Weber 6a14 build still isn't right

[5thumbs]

Hi Gang,
I have been trying to find the issue with this amp since I finished wiring it! The amp has very low output volume and it is distorted. I have changed all the tubes and traced all the wiring which seems fine. I actually traded out the OT for another one I knew was good and still the same problem. I tested the preamp plate resistors and they all checked out fine. The plate voltage on the V1-12AX7 is 140vdc on pin 1, and pin 3 is .89 dcv, pin 2 is 0. 6,7,8 is also 136,0,.94 respectively. B+ is 200vdc coming to the plate resistors and the 100k resistors drop it to 140v going into the tube. Isn't this too low? I can't seem to find any voltage charts on the Princeton Blackface Reverb, so I am a little lost. The dropping resistors by the filter cap can all check good. V3-12AX7 is also at the same voltages and it also seems low. The 6v6's seem fine with 310 vdc plate, 304 vdc screen grids, and -26 vdc on the control grid. What am I missing and what should I suspect?
Help!
Tim

[loudthud]

Could be a short at the speaker jacks. Turn amp off. Unplug the speaker and measure the resistance right at the speaker, it should be about 6 ohms. Plug the speaker into the amp and measure the resistance at the speaker again, it should be pretty close to a dead short. Unsolder the green wire that goes to the output transformer and measure the resistance, it should now measure 6 ohms again. If it still measures a short there is some kind of short at the jacks.

What are the voltages at V4 pins 6 and 8?

-26V on the grids of the 6V6s seems a little high. Do you have any way to measure the bias current?

[5thumbs]

Hi Loudthud,
Thanks for your help!

Okay, did the speaker test...I got 7.4ohms on just the speaker, .9 ohms on the speaker connected to the circuit, 7.4ohm with the green OT wire unsoldered.

The voltages on V4 are: pin 6=143 vdc and pin 8=56 vdc (pin 7 also was 56vdc)

I used a Weber Bias Rite from a friend and got: tube A 300 vdc and 16ma, tube B 300 vdc and 16.2ma. I could dial up the bias setting to around 325 vdc and the Ik reading would go down to ~9-10ma, when I dialed the bias pot down all the way it would be around 270 vdc and the Ik would be 18 & 27 respectively. I have only built tweed style cathode biased amps up until this point and am still pretty confused by the whole bias setting thing. ;-)

[loudthud]

Plenty of threads here on biasing, maybe an overwhelming number. In general, a 6V6 with 300ish B+ would like to be around 30mA (per tube). Try that and see how it sounds. The V4 voltages look good, that's your phase inverter. Look on some of the Fender layout diagrams for voltages. Those preamp voltages are on the low side, probably because the B+ is low. I would change R29 to 4.7K and R46 to 10K to bring the B+ up.

[5thumbs]

Hi Loudthud,
That is my problem! I would like to run the amp at 275 VDC plate voltage with a bias current around 27ma. But I can't get there from here! ha! When I turn the bias pot up to 300+ volts the bias goes down around ~16ma, when I turn the bias pot all the way down around 270 VDC the bias current no longer balances and I get 18 & 27ma respectively. I will try a new set of 6V6's tomorrow, but that didn't help last time I tried. Does the low voltage from the V1 preamp tube cause a low grid current on the output tubes? I actually went back in to the build and changed the PT B+ leads from the Red/Wht (540V) to the Red leads (680V). This helped to raise the plate voltages of all the tubes, does this hurt anything? I will pick up some power resisters tomorrow and try your suggestion. Thanks for sticking with me through this and I am going to read a little more about how bias current is created and effected from the circuit.

Tim

[MWJB]

"That is my problem! I would like to run the amp at 275 VDC plate voltage with a bias current around 27ma. "

What an earth for? Princetons run well over 400vdc on the 6V6 plates, though anything from 400v upwards could be considered ball park.

In a typical Fender preamp stage with 100K plate resistor and a 1.5K (per triode) cathode resistor, you will see roughly 68% of the dc voltage applied to the 100K plate resistor at the tube plate...so 140vdc on the plate, from 200vdc at the B+ rails is spot on.

