Prototyping issues

[Paul1453]

It seems that any time I try to proto a circuit with more than 1 12XX7 tube in the pre-amp section I have terrible hum problems.  I don't seem to have the same problems when I use say 2 6SJ7's in the pre-amp.  Is it the increased gain of the 12XX7 tubes amplifying the hum that is the issue, or more likely my wiring of the 12XX7 tubes?  I tried numerous different wiring layouts, trying to move my AC heating wires as far from the signal path as possible and still no luck.  I can make a completely quiet 6SJ7 6V6 circuit no problem.  When I try to put in some 12XX7 pre-amp tubes in, it all goes to shit.  Any suggestions?   

[Willabe]

Have a drawing of the circuit including the power supply (PSU)?

[Paul1453]

It's page 3 of the Hoffman 5E3 Tweed Deluxe, I'm trying to just post that page here.  I just cut out from C4 and below to remove the 2nd input and tone circuit.  I'm using my bench adjustable PS for the 6.3VAC and output of the 5Y3 and following the rest of the schematic as closely as I can with the parts I have in inventory.

This was also my attempt at subbing 6AQ5s for the 6V6s in a PP output circuit.  It seemed to be working, in that I could hear the guitar signal underneath the terrible hum.  It was quite a bit louder than the SE circuit I successfully did.  I only dialled the B+ up to 150V before it was too loud with the hum.

[Willabe]

I was wondering if you did have enough decoupling on the preamp tubes, but I think your fine.

Can you post a few pics of the prototype your having trouble with? It may be something in your layout that 1 of the guys might see?

[Paul1453]

If you promise not to laugh.  I have a 12AX7 mounted in the shielded set-up with cat 5e solid core network wire attached to the socket pins.  Heater wires separate from the signal wires.  I've already removed the other 12XX7 tube from the circuit in this picture.  Initially had my PS RC configuration on the little copper board showing.  Tried using one of my new can caps to get rid of the hum, no better no worse.  I am short on high wattage resisters and have to resort to cobbling together the ones I have to get something close to what is called for in the schematic, another potential source of problems.  It seems that if I were to design this to use a 6SJ7 as the pre-amp and PI the hum would go away.

[DummyLoad]

ditch the breadboad. seriously. they don't work for anything over a couple dozen volts.

--pete

[sluckey]

ditch the breadboad

Agreed.

I don't think the type of tube is the problem. How do you have the filaments connected to the 9 preamp tube? Which pins? I don't see an artificial center tap for the filament string. You need one. Pull the conductors out of that cat 5 cable so you can separate the individual wires, or toss the cat 5 altogether.

[Paul1453]

ditch the breadboad

Agreed.

I don't think the type of tube is the problem. How do you have the filaments connected to the 9 preamp tube? Which pins? I don't see an artificial center tap for the filament string. You need one. Pull the conductors out of that cat 5 cable so you can separate the individual wires, or toss the cat 5 altogether.

Breadboard replacement suggestions?

I've got pins 4 and 5 tied to the red wire and pin 9 on the green wire coming out of the cage.  I only had 1 high watt 100 ohm resistor, so I used two 400 ohm 10W resistors tied to Gnd on my PS as the artificial center tap.  Is this bad?  I can pull the wires out of the cat 5 cable to separate them.  I thought it was easier to keep them together, but can see how being so close and twisted together in the casing could be an issue.

Any other obvious newby mistakes?

[sluckey]

I only had 1 high watt 100 ohm resistor, so I used two 400 ohm 10W resistors

You don't need power resistors. Half watt is fine for the 100Ω.

[Paul1453]

1/4 W OK?  I've got an assortment of 1/4 W resistors, and some higher W ones on the way.  I thought since the heaters pull quite a bit of juice I needed the higher wattage ones for the tap.  I do realize that the juice shouldn't really go through these resistors if the heater filaments are not open though.

