Tremolus bias tap

[jjman]

-5.5vdc bias is way to weak. This is why the tubes are close to exploding. The scheme calls for a target of -45volts. You can read the dc volts on the grid while turning the bias control to see the range available. Check this after fixing the circuit before you put any power tubes back in.

If it were my amp I would change all the components in the bias supply and be reluctant to use those particular 6L6s again. At best I'm thinking they are no longer matched, assuming they were in the past.

Thank Leo for using an output tranny that can take that level of current.

[Tubetwang]

-5.5vdc bias is way to weak. This is why the tubes are close to exploding. The scheme calls for a target of -45volts. You can read the dc volts on the grid while turning the bias control to see the range available. Check this after fixing the circuit before you put any power tubes back in.

If it were my amp I would change all the components in the bias supply and be reluctant to use those particular 6L6s again. At best I'm thinking they are no longer matched, assuming they were in the past.

Thank Leo for using an output tranny that can take that level of current.

WOAH!!!!!!

Good call JJ...

Thanks for the tip...

I will change all the components in the bias supply and report back...

I'm thinking of building Leo a small shrine on my front yard.

Will also get some "I love Leo" car plate.

I already have a Leo's bed pillow and a Leo's toohbrush.

You won't catch me without a Leo's shirt pen pocket...ever!

He was the man!



I will

[Tubetwang]

I put in a new diode, 25uf 50v electrolytic, and a 220R 1w.

After smoking the 220R 1w resistor, i realized my mistake and put in a 220K 1w and got a reading of -5.60.

Tried a 330K uf 1w and got -6.30 on my Volt DC.

Tried a 1M 1w and still got -6.00.

That is still far from -45

I should get the new transformer tomorrow...in the mean time i'm stuck and don't know what to do...

Time to Google i guess...

[Structo]

220uf 1w resistor

[Tubetwang]

Sorry.

I meant 220R 1w resistor...

[jjman]

http://www.schematicheaven.com/fenderam ... _schem.pdf

You are keeping with a 22k in the 22k location, yes?

You are grounding the + side of the cap, yes?

I would have expected the voltage to get weaker when you moved to the 1meg, not stronger.

[Phil_S]

+1 on what JJman says. Check against the schematic. Try a 180K or 150K instead of the 220K. You need more negative voltage, not less. Raising the value of the R cuts back on what goes through it. Think of it like a water pipe. If you 220,000 little marbles in the pipe, not much water gets through. If you cut it back to 150,000 little marbles in the pipe, more water gets through.

Don't put that new PT in there until you have bias from the old one. You don't want to toast the new one and something isn't right. I don't believe there is anything wrong with the PT, as I think you reported plate voltage that was reasonable.

Putting in the new PT introduces a whole new set of variables at one time and I'm not sure it is a wise move.

[Tubetwang]

Ok lads!

Tonight i took all the old carbon resistors out and put in all new carbon film resistors everywhere. Ceramic caps were changed for Mallory film caps and Silver Mica.

I also got my new tranny today and i was about to proceed to wire it up and decided to come and check on you guys...

Good thing i did...

I'll put the old 273X Hammond back in and will try the resistor trick tomorrow.

Thanks for the help. Mui appreciated!!

I will be back!!!!!!!:lol:

[Tubetwang]

http://www.schematicheaven.com/fenderam ... _schem.pdf

You are keeping with a 22k in the 22k location, yes?

You are grounding the + side of the cap, yes?

I would have expected the voltage to get weaker when you moved to the 1meg, not stronger.

Yes JJ, but i put in a new 22K. I've got all new Mouser resistors everywhere now...

The + side of the cap is grounded. I had an Atom 25uf 50 volts in there. I saw a guy overalling a Deluxe Reverb and he put in a 100uf 100v. I took the Atom and put in a bi-polar 56uf 100v that i had.

Will come back tomorrow...stay tuned...

