How do I raise the overal B+ voltage in a Fender?

[JubileeMan 2555]

I have 3 amps in front of me.

All are the same Fender AB763 circuit.

2 have low B+ voltages and 1 has high B+. In fact, ALL the voltages from the preamp to the power section are lower on 2 and higher on one.

The one with the higher voltages sounds much better.

What is it that controls these voltages? Is the Power Transformer just putting out more volts in the 3rd amp?

[Tone_Terrific]

I have 3 amps in front of me.

Is the Power Transformer just putting out more volts in the 3rd amp?

Meter it. Unloaded.

[JubileeMan 2555]

Meter it. Unloaded.

Can you quickly tell me how to do this? What connection do I dissconect, where do I measure, and does the amp need to be off or on?

[jjasilli]

1. AB 763 is just about every type of 2 channel amp that fender made. Yes, they had different trannies, etc.; and they all sound different

2. It may not be the voltage that's making the amps sound different.

3. Different rectifier tubes put out different voltages. Solid state rectification puts out most of all. Ted Weber and others make solid state rectifiers to plug into the rectifier tube socket.

However, I suspect that focusing like a laser beam on voltage is not the answer.

[JubileeMan 2555]

1. AB 763 is just about every type of 2 channel amp that fender made. Yes, they had different trannies, etc.; and they all sound different

2. It may not be the voltage that's making the amps sound different.

3. Different rectifier tubes put out different voltages. Solid state rectification puts out most of all. Ted Weber and others make solid state rectifiers to plug into the rectifier tube socket.

However, I suspect that focusing like a laser beam on voltage is not the answer.

... All three amps are super reverbs. But all three amps have different power transformers. But all three amps have similar components (including values and brands) and all three have SS Rectifier.

I want to know how to raise the voltages in these amps for educational purposes as well as possible troubleshooting where the tone is failing on the two. its just too much of a coincedence in my mind that the one amp that sounds sweet has much higher voltages throughout.

[JubileeMan 2555]

Take out all preamp tube and power tubes, leave the rectifier in, and turn the AC and DC power 'on'.

Meter the B+ at the first filter cap, or an equivalent place in-circuit. Make sure your filter caps are in good condition though, sometimes unloaded amps can exceed the voltage rating of certain caps.

As an example of the differences in measurements, my 5E7 Custom measured 500VDC unloaded, 423VDC loaded.

Thanks bud!

Found out

Good super: 525v (original '67 tranny)
Super 2: 512v (some no name tranny)
Super 3: 491v (Hoffman tranny)

So... apart from finding power trannies that have higher unloaded voltages, is there anything I can do to up the amp's volts?

[phsyconoodler]

Again,the voltage is likely not the key to the sound being better.There is no way to up the voltage other than changing the PT.
Too many other variables are present to say it's the higher voltage.And 491v to 525v can be explained in many ways.Lower bias,different tubes,more load somewhere in the circuit,etc.
You are barking up the wrong tree here.
And raising voltages brings in a host of other potential problems that you won't know about until your amp melts down on a stage in bumf**k N.D.
What about speakers,coupling caps,output transformers,power tubes,preamp tubes,tone stack,lead dress?

[JubileeMan 2555]

...What about speakers,coupling caps,output transformers,power tubes,preamp tubes,tone stack,lead dress?

All are basically equal. The only one thats the odd man out is the "super 3" which is a custom build from a private amp builder. But they all basically have the same variables you just mentioned.

One factor, which is kind a ?? in my book is the SS rectifiers. All three have completely different versions. Amp 1 had a hand made one that looks like just a bunch of diodes. amp 2 has a no-name black stubby thing. and amp three has a weber copper cap.

here is the pic of what I'm using as the SS rectifier in the higher "good sounding" super:

http://www.shelleygrund.com/images/ssrectifier.jpg

[jjasilli]

1. If these are old amps, the values of components may have drifted - resistors usually go up in value. Caps can go either way, sometimes by a lot. This is random and can greatly affect the tone of the amp.

2. Even with new components there can be a 20% variation in part values either way. E.g.: a 100K resistor may actually be anywhere from 80K - 120K ohms. This may affect tone.

3. There may also be different tubes; speakers; bias settings; tubes matched or mismatched; choke or no choke.

There are numerous variables. Anyway as said above you cannot increase the B+ voltage without replacing the power transformer. But if the SS rectifier has a resistor in it to simulate a tube rectifier, you can replace it with one that has no resistor - also as said above.

[phsyconoodler]

Show us some pics of these 'basically equal' amps inside.I bet there are some glaring differences.
The Rectifier is not the issue here either.However you string together a few diodes to get a rectifier is not important.
I have neard many supposedly 'identical' amps that sound way different from each other.the output transformers and tubes being the major factors,not to mention speakers.Speakers can be eliminated from the equation by plugging all the amps into one set of speakers for A/B tests.