[sluckey]

1/4W is OK

[Willabe]

Breadboard replacement suggestions?

Some barrier strips like these, they used to stock singles, says new stock coming;

http://www.newark.com/molex/39100-1812/terminal-block-eurostyle-12-position/dp/83X8170

[Paul1453]

Thanks for the info, those look like I could figure out how to make them work as a breadboard.  I see the much higher voltage and current specs that you have pointed out that I need.  Unfortunately, they show a minimum order of 250 pieces.  I guess I'll have to look for something similar on Ebay.

[Willabe]

Yeah there out of stock but it says date when their supposed to get restocked.

When I got mine you could buy just 1 if you wanted.

Do a search for that company and the model # and specs, some else probably sells them. They make the same in a 300v rating but for pennies more I bought the 600v version.

[Willabe]

Here's a link for my bread board, scroll down for (many) pictures;

http://el34world.com/Forum/index.php?topic=18354.0

DL and Silvergun both have started threads on their bread boards with pics. Jack Hester posted his drawings for his BB in SG's 2nd thread "Son of Bread Board".

[Paul1453]

I checked for the Molex ones on Ebay, no luck.  They look like they are much higher quality than the others I've found from China.  These look like much lower quality but extremely cheap.   http://www.ebay.com/itm/10-Pcs-12-Position-Barrier-Terminal-Strip-Block-X3-1012-10A-Plastic-Terminal/231680885030?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D34008%26meid%3D7618126547fa40c8a79ee3df06fd524a%26pid%3D100005%26rk%3D2%26rkt%3D6%26sd%3D171907335464
Would they be OK?
I wish I knew about this before I ordered 5 different colored breadboards this weekend.

[Willabe]

Can you return the BB's you ordered?

They look like they are much higher quality than the others I've found from China.  These look like much lower quality but extremely cheap.

They look all right to me?     Only rated at 360v but their also rated at 5A's or more so they shold work because your running much lower current.

You have to also make sure they clamp down on a single 22gauge wire lead.

Keep looking, you'll find them.

[PRR]

> two 400 ohm 10W resistors

Learn to estimate resistor power.

We have two equal resistors across 6.3V.

We predict that each one takes half, 3.15V.

3.15V across 100 ohms is 3.15/100= 0.0315 Amps.

3.15V times 0.0315 Amps is 0.0992 Watts.

0.1W appears OK.

If you want it to work more than a month straight (a breadboard may not), double the calculation. 0.2 Watts.

The 1/4W (0.25W) part is the next-up value.

There is a got-cha (there's always a got-cha). *IF* one side of the heater gets shorted to ground, one resistor gets the full 6.3V. This works out to 0.4 Watts. Heater shorts should be rare, so the tip is to order some 100r 1/2W before a short happens. Maybe today, or maybe not until 2025, possibly never. On the breadboard, the gig is not ruined and the smoking resistor calls attention to itself, so I do agree "1/4W is OK".

But 10W was just over-over-kill (especially at 400 Ohms).

Anyway, when fooling with 6AQ7s, a handful of 100r 1/2W is handy for dialing-in your cathode bias resistor.

[printer2]

Try changing holes in your breadboard. I use one and sometimes it is a contact issue. I also leave a couple of strips between traces if the voltage between them is high.

[Paul1453]

Try changing holes in your breadboard. I use one and sometimes it is a contact issue. I also leave a couple of strips between traces if the voltage between them is high.

This is why I ordered 5 different colored boards last weekend.  My idea was to separate different parts of the circuit on to different boards.  Also to isolated the B voltages to a separate board and only attach the voltage needed to the separate circuit boards.  It only cost me $2.92 including shipping for these so if this separation idea does not work as planned I'm not out much.  I will be getting some of the higher voltage/current strips soon.

[DummyLoad]

barrier strips with the wire clamp screw are easier on the part leads. below is 10 position part.

1-1546307-0

http://www.onlinecomponents.com/

--pete