What is typical plate voltage on a SF Pro reverb??

[sidehatch]

Sorry to keep on adding seperate biasing questions but now IM talking about a different amp.

Its an early 70's and does have the 5u4GB rectifier and says 45 watts on the back.

I was just turned onto a cool website that all you have to do is enter the plate voltage and type of tube and it gives you the ma reading.

Also, could anyone point out where you actually measure the plate voltage on that amp?

If you have to wire in resistors like I've read on lord valves method I'll pass and just guess based on typical pro reverb plate voltages(at least for now).

Thanks!!!

[davemccarthy707]

You can only balance the tubes in this circuit. Typical plate voltage is about 460. I usually bias my Blacface/silverface amps to 33ma . But thats me

[sidehatch]

Thanks for the reply.
I should have added that I have had the amp blackfaced, Bias adjusted converted and hum balance completely removed.

I just mentioned SF and the 5u4Gb in case the the plate voltage is different than the 5ar4 models.

So are you thinking that mine should be around 460 volts on the plates?

How does someone check that with a voltmeter or does it need the resistors and stuff added?

[docmh]

What's the website you mentioned that gives the ma reading? Thanks

[TweeDLX]

Originally Posted by sidehatch

Thanks for the reply.
I should have added that I have had the amp blackfaced, Bias adjusted converted and hum balance completely removed.

I just mentioned SF and the 5u4Gb in case the the plate voltage is different than the 5ar4 models.

So are you thinking that mine should be around 460 volts on the plates?

How does someone check that with a voltmeter or does it need the resistors and stuff added?

To measure the plate voltage, look at the power tube sockets. Each socket will have either a blue or brown wire coming from the OT connecting at pin 3. That's the terminal for the plate. Set your DMM to measure DC voltage. Place one tip of your DMM very carefully on pin 3, and one against the bare metal of the chassis (to ground it). It's tight, and if you slip, you can short something out. Plate voltage on an amp with a 5U4GB will differ from that of a 5AR4. If you converted your bias balance to a standard bias pot, you don't need to add resistors (unless you don't have the bias range you need).
Mike

[sidehatch]

Thanks!!! I'll open it up later and get the exact reading.


That website calculator is:

http://ax84.com/contrib/biascalc.php


All you have to do is type in plate voltage and it gives you the ma to set.

If this works for me I'll be able to bias my super reverb and pro reverb and all my adjustible bias amps without buying anything!!!

Originally Posted by TweeDLX

To measure the plate voltage, look at the power tube sockets. Each socket will have either a blue or brown wire coming from the OT connecting at pin 3. That's the terminal for the plate. Set your DMM to measure DC voltage. Place one tip of your DMM very carefully on pin 3, and one against the bare metal of the chassis (to ground it). It's tight, and if you slip, you can short something out. Plate voltage on an amp with a 5U4GB will differ from that of a 5AR4. If you converted your bias balance to a standard bias pot, you don't need to add resistors (unless you don't have the bias range you need).
Mike

[RussB]

For the 12,285 time, there is no "mA to set" the tubes at.


You set them within a range that yeilds good tone and acceptable life. This can be anywhere between 20-30% and 70% dissipation, give or take

[sidehatch]

yes, I see where you are coming from but this calculater will tell you the ma for 60%,70% all the way to 100%.

I think its cool that I can easily know what the ma range will be without having to buy any tools.


Thanks

Originally Posted by RussB

For the 12,285 time, there is no "mA to set" the tubes at.


You set them within a range that yeilds good tone and acceptable life. This can be anywhere between 20-30% and 70% dissipation, give or take

[davemccarthy707]

Originally Posted by sidehatch

yes, I see where you are coming from but this calculater will tell you the ma for 60%,70% all the way to 100%.

I think its cool that I can easily know what the ma range will be without having to buy any tools.


Thanks

I find in most blackface/silverface amps 33ma seems to be the golden spot for me.

[steveb]

I have a '72 Bandmaster Reverb that I recently acquired, & have also modified the bias control to blackface.

