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BC 30 is missing and by-passed 5ar4 tube ?!?!


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Hello. This is a great place to get info. New to the forum and all this. My problem is that I bought a used BC30 with noise and no reverb. Yes, I did this on purpose. Got it home and played it for awhile. Noise got worse as it got warmer. Decided to open it up and check out my toy. Found ground wire loose on stud, added a nut and a lot of the noise went away. I also found that I have no reverb tank, no wires for it, not only is the the 5ar4 tube gone (socket still there) but the socket has been diode bypassed. The stand-by switch is not wired completely - one of or more of the transformers has been replaced and wires just hanging loose.

 

OK, I have replaced all 12ax7, 5881s, installed a reverb tank and have a new 5ar4. I don't know enough about this to know if this is a correct replacement/removal of the 5ar4. I read that it also powers the reverb. The transformer issue is that I don't quite know which of the transformers does what. I have a link to my photos, if it will help. Being a mechanic for a living, I can rebuild an engine or a transmission and troubleshoot most electrical, but this may be out of my league.

 

Thanks for letting me vent and all. Any help or advice is appreciated. Don't want to take it to a tech...waaaa... Thanks again. btw I am in Tennessee.

 

Link to photos: http://www.flickr.com/photos/marlajap/sets/72157627583934150/

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  • 1 month later...

Hi,

That internal tranny behind the power switches is not stock and should not be there, also it looks like the slightly wonky large power tranny is not stock, though it could be, mine are UK 50Hz type so I don't know what yours should look like.

 

It does seem like the rest of the amp is stock, judging from those messy diodes you can probably tell what else has been changed.

 

As for the wiring to the power tranny, you could go by the schematic. BTW while your at it you can ignore the standby switch and leave it disconnected. It's a liability, it causes nothing but trouble.

 

 

Andy

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Sorry - just getting back to this, busy days lately.

 

Andyroid - I will PM you my e-mail address tonight - thanks for the tips!

 

Still in search of figuring out the 5ar4 bypassed tube and the reverb tank not working correctly. Any tips welcome! Will try to check in more often.

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Hi, the best website I've found for explaining the workings of amps in a clear way is the Valve Wizard site. The two-phase full-wave rectifier page is what you want to read http://www.freewebs.com/valvewizard/fullwave.html . BTW, the rectifier simply provides high voltage DC to the valves, V3 (the third valve from the guitar input end) powers (sends to)the reverb, V5 recovers (listens to)the reverb signal.

 

If you really do want to know what you're doing, I recommend you buy at least his first book, it's a real eye opener! It covers pretty much everything you can do with small signal valves (except tremolo). His second book is more difficult to get to grips with, it's got far more maths and electronic theory as it's all about making the perfect power supply.

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Andyroid thanks will check out the web site in a few. Wishing for more hours in a day here. Thanks again for info. I am afraid I may have put the wrong reverb tank in I know the one I got is ohm correct but i got a short one instead of the 16". Would that make a difference in sound output level?

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Hi, hope things are going well for you! If you fix it up right, you've got a great amp there!

 

As for the reverb, I don't think the physical size of the tank matters too much, I think it's all about what the springs actually do (and what the valve stages do to the signal). You need a tank designed for 8ohm input. As long as you've got that, you wont damage the driver valve. If you need to boost the reverb volume, you could try changing R31 from the stock 470k to maybe 100k. That should attenuate the reverb signal less. The tricky thing (for me)that I can see is that the reverb signal is mixed with the dry signal in a typical passive way. So reducing R31 also reduces the resistance to ground for the dry signal (look at the schematic). So, I think an alternative might be to increase R29 from 220k to 1M, this should increase the input signal to the recovery stage, thus increasing the reverb volume without affecting the dry signal. Unfortunately right now I don't have time to check this stuff for you, though I genuinely would love to. You can undo it all easily if it doesn't work for you, but I reckon it will increase the volume a fair bit.

 

Also, you could replace VR8 with a 1M pot, so on full reverb volume there is more separation from ground. Again, this will subtly affect the dry signal.

 

One thing I wouldn't recommend is reducing R25. I could be wrong, but I think that some of the trademark BC30 mushy reverb sound comes from the driving signal being too strong. So I think it's probably best to just recover the reverb best as possible rather than driving it harder.

 

Best of luck,

Drunk Andy

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Andyroid, thanks again for your input. I am planning on restoring it and "adjusting" somewhat, But for now life is not allowing the fun. Seems like I get started on it and have other more important things come up. But, hey, thats why its a hobby. Thanks again.

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