Help with 5E3 build...

[macdillard]

You are getting close now..The wiring around the power tubes is still wrong. Pin 3 on the power tubes should be about what B+ is that is around 350 VDC. The only drop you have is the resistance of the OT primary windings. Pin 8 is still way too high. It should be around 21 VDC. The jumper from the junction of the 220K resistors goes to ground. Pin 4 of the 6V6's goes to the positive end of the second filter cap. should be around 330 VDC. Getting that jumper corrected may fix it all. Ain't this fun. . When you get the power tubes right you will have to look at the pre-amp tube wiring. Those voltages are not right

[tubeswell]

I removed the tag board to check some connections and there's a jumper from the 2 220K resistors that isi tied back to the second filter cap (+) end, then on to pin 4 of the power tubes. My layout calls for that jumper to go to ground.

Sounds like you are talking about the ground returns from the 'bottom' of the 220k grid load resistors and the 6V6 cathode resistor and the screen node reservoir. For the moment it would be okay to leave them where they are - I don't think that would be causing the fizzy sounding notes

As is, voltages read:
For first (input) pre-amp tube:
Pin 1 =55.2
Pin 2 =0
Pin 3 =2.01
Pin 6 =67.7
Pin 7 =-2.16*
Pin 8 =2.01
Pin 7 is not connected. In an earlier post i noted that this was built as a single-input amp. The builder had jumped pin 2 to pin7, I have since removed this jumper and associated downstream wiring for now.

55 and 67 are low plate voltages for the first stage. You'd normally expect around double this. Sounds like there isn't enough voltage on the B+ rail at this point (which is probably caused further back in the power supply chain)

For second preamp tube:
Pin 1 = 108.9
Pin 2 = 0
Pin 3 = 0.786
Pin 6 = 132.8
Pin 7 = 10.38
Pin 8 = 28.57

Similar problem as noted above - these voltages are low. (Incidentally for the driver cathode (pin 3) what value plate resistor have you got there? - should be 1k5.)

For first 6V6:
Pin 3 =268
Pin 4 =228
Pin 5 =53.6
Pin 8 =52.3

The second 6V6 has very similar measurements.

Well the low voltage problem is even manifested here. The plates (pin 3) should be nearly what the B+ is. Makes me wonder - did the amp maker put in another (sag) resistor and filter cap in the power supply between the rectifier and what would otherwise be the first filter cap? That could possibly explain low plate voltage. Got any gutshot pics?

The B+ at first filter cap supply is 359.

This is normal - but as queried above, is the first filter cap where the 6V6 plate/OT centre tap supply is taken from, or have they added an intermediate filter capand or sag resistor in between the rectifier and that point?

[mnexsen]

One down...

OK, I relocated the jumper from the 2 220 ohm resistors to ground. Things got some better: bias voltage seems in spec now and some of the voltages came up.
Power tubes, vdc to ground:
3=355
4=333
5=0
8=21.2
Other tube similar values.

1st (input) Preamp tube
1=80.0
2=0
3=3.042
6=73.6
7=-2.5
8=3.042

2nd preamp tube
1=153.4
2=0
3=1.116
6=186.7
7=-2.3
8=3.044

The amp sounds better, blocking distortion is gone. I now have pronounced ghosting on low notes (low E string.) I plan to go through the layout again now that I've puled the tag board. Tubeswell, the "top" of the layout all seems to match the schematic, with these exceptions: The 5K, 5 watt resistor between the first and second filter caps is a wirewound resistor. The amp was built as a single input, originally wired with 2 volume controls and the 12AX7 stages jumped together. I removed the jumper and second volume from the circuit, at least for now. Otherwise, no extra or missing components, and I have verified the resistors against the layout with a meter.

[macdillard]

See what the voltage is at the third filter cap. It should be about 250 VDC. Then check the voltage at the junction of the two 100k resistors that go to pin 1 and pin 6 of the 1st pre amp tube. It should be the same. (250VDC). You want to see about 120 VDC on pin 1 and 6 after the drop through the 100K resistor of each. You are pretty close on the second pre tube with the exception of the reading at pin 8. It should be about 50 VDC.

[mnexsen]

Changed preamp tubes...

I made the first set of measurements below with the 12AU7 that had been in the amp all along. Then I rechecked with a new 12AX7EH:

12AU7
Voltage at 3rd filter cap: 229.3, also at the junction of the 2 110 K resistors.
Pin 1 = 80.4
Pin 2 = 0
Pin 3 = 1.485
Pin 6 = 75.6
Pin 7 = -1.17
pin 8 = 1.56

12AX7EH
Voltage at 3rd filter cap: 262, also at the junction of the 2 110 K resistors.
Pin 1 = 191
Pin 2 = 0
Pin 3 = 1.621
Pin 6 = 170.7
Pin 7 = -0.58
pin 8 = 1.63

I also checked plate voltages on the 2nd preamp:
12AU7
Pin 1 = 154.7
Pin 6 = 188.2

12AX7
Pin 1 = 176
Pin 6 = 214

The amp sounds really bad with the 12AX7-everything is distorted at any volume level.

