Biasing a 59 Bassman Ltd

[analogopher]

Hi,

I'm not a builder, but i decided to bias my 59 Bassman Ltd myself. I want to use a multimeter to measure mV at the provided bias testing point. Fender recommends about 60mV but most people out there feel that's too cold. I would love to go as HOT as possible, SO, how far can i safely go in mV's?

thanx

[vibratoking]

I don't think there is a clear answer to your question. As you increase the bias current, you shorten the tube life. So the question becomes, how much tube life are you willing to trade for increased bias current? Not that anyone can tell you the exact answer to that question either. Many of us could throw an answer at you, but how will we know if it is hot enough for you? Be ready to buy some tubes, experiment, and buy some more tubes. Repeat the experiment until you find acceptable results for you.

Tell us what tubes you want to use and you may get some more satisfying answers. Are you using 6L6s?. 6L6WGC, 6L6GC, 6L6WGB/5881?

[rdjones]

Hi,

I'm not a builder, but i decided to bias my 59 Bassman Ltd myself. I want to use a multimeter to measure mV at the provided bias testing point. Fender recommends about 60mV but most people out there feel that's too cold. I would love to go as HOT as possible, SO, how far can i safely go in mV's?

thanx

The easy answer is you can go up to 100% rated dissipation or essentially Class A.
The Bassman is not designed as Class A circuit.
Running Class A in a pushpull amp is wasteful and inefficient, and may not gain you what you are hoping for as far as tone.

Most amps are biased on the cool side, so I'd listen closely as you make small adjustments upward to hear if you are going in the direction you are expecting.

Measure the current sensing resistor to verify it's value first (with the power off) then measure your plate voltage, and calculate where you are for % dissipation at idle before making any drastic changes.
Set the bias at 70% and judge the tone there before going any further.

rd

[roberto]

I also would like to point out that fender amps don't sound so good with bias on its max. Part of their sound is the sparkyness of a bias around 50-60%.

[Structo]

Yes, forget everything you have read or been told by friends.

Many times an amp will sound it's best well below the golden 70% fixed bias number.

Also you should know your power tube plate voltage since the bias current and voltage go hand in hand.

Your amp has two 6L6GC tubes, so the 60ma you measured is split between the two, so 30ma each.

Here is the bias formula.

(W * .7) / V = I

Here W is multiplied by .7 to give us 70% of the tube's maximum plate dissipation.

This number is divided by the actual B+ plate voltage in the amp to determine what idle cathode current to adjust the tubes for in AMPS.

An example:
In an amp with 480V B+ and using EL34 output tubes (25W max plate dis.).

(25W * .7) / 480V = .036A

17.5 / 480 = 36mA

That is for one tube.

I would sneek up on 70% and start at 50%, then 55%, then 60%, etc.

[M Fowler]

Yeah I was talking to Mike Soldano about a Jet City 100 that I felt was suffering from bad bias pot which I did replace and that stopped the red plating problem. But I wanted to go up to 30mA per tube see what Mike said to me below.

Hi Mark,

Happy New Year!

Are all four tubes going red plate? If so, it could be a bad bias trim pot OR the filter caps on the bias supply - you'll have to test to see. If only one pair are going red plate, then I suspect a bad tube in that pair. When one tube gas shorts, it pulls a bunch of grid current which then pulls down the bias voltage for that pair of tubes thus causing them to draw excess current and red plate.

By the way, 25ma per tube is fine. I prefer to set the bias as "cold" as possible - once you've got a good sine wave with no zero crossing, you are there. This gives the power amp its maximum headroom and output, as well as best tube life. There is no reason to run the tubes any hotter.

I hope this helps!

Michael Soldano
Jet City Amplification

[analogopher]

Thanx everyone!

I'm getting no more than 56mV reading at the test point, when the bias pot is maxed. (Yes, i know i should also be measuring plate current, but i don't have a bias probe and i don't trust myself yet to do the other method.)

I understand (i hope) that when measuring mV across the 1 ohm resistor at the bias point, i'm actually getting more or less the plate current of both tubes (they are matched) combined. SO...that would mean about 28mA, and i'm using stock GT6L6GE.

WHY CAN'T I GET A HIGHER READING??? Is this the way the amp comes out of the factory? A faulty bias pot? Weak tube? Bad screen resisotr (...and no, it's not the Multimeter battery).

Also, what is the green lead connected to the back pannel with a screw? A ground of sorts? Can i screw it out of the back pannel? It's most uncomfortable to work like that with the back pannel still connected.

HELPPPPP!!!!!!!!

thanx

opher

[martin manning]

The green lead attached to the back panel is there to ground it to the chassis so that it is effective as an electrostatic shield. You can temporarily disconnect it to get the panel out of the way.

