Problem: Weber 5F6A with very low output!

[jibe]

Hello!





I've built me a 5F6A from a Weber kit but the gain is very low and it starts to distorts at low volume. This is my first ever amp build.
I've tried changing preamp tubes with no luck, problem persists!


Links to layout and schematic, also PT and OT specs:

Layout 5F6A
Schematic 5F6A
Power Transformer
Output Transformer

I'm a bit confused by the layout of the bass pot. Doesn't really match the schematic in my opinion, should be a connection between the middle en right pin of the pot. Is that layout correct or should I connect the pins?

Plz help, I've had this kit finished for a year now and can't use it, it's gathering dust and taking up space instead of making rock'n'roll.

Why do I get low output, up to 4 on the volume knob I get a nice clean sound, after that distortion, but no gain, no power.
Can play on full volume, it's like I only get 2 watts or something.

measurements:

5UA4 Rectifier
1:Unconnected
2:425
3:Unconnected
4:-5.4
5:Unconnected
6:-5.4
7:Unconnected
8:425

Output tubes, China 6L6GC:
Same on both tubes
Pin
1:Unconnected
2:Fil
3:417
4:419
5:-33
6:427
7:Fil
8:0

PreAmp tubes, China 12AX7B

V1
1:232
2:0
3:1.64
4:Fil
5:Fil
6:230
7:0
8:1.64
9:Fil

V2
1:193
2:0
3:1.16
4:Fil
5:Fil
6:332
7:192
8:193
9:Fil

V3
1:250
2:332
3:43
4:Fil
5:Fil
6:258
7:28
8:43
9:Fil


//jibe

[erndawg]

FIRST, make certain that you are qualified to troubleshoot and repair this amp and are thoroughly aware of the LETHAL and DANGEROUS voltages present in the amplifier - even when the power is turned off. If you have ANY doubts about your skill or ability, or Safety concerns, then send the amp to a qualified repair shop.

First thing I would do, immediately, is set your bias to as cold as possible.

According to your voltage list, your bias is set at -33 VDC. Adjust the bias pot to as cold as possible. That means, you need to set it more negative. Set it towards -50 VDC or more negative (-52 to -55) if you can. At -33 VDC, it is way too hot and will stress your power tubes. We'll get back to the final bias at another time. For now, cold bias is a good thing.

Your concern regarding the bass pot interconnect in the layout vice schematic not being in disagreement is unfounded. The layout and schematic agree. Yes, only two lugs are soldered to. No worries there.

So, it sounds to me that it boils down to the volume being extraordinarily low. Is that your primary concern? If so, follow these steps to determine if the signal loss is in the power amp section or the preamp section:

1) Power up the amp, and verify there is no voltage between the wiper (center lug) and ground of the Treble pot. Make SURE that is true.

2) If you have an audio signal generator, inject a signal between the treble pot's wiper (center lug) and ground. If you do not have a signal generator, you can use a portable CD/MP3 player's headphone out (either right or left headphone out and ground) and slowly increase the volume of the CD/MP3 player. The 5F6's volume and tone controls will have no effect on the volume and tone of the CD/MP3 source. The presence pot will have an effect.

If the volume gets very loud - you probably know what 50+ watts sounds like - then you know the power amp is working and we need to troubleshoot the preamp. Conversely, if the volume is very low, we need to troubleshoot the power amp.

If you do not have access to a signal generator, you could touch the Treble pot's wiper with your finger instead. (Do NOT touch any voltage points!) It will be *similar* to touching your guitar cable's plug tip without being plugged into a guitar. But the volume output can be somewhat hard to quantify unless you have experience with this simple test. It will react differently than touching the guitar cable.

Report back with the results. Good luck.

[erndawg]

I also notice in your voltage list that V3 pin 2 is way too high. It should be similar to pin 7, about 20 or 30 volts. Check that voltage again and report back.

[tubeswell]

I'm a bit confused by the layout of the bass pot. Doesn't really match the schematic in my opinion, should be a connection between the middle en right pin of the pot. Is that layout correct or should I connect the pins?

Doesn't matter. That pot is wired as a variable resistor. So if you connect the wiper to the 'input' end you achieve the same result.

