NEW EXPRESS BUILD!! A FEW PROBLEMS!! NEED HELP!!

[prslespaul]

I finally wired up the rest of the amp and decided to plug it in.. Nothing blew up, so were doing good! Then i put the tubes in, they fired right up. Only problem, I didnt get any sound at all so i looked at the OT and realized i forgot to ground it. I unplugged it, grounded the OT and BAM worked like a charm! It sounds great! Only one problem with the sound, the presence knob doesnt really sound like its doing what is should, It seems to be wired correctly and it is going out to the 8ohm output. When the presence is turned down (anywhere from 1-9, and i pluck the strings real hard, it makes like a pop sound and gets a little noisy (hum when not playing). When i turn it allll the way up to 10 (no less) it sounds great, and the noise (hum) disappears. I am using a 6800ohm Weber OT. Any Ideas??

Also, I bought a Toroidal Transformer for the PT

https://www.antekinc.com/details.php?p=58

I have the primaries wired together to give me 8A on the heaters and to give me a center tap for the 300V. It seems to be working good, it just gets really hot. Im not sure how hot these are supposed to get, but i played it for a good 10 minutes and it was too hot to touch, but not much hotter than too hot to touch.. It seemed to be sitting at the same temperature, but it starts to dispense the heat through the chassis making the chassis super hot too. That i dont really like because i dont want any of the solder to the grounds breaking loose or melt any insulation on the wires. Any ideas on that?

Other then that, the amp sounds fantastic, I will post clips and pics soon! Thx guys!!

[paulster]

For your presence control, lift one end of the cap and ensure that the amp sounds okay without it. Assuming it does then you've probably got a leaky cap and are affecting the bias of the PI depending on the position of the pot.

Pull the tubes and let your amp run for a bit in standby mode, so the transformer has no load whatsoever. If it still gets really hot then there is the possibility that you have one of the pairs of windings that have been tied together out of phase. I wouldn't have thought it was likely as I wouldn't expect to get anywhere near normal operating voltages, but it's also an experiment I've never tried(!) so worth ruling out all the same.

If it's cold or cool then add the tubes and keep it on standby, so it's only driving the heaters and see how that goes.

You are using the rubber washers either side of it, aren't you? It's important that there isn't a metal plate either side of the core that will cause a magnetic short on a toroid.

[keewee]

Hi, I was just reading your post and have been contemplating using a toridial also, I think the toridial does't have a center tap, so you must use a bridge rectifier and must wire the two secondaries in parallel to get the full current capabilities, otherwise using it may be current starved explaning the excessive heat.
Did you use there output transformer as well?

[paulster]

It's a 200VA transformer so it should be okay used in a full wave configuration for a Wreck build.

Going to a bridge would make more efficient use of the HT secondary but then you'd have to use a capacitor-coupled bias circuit which adds a degree of complexity.

[prslespaul]

You said having your heaters wired out of phase would cause it to overheat like that? I actually did wire some of them out of phase because i knowticed on some of the originals the heaters were wired out of phase so i tried it to see what would happen. Do you think i should switch them all into phase then?

I pulled the tubes out and just ran it on standby mode. it still got hot, and it got hot fast. What do you think is the cause?


Heres a clip of what the noise sounds like.

https://dl.dropbox.com/u/2727776/Trainwreck.mp3

[paulster]

It sounds like a possible magnetic short caused by having one winding connected out of phase with another (most likely your two primary windings or your two heater windings).

Disconnect your HT secondary connections and cap them off, and disconnect the heater windings and cap them off. Make sure that the two heater windings are disconnected from each other.

Then fire up the transformer and see if it gets hot, as it could be a phasing issue on the two primaries.

If that's okay then connect two of the heater wires (one from each winding) together exactly as you had before but leave the other two disconnected. Then meter between these two disconnected wires on AC volts and ensure that you get 0Vac. If you get 12.6Vac then you've got the phase wrong.

Then you can reconnect the heaters.

Finally, meter between the two HT secondary windings with the centre-tap re-established and ensure that you are getting 600Vac from one end to the other, and that you've got a meter that can withstand 600V.

[prslespaul]

The HV was correct it was at 600V but when i measured the heaters they were at 8.4v and i had the secondaries hooked up parallel. I figured that wasnt right so i disconnected them and messed around with a few different wiring schemes and i found it only to read right when it was in series so thats how it stayed. It reads 6.3V now and it is like 11-12 amps. I dont understand why the amperage is so high. That wont hurt anything will it? Regardless of the high amperage, the PT stays cool, it also cleaned up the bottom end a lot.

