Up and running

[fred]

It's working and it sounds great, but I have some concerns. Here are my voltage readings. V6 1 - 297V (AC) 7 - 298V (AC) 3 - 328V
V4/V5 3 - 10.6V 7 - 314 9 - 308V
V3 1 - 76/80V 3 - .88/1.03V 6 - 134/154V 8 - .88/1.03
V2 1 - 193V 2 - 38V 3 - 80V 6 - 194V 7 - 38V 8 - 79V
V1 1 - 150V 3 - .91V 6 - 150V 8 - .87V

V4 & V5 get pretty red, but I don't know if it's abnormally red. I've been trying to follow all the math stuff, but I'm not really sure what volatges your talking about (plate, cathode volatges etc etc). Here what I got out of the posts. I think I'm dissapating 28.6 watts from both tubes ( a little high?).
Did I do this right, and do I need to change R20 (it's 125 ohm now) to a higher value?

I ran the amp through the speaker in my Gibson Discoverer amp. It's a Jensen 12" w/alnico magnet (don't remember what model), and then I ran it through my Carvin Belair with 2 12" V12 speakers. I think I liked the Jensen better. Right now the 18 Watt is my favorite.

Now on the the cabinet.


Fred

[fred]

Thanks Paul. Could you explain to me by using my tube voltages how you came up with your numbers. These reading I took were for a pair of old RCA (6BQ5) tubes I had taken out of my Gibson. At first I had a pair of the russian tubes in and has readings of: pin 1 - 10.09V 7 - 307V 9-300V
The filaments didn't seen to be as red on those.

Fred

[fred]

After reading all this stuff, this is how I think you get the numbers.
My readings on V4/5 pin 3 - 10.59V pin 7 - 314V pin 9 - 308V

Subtract 10.59 from 314 = 303.41
Divide 10.59 by 125 = .0872

Then multiply .08472 X 303.41=25.705
Then divide 25.705 by 2

And should you measure the 125 ohm resistor due to the fact that it probably a 10% tolerance part, and use that number?

[fred]

Thanks again Paul.
Another slight problem I'm getting is a slight humming noise from the amp when I plug in the guitar, It gets worse when I turn the lights in the room on. Is there anything I can do for that.

[fred]

It's one of Graydon's chassis, so it's open, and I just have it sitting on the table. I do have shielded cable on the regular channel inputs, but not on the Trem channel inputs. Does it matter which end you ground? I grounded the tube end. I think one of your recent messages said that you grounded the input jack end. Will putting in the 68K resistors in line with the inputs help this?

[fred]

I'm using a footswitch to turn the tremelo on and off, and I'm getting the poping noise when I turn it off even with a 150pf cap in place. And it seemed to develope a new problem yesterday. With the themelo on and the footswitch plugged in it makes a loud humming noise. It goes away when the switch is unplugged (trem is on). I've switched tubes, and moved wires around. The next thing I was going to do was to hardwire the footswitch in, thinking maybe it doesn't like my jack (it's not the plastic Cliff
jack, but a cheapo from Radio Shack placed right between V2 & V3).

I also want to mention the startup info on Paul Ruby's web page. It was very helpful, and I had no problems with getting this thing running (except for this minor stuff I just mentioned). I also get some minor humming noise, but all my amps seem to do this. I think it's the old wiring in the house.

[zaphod_phil]

With the themelo on and the footswitch plugged in it makes a loud humming noise. It goes away when the switch is unplugged (trem is on). I've switched tubes, and moved wires around. The next thing I was going to do was to hardwire the footswitch in, thinking maybe it doesn't like my jack (it's not the plastic Cliff jack, but a cheapo from Radio Shack placed right between V2 & V3).

Check that the jack is fully insulated from the chassis. Also, check that the hot and ground side of the jack are wired the right way round.

[fred]

I had the jack hooked up correctly. I even hard wired the footswitch in trying different ground points, and then even disconnected the switch. For some reason it doesn't like the cable. As soon as I hook it up it makes a humming noise. I tried switching tubes I have some russian ones from my carvin belair, and some JJs (all the JJs motorboat). I may just try another brand tube. I hooked up a short (12") section of wire and connected it to an old wall light switch I had and that seemed to work ok. I'm about ready to just give up.

I did change my cathode resistor to a 150 ohm and i'm running around 11.2W plate dissipation thru each tube.

I'm also using the star ground method that is shown on one of the layout drawings. The only thing that is using the chassis for a ground path are all the POTS. Should I ground them with a buss wire?

[drgork]

Fred,

I'm a little late to this thread, but before you change the Cathode resistor, you may want to try a set of new matched EL84's. I had a customer try a couple of sets of "matched" NOS tubes and they red-plated like crazy. Even though the tubes may be "matched" in some way, they may not be able to handle the voltages in the 18. There has been some discussion about this in the past. Matched JJ's and Ei's have both worked well. Certainly tweaking the Cathode resistor has helped several of us in the past.

Just a thought.

BTW, thanks for the math Paul. Very helpful!

Danny

[MarkB]

Fred,

If you're doing a wire (star/buss) ground, the pot cases are grounded to the chassis, and the lugs are grounded to one of the stars. Usually, a volume or tone pot is grounded to the cathode resistor ground of the next tube stage. The wire - signal - ground is connected to the chassis in one and only one place.

MarkB

[zaphod_phil]

Fred,

I'm a little late to this thread, but before you change the Cathode resistor, you may want to try a set of new matched EL84's. I had a customer try a couple of sets of "matched" NOS tubes and they red-plated like crazy. Even though the tubes may be "matched" in some way, they may not be able to handle the voltages in the 18. There has been some discussion about this in the past. Matched JJ's and Ei's have both worked well. Certainly tweaking the Cathode resistor has helped several of us in the past.

Danny, the issue is related to variations between different manufacturers' EL84s, not tube matching. Some can handle higher B+ voltages and over-warm biasing better than others. That's the key, as well as taking care with your cathode bias resistor value. The benefits of tube matching are actually quite debatable - as long as you don't have a pair of tubes that are very different from each other in their characteristics. The vintage Marshall, Vox, Fender amps we try to emulate weren't built with matched tubes.