[Tubetwang]

Against my better judgment, i wired the new power tranny, and, the bias is now at -45 with room to play on both sides...

But...

The rectifryer is the only tube's lit...bulb is also in the dark...

Some more Googling is in order...

As a side note...

Steve Landry at Hammond told me this morning that this here Hammond 290CX drop-in replacement PT is built by FENDER for Hammond.

[jjman]

I'm surprised to hear that Fender is making trannies. I don't think they ever have in the past?

Sounds like the 6.3v heater circuit is now a problem. Should be easier to diagnose and fix than the previous problem. Does the newest tranny have a 3-wire setup with a "center tap" for the 6.3v heater winding? (Perhaps green with a stripe on the CT wire?) Did you solder the new green wires to the same locations as the old ones? Was that directly to the light bulb socket? Any pics?

[Tubetwang]

Yes JJ...

i rewired the green wires going at the bulb and on to pin 2 and 7 of the 6l6 as before...

I called Hammond this morning to inquire about the orange PT wire (shield), not knowing if i should solder to chassis as a ground or leave it heatshrinked.

He told me to do either, but best to ground it to chassis...

The red-yellow and green-yellow were grounded to chassis as the old pt.

Here's some pics...

[Phil_S]

Here's the cold truth as I see it, no sugar coating. It's a damn shame you replaced all those caps and resistors. You just devalued that amp significantly and produced no result. There was no reason to do that work. You are shooting in the dark rather than concentrating on the most apparent problem, which is lack of bias voltage.

Ah well, it's your amp, so you do as you see fit. If it were mine, I'd figure out the bias voltage problem. It well could be that is the only problem, so concentrate, don't go all over the lot. The bias problem isn't hard to solve. When you tap the HV winding for bias voltage for bias, the only difference required is a dropping resistor, instead of 470 ohms, needs to be in the 150K to 270K range depending on what the PT is generating. I'm thinking you need 180K or 220K to get it to the mid 40's. Make sure everything else in the bias circuit works as it should.

BTW, I think you mis-heard them at Hammond. Hammond is now making a drop in replacement for a Fender, not the other way 'round. I seriously doubt Fender is winding it's own.

[Tubetwang]

Here's the cold truth as I see it, no sugar coating. It's a damn shame you replaced all those caps and resistors. You just devalued that amp significantly and produced no result. There was no reason to do that work. You are shooting in the dark rather than concentrating on the most apparent problem, which is lack of bias voltage.

Ah well, it's your amp, so you do as you see fit. If it were mine, I'd figure out the bias voltage problem. It well could be that is the only problem, so concentrate, don't go all over the lot. The bias problem isn't hard to solve. When you tap the HV winding for bias voltage for bias, the only difference required is a dropping resistor, instead of 470 ohms, needs to be in the 150K to 270K range depending on what the PT is generating. I'm thinking you need 180K or 220K to get it to the mid 40's. Make sure everything else in the bias circuit works as it should.

BTW, I think you mis-heard them at Hammond. Hammond is now making a drop in replacement for a Fender, not the other way 'round. I seriously doubt Fender is winding it's own.

I understand, agree and appreciate all of your comments Phil...

I kept all the old parts in a bag.

Resistors were out of values anyway...

I only changed the ceramic caps for silver mica.

It's not has if i butchered the amp. I can easily put them back in if a collector is eventually buying it...


I got the bias under control now...

The Hammond 273X is a very old one with the Hammond metal badge on it. It will go in my parts bin...

I will bring this amp to the repair dude.


I asked Steve twice after he told me that Fender was building these trannies for them...Fender is building these trannies for you?

Either it's true or Steve shoots the shit...

One thing i know for shure...i know i'm not worthy!

My bench will be dismantle today and stored for better days...

I better concentrate on me playing...

[Tubetwang]

Update...

Mr Tube (the repair dude) just called me and told me that the 290CX P.T. green wires read zero...

So much for a brand new transformer...