It is currently Mon Mar 16, 2020 4:54 pm

All times are UTC - 7 hours



Post new topic Reply to topic  [ 72 posts ]  Go to page Previous  1, 2, 3, 4, 5  Next
Go to page Previous  1, 2, 3, 4, 5  Next
Author Message
Post subject: Re: Bassman 100 bias problem ?
Posted: Wed Aug 14, 2013 7:01 am
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
The 470-ohm ones (screen grid resistors) got toasted. Typical, even when only ONE 6L6GC goes ballistic. One bad output tube can draw enough current to effect all the other 6L6GC. And the resistors attached to them. That's why you need to replace all the resistors on the sockets.


" soldering those resistors to the sockets is going be annoyying as its very crowded there and im somewhat very clumsy "


Yeah, I hear you. Part of the problem is the sockets are right next to the back of the chassis. I sometimes unsolder the other leads to the socket and pull them from the chassis. So much easier to wire up those resistors with the sockets out of the chassis. Just remember to note the original wiring positions. Photos really help.

Good luck with that!


Top
Profile
Fender Play Winter Sale 2020
Post subject: Re: Bassman 100 bias problem ?
Posted: Wed Aug 14, 2013 7:39 am
Offline
Hobbyist
Hobbyist

Joined: Tue Aug 13, 2013 12:26 pm
Posts: 29
i've changed all 8 resistors successfully, but unfortunately the readings are the same.

im not sure what to do now, bias pot fully counter clockwise: first tube from the left if you're looking from the back reads 49ma, 2nd tube 0ma, 3rd and 4rth read 6ma. they all have 500 volts across pin #5, could the tubes be bad ? :(


Top
Profile
Post subject: Re: Bassman 100 bias problem ?
Posted: Wed Aug 14, 2013 7:56 am
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
Subway_savage,

Pull the tubes. Have amp with load (speaker connected), no input. Check the BIAS SUPPLY voltage just after the 3.3k-ohm resistor in the circuit (See schemo). Shoud be around -52 VDC.

If good... dial bias pot to about half way point. Read the voltage at the junction of the two 1500-ohm grid stopper resistors. Should be in the mid -40VDC. If not, check the components in the BIAS SUPPLY for out-of-tolerance items.

Good luck! Keep us posted.


http://www.webphix.com/schematic%20heav ... _schem.pdf


Top
Profile
Post subject: Re: Bassman 100 bias problem ?
Posted: Wed Aug 14, 2013 8:22 am
Offline
Hobbyist
Hobbyist

Joined: Tue Aug 13, 2013 12:26 pm
Posts: 29
ok i cheked the 3.3k resistor it reads -67v (the end near the balance pot).

i turned the put half way and checked the junction of the two 1500 resistors and it reads 0 !

i sense something stupid is wrong, waiting for more Trouble shooting advice!

thanks


Last edited by Subway_savage on Wed Aug 14, 2013 8:44 am, edited 1 time in total.

Top
Profile
Post subject: Re: Bassman 100 bias problem ?
Posted: Wed Aug 14, 2013 8:31 am
Offline
Rock Icon
Rock Icon
User avatar

Joined: Wed Mar 31, 2010 12:48 am
Posts: 26417
Location: Tombstone Territory
Just the power tubes.

And remember, your amp has a "bias balance" pot -- not as with the traditional Fender models where adjusting the pot affects the bias-supply voltage to all tubes equally. Some variation between V4 and V7 is to be expected.

Arjay

_________________
"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


Top
Profile
Post subject: Re: Bassman 100 bias problem ?
Posted: Wed Aug 14, 2013 8:57 am
Offline
Hobbyist
Hobbyist

Joined: Tue Aug 13, 2013 12:26 pm
Posts: 29
ok i cheked the 3.3k resistor it reads -67v (the end near the balance pot).

i turned the put half way and checked the junction of the two 1500 resistors and it reads 0 !

i sense something stupid is wrong, waiting for more Trouble shooting advice!

i checked the 1500 resistors and they read fine, there only seems to be 2v entering those resistors from pins #3 of the two pre amp tubes.


