68 12,000 with 2" high impedance OT

[JimiJames]

Leviton sockets can be had at Home Depot, Lowes, Menards or home inprovement stores, no?

[emmjaydubya]

Yeah you know, they used to, at least at Home Depot. I know I bought them there years ago here in Austin but last year when I needed them they were discontinued, I guess they didn't sell enought of them? So I had to go the online route. Maybe you guys would have better luck at the big stores in your areas?

[emmjaydubya]

Well tonight I started to reform the NOS caps on this thing with the variac, after 6 hours the voltage is hovering around 33V and I'm REALLY bummed. I highly doubt that it will fall to 10V let alone the desirable 5V by 24 hours, it's just not moving down enough. I'm not sure which caps aren't reforming, the pair of 100uf Eries in the mains or the 32+32 Hunts in the pre, or maybe all three. I'll wait till tomorrow and reserve judgement but I have a feeling I'll be stuck with putting some F&Ts in there, I'm just not sure how to determine what has formed and what has failed to form. Any ideas?

[vh junkie]

Did you remember to unhook the PT secondary center tap? And remove all the tubes? As you raise the voltage the only filter cap that is "alone" (not sharing the voltage in series is the PI cap... do not exceed it rating...

[Strat78]

What is the voltage at the junction of V1A cap and resistor? I ruined two erie 32/32's by letting the voltage climb above 400v there, at least I think that's what happened. I have not had a problem since though. Never had a problem with the erie 100uf's. Let the forming continue another twenty hours to insure that the other cans are properly formed, then start looking for the culprit.

[emmjaydubya]

I read the forming thread ad nauseum and I never actually hooked up what I shouldn't have so I'm positive it's right. I was worried about what voltages the caps would see so I started forming at 100V input. I checked each cap and voltages were well under 250V, so I gradually increased the input to 120V last night, measuring each cap. The cap that was seeing the most was the Hunts 32+32 preamp cap, it measured around 300V, so I figured it was fine and went to bed.

This morning I woke up and voltage had fallen to around 30V across the forming resistor, but when I checked the caps, the pre had climbed to 385V!!! Don't know how long it had been that way, but I brought the input way down to 90V which brought the preamp voltage to 350V. I guess it will have to finish forming at 90V instead of 120V. I checked the forming resistor and it shows 6V!! Perfect!! Haha!

Now I'm not sure how to properly measure it since it's obviously reading way low. I'm thinking NOS caps was a bad idea.

[emmjaydubya]

Well looks like I figured out the forming problem! Phil gave me a heads up on how to test each cap section and it worked great. Turns out the mains (Eries), screens (Rifas) and P.I. (F&T) all formed perfectly! Voltage drop across the 100k forming resistor for all three sections combined (pre-cap removed) was 3.7V last I checked, which amazes me. I wasn't sure about the Eries since I got a good deal on 'em, but looks like they will work as they should.

As I suspected, the Hunts 32+32 was the problem. It didn't seem like it was going to form, like I said before, after 16 hours it was hovering at around 30v, and that's also when I discovered it was getting 385V! So either it wouldn't have formed or I fried it to hell!! Either way it's getting yanked, I have an order in to Valvestorm for an Erie 32+32 that's formed and ready to drop in.

Now it's finish a few wires and testing, then tubes, then put in the pre-cap. I am soooo ready to play this thing!

[Strat78]

Seems like 380v should not be too far beyond the 350v rating but who knows, perhaps over the years as the resin pools to one side, the rating of the cap drops even further. The old dual pre caps are more hit and miss than the NOS Erie 100uf's. I had a Hunts 16/16 and two erie 32/32's that would not go lower than 17v, so I chucked them. Although I did exceed 400v on one of them, but sill. You just need a drawer full of extras because one is bound to work. I remember Larry mentioning that sometimes he likes to submerge the caps in warm water before forming, especially if it is a NOS cap. Guess that loosens things up before it gets hit with the juice. I used a hair dryer once but you've got to stand there for 15 minuets.

