Homebuilt 5F2-a Squeal

[Tele_MEL]

Hi,

I just completed my homebuilt ( not a kit ) 5F2-a, and things are pretty good... but not without a problem.

There is a squeal related to the tone pot.

With NO guitar plugged in, tone pot turned all the way left<down> - Volume turned all the way down, a squeal appears immediately on turning the volume up.

WITH a guitar plugged in - and the tone pot turned ALL THE WAY DOWN, I can turn the volume up to about 2/3 before the onset of squeal, which increases if I continue to turn volume up.

There is a sliding relationship to - the more I turn the tone UP, the less I can turn the volume up before onset of squeal.

If I stay at settings that don't create the squeal, the amp sounds good, and gets very loud ( through a 12" speaker, ) and is very quiet ( no hum ) .

Some data:
I did wire the NF ( with 22k resister ) to a switch so I could take it out of the circuit. If I do, it INSTANTLY squeals LOUD.

6V6 Plate voltage is 380

I'm using a cap can, with 20uf 450, 20uf450, 10uf 450 and 10uf 450.

After the 4th cap, voltage is 240 volts,

and after the 100k dropping resistors... the 12ax7 plate voltage is about 105 volts ( this seem low ? )

One of the only deviations I know of was
V1a 12ax7 cathode bias called for 25uf cap, I used 22uf ( with 1.5k resistor )

V2 cathode bias called for 25uf cap and 470 ohm resistor - I used 22 uf and 500 ohm.

Other than that I I think was accurate with parts and wiring.

Aside from suggestions forthcoming, would it be a troubleshooting approach to just remove the tone pot from the circuit altogether?


Thanks in advance for any suggestions

[mojo2001]

Is the case of the tone pot grounded?

It needs to be grounded.

[getbent]

oscillations! whoa yeah!

Check your grounds as the good Mojo2001 said. Make sure they are really grounded (chopstick test should work)

I suspect you may have your input jacks wired backerds. Double, triple check that'n.

Be really careful... discharge before working and maybe just use one hand!

I found adding a grounding post for my vol & tone took away my oscillations.

here is a question--> what happens if you plug a guitar in? how about input 2?

[Guitarslinger1]

Any chance you could post photos of your wiring?

Also, though it may not seem "50s vintage cool", using shielded wire on the preamp tube grids (pins 2 and 7), kept as short as reasonably possible, can save a lot of headaches. Unfortunately, I think your parasitic is caused by something more than that will fix.

As Get Bent suggested, double check that wiring.

[Tele_MEL]

Hi

Thanks so much for reiterating discharging the caps, ( am doing that right now ). Can't be too careful, as there is only one of me...

I will take some photos. in the interim:

Here is a verbal of the wiring of tone/volume pots: ( 1 meg linear )

viewed from rear of the pot - I'm calling three tabs, from left to right 1 , 2 and 3.

Volume:
Tab 1 ( leftmost ) - I've run a wire to ground. I did NOT wire it to the pot case!
Tab 2 ( middle ) - Receives wire from Tone tab # 3 ( has 500uf cap ) - and continues to pin 7 of 12ax7.
Tab 3 ( rightmost ) receives wire from pin #1 on 12ax7 ( thru the .02 / 400 cap )

Tone:
Tab 1 ( leftmost ) I've run a wire to ground thru a .005uf cap ( Did NOT wire it to the pot case! )
Tab 2 ( middle ) Receives wire from Tab 3 of volume.
Tab 3 ( rightmost ) Goes thru a 500pf cap to tab # 2 on volume.

Re: Jacks.
Same situation occurs with guitar into both jacks, can't turn the volume up as much with jack # 2.

and I no issues re: ' vintage ' .. . ( well at least not for this particular project) ... if shielded wires to grids helps- I'll do it.!

[Guitarslinger1]

Originally Posted by Tele_MEL

Hi

Thanks so much for reiterating discharging the caps, ( am doing that right now ). Can't be too careful, as there is only one of me...

I will take some photos. in the interim:

Here is a verbal of the wiring of tone/volume pots: ( 1 meg linear )

viewed from rear of the pot - I'm calling three tabs, from left to right 1 , 2 and 3.

