Help! Blown transformer on blackface champ

[evanjackson]

I think I may have blown the output transformer on my blackface champ. It went from sounding OK to drastically losing power while playing to making crackling sounds and now it makes no sound at all. I replaced the tubes with a different set and still nothing. Using my handy troubleshooting chart says this seems to point to the OT. Does this sound logical? About how much should I expect to pay to get it fixed? Any suggestions for a good amp tech in the suburban Philadelphia area?
Thanks!

 

[mc5nrg]

There is a person by the name of Jim Walton in NE Philly who is a long experienced tech.He may be in the phone book or if you search Google groups you will turn up his contact info.

Jim Martin is a reputable tech-you can hook up with him through DiPinto's in second street.Editops,should be John Martin

Allessandro does repairs for Dtown Guitars,formerly Martyn's Guitars of Lansdale now of Doylestown. www.dtownguitars.com

Cintioli Music in NE Philadelphia does in house repairs and is worth checking out for inventory,very competitive prices and some old school Philadelphia vibe in general.

 

[John Phillips]

Originally posted by evanjackson
I think I may have blown the output transformer on my blackface champ. It went from sounding OK to drastically losing power while playing to making crackling sounds and now it makes no sound at all. I replaced the tubes with a different set and still nothing. Using my handy troubleshooting chart says this seems to point to the OT. Does this sound logical?

It's possible, but don't panic yet - it could be a few other things. The most likely would probably be a blown cathode resistor for the 6V6 - the original is a carbon-comp and they do tend to fail eventually. It may take out the cathode cap as well if it blows.

If it's that, ask the tech to replace it with a wirewound 5W resistor - which will never blow again - and possibly to rebias the amp. The stock 470-ohm resistor is way on the low side and is very hard on the 6V6, as well as reducing the headroom of the amp and (IMO) producing a less-than-ideal overdriven tone. The right value can be as high as 1Kohm. You'll need to increase the voltage rating of the new cathode cap to 50V, and preferably double its value to 50uF as well to maintain the same bass response with the new resistor value.

Another possibility would be a failed B+ chain resistor - probably the first one (1Kohm) - these are also carbon-comp and again should ideally be replaced with a wirewound, of the same value this time. This won't change the tone but it will give better reliability. This resistor gets stressed if a 6V6 blows since it supplies the screen grid - the champ doesn't have an actual screen resistor, and fitting one is a very good idea. A 1K is also better than the 'standard' 470-ohm used in most Fenders.


Hope that helps! A good tech should know all that anyway.

 

[WailinGuy]

The right value can be as high as 1Kohm. You'll need to increase the voltage rating of the new cathode cap to 50V, and preferably double its value to 50uF as well to maintain the same bass response with the new resistor value.

If you double the value of the cathode resistor, wouldn't you want to halve the value of the bypass cap to maintain the same bass response? In other words, if you increase the cathode resistor to 1K, a 10uF or 15uF cathode bypass cap would be in order. (Although in actual practice, I think still using a 22 or 25uF cap would be perfectly OK.)

 

[John Phillips]

You want to increase the value of the cap since the effective impedance of the cap/resistor parallel pair has increased when you raise the resistor value, especially at low frequencies. So to bring that back to where it was, you need to decrease the impedance of the cap, which means increasing its value.

I'm not sure you really need to either, but since you DO have to change the cap anyway (the cathode voltage goes up to around 30V with a 1K resistor) you may as well use a larger value while you're at it. IMO, anyway .

 

[evanjackson]

Thanks you guys very much for the help!

 

[WailinGuy]

You want to increase the value of the cap since the effective impedance of the cap/resistor parallel pair has increased when you raise the resistor value, especially at low frequencies. So to bring that back to where it was, you need to decrease the impedance of the cap, which means increasing its value.

In general, don't higher impedance circuits require smaller cap values for a given bandwidth to be passed through? (A classic example is the 10pF cap in parallel with the 3M3 resistor in the reverb channel of Fender amps. Because the resistor is such a large value, a very tiny cap value bypasses enough high frequencies in the audible range to make the tone noticeably brighter.) So how is the situation with cathode bypass caps different?

 

[John Phillips]

Originally posted by Jim Salman
In general, don't higher impedance circuits require smaller cap values for a given bandwidth to be passed through? (A classic example is the 10pF cap in parallel with the 3M3 resistor in the reverb channel of Fender amps. Because the resistor is such a large value, a very tiny cap value bypasses enough high frequencies in the audible range to make the tone noticeably brighter.) So how is the situation with cathode bypass caps different?

In the stage you're refering to, the direct signal is deliberately cut drastically (to bring the signal down to around the same level as the reverb return), then the whole lot is reamplified. It's only the relative levels that matter, not the current or power.

In a cathode-bias power stage, there is no subsequent reamplification. The output bass response is directly dependent on the low-frequency impedance in the current path, not the relative levels. If you raise the cathode resistor value, you decrease the bass response, because at low frequencies most of the current goes through the resistor, not the cap. To get that back, you need to increase the cap value.