5f1 new build volume question

[cubby01]

Hi all.
I had built this amp as a head in an odd chassis and was having some issues... (covered in this thread http://music-electronics-forum.com/s...ead.php?t=7293)
...so I totally rebuilt it in a self fabricated chassis and cabinet. Used all the same parts. Just unsoldered everything and put them into a new fiber board and chassis. Generally followed the weber layout. Really happy with the way it turned out aesthetically but now have an issue (maybe) with the output.

With this new build the volume just isn't anywhere close to where it was before. I didn't check it with a meter but in it;s earlier incarnation it was plenty loud for a small practice amp, almost too loud for what I wanted. But now I keep reaching for the volume pot on the guitar wanting to crank it up more.

I swapped out the 6V6 and the 12AX7, no change. Where should I look next?

Thanks,
Cubby

[Paul P]

It would help a lot if you supplied a list of the various voltages throughout
your amp. Power transformer secondaries, B+ at the different power supply
nodes, plate voltages and cathode voltages. Clip your multimeter's ground
lead to a good ground and take your measurements with one hand in your
pocket or behind your back (and don't move it from there for any reason).

An amp is just a series of voltage amplifiers (up to the output transformer)
so any lack of output caused by something wrong before the output
transformer should show up in the voltages somewhere.

Are your filter caps holding their charge this time 'round ? Make sure
you have a dependable multimeter.

Paul P

[cubby01]

Hi Paul, Thanks up front.

All measurements with negative meter lead clipped to ground.
Attached here is a labeled layout with the readings and where I took them.

Power transformer secondaries
5Y3GT - pin 4 : 320 vAC
5Y3GT - pin 6 : 320 vAC

B+ at the filter caps: +262 at 16uf ; +313 at 10uf ; +268 at 2nd 10uf (see attached)

6V6
Plate (anode) - pin 3 : +352 vdc
Cathode - pin 8 : +17 vdc

12AXT
Plate (anode) Triode-2 - pin 1 : +169 vdc
Cathode Triode-2 - pin 3 : +1.4 vdc
Plate (anode) Triode-1 - pin 6 : +165 vdc
Cathode Triode-1 - pin 8 : +1.4 vdc


I've got some pictures of the cabinet and the guts that I'll post later.

I still see the +B voltage drain away when I turn off the amp. It's not an instant drop. I was told in another forum that this is not uncommon if the tubes are hot.

I have checked things with a second meter and I'm convinced it's not a bad meter.

Anyway after the rebuild it sounds better. There isn't the hard crackup I was getting earlier. Tone seems better. Just not loud as I've heard these should be. The rectifier and 6v6 do get quite warm after playing a bit.

Again, thanks for your help.

[cubby01]

Hmmm, noticed something else. I've got the heater wires in parallel to the filaments on the 6v6 and 12ax7. Had artifical center tap off the 6v6 running to ground.

I checked the heater voltages and they were only reading 3.0 vAC or so on each side. So I moved the center tap grounding to pin 8 on the 6v6 for "elevated ground". That brought the them up to 4.6 vAC. More volume but also brought up the noise floor from something ramdom scratchy noise. I think I may go back to a traditional 50s filament wiring and see what that does.

[Paul P]

I wrote the following just before you posted your last message :

Everything looks pretty good to me, your voltages are just about perfect.
So what was your problem again ?

I don't know what you're comparing with but I wouldn't expect your 5F1
to be as loud as later Champs which might have 200v or more on their
plates. Maybe you could try different speakers.

And you could disconnect the 22k feedback resistor and see what happens.
SHould be louder and may sound better (but may be noisier).

-----------------

Ok, now for your last post :

I don't think elevating your heater circuit should increase the AC, unless
there's already some on your power tube cathode. Did you have some
hum at the output before doing this ?

What are you using for a volume pot ?

Post some pictures of the insides if you get the chance.

Paul P

[cubby01]

But the heaters should be showing 6.3 vAC right? That puzzles me.

Hum before was minimal. Fairly happy with the level of hum. There is some but not really an issue for me.

I had earlier disconnected the feedback. Was only slightly louder and not as clean.

The volume pot is from an old tube console. It's a 1 meg audio taper I had on hand so I used it.

Here are some pics. These were taken after I added the center tap on the heaters and ran it to ground - so was before I ran the artifical center tap to the 6v6 cathode.