[5thumbs]

Can I really run the 6V6's that hot? I was told that Fender biased the Princeton Blackface Reverb amps around 275-300 vdc. Of course that is just what someone told me, I haven't really found any voltage charts for this amp. I am going to get the smaller power resistors and I will try to boost up the B+ voltage.

Thanks!
Tim

[tubeswell]

Can I really run the 6V6's that hot? I was told that Fender biased the Princeton Blackface Reverb amps around 275-300 vdc. Of course that is just what someone told me, I haven't really found any voltage charts for this amp.

Thanks!
Tim

Hi Tim

Take a look at the Fender AA1164 schematic, plates are at 410 - screens at 400

[5thumbs]

Hi Tubeswell,
How do you get 410 to the plates when the Power Transformer only puts out 340 per leg? Does Weber supply an undersized transformer? Please understand, I am asking hoping I can learn some of your knowledge about this because I know enough to follow the layout when I wire these things up! ha!

Thanks for your help,
Tim

[tubeswell]

Hi Tubeswell,
How do you get 410 to the plates when the Power Transformer only puts out 340 per leg? Does Weber supply an undersized transformer? Please understand, I am asking hoping I can learn some of your knowledge about this because I know enough to follow the layout when I wire these things up! ha!

Thanks for your help,
Tim

Hiya Tim

Not sure which Weber PT you have - the one I had in my 6A14 build was a 025130EU which had a choice of secondary taps for the HT winding of either 270-0-270VAC or 340-0-340VAC IIRC.

When you rectify 340-0-340VAC with a 5U4G you get about 410VDC B+. The rule of thumb multiplication factor for B+ from a 5U4G is 1.21 x the VAC of one side of the HT winding: so 1.21 x 340 = 411 which is in the ballpark

[JoeM]

How do you get 410 to the plates when the Power Transformer only puts out 340 per leg?...
Tim

Because you're mixing up the AC volts from the PT with the (rectified) DC.

[5thumbs]

Aha! So the Rectifier Tube amplifies as well as rectifies! That is one strange tube that has the output on the same pin as the heater! Who came up with all of this? ha!

Do you think I may have a bad rectifier tube that is not supplying enough voltage? I am using the Red wires from the Weber transformer (WO25130) which are 340 per leg. I assumed the tube was either working or not (I guess maybe I should say on or off) but can it be partially working or converting voltage without boosting it? If so, I will have to try and track down a new tube on Monday.

Thanks,
Tim

[JoeM]

Aha! So the Rectifier Tube amplifies as well as rectifies!

The rectifier tube doesnt amplify, it can only rectify.

[tubeswell]

Aha! So the Rectifier Tube amplifies as well as rectifies! That is one strange tube that has the output on the same pin as the heater! Who came up with all of this? ha!

Do you think I may have a bad rectifier tube that is not supplying enough voltage? I am using the Red wires from the Weber transformer (WO25130) which are 340 per leg. I assumed the tube was either working or not (I guess maybe I should say on or off) but can it be partially working or converting voltage without boosting it? If so, I will have to try and track down a new tube on Monday.

Thanks,
Tim

340VAC is an RMS voltage (the 'averaged' voltage of the AC sine wave). A full wave rectifier takes the negative AC swing turns it into an intermediate positive AC swing, (so instead of having the original positive AC swing, followed by a negative swing, followed by another positive swing and so on, you now have a whole lot of positive swings immediately one after another). These swings are smoothed out mainly with the 1st (reservoir) filter cap, and thereafter turned into a virtually flat DC voltage by the rest of the filter cap system. The DC voltage corresponds more or less with the peak rectified voltage.

To get the theoretical peak voltage of the AC cycle swing you multiply the RMS voltage by 1.4142 i.e.: 340 x 1.4142 = 480.8, which is about the DC voltage you would get if you used FW SS rectification on the 340-0-340VAC, because diodes only drop 0.6V each. However a tube rectifier drops more voltage, hence that rule of thumb I mentioned (1.1 for a 5Y3, 1.21 for a 5U4, 1.31 for a 5AR4).

[5thumbs]

Finally it is starting to make a little more sense to me! Thanks Tubeswell!


Tim