5UA4 Rectifier
1:Unconnected
2:425
3:Unconnected
4:-5.4
5:Unconnected
6:-5.4
7:Unconnected
8:425

These are in the ballpark

Output tubes, China 6L6GC:
Same on both tubes
Pin
1:Unconnected
2:Fil
3:417
4:419
5:-33
6:427
7:Fil
8:0

Those look okay (but better if the screen is slightly below the plate) 3 is the plate, 4 is the screen, you could try a single 100R-330R in series with the screen supply and the 'first' pin 6 (i.e. so that it knocks down the screen voltage on both screens) (- and it might be a good idea to put a 10uF 450V decoupling cap to ground at that 'first' pin 6 if you are going to try that resistor)

PreAmp tubes, China 12AX7B

V1
1:232
2:0
3:1.64
4:Fil
5:Fil
6:230
7:0
8:1.64
9:Fil

The plates (1 and 6) are somewhat high compared to the original circuit - see the 5F6A schematic here:

http://ampedia.redbeartrading.com/da...f6-a_schem.gif

But nevertheless within the parameters of a 12AX7. The original uses a 12AY7 in V1. (Have you tried plugging in a 12AY7 and seeing if that makes a difference in distortion levels?)

V2
1:193
2:0
3:1.16
4:Fil
5:Fil
6:332
7:192
8:193
9:Fil

These are in the ballpark for the driver/CF stage

V3
1:250
2:332
3:43
4:Fil
5:Fil
6:258
7:28
8:43
9:Fil

Your input grid (pin 2) is way-way wrong. It should be about the same (give or take a couple of volts), as pin 7. Are you sure you measured that in the right place? Either you didn't, or you have it wired wrong. Go back and check your layout

(PS Oops, as I was busy replying to this I see erndawg has double-posted)

[jibe]

First of all, I am well aware about the dangers when poking around in an amplifier of this magnitude. Since I don't have the experience I make a point of not doing anything without triple checking and draining caps. If I'm not sure I don't touch.


I have a couple of leads to follow up. Cold bias, Pin2 of V3 and a way to determine where the problem lies, power amp or preamp. I don't have a audio signal generator but i can sure make one from an old portable CD. If I'm understanding this right the output, if the poweramp is working, could be high enough to damage things? Enough to damage my speakers? I have a homemade cab with 2 Celestion blue's in them. 8 ohm, 30 watts.

I feel like I need to clarify some things, I use a 2-4-8 Output Transformer and my amp is a Head model. It's wired so that I can use either a 2, 4 or 8 ohm Cab, using the stock RCA connectors. That part of the schematic is not "stock", I'll get back to you with a schematic for that.


Thanks for all the input, I'll get cracking (carefully) and get back to you all as soon as possible.

PS. I'm not sure I feel comfortable experimenting around the wiper of the treble since it seems I have unusual
high voltages there, +300 instead of +20. DS.



//Jibe

[MWJB]

V1 voltages are fine, Jibe has a 12AX7 in V1 NOT a 12AY7 like in the original schem.

If you rebias your power tubes as Erndawg suggests your plate voltage (6L6 pin 3) will rise and probably end up the same or a little higher than the screens, modification to the screen supply circuit is not necessary.

Your rectifier is wired wrong, the hi voltage AC (red wires) should be at pins 4 & 6, the yellow 5.4VAC should be at pins 2 & 8.

Double check the wiring of the phase inverter, if you have 332v on the grid there should be something pretty obviously wrong here, post a pic if you can.

[jibe]

Pin 4 & 6 of the Rectifier are connected to red and
2 & 8 to yellow, but I think I measured DC.

I get 425 DC FROM the Standby Switch connected to pin 8.

I'll triple check my connections and voltages around V3 too see what
makes the high voltages. Maybe even reflow some joints to see if I
have a cold joint or something.


//Jibe

[MWJB]

425v dc at the standby is way low, rebias to 35mA per 6L6 tube and see what you get then.

All voltages going IN to the rectifier FROM the PT are AC (red & yellow secondaries), you should only be reading dc at pin 8 (rectifier output). Transformers run on AC, they only produce dc after a rectifier (GZ34 & bias diode).

[loudthud]

The low voltages on V3 pins 2 and 7 are due to the 1meg resistors and your meters input resistance. Just measure across the 1megs, it should be close to zreo volts, and then assume the voltage is the same as that at the junction of the 1 megs. Verify that you are getting a good heater voltage, 6.3VAC on all tubes. Check for a short at the speaker connections. Disconnect all the speakers and one wire from the OT and then check with your meter. Check each speaker for a short in the wiring.

[jibe]

I've got some work to do!