[tubeswell]

Nice looking amp. All it needs to make it complete is a carry handle ;-)

On the heater qn, when you had the heaters wired in parallel, did you connect pins 4 and 5 of the 12AX7 together on one side (with one wire) and pin nine with the other wire? If you only had either pin 4 or 5 connected, you would only be warming up one of the cathodes (which would probably account for a volume drop).

Also where you have got the heater circuit running, doesn't look to be far enough away from some of the signal path wires in one of you rpics. Try edging the heater wires further away from the signal wires, or perhaps better still have the heater wires positioned differently so they are tucked down the bottom at the rear (open end) of the chassis well away from the rest of the wires

[Enzo]

The tubes need 6.3v ACROSS the heater, NOT 6.3V to ground.

The center tap of the 6.3v winding on the power transformer is exactly that - the center of the 6v. There are then 3v to either side of that center tap, so if you ground it, there will be 3v to either end. But at the tube ther will still be 6v across the tubes. There is still 6v from one end of that winding to the other.

If the tubes light up and the amp makes sound, then the heaters are working.

[Paul P]

Nice amp. Champs don't have much in the way of ventilation for the
tubes, do they.

What kind of soldering iron are you using ? I ask because some of your
joints look as if the iron wasn't quite hot enough for the solder to flow
properly. This can lead to high resistance joints. For example, in your
last picture, the four eyelets at the top right of your board, the solder
didn't flow well into the eyelets but stayed more or less on top with a
blob formed as you lifted the iron. The eyelets should be filled with a
puddle that will flow flat when you remove the iron.

For some reason a soldering iron heats much better right after you
tin the end with some solder, which probably makes for better contact
between iron and parts. The procedure I follow for each joint, or maybe
two if they're close together, is : wipe the tip on a wet sponge to clean
it (cutting some slices into the sponge with a razor will help) , tin the tip
with some solder, shake off the excess solder, immediately apply the tip
to the part, wait maybe a second, then start feeding the solder onto the
part, not the iron tip (or maybe right at the junction of the two). Repeat
for the next joint. Don't blow on the joint after.

I use a 50w Weller WS51 soldering station set at about 75%. This works
much, much better than the cheap 25w soldering pencil that I used before
which required waiting after each joint for it to get back up to the proper
heat (which you'd lose the minute you cleaned the tip).

Solder containing lead is easier to use than the new silver solders.

Paul P

[cubby01]

Nice looking amp. All it needs to make it complete is a carry handle ;-)

Thanks much. Ha, yes, handel is next on the list. I was going for a rustic look and think I achieved it. The resistors and caps are new but most everything else in the amp is close to 50 years old. The pine board maybe 20+ was already stained and I just touched up the round-overs and cut edges to match.

On the heater qn, when you had the heaters wired in parallel, did you connect pins 4 and 5 of the 12AX7 together on one side (with one wire) and pin nine with the other wire?

Yes and I checked continuity and even swapped tubes.

Also where you have got the heater circuit running, doesn't look to be far enough away from some of the signal path wires in one of you rpics. Try edging the heater wires further away from the signal wires, or perhaps better still have the heater wires positioned differently so they are tucked down the bottom at the rear (open end) of the chassis well away from the rest of the wires

Maybe an artifact of the photo. They are fairly well away and hum is low. And pushing the wiring about with a chopstick doesn't seem to highlight any issues.

The tubes need 6.3v ACROSS the heater, NOT 6.3V to ground.

Enzo, Thanks I should have known that

I'm going to move the heater CT grounding from the cathode back to a straight ground. Tone seems better there and for some reason it appears to raise the vDC voltages measured at the caps. Not sure why. I haven't yet double checked this. Will this evening.

[cubby01]

What kind of soldering iron are you using ?

I'm probably using the wrong tool for the job but it's a older Weller soldering gun. Not looking at it right now but think it's an 8200. Gets real hot. Maybe too hot. I've got a a cheap pencil style iron from Rat Shack that I've never been happy with. Will take your advice and reflow some those joints.

[cubby01]

Regarding the high voltage readings. Paul you hinted several times you thought it might be the meter. I'm now convinced it is the meter. Piece of crap meter it is too. Seems the battery in the meter was low and reading incorrect voltages. I'm confused as to why it read correctly some times and not others but I'm not depending on it any more.