Volume:
Tab 1 ( leftmost ) - I've run a wire to ground. I did NOT wire it to the pot case!
Tab 2 ( middle ) - Receives wire from Tone tab # 3 ( has 500uf cap ) - and continues to pin 7 of 12ax7.
Tab 3 ( rightmost ) receives wire from pin #1 on 12ax7 ( thru the .02 / 400 cap )

Tone:
Tab 1 ( leftmost ) I've run a wire to ground thru a .005uf cap ( Did NOT wire it to the pot case! )
Tab 2 ( middle ) Receives wire from Tab 3 of volume.
Tab 3 ( rightmost ) Goes thru a 500pf cap to tab # 2 on volume.

Re: Jacks.
Same situation occurs with guitar into both jacks, can't turn the volume up as much with jack # 2.

Where exactly is the ground you are using in lieu of the pot housings?

And the cathode cap and resistor connected to pin 3 of the 12AX7, where are they grounded?

Did you use a brass plate under the controls, or a ground buss, or...?

[mojo2001]

I'd say take a bare wire and solder it to both pot cases then connect it to the chassis as a first step.

An ungrounded pot case is adding spurious capacitance at a high impedance point in the circuit which can create freaky effects, including hum and oscillations. This needs to be done even if it doesn't solve your major problem.

Ground leads must be SHORT, or else they are not really grounds at high frequencies!! A couple inch lead to ground from the pot is a big no-no!

I would go with the Fender layout, grounding to pot cases, then run a 18 ga or so wire between the cases to chassis ground.

As Specialty insinuates, the ground for those pots should be at the same potential as the cathode grounds for the preamp tube, and the input jack ground should also be at the same potential.

A thick wire, plate, or piece of copper tape can do it properly. The chassis can be used to connect those points but this can be unpredictable compared with a high conductivity, low impedance ground plane or ground buss.

[Tele_MEL]

I used terminal strips for some of my parts,

I sanded the chassis where the mounting tab are screwed to the chassis, and used them as grounding points.

The Volume pot left tab has about a 3" solid core wire running to one of the mounting tab sold lugs,

and the Tone Pot left tab has about a 4" wire running to a different mounting tab.

The tone pot ground wire does run very close to the 6V6 cathode bias resistor / cap pair...

[Guitarslinger1]

Originally Posted by Tele_MEL

I used terminal strips for some of my parts,

I sanded the chassis where the mounting tab are screwed to the chassis, and used them as grounding points.

The Volume pot left tab has about a 3" solid core wire running to one of the mounting tab sold lugs,

and the Tone Pot left tab has about a 4" wire running to a different mounting tab.

The tone pot ground wire does run very close to the 6V6 cathode bias resistor / cap pair...

OK, like Mojo is suggesting, run a ground wire (buss) from the input jack grounds, across the backs of, and soldered to, both of the pots. Ground everything preamp related to this buss.

Solder the volume pot leg that gets grounded to the volume pot housing, and solder the tone control cap to the tone control housing.

[Tele_MEL]

Aye Aye! Will do as suggested, and advise!

Thanks for the fast responses.

MEL

[Guitarslinger1]

Originally Posted by Tele_MEL

Aye Aye! Will do as suggested, and advise!

Thanks for the fast responses.

MEL

Sure thing.

And remember to keep all of those wires coming off the board to the ground buss as short as possible.

The preamp filter cap ground goes to that buss as well.

[Tele_MEL]

OK. This will be long, but will try to provide as much as I can to anyone with the patience to read thru it.

Some updates. Still a squeal

First I added a ground lug right below the tone pots.
Since I didn't have a heavy enough soldering iron ( at the time ) to solder to hte pot cans, I just ran the grounds from the pot lugs to this new ground lug.

And I ran the ground from pin 9 of the 12ax7 to this new lug.

The Filter Caps I am using are in a can, and there are 4 ground lugs, and my understanding is the lugs are all common, but I still wired the cap can ground lug nearest the 4th cap to this new ground lug.

Fired it up, and it hummed BAD, ( whereas it had been very quiet before ) and no improvement on the squeal.

So I undid all those changes.
Ran the 12ax7 pin 9 ground back to the terminal strip where it was before.
Cut the wire that was grounding the 4th cap can ground lug. ( 2 of the four cap can ground lugs remain grounded to a terminal strip mounting lug).