I've read up on the fixed bias procedure.
Solder 1ohms resistor pin 8 to ground of output tubes to read bias current.
I have the bias pot already in place.

I need to make schematic over the 2-4-8 connections of the
output transformer. I had an amp-tech help me to start it up for
the first time and he's the one who fixed my output connections.
I wasn't sure how to fix that.
Maybe I should remove 2-4 and only connect the 8 and ground?
Narrows it down a bit if nothing else.

The cab seems to be working fine with other amps so that shouldn't be
a factor, no shorts or anything there.

Strange that I get 425v on pin 2 of Rectifier, how come? If I should
only get DC on pin 8.
I'll make some AC readings as well.

Strange to get 332v on pin2 on V3 When pin7 is only 23v?
Have to double check that but I'm pretty sure about the measurement.

A question: How long does it take for power caps to drain?




//Jibe

[loudthud]

So you have an impedance switch. What kind of speaker jack(s) do you have? It's easy to make a short you can't see on those phono jacks that are standard. And you can't really measure for a short with the transformer connected. While one side of the OT is disconnected, measure/verify the 56K feedback resistor. Your voltages look good enough you should be getting more sound. The signal must not be connected as it should or it is shorted to ground somewhere.

[erndawg]

I've read up on the fixed bias procedure.
Solder 1ohms resistor pin 8 to ground of output tubes to read bias current.

For now, forget about installing bias measuring resistors on the power tube cathodes. Just set the bias voltage very negative as I explained in my first post.

PS. I'm not sure I feel comfortable experimenting around the wiper of the treble since it seems I have unusual
high voltages there, +300 instead of +20. DS.

Are you certain you're reading 300 VDC on the treble pot's wiper? There are DC blocking caps all over that circuit that should prevent that. Check again.

Maybe I should remove 2-4 and only connect the 8 and ground?

Yes, do that. That will eliminate any wiring error there that your tech friend may have introduced. Put electrical tape on the 2 and 4 ohm taps. Use a standard 1/4" jack (non shorting) as your speaker connector for now.

Strange that I get 425v on pin 2 of Rectifier, how come? If I should
only get DC on pin 8.

Not strange at all; that's how it is supposed to work. Pins 8 and 2 are the rectifier's cathode; they are common to each other in the tube and also through the 5 VAC windings. Note how the 5 VAC winding floats above ground. It has no ground reference like the HV AC and the 6.3 VAC. That is normal operation.

Strange to get 332v on pin2 on V3 When pin7 is only 23v?

Yes, double check that reading and triple check the wiring per the layout.

A question: How long does it take for power caps to drain?

I DO NOT like the fact that these kit companies do not put safety bleeder resistors on the B+. That is inexcusable. Fender got smart on that 45 years ago. What method do you use to bleed the caps?

[erndawg]

Just to make sure, are you reading all DC voltages with respect to ground?

That is, when reading DC, do you connect your Digital Multimeter's negative (black) probe to chassis ground?

So, in the case of reading the Treble pot's wiper, you would have the black lead to chassis ground and the red lead on the Treble pot's wiper.

[jibe]

I don't have a switch, it's wired to 3 of the 4 RCA connector that came with the kit. Then to a switchcraft phono jack in the cabinet. I changed all jacks to switchcraft. I tried to get one today but the ones at my local dealer didn't fit my needs, might have to order in from the US. Mouser or somewhere.

I haven't measured 300 on the wiper, but on pin2 of V3 and that leads to
the wiper, but as you say, there are blocking caps in the way. It just makes
me really casious (read nervous) to be so close to the danger zone.

I touch a 100k wired to ground on each of the power filter caps for about one minute. ( Then I let it rest for about a week since I'm new to this )
If I want to install a bleeder resistor, I should put it across C12 on my schematic? 100k 2W? Right?

Yes! I have the black stuck to chassi and the red on the place I measure (and one hand behind my back).
//Jibe

[tubeswell]

Hi there,

Sorry I missed the fact that you measured the rectifier in DC. The PT secondary windings all put out AC so that's what you meed to measure. However you will get a high DC reading on Pin 8 (or 2, whichever you have connected to the B+, because this is connected to the reservoir cap). The original Fender 5F6A schematic shows 325VAC coming off each side of the HT winding (red wires), so you ought to get something like that with the Weber version (depending on the PT it uses). 425VDC B+ is a teency weency bit on the low side for that amp, but is still within the ballpark. You need to solve the riddle of why you have a high voltage on pin2 of your PI.