scratch build 5e3 help please

[cubby01]

This is my second amp build - first was a champ clone. Did the last solder joint and fired it up. First off it's LOUD. Second is problem areas.

Noise floor is very high. I get white noise type hiss that is there as soon as the tubes warm up and increases with volume increase. Sounds like am radio between stations. If I pull the preamp tube it's still there although at a lower level. If I pull the PI it's gone.

Another thing is that if I tap on the fiber board I hear it in the speaker. Also I hear it if I tap on the preamp tube. I've swapped out all tubes and this doesn't change.

I have gone though and reheated most solder joints. Added solder to some. No change. I've retraced my wiring over and over again.

Voltage off my PT measured secondary HT was a bit under recommended. At the time I didn't think that would be a big problem. I used a tranny from a donor amp that gave 320-0-320 out of chassis. In fired up 5e3 it's giving 318-0-318. As result DC voltages at filter caps are down a bit below what they should be but I'm guessing that's not what's causing my white noise.

5e3 RI voltages pdf -> http://www.fender.com/support/amp_sc...0JUN%2007).pdf

I've been poking around comparing voltages to this 5e3 RI pdf and I see a couple concerns. AC voltage at my OT primary to 6v6 plate are WAY high. pdf says 190AC and my DDM runs up and pegs out.

Also the pdf says to expect 16.7VAC off the .1mf caps that lead to pin 6 of each 6v6. I get 0VAC (zero). That seems strange. Is that an indication that my .1mf caps are bad?

Anyway from what I've heard this amp has great potential. Just need to figure out what's screwed up. Any help or direction is appreciated.

[cubby01]

Oh, it occurred to me that I deviated from the parts list in that I used 1.2meg volume pots instead of 1meg. (I had the 1.2meg on hand and I'll swap those to 1meg when I can get them ordered). The tone pot remains 1meg.

[tubeswell]

Hi cubby01

What are the idle voltages?

Did you check continuity with your meter and check your resistor values?

Hiss can be a plate resistor 'problem' - some hiss is normal but can be alleviated by using higher power rated resistors or C F or MF resistors instead of CC (even tho this may kill some of the 'mojo')

In particular maybe check the plate resistors on the PI - what did you use? (CCs are very prone to overheating when soldering them in, and can easily be damaged resulting in the kind of symptoms you describe)

As a further thought, also check the socket pin tension.

[cubby01]

I'll list the idle voltages a bit later. What points to check? DC only or any AC values? When you say idle you mean on with all tubes in but no input and no volume? (Thanks)

I ordered all resistors and caps through Weber's site using Weber's 5e3 BoM. All resistors are MF, none are CC.

I checked continuity with a DMM but will throw in a fresh battery and recheck.

[MWJB]

"Noise floor is very high. I get white noise type hiss that is there as soon as the tubes warm up and increases with volume increase. Sounds like am radio between stations. If I pull the preamp tube it's still there although at a lower level. If I pull the PI it's gone." Suspect grounding/layout issues around V1/V2. Check gounded components do not show whole ohms when referenced to the chassis with an ohmmeter, re-route grid wires away from plate wires & 6.3VAC heater wires.

"Another thing is that if I tap on the fiber board I hear it in the speaker. Also I hear it if I tap on the preamp tube. I've swapped out all tubes and this doesn't change.This doesn't surprise me, components in grid circuits will be noisy when tapped, as will V1. This could be normal depending on the level of noise.

Low PT voltages will not be the cause.

1.2Meg pots are not the issue

"lso the pdf says to expect 16.7VAC off the .1mf caps that lead to pin 6 of each 6v6. I get 0VAC (zero). That seems strange. Is that an indication that my .1mf caps are bad?" If you get zero VAC here your amp would not be loud, recheck. 0vdc is normal here however. If you suspect the caps then it's easy & quick enough to change them.

If you have zero VAC at the 6V6 pin 6 (& subsequently pin 5), then how are you getting way high AC between the 6V6 plate & OT? You have 100mVAC at the Normal/Mic 1 input and all controls set to 50% rotation?

Can you post some pics?

