Booting Up a 5C8 Works! But Odd Low Heater & 6J5 Plate V

[rp]

My 5C8 booted right up Sure is sweet when you don't get a bright bulb or smoke or no signal... I reversed the pots but odds of that are running 50/50 for my builds. From now on I swear on a stack of RCA tube manuals I'll meter the pots and work it out first. I also had the OT taps reversed - thanks MartinM for reminding me to disconnect NFB before boot.

Got some big hum at first but I had both trannies on the outside jumpered in, I was a little worried I had hit octal hell but have learned patience and not to be negative. When I bolted in the OT and buttoned her up with her shielded sides and base the hum was minor and only when cranked and no more than expect from old octals.

I'm using a Marstran #118 PT to test it as it has dual HT taps ~290 & ~ 325 as I'm trying to find what's best here. With a 5U4GB and the 325V taps I get mostly spot on voltages to the Fender layout - 375V on the plates and 300V on the grids of some big bottle 6L6Gs. Preamp tube voltages also match the Fender layout within 3-4 volts. Pretty dang good, I'm happy. Except:

The 6J5 reads only 77V instead of the 160V called for and my heaters are oddly low at 5.5V? I tripled checked everything and all looks good. Unloaded (tubes pulled) the heaters read 6.4V at all sockets. When I put in the tubes it drops. Causes?

For the low plate V of the 6J5 should I assume it's just a bad or failing tube? The plate resistor and cathode resistor measure correct ohms. I can't think of anything else. Is this something bad tubes do?

And, what's causing the heater drop??? Underrated PT? Tubes here = 3A total, the PT is for a JTM50. Bad PT regulation? Bad tube(s)? Is it a heater wire issue (wound too tight on the drill?) I checked wire resistance to ground and across the wires, all looks good.

I need to order a custom PT but need to solve the filament problem first. So what next? I have no octals to roll either - cursed relics... and I'm in the Italian Boonies... maybe a NOS 6J5 will turn up in a plate of pasta?

BTW even a bit off it sounds very promising. Bright mother, no need for bright caps.

[martin manning]

Will pulling any one tube restore the heater voltage, like that 6J5, for example? Low plate voltage there means it's drawing a lot of current, which you could verify by measuring cathode voltage. Calculate the current and see if it seems to be operating like the curves on the data sheet say it should be.

[Phil_S]

+1 on what Martin says.

I am wondering:
What is heater voltage is with no tubes?
What is the primary rated for?

What I am thinking:
You have a PT rated for 240v and your actual supply is 220V. To make matters worse, your 6.3V winding may be off by one turn or so, carving off a few tenths that you really want and need.

Congratulations on the build.

If it's what I'm thinking, either you can try to exchange the PT or just get a small filament transformer.

If it's a tube with a bad heater pulling too much current (what we hope for), you just replace the tube.

The 77v problem could also just be a bad tube, or it could be an incorrect resistor. Look for where you may have placed a 1K or 10K instead of 100K or something like that.

Good luck.

[rp]

I am wondering: What is heater voltage is with no tubes?

A nice 6.4V

What is the primary rated for?

Marstran doesn't say but as it's a JTM/JMP50 PT it should be more than adequate.

What I am thinking: You have a PT rated for 240v and your actual supply is 220V.

Nope, I have 235Vac at the wall and it's a multi-tap PT. I used the the 240V tap. I also tried the 230V tap to get a boost and got 6.6V but still just 5.8V on the loaded heaters.

The 77v problem could also just be a bad tube, or it could be an incorrect resistor. Look for where you may have placed a 1K or 10K instead of 100K or something like that.

4X re-checked. I also meter my resistors before installing (and give the leads a quick pass with steel wool) to avoid mistakes that come with 2am soldering.

arrrgh, I never found myself w/o a single tube to swap. Back in the day I was ignorant enough to stash plenty of the common 9 pins and 6L6/El84/EL34 and avoided all the really cool relic tubes. Now I must go on eBay and ePay... and eWait...

Will pulling any one tube restore the heater voltage, like that 6J5, for example?

I'll report back on what happens when I try pulling one tube at a time. Here's a little teaser:

[rp]

Couldn't wait till tonight, I pulled one tube at a time, first the pre's then the PI: heater V stayed around 5.5-5.7V at the end with just the power tubes still in. Lastly, I tried all the pre's and PI installed and just the power tubes out - got only 5.9V.

WTF? Bad PT??? I don't believe it, too damn rare. And that low plate V on the 6J5 is weird as parts and wiring are fine, all other voltages look good, and it plays well. It's a nice looking 6J5 too - clean, crisp, perfect lettering and getter and it had nice orig box. Not sure where I got it but it looks like a solid NOS tube.

hmmmm???

I got an extra Hammond PT too, I might try the heater taps off it. Thought I was done when it played, but it's never that easy is it? And why is it always a new unfamiliar problem I want my kitchen table back so I can eat sitting down.

[martin manning]

The heater voltage drop loaded to unloaded doesn't sound extreme, but 6.4V unloaded sounds a bit low to me.

I don't know what to think about the 77V on the 6J5 plate. Can you measure and post the voltages on all the pins and the supply voltage on the other side of the plate load resistor?

Looks cool with the big bottles!

[Phil_S]

To me, the tube pulling exercise suggests the filament winding is too soft for 6.3V and isn't even good for a couple of preamp tubes. I'm guessing you can't return or exchange it. If this was my build, I'd see if I could add a small filament transformer. So sorry to hear this.