On the negitive side..., the clipping noise is still there. I disconnected the .1uf cap on the presence knob and it still makes that noise. That clip posted above is from when the knob was turned all the way up. When i turn it down it gets worse and worse untill it eventually starts to squeal. I am still pretty clueless on what is causing this, everthing seems to be measureing up fine and the schematic looks good. The amp is virtually noiseless even at high volumes, its just when i play it makes the noise. Hopefully that helps you. Thanks a lot Paulster for all your help, this build wouldnt of been possible without you.

[keewee]

Hi, It's me again, if you use that 600vac I think that translates more than 700vdc filtered properly if I recall my electronic theory, I hope you have some sturdy tubes. 300vac should give you about 360vdc (i've misplaced my calculator). I think the express uses about -33vdc fixed bias, that is why I am going to build a rocket (cathode biased) with the bridge rectifier. I'm really interested to see how you do it without a centertap on your secondary if I recall my power supply theory.

[paulster]

Hi, It's me again, if you use that 600vac I think that translates more than 700vdc filtered properly if I recall my electronic theory, I hope you have some sturdy tubes. 300vac should give you about 360vdc (i've misplaced my calculator).

This transformer has two 300V secondaries. Put them in series, as prslespaul has, and you have effectively a 300-0-300V centre-tapped transformer.

That's why you read 600V between the two ends and use a full wave (not a bridge) rectifier exactly as the Express always has.

[paulster]

The HV was correct it was at 600V but when i measured the heaters they were at 8.4v and i had the secondaries hooked up parallel. I figured that wasnt right so i disconnected them and messed around with a few different wiring schemes and i found it only to read right when it was in series so thats how it stayed. It reads 6.3V now and it is like 11-12 amps. I dont understand why the amperage is so high. That wont hurt anything will it? Regardless of the high amperage, the PT stays cool, it also cleaned up the bottom end a lot.

That doesn't make a lot of sense, particularly how you're getting the 11-12A figure. How did you measure this, and can you post some photos?

The heaters will read higher unloaded but in series you should get 12.6V loaded and even higher unloaded so I'm very confused!

On the negitive side..., the clipping noise is still there. I disconnected the .1uf cap on the presence knob and it still makes that noise. That clip posted above is from when the knob was turned all the way up. When i turn it down it gets worse and worse untill it eventually starts to squeal. I am still pretty clueless on what is causing this, everthing seems to be measureing up fine and the schematic looks good. The amp is virtually noiseless even at high volumes, its just when i play it makes the noise. Hopefully that helps you. Thanks a lot Paulster for all your help, this build wouldnt of been possible without you.

Photos!

[prslespaul]

I measure everything from one side of the heaters to the other side of the heaters and it read 6.3V. When i had them in parallel it was measuring like 8.4V or something retarded and my power supply was getting way too hot. I tried to wire it every which way in parallel but it would blow fuses, even 2A fuses. The only way it didnt blow, it got hot, so i rewired it in series and it worked first try. Keep in mind i do have a crappy multimeter so who knows. Here are some pictures though if you wanna see the build.

http://s205.photobucket.com/albums/bb25 ... rainwreck/

[prslespaul]

Anything in the pictures seem to stick out? I am puzzled on the noise, I cant use it in the studio with that clipping noise, do you think one of the filter caps can be leaking? I am using weber caps.

[paulster]

The most obvious potential problem is that you've run your two power tube grid wires via different routes rather than right alongside each other, but also you've run your negative feedback wire to the presence pot right alongside one of them.

The NFB wire is one of the trickiest wires in an Express build as it can cause all sorts of noise and oscillations.

The mp3 sounds like it could be a leaky cap, but before exploring that I'd certainly sort out the lead dress around the power tube grid wires and the NFB. Take a look at the Francesca photos if need be to get a good example of how to do it.

[bigbeck]

Just a couple of questions on the PT. The link that you posted shows one of the secondaries as being 3V and the other being 300V. The spec sheet shows two 300V secondaries and two 6.3V secondaries. So I figured the 3V figure on the description page is a typo. So if you hooked both 6v secondaries in series, you would end up with 12V,right? Not 6.3 V - unless the heater taps are 3V like it shows on the PT description page. I'm confused. Maybe it's a defective PT?

[Structo]

Man, $35 for that PT?

That seems very inexpensive.
I suppose when they don't have to have endbells and laminations it makes the cost much less.

How come more guys aren't using these?

Is there something wrong with using a toroid PT?