Top
Profile
Post subject: Re: Bassman 100 bias problem ?
Posted: Wed Aug 14, 2013 9:46 am
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
Subway_savage,

Arjay is correct. You amp has a BALANCE POT and not a BIAS POT. So, turn the pot to mid-point. Insert the 6L6GCs. Read the idle current of each tube. As you turn the BALANCE POT you will notice one pair of tubes going up in mA, while the other set of push-pull pairs will go down. You want both set of tubes to balance out to the same bias point. V4 & V7 are parallel as one set of tubes. And V5 & V6 the other. In other words, the pot will affect V4 and V7 in one bias direction and V5 and V6 in the opposite direction.

See if this is true. If not --- there is something wrong with your BIAS POWER supply. Perhaps one or both 68k-ohm resistor that connect to the 1500-ohm resistors got toasted, too.

Good luck! Keep us informed.


http://www.webphix.com/schematic%20heav ... _schem.pdf


Top
Profile
Post subject: Re: Bassman 100 bias problem ?
Posted: Wed Aug 14, 2013 3:01 pm
Offline
Hobbyist
Hobbyist

Joined: Tue Aug 13, 2013 12:26 pm
Posts: 29
so the rattle noise went away after changing the resistors, but i still have that one socket without any chathode current, i swapped the tubes around but still same socket 0ma, the rest of the tubes react ok with the balance pot that all three averaged out at 14-15ma (is this cold ? or what). so now its somewhere from the socket to the board. im guessing there's something wrong with the socket, since all the connections are in parrallel so if there's something bad in the components it would affect both tubes right ? ill try to take it off and clean it or mess around with it and see.


Top
Profile
Post subject: Re: Bassman 100 bias problem ?
Posted: Wed Aug 14, 2013 3:17 pm
Offline
Rock Icon
Rock Icon
User avatar

Joined: Wed Mar 31, 2010 12:48 am
Posts: 26417
Location: Tombstone Territory
Possible wonky socket with an amp that age. Inspect the insulator carefully for cracks or missing ceramic chunks and check the continuity of all wires feeding that socket.

I have a Bassman 135 (similar in architecture to your Bassman 100) and my Tung-Sol STR's are biased significantly hotter than the 15mA you're seeing. Dittos with my '67 Showman top running a matched quartet of Sylvania JAN's.

Arjay

_________________
"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


Top
Profile
Post subject: Re: Bassman 100 bias problem ?
Posted: Wed Aug 14, 2013 3:30 pm
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
O0k.. now go back and inspect and test every component in the BIAS SUPPLY (done statically, no power). I would simply relace the 80mfd/75VDC cap (I like going to 100mfd/100VDC). And the diode. Use a 1N4007 or UF4007. Watch the polarity!

Also replace the 470-ohm resistor just before the diode.

That done, now check the tightness of each socket. You may need to re-tighten them. I use a dental pick -- BE SURE TO DISCHARGE MAIN CAPS, FIRST. The tubes must be snug, not wobbling around. Apply De-Oxit GN5 to the tube pins and sockets. Now, try measuring the idle bias again.

If you decide to change the sockets, I would suggest all of them. This will save you future socket headaches. Put the grid stopper and screen resistors on first. Then, attach the new socket and other wiring. Use good sockets. Liek Amphenol or Beltron. Be sure to measure the diameter of the hole and distance between the screw holes. Fender used both 1-1/16 and 1-inch sockets during this era.

