5E3 Almost there, just a couple of questions before power up.

[Leif H]

I'm all stoked, after a month, I've got my transformers! Ipve pretty much built this from scratch, the Chassis was from ebay, I built the cabinet from scratch, finger jointed pine, tweeded myself with two coats of amber shelac.
there is a sag in the grill cloth that I need to tidy up, but the remainder seems
Tonight, I'm going to be putting them in the chassis, and powering things up.

I have two things I'm wanting to do before I finish, first is a half power switch, which I've copied from this forum, Geezer's circuit diagram.
I'd like to know if the 100k & 330ohm resistors need be anything other than 1/2 watt, as the two bias resistors are going to be 5w each?

Secondly, I just would like to check on a the power up procedure.

I have this in mind, and correct me if I'm wrong;

Power up without tubes, check heater voltage,
Install rectifier and check voltage A, B and C voltages (working from Weber's circuit diagram),
then install the tubes and check idle current on power tubes.

Lastly, when checking the voltages, are both the volume's set to full with no input?

[Leif H]

Here's a picture it turned out almost as nice as I wanted. the worst thing is that when you build something, you know where each flaw is, and have all the "should've" issues.
The tweed box to the left is to house my tube screamer clone which is the next project.

[Jazz P Bass]

I would think that you would want to check the cathode voltage of the output tubes. Something like 21 volts dc.
A 100 mv input signal, all controls at 50%, this should give a 10 Vac rms output across an 8 ohm load.
That is 12.5 watts.
happy testing

[Leif H]

Originally Posted by Jazz P Bass

I would think that you would want to check the cathode voltage of the output tubes. Something like 21 volts dc.
A 100 mv input signal, all controls at 50%, this should give a 10 Vac rms output across an 8 ohm load.
That is 12.5 watts.
happy testing

I'm fricken ecstatic! I've been jones-ing to have at this for almost a month! I cant complain, as the Transformers didn't cost me due to the month wait.

[tubeswell]

Originally Posted by Leif H

Lastly, when checking the voltages, are both the volume's set to full with no input?

I check idle voltages with the vol killed. That way there is least hiss/no static in the signal path to mess with your idle voltages

[Leif H]

Originally Posted by tubeswell

I check idle voltages with the vol killed. That way there is least hiss/no static in the signal path to mess with your idle voltages

Thanks, I spent the evening making up an adapter plate, as the chassis is for a MM PT, of course the hammond is different mounting footprint!

[Leif H]

Ok, I seem to have AC hum even with volume killed.

I've checked my grounds, my, twisted all AC wiring.

Where do I go now?

[txstrat]

OK, besides not making any sound at all, the hum is a very common problem in tube amps. Kidding.
What if you pull the preamp tubes, still hum?
Have you separated the ground for your main filter caps and the preamp/PI filter caps?
Just a thought. Oh and you might wanna try some different tubes.
A layout plan or some pics of the innards would be helpful.

[Leif H]

Originally Posted by txstrat

OK, besides not making any sound at all, the hum is a very common problem in tube amps. Kidding.
What if you pull the preamp tubes, still hum?
Have you separated the ground for your main filter caps and the preamp/PI filter caps?
Just a thought. Oh and you might wanna try some different tubes.
A layout plan or some pics of the innards would be helpful.

I've tried pulling V1, no change, V2, the PI removes almost all of the hum, and I've tried several different tubes in V1, V2, V3 & V4.


I thought it was the fact that I've not a "pair of matched tubes". 34.9ma and 37.8ma are my bias current with 19.7V across the bias resistor 270 Ohm.

By separating the Filter Cap grounds, should they all be individually soldered to the brass plate? I've one single point of ground for everything. Star Ground?

The connection point is between the tone and volume 1 control... and I'm thinking this may be the issue?

I'm following Leo's / Weber's layout minus the standby switch.

Even with the volume's at 0, i'm getting feedback?

My voltages are as follows, From the first filter cap 369V, 313V & 201V. The 201V seems low to me, I thought it was supposed to be around 250V

[Leif H]

Still have hum....

Normal channel (2) seems to be much noisier than the Bright Channel (1).

I've taken the grounds from the filter caps and bypass caps and connected them to the star ground by the PT, now, all go to the star ground plus one wire from the brass plate goes to the star ground.


I get hum as soon as the rectifier tube warms up, and have on occasion had some feedback.

if I pull the preamp tube, the hum quietens significantly, and this sounds like an amp should... slight hum.

I remeasured the Filter Cap voltages and now have 358, 332 & 251.
I have the following voltages on the following tubes:

V1 12AX7
1: 153V
2: .1V
3 & 8: 1.25V
6: 156
7: .1V

V2 12AX7
1: 151V
2: -.5V
3: 1.1V
3:186V
7:15.7
8: 42.5

V3 6V6
Pin:
3: 310V
4: 265V
5: 0.4V
8: 19V


V4 6V6
Pin:
3: 322V
4: 263V
5: 0.4V
8: 19V

V5, 5Y3GT

Pin:
2: 349V
4 & 5: 676VAC
8: 346V

The only thing I haven't been able to swap out is the rectifier, which I don't have a second one.

I've attached a picture to show how we are set up now.

[Jazz P Bass]

Can you measure the AC voltage that you are calling hum.
Measure at the load.
Are you sure that your input jacks are grounding when nothing is inserted?
Is your output transformer wired correctly?
Take note of the third grounding wire.
Maybe try to reverse the plus OPT leads.
I have attached Fenders reissue schematic.
It has voltage levels to check.
AC & DC.