[macdillard]

What is the voltage at pin 8 of the second pre amp tube (phase inverter). It should be about 50 VDC. That tube should be a 12ax7. Make sure that is what you have in that position when you check it..I am not famillar with the 12au7. the first preamp tube is suppose to be a 12ay7. I am sure you can get away with a 12ax7 there but don't know about the 12au7. Maybe tubeswell will chime in and help us out.

[mnexsen]

Preamp tube 2 voltages...

Preamp 2 with 12AX7's in both positions:
1 = 176.0
2 = 0
3 = 1.278
6 = 213.9
7 = 16.33
8 = 46.7

I'm wondering about the input configuration since the voltages seem to be getting closer: There is no 68k input resistor. The single input jack has a 1 meg to ground, then straight to pin 2 of the first preamp tube. I previously removed a jumper between the sections of the first preamp tube and the output from the second preamp tube. Is this the source of the distortion?
I got my camera back-going to try to post a picture...

[Bruce / Mission Amps]

I feel sorry for that B section triode in the first preamp tube.
It is running under zero bias, it might even be trashed out now.
If you aren't going to bias it with a proper grid load resistor, at least undo the wire from lug 6 to the coupling cap on the eyelet board.... and then watch your B+ on lug 1.
You'll see it move up from that low 70v... especially if you use the proper tube, a 12AY7... not an AU7, not an AT7 and not an AX7.
Or, just wire in two input jacks correctly with some 500K to 1M grid load resistors on the jacks.
Run one wire from each jack to lugs 2 and 7 respectively and use a 10K to 30K resistor on each socket as grid stoppers on on lugs 2 and 7.

[macdillard]

Thanks for jumping in Bruce...Now Mnexsen you have some real help. Do what Bruce says and you will have one heck of an amp. He knows what he is on about.

[tubeswell]

Thanks for posting those pics. I see now that you only have 1 input going to one 1/2 of V1 and that the grid on the other half is floating (which will bugger up that triode as Bruce pointed out). Another option to think about might be wiring the grids for both those V1 triodes in 'parallel' (i.e. tying the grids together). You could then experiment with different tonal variations by adjusting the volume pots. As it is I would guess that the ungrounded grid is stuffing up the signal in V1 altogether (by seriously affecting the tube current in that triode, and thereby having a flow-on effect to the plate and thence into the VTS section, maybe the cause of at least some of your troubles).

[macdillard]

Looks like the builder did install a resistor on pin 2 of the pre amp tube..Appears to be mounted right on the pin and covered with heat shrink.

[mnexsen]

Thanks...

and I appreciate all your patience. I'm working screwy hours right now, hopefully I can get some bench time this weekend. I'm not sure that's a resistor under that shrink, but I'll check it.
If I put in another jack, how are the 500K to 1meg resistors wired? Are they between the jack and ground or in series with the grid?
The amp actually came with the grids tied; i removed the jumper early on in the troubleshooting. A little knowledge...
If I tie the grids, do I need to add any components between the grids? I like the notion of having another gain stage, but I understand that tweeds have plenty of gain as is.

[tubeswell]

If I tie the grids, do I need to add any components between the grids? I like the notion of having another gain stage, but I understand that tweeds have plenty of gain as is.

No all you need to do is connect the grids. This will make both stages take the same signal and then you can use the vol controls to achieve different blends of parallel vs signle triode signal passed on to the next stage. But it might be sensible to add another 1M grid leak resistor from that grid to ground (because with both grids otherwise sharing a single 1M grid leak resistor, you effectively have twice as much grid electron replenishment going on, which is the same effect as one triode working with 2M2 grid leak resistance, and although theoretically some 12A_7 tubes can handle up to a 2M2 grid leak resistance for a single triode, it might be better not to push the limit.)

If you want even more tonal variation you can put a 220k plate resistor on one of the triodes, and then the vol controls will let you blend between a sort of typical 5E3 front end, and another front end with more grind to it. (or go the other way and put 47k on that triode and get a more jazzy tone (although for that option you might have to change the coupling cap to a lower value to limit possible excess grid current in the following stage)

[macdillard]

If he was to undo the wire from pin 6 to the board want he have killed the b half of the tube? If so then the amp should operate correctly. That is a quick fix. Think I would try that first and then start playing with the grids. Really, being dumb as I am I would add the second input with Rg correctly, only problem with that is that you have to drill the chassis. mnexsen, let us know what happens.

[mnexsen]

I took the easy way out...

Lifted the pin 6 wire and replaced v1 with a 12At7. All's well, although there is some pretty serious hum. I'll run that down now that the rest of the amp is working well-maybe put the artificial center tap back on the filaments. I'm looking forward to playing with the extra triode. Thanks again for everyone's help.