Help with 5E3 build...

[mnexsen]

I bought a homebuilt 5E3 on Ebay and I'm having some problems with it: The amp has a severe distortion present at anything above 2 on the volume. It doesn't break up, it farts out on the attack and stays severely distorted until the note decays.
The tech work on the build looks nice: pretty solder joints, nice lead dress, generally good components. I've tested every resistor with a meter against the schematic and they're all good. The builder subbed 20 uf/25 volt for the 25/25 called for on the schematic but otherwise looks OK. Voltages seem very close to what others have posted here. I haven't completely ruled out the coupling caps, but they're new-looking Spragues and I can't decipher the markings and don't know how to test caps.
He only used 1 input and had jumped pin 2 to pin 7 on the first preamp tube. I removed the jumper and associated wiring back to the second volume pot but no real improvement. He did omit the 68K input resistor but with only a single input jack I'm thinking that isn't a problem.
I'm pretty much down to thinking the issue is either in the OT, which looks pretty old and cruddy or the bias circuit which I don't really understand.The builder made a faux center tap with resistors off the filament wires, but I'm pretty sure the PT (Magnetic Components Deluxe) has a bias supply tap available. Anyway, can someone explain the bias circuit to me and tell me where to measure the bias supply voltage. I'll go check the bias in the meantime with a Swamp bias adapter.
Funny thing, I was going to build a kit amp but this one looked so good i thought I'd be better off with it.

[macdillard]

The Fender 5E3 is cathode biased not fixed biased. To measure the bias voltage check from pin 8 to ground on either of the 6V6 power tubes. Will probably be something around 20 VDC. The 250 ohm 5 watt resistor (on the schematic) that is connected to pin 8 sets the bias.

[mnexsen]

I'm starting to understand why I'm confused. The builder wired the amp with what looks like 2 different bias supplies, although there's no way to switch between them: He's got the 250@5watt/25mfd cap wired to pin 8. Then he's taken 2 filament wires, put a resistor in series with each to ground, and tied this back thru pin 1 and 8 of the power amp tubes. I measured 64 vdc from pin 8 to ground. I'm going to remove the wires from the filament taps to pin 8 and see what happens.

[macdillard]

Are the power tubes 6V6's? If so then you are going to burn them up at that voltage. Sounds like what he has tried to do is create a CT for the filament supply. You do that by installing 100 ohm resistors from each leg and taking the resistors to ground. What he has done is tie the resistors into the top of the cathode. This gives you DC elevation for the heaters..It is a method of reducing noise. If I were you I would take a schematic and a layout (if the layout was followed) and check the amp.

[mnexsen]

Yeah, I thought that what what he had done, except he didn't go to ground. He took the filament taps and tied a resistor to each leg, joined the 2 legs, then on to pin 8. There is no ground connection.
I'm OK with going back to the classic 5E3 checmatic and layout but I have a couple of questions about that:
1. Where does the bias volatge come from? I see the 250 ohm resistor and it's tied back to pin 8, but I don't see where that gets a voltage supply.
2. If i go strictly by the Fender layout, they show the pin 7 filament wiring going to ground. In it's current state there is no ground connection there. Measuring before the artificial center tap resistors I get 129vac on one leg and 122 on the other leg of the filament wires, 6.3vac between them. Does that sound right?
3. I removed the artificial center tap from pin 8 and still get 65 volts pin 8 to ground and a bias current of 128 ma! I measured 125 vac on the wire I removed from pin 8.

[macdillard]

1. bias voltage comes from pin 8. it is what is left of the anode voltage as it drops through the tube to the cathode. this is a cathode biased amp. not fixed biased.

2. forget about the ct for the heaters just now.. one leg of the filament wiring goes to pin 2,other leg goes to pin 7 of the 6V6's. The 129 and 122 vac on the legs of the filament wiring with 6.3 between the legs can not be. You should read about 3vac to ground from each leg.

3. 65 volt on pin 8 and a bias current of 128 ma...how are you getting bias current? bias current is cathode voltage divided by the cathode resistor or in your case it would be 65/250 = 0.260 amps. Don't think so. the tubes will not take that.

take the heater wires loose from the tubes and measure the voltage on wires directly from the transformer. Should be 6.3 VAC or thereabouts. If you have 129 VAC there then there is a problem with the PT. Be careful. remember ever thing is powered up.