I borrowed a heavy soldering iron, and wired a bare wire across the pot cans and connected it to the ground lung on Jack#1; and I wired the pot terminals ( left hand of each pots 3 terminals ) directly to the pot can ( the tone pot going thru it's .005 cap )

I cleaned up and shortened some other wiring:
Ran the Center Tap of the PT secondary right to a ground lug attached to a transformer mounting bolt ( where it had been connected to a terminal strip mounting lug)
I wound the 6.3 AC filament pair cleaner and tighter all the way to the 6V6, and connected one of the wires to a tube socket ground lug, where it had been wired to a terminal strip mounting lug.

Powers up fine, gets loud, and I'd say very quiet -
but:

WITH GUITAR PLUGGED INTO EITHER JACK.
With the Tone to ZERO, can only turn up volume to about 1/2 before onset of squeal.

With tone to 1/4, can only turn up volume about 1/4 .


NO GUITAR:
With tone to zero, can't turn volume hardly at all, and big squeal.

and: If I turn OFF the NFB, instant ear piercing squeal!

I am working in very small space, but I know my wires can be shortened!
and I note the input jacks are VERY close to the tone pots.

Pictures:

Initial wiring.
http://picasaweb.google.com/marklahaie/5F2AInitial

After this first batch of mods.
http://picasaweb.google.com/marklahaie/5F2AAfterWork

It's a great learning experience, so thanks for any help, and will keep everyone posted!

MEL

[Guitarslinger1]

Mel,

I don't know where to start.

No offense, but that lead dress needs to be tightened up quite a bit. It's unlikely the amp will work with a rat's nest of wiring like that. You pretty much have a box of noise antennas going on there.

For the most part the wires, especially the preamp wires need to be as short as possible, and running along the chassis when possible.

I'll see if I can find some photos of proper lead dress.

Maybe some others can chime in here with ways to explain how to clean up that lead dress.

[Wally]

I am curious as to the suggestions and cure on this one, so I am overtly lurking....
Good luck with this one, Mel.

[Tele_MEL]

No offense taken!

I took a previously begun project ( chassis, cap can, tube sockets and transformers all mounted )

and wired according to the schematic.

Tried to replicate as much of a 'tag board' component placement as possible with the terminal strips , but little room, and wire runs to and from pots, tubes dictated by their locations.

Even if I just use the Transformers and tubes and completely get a new chassis, and use a turretboard layout - I've learned a lot!

Tnx,
MEL

[Guitarslinger1]

Originally Posted by Tele_MEL

No offense taken!

I took a previously begun project ( chassis, cap can, tube sockets and transformers all mounted )

and wired according to the schematic.

Tried to replicate as much of a 'tag board' component placement as possible with the terminal strips , but little room, and wire runs to and from pots, tubes dictated by their locations.

Even if I just use the Transformers and tubes and completely get a new chassis, and use a turretboard layout - I've learned a lot!

Tnx,
MEL

Mel,

If you could, it would sure be easier for you to start with a genuine 5F2 chassis. The arrangement of the components is such that it facilitates (with a few exceptions) proper, short lead dress.

Then all you'd have to do, is follow the diagram. Notice there are no preamp wires running through the air. Not that that can't be done, but in general it's best if leads are short and running along the chassis, with as few crossing each other as possible.

[Tele_MEL]

Yep, a specific chassis and turret board placement according the layout will be the next project.

I had this layout printed out, and used it to visually confirm component connection as I looked at the schematic - as I'm also trying to understand how the electrons are moving! in particular trying to get my head around how B+ voltages and signal voltages can share wiring.

FWIW, the wires were cut and organized to all run along chassis wall ( that's why they're longer than need be ) and aren't 'flying' in mid air when tested.
I did pull the wires out for the photos, just to help note what is connected to what.

Actually - continued learning will be to take this chassis, and not try to mimic the ' turret board ' layout of parts, ( which is forcing me to have long wire connections ) ..

I'll just do point to point - and absolutely try to minimize lead dress.

I'll sure get my soldering practice !

Thanks for looking!

MEL

[mojo2001]

OK...you're getting there. It is a mess but kudos for sticking with it. You will be an amp builder before you know it, in fact you are likely already off the deep end. .

You have to shorten the leads on the input and that first 12AX7 section.

Try this...

1) Ground BOTH jack grounds off that buss you ran across the pots. Run a thick wire from the jack grounds to the cap can.

