Amp Bias Help Request

[GAD]

SO I have this 1963 Bassman that had a Bias pot professionally installed. When I got it, it had Mesa 6L6GCs in it. I put a set of NOS Tung Sol 5881s in there and proceeded to re-Bias the amp.

I measured 420VDC on Pin 3 of the power tubes. The Tung Sol 5881 max plate dissipation is 23 Watts (http://lib.store.yahoo.net/lib/thetubestore/ts5881.pdf)

23 / 420 = .0547, so 54.7 mA
70% of 54.7 = 38.33 mA

With the tubes nice and hot, I measured before adjusting anything. I got 24.91mA

Turning the bias pot all the way clockwise results in a reading of 34.47 mA using my Eurotubes bias probe.

If my math is right, this translates to 63% of max plate dissipation, which seems kind of low.

What I'm confused about is that from what I've read, bias below 70% translates to more headroom, but if I reverse the math (.03447 * 420) I get 14.477 watts, which seems to indicate that I would get break up at a nice low volume.

Is that 14.477 watts per tube? That would make the amp 28.95 watts which makes way more sense...

Obviously I'm fuzzy on this stuff...

 

[mad dog]

GAD:

We're deep into murky here. You might enjoy this page:

http://www.webervst.com/tubes1/calcbias.htm

I use it all the time in my own biasing efforts. I'm no expert either, so please bear this in mind. My understanding is that the 70% figure is a guideline only, a suggested maximum. Practically, tonally, that's often on the hot side. I usually go more towards 60%. That varies tube to tube. Some seem to sound better hotter, like JJ 6L6s.

Even murkier is the bias effect part. I hear a too cold bias as a stiffness, a lack of response and depth. Whereas a warmer bias sounds lively. I don't know about higher bias levels cause I don't go there. It risks tube health and possibly amp health. Bottom line is, your max bias possible on those particular tubes might be just right.

I'd be looking more to tube choice than to bias for adjust amp breakup. As in, a properly biased 7591A tube pair in a specific amp tends to be louder, also cleaner longer, than a properly biased 6L6WGB (sort of 5881) in that same amp. Speaker choice is also a highly effective way to postpone or hasten breakup.

MD

 

[GAD]

Cool - thanks.

 

[capnjuan]

... Is that 14.477 watts per tube? That would make the amp 28.95 watts which makes way more sense...

Hi Gary; I think you're confusing 14.47 watts of measured plate dissipation for power output. If you have a single-socket bias tool, then the results only apply to the tube you're testing. Not sure which Bassman model you have but this is the schematic for the 5F6 model: twin 5881s / 430V on the plates. I don't know what this model's rated power output is but +/- 40 watts ought to be about right; if the plate voltage were in the 460V-470V range, then it'd be closer to 50 watts.

23 watts is the tube's maximum plate dissipation and 70% represents de-rating for safety purposes. As MD has suggested, 70% represents a not-to-be-exceeded ceiling and I further agree with his remarks about cold v. hot bias and the effect on tone. The pot is there to let the player manipulate the bias voltage to get subtle changes in tone.

Some techno-breakdancing below; you don't have to read it if you don't want to.

The Bassman is a fixed-bias amp; 'fixed' in the sense that the bias circuit 'fixes' or sets the primary bias supply to the grids (where the music signal goes in) at -48 volts. In the pic, the red arrow in the lower left is the AC bias voltage from the transformer. It hits that diode (red box) which keeps the AC from cycling and passes only the negative half of the AC waveform. Very briefly, for the tube to conduct, the grid (where the music goes in) must be more negative with respect to the cathode or else nothing happens. To ensure that the grid is more negative than the cathode, this design injects a negative voltage; initially -48 volts further reduced by the 220K resistors.

Once past the diode, the two capacitors filter out ripple and the 15K and 56K resistors set or 'fix' the bias voltage at -48 volts ... not milli-volts, not milli-anything; plain old volts. The green dot is where I think your pot is in the circuit. The two 220K resistors drop the voltage further to something in the -35 to -42 volts range; you can measure it.

I don't know the value of the pot; it could be anything from 100 ohms to 1.5K ohms and unless you find out whether the dropping resistors are design value and whether the two 220K resistors are within tolerance, I'm not sure I know why the pot won't allow a lower bias voltage which = less milliamps of current = less plate dissipation in watts. Good luck with your amp!

 

[GAD]

Thanks. The amp is a '63 6G6-B amp. The bias pot was added by someone before I got it.

I'm no electrical engineer, but I can (barely) read schematics and such, and I have another question.

Looking at this: http://www.gad.net/Blog/wp-content/uplo ... Layout.jpg

It seems to me that there are four preamp tubes, and it looks like:

One for Base channel
One for Bass tone
One for Normal Channel
One for Presence

Is that accurate?

 

[Default]

It seems to me that there are four preamp tubes, and it looks like:

One for Base channel
One for Bass tone
One for Normal Channel
One for Presence

Is that accurate?

The 7025/12ax7 closest to the 6L6s is the phase inverter that splits/flips the signal allowing the amp to operate in push pull mode. the rest are gain stages for the bass and guitar channels. Fender tone controls are all of "bleed frequencies to ground" type.

 

[capnjuan]

Post deleted.