Filmosound conversion problem

[baddog]

Anyone out there?

I removed the first tube V1 and the extra 6v6 from the signal chain and connected a standard fender input to V2. I biased the amp to 31ma and have 355v on the 6v6 plates . The amp sound good except when the amp is turned up past 8. My notes cut out for a second when i hit a note hard but it it dousnt happen when i hit the same note soft. this happens mostly on the E,A & D strings. I am using old tube organ 6V6's and was wondering if that could be causing the problem because of the higher plate voltage. I have tried other tubes ( old ones) but have the same problem. I installed a 500k volume pot at the input of the phase inverter in place of the 450k resistor to ground. I also recaped the amp .
Any sudgestions welcome

thanks
BD

[Dave Curtis, dB AudioTech]

Perhaps it is the sound of the power supply collapsing under load. How old are the PS capacitors (electrolytics)?
Aha, I re-read that you recapped the amp...

[baddog]

Hi Dave

The amp was recaped when i got the amp for repair. I even changed some of the caps because he had sprague caps that where physicaly to big to fit in the chassis comfortably.

Thanks
Bd

[loudthud]

Still have the 500 ohm pot connected to pin 8 of the 5Y3? That will cause a big sag in the B+. Connect a wire around it.

There is feedback around the power amp. See the 22k resistor and cap connected to the 16 ohm tap on the output transformer? Try disconnecting it or hooking it to the 8 ohm tap. The value of that resistor may need to be increased or removed altogether.

To improve the tone controls, connect pin 2 of the 1st 12AX7 to the center terminal of the treble pot and install a 330K resistor between the treble pot and bass pot where a wire is now.

Got any 12AT7s? Their lower gain might help eliminate the unwanted blocking type distortion.

[baddog]

Hi Loudthud

The pot at the rectifier is actually a 500 ohm 10watt resistor. What is giong to happen when i bypas the resistor? More headroom ?

I was thinking about just rewiring the preamp altogether. Any thoughts

I dont have the feedback loop connected to the 8 ohm tap just the 16ohm. I am using the 8 ohm for testing.

It would be cool to be able to use the 5879 tube and conect it to a switch. I didnt remove it totally from the chssis just the signal path. When it was hooked up i had all of the distortion & feedback i ever wanted . Jimi Hendrix style

Thank
BD

[baddog]

Hey loudthud

Thanks for the advice it made me check my work again anf i found the problem. When i replaced the FBL i installed a .002 instead of a .001 cap and the amp works fine now. I also took the advice from a guy on the Weber Forum and replaced the plate B+ resisters for the 12ax7 preamp with 100k 's and now have nice distortion. IF i remove the 500k 10 watt adjustible resistor wont that bring the plate voltage up to high for my 6v6's ?

Thanks Again
BD

[loudthud]

If you are concerned about the B+ being too high, a zener diode will drop the voltage but it's constant voltage drop won't contribute to sag. Usually a 5Y3 has enough sag by itself IMHO YMMV. 6V6's are frequently used above 400V, upto 450V in some cases so things should be ok. There are quite a few threads on Ampage that discuss using a zener to drop the B+, do a search. Most often the zener is installed in series with the PT's center tap.

[baddog]

I will try bypassing it to see what the voltages are and see how it sounds. If its sounds nice enough maybe i can conn my friend into buying new tubes . JJ's or something that can handle the voltage. He doesnt want to spend to much more money on the amp. It was saposed to be a project for him to mess around with that got out of control.

Thanks again for the help.
BD

[Enzo]

Maybe a couple grid stoppers...

[amp boy]

might help to send some word........

Email Antonio.
He has a Filmosound-ish project completed.
http://www.teixeiraamps.com
there ya go.
: )

[Leif H]

Hi Dave

The amp was recaped when i got the amp for repair. I even changed some of the caps because he had sprague caps that where physicaly to big to fit in the chassis comfortably.

Thanks
Bd

baddog,

I've a few quwestions if you could...

What size or kind of caps did you go with? I'm getting to that point in my quest to remove hiss from my amp, and I'm open to suggestions.

I find the tone control kind of drastic when it finally does work, but I think that maybe new caps could help, if not, I'd like to begin looking at a beter tone stack.

Also, for the cap on the FBL did you use a .001 uF 350V?

Did you figure out how to switch in the 5879?

[baddog]

I No longer have the amp and dont remember what i used . Soory