Voltage on 5E7 seems way off....

[playonit]

I just got new tubes (5881) from the postman. I put them in and went to bias (adjustable) and couldn't get below 45mv. I checked voltages and they are pretty high.. I went through a few checks re-checked and no difference. I have included my voltage chart for the 5e7..

my voltages are on the right.. I have a Weber PT 720vct....

[Phil_S]

Well, you'd need to increasee the 6.8K bias resistor to get the bias voltage to go less negative. I don't actually believe this is the problem here, as all the voltages are kerflooie.

Normally, when bias voltage is too cold, you'd expect plate voltage to rise. That makes perfect sense for the power tubes and if that was the only problem I'd say to fiddle with that 6.8K.

It's the preamp tube changes that strike me as weird. I'm wondering if your rectifier isn't dropping enough voltage or if there is some other power supply rail problem. Your rectifier is spot on at 1.2x 360 with respect to expected output.

Have you checked the voltages on each side of the choke to see if it is doing it's job properly and the same for the dropping resisters in the B+ rail?

[playonit]

The thing I find funny is my Magnetic components choke is rated at 90mA /4HY and my Weber choke is rated at 200mA / 5-8 HY. here are quoutes from both with regards to their chokes..

Weber - Choke for Tweed Fenders. 5-8H @ 200ma

Mag Components - This is the most common 90mA DC rated Fender* type choke used in many of their amplifier applications


I am using the Weber choke with my MC's PT and OT on my other 5e7 with the voltages in line with where they should.. I am using the MC choke on the amp that is showing the high voltages....... Under choked ??

The Weber PT in this build is rated at 720vct @ 200mA

[Luthierwnc]

You need to drop the value on the 56k bleeder. Since this isn't a vintage restoration, put in a 25k trim pot (wired as a variable resistor) in series with a 47k (order a 41k too) resistor to get those volts down. It doesn't matter which order you put them in.

The "E" series Fenders have the choke in front of the plates so you need a more robust choke than the tolex models. I just sent the choke for a '55 Bassman (pretty similar to your 5E7) to Mercury Magnetics for rewinding. It is beefier than even the choke for a modern Twin. You need that. With a 720VCT trannie, a LC filter is going to knock the volts down to a still frisky 430VDC. In a modern filter arrangement, you are looking at a very non-tweedish 460. This is my favorite circuit and I wouldn't want the plate volts over 440 -- especially with 5881s. sh

[Phil_S]

IMO, you don't need the 200mA choke. It sits between the plates and the screen supply. Either one should work OK.

If you are not otherwise seeing a problem, why don't you tack in another 6.8K in parallel to the existing one in the bias circuit and see what happens. Sometimes things are just a certain way for reasons that we don't fully understand.

[playonit]

You need to drop the value on the 56k bleeder. Since this isn't a vintage restoration, put in a 25k trim pot (wired as a variable resistor) in series with a 47k (order a 41k too) resistor to get those volts down. It doesn't matter which order you put them in.

The "E" series Fenders have the choke in front of the plates so you need a more robust choke than the tolex models. I just sent the choke for a '55 Bassman (pretty similar to your 5E7) to Mercury Magnetics for rewinding. It is beefier than even the choke for a modern Twin. You need that. With a 720VCT trannie, a LC filter is going to knock the volts down to a still frisky 430VDC. In a modern filter arrangement, you are looking at a very non-tweedish 460. This is my favorite circuit and I wouldn't want the plate volts over 440 -- especially with 5881s. sh

I am running a 25K Pot already off a 47k r with the pot turned all the way down I'm still running over 45mA on bias I built this using the Ceriatone and Weber docs.. I have another same specs and voltages are in line.

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[gingertube]

Just a comment on chokes for your info.
The inductance of chokes drops as you put more DC current through them so it is generally a good idea to use over-rated chokes.
While the 70mA job perfectly adequate to supply the output tube screens and the preamp, the 200mA rated one will be better.
These are also reasonably expensive devices so a lot of suppliers/manufacturers will only stock a limited range and provide the same 200mA rated choke for any project which requires a choke of that value with any rating up to 200mA.
Cheers,
Ian

[Phil_S]

I am running a 25K Pot already off a 47k r with the pot turned all the way down I'm still running over 45mA on bias...