I have 6L6GC tubes in mine, & I'm reading 418V for the plates. According to the chart on the link, 70% would be 50.24 mA. I didn't realize there was such a difference between 6L6 & 6L6GC tubes.

Anyhow, do as TweeDLX suggested to get the readings. I use alligator clip test leads to connect from the measurement points to the meter before turning the amp on. That way I don't have to worry about my hand slipping & touching the wrong spot. I also have the amp connected to a power strip to turn it on & off without having to touch the amp itself.

[Prairie Dawg]

I'm of the belief that the much maligned hum balance is a pretty good setup and here's why. Resistors drift over time, as do tubes. If you want your perfectly bench matched tubes to stay matched in the sockets you put them in, your bias network has to be accurate. You can get there in two ways. First, rebuild the bias supply with new resistors that you have carefully matched, and then set your level with an adjustable control or fixed resistor. Or, you can use a balance setup like Fender did which allows you to equalize both sides and compensate for differences in component values and tubes.

My own personal preference is to have the balance control available, and a means for adjusting the overall level. I have heard that some amps have individually adjustable bias for each power tube. That seems like the best of all possible worlds.

[sidehatch]

Im getting 891v on the blue and brown wire on each tube. Is this right? To I devide that by 2 and get 445v?



About the hum balance:

I am actually, in a way, with you. I converted this amp to bias control and cant tell much difference if any in actual tone. The tone didnt reall y start changing til I did the speakers which was 80% better. New tubes was 15% and a few choice cap upgrades was the remaining piece of the puzzle.

IMHO, There is a million times noticible difference to my ear from me adding a negative feedback pot.

That sucker makes a big difference when you turn it. The bias deal sounds slightly different . At one end of the spectrum its different to the other in terms of distortion but in hte middle IE: 40ma to 70 ma not so much.

I did it because you read "blackface is better" so I went with it even though the best stock fender I have ever owned and still do is a 73 non master super reverb. Kicked several BF supers I've owend to the curb!!!

The hum balance thing was actually effective though its now gone. It may have been a wild card on tube life but I popped a few sets of tubes in and just turned it til it had no hum.

Originally Posted by Prairie Dawg

I'm of the belief that the much maligned hum balance is a pretty good setup and here's why. Resistors drift over time, as do tubes. If you want your perfectly bench matched tubes to stay matched in the sockets you put them in, your bias network has to be accurate. You can get there in two ways. First, rebuild the bias supply with new resistors that you have carefully matched, and then set your level with an adjustable control or fixed resistor. Or, you can use a balance setup like Fender did which allows you to equalize both sides and compensate for differences in component values and tubes.

My own personal preference is to have the balance control available, and a means for adjusting the overall level. I have heard that some amps have individually adjustable bias for each power tube. That seems like the best of all possible worlds.

[TweeDLX]

Originally Posted by sidehatch

Im getting 891v on the blue and brown wire on each tube. Is this right? To I devide that by 2 and get 445v?

About the hum balance:

I am actually, in a way, with you. I converted this amp to bias control and cant tell much difference if any in actual tone. The tone didnt reall y start changing til I did the speakers which was 80% better. New tubes was 15% and a few choice cap upgrades was the remaining piece of the puzzle.

891VDC is way too much. One tip one the blue or brown wire (pin 3) and one on the chassis, right? You should be reading somewhere in the 400's. Is the DMM set correctly? DC volts.

The bias pot itself isn't designed as a "tone" mod, but used correctly, makes for better tone overall because your tubes are running at the optimum dissipation point. Turn it too low and your tubes run cold, too high and you can red plate them, or at the least lose any chance at tube longevity.

Mike

[sidehatch]

Yep, Something is wrong. I wonder if my DMM isnt set right or wont do this?

I put it on VDC and one tip to the blue wire on a tube which I think is pin 3. The other tube has a brown wire in the same spot, both coming form the tranny.

PIn 4 has a big blue resistor going to another pin.

I wonder what Im doing wrong?

[sidehatch]

Her's a pic of that area. Is this the same blue and brown wires we are talking about?? Thanks for your help!!!