2) Take the cathode resistor and cap and connect it from the tube socket socket terminal to the closest terminal strip lug (marked #1). Then run a wire to the cap can lug from lug #1. Observe correct polarity. SKIP the nice looking terminal strip scheme and go for SHORT.

3) Take the 68k input resistors and connect directly from input jack terminals to terminal strip lug marked # 2, then on to grid of 12AX7.

Better yet-- go from the jack straight to the tube socket grid terminal with the 68ks. Might get a bit tight in there.

Right there, you save about 10 inches of wire.

You can also scrap one of the input jacks for now until you get it working and go straight into the 12AX7 grid from the jack with a 68k.

It is also a good idea to turn those pots around to get the terminals as close as possible then use the shortest wire you can, run along the chassis.

Make sure the cap can, which is your common ground, makes contact with the metal of the chassis somehow. Sand off the paint or whatever.


PM me if you need specific advice.

Joe

[Tele_MEL]

Thanks mojo2001,

I agree I'm hooked on the amp building thang.. . Great fun. and I've yet to stand back and be the proud papa ...

After initially being bummed - it's quite clear to completely abandon the terminal strip/turret board style layout and just connect the parts in as short a distance as possible.

Since I know what is supposed to be connected to what and where, no reason not to just connect direct to Jacks or Sockets - with maybe just one stop at a terminal strip if necessary.

Means tearing it apart and rewiring basically everything... but .. so what. That's the whole idea is to learn what works, and what doesn't, and WHY when it doesn't work!

If the assessment is too long of leads.. - that can be remedied - since everything is very close..

I will repost as I make progress!

Thanks for your help, and I'll give you a holler if I get myself into a corner!
MEL

[mojo2001]

MEL:

Yeah, terminal strips look nice but there has to be electronic logic in the layout too.

Note how in turret board Fenders like tweed Champs there are tubes on one side, jacks and pots on the other, and the board sort of bridges the gap. In your layout, the terminal board is off to the side and you have to run long leads to get to it.

BIG DIFFERENCE.


The first hi-fi amp I built 25+ years ago, I used these cool terminals that I found at a flea market to lay everything out. Fired it up and BLAM, smoked a fuse. Could not get it to work.

Only after a LOT of head scratching did I figure out that the "terminals" were actually unmarked stud mount diodes and the amp was shorted to ground in about 25 places!!

In your case, start at the input and shorten everything, triple check grounds, and you should be cool.

You can also simplify by using one input jack, take out the tone circuit, and just get the dang thing to work, then put those extras back in.

If you fix the first stage and the tone circuit grounding and layout, the rest of the amp should be relatively insensitive to these probs and should work OK.

This is the way you're supposed to learn. Try and screw up, ask questions, etc.

I used to run a tube audio DIY magazine. I saw 100s of folks go from zero knowledge to crazed, highly advanced builders over the years. It is not that hard but it is a new thing that takes some mental adjustment and basic knowhow.

Getting started is being half way there...

[Guitarslinger1]

Originally Posted by mojo2001

OK...you're getting there. It is a mess but kudos for sticking with it. You will be an amp builder before you know it, in fact you are likely already off the deep end. .

You have to shorten the leads on the input and that first 12AX7 section.

Try this...

1) Ground BOTH jack grounds off that buss you ran across the pots. Run a thick wire from the jack grounds to the cap can.

2) Take the cathode resistor and cap and connect it from the tube socket socket terminal to the closest terminal strip lug (marked #1). Then run a wire to the cap can lug from lug #1. Observe correct polarity. SKIP the nice looking terminal strip scheme and go for SHORT.

3) Take the 68k input resistors and connect directly from input jack terminals to terminal strip lug marked # 2, then on to grid of 12AX7.

Better yet-- go from the jack straight to the tube socket grid terminal with the 68ks. Might get a bit tight in there.

Right there, you save about 10 inches of wire.

You can also scrap one of the input jacks for now until you get it working and go straight into the 12AX7 grid from the jack with a 68k.

It is also a good idea to turn those pots around to get the terminals as close as possible then use the shortest wire you can, run along the chassis.

Make sure the cap can, which is your common ground, makes contact with the metal of the chassis somehow. Sand off the paint or whatever.


PM me if you need specific advice.

Joe

+1 Right on Mojo. Well done.