New 5F6A Build with Odd Voltages and Loud Buzzing

[jim911]

Hi all, I have been building a 5F6A Tweed Bassman over the past month and got to the point of start up this weekend. I want to thank all who's posts I have used as reference for different steps of this build to get to this point.
This forum is a terrific resource as a new builder.

I am experiencing what I believe are some sort of odd and high voltages and very loud buzzing with no guitar signal (had guitar signal for a little while but still with very loud buzzing).

I'm using the Heyboer power trans HY022798/HTS-9405 from Weber and a Heyboer output transformer as well.

I have the following voltages without the power tubes in (its is buzzing to loud to test the voltages with the power tubes in and the family at home).

5AR4 I have 5.4 volts AC between pins 2 & 8 and on pin
4: 382 vdc
6: 382 vdc

On all filaments when I measure between pins 2 & 7 on the power tubes and between 4/5 & 9 on the preamps I get right around 6.9 volts.

Here are the DC voltages on other tubes (once again the power tubes are not installed at this point).

V1
1:268
3:2.26
6:268
7: 0
8:2.29

V2
1:198
2:0
3:1.65
6:421
7:201
8:205

V3
1:343
2:59.3
3:176
6:340
7:67.4
8:176

Power tubes are both the same at:

3:527
4:528
5:-47.7
6:526

In reference to the buzzing I have grounded per the fender layout to the brass back plate. I haven't taken any common ground from that plate to a bolt anywhere....Do I need to?

Also jacks I assumed grounded themselves by being bolted to the chassis is this a correct assumption?

The buzz is very loud whether volume, presence and tone controls are up or not...whether guitar is plugged in or not.

I did get guitar volume out of it for a shrt periond of time but now nothing but buzzing. I went throught each component and all wiring again this morning and double check with the layout and with the exception of what is under the board all appears to be correct.

I aprreciate any idea's I could try to trouble shoot where the problems may be.
Thanks

[loudthud]

Those voltages seem really high. Info from Weber says the transformer has 120V and 125V inputs. Which input voltage is your amp wired for?

Measure the resistance to ground (with power off) from V3 pin 3 or 8 to ground. It should be 15.4K. If you measures around 66K, you are missing a ground at the presence control.

[jim911]

Loadthud,
Thanks for the heads up on the Presence Control ground it was indeed bad. I went through and grounded all of the additional pots to the brass grounding plate per the layout, which I hadn't done originally assuming (I guess incorrectly) that the pots would be grounded to the brass plate and chassis when they were tightened in place. Please let me know if I now have that missing portion of the grounding scheme correct as it seemed to have lowered the deafening buzz down to just medium at this point.

It still buzzes quite loud but I have guitar signal back through the speakers which is a plus. The buzz now seems to be adjustable with the volume controls seeming to lesson the buzz with the bright channel volume around 2 and chhannel set between 4 and 5.

Also, I have taken new voltages with the tubes in place which are below but one thing I have notice is that on V3 when I am taking the voltages when I touch either pin 2 or 7 the buzz completely goes away. This seems that it somehow is the source of my buzzing problem and the voltages on those pins seem to be very low. I wonder if I have a bad tube or socket?

V1
1:244
3:2.14
6:248
7: 0
8:2.14

V2
1:205
2:0
3:1.4
6:387
7:205
8:215

V3
1:277
2:16.4
3:55.3
6:272
7:18.5
8:55.5

Power tubes are both the same at:

3:500
4:499
5:-46
6:499


In reference to the Heboer PT being 120 or 125 I am not sure as when I contacted Weber for the specs they sent me to Heybour who proved an email with the following info only:

HY022798

Primary Black & White
Hi Volt 370 - 0 - 50 - 370 volts
Red - Red/Yellow - Red/Blue - Red
5 Volts Yellows
6.3 Volts Browns

Once again thanks for the helpful advice and it seems like I am making some progress.

[tubeswell]

Pin 2 and 7 of a 12A_7 type tube are grid pins. If diverting pin 2 to ground reduces the buzzing it could be something in the stage in front of it or a bad tube. Did you try swapping the tubes?

[MWJB]

Voltages are in line with the PT specs & marginally high for a 5F6A, try a 5U4G rectifier.

The Fender layout does not contain adequate grounding info to build a properly functioning amp.

OK for jacks to ground at chassis. Check out Hoffman's grounding scheme at Hoffmanamps/EL34 World. Post some pics if you can.