Good luck! Keep us posted. :mrgreen:


Top
Profile
Post subject: Re: Bassman 100 bias problem ?
Posted: Wed Aug 14, 2013 4:34 pm
Offline
Hobbyist
Hobbyist

Joined: Tue Aug 13, 2013 12:26 pm
Posts: 29
i cleaned the socket pins, re-tightened them, the tubes are sitting flush and very snug in there. i also checked the continuity from the top of the socket and all pins show same continuity across all sockets. they also look very good. the 75 vdc caps are already replaced (replaced all electrolytic caps with F&T caps).

there's one thing i noticed i'm not sure if it matters. the 4 1-ohm resistors installed, 3 of them read 2-ohms+/- in the circuit except for the suspected socket where it reads 1-ohm. does that mean anything ? i checked the continuity on that pin and it gives me a reading so its def connected and grounded.


Top
Profile
Post subject: Re: Bassman 100 bias problem ?
Posted: Wed Aug 14, 2013 4:51 pm
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
Subway_savage,

" 1-ohm resistors installed, 3 of them read 2-ohms+/- in the circuit except for the suspected socket where it reads 1-ohm. "

There is something amiss. If you read across a 1-ohm resistor, then the mV read = the mA at idle. With a 2-ohm resistor, to get the same mA of current you'd need to read twice the mV. Pull a leg off the resistors and re-read the values. If they all go back to 1-ohm, then where they are soldered to the chassis there is an issue.

Are you using a terminal strip to attach the resistors to the chassis? It's about the only way to assure all resistors have equal contact. Even the slightest problem here makes a world of difference to your readings. Think of it, a one-ohm extra resistance in the ground point is all you need to get your results.

It's critical that these resistor read within a 5% tolerance if you expect to get a 5% tolerance on the bias. That's why I use 1% heat-resistance resistors.

Get back to us, with results. Good luck!

EDIT: This is why many ppl use 10-ohm resistors from cathode-to-ground. For each 10mV read = 1mA of current. But, more important, small differences in contact don't have as much effect (1/10th the effect) as 1-ohm resistor. IMPORTANT: you need to go up on the wattage rating of the resistor, as a 10-ohm resistor will dissipate 10X more heat compared to a one-ohm resistor. A 5 watt Mills wire-wound 1% precision resistor is good for this function.

Image


Top
Profile
Post subject: Re: Bassman 100 bias problem ?
Posted: Wed Aug 14, 2013 5:28 pm
Offline
Hobbyist
Hobbyist

Joined: Tue Aug 13, 2013 12:26 pm
Posts: 29
i pulled em out, they became 2 ohms all 4 of them, looks like they cant handle the heat. ill def get some chunky ones after i wake up, also look for a terminal strip. those damn ground terminals will never stick properly i might just have to weld them together ! :x :x

i don't know about the 0ma socket though, will see if it will clear up with a good solid connection.

thanks alot !


Top
Profile
Post subject: Re: Bassman 100 bias problem ?
Posted: Wed Aug 14, 2013 6:21 pm
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
Clean the beejebers of the strip and the point where you are going to attach it to the chassis with a wire brush (I like brass brush) & then rubbing alcohol. Dry.

Then, bolt the terminal strip to the chassis. Use a lock washer & washer to assure good contact.

Like the ground point in the middle of this unit.

Image


Top
Profile
Post subject: Re: Bassman 100 bias problem ?
Posted: Wed Aug 14, 2013 6:46 pm
Offline
Rock Icon
Rock Icon
User avatar

Joined: Wed Mar 31, 2010 12:48 am
Posts: 26417
Location: Tombstone Territory
+1000, Steve.

On all salient points.

8)

(you're getting a "rawk on" emoticon for saving me from typing a whole pisspot fulla procedural checks)

:lol:

Arjay

_________________
"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


Top
Profile
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 72 posts ]  Go to page Previous  1, 2, 3, 4, 5  Next
Go to page Previous  1, 2, 3, 4, 5  Next

All times are UTC - 7 hours

Fender Play Winter Sale 2020

Who is online

Users browsing this forum: No registered users and 2 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum

Search for:
Jump to: