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

[Structo]

That is strange.

Every schematic I have for the 5C8 shows a 100K plate resistor on the 6J5.

But since the measured voltage is half of what is expected I think I would sub in a 47K resistor to see if that changes it to the expected 160v.

[rp]

Tom, the 1.5K cathode 100K plate set up looks correct, here's a Gibson. Next few days I'm going to pull out the soldering iron and I might try that if still unresolved. Might just swap out that 100k, but I'm 100% sure it's fine.

BTW that's 2 6J5's in the Gibson not a twin triode! Had me going for a split second.

Must put this away and go outside.

[Structo]

Yeah, I didn't think about the plate to cathode ratio.

Puzzling.

It will most certainly turn out to be a simple thing to fix, it's just the journey getting there that is aggravating.

[Firestorm]

As to the low heater voltage: have you tried substituting a different 6L6 pair (even modern ones)? If a tube has significant heater-to-cathode leakage, it might be enough to pull down the heater supply.

As to the 6J5: what happens if you ground the grid (as a test)? If there's a high-frequency oscillation present, it might drive the tube hard enough to pull an extra mA of current through the plate resistor.

Just grasping at straws here.

[Phil_S]

On the 6J5, I wonder what disconnecting the two coupling caps from the plate will do for the plate voltage (.047 OD cap, and the .00025 that must be on the pot). I think we can determine whether the subsequent stage is somehow loading down the 6J5. I think I would want to try it 3 ways with both caps disconnected, with just the .047, and with just the .00025.

If you have 90% of 6.3V on the heaters, that is within the relevant range. What you have at 5.8V is at the bottom end of what we might be willing to accept. If you really think it is the heater wire, consider running a new string. It is parallel wiring, so making a "Y" branch mid-way is not likely to be an issue.

[martin manning]

Everything about the 6J5 operating point looks spot-on. Exactly what you should see for the B+, Ra, Rk, and 2.7V on the grid according to the 1943 RCA plate curves.

[martin manning]

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?

I think yes.. If they are all in parallel how can this be?

[Phil_S]

If that is really happening, I would replace the string reading 6.0, and I'd make sure to do a good job cleaning up the old solder. It is strange.

Can you give us a photo of where you connected to the PT?

[rp]

Can you give us a photo of where you connected to the PT?

Phil, there are big yel & green jumpers on the fuses that's the AC primary. The other smaller yel and green jumpers are connecting the PT to the 2 heater strings together.

I think yes.. If they are all in parallel how can this be?

Martin they are all in parallel every way, the tubes in each string and the 2 strings together. First string has V1 V3 V5 V6 in // the next V2 V4 in // and then the 2 strings connect together in // at the yel/grn jumpers to the PT. The two strings though have a different voltage when I measured each out of curiosity. The one with the lower V has the power tubes which is interesting but I had Chinese tubes in at the very start with the same voltage and just tried a set of Philips 6L6GCs and got 5.7V again - RATS!

This has the makings of one of those unresolvable conundrums and damn if I go back and redo the heaters until I've exhausted everything else. Tomorrow I swap OTs - who knows, can't hurt. Maybe there is something wrong with the Stancor that's pulling everything down?

On the 6J5 are you saying 80V on the plate is the nominal datasheet value? If yes, maybe Fender has the measurement wrong? Maybe Cliff will show up and check the 6J5 on his 5C8 and report back. Firestorm, don't you have a real one? Maybe your amp needs a tune up and you need to go in there soon

On the 6J5, I wonder what disconnecting the two coupling caps

Top of the punch list.

[rp]

Just realized it can't be the OT, when I checked the heater string(s) I had the rectifier out - Big fat ugly rats!

For me 5.8V is too low and needs to be resolved. This goes on the shelf if I have to redo the heater wires, and I know exactly what will happen after that agony - I'll still get 5.8V! I'm a big blue funk!

For now I deal with the 6J5. And order me up a 330-0-330 PT.

Buona notte.

[martin manning]

The 6J5 is fine. I calculate 1.8 mA from your voltages and both Ra and Rk. That intersects a 100k load line at your plate voltage, and agrees with the location of a 2.7V grid line. I'll post it later...

Here's the load line plot- the only thing that doesn't agree is the 160V on the layout.

[Phil_S]

Do you have a pilot lamp on the heater string? If so, disconnect it (or maybe just remove the bulb). What happens?

[Firestorm]

Firestorm, don't you have a real one? Maybe your amp needs a tune up and you need to go in there soon

Mine's transitional; has 12AY7s and 12AX7s; more D8 than C8.

[martin manning]

I think yes.. If they are all in parallel how can this be?

Martin they are all in parallel every way, the tubes in each string and the 2 strings together. First string has V1 V3 V5 V6 in // the next V2 V4 in // and then the 2 strings connect together in // at the yel/grn jumpers to the PT. The two strings though have a different voltage when I measured each out of curiosity. The one with the lower V has the power tubes which is interesting but I had Chinese tubes in at the very start with the same voltage and just tried a set of Philips 6L6GCs and got 5.7V again - RATS!.

Wait, are you saying the two strings have different voltages when all the tubes are installed, or when just the tubes on each string are installed? The latter is ok because the loads are different, but if it's the former then something is wrong.

[rp]

The 6J5 is fine. Here's the load line plot- the only thing that doesn't agree is the 160V on the layout.

Thanks for the confirmation, my sophomoric reasoning was also leading me to believe Fender got the V wrong on the layout, it's the only reasonable explanation. It's the simplest stage possible and just a triode, parts are 100%, there simply can't be anything else and all the other Vs (except the heaters) are dead on and it plays well. Hoping Cliff chimes in, I'll wait a while and then PM him. He could settle it.

At least I know the Stancor is good and can stay in (as I already drilled the fancy chassis for it

Now the heaters

Martin, it's each string energized separately that I measured. When tied together I get 5.8V. All the 6l6 amps I've build have ~ the same load and only drop from ~6.6 unloaded to ~6.3. First time I've ever had such a huge drop, it can't be right. I want to clean up, get my kitchen table back, I want to order it's dedicated PT but would like to be certain of the voltages first.

BTW going (hopefully one day) from 5.8 to 6.3V shouldn't affect the plate voltage should it? I can safely order a 330-0-330 PT?