5F6-a Bassman not working..

[silat]

Thanks again all! I just need to get that biased right now...

[Willabe]

Do you know how to bias the amp?

[silat]

It is fixed so the only way is to put a different value resister or change it to be adjusted with a bias pot.

[sluckey]

Number 8 pins they are also tied in to number 1 on the power tubes are not grounded to chassis.

I did have them(The 6l6's) grounded to chassis but they red plated my power tubes. I never thought anymore about it....

Now that was a BIG missing piece of the puzzle. Kinda like withholding evidence. Would have been nice to know about 200 messages ago.

[silat]

Number 8 pins they are also tied in to number 1 on the power tubes are not grounded to chassis.

I did have them(The 6l6's) grounded to chassis but they red plated my power tubes. I never thought anymore about it....

Now that was a BIG missing piece of the puzzle. Kinda like withholding evidence. Would have been nice to know about 200 messages ago.

Hindsight is 20/20... I would have had to brain-storm to remember all that and reveal it. It is easy for you all to think of because you all do this all the time, I am just learning.I apologize but I would have never thought of that in a million years. I was not purposely "with holding evidence"  as I am with you and it would have been much nicer to think about it from the start and get it fixed then.... Again Thank you!

[plexi50]

I am glad you have it working.  You may try another pair of 6L6GC / 5881 tubes. You may have damaged them during previous red plating and other tubes may resolve your bias problem. -48 is what your shooting for.

[silat]

I am glad you have it working.  You may try another pair of 6L6GC / 5881 tubes. You may have damaged them during previous red plating and other tubes may resolve your bias problem. -48 is what your shooting for.

Putting new tubes in still gave me -38

[Willabe]

That's where Doug's tracing method is very helpful.

If you take your time and are careful you find/remember things like the power tube K's not being connected. 

Glad you have it up and running! 
 

[plexi50]

My 5F6A PT is supplying 59VAC for the bias. Yours is supplying 45-46VAC. I was think you may have to use an odd ball resistor value above 56K. Try the 82K again. Your bias voltage came way up in the -40 range last time you tried that. Your plate voltage will go up though.

I myself went with a 325-0-325 PT and am using a 5U4GB rectifier instead of a GZ34/5AR4.
I wanted my plate voltages to be lower producing and smoother mellower sound.

[silat]

My 5F6A PT is supplying 59VAC for the bias. Yours is supplying 45-46VAC. I was think you may have to use an odd ball resistor value above 56K. Try the 82K again. Your bias voltage came way up in the -40 range last time you tried that. Your plate voltage will go up though.

I myself went with a 325-0-325 PT and am using a 5U4GB rectifier instead of a GZ34/5AR4.
I wanted my plate voltages to be lower producing and smoother mellower sound.

I put a 5u4G rectifier in and got it down to 432 but my Bias still remained -38

[plexi50]

That should be because your PT bias tap is putting out 46 volts a/c vs my 59 volts a/c. Try 82k or 100k bias r again.

Some of the Power transformers i have bought are not exactly as advertised. The specs are off a bit sometimes. The current draw is different in some of them under actual working load.

[silat]

That should be because your PT bias tap is putting out 46 volts a/c vs my 59 volts a/c. Try 82k or 100k bias r again.

Some of the Power transformers i have bought are not exactly as advertised. The specs are off a bit sometimes. The current draw is different in some of them under actual working load.

I put a 93 k and Got - 43  my pins 3 on the 6l6's  went  up to 450

[silat]

My 5F6A PT is supplying 59VAC for the bias. Yours is supplying 45-46VAC. I was think you may have to use an odd ball resistor value above 56K. Try the 82K again. Your bias voltage came way up in the -40 range last time you tried that. Your plate voltage will go up though.

I myself went with a 325-0-325 PT and am using a 5U4GB rectifier instead of a GZ34/5AR4.
I wanted my plate voltages to be lower producing and smoother mellower sound.

I put a 5u4G rectifier in and got it down to 432 but my Bias still remained -38

Put a 5y3 and got -42 and 424 on both pins 3 of the 6l6's

[silat]

I see why it went up, it was not a 5u4... It was a 5AR4/Gz34 I did not look at it close enough. But the 5y3 it likes!

[Willabe]

The 5Y3 will not like it, it can't deliver enough current for a pair of 6L6's in bride bias and will burn up.

[plexi50]

Don't use a 5Y3. It can can't take that high voltage for long. I think the power transformer you have is not really 5F6A material.
450 on the plates is fine but i better let someone else chime in here and take over on the bias and plate dissipation factor.

Edit: changed can to can't as that's what I think he meant? But the real problem is a 5Y3 doesn't have enough current capacity for this amp.  Willabe

[plexi50]

The 5Y3 will not like it, it can't deliver enough current for a pair of 6L6's in bride bias and will burn up.

Isn't a 45 volt bias tap on the weak side for a transformer like this?

[silat]

Don't use a 5Y3. It can can't take that high voltage for long. I think the power transformer you have is not really 5F6A material.
450 on the plates is fine but i better let someone else chime in here and take over on the bias and plate dissipation factor.

Edit: changed can to can't as that's what I think he meant? But the real problem is a 5Y3 doesn't have enough current capacity for this amp.  Willabe
[/quote
I got rid of the 5y3 and put the 5AR4 back in it.

[Willabe]

Trying to bias an amp by the -dcv listed in an old schematic is not a good way to bias it.

Fender got away with it back then because they were using vacuum tubes from the golden era of tube manufacturing. And they were using RCA which were great tubes. They were very consistent in their current draw. So Fender could just pop them in and they were close enough.

Different tubes from different makers will bias up differently, draw more or draw less current with the same -dc bias voltage.

You really should add a 1 ohm R in between each K and ground. Then you can measure for the tubes current draw without messing with hi B+ dcv. Much safer! And it's also good to change the non-adjustable bias that's in that amp to adjustable bias.

Here's the link for information on biasing an amp from Doug's Tube Library of information.

Since the bias of an amp is crucial to keep the amp from burning up you should read the whole thing on biasing.

Further down he tells you how to add the 1 ohm R's to the K's, how to make the bias adjustable and how to do the math for total power tube current draw.

After reading it through if you have any more questions on it post them. 

http://el34world.com/charts/fenderservice5.htm 

[Willabe]

Isn't a 45 volt bias tap on the weak side for a transformer like this?

Maybe a little, but a bias circuit doesn't draw much current, only the bias caps charging current so it doesn't sag down like the B+ wind.

45v x 1.4 = 63v

[plexi50]

Isn't a 45 volt bias tap on the weak side for a transformer like this?

Maybe a little, but a bias circuit doesn't draw much current, only the bias caps charging current so it doesn't sag down like the B+ wind.

45v x 1.4 = 63v

Ok i get that. Maybe i will stick a GZ34 after i order one and compare voltages throughout
Those NOS vintage GZ34 's cost a small fortune. I have not decided if i want to spend $150+ yet for a NOS Mullard GZ34.

[Willabe]

Maybe i will stick a GZ34 after i order one and compare voltages throughout.

Be interesting to see, Doug sells a good newly made version. 

Those NOS vintage GZ34 's cost a small fortune. I have not decided if i want to spend $150+ yet for a NOS Mullard GZ34.

Too rich for me.   

[sluckey]

.

[silat]

.

I have -64 @ the diode.

[Willabe]

Sluckey, is what you just posted backwards?

[Willabe]

I have -64 @ the diode.

That should be able to get you enough bias -dcv once you get the 2 R's adjusted. Or better yet make it adjustable bias.

[silat]

I have -64 @ the diode.

That should be able to get you enough bias -dcv once you get the 2 R's adjusted. Or better yet make it adjustable bias.

Yeah I  just read those articles and I am thinking about making it an adjustable bias. I bought a little pot just for it in case that was what i wanted later on.

[Willabe]

Yeah I  just read those articles and I am thinking about making it an adjustable bias.

Great. 

[silat]

I just cannot say  Thank you enough! Willabe, Sluckey, and Plexi50! You all helped me tremendously. I hated that it took  me so long to figure this all out but I learned a lot in the process.

[plexi50]

I just cannot say  Thank you enough! Willabe, Sluckey, and Plexi50! You all helped me tremendously. I hated that it took  me so long to figure this all out but I learned a lot in the process.

See that's what this is all about. Pulling your hair out struggling to understand why an amp doesn't work or work properly. Just swapping out tubes does not tell you why a tube may have failed. The health of the resistor & capacitor in a bias situation is something you have to ruel out by taking voltage readings. It may seem like we are harsh at times but it's anything but that. So happy you have it working. The rest is minimal tweaking at this point. 

[sluckey]

Sluckey, is what you just posted backwards?

Which part? All of it? What do you mean?

[Willabe]

Which part? All of it? What do you mean?

"If it is higher than the voltage you want at pin 5 of the 6L6s you can lower the value of the 15K OR increase the size of the 56K to achieve your desired voltage."

Won't both of those increase the -dcv not lower it?

[sluckey]

Which part? All of it? What do you mean?

"If it is higher than the voltage you want at pin 5 of the 6L6s you can lower the value of the 15K OR increase the size of the 56K to achieve your desired voltage."

Won't both of those increase the -dcv not lower it?

Absolutely. That's the objective here. When I wrote that he only had -38v and plexi was trying to get him to increase it to -48v. Makes sense to me. Maybe I didn't state it clearly?

[Willabe]

I see it now. If the -dcv at the diode is higher than what you want/have at pin 5 then you can get more by changing either/both of those 2 R's.
 
I misread it as if the -dcv is higher pin 5.

[plexi50]

I first used a 56K that put me @ 418VDC plates with a -49.8 grid
Then i put in a 47K and that gave me 408VDC plate with -48 dead on.

I can live with 430PV if i had to but i wanted the lower plate voltage.
Tired of 460VDC 6L6GC amps. Their just LOUD to me and that's it.

When my GZ34 arrives i will do all voltage comparisons.

The lower B+ for me though really allows for a more musical tone.

I wonder if Silat used all old tubes would he not have this bias issue? Hard to say i suppose.

[Willabe]

That's fine to get your plate voltage down some as long as the tubes won't red plate.

But I would still double check the total current draw on each tube so their not over 70% at idol. 

[Willabe]

I wonder if Silat used all old tubes would he not have this bias issue? Hard to say i suppose.

Maybe?