Tremolus bias tap

[Tubetwang]

i just noticed that my AB763 1965 Tremolux has a weird connection.

Previous owner had changed the PT for a Hammond 273X.

This here Hammond does not have a Bias tap.

The wire to the bias 470 1W resistor comes from rectifier pin 4.

There should be a tap on the original transformer...a white wire that would go to the bias 470 1W resistor.

Any one in the know?

Is this all right or should i be worried?

I've been Googling all day...

Will keep on trying to find out what is going on here...

Thanks for any tips.

[gahult]

So that puts 1 leg of 350v AC there
What do you check it at
What do you get at the Neg. end of the diode?
Not that I know what to tell you but, just wondering.

Gary

[Tubetwang]

Every word helps Gary...

I'll be on it, until i fix this amp, even though...i'm not worthy... :roll:Ho Yes!!
I will !!!

I noticed that the 6L6 pair were pretty red...

I dumped all electrolytics for new ones.

Tremolo did not worked.

Ordered a few tremolo roaches and a Compu-Bias per Allyn's recommendation.

Have'nt received the tremolo yet, but Compu-Bias made it home.

After the cap job, the power tubes were still glowing red.

I took the measurements down from the Compu-Bias and here's the numbers ...

VP 370 - IK 200 - W 73
VP 365 - IK 225 - W 81

I shoul of got 15 to 21 W per power tubed... 73 and 81 W looked suspicious...

I also noticed that turning the bias pot made no change on the readings.

That is when i discovered that there was no bias tap.

I won't put tubes back in until this amp has been fixed.
Ho no, i won't!!

I will order a proper P.T., yep-yep, and will measure every resistors and what not.

I'm not one to change parts needlesly, but, this amp has been worked on by who knows who, and i figure that some resistors may be of the wrong values...or worst.

I have too much respect for M. Leo Fender and his work...to let one of his amp go down...

I don't believe in God...but i believe in Leo!!

For the record, i will report on my findings as they come along...

[Phil_S]

Plenty of amps tap the high voltage supply for the bias circuit. Look for example, at the Marshall JTM-45. Your particular problem now is that no one changed the 470 ohm resistor. I am too lazy at the moment to calculate what you'd need in there, but it is probably 220K or 270K depending on how much your PT puts out. You might go as high as 330K to get the right voltage. Once you get the bias voltage into the mid 40's you should be just fine.

If you want to replace the PT, by all means, that's your choice. If your motive is simply to fix the bias circuit, that's unnecessarily expensive.

The tremolo won't work on this amp unless you've got a footswitch plugged in. If you don't have a f/s, make yourself a dummy plug with a SPST switch between the tip and sleeve, but you must have something operational in the jack. You also need the correct tube. The schematic calls for a 12AX7. A 5751 will work. The other 12A?7 family of tubes have too low a plate resistance to trigger the required oscillation. If someone stuck a 12AT7 in there, that's part of the problem.

While you are in there, get rid of that wire nut. You can splice, solder and cover with heat shrink, which I think is the best thing to do. You could also take advantage of an unused pin on the rectifier tube as a tie point, but I'm not fond of this for the mains supply voltage.

[Tubetwang]

Hey thanks a lot for the tips Phil!

Wow...i did'tn know that.

I've check it again quickly and everything works.

The 6L6 go cherry red...i guess because of that 470 resistor that makes for a bias that is too high?

I already ordered a proper tranny this morning...we'll see what's up when i get it.

My schema ask for a 12AU7 in v4.
Both 12AX7 and 12AU7 work though.

I'm going to get that bias under control...
I will...

I wonder if i should install a new diode or leave it alone?
Do they go bad with time?

[Phil_S]

The 6L6 go cherry red...i guess because of that 470 resistor that makes for a bias that is too high?

You are going burn up those tubes, if you haven't already, with the bias set like it is. Sub a 220K for the 470 ohm and it will work properly. This will allow you to work on the amp, and probably play it, too.

I already ordered a proper tranny this morning...we'll see what's up when i get it.

