were lots of things I was unsure about, so I made all wires long in-case I needed to relocate them.

On another note, the amp sounds really good.  I cannot i9magine how good it will sound once I have the proper tubes, bias and voltages.

[phsyconoodler]

Ok,FIRST OFF STOP READING VOLTAGES IN ACV!!!

The amp uses DC volts after it's rectified.
  It may be a typo,but please use the correct terminology or it's hard to help you.If you found 'many mistakes' it's very likely you have some more.If it was wired right it would be working,wouldn't it?

[sluckey]

My filter caps are not in the 400 range, they are in the 330 to 300.  I am using a 5u4gb tube.  My rect readings on 4 and 6 are 338 acv on both.

Aldo the reading at the bias cap is -21.

I suspect the low bias voltage and low filter cap (B+) voltages are related. It's very possible that the low bias voltage is allowing the 6L6s to conduct very heavily, loading the B+ down.

The problem was from the wire coming from the intersection of the 2, 220k that feed the diode has fallen off. Not sure what this is called.

Yikes! that's the bias to the 6L6s. Your bias voltage was not just low, it was totally missing! This can cause the output tubes to really conduct heavily.

I suggest you pull the 6L6s and lay them aside. Don't put them back in until you can measure approx. -40vdc on pin 5 of each output tube socket. Also, with the tubes pulled, measure the B+ on the filter caps again. The readings will probably be above 400vdc now.

Fix the bias voltage. The Ceriatone layout works. Double check your wiring and be sure the positive end of the bias cap is connected to ground.

One other thing. That choke has all B+ dc current flowing through it. This includes the high plate current as well. You need a heavy duty choke that can handle this current. Are you using the correct choke?

Which layout did you use for the amp?

[Ed_Chambley]

Yikes! that's the bias to the 6L6s. Your bias voltage was not just low, it was totally missing! This can cause the output tubes to really conduct heavily.

I did remove the tubes as they got very hot.  I tested them and they are trash.  Doesn't matter, I was using some very old Sovtek 5881.  It ruined the rectifier tube as well.

I suggest you pull the 6L6s and lay them aside. Don't put them back in until you can measure approx. -40vdc on pin 5 of each output tube socket. Also, with the tubes pulled, measure the B+ on the filter caps again. The readings will probably be above 400vdc now.

Fix the bias voltage. The Ceriatone layout works. Double check your wiring and be sure the positive end of the bias cap is connected to ground.
The positive end is connected to ground, but not directly, it is attached to the filter cap which is connected to ground.

One other thing. That choke has all B+ dc current flowing through it. This includes the high plate current as well. You need a heavy duty choke that can handle this current. Are you using the correct choke?

I am using MOJO777, Mojotone choke for 2 6L6, the one that comes with their 5F4 kit.  I got a build list of materials from them.  I sourced parts from Hoffman and Weber.

Which layout did you use for the amp?
I used the Weber 5F4 layout.

[Ed_Chambley]

In the Ceriatone layout, what is the purpose of the 47k resistor attached to the positive side of the cap?

[sluckey]

It is part of the voltage divider for the bias voltage. It sets the lower limit that the bias voltage can be adjusted with the pot.

Did you get the bias voltage up to about -40vdc yet? Is the B+ voltage higher with the 6L6s pulled?

[Ed_Chambley]

I am at work.  I will have to do it tonight.
Is it possible the diode could be causing the problem?  I did install it backwards to begin with.  I checked the resistance and get "0" and 1.875.  I do not have another 1n4007, but I do have a 1N5408/1000V but I don"t know if it is compatible.  If I am correct, all you have is a 6k8 resistor to a 1n4007 to achieve the -40.  What else could it be other than the resistor or diode?

[sluckey]

Is it possible the diode could be causing the problem?  I did install it backwards to begin with.

Sure, the diode could be bad. Just replace it with the 1N5408. The 5408 is tougher than the 4007.

If I am correct, all you have is a 6k8 resistor to a 1n4007 to achieve the -40.  What else could it be other than the resistor or diode?

