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Post subject: Phase inverter resistors in Bandmaster tfl 5005D
Posted: Sat Oct 17, 2015 9:46 pm
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the resistors coming off the bias balance pot are 100k for left .1 uf cap and 68K for the right .1 uf cap. the schematic says bothe should be 100k. They look original. Did Fender ever use a 68K on the variable tube?


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Post subject: Re: Phase inverter resistors in Bandmaster tfl 5005D
Posted: Sat Oct 17, 2015 10:50 pm
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Reviewing the schematics for the AA768, AA1069, and AA270 circuit revisions, all three drawings show 100kΩ resistors for each leg.

That said, I have seen wiring variations for this amp that looked to be factory original work. It's possible that yours is one of those.

Arjay

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Post subject: Re: Phase inverter resistors in Bandmaster tfl 5005D
Posted: Sun Oct 18, 2015 4:30 am
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Both legs of the BALANCE pot should be 100K-ohms.

However, I've seen the PHASE INVERTER plates have asymmetric 68/100k-ohm load resistors.


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Post subject: Re: Phase inverter resistors in Bandmaster tfl 5005D
Posted: Sun Oct 18, 2015 8:23 am
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The resistors are original. They have the wax on them. The filter caps have the 10K and 2.2K 1 watt resistors like a Dual Showman. So it wouldn't surprise me that it's a variant. The amp is biased cold, 33 ma for most average rated tubes. The filter caps are old, not original. Could that be a cause? My Silverface Super and Twin bias around 38ma. They both have fresh filter caps. Thanks gentlemen.


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Post subject: Re: Phase inverter resistors in Bandmaster tfl 5005D
Posted: Sun Oct 18, 2015 9:15 am
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Fresh electrolytics in the filter array and bias-supply circuit may perk things up. You might also check your rectifier tube's performance. Replace that if needed as well.

Arjay

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Post subject: Re: Phase inverter resistors in Bandmaster tfl 5005D
Posted: Sun Oct 18, 2015 1:44 pm
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Krankster wrote:
The resistors are original.

They have the wax on them.

I never saw wax on resistor

The filter caps have the 10K and 2.2K 1 watt resistors like a Dual Showman. So it wouldn't surprise me that it's a variant. The amp is biased cold, 33 ma for most average rated tubes. The filter caps are old, not original. Could that be a cause?
No

My Silverface Super and Twin bias around 38ma. They both have fresh filter caps.
it is not related

Thanks gentlemen.


Bad filter = hum at speaker or / and you may loose few volts. Nothing to worry about .
New filter caps may give more punch to your amp and amp could be more reliable if you gig with


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Post subject: Re: Phase inverter resistors in Bandmaster tfl 5005D
Posted: Sun Oct 18, 2015 7:21 pm
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Most preamp boards in the 70's were dippied in wax probably. Off topic, do you guys form your new filter caps with a variac of some homemade device.


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Post subject: Re: Phase inverter resistors in Bandmaster tfl 5005D
Posted: Mon Oct 19, 2015 2:38 am
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Krankster wrote:

Off topic, do you guys form your new filter caps with a variac of some homemade device

sometimes I do sometimes no


.


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Post subject: Re: Phase inverter resistors in Bandmaster tfl 5005D
Posted: Tue Oct 20, 2015 9:14 am
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As a by the hour tech, I don't have time to mess with forming new caps. Old caps may benefit from forming, but at that point, I recommend replacement.
Vishay capacitors has this to say
"STORAGE LIFE
During transport or storage, the temperature of electrolytic
capacitors is allowed to fall below their lower category
temperature and reach a minimum of -65 °C, while their upper category temperature may not be exceeded.
Depending on the design and the purity of the materials
used, electrolytic capacitors offer very good storage
properties. They can be stored in dry rooms at temperature
ranging from -40 °C to +40 °C
(preferably between 0 °C and
+25 °C) for up to three years without any restriction. Within
that period it is possible to apply the fully-rated voltage to the capacitors without any further preparation.
This procedure neither impairs the capacitor’s operational reliability nor its life expectancy.
All electrolytic capacitors have a leakage current when a
direct current is applied. This leakage current depends on
time, voltage, and temperature.
After long dead storage this leakage current will increase and, for a short time, can be
10 times greater at the time of reuse.
The capacitor will not be damaged and its life expectancy will not be impaired if the rated voltage is applied directly after long storage.
In general, the expected continuous operating leakage current
will be re-attained or fall below its value after about
30 minutes. Any operation below the rated voltage will result
in a significantly lower leakage current.


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Post subject: Re: Phase inverter resistors in Bandmaster tfl 5005D
Posted: Tue Oct 20, 2015 11:50 am
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Ramping up new electrolytic caps will prolly NOT increase the life of the caps. If anything, using a Variac or current-limiting light bulb setup will help spot miss-wired shorts, before too much current passes and more damage is done. A good thing to do on first fire-up.

Ramping up new lytic caps prolly won't hurt. Remember, if the amp has a slow-start, indirect heated rectifier (like a GZ34), you should replace it with a direct heated one (like a 5U4GB). This allow the lower voltage to pass into the caps. A GZ34 won't start conducting much current into the circuit, until around 100VAC, IME.

I really like to replace the resistors in the power rail with appropriate metal oxide or wire-wound types. Better tolerance to heat and lower noise. Carbon comps have no real advantage in the PSU.

Now, back to that bias circuit. I've redone quite a few 6L6GC silverface era amps. I've never seen one with a 68k-ohm on one leg and a 100k-ohm on the other leg of the balance pot-to-grid stopper. On the surface, this would not make sense. Esp if you are using "matched" output tubes. As one 6L6GC tube (or pair of tubes, in a TR or Showman) would get less bias voltage than the other tube. For these resistors, Fender liked to use 5% tolerance types to keep the two legs at similar voltage range. One of the few places 5% tolerance type were used.

You sure you aren't looking at the resistors that feed the anode plates of the 12AT7 phase inverter? These are the two resistors on the board, that form one of the V-shapes --- very close to where the resistors of the bias supply are located. The two plate resistors are 47k-ohm in the diagram below. I've seen amps with 68K on one leg and 100k-ohm on the other. Amps with BIAS pots have two 220k-ohm, 5% tolerance resistors in a V-shape layout --- where one 100k-ohm BALANCE resistor now lays on your amp. The other 100k-ohm resistor is usually on A SEPARATE terminal strip next to the main circuit board.

Plate resistors have red dot next to them. The bias resistor has purple dot:

Image


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Post subject: Re: Phase inverter resistors in Bandmaster tfl 5005D
Posted: Thu Oct 22, 2015 11:08 am
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BMW2002Ti wrote:
I really like to replace the resistors in the power rail with appropriate metal oxide or wire-wound types. Better tolerance to heat and lower noise. Carbon comps have no real advantage in the PSU.


+1

Dittos with the 470Ω screen-grid resistors on the power-tube sockets.

Arjay

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