Moderators: pompeiisneaks, Colossal
I agree 100%, another PT is a waste of coin.hans-jörg wrote:Hello,
I´m quiet sure you don't need a higher mA rating with the PT.
120 mA for 2 EL84, 1 EZ 81, 2 ecc83 is the common rating for PT. With 150 mA you are more than in game, which don´t hurts
Best,
Hans-Jörg
Yes, but you are ignoring the fact that the amp is drawing MORE than 150mA.hans-jörg wrote:Hello,
I´m quiet sure you don't need a higher mA rating with the PT.
120 mA for 2 EL84, 1 EZ 81, 2 ecc83 is the common rating for PT. With 150 mA you are more than in game, which don´t hurts
Best,
Hans-Jörg
) and kill all connections with the tremolo circuit before measuring.This is where I started. The only difference being I am using a new Mercury Magnetics transformer with a 110V primary. The original PT was 220V.SilverFox wrote:hans-jorg: "but that's not the fault of the PT. You have to find the "wound" before you heal."
I noticed the OP states: " He likes how it sounded (before the last tech's attempted conversion)
What if it was returned to the original design, original schematic?; and then go from there?
Silverfox.
It's not a customer per se. The owner of the amp is one of the owners of the shop, so he understands the mess of a project I've been handed. Other than the frustration of this amp being a PITA, this is actually quite the perfect situation - the owner wants it fixed no matter the cost and I have no time constraint. So, really, the whole point here is to bring back to life the amp he loves. Simply giving him a Vibrolux in its place isn't what he wants, although I completely understand your motivations in suggesting so.Phil_S wrote:LOL, I wouldn't have imagined an oldie like this was for a customer. You, me, the rest of us have no idea how it sounded or what the other "tech" did. You've been asked to put the toothpaste back in the tube. As if that's not hard enough, the toothpaste is on some other guy's bench.
It will be faster/better to put a variant of a known good Fender circuit in that chassis, with the right features and number of knobs. I'm thinking you can cobble together something like a simplified 5E11 Vibrolux variant with cathode bias and EL84's. Heck, just make it fixed bias and get it over with because we know the that's the way the tremolo will work right.
Maybe it is time to have a heart-to-heart talk with the customer. When he brought you the amp it was already hosed. What were the expectations? He is the one who messed up by taking it the the other "tech" and now he really can't have what he wants because no one knows what that is. Who wouldn't want a Vibrolux?
Keep in mind the 166mA draw was with the silicon diode rectifier. The current draw with the EZ80 was around 132mA max. However, I agree with you that there must be at least a semi-short somewhere. Thank you for the link to the super secret transformer test.Phil_S wrote:There's something here bothering me, the reported draw of 166mA pushed with a signal. RC-30 says 92ma max with signal. Add 3mA for the preamp tubes (we know this from Vdrop on the B+) and round up to 100mA for good/bad measure. Granted, specs are imperfect, but I have been pondering what gets it go to 66+mA more. This hardly seems possible.
I'm thinking there's a short. In particular, maybe a short in the OT winding where the varnish on the wire is thin and arcing occurs. For this, the test: http://www.premierguitar.com/articles/t ... r-tester-1 and a bad OT might just explain the crappy sound, too.
Beyond that, it might be more pedestrian like an unintended ground, a leaky cap to ground, or an unintended cross connect, but none of these are setting off bells for me.
I know I've put in more than my 2 cents worth on this, so here's another nickle for the jukebox https://www.youtube.com/watch?v=IAn8jIkNiZc
BungleSim wrote:LOL, there's no such thing as a semi-short. I'll ad this to my list of non-expressions which so far only has "a little pregnant" on it.Phil_S wrote:I agree with you that there must be at least a semi-short somewhere.
Curious history. Interesting project. Unlimited budget and only a schematic to go by. Good luck figuring this out.
After testing the OT, this is what's I'd do at this point. Working from the schematic, I'd check everything from the input jack to the output transformer, one component at a time. Determine that it's value is correct and that it's connections are correct. Then, I'd do it a second time, but this time start at the OT and work towards the input jack. The method scrambles the brain in an attempt to make you see what is really there instead of what you think you want to see there. If you need to go around a third time, flip the chassis 180 degrees and do it again.
Also, I think you've got to stop using the EZ80. It isn't up to a pair of EL84's or it's very marginal. EZ80 is rated at IRec = 90mA. Go to EZ81/6CA4. Perhaps I'm wrong about this and the EZ80 is part of the charm of this amp. <shrug>
I really don't think anything regarding the original transformers has anything to do with it because the voltage input only concerns the primary winding of the PT. If the HT is supposed to be 300V, then that's what I need to provide. I spoke with Mercury Magnetics, told them my voltage and size requirements and we agreed upon a 290V HT transformer. The only other transformer they had that would fit the mounting setup had a 310V HT. I erred on the side of the lower voltage as I knew the spec for EL84s is 300V.SilverFox wrote:This is becoming a very interesting topic if for no other reason than the "Mystique" factor.
Phil_S: "So he never played it in Britain. He had it here in the US but it had the original 220V PT in the amp. He had the shop's previous tech put a step up transformer in it to convert 110V to 220V."
Don't know if this is part of the problem or not but I found it interesting while checking out the Mesa Snake King. See the 7 min mark regarding British voltages. Ends at 8:30.
http://www.youtube.com/watch?v=-mD03JT1AgY
So what were the original PT specs? Could there have been a mis matched voltage that caused a failure? Or is the amp now running over under-voltage according to design?
Silverfox.
Yes, yes, no need to be pedantic. By "semi-short" I mean not a dead short.Phil_S wrote:LOL, there's no such thing as a semi-short. I'll ad this to my list of non-expressions which so far only has "a little pregnant" on it.BungleSim wrote:I agree with you that there must be at least a semi-short somewhere.
The volume is not pulsating anymore since I took the tremolo circuit out.hans-jörg wrote:Odd situation, but I´m still focused on the tremolo circuit. This whawha effect remembers me a similar situation.