All you really know at this point is that the adjustable bias circuit doesn't have enough range to get more than 56 mV across the current sensing resistor with that particular set of tubes installed. If the resistor is within tolerance, then you can say that you can't get more than 56mA total current (plate + screen) for both tubes together.

56mA sounds low considering that the expected plate voltage is around 450, but you don't know what the plate dissipation is (and this is what really matters) until you know the plate voltage, so measure that.

Since Fender didn't want to spend the extra pennies to include individual current sensing resistors in the amp, you don't know how that 56mA is split between the two tubes (i.e. how well the tubes are matched). You can determine that by measuring the current (or better yet measuring resistance and voltage to calculate current) flowing through each half of the output transformer.

If you determine that this pair of tubes is well matched, and the available bias voltage range is just not big enough to get them into the desired plate dissipation range, you can likely extend the range by changing a resistor in the bias circuit.

Anybody have a schematic specific to the Bassman Ltd.?

[analogopher]

thanx Martin and all the others! Great help. I'll keep on trying.

[martin manning]

Is this the right document for the amp?

Is the plate voltage somewhere around 490v?

No, that's what I meant by a schematic specific to the Ltd. Note there is no bias trimmer or current sense resistor in the one you posted.

[Structo]

OK, I deleted it.

[pdf64]

the easy answer is you can go up to 100% rated dissipation or essentially Class A.

That's contrary to my understanding on the following counts.
1/ An amp operating in class A will have max plate dissipation under static conditions, as signal is applied and increased the plate dissipation will reduce. Whereas the plate dissipation of a class AB amp is likely to increase over its static level at some signal levels, maybe reaching a peak midway on the power output. Therefore, if a class AB amp is biased to 100%, the average plate dissipation may end up over 100% under some modes of use. Hence the 'don't bias above 70%' guidance http://www.aikenamps.com/Why70percent.html
2/ Biasing such an amp to 100% plate dissipation does not equate to class A operation; that combination of tube type, plate voltage and load is unlikely to facilitate class A. Plate current and dissipation are likely to increase over the static levels under some signal conditions; the tubes are likely to enter cut off before the amp hits clipping.
By the same token, a different combination may operate in class A at a static plate dissipation of less than 100%.

Sorry if I've got muddled up on this.
Pete

[rdjones]

the easy answer is you can go up to 100% rated dissipation or essentially Class A.

That's contrary to my understanding on the following counts.
1/ An amp operating in class A will have max plate dissipation under static conditions, as signal is applied and increased the plate dissipation will reduce. Whereas the plate dissipation of a class AB amp is likely to increase over its static level at some signal levels, maybe reaching a peak midway on the power output. Therefore, if a class AB amp is biased to 100%, the average plate dissipation may end up over 100% under some modes of use. Hence the 'don't bias above 70%' guidance http://www.aikenamps.com/Why70percent.html
2/ Biasing such an amp to 100% plate dissipation does not equate to class A operation; that combination of tube type, plate voltage and load is unlikely to facilitate class A. Plate current and dissipation are likely to increase over the static levels under some signal conditions; the tubes are likely to enter cut off before the amp hits clipping.
By the same token, a different combination may operate in class A at a static plate dissipation of less than 100%.

Sorry if I've got muddled up on this.
Pete

No, you are right.
I hope the OP didn't think I was saying he should bias at 100% (or even 70% for that matter).
Just that there is a maximum rating for the tube that should not be exceeded for safe operation.
I do not recommend biasing an amp hotter than it's mfgr suggested operating point.

Mark, Tom and Martin gave additional guidance that I generally agree with.
I fully believe that a large part of the Fender sound is the higher voltages and moderate bias point.

Pete, thanks for calling me out on this and giving me a chance to clarify my statement.
Hopefully the OP will not blow his amp up based on my initial reply.

reddog Steve

[bluesky636]

Here is the correct schematic for the 59 Bassman LTD.

I bias both my 5F6A Bassman clone and highly modified Fender Blues Deluxe Reissue at just under 70% and they sound fantastic.

BDRI:

CATHODE CURRENT 53 mA
PLATE CURRENT 50 mA
PLATE VOLTAGE 403 VDC
% MAX PLATE DISS 67%

5F6A clone:

CATHODE CURRENT 50 mA
PLATE CURRENT 48 mA
PLATE VOLTAGE 429 VDC
% MAX PLATE DISS 68%

[martin manning]

Bluesky, thanks for posting the Ltd schematic. I've been curious about that version for some time.

Re class distinction it's all about the conduction angle. I don't think it is likely that a typical class AB amp could ever be biased hot enough to operate in class A at full power (at the onset of clipping). Even using AC 30 B+ and Zaa I estimate it would have to be idling at ~240% of Pa max. The only way to get there would be to reduce the plate voltage dramatically.