[John Phillips]

Take out all preamp tube and power tubes, leave the rectifier in, and turn the AC and DC power 'on'.
No, do NOT do that!

If you remove all the preamp tubes, no current will flow through the resistors in the B+ chain, and the voltage at the far end of the chain will rise to the full unloaded B+ at the rectifier end, which in many amps will be well over the filter cap rating - that's why the first cap stage (and sometimes the second) is done with two stacked caps.

If you have a modern amp with 500V caps throughout, you're likely to be OK, but not in many older amps with 450V caps further down the chain (and sometimes only 350s).

And in fact, with two of these particular amps with solid-state rectifiers and unloaded B+ of over 500V, it's a risk even with 500V caps.

If you're metering unloaded DC voltage with the tubes out, you must leave the standby switch open. Better is not to remove the preamp tubes, or certainly not V1.

If you've already done this and didn't blow a cap, you're lucky. I found out about this problem the hard way.


FWIW, the amp with the original '67 PT was designed for slightly lower line voltage and so will produce higher secondary voltages than the original intended spec when driven with modern voltage.

The Weber Copper Cap should have some resistance in it too, unless it's specifically the 'no sag' type. Under no-load conditions it won't show up as much.

Amp #2 is the unknown quantity, since it has an unidentified PT and an unidentified rectifier, which may or may not (but probably not if it's small) have a resistor in.

[mark norwine]

"Voltage" doesn't mean much........it's VA that matters.

[JubileeMan 2555]

"Voltage" doesn't mean much........it's VA that matters.

What?

[BJF]

Hi,

Voltage will matter downstream setting the workingpoints of all tubes in the circuit since all are derived from the B+.
There would be two basic ways: either go by part or by ear
The absolute value at the anodes is a bit hard to address without changing power transformer allthough there are ways.......

but if you map out all working points of the tubes and the B+ taps on the one you think sounds best and compare that with the other two, you can adjust the voltage at the B+ taps to desired level.

This is easily done with a Kirschoff walk: the current through the first dropping resistor, closest to rectifier would be the sum of all currents through the B-tree.
You can then adjust the voltage dropping resistors to give approximate voltage at each tap, which will go a certain way....maybe enough.

Another way of addressing this would be to adjust the individual workingpoints of all tubes down stream
-also take out and measure all cathode decoupling capacitors and if you want greater control make sure each triode in the preamp has its own cathode resistor and so workingpoint can be set for each tube.

If only the anode and screen voltage would drop in a circuit like this and all other voltages stay the same, you'd not hear much difference until you reach the maximum levels of the poweramp; but a voltage change throughout the amp affects many things, including the small parts of distortion hardly recognizable as distortion but as shimmer.

If I'd work on an amp like the less good sounding ones I'd listen to what sucks in the sound and adjust accordingly throughout the amp, accepting the lower maximum voltage but correct all else until it sounds fine, quite regardless of the great sounding amp you have that is fine as is.

Oh and don't take out all the tubes when measuring: firstly decoupling capacitors may not be able to take the voltage then and secondly it won't tell you much if anything, because you need to see all the currents.

If you'd listen to the less good sounding amp and define what is missing or that there is to much of or just generally what sucks you can address every such point.

An example if the working point of stage three on the vibrato channel is low it may saturate harshly this also due to the dynamic load it sees- something that is fine as long as there's no distortion, but that yet gives a 'hard' quality to sound. Now this stage will be on the brink of overload while amp is still running 'clean' due to the signal level having been amplified by stages preceeding it.


Just a few thoughts
BJ

Affiliations
www.bjfelectronics.com
www.mpamp.com

[JubileeMan 2555]

To better illustrate the issues here is an image of a bunch of voltage readings.

GREEN= good sounding amp
PINK = amp im working on

http://www.shelleygrund.com/images/Voltage_readings.jpg

[tlpruitt]

Have you tried switching rectifiers from amp to amp?

Have you tried switching tubes from amp to amp?

Are all 3 amps biased the same?

Are you listening to the amps through the same speaker cabinet or playing each amp through its own speakers?

[JubileeMan 2555]

Have you tried switching rectifiers from amp to amp?

Have you tried switching tubes from amp to amp?

Are all 3 amps biased the same?

Are you listening to the amps through the same speaker cabinet or playing each amp through its own speakers?

I've tried that all...except the rectifier swaps. Thats what I'm going to be doing this evening.

Also... the BIGGEST difference in tone is really the feel of the amps. the best sounding one is smoother and transitions into distortion much better.