Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Simon on October 15, 2019, 08:48:14 pm
-
Hi folks
This is my first post here, so I hope it's OK to just jump in and ask for a bit of help?
First of all, I am no electronics expert and have never built or repaired an amp. I'm just a musician (or guitarist, anyway) who recently bought a Trace Elliot Velocette (a beautiful little amp). Unfortunately, the amp has a small, intermittent issue and I'm having trouble finding anyone in my area to work on it.
Every now and then, while I'm playing, the amp will kind of fade out and drop right out. When it does this, I have noticed that one of the two output tubes (EL34s) will be glowing much more brightly than the other, but it's not always the same one that does this.
Is anyone able to offer any suggestions as to why it might be doing this, what kinds of things I should maybe test (and how) and even, perhaps, whether might be anything that an inexperienced person such as myself might be able to repair?
I am attaching circuit diagram in case it might help.
Anything anyone can off would be hugely appreciated, as I have quite a few gigs coming up that I would love to be able to use this amp for.
Thanks
Simon
-
First, if you've never had your noggin inside an amp with LETHAL voltages, do some research, look over the shoulder of an experienced tech, and ask questions. Do not take any chances. There are some things you can do right off the bat without getting into the danger area. Since the problem seems to be intermittent and possibly thermal related, try this first. Unplug the amp. Remove the EL34's. Spray some contact cleaner into the socket pin openings and immediately insert the tube, working it down in a circular motion. Do both sockets. Switch tubes when you go to reinsert. Wait for cleaner to evaporate. Fire it up and see what happens. If it does it again, does it follow the tube? Could be a bad tube. Many times tube redplating can be attributed to sockets with tension issues or they are just dirty. Always do the easy stuff first.
Jim
-
Sorry, just noticed you said it was not always the same tube. Still do the same.
Jim
-
The output tubes are EL84s, not EL34s.
-
The output tubes are EL84s, not EL34s.
True. Of course. Sorry. My Typo. I was just exited at finding this forum and a bit distracted. :-)
-
Sorry, just noticed you said it was not always the same tube. Still do the same.
Jim
Hi Jim.
Thanks for your input. Makes a lot of sense. Thing is, I already had a guy service the amp and one of the things he put a lot of time time was cleaning connections such as the valve pins.
-
Is anyone able to offer any suggestions as to why it might be doing this
the typical;
bad tube
bad socket/soldering
bad bias
bad bias supply
OT breaking down with heat
All the above can light up your life if NOT carful
-
Every word on this page can—without much exaggeration—save your life: https://robrobinette.com/Tube_Amp_Safety.htm
-
my brains on snot so......
the weird "links" in your OT secondary going to the speaker
the Cathode R for the PA tubes, or bypass cap.
PI circuit if it only happens with constant signal - play for 2 min, let set (powered up, guitar vol 0) for 10min, play again 2min, do it enough to "see" if it degrades, or stays happy.
IF it stays happy, play for 10min, wait 2 min, play for 10.......
when you say;
will be glowing much more brightly than the other
are you talking filament glow, or more ominous like plates getting hot?
-
The Velocette is known for having problems with output transformers. I read there were even some that had the wires wrong - 16 and 8 ohm leads switched and sometimes even the ground was switched... something like that. And yes, the transformer has both 8 and 16 ohm outputs but only the 16 ohm is hooked up by default. There is an internal jumper that is soldered to the circuit board that determines which is being used... at least that is the way it was on the 12R version (the one with reverb). I changed my amp from 16 to 8 ohms.
Anyway... could be you have one with a bad output transformer. Someone else with more technical knowledge can chime in to say if that would cause the issues are seeing.
-
Hi
Yes, I'm aware of the output transformer issues, and I wondered if that might be the problem here, but I'm pretty sure the OT has been changed at some point before I bought it. Apart from anything else, the amp is twenty years old, and I doubt it would still be going if the OT hadn't been changed at one point.
I managed to get hold of a guy who can look at the amp for me in a couple of weeks, and I know he's good at his stuff. When I described the issue to me he said there is a design fault in these amps - something to do with a resister in the heater circuit (at least I think that's what he said). He reckons that the best way to fix it would mean changing the amp from class A to AB, and I really don't know what to think about that. I have no idea how much the sound (which I love) might change.
The Velocette is known for having problems with output transformers. I read there were even some that had the wires wrong - 16 and 8 ohm leads switched and sometimes even the ground was switched... something like that. And yes, the transformer has both 8 and 16 ohm outputs but only the 16 ohm is hooked up by default. There is an internal jumper that is soldered to the circuit board that determines which is being used... at least that is the way it was on the 12R version (the one with reverb). I changed my amp from 16 to 8 ohms.
Anyway... could be you have one with a bad output transformer. Someone else with more technical knowledge can chime in to say if that would cause the issues are seeing.
-
when you say;
will be glowing much more brightly than the other
are you talking filament glow, or more ominous like plates getting hot?
Thanks for your input.
I'm not sure I can tell. When it does it, I'm in such a hurry to switch it into standby in case some real damage might be occurring that I haven't dared to take the time to look more closely. If I did, how could I tell which it was?
-
not sure about uniqueness of this amp, but it's self biased (typically way "safer" than fixed bias)
I didn't see anything in the filament circuit that seemed "flawed" except maybe the use of connectors? to feed tubes filament volts.
I would ask the tech to verify PA DC voltages, PA power dissipation, AND any stry DC coming in at grid of PA tubes, move on from data results
-
Simon,
If this crazy hobby is something you are interested in, ask the tech to call you when he is working on your amp. Tell him you want to watch. Ask a lot of questions. If you are still interested, buy a kit. Do some builds and maybe some repairs when you are comfortable and aware of all the safety hazards. It is a lot of fun and very addicting. Especially when you fire your new build up for the first time and it sounds better than anything you have ever played! OR, you spend the next month chasing that one mistake you made! :BangHead: ....that you will never make again!
Jim
-
Thanks, Shooter. If nothing else, this discussion has made me realise just how out of my depth I'd be if I tried doing anything to it myself. I guess I'll just have to wait until this tech has time to take a look at it. And yes, I might see if I can watch him work on it.
not sure about uniqueness of this amp, but it's self biased (typically way "safer" than fixed bias)
I didn't see anything in the filament circuit that seemed "flawed" except maybe the use of connectors? to feed tubes filament volts.
I would ask the tech to verify PA DC voltages, PA power dissipation, AND any stry DC coming in at grid of PA tubes, move on from data results
-
Thanks, Shooter
:laugh:
my last "trainee" went screaming out the door his 1st day, AND he was my 2nd person safety guy :icon_biggrin:
hang in there, read, decide if it sounds fun otherwise keep making 'em sound good :laugh: