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SVT 3 PROblem

G

Gint

Sep 12, 2009
10
0
#1
Couple of questions
Powering up, preamp working but no sound coming out. Checked leads cab etc, all working.
1: Should all the valves light up? (all the 12ax7's are lighting up but not the 12au7).
2: Should the fan always be on, its not going at all (could this be related top the tube?)
Would the fan not be working because of something else, or would the sound not be coming out because of the fan. Where can replacements be found?
Any suggestions?
Thanks.
 
#2
Hi Gint.

Welcome to TalkBass.

While a working tube always glows (but a glowing tube doesn't always work ;)), not all glow enough to be noticeable in normal lighting.

If all the fuses are all right, and swapping the 12AU7 with a new one doesn't bring the sound back, then I'm afraid it's a trip to a tech for that amp.

Regards
Sam
 
#5
Hey Gint,

Yes. The fan should be on at all times. Not having the fan on could have resulted in something burning out in the amp. Sam and NKUSigEp both offer good advice here as well so heed that. First thing I would try is replacing the 12AU7 as Sam suggested.

A couple of things to check as well. Are you sure the Preamp is okay and not the power section? Have you tried driving another power amp with just the PREAMP OUT from the 3 PRO? Another way of checking the power amp (and this should only be done with extreme caution) is to set up your rig but instead of plugging your bass into the front input on the 3 PRO, plug into the POWER AMP IN on the back of the amp.
*****VERY IMPORTANT!!!!*****
Be absolutely sure that the volume on your bass is all the way down and the amp is turned off. Once every thing is connected, turn the amp on, wait the 30 seconds for it to click on, then SLOWLY turn the volume on your bass up. If you don't hear anything, the power amp needs help. You should hear signal as you turn up your bass.

What you're doing here is completely bypassing the 3 PRO's preamp and plugging your bass directly into the power amp (which is wide open at this point) The only thing attenuating the volume is the volume knob on your bass. Thus it's very important to have your bass volume all the way down when you do this.

Let us know what happens!

Best,
Dino
 
#7
The 12AU7 and the last 12AX7 share the same filament supply, in series. The fan is also part of that circuit.

IF the 12AU7 filament is open, OR that 12AX7 filament is open*, then the fan won't run, the power amp won't pass signal, and neither that 12AX7 NOR the 12AU7 will light up.

I'm thinking maybe you overlooked that one 12AX7. That would make sense. You could find out if IT is the problem by swapping one from the preamp. if the poweramp then starts working and the pre is dead, you've got it nailed, the one you swapped-into the pre is dead.

For that matter, you could swap the 12AU7 with a preamp tube and check same way... you don't need the right gain, you just want to know if sound comes out.....

* could also be Q17, (voltage regulator for the filaments), or associated parts, etc, etc, etc, which would mean tech time if true.
 
#8
Gint said:
...Can you recommend where to get a replacement fan? Thanks for the reply Dino.
Click to expand...

Radioshack usually has them in stock, pull the fan from the amp and bring it with you to the store so they can match it up for you. It's a pretty simple fix if you have any sort of knowledge of electronics - just some basic splicing and soldering.

Read the above post by Jerrold Tiers. He seems to have some good info to check on too. :D
 
#10
Thanks again for everyones replies. I tried plugging the bass straight into the power amp in and the speaker out to the speaker and no sound came out. The 12ax7 on the power amp is definitely lighting up. I haven't been leaving it switched on too long because of the fan not working, would the 12au7 take longer to light up or only light up if there is sound going through it? Radioshack hasn't quite made it to Ireland yet, i've been looking on the net for a fan with the exact same spec as the one thats in it i.e. 24v 14a 3.4w ac fan 80mmx80mmx32mm (I think) but don't seem to be able to find 1.
 
#11
If the 12AX7 on the power amp is lit up, then most likely the 12AU7 is also.

There is a small chance of a short circuit that might allow the one tube to work and not the other. That same fault would shut off the fan. Naturally, no sound would come out with one tube off.

If the temperature sensing circuit were bad, it COULD shut off the fan, and should cause no-sound as well. But both power amp tubes should light up.

When you turn it on, is there a soft 'click" a few seconds AFTER you turn on the switch?
 
#13
In that case the entire output mosfet section is probably working fine, the temperature sensor is OK, and the fan circuit SHOULD be working.

Maybe the short circuit I mentioned as a possible problem at Q17 is the actual problem.

That's if you are SURE the 12AX7 is working and the 12AU7 is NOT. I can't see a good way in which one can work and the other not, since the filaments are in series.

For one last sanity check, you could see if the 12AU7 gets hot. Carefully...... leave the unit on for a while, then turn off, and check if the glass of each and every tube is about the same temperature. Be ready for them to be hotter than you want to touch..... I use a very quick touch or just hold fingers near the tube.... or you can turn off and leave it for a few minutes, then check if both are the same warmth

If both tubes seem to be hot, they both are probably working, not all filaments are visible. In that case, you have a strange problem.

If the 12AU7 is NOT hot, there may be a short to ground at Q17 collector... that would shut off the fan and the output.

probably tech time either way, you may not need to bother checking the tube heat, unless you just want to know what your problem is.

You can also, as mentioned, just swap the 12AU7 with a preamp 12AX7 and see if the preamp still works somewhat (it will have low gain). That will check it, and also check the power amp, in case we are all crazy and the tube really does fix it.....
 
#16
Techs always have lots to say.....

I had a guy write me a letter once, patiently explaining to me several "glaring defects" in a PA amp design..... he closed by saying that "musicians can break an anvil, you really should field test units before putting them into production". Now, that's a nice rant, but the odd thing is that at the time, the unit had been made for 10+ years, had an excellent warranty record, and was later discontinued only when the transistors used became hard to find.... we had made many many thousands of them when he wrote :D

OK, now that "I" have had my rant.....

The tube filaments make a nice dropping resistor, avoiding the waste of power that would otherwise occur....and doing that is no worse than using various resistors in the plate power circuit to drop the finals plate voltage down for the preamp.... a practice with a long history of use in successful tube products.......

And we are clearly not the first or only to ever use the filaments like that. In fact, the filament current draw of those tubes is controlled and specified (or used to be, with *good* tubes) specifically so that CAN be done.....

Don't worry about it..... of the few real problems that seem to come up on the 3PRO, THAT part of the design is definitely NOT one that is an issue service-wise....
 

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