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Post subject: Bassman 100 bias problem ?
Posted: Tue Aug 13, 2013 12:34 pm
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Hi there,

I had a bassman 100 head laying in the store for so many years and i decided to restore it, i changed the electrolytic caps and filter caps and got 4 matched 6L6 tubes, when i checked the cathode current through the 1 ohm resistor the first tube (leftmost) reads 35ma which is fine, the 2nd tube reads 0ma, the 3rd and 4rth read around 8 to 14 max. i have no idea whats the problem, i measured the voltage accross pin 3 and they all give around 500v.

what could be the problem ?

thanks !


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Post subject: Re: Bassman 100 bias problem ?
Posted: Tue Aug 13, 2013 1:05 pm
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Subway_savage, Welcome to the Forum. :D

Each tube has a 1% tolerance 1-ohm resistor from the cathode-to-ground (pin 8 to ground)? No shared resistors, right?

I usually select a match set of 1% tolerance-1 watt resistors for this function. It's really important that the resistor reads 1-ohm. But your readings are further than 5% off. I'd double check the solder points. Esp the one onto the chassis. It maybe better to tie a terminal strip to the chassis and ground it well with a hefty soldering iron (like 100watt one). Then tie each resistor lead to this terminal.


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Post subject: Re: Bassman 100 bias problem ?
Posted: Tue Aug 13, 2013 1:51 pm
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BMW2002Ti wrote:
Each tube has a 1% tolerance 1-ohm resistor from the cathode-to-ground (pin 8 to ground)? No shared resistors, right?

I usually select a match set of 1% tolerance-1 watt resistors for this function. It's really important that the resistor reads 1-ohm. But your readings are further than 5% off. I'd double check the solder points. Esp the one onto the chassis. It maybe better to tie a terminal strip to the chassis and ground it well with a hefty soldering iron (like 100watt one). Then tie each resistor lead to this terminal.


+1

As well, check the 470Ω/1W screen resistors at each power-tube socket. These bear the brunt of thermal abuse over the years and require periodic replacement.

Arjay

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Post subject: Re: Bassman 100 bias problem ?
Posted: Tue Aug 13, 2013 2:33 pm
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yes 1 ohm resistor for each tube, from pin 8 to ground. the ground connections are not the best but its def grounded, that heavily oxidised ground terminal doesn't like my 50w iron or my solder.

i found one 470ohm resistor on the first tube split in half, not sure when that happened ! lol

the other ones seem ok, would this fix the problem or could it cause some other parts to over stress/fail ?

i also has alot of noise coming from the speakers.


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Post subject: Re: Bassman 100 bias problem ?
Posted: Tue Aug 13, 2013 2:58 pm
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Subway_savage wrote:
i found one 470ohm resistor on the first tube split in half, not sure when that happened !


Thirty or forty years of baking at 300º tends to do that.

Replace all four -- recommend metal-film types for their resistance to thermal exposure. Do not over-spec them, the 1-watt rating is intended to protect the amp from a runaway tube.

Arjay

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Post subject: Re: Bassman 100 bias problem ?
Posted: Tue Aug 13, 2013 3:18 pm
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Arjay,

Actually you want to have over-spec'ed screen resistors, because when they pop you lose the screen's function to control internal oscillations between the tube's grids. By limiting electron backflow in the tube.

Plus, a larger wattage resistor on the screen will help control excess transient current flow within the tube. Kinda like a current soak (converting this excess into heat)-- just when the tube starts to go out of spec, before going ballistic.

I like Mills 5 watt, wire-wound or good tolerance, heat resistant resistors like metal oxides. Wire-wound (like the sandbox type on Jerry's Super Reverb) have the advantage of superior performance under heat and inherent low inductance.

As for the bad noise from the amp-- I used to get that too. Then I took guitar lessons. :lol:

Ok.. sorry the Devil made me do that.

Seriously, you need to go through the entire output stage. Replace both screen and the 1.5k-ohm grid stoppers. The grid stoppers can be 1-watt rated metal oxide. No problem here, as the grid should not be seeing more than a few mA. Not the 20-30mA a screen grid sees.

