Fender bassman 135 biasing

[cheveatter]

I am amateur at this. I have a bassman 135 that i have replaced the screen resistors and sockets and electrolytic caps. Put new tubes in and installed 1 ohm resistors on pin 8.
I have readings on grid pin 5- -57.9v plate pin3-502v cathode pin8- 20mv and i multiply pin 3 and pin 8 and come up with 10w static dissipation.
My question is that except able or should i try to get a higher reading on pin 8. i will be using the amp for guitar. It doesn't sound bad, but i would like to have things correct.

[xtian]

Here is a nice bias calculator AX84.com - The Cooperative Tube Guitar Amp Project

For 6L6 at 500v plates, the 70% target is ~26mA per tube. So if you are seeing 20mA (20mV across your 1R bias resistors) you're running a bit cool. But so long as it's not too hot, let your ears be the judge.

[Pedro Vecino]

Xtian: 70% of 30 watts is 21 watts. With 500V, 42mA. An excessively high measure for this amplifier.
In this model, with 28 mA sounds perfect. No need to exceed 50% dissipation.

[g1]

Depends what you consider "correct", which seems to be a matter of opinion and taste.
From my experience, the 50% that Pedro mentioned is probably around the way they were biased when they came out of the factory.
Longest tube life will be from cooler bias. So if you don't think it sounds bad, you should be good.
I'm guessing that the amp doesn't have a bias adjustment anyway, just a balance/matching pot for minimizing hum.

[mac dillard]

Depends what you consider "correct", which seems to be a matter of opinion and taste.
From my experience, the 50% that Pedro mentioned is probably around the way they were biased when they came out of the factory.
Longest tube life will be from cooler bias. So if you don't think it sounds bad, you should be good.
I'm guessing that the amp doesn't have a bias adjustment anyway, just a balance/matching pot for minimizing hum.

The schematic shows a balance pot and -58vdc at the bias circuit. I would think he is right on factory spec.

[pdf64]

To give modern 6L6 an easier life, I suggest that 1k screen grid resistors are used.
I usually find that modern tubes run rather hot in these SF Fender non-adjustable fixed bias models.
The FWB rectifier diodes can get popped occasionally; best to replace with a 3A type.

[cheveatter]

Thanks guys for answering my post. Yes it just has a balance pot. I was trying to decide whether to add a bias pot to the amp. But as a newbie at this was not for sure how to go about it. I don't find to much information on this amp on the net. All your opinions and advice is very appreciated.

[pdf64]

Here is a nice bias calculator AX84.com - The Cooperative Tube Guitar Amp Project

For 6L6 at 500v plates, the 70% target is ~26mA per tube...

I think you should have selected 6L6GC on the calculator

I was trying to decide whether to add a bias pot to the amp

That would be beneficial in the long run.
To do that, the bias supply voltage should be increased a bit; reduce the 1k2 to 100 ohms, if that doesn't increase it much then try reducing the 2k2 to eg 1k5.
To get level adjustment, replace the 33k (between the bias balance control terminal and chassis 0V) with a 20k trimmer wired as a variable resistor in series with a 22k resistor.

http://bmamps.com/Schematics/fender/..._135_schem.pdf

[Pedro Vecino]

I always measure currents through the anodes. Those 28mA with 500V would represent approximately 30mA. through the cathodes. With a good matched quartet in my opinion not need more, although a 12AT7-6201 GE in the driver always comes in handy to give a good sound.

You can replace the 33K resistor connected to the balance potentiometer by a 25K adjustable resistor and 15K/18K fixed resistor in series.



EDIT: We have posted at a time.

[cheveatter]

Thanks guys!! That looks simple enough, I will get parts ordered and finish this project.

[pdf64]

I always measure currents through the anodes...

Just to note that this method may not be universally recommended, due to the hazard presented by current measurement in HT circuits, and the potential for stray parasitic coupling between the test probes/leads/meter (when connected to the power tube plate circuit) and earlier stages of the amp, which can result in instability.
A competent tech should be aware of, and able to mitigate for, both these issues, whereas someone less experienced may put themselves/their test gear at risk whilst taking a corrupted reading.
Taking readings from current sense resistors in the cathode circuit may be seen to be rather less problematic, and so more suitable to the occasional visitor to amp innerds, who perhaps may be a non-technical, practically minded muso.

[Pedro Vecino]

Yes. You are right. In fact never recommend it. I said only to differentiate measure through the anode and through the cathode with one element added: the resistor tolerance. With 6L6 / EL34 / 6550 ... and wires 15" long direct to an ammeter I have never observed problems. However, with EL84 and certain amps (a pair, no more) I have seen instabilities in one side. Blues Junior is one of them. Thanks for the advice.