On the 77V, as Martin says, more info and pictures of the inside will help. Something isn't done the way it should be or its a bad tube. When you take the voltage readings at the socket, will you also do it with the tube removed?

[rp]

To me, the tube pulling exercise suggests the filament winding is too soft for 6.3V and isn't even good for a couple of preamp tubes. I'm guessing you can't return or exchange it. If this was my build, I'd see if I could add a small filament transformer. So sorry to hear this.

Before heading out I used the filament taps of a Hammond PT I have w/ 6A heater taps. Initially I got 6.2V and thought I had it solved but after a 3 minutes and a full warm up it went back down to 5.5V with the Hammond too, so that rules out the Marstran PT. Real glad I don't have to email Brian and send it back from across the pond.

The heater voltage drop loaded to unloaded doesn't sound extreme, but 6.4V unloaded sounds a bit low to me.

I'm now thinking all the voltages are a little short. The 5V tap is also like 4.9V, and I think the B+ should be higher with a 5U4GB and ~325Vac on the secs loaded. I also tried the 220V tap w/ 235Vac at the wall to goose it up and got 355V! on the secs loaded and 395V on the plates, again, you'd think higher if all was right. Something's eating up a bit of current somewhere.

On the 77V, as Martin says, more info and pictures of the inside will help. Something isn't done the way it should be or its a bad tube. When you take the voltage readings at the socket, will you also do it with the tube removed?

Will do all, a complete blueprint and picts. I'll post the PT's Vs loaded and unoaded and the wall voltage and primary tap used to be clear. Nuts, I was hoping for a Grande Unveiling, now I just get a Grande Troubleshoot

BTW it's a Stancor A3851 OT I got off eBay - NOS apparently with long newish leads. It's an odd OT with Tertiary Feedback windings which I heat-shrunk and buried in the bell. Cliff used one on his amp and was OK maybe mines got a fault??? Both PT and OT run cool after a half-hour.

I got a 30W 6.6K Edcor I could jumper in. One step at a time... Here's an innards preview.

[Phil_S]

I'm not quite sure what to think about this. It would be good to hear what Martin has to say as he's considerably more accomplished at this than me.

After trying the second filament supply and knowing that voltage drops after warm up, I am ready to suspect one or more tubes that have some sort of filament problem, but I don't know enough to understand how this can happen.

I don't imagine any way the output transformer can influence heater voltage. As for the 77V problem, its in the preamp, so also hard to figure the OT has anything to do with that. I guess I wouldn't fiddle with the OT, at least not yet.

I think that 4.9V for a 5.0V winding is not significant and unrelated to the 6.3V problem.

[Phil_S]

Looking at your photo. I think I see a black wire near the 6J5 socket that is grounded under the terminal strip bolt. I can't see the other end, but it seems to go under the 1.5K cathode resistor and then disappear. What is that?

[rp]

Looking at your photo. I think I see a black wire near the 6J5 socket that is grounded under the terminal strip bolt. I can't see the other end, but it seems to go under the 1.5K cathode resistor and then disappear. What is that?

Pin 1 shield ground, to chassis rather than the bus bar. Each tube has one including the power tubes in case I want to do an all metal amp - heavy metal Tomorrow is a holiday and I'll document better. Happy Ferragosto. If TAG was Italy this forum would shut down for 2 weeks and go to the beach

[xtian]

If TAG was Italy this forum would shut down for 2 weeks and go to the beach

I would LOVE to go to the beach. But I read TAG even when I'm on vacation, man, so I vote for no shutdown.

[rp]

Here's the details, I did a nice chart so as not to give anyone a headache. Except for the 6J5 Vp and the dang heaters everything is spot on. I was very close to hitting it out of the park - one day.

Here's a notable observation: I tried installing all the tubes except the rectifier one at a time, starting w/ V1 ending with V6 while monitoring the heater V. With each tube the V dropped a bit, with the last tube it arrived at 5.8V. Also, w/ just the preamp tubes & PI the drop was to 6.3V, add the power tubes and it's 5.8V Again, this is with no rectifier installed.

Is there any chance it's the heater wiring? A stray strand shorting across or to ground. I used Kimber hook-up which has few and thick strands so it doubt it. Maybe the wires were spun in the drill too tight and stretched the teflon coating out? But if this was the cause wouldn't this have the same effect tubes in or out? Again, heaters check open across and to ground.

BTW my heater wiring is kind of strange (not like I planned in a previous post) it's all parallel and hum is good but to snake around the terminal strips I ran V1, V3, V5, V6 in one string and V2, V4, on an other which then combine at the PT. The first string's tubes read 6.0V, the 2nd string reads 6.47V. Curious? Maybe not?

Here's my nice chart and the link to more detailed images. The pesky 6J5 is included below too.

Images:
https://picasaweb.google.com/1041232856 ... directlink

[rp]

I don't know what to think about the 77V on the 6J5 plate. Can you measure and post the voltages on all the pins and the supply voltage on the other side of the plate load resistor?

100K R on V3 = 76V tube side, 258V B+ side (@ terminal).

[rp]

Went into my trunk-o-tubes, found a few more 6J5s and 6CS7s, all crisp and shiny in perfect boxes, so I'd say these are good.

Sub'd the 6J5 and got very close to the same readings; it's not the tube. Hmmmmmmm... and hmmmm again.... could Fender have that 100K value wrong on the schematic? Or the voltage marking? I'm been through a lot of Fender schematics, I've seen some unlisted changes but never wrong. hmmmmm....