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Help with '64 Vibroverb

2/16/2000 2:22 PM
Tim C.
Help with '64 Vibroverb
I have a 1964 Fender Vibroverb on the bench that was brought in because it has an unusually glassy treble sound.  
The filter caps were changed because they were mushrooming at the ends on a couple of them. It has been retubed as well.  
Someone has been in the amp before and changed all the cathode caps to 47mfd's at 50 volts. I have changed them back to stock values. The output transformer has been changed as well. The one in it now has the paper bobbin exposed (no cover over windings). I replaced all the 100K plate load resistors. Also the tone caps have been changed. I noticed that the output caps to the power tubes were .1 mfd @200 volts so I changed those to .1 @ 600 volts.  
The stock JBL has been removed and a standard 15" speaker installed. The amp still has the ice pick in the ears sound only not quite as pronounced.  
Any suggestions as to what could be causing the annoying high treble sound?  
Tim C.
2/16/2000 2:50 PM

Sounds like a tube issue to me. What tubes are installed? are they biased correctly?
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2/16/2000 4:00 PM
Scott Swartz

I recommend bypassing the preamp and seeing what that does; if its still bright you know its in the power section. Disconnect the inputs to the PI and inject the geetar signal; works better if you use say a clean boost pedal to get the voltage up. Most preamps tend to introduce a midrange dip, but I think you have the one with the tapped treble pot which is capable of fairly flat response, so if a direct guitar signal sounds OK, the problem must be in the preamp.
2/16/2000 5:58 PM
John Stokes
We're talking a BF Vibroverb, right? First off, are the bright switches off? Is the excess treble present in both channels? If you inject a low frequency signal, can you observe on your scope the proper operation of the Bass controls? Does the preamp have the characteristic midrange dipCthat paralleled 3.3 meg and 10 pf cap. Make sure the 3.3 meg hasn't opened or has drifted way up in value.
2/17/2000 4:58 AM
Tim C.

Thanks for your posts.  
Hi John. Yes it is a blackface dated inside April of 1964. It is a Fender Electric Instrument model. It came here from the Pheonix area. The bright switches are not the problem. It did it on both channels. I checked the resistors you mentioned and found them to be okay.  
I originally thought it would be a tube problem as rebel mentioned but that wasn't it. It has new Ruby Svetlana's that are biased properly. I did try other new 6L6's just to be sure. I put in new Sovtek 12AX7WB's in all sockets except one where I used a Tesla 12AX7 from Ruby.  
I checked the plate voltage on the 6L6's and found 456 volts present. I went ahead and changed all the coupling caps with good pullouts because the guy wants old parts only in it. (other than filter caps)That seemed to help emensely. I put the stock JBL back in and it seems to be much better. There is plenty of bass now. I guess the best way to describe the annoying sound was like a microphonic treble ringing. It was one of those things that was hard to describe unless you hear it. It still has a slight microphonic treble sound that I am not fond of. But it is tollerable at least.  
I am wondering if the output transformer could cause this type of problem. It has been changed at some point and the leads are somewhat stretched to the plates.  
Thanks all,  
Tim C.  
2/17/2000 6:18 PM
John Stokes
Tim, I would urge you to throw the thing on a scope and look for ringing ot other extraneous crap on the output. You do have a scope, no??  
Is the problem present on both channels? Is it a function of the reverb level? I assume you have eliminated the possibility of a microphonic tube.  
IMO those amps sound great.  
2/17/2000 6:41 PM

It could be the OT.. esp if it's not original. I would try other output tubes (just because the Svets are cool does not make the always work great in every amp.) OT is same size as Bandmaster so it may have a much later piece in there which could do what you describe. Major "ring" factors IMHO (it's my big pet pieve with any amp - the HF response) Tubes, speaker and OT. Caps and resistors can help a little, but not nearly as much as the "big 3" IMHO, YMMV, etc, yadda yadda yadda. ;-)  
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