Be sure that both screen and grid stopper resistors have the shortest length possible (that's why Fender puts them onto the sockets -- fitting between appropriate grids and empty pins). Any added inductance and/or capacitance due to the lead lengths is enough to actually induce instabilities.


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Post subject: Re: Bassman 100 bias problem ?
Posted: Tue Aug 13, 2013 3:57 pm
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BMW2002Ti wrote:
Arjay,

Actually you want to have over-spec'ed screen resistors, because when they pop you lose the screen's function to control internal oscillations between the tube's grids. By limiting electron backflow in the tube.


I most certainly do NOT.

If/when they "pop" (as you infer) there aren't going to be any oscillations because that tube will likely cease functioning and the amp's tone and volume will immediately suffer.

Over-spec if that is your wont but IMO there are few amp tech's who have more insight into Leo's tube creations than Leo himself. Everything he did was based on solid engineering principles and countless trial-and-error testing, both on the bench and in the field. Woe be to anyone who assays to second-guess the master.

Arjay

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Post subject: Re: Bassman 100 bias problem ?
Posted: Tue Aug 13, 2013 4:16 pm
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Arjay,

I think you maybe mistaking the grid stopper with the screen grid. The signal will stop if the grid stopper pops. And since most instabilites are induced by the signal, no signal usually controls the oscillations.

But, the tube will still work with no voltage on the screen grid. In essence you convert the tube from a Pentode to a pseudo-Triode with no voltage on the screen grid.

What some companies do is put a 10-ohm or 20-ohm resistor (lower the resistance, the less effect on tone) inline with the anode plates. If this resistor pops --- the tube stops working. Or a resistor between the cathode and ground will do the same thing.

Looked for a good reference, think Aiken had one. Valve Wizard mentions it near the bottom of the link.


http://www.valvewizard.co.uk/gridstopper.html


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Post subject: Re: Bassman 100 bias problem ?
Posted: Tue Aug 13, 2013 4:38 pm
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Trust me, I know the difference between a control grid and a screen grid. And you are correct that if the 1.5KΩ control grid resistor fails the input signal is removed. But if the screen grid resistor fails the tube may still conduct, but without the safeguards that this resistor provides.

Result?

Runaway tube.

I rest my case.

Arjay

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Post subject: Re: Bassman 100 bias problem ?
Posted: Tue Aug 13, 2013 4:44 pm
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" But if the screen grid resistor fails the tube may still conduct, but without the safeguards that this resistor provides. "


Exactly! That why you don't want it to pop.

:D


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Post subject: Re: Bassman 100 bias problem ?
Posted: Tue Aug 13, 2013 6:14 pm
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BMW2002Ti wrote:
Exactly! That why you don't want it to pop.


......Except in the case of abnormally high plate voltage. I believe Leo calculated a 10% fail-safe point and his figures correlated to just over 1 watt across those screen-grid resistors. Hence, the 1-watt spec.

Arjay

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Post subject: Re: Bassman 100 bias problem ?
Posted: Tue Aug 13, 2013 8:47 pm
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Reference this old thread of mine:

viewtopic.php?f=13&t=74557

The screen grid of one 6L6GC shorted which took out the associated 1 watt screen grid resistor. The amp still played and actually sounded good. I replaced the one watt resistors with 2 watt metal oxide. No complaints, no problems.

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Post subject: Re: Bassman 100 bias problem ?
Posted: Wed Aug 14, 2013 12:11 am
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hi, ill get a bunch of resistors 4x 470 1w and 4x 1500 1w, ill stick to the original values they worked fine for decades, i also found a 100w gun soldering iron ill try to make a solid ground connection for the 1ohm resistors and ill check again.

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

cheers.


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Post subject: Re: Bassman 100 bias problem ?
Posted: Wed Aug 14, 2013 12:27 am
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The 1.5KΩ resistors need only be ½-watt rated. Both they and the 470KΩ resistors, 10% tolerance.

Arjay

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Post subject: Re: Bassman 100 bias problem ?
Posted: Wed Aug 14, 2013 5:48 am
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i removed the resistors, the 1.5k ones read fine, but the 470Ω ones are way too high !, they all read almost 800Ω ! :shock: i'm replacing them right now, once i'm done ill check the cathode current again and see if there's any difference and ill report back here.

thanks all for the help ! :)


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