Blown C5 Cap

[77seriesIII]

Shannon,

I'm gonna show my ignorance...test the resistance on the secondaries/filaments power off correct?

/e

[Shannon Parks]

Shannon,

I'm gonna show my ignorance...test the resistance on the secondaries/filaments power off correct?

/e

Definitely, and unplugged and well drained.

Disconnect them from the circuit and measure the resistance across the winding. Then you can take one leg of the winding and compare it to a leg on the secondary.

77, sorry for brain storming so much, but have we eliminated a 6AX5GT failure?

[EWBrown]

Interesting little observation.... ANd totally nothing to do with GSG...

I'm ginning up a simple PP 6V6 based guitar amp on the old Grommes "Little Jewel" chassis.

The original rescifier tube is 6X5GT. I decided to do a little experiment, and since I have some Dumont brand 6X5GTs and 6AX5s, I'd try a little comparison. The power trannie is the "default standard" 350-0-350 @ 90 mA, etc, and I wanted to see if the 6AX5 would be a better fit. Nice and easy-breezy, since both have the same pinout Yellow_Light_Colorz_PDT_03

Well, it turns out that the 6X5 delivers a significantly higher B+ voltage than the 6AX5s (and I tried several of each, all NOS Dumonts,, in order to eliminate any "sick tube" factors).

The 6X5GT delivers 362VDC B+, and the 6AX5 delivers 331 V B+, measured at the OPT primary centertap. I'm running 33-34 mA through each tube (620 ohm cathode resistor for each) and the G1 voltage is around 21 vDC.

Still working out the driver section, I'll probably just copy the "18 watter" design, a couple of 12AX7s, or just to be contrarian, 6N2Ps...


I'da thunk it would have been the other way around , maybe I have to print out the data sheets and look at teh curves again... 6X5GT doesn't have enough current capability to feed a GSG, it will get the dreaded glowing red plate syndrome, I'm sure...

/ed B in NH

[TomMcNally]

Ed -

I think I recall reading someplace that the 6X5 had
a reputation as a transformer killer. It supposedly
would die in a shorted state ??

... tom

[77seriesIII]

shannon....I have tried another my one other 6ax5gt but in a nutshell...##### I havent done the simple thing first. I've been staring at my ss rect for the past week thinking what am I going to do with that?

I'll run the tests after doing the OPT and PT tests.

/e

no worries on the brainstorming...quickest way to find the cure!

[TerrySmith]

Has anyone done a comparison between a normal 6AX5 and the GE 6AX5? The GE version looks like a 5AR4 inside, and I would SWAG it's more robust.

My plans for a 300B version is use the 6123hs outputs, and for power a Heathkit AA100 pt, 375-0-375v, two 6.3v, and one 5v windings. For the rectifier I will rotate, cut&jumper the socket to use a 5AR4 or maybe a JAN-5R4 "tater masher", that would look cool!

[77seriesIII]

Shannon, here are my measurements:

OPT Both Ω reading between 8Ω and GND = 1.8
OPT Both B+ to UX4 pin 2 OPT1= 280Ω OPT2 279Ω

Across 6.3v Secondaries .1Ω all of them
Across Hv 105Ω
Hv1 to GND 54Ω
Hv2 to GND 51Ω
J4 Pin1 & 2 to GND Both 3.25kΩ
J3 Pin1 & 2 to GND Both 3.27Ω

The Interesting part...Inductor read 1.03H SUPPOSED to be 1.5H

Ok. Tested the SS w/ 1 300B in...the Hum started WAY to fast, even w/ my hand on the switch, I had to yank the tube out before the noise became deafening.

I did order all the parts to replace what is on the pcb, resistors, caps. I dont want to do it but am thinking start w/ caps and replace two pieces at a time and test. when Hum is gone culprit found.

/e

[EWBrown]

I've been using the GE 6AX5 in my GSGs. Much nicer, and looks something similar to a 5Y3GT. The Dumonts were "wimpier".

I just ran those specific tubes in my 6V6 test because I had'em and they were the same manufacturer. The Dumonts are the "exposed cathode" tye with the two parallel flat plates.

6X5 is pretty much just an octal 6X4. Even the "metal bits" inside look nearly identical... The spacing between the cathode and the two plates is VERY close.

A 6X5 in a GSG, the PA774 would probably blow it out of its socket, if it didn't short out first and pre-emptively kill the PA774...

/ed B in NH

[Shannon Parks]

Across 6.3v Secondaries .1Ω all of them
Across Hv 105Ω

Thanks for taking those measurements, 77. Did you measure from each of these taps to each other? For example, Filament #1 to HV, Filament #1 to Filament #2, etc.

[77seriesIII]

no missed that. let me test.

[77seriesIII]

shannon,

Took the measurements:

Hv 1 & 2 to each of the secondaries was 0 Load, no connection.

I think I may be missing something that you asked but not sure what.

On the choke measurements, is it common that the actual H of the choke is lower than advertised/stated on the choke? My vm can measure H and mH and the choke measured out a full .5 H less than than stated for a 1.5H choke that is 33% to poorer. The choke was supposed to be 1.5 but only measured 1.0 H. I'm not sure I want to try testing the amp w/o the choke installed and I'm not sure what the results would be if I did this. Suggestions?

/e

[Shannon Parks]

I think your choke is OK, but check the DCR (DC resistance). Is it where it was supposed to be (if you know)?

[77seriesIII]

the choke is a hammond I'm not sure what it is supposed to be, I'll have to check.

[77seriesIII]

the choke measured out to 51.9Ωs.

[EWBrown]

That is in the good range for the 156R or C354, which is nominally 56 ohms +/- 10% DC REsistance..

/ed B in NH