why put 150R on cathodes a Bandmaster Reverb

[PRNDL]

I'm fixing a Bandmaster Reverb that looks pretty much in original condition except for a few minor changes:
- Coax for the 2nd channel input to the grid
- Orange drops in a few places (reverb mostly)
- Two 150-ohm ceramic resistors at the cathodes of the 6L6's

The initial problem was a loss in volume, which turned out to be a bad 80µ cap in the power supply. I've ordered a new set of Sprague Atoms, which will arrive Monday.

My plan is to remove those 150 ohm resistors, as well as the 2000pF snubber caps across the 6L6 grids (I may leave them in place just in case the amp oscillates). I may also snip out the death cap while I'm at it.

Does anyone know if those 150 ohm cathode resistors were some kind of mod for improved tone?
Is it a way to combine fixed and cathode bias?

Are there any easy mods to improve the tone of this amp?
I really like it ... a pair of 6L6's with reverb, sort of like a Vibroverb in a head.
The speaker cabinet is almost as big as a refrigerator though!

Thanks!

[acorkos]

if those resistors are between the cathode (pin 8) and ground, they are probably 1 ohm (brown-black-gold) and not 150 ohm (brown-green-brown) and are used to measure bias voltage/current and have no effect on tone. i would leave them.

a lot of people like the way the aa1069 bandmaster reverb sounds, and it will probably be best for its resale/investment value if left stock with as little changed as possible. but if you want to experiment, consider making changes in your amp to bring it more like an ab763 vibrolux reverb....http://www.schematicheaven.com/fende...b763_schem.pdf
especially in the phase inverter and bias adjust circuitry

[Bruce / Mission Amps]

if those resistors are between the cathode (pin 8) and ground, they are probably 1 ohm (brown-black-gold) and not 150 ohm (brown-green-brown) and are used to measure bias voltage/current and have no effect on tone. i would leave them.

a lot of people like the way the aa1069 bandmaster reverb sounds, and it will probably be best for its resale/investment value if left stock with as little changed as possible. but if you want to experiment, consider making changes in your amp to bring it more like an ab763 vibrolux reverb....http://www.schematicheaven.com/fende...b763_schem.pdf
especially in the phase inverter and bias adjust circuitry

Nope, the 150 ohm power resistors are real.
That was some kind of weird Fender design thing where they were trying to get fixed bias mixed with a little cathode biasing going.
and lots of tweakers yank them out, set up the negative voltage bias supply and go full bore fixed bias.

[PRNDL]

They are square white ceramic resistors with 150 ohms typed clearly.

It also has a negative bias circuit that was set at -22 volts (if I remember correctly). The bias was something like 4 mA each tube, but that would change when the PS cap is replaced and the B+ rises from 250 to 400 or so.

[Rob Mercure]

Used to be the "wisdom' was to pull out those resistors and go to usual Fender fixed bias with "all associated with CBS was bad." But in the past few years I've had two customers that got me to reinstall the "half-n-half" after I "blackfaced" the amps (always remove vintage parts carefully, put them in a ziplock bag, identify them, and tape them inside the chassis - you never know who might cuss you in the future). The interesting thing in both cases is that the musician was a bass player using the head to push a tall bass cabinet (don't remember the cabinets specifically but do remember that they were dissimilar).

Funny world.

Rob

[Regis]

My 68 Pro drip edge had that shit in it, it was some sort of combination Cathode/Fixed bias. I pulled it all out along with the snubber caps and did a BF bias setup.

[Jag]

The most important thing to remember is to mod the bias supply to provide the appropriate negative voltage. Left stock, that bias supply probably won't go negative enough to bias the tubes properly. That's because, as Bruce said, the 150's provide some of the bias offset.

[stokes]

It was,as Bruce pointed out,a combination fixed/cathode bias thing Fender did on a few amps.I first saw it on an early SF Bassman,it was one of the "tansitional" SF's it had a lot of the BF circuitry with the SF style cab and grill cloth.A lot of people dismiss it out of hand because it is CBS related,but it can sound pretty good if you tweak the cathode resistor.I added it to a couple of amps I built and it has some good qualities.At full bore it takes on more of the cathode bias singing quality,but even clean it adds a little texture.

[PRNDL]

I'm glad I asked!

I'm going to leave it as is since it isn't my amp. Also, the amp is in fantastic shape and doesn't appear to have been altered at all.

I figure some new tubes with the electrolytics and it will sound great.

[Rob Mercure]

PRNDL,

While I did reinstall the resistors for my customers I didn't replace the "tone killer" caps and I've probably at least disconnect one end and see how the amp sounds if not just remove 'em.

Rob

[PRNDL]

Thanks for all the great suggestions.

After replacing the electrolytics and removing a bad preamp tube, the amp started up fine with the tubes biased around 30 mA each, which is a tad cool, but nice.

I'm leaving the 250 ohm cathode resistors in place, since removing them would require a complete rework of the bias circuit. Besides, it biased itself so easily and a quick turn of the pot made equal current in both tubes. I can see why they did this.

I suppose a real simple mod would be to put a trim pot in line with the 1.8K resistor on the bias board.

Tomorrow I'm going to try it out with a good speaker (an old EV SRO 12") and see what effect removing the tone caps has.

[Sweetfinger]

I've seen a few of these over the years and was just barely smart enough to actually LISTEN to those amps instead of listening to the "gurus" who advocated tearing out the combo bias just on principle. I leave the resistors in.

[PRNDL]

Thanks Sweet ... the amp sounds fantastic!

The reverb is weak. At ten you can barely hear it, but it does seem to be there. I'll have to check the tube voltages and resistor values.

I'm wondering whether to work on it, since the owner didn't want to spend much or anything on the amp. He sold it at a very low price and may be shocked when he hears it. The buyer is going to be very pleased!

--------

Well the reverb problem turned out to be the wires were swapped ... both connectors had red paint, with one having more red paint than the other.