Throwing metal film resistors in Fender amps

[J M Fahey]

Your hiss and hum levels are "normal" for that kind of amp, *BUT*
1) your camera audio system is heavily compressing all the time, to catch everything from a roar close up to a whisper 20 ft away, so when you record something in a silent room hiss and hum "look" 10X as loud as they would for anybody present there.
So it's not as bad as it sounds in your video.
2) your amp, which was not designed as a high gain amp to begin with, has an extra gain boost added, either an extra gain stage or some NFB killer.
Both will "increase" hiss and hum big way.
You'll have to live with that, unless you are ready to spend 3X the amp cost with a Boutique Tech who will rewire it, elevate filament voltages, reorient the reverb tank, replace caps and resistors, select tubes, all the tricks in the trade, to optimize it.
Personally I'd avoid using that added gain boost and instead add a clean boost (Think MXR clean boost) ahead of the amp.
Once your signal has mixed with hum and hiss at V1, not even the Devil will separate them.

[Wil]

Your hiss and hum levels are "normal" for that kind of amp, *BUT*
1) your camera audio system is heavily compressing all the time, to catch everything from a roar close up to a whisper 20 ft away, so when you record something in a silent room hiss and hum "look" 10X as loud as they would for anybody present there.
So it's not as bad as it sounds in your video.
2) your amp, which was not designed as a high gain amp to begin with, has an extra gain boost added, either an extra gain stage or some NFB killer.
Both will "increase" hiss and hum big way.
You'll have to live with that, unless you are ready to spend 3X the amp cost with a Boutique Tech who will rewire it, elevate filament voltages, reorient the reverb tank, replace caps and resistors, select tubes, all the tricks in the trade, to optimize it.
Personally I'd avoid using that added gain boost and instead add a clean boost (Think MXR clean boost) ahead of the amp.
Once your signal has mixed with hum and hiss at V1, not even the Devil will separate them.

Thank you for the answer. Even if you're right about the compression of the camera, he amp is way noisier than the Deluxes I had the chance to test out in store. I think I'll build a new hoffman style AB763 board and throw it in there. Measure if the trannies etc, are in spec and rewire everything as clean as I can.
It might cost some money, but spending money to learn new things is, to me, money well spent.
Thanks again!

[Gingertube]

Wil,
My advice on resistors.
Carbon Composition resistors are always very bad. These resistors are noisy and drift badly in value with age . They should NEVER be used in the build of a new amplifier and you will most likely not see them except in amplifiers built before 1970 (unless an "dodgy" builder was using cheap old parts to cut costs).

ASIDE: "Dodgy" is the english common expression for disreputable, some one likely to do things the most inexpensive way at the expense of reliablity. We make jokes about "the Dodgy Brothers" motor mechanics or the "Dodgy Brothers" plumbers and the like, so "dodgy" is anything a little (or a lot) suspect.

Carbon Film resistors generate more noise than metal films. The amount of noise is generally proportional to the current through the resistor. The resistors which are in the signal path and have most current though them will be the anode load resistors and the cathode bias resistors. The cathode bias resistors will generally have a bypass capacitor which should shunt the noise, so concentrate on anode load resistors as the usual source of noise.

The highest source of noise (from a resistor) in most amplifiers is the grid stop resistor (usually 68K) on the input tube. This is one of the things that is addressed in Merlin's "Designing Tube Preamps for Guitar and Bass" book. His suggestion (which I agree with) was to reduce that grid stop resistor (from the typical 68K) to 10K and keep the same high frequency roll off (from this resistor + Miller capacitance at the tube grid) by adding an additional capacitor from grid to 0V on that input tube. I add 100pF but about 470pF would be required to keep the original high frequency roll off.

Common sense tells us that when looking for noise problems we should concentrate on the sections which have the highest amount of gain following them, that is, the input stage and the reverb recovery stage.

For new builds I use ONLY metal film resistors.

Is there any sonic benefit from using Carbon Film resistors? The answer is maybe - Carbon Film resistors have a slight voltage dependency to their resistance value. In the particular case of anode load resistors their resistance changes with the signal as the total voltage across the resistor changes with signal swing. This will impart some second harmonic distiortion (in addition to what is generated by the tube itself). This is a low level effect but those with "golden ears" MAY notice it. It is unfortunate that Carbon Films as anode load resistors is about the worst place to use them with respect to noise.

These are my opinions, I do not expect that everyone will agree.

Cheers,
Ian

[Alan0354]

Hey Ian, even if the carbon resistor voltage dependency give a little of the even harmonics, It is pale compare with the even harmonics generated by the triode. The voltage dependency of the plate cure introduce so much even harmonics that the tiny amount of voltage dependency of the carbon resistor is going to be drown out.

I never did an A/B side by side comparison between MF and carbon resistor as it is very hard to compare between two amps. The 10% tolerance of the old carbon resistor make it impossible to eliminate all the variables UNLESS, you measure every resistor and match to the exact same value. 10% change of the anode resistor causing 10% change of the plate voltage and that alone can make the tube working slightly on a different plate curve already and sound slightly different.

Then the individual tube make a difference in sound, so it is impossible to have a real comparison. But my bet is nobody can tell the difference between carbon and MF resistor in an ideal identical comparison.

[Alan0354]

Thank you for the answer. Even if you're right about the compression of the camera, he amp is way noisier than the Deluxes I had the chance to test out in store. I think I'll build a new hoffman style AB763 board and throw it in there. Measure if the trannies etc, are in spec and rewire everything as clean as I can.
It might cost some money, but spending money to learn new things is, to me, money well spent.
Thanks again!

One video speak a thousand words!!!! You never mentioned you open the NFB loop, you can't even start to compare when you modify the amp already. You increase the gain by opening the loop like 4 times, your noise voltage is going to be increased by 4 times!!!

[Gingertube]

Hey Ian, even if the carbon resistor voltage dependency give a little of the even harmonics, It is pale compare with the even harmonics generated by the triode. The voltage dependency of the plate cure introduce so much even harmonics that the tiny amount of voltage dependency of the carbon resistor is going to be drown out.

I agree which is why I said "MAYBE".

The change of tube internal rp with signal current (which is the source of tube distortion) will swamp any distortion from the change in Ra resistor due to voltage across it.
I did not do the analysis to see if the change in Ra with signal would add to