If it were mine, I'd lower the value of the bleeder by tacking in another 47K in parallel to the one that's in there. That will change your range from 47K-73K to 25K-48K. That's the next notch down. If you've got the pot at 0, then you are already at 47K.

[playonit]

Thanks guys.....

[rdjones]

5E7Ceriatone_100508.jpg

Another Ceriatone layout with errors and/or safety violations in it.
Let's see who can find them ...

{hint} the fuse and the rectifier are wired correctly in this one.
There is at least one safety issue, and two wiring errors (same error twice).

Takers ?

rd

[gingertube]

Well the Output Tranny power feed is from the output side of the choke which means that the 200mA rated choke is a must. Also unless the colors on my screen are a bit sus then one side of the OT seems to be wired to a screen instead of a plate - did'nt really look any harder.
Cheers,
Ian

[playonit]

5E7Ceriatone_100508.jpg

Another Ceriatone layout with errors and/or safety violations in it.
Let's see who can find them ...

{hint} the fuse and the rectifier are wired correctly in this one.
There is at least one safety issue, and two wiring errors (same error twice).

Takers ?

rd

IEC socket grounded on PT mounting bolt.

The 2 under board wires from the 220k going to V4 & V5 5 pins instead of 6??

[rdjones]

IEC socket grounded on PT mounting bolt.

The 2 under board wires from the 220k going to V4 & V5 5 pins instead of 6??

DING, DING, DING We have a winner !!

Yes, the Earth ground should get it's own separate chassis bolt.
This is less of a risk when used with an IEC AC connector, but to my understanding is still part of safety codes.

With the grid feeds going to pin 5 the grid stopper is out of the circuit.
This is part of the reason I don't care for using N/C tube socket lugs as tie points.
The grid stoppers are most effective when used as close as possible to the socket.
The alternatives are to use a terminal strip next to the socket(s) or fly the resistor.
But that's the way Leo did it so we're stuck with it.

Ceria was kind enough to ground pin 1 on the outputs, which in the years before Leo changed things and used that pin as a tie point it was convention to ground pin 1 because the outer shell of metal tubes was on pin 1.
On a lot of older designs you will see pin 1 grounded whether it needs to be or not. <history lesson>

On other Ceria layouts I've seen the fuse wired with the line to the ring instead of the tip, and/or the B+ taken from pin 2 instead of pin 8 on the rectifier.
This pin 8 error is a mistake copied directly from a Fender schematic without being corrected.

As far as safety issues go ... Yeah yeah, I get that Ceria is half way around the world and doesn't give a crap about other countries safety codes.
It's just that now we are in a sort of Renaissance of tube amp building and I feel more of a responsibility to be a "Safety Advocate".

sorry for the thread hijack, carry on ...

rd

[Luthierwnc]

"I am running a 25K Pot already off a 47k r with the pot turned all the way down I'm still running over 45mA on bias I built this using the Ceriatone and Weber docs.. I have another same specs and voltages are in line. "

Couple of big things; 45ma per side? If you are too high on your bias amperage, you need to increase the resistance on the bleeder -- not decrease it. I got the impression your bias voltage was too high. They work opposite. More negative voltage decreases the current flow.

Phil S: Take another look. This uses a choke input filter. All the B+ is on the other side of the choke -- plates, screens, preamp. At normal idle this puppy will be pulling 80 ma. Actually playing it is more.

720VCT is too high for that particular tone anyway. 660 is more like it. I'm not saying it won't sound good -- just that the last of the cathodyne inverters wasn't designed around silverface voltages. Certainly 5881's aren't. If that trannie was spec-ed for modern power supplies and the bias winding is on the HV supply, chances are good the LC filter is knocking the bias tap volts down as well. Fewer volts, more amps. sh

[Phil_S]

OK, I looked again at the schematic. Oops, my bad on the choke. Sorry and thanks for setting the record straight.