[tubeswell]

Originally Posted by cubby01

I'll list the idle voltages a bit later. What points to check? DC only or any AC values? When you say idle you mean on with all tubes in but no input and no volume? (Thanks)

I meant DC (to see if you had a voltage-related problem). Check all the plates, screens and cathodes, and the grid of the PI (and yes 'idle' means with no input signal).

[cubby01]

Here are the voltages

Off 5Y3 rectifier - Pin 8 , 1st filter cap - 312 vDC
2nd filter cap - 277 vDC
3rd filter cap - 183 vDC

(V4) - 6v6
pin 3 - 304 vDC
pin 4 - 278 vDC
pin 5 - 50mvDC
pin 6 - 50mvDC
pin 8 - 17.6 vDC

(V3) - 6v6
pin 3 - 304 vDC
pin 4 - 278 vDC
pin 5 - 90mvDC
pin 6 - 90mvDC
pin 8 - 17.6 vDC

(V2) - 12AX7 (note, it's an old mullard. Is not an AX7b )
pin 1 - 112 vDC
pin 3 - 95mvDC
pin 6 - 120 vDC
pin 7 - 4.7 vDC
pin 8 - 38.4vDC

(V1) - 12AV7 (note the V instead of Y, should just be a slightly lower amplification factor)
pin 1 - 47.6 vDC
pin 3 - 2.1 vDC
pin 6 - 60.1 vDC
pin 7 - 68mvDC
pin 8 - 2.1 vDC

If I put in another 12AX7 in V1 then I get *slightly* different reading on the PI but not much, the V1 readings with 12AX7 are
(V1) - 12AX7
pin 1 - 104 vDC
pin 3 - 1.2 vDC
pin 6 - 137 vDC
pin 8 - 1.2 vDC

[cubby01]

Oh, and filament is an artificial CT and showing pretty close to 6.3vAC across them.

[cubby01]

Originally Posted by MWJB

Low PT voltages will not be the cause.
1.2Meg pots are not the issue

Thanks I didn't think so.

Originally Posted by MWJB

"lso the pdf says to expect 16.7VAC off the .1mf caps that lead to pin 6 of each 6v6. I get 0VAC (zero). That seems strange. Is that an indication that my .1mf caps are bad?" If you get zero VAC here your amp would not be loud, recheck. 0vdc is normal here however. If you suspect the caps then it's easy & quick enough to change them.

Will try that soon as I pick up some to change in.

Originally Posted by MWJB

If you have zero VAC at the 6V6 pin 6 (& subsequently pin 5), then how are you getting way high AC between the 6V6 plate & OT? You have 100mVAC at the Normal/Mic 1 input and all controls set to 50% rotation?

I may have misspoke when I said "plate"?. I was refering to pin 3 on the 6v6 which I thought was the anode/plate. These pins from each 6v6 are connected to the OT push pull primary windings. The link I reference in the first post (first page of the pdf) indicated I should expect 190 vAC there. I'm showing 660 vAC from pin 3 to ground on both 6v6s and also from the 5y3 pin 8 where the PP OT primary center tap connects. I don't personally understand how I can be getting a value that high but I rechecked with an old analog meter and got similar reading.

For what it's worth both the PT and the OT are vintage but were working fine in their original chassis.

TIA

[MWJB]

"I may have misspoke when I said "plate"?. I was refering to pin 3 on the 6v6 which I thought was the anode/plate. These pins from each 6v6 are connected to the OT push pull primary windings. The link I reference in the first post (first page of the pdf) indicated I should expect 190 vAC there. I'm showing 660 vAC from pin 3 to ground on both 6v6s and also from the 5y3 pin 8 where the PP OT primary center tap connects. I don't personally understand how I can be getting a value that high but I rechecked with an old analog meter and got similar reading."

Pin 3 is the plate (anode).

How did you determine which secondaries were the 8 ohm speaker tap? Your OT may have a completely different primary impedance compared to the 5E3 OT (8.5K into 8ohms). Budget brands used whatever values were handy, sometimes half the primary Z you might expect, other times double.