The  cap has definitely been stressed and is likely shot if you had the diode backwards for any length of time, so replace it too. The pot and the 47K resistor are necessary too, but they are not likely damaged.

[Ed_Chambley]

I have already replaced the cap.  It was damaged.  Thanks for the suggestion and also letting me know the diodes are interchangeable.

[Ed_Chambley]

Is it possible the diode could be causing the problem?  I did install it backwards to begin with.

Sure, the diode could be bad. Just replace it with the 1N5408. The 5408 is tougher than the 4007.

Replaced the diode and now have -49 dcv at the bias.

If I am correct, all you have is a 6k8 resistor to a 1n4007 to achieve the -40.  What else could it be other than the resistor or diode?

The  cap has definitely been stressed and is likely shot if you had the diode backwards for any length of time, so replace it too. The pot and the 47K resistor are necessary too, but they are not likely damaged.

[Ed_Chambley]

Now all the voltage readings are correct.  Thanks for the help.

Now, I have a couple of other things to iron out.  I installed meter taps to check the bias on the back of the amp.  I wired the hoffman resistors 1 ohm 1 watt resistors 1% tolerance through pin 1 and pin 8 then to ground then to the bias probes.  The red and black things, not sure what they are called.  Each 6l6 has one going to red, a total of 2 and 1 black going to ground.  I should be able to measure MA, but I get nothing but 000.  Is it correct to ground them and then measure MA.

Next, can you install a channel jumper switched across pin 2 and 7 of v1, or will it be out of phase?

[sluckey]

I should be able to measure MA, but I get nothing but 000.

Set your meter to measure millivolts, not milliamps.

Next, can you install a channel jumper switched across pin 2 and 7 of v1, or will it be out of phase?

Sure, go for it. Phase is not a factor.

An easy way to do this is plug your guitar into the #1 (hi) input of a channel. Then connect a patch cord between the #2 (low) input of that same channel and the #1 (hi) input of the other channel.

[Ed_Chambley]

Is it normal for the bass pot to control so much of the volume in the 5F4 circuit?  If I turn the bass to all the way down, there is almost no volume.  Also, the presence control seems almost useless. Any suggestions on a way to liven it up?

Also, shielded cable.  If I install it from the inputs to v1, pin 2 and 7, the shielding should be shrink wrapped and not connected at the tube.  Where would the shielding be connected at the inputs? I am assuming at the G/sleeve as this is where the shielding from the instrument patch cord is connected.  Is there any value in using shielded cable anywhere else in the circuit?

I have read where using shielded cable from the inputs to v1 has drawbacks, but I cannot understand how there can be nothing but benefit.  Your guitar patch cord is shielded so in essence to continue that shielding to the tube seems like a no brainier.

I have some NOS 12Ay7's on the way, right now I have tried ax7 and av7.  The 12Ax7 seems not quite right and the av7 I left in as they cleaned up the amp, but lack punch.

As for power tubes, I collect old super reverbs and It seems the Winged C's sound best.  I have some RCA Blackplates which are considered the holy grail of the 6l6, but I think the Winged C's got them beat in Super Reverbs.

What do you like in a 5F4?

[stingray_65]

Also, shielded cable.  If I install it from the inputs to v1, pin 2 and 7, the shielding should be shrink wrapped and not connected at the tube.  Where would the shielding be connected at the inputs? I am assuming at the G/sleeve as this is where the shielding from the instrument patch cord is connected.  Is there any value in using shielded cable anywhere else in the circuit?

That is correct, it is common practice to ground the shield at the source end, so attach the shield to the sleeve lug.

A properly shielded cable will be grounded on one end only to prevent ground loops.

I have not heard of any disadvantages to using shielded cable (in this application) except cost and complexity of labor. (very minor in this case)

In high gain amps  it is not uncommon to shield wires to and from the control pots. Those (shields) are typically grounded at the pot or in most cases the controls will have a ground bus.

Also in high gain pre amps, it is not uncommon to attach the grid stop resistor directly to the pin of the pre amp tube

Ray