[txstrat]

Originally Posted by Leif H

I thought it was the fact that I've not a "pair of matched tubes". 34.9ma and 37.8ma are my bias current with 19.7V across the bias resistor 270 Ohm.

All my matched tubes so far had a difference of 2ma up to 5ma. No prob.

Originally Posted by Leif H

By separating the Filter Cap grounds, should they all be individually soldered to the brass plate? I've one single point of ground for everything. Star Ground?

Your goal is, that no currents have the same path to this point of the chassis, the star ground. This point shoud preferably be at one of the PT bolts as you did. The main filter cap and the second can share a ground (doesn't hurt if they have their own). The preamp filters share ground as well. Some say that the preamp filters should be connected to the preamp ground some say that's BS. You might wanna try. Do the cathode resistor and the main filter in your build share a ground wire?

[Leif H]

Originally Posted by txstrat

All my matched tubes so far had a difference of 2ma up to 5ma. No prob.

Your goal is, that no currents have the same path to this point of the chassis, the star ground. This point shoud preferably be at one of the PT bolts as you did. The main filter cap and the second can share a ground (doesn't hurt if they have their own). The preamp filters share ground as well. Some say that the preamp filters should be connected to the preamp ground some say that's BS. You might wanna try. Do the cathode resistor and the main filter in your build share a ground wire?

Yes, the cathode resistor, the main filter share a ground, as do the 2nd & third filter caps.

The resistors and bypass caps on the pre amp tubes each have their own wire to the star ground.

I'm going to remove the input from channel 2, to see if there is something in that signal path making the noise, then work my way through to the phase inverter.. the more I think of it, I wonder if I have a cap that is bad.

[txstrat]

Here's what I usually do.
1) One wire from the first and second filter cap (PI) to ground.
2) One wire from the input jacks along the pots to the star ground. To this wire I solder the filter caps and bypass caps of the preamp, just there where they are on the layout. You can combine the cathode grounds of one tube to one wire.
3) One wire from the speaker jacks to ground.
4) One wire from the bias to ground (you don't need this, since you got a self bias with cathode resistor) but I would use a separate wire from the resistor to ground.
All other grounds (PT center tap, bridge rectifier ground, etc.) by separate wires to the star ground.
This works for me.
Make sure, the bias circuit is not too close to the B+.
Hope this helps.

[Leif H]

We're getting there.... a little less hum, but I still have summm...

At this point, I'm not sure if it's ac hum or ground hum.

I have about 36mVAC on the speaker with all the vol / tone pot's at 0.

The odd thing is, when Normal / volume 2 is turned up to about 1, the hum quietens a bit. The tone control also seems to amplifiy the hum and it was quieting down at about 8 or 9.

I removed chanel 2 from the circuit last night, no change. I tidied up the wiring to V1 & V2, trying not to cross wires, but it's still a humm machine.

I'm going to try a co-ax from the tone control to V2, as removing V1 does not change the hum.

Lasty, I'm wondering what are the chances it's one of the filter caps?

Thanks for you help, much appreciated.

[txstrat]

If it is one of the filter caps you could take two leads with clips and clip another cap in parallel to the suspicious one. If that makes a difference, it's probably a bad cap.

[Bruce / Mission Amps]

Originally Posted by txstrat

...Your goal is, that no currents have the same path to this point of the chassis, the star ground. This point should preferably be at one of the PT bolts as you did. ...

I sort of agree with the first part to some degree but why should this "tranny bolt" be preferable?

[Leif H]

I still have hum,

I took a cap and jumped around the filter caps, with no change, bypass caps, no change and coupling caps, still no change.

I removed the tone stack, no change, possibly worse.

I'm at my whit's end here. and things seem to be getting worse. the tone was almost acting like a volume, and if turned up, the hum is magnified drastically.

Now it sounds like it's really buzzing rather badly when playing though it.

I'm to the point where I want to tear it out and start one channel at a time, front to end, and give it a go.

[txstrat]

Originally Posted by Bruce / Mission Amps

I sort of agree with the first part to some degree but why should this "tranny bolt" be preferable?

As far as I read the star ground should be far away from the input jacks.
That's where the PT mostly is. And there are 4 bolts you can conveniently use for your ground lug.

[txstrat]

Originally Posted by Leif H

I'm to the point where I want to tear it out and start one channel at a time, front to end, and give it a go.

I'm at this point as well with my Portaflex clone.
If you followed the weber layout there should be no hum or at least no hum to worry about. Check if you forgot a ground connection at some point. Check the solder joints and if all connections are soldered properly. Check the heater wires (do you have a heater CT or virtual tap? - if it's a VT are the resistors OK?).
Get a closer look around the PI (and the power tubes), since pulling the PI tube reduced the hum while pulling the preamp tubes didn't change anything as you mentioned before.

[Leif H]

Originally Posted by txstrat

I'm at this point as well with my Portaflex clone.
If you followed the weber layout there should be no hum or at least no hum to worry about. Check if you forgot a ground connection at some point. Check the solder joints and if all connections are soldered properly. Check the heater wires (do you have a heater CT or virtual tap? - if it's a VT are the resistors OK?).
Get a closer look around the PI (and the power tubes), since pulling the PI tube reduced the hum while pulling the preamp tubes didn't change anything as you mentioned before.

Don't say that, that's one of my next projects, B15N pre-amp for a bass amp.

[Leif H]

We're done, the hum beast is silenced!

THANKS GUYS!

I fixed the star ground, tidied up the heater wiring and put the virtual center tap on the heater wires. And was done in time to "warm her up a bit"

I'm grinnin like the cheshire cat.

Thanks again for all your help, this board is a great resource and full of people who can offer the obscure that comes only with experieince!