Bias in Fender amps - questions about voltages

[überfuzz]

I'm about to fix some things in a Fender amp, like bias. Up till now I've only seen bigger power stages, 4*6L6. This amp has 2 6L6 with the same cathode bias as I've seen before. The thing the got me wondering is the plate voltage. It's lower in this amp approximately 430 V. When I'm googleing around I see plate voltages down to 415V in the same power stage set up. I seem to remember seeing plates close to 500V.

Why did they, Fender, alter the B+ so much in different amp even if the board was the same? Does it affect the tone in the preamp a lot..? What's the functional limits, max min plate, for this power tube set up?

Edit, What wattage are you aiming at when you adjust bias in Fender amps. I often go quite low. I read that Fender amps are at their best with low bias, well somewhere on the net someone said so, I've stuck to that since.

[JoeM]

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Edit, What wattage are you aiming at when you adjust bias in Fender amps. I often go quite low. I read that Fender amps are at their best with low bias, well somewhere on the net someone said so, I've stuck to that since.

I dont have an answer for the first part of your post. But as far as bias current, I've noticed my SFSR and DRRI dont seem to have as much change in tone with different bias current (i.e. idle current dissipation) as my 5F6A clone. Maybe the different NFB has some effect there? My clone (5F6A) usually sounds good +/- 60%. But the other Fenders sound good to me at levels closer to 50%. I'm usually going for a good clean sound though, so I guess it's more up to personal tastes. (As long as they're not set too hot). Also, I'm not sure what newer production tubes will hold up to, so I bias more conservatively.

Back when I first starting playing, I dont think players even gave it much thought. I didnt even know what 'bias' was. You just went to the local electronics store, bought the tubes and put the new tubes in the amp.

[ThermionicScott]

I'm about to fix some things in a Fender amp, like bias. Up till now I've only seen bigger power stages, 4*6L6. This amp has 2 6L6 with the same cathode bias as I've seen before. The thing the got me wondering is the plate voltage. It's lower in this amp approximately 430 V. When I'm googleing around I see plate voltages down to 415V in the same power stage set up. I seem to remember seeing plates close to 500V.

Why did they, Fender, alter the B+ so much in different amp even if the board was the same? Does it affect the tone in the preamp a lot..? What's the functional limits, max min plate, for this power tube set up?

Edit, What wattage are you aiming at when you adjust bias in Fender amps. I often go quite low. I read that Fender amps are at their best with low bias, well somewhere on the net someone said so, I've stuck to that since.

What amp circuit is this? Generally, the plate voltage in cathode-biased Fenders is shown as 400V or lower, and they're biased on the hot side, since the bias will shift colder when a signal is applied.

It was the later fixed-bias circuits that had 400-500V on the plates.

- Scott

[überfuzz]

Thinking about it, I come to the conclusion that the power section might be well of with quite a big difference in plate. (Please correct me if I'm wrong.) You should be able to adjust using the bias. But the preamp stage...

I've been botanising Fender ab763 schematics to get a better idea. While comparing a deluxe reverb with a super reverb you have quite a difference in plate voltage.
Deluxe Reverb : 230V
Super Reveb : 410V

I'm not an expert by any means but this must sound very different. Look at the load line for these two 12xa7 stages, both the same ab763 circuit. (See attached picture.) Cathode line would be the same, see the schematics.

[ThermionicScott]

Alright, I'm confused. Are you asking about the power tube voltage/bias, or the preamp?

If I'm parsing the question in your last post correctly, the lower voltages in the DR preamp may have been a deliberate move to keep gain down, as it only needs a 70V pk-to-pk signal to drive the output tubes, compared with 104V for the SR.

- Scott

[überfuzz]

The idea was to address both preamp and power amp. Could you elaborate the 70 - 104 peak to peak reasoning..?

[ThermionicScott]

The DR is biased to -35V, and the SR to -52V, so for a push-pull amp you double the cathode-grid difference to find the input headroom. The primary goal of the preamp is to produce enough voltage swing to drive the output tubes to full power, after all.

- Scott

[JoeM]

I've been botanising Fender ab763 schematics to get a better idea. While comparing a deluxe reverb with a super reverb you have quite a difference in plate voltage.
Deluxe Reverb : 230V
Super Reveb : 410V

Are reading off the schematics? Actual values may differ. I think my SFSR has pres about 230 and the DR around 180, measured at the plate.

[überfuzz]

JoeM - Off course, but when Fender constructed their circuits, like for instance the AB763, they probably did it on the drawing board.

About the terminology confusion. If you look at the picture I posted, in the diagram showing the plate curves, you'll see why I called B+ supply plate voltage. Further if you have a anode voltage of 230V you can read the plate, it should be in the realms of 360V. 1. Find 230V on the x-axis. 2. From here straight up to the cathode line, black. 3. Now you just follow a path parallel to the red and green down to the x-axis again. Looks like you end up at approximately 360V.

[Amp Kat]

You are talking Cathode bias or fixed bias. Not sure I've seen a 2 6L6 Cathode biased Fender. Are you sure maybe it's not like a 5E3 that is a 6V6 amp that someone put 6L6's in ?