[mnexsen]

Took the filament wires out...

With the filament wires removed from the circuit, I measure 6.3vac leg-to-leg; but 167vac and 173vac leg-to-ground. I double checked and checked again. There's no voltage present until the main power is turned on, then it's as above. The leg-to-ground voltage was lower with the pilot lamp in the circuit: 121.7 and 128
Transformer is a Magnetic Components Inc 40-18016, looks like new. All transformer wiring looks good; no fraying or burns.

[macdillard]

If you haven't already done it, check/swap out your power tubes and check the cathode cap and resistor.

[mnexsen]

5E3 schematic questions...

I'm thinking the leg-to-ground measurement might be a red herring, since they don't go to ground. But, the Fender layout shows pin 1 tied to pin 2 which is grounded. Should I ground pin 2?
I have measured resistances of every resistor in the amp, including the cathode resistor and they're all in the ballpark. In my amp, the builder used a 300 ohm rather than a 250, and he subbed 20 mfd caps for 25's throughout. I don't know how to test caps.
FWIW, I do have an oscope available. It's and old single channel, no bells and whistles, but if we ever get to a place where it's useful...
BTW, thanks for your help and patience on this one.

[macdillard]

Think you are correct on the leg to ground measurement. If you have 6.3 ac leg to leg then you are correct. What is your plate (anode) voltage at the 6v6's. You are the one who has to be patient. You will find the problem and it will be something simple. Can you change out the 6v6's? The reading you are getting at pen 8 is way out of spec. Don't ground pin 2. run one leg of the heater supply to pin 2 and the other to pin 7. Leave pin 1 along. The 40-18016 is probably going to give you about 390 vdc on the anodes. maybe more. according to what you have as the rectifier. By the way, what do you have as the rectifier, 5y3?

[macdillard]

go to http://www.kilback.net/homebrewtweak...ematic_r3v.gif and take a look at that schematic. It will show you what was being done with the resistors in the filament leads and the connection to pin 8. Also will show you about what voltages you should have.

When all else fails go over to www.paulrubyamps.com and go through the 1st power up procedure for an amp..You will find the problem..

I am just about convinced that you have one or more bad power tubes.

[mnexsen]

No resolution, but lots of data...

I have 2 complete sets of tubes: RCA 5Y3, RCA 6V6's, and assorted preamp tubes in one set; Sovtek 5Y3, JJ 6V6's, and EH preamps. I removed the artificial center tap and soldered the filament leads direct to the tubes, no ground on pin 2. I ran the tubes as sets, and tried the Sovtek rectifier with the RCA power tubes. I measured both tubes in the pair and the readings were within 1 volt.

PIN Sovtek RCA Sovtek
JJ RCA RCA
3 327 268 330
4 255 228 278
5 66.7 54.3 64.7
6 67.6 55.4 71.3
8 64.5 52.6 67

The distortion is still present. Unless I'm missing something, the amp is now wired according to the Fender schematic and layout. I can work on the artificial center tap if hum is an issue after I get the distortion sorted out.
I'll try the paulrubyamps procedure later this week.

[macdillard]

Check the wiring/components around pin 5 and 6. Don't think they should have any DC voltage or it should be in the mv range.

[tubeswell]

PIN Sovtek RCA Sovtek
JJ RCA RCA
3 327 268 330
4 255 228 278
5 66.7 54.3 64.7
6 67.6 55.4 71.3
8 64.5 52.6 67
.

These numbers are a bit meaningless. Could you please measure the DC voltages from each pin to ground when the amp is idling and re-write them as:

For each pre-amp tube
Pin 1 =
Pin 2 =
Pin 3 =
Pin 6 =
Pin 7 =
Pin 8 =

For each 6V6
Pin 3 =
Pin 4 =
Pin 5 =
Pin 8 =

B+ at the first filter cap supply (OT primary Centre Tap) node =

That will make it easier to help

Forget about the filament voltages for now - as long as you have about 6.3VAC (give or take 10%) between the two sides of the heater winding, it should be fine.

[mnexsen]

OK, I think you may have found it...

I measured a high resistance from the junction of the 2 220K resistors to ground, so I tried jumping the connection to ground but then got no voltage on pin 8. 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.
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.

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

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

The second 6V6 has very similar measurements.
The B+ at first filter cap supply is 359.