50-Watt kit - bias voltage way too negative

[hourglass]

I built this kit with a Metroamp board & rectifier and mounted a 2nd 50/50 can cap inside the chassis (a lot like the orange amp Granger has in their gallery, 2nd from the top).

I kept the Weber chassis and transformers. I built it to 1971 metal panel specs with two 47uF caps mounted to the board in place of a 3rd 50/50 can on the chassis.

The problem I'm having is that I'm reading -190 VDC on pin 5 of each power tube socket. This is with the power switch engaged but the standby switch UP so there is not anything flowing to the B+ circuit.

The Weber transformer has a brown 120V primary and a blue 125V primary.

I used the 120V brown wire since I'm measuring 117 VAC coming out of my wall socket and at the live terminal on the IEC connector.

I measure 340 VDC on the + terminal of the rectifier bridge.

If I install a tube in V1, the filaments light up. I took this out after testing to see that part was working.

If I work the bias pot, the voltage on pin 5 sweeps between -185 and -200 VDC.

Flipping the standby switch to PLAY shows I have 498 VDC on the B+ fuse. Pin 5 doesn't change. I flipped the standby switch UP after quickly testing those two voltages.

Since this board uses a bias circuit more like the Marshall one, I referred to the schematic. Where the Metro board calls for a 150K next to the diode in the bias circuit, the schematic calls for 220K.

I changed this to 220K and the pin 5 voltages dropped to -85. Sweeping the bias pot gets me from -80 to -90.

Since the B+ voltage is so high should I try the Blue 125V tap on the power transformer? Or does anyone have any other ideas? I'm thinking that the Weber transformer might simply require a different value for the 150K or 220K resistors that I already tried, but I wanted to post to be sure I wouldn't simply be masking some bigger problem by changing the resistor to get the bias voltage to the correct range.

Here's a pretty high-res photo of the guts in case anyone sees something I missed. As I'm color blind I have a tough time with resistor bands. However, I measured every single resistor before soldering it into place.

Thanks in advance for any advice.

http://www.guitar-hardware.com/ampkit/IMG_0517.jpg" onclick="window.open(this.href);return false;

Ryan

[neikeel]

Assuming that is the stock 25k bias pot if you are using that 150k bias resistor (on an angle betwwen the bias caps) it is well out of kilter with what is normal. I would try a 68k or 56k there.

Oh, and particularly if Larry is moderating, you might want to say hello and introduce yourself properly before you get severely chastised and have to say a few Hail Mary's to apologise and atone for using a Weber kit, but asking for hepl on a Metro forum

[hourglass]

Please accept my apologies for not introducing myself.

As for me, I'm 35, work a computer job, build guitars (and now, hopefully, amps) and play lots of loud noisy music to annoy the neighbors.

As for the Weber kit, it was a gift from my family for Christmas so I decided to make the best of it by upgrading as much as I could. Aside from the chassis, trannies, tube sockets, and pots it's all Metro.

If it helps at all, here's a jpg with the specs of the weber transformer: https://taweber.powweb.com/store/022798sch.jpg" onclick="window.open(this.href);return false;

Are you saying you see a 150K resistor between the two blue bias caps in the picture I posted? The resistor I was talking about is the one at the top left edge of the board. That started out at 150K and is 220K in that picture. If the one you are talking about is 150K, then yes - the layout shows that it should be 47K.

Thanks for the help.

Ryan

[neikeel]

Yes it is the 150k that is not right, 47k - 68k is usually the range. I just made a guess of 56-68k on the voltage you got with the pot dialled back, try any of the above. I like to set up so that at first you can get around -40v with the pot in the mid position, gives you leeway for when the valves burn in (few hours of decent use) or if you change the valves at a later date. The 220k bias feed (in series with the diode is correct as is the 15k between the two caps.

Welcome by the way, lots of friendly helpful people here

[hourglass]

Neil,

Thanks a ton! You spotted the exact problem. In my excitement to finish last night I managed to swap the 47K and 150K resistors in the bias circuit. When I tested the supposed "150K" bias supply resistor I found that it was the 47K.

I put them in the right spots and my grid voltage was spot on.

I continued checking it and found that the voltages for V1 seem a little low. I powered it up with all tubes in it and biased it. Running at 17.5 watts with 498 volts on the plates, my brain says to bias it at 36ma. When I first checked it was in the 20's. As it went though the 30's it started a low thumping oscillation up until about 41. I set it back to 36 and then went about trying to see what might be thumping. After turning all the knobs, I found that in the middle of the sweep on the treble pot causes the oscillation. At 0 or 10 it's fine without any noise, even with the volumes dimed. If anything, I suspect the treble pot. It seemed like it was coated with lacquer and was extremely difficult to solder to the back of it so it could have gotten overheated.

Plugging a guitar in, I found that both the upper and lower jacks for input 1 worked and were controlled by the volume 1 pot. However, the upper jack for input 2 also seems to be affected by the volume 1 pot. The lower jack for input 2 is only affected by volume 2. Any idea why this would be?

Despite those couple of issues, the sound of the guitar coming out of it made me grin like a kid at Christmas.

Ryan

[arledgsc]

I see the problem. You have a 150K ohm resistor between the BIOS pot and 15K ohm dropping resistor after the diode rectifier. Change the 150K resistor to something like 47K ohm. (Doh... Already answered by Neil. Good find).

On the input issue did you forget the green jumper wire on between the top and bottom CLIFF jacks? It's hard to see if it is there on Input 2.

Scott

[guilds100]

I too tried to marry a weber kit to a metro layout with less than perfect results. I would use the red/white leads on the output side and the blue lead on the input of the power transformer to get your b+ down closer to the range of the metro 50 watt circuit. Mine started at 505v and was down to 435 after the changes. Also be careful with the pots. I just had a lengthy trouble shooting session that turned out to be a bad pot.

[hourglass]

I tried a new treble pot but the weird oscillation was still there. I also realized that turning the treble knob changed the bias setting.

So, I turned my attention to the presence knob and found that I hadn't grounded the capacitor. As soon as I did that the osciallation stopped and the amp got a lot louder. It now works perfectly.

Thanks for pointing me in the right direction.

Ryan