As I said, that's up to you, but isn't needed here.

My schema ask for a 12AU7 in v4. Both 12AX7 and 12AU7 work though.

V4 is the phase inverter. 12AU7 is very similar to 12AT7 but lower gain. It will deliver a cleaner tone and less drive; possibly less volume. But that's not the tube to which I refer.

V3, the 12AX7 is what drives the tremolo. For that you need a tube with high plate resistance or you won't trigger the desired oscillation for the effect. Make sure what's in that hole. BTW, 7025 and 12AX7 are essentially the same tube, with 7025 (in the good old days, at least) being a low noise version of the tube. Don't forget to plug in the foot switch.

I'm going to get that bias under control...
I will...

Of course you will. This isn't rocket science! Put in the 220K. It will take you all of 60 seconds to get it done. You can lift one side of the 470 and leave it there if you like, so you can reconnect it when you get the new PT. Put some heatshrink over it to insulate it.

I wonder if i should install a new diode or leave it alone?
Do they go bad with time?

Well, at about 10 cents each, you can replace the diode if there are any questions, but really, it should last forever.

[Tubetwang]

Well...

In a few sentence you have done more for me than 3 hours of today's Googlin'...

You're the man Phil, and i thank you for it.

Cheers!

[Phil_S]

Twang: someone else taught me all of this. Now it's been passed on to you. This is a good thing. Don't think I invented this wheel; I'm not that smart. Go fixt the amp. Let us know how it works out.

See if you can save that new PT for a nice homebrew. You really don't need to make the change, as it won't do Jacques Chit for your tone.

[jjman]

Isn't it likely that the bias cap (and perhaps diode) are toasted? If they are not toasted, wouldn't the tubes be "shut off" from the too-strong negative bias voltage from the stronger voltage tap being used? Has the DC voltage on the grids been been measured? Maybe a few hundred volts of ac are making their way to the output's grids after all?

[Tubetwang]

Isn't it likely that the bias cap (and perhaps diode) are toasted? If they are not toasted, wouldn't the tubes be "shut off" from the too-strong negative bias voltage from the stronger voltage tap being used? Has the DC voltage on the grids been been measured? Maybe a few hundred volts of ac are making their way to the output's grids after all?

Hum...

I'll measure the DC voltage on the grids and come back tomorrow with it...

Been a long day...i'm hitting the sack...zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz

[M Fowler]

Maybe I am missing something here but I have attached the Hammond specs and hookup diagrahm and it shows a center tap to me? Or maybe not bias tap.

Mark

[Tubetwang]

JJMAN

I just took the DC voltage on rectifier pin 4 and i got 373.3mv.

[Tubetwang]

Mark,

to ad to the confusion, my 273X is a 1995 model and has an extra wire (green-yellow) that is soldered to the chassis along with the red-yellow...

I should get the 290 CX tranny today...I will post a pic for your viewing pleasure...

I'll use the 273X for a Princeton Reverb build...so it's no lost.

[jjman]

With the output tubes removed, what is the DC voltage on the output tube sockets’ grid terminals (pin 5) ? (measuring each separately.)

Is it negative or positive voltage?


With the output tubes removed, what is the AC voltage on the same output tube sockets’ grid terminals? (measuring each separately.)

I mention "with the output tubes removed" since it would be best to keep them out until more is learned. The output tranny could blow under these circumstances, until resolved. The tubes may also be damaged.

[Tubetwang]

With the output tubes removed, what is the DC voltage on the output tube sockets’ grid terminals (pin 5) ? (measuring each separately.)

Is it negative or positive voltage?


With the output tubes removed, what is the AC voltage on the same output tube sockets’ grid terminals? (measuring each separately.)

I mention "with the output tubes removed" since it would be best to keep them out until more is learned. The output tranny could blow under these circumstances, until resolved. The tubes may also be damaged.

OK JJ,

we've got (negative)-5.57DC and 0.015AC on one 6L6 and
(negative)-5.45DC and 0.039AC on the other 6L6

Does it make any sense?

Thanks