New 18-Watt Works! - well almost...

[nigelb]

Just checked out and fired up my 18-watt chassis today. Its not meant to be a clone as such but will be built into a head. I was first greeted with ultra loud motor boating. I have been visiting this forum for a few months now and I have got the impression that the trem channel can be a bit troublesome. So I removed V3 and it disappeared, normal channel works fine. Then found out that shorting the trem out (turning it off) stops the motorboating and the channel then works fine, but of course without trem.
I already have a screened cable to the footswitch jack and I have tried a 47pf across the footswith but this just makes it motorboat when switched on or off (though the sound is different).

I do not have the input to V3/7 screened, is this necessary? It is not shown as screened on any of the layouts.

I have to admit the sound through my 1x12 cab with Vintage 30 is wonderful and so, so loud.....I am well pleased, I simply gotta get the trem working properly. Any other ideas guys? BTW the valves are all JJ/Tesla, except for EZ81.

Also I am a little concerned about the plate voltages on the EL84s. I am using a Hammond 372FX 300-0-300 transformer (I said it wasn't a clone)but am getting 370v out of the EZ81 so the plate voltages are about 360v. Is this too high? I notice that on the voltage chart that Gabi put up a few days ago the highest is 342/343v. Should I be concerned?

Cheers

Nigel

[nigelb]

Five minutes after posting this I decided to try a cheap crappy old EI ECC83 that I had lying around in the V3 position and.......

the trem channel now works!!!!

This valve sounded awful in the EL84 P1 that I recently built, but it actually sounds sweet here. So false alarm guys. The JJ isn't faulty, its fine in other positions. Is this common?

But I would still appreciate some advice on the other matter.

Yippee

Nigel

[EBE]

Yes, this is common. I use JJs in V1 and V2 and sovteks in V3. I still have a few of my precious RCAs left and they do not work in V3 either.

Norm

[EBE]

Another thing I do know is twist the wires off the 100ks to V2. I learned this trick from Richie. To be honest I am not sure if it makes a difference but every amp I have built since has worked so I continue the practice.

Norm

[loverocker]

Can't help on the voltage, but just wanted to say !

[zaphod_phil]

Also I am a little concerned about the plate voltages on the EL84s. I am using a Hammond 372FX 300-0-300 transformer (I said it wasn't a clone)but am getting 370v out of the EZ81 so the plate voltages are about 360v. Is this too high? I notice that on the voltage chart that Gabi put up a few days ago the highest is 342/343v. Should I be concerned?

There some folks running their amps at higher anode voltages, and some EL84s seem to be capable of handling those voltages better than other. If you don't hear crackiling noises when cranking up the amp and playing loud (which would indicate arc-over), you're probably OK. First thing to sort out however, is your anode dissipation, which should be roughly somewhere around 75% of the EL84's rated 12W. You may need to increase the cathode bias resistor to something like 180ohms or so. In fact 18watters with more normal voltages really need to have a 150ohm resistor in there to stay closer to spec, but some builders will prefer to run hotter and put up with the reduced EL84 life span....

[Plexi]

To go back to some of the trem problems... I found that the wire going to pin 6 of the trem channel tube..use shielded cable,it takes out a lot of problems you can have. And if you don't use shielded cable.. that lead or wire, can cause noise problems. Try to keep it close to the chassis. If its sticking up in the air off the boiard, even the slightest placement of this wire can pick up noise,ticking,and alot of things. I also like the trem alot better with the slight value changes..it comes on without delay, it has a good speed range, and sit removes a lot of the speaker thumping.
I sent these to Gabi,if anyone wanted to use them..here is the change
very simple.
820 ohm to 1.5k
1M that goes to ground change to 470k
use a .022 cap in place of the .01 {the one attached to the 2.7M end}
2M pot to 1M Linear taper
1k pot to a 1k Linear
i like the .05 cap just a slight difference useing the .047 cap a very slight difference in speed. thats about it..

But that tube is also the first thing to try many different ones.. i also have done this quite a bit..and found the Sovtek WA to seem to be the most consistent.. Its just weird that other really good tubes just won't work in this V3 spot..but will work fine in V1 and V2

Richie

[nigelb]

Cheers guys

Phil, I had already decided to up the cathode resistor and got 2x330R 3w resistors this morning from my local Maplin. 165R should at least be safer until I get around to doing the measurements and calculations. Otherwise I'll leave as is. As for cranking up the amp and listening for crackling noises, I can't run it over 50% in my office/workshop as eveything bounces off my shelves and the windows and doors rattle!!

Richie, Those mods you describe may well be useful, I'll give them a try. After the elation of getting the trem channel to work, I realise now that there are still problems with ticking and motorboating at certain positions on the controls (tone and intensity at min is one). The trem channel will be of limited value to me but it is nice to get it correct.

But I am still happy with the amp, it sounds awesome. I couldn't have done it without 18watt.com - cheers

Nigel