[cubby01]

Originally Posted by MWJB

How did you determine which secondaries were the 8 ohm speaker tap? Your OT may have a completely different primary impedance compared to the 5E3 OT (8.5K into 8ohms). Budget brands used whatever values were handy, sometimes half the primary Z you might expect, other times double.

The OT is a pull from a vintage extension speaker that was running 6v6's push pull. The original speaker was 8 ohm. There aren't multiple options on the OT. Just the three primary leads (blue, brown and red-ct) and two secondary (green, black). This is pretty standard color coding on the inputs but I also measured each winding to confirm primary/secondary and then measured AC with a variac to get turns ratio and then calculate impedance ratio. I got 1000:1 or 8K:8ohm so should be good with an 8 ohm speaker.

Thanks for continuing to troubleshoot with me.

[tubeswell]

Hmmm... those 6V6 plate voltages look a little on the low side for 6V6s in a guitar amp. You may want to get it to around 340-360. Have you tried it with a 5R4 or GZ34 rectifier?

The cathode voltage on the driver stage before the PI looks wrong (Pin3 of V2) - it should be about a couple of volts. What size cathode resistor have you got in there? (Also could be a bad/leaky coupling caps after V1. Are any of the volume pots scratchy?).

Also the PI voltages look low. Maybe these will come up if you have a stronger rectifier tube. What size dropping resistor have you got in the B+ rail after the screen filter cap node?

[cubby01]

No 5r4 handy. I have other 5y3 and a 6ax5. Believe the latter would have less voltage drop but the 6ax5 would need the 6v heater and I don't know if the 6.3 winding has the current available.

I used resistor values shown in this layout. This layout and schematic are from the weber kit site. I believe the values are same as a classic fender 5e3.
Layout -> https://taweber.powweb.com/store/5e3_layout.jpg
Schematic -> https://taweber.powweb.com/store/5e3_schem.jpg

I'm suspecting the .1uf caps because I was concerned earlier about too much heat with the iron. Unless Radio Shack has them locally it will be days before I can replace.

(Lessons learned: To keep up this hobby order parts for redundancy until I build up some inventory. Build up recto inventory.)

[cminor9]

Originally Posted by cubby01

(Lessons learned: To keep up this hobby order parts for redundancy until I build up some inventory. Build up recto inventory.)

Yes, good lesson. Parts are expensive. You probably have your fave suppliers as does everyone, but I have found a local surplus place here in town. They don't have everything, but what they do have is super cheap. You might be able to find something like that to keep yourself stocked up.

[cubby01]

Originally Posted by cubby01

I'm suspecting the .1uf caps because I was concerned earlier about too much heat with the iron. Unless Radio Shack has them locally it will be days before I can replace.

I had some .1uf caps but of 250 vdc rating so tried those hoping to rule something in or out. No difference in the noise. So I don't think the .1uf caps are bad.

The noise is like the roar of a distant waterfall. It is gone with the volumes both at zero. Comes on when either volume is turned up a bit and is louder as volume increases. Noise is there whether or not there is any instrument plugged in. When volume is zero turning the tone control has no affect. If the volumes are above zero then turning the tone control is like adding hiss to the noise.

I think I must have made some stupid dumb mistake putting it together. I just cant figure out what it is. But the amp does amplify. If you were to ignore the noise it doesn't sound not too bad. I did notice that when volume is set to about 3/4 and hitting some power chords in rapid succession, it will cut out briefly. Something is overloading something somewhere.

[tubeswell]

The cathode voltage of the PI driver is wrong. You must have something wired wrongly. Go back and triple check everything carefully, and test the circuit for connectivity with your Rmeter. You should get ~1k5 between the cathode and the chassis.

[cubby01]

I get ~1k5 between cathode and ground. From what I gather all voltages appear to be somewhat low. I'm going to see if I can dig up a more appropriate PT. In the mean time I'll keep rechecking.

[MWJB]

As Tubeswell says, you voltage at V2 pin 3 is too low, fix this first. you sgould have arounda volt there.

Changing PTs won't address the problem. Again, as Tubeswell states a different rectifier would be a simpler solution than changing the PT.

How did you get on checking grounds & layout?

Some pics would be really helpful, 5E3 can be very picky regarding layout.