[emmjaydubya]

Yeah I've heard that about the dual caps. Voltage went high, but not that high. Like you said, maybe the tolerance dropped, or maybe it was just a bum cap from the start. Neil was nice enough to send it just because, along with some other goodies (including the .68 chicklet I used at V1) when I bought the chassis from him. I had hoped it would be on the board.

Maybe the hair dryer would be safer than water? I know what your talking about, Larry uses LCRs, do they have a coating around the entire cap? I would be afraid to stick an Erie in water, both ends are open on the insulated blue covering, seems like water would seep in and be big trouble at high voltage.

[emmjaydubya]

Progress!! And then...

Swapped out the pre cap today with a 32+32 Erie, along with moving the line side of the input to the mains fuse, instead of the neutral being fused. Put pre tubes in and started testing voltages, finally!

Everything seems okay EXCEPT for V2! I'm reading 256V @ pin 1/7 and 254 @ pin 6! George's 100 watt chart shows 168V @ pin 1/7 and 303 @ pin 6. I pulled the tube and tested the 100k resistor across 1/7 and it reads 99.7k, so it's fine. Also swapped the tube and it reads the same. Why such a big discrepency? Any ideas?

[neikeel]

Progress!! And then...

Swapped out the pre cap today with a 32+32 Erie, along with moving the line side of the input to the mains fuse, instead of the neutral being fused. Put pre tubes in and started testing voltages, finally!

Everything seems okay EXCEPT for V2! I'm reading 256V @ pin 1/7 and 254 @ pin 6! George's 100 watt chart shows 168V @ pin 1/7 and 303 @ pin 6. I pulled the tube and tested the 100k resistor across 1/7 and it reads 99.7k, so it's fine. Also swapped the tube and it reads the same. Why such a big discrepency? Any ideas?

Without seeing pics you need to check which side of the two preamp board10k droppers you are taking the plate feed.

[emmjaydubya]

Thanks Neil, for making me take a different look at this, wow what a dumb-dumb I am:



I put the blue plate feed wire onto pin 7 (circled in red) instead of pin 6, no wonder the readings were all crazy. Funny that you wonder how you didn't see something sooner, AFTER you've seen it. Guess I'll double-check all the voltages again.

Tonight WE ROCK!!

[Strat78]

Love the pic! 100k Iskra

[emmjaydubya]

IT'S A-LIIVE!!!

Wow, as said many times before, nothing like those first chords on a fresh amp. I got it biased at 90V on the Variac last night (took a while) but only got to open it up this morning. Played it on 8 for 15 minutes or so, watching for red-plating but it was fine.

Here is the biasing method I used per VH junkie, since I don't have the 1 ohm resistors on the power tube sockets.Tell me if I'm on target here. The formula:

OT_VOLTs/OT_RESISTANCE * plateV = target wattage (17.5w @ 70%)

Here is the last calculation I made (7 times!), since you have to measure and re-calculate every time you adjust the bias:

2.23/45 ohms * 346V DC = 17.14, with the target being 17.5 for EH 6CA7s.

After playing it for 15 minutes:

1.75/ 45ohms * 351V DC = 13.65

So it went cold, gonna mess with it some more and get it back up in range again, but I'm approaching the pot being maxed, we'll see. If it wont get up there I can raise the input to 95V or change the 46k bias resistor to 33k. Opinions?

[Strat78]

13.65 is the resistance between pin3 and hot fuse when powered off? Isn't your target 1.75 or 2.23 between pin 3 and HT fuse when powered up? I got 17 between hot fuse and pin 3 of V5 when powered off. So here is how I calculated mine. 17.5 (standard for el34's or 6ca7's) devided by 368plates X17 X2= about 1.65 between pin three of V5 and HT fuse (when amp is powered on). I bump it up to 1.90 or 2.00 just to get those 6CA7's working a little harder.