[jim911]

Tubeswell...I have swapped in a different 12ax7 into v3 and no improvement in buzzing or changes to the pin 2 voltage of 16.4 and pin 7 of 18.5

MWJB...did a quick read through of the Hoffman grounding info and that looks like a good guide to follow...it would be a pretty major change from using the brass ground plate which I know others have gotten to work but if I am unable to find other ways to improve the problem may be the route I need to try. I'm certain my grounding could be improved up but it seems that the low voltage on the grids of v3 seem to indicate some additional problem.

I will pull a 5U4G out of another amp this evening and see if that helps lower the overall voltages.

I will try to attach some photo's as well

[jim911]

some additional photos of the other side

[loudthud]

You can't directly measure the voltage on the grids of V3. The impedance of the meter changes the circuit because it pulls against the 1Meg resistor towards ground and that tends to reduce current through the tube. The best you can do is measure the voltage at the junction of the 1Megs, the 470 and 10K and then measure from that point across the 1Megs which should be close to zero volts.

What's the arrangement of the caps in the "dog house" with respect to grounds? That's where most hum problems originate. Study how Hoffman suggests you split the grounds at the filter caps.

[Mandopicker]

I cannot see from the photos, but I am curious to know if your power transformer has a Filament CT or if you are using an artificial design with 2 x 100 Ohm resistors to ground.

Thanks in advance...looks great...your almost there.

[jim911]

Loudthud...The voltage at the junction of the 1 megs, 470 and 10k resistor is52.5. To measure across the 1 megs am I using both probes in the amp (I have been keeping one hand in my back pocket) one on each end of the 1meg resistors? Power off or on?

In reference to the caps and grounding I have attached a photo but I have run the positive side per the attached Weber layout and the grounds go to ground rings attached to a small stainless bolt I tapped into the chassis through the card that insulates the caps from the chassis. Maybe this is the source of a poor ground?

Also on the weber layout I ran the main power caps from the points labelled A, B and C per the drawing. I do have a question regarding this layout as it shows just beyond point C which feeds the first cap there appears to be a 4th wire running to the same first 22uf 450 cap. Is this correct? When I tried to wire it as show I instantly burned up the 4.7K 1watt resister tied to the choke. So I question whether this connection is correct. I noted that this 4th wire or connection doesn't appear on the original Fender layout.

[jim911]

whoops posted the wrong Weber layout

[loudthud]

I wouldn't lose any sleep over measuring the voltage across the 1Megs. If your meter has some kind of ground clip, clip it to the junction and measure to the end of the 1Megs with one hand. I don't think that's the problem here. Voltages are measured with the power on and yes, it's dangerous! Use all due caution.

The ground for all the caps on the standup board looks like it comes from the "point C cap" past points A, B and C, solders to an eyelet and then a short wire connects to the brass plate. It's ground so it will burn up the 4.7K poof!

Trying to improve on Webers grounding scheme:

Look at the Hoffman article and note how the first filter cap has one ground and the cap after the choke has a different ground. That's what you need to do here. The Weber layout tried to do the same thing by separating the ground for the 8uF cap on the main board. I would remove the wire from the screw to the two black caps in parallel and run a new ground wire from those two caps to where the red wire with the yellow stripe from the transformer gets it's ground. If that doesn't make enough improvement, run a wire from the other two caps to the brass plate.

[jim911]

Mandopicker

I deducted (I hope correctly) from the specs provided below from Heyboer that the -0 Hi Volt secondary wire which is described below as the Red/Yellow wire was my center tap and I have it grounded to one of the power transformer bolts.

Heyboer

HY022798

Primary Black & White
Hi Volt 370 - 0 - 50 - 370 volts
Red - Red/Yellow - Red/Blue - Red
5 Volts Yellows
6.3 Volts Browns

[MWJB]

Rewire main stage filters as 2x100uf 350v in series, each cap bypassed with 220K 2W resistor, see Super Reverb schem for the idea. You have no voltage headroom on your main filters.

Ground volume pot tabs to the pot body, not to the brass plate, likewise the presence cap. Ground your power tube cathodes to the same PT bolt that you are going to use as a ground point for main & screen filters & bias supply.

[djembutz]

You should do what MWJB sugested. I thought it just grounding problem.
From picture i saw in powersuplly can caps section, it's ground to the body. I'm right ?
I sugesting that you properly ground that powersupply can caps in the right place. Just search starground or better galatic star ground for refference.