• TGP is giving away a Strat, Tele, and Jazzmaster. Click Here for full details.
    Click Here to upgrade your account and enter today!

Vibrolux Reverb build - problems

Messages
7
This is my first post here, so hello everyone (or everyone who reads this). I've been referring to the Gear Page for ages looking for information, but only just registered because I've hit a dead-end with my Vibrolux Reverb build. If anyone can help, I'd be incredibly grateful. I'm a relative beginner - I've only built a Champ and a 5E3 Deluxe before this one, though I did manage to mod the 5E3 with reverb. There are big gaps in my knowledge, so please be tolerant. Anyway, here goes...

I've built a Vibrolux Reverb AA964 with a few mods. It has the vibrato circuit from a Vibro Champ, which works on the cathode of the triode immediately preceding the PI. I've added a PPIMV (the fixed bias version designed by Bruce Collins), which works a treat. I've got rid of the normal channel. And I've added a raw control which bypasses the tone stack. After a few teething problems the amp worked fine - I've put it in a 4x8 combo, and it sounds amazing. However, I am now having problems with the amp suddenly howling with feedback. Here are some details:

1. The feedback is mid-rangey - it's not a squeal, but a howl.
2. The master volume turns the howling down, so presumably it is not the output valves or OT.
3. It is intermittent - it did this a few days ago. I fiddled about with the negative feedback and OT secondaries, which I thought might be causing it. It stopped for a day. Then it started again. Before starting again, there was scratching and buzzing on the treble and bass pots, and there was farty distortion when playing an open A (which also vibrated the whole cab a lot, and made the valve filaments rattle). Also, both times it has worked before the howling starts, but at greatly reduced volume levels.
4. The howling occurs if I remove the preamp, reverb driver and vibrato valves, turn it up, and knock the cab.
5. I've switched valves around - none are bad.
6. The voltages are on the high side, but not stupidly so.

I've checked the circuit many times for continuity, but the problem is intermittent and seems to be caused by vibrations, so continuity wouldn't indicate absence of the problem. I'm a bit stumped. Please help!

Thanks,

Paul

PS Sorry about the essay, but I thought it better to be thorough.
 

Blue Strat

Member
Messages
30,067
Did you change anything between the time it worked well and it started howling?

Can you post a schematic? Repairing known amps over the web is difficult. Repairing unknown amps over the web is nearly impossible.
 
Messages
7
I didn't change anything before the howling started, only in reaction to the problem returning.I will post a schematic when I get home tomorrow. Yes fixing unknown amps online is pretty tricky I imagine.

I haven't tried new power valves as I don't have any. I really thought it was the 6L6s - they became very rattley after a few hours use at modest volume - but how can it be if the PPIMV reduces the volume of the howl? I suppose I'd better order some more, but I don't really want to destroy another pair of valves if the problem is that my amp destroys valves. It's biased conservatively by the way, with no red plating. They are brand new JJ 6L6GCs if that matters.

Thanks for the input so far both of you. I appreciate it.
 

Blue Strat

Member
Messages
30,067
You're right, if the master kills the howling, it must be coming from the preamp.

So, what are the minimum, and particular, preamp tubes that have to be in to make the amp howl?

BTW, a wild guess, but if the amp has negative feedback (stock versions of Vibrolux Reverbs do), try disconnecting it temporarily.
 
Messages
7
I've got a hunch that it's the negative feedback. I will try disconnecting it. Also, the speaker jack is not isolated from the chassis, which I have heard can cause problems when there's a NFB loop. Maybe I should try isolating the jack and grounding it on the main bus. I'll test the amp for minimum preamp valves required for howling too and post results. Thanks Blue Strat.
 
Messages
7
Well, after finding that it was the reverb recovery/driver valve that was causing trouble (thanks for the trouble-shooting tip, Blue Strat), I then discovered what the main problem was - a dry joint on the grounding of the cathode biasing resistor and bypass cap. My grounding scheme was not very good anyway, so I re-did the whole lot, and there is no howling or cutting to low volume evident.

However, the nasty distortion which I thought was a symptom of the same problem is still there. This occurs when the pre-amp, reverb driver and reverb recovery/driver valves are in, and when I turn the pre-amp volume up beyond 4 or 5. It is not dependent on the master volume. In fact, and weirdly enough, if I remove all the other valves (except the rectifier of course) I can hear it farting away acoustically from inside the chassis. I'll get the chassis out again and chopstick it to try to find out what's causing the noise - probably a bad cap isn't it? I've already had to replace one silver mica cap which was leaking DC, so maybe this is the 500pF going between the reverb circuit and the recovery triode. I'll post again if and when I find out. If anyone has any experience of such things please share - I had absolutely no idea it was possible to hear components behaving badly like this.

Cheers,

Paul
 

Blue Strat

Member
Messages
30,067
Silver Micas have a bad rep for blocking DC. The fix is to precede them with a ~.2uf film, or whatever, cap. The added cap blocks dc but lets all audible signal through.
 
Messages
7
I thought I'd wait until testing the amp at volume before posting - and it appears to be working fine now. Thank God, I thought I was going to do mad. I tried a .1uf cap before the 12AT7 reverb driver (after the 500pf mica). It stopped the buzzing - I could here it charging up when the preamp was pushed - but then the reverb stopped working, or began oscillating. If I took a DC reading on the grid of the valve it read 12V, and then discharged through the DMM. Following this everything worked fine again - until I cranked the preamp again. Rather than fiddle around with trying to get this to work, I just removed the cap and bizarrely it worked fine. There must have been a bad solder joint at the 500pf cap. Anyway, just thought I'd pass it on. I used the amp at a rehearsal last night and it sounds absolutely great, even with the speakers hardly broken in. The 4 x 8 configuration works very nicely. Thanks very much to those who helped me - now I wonder what I should build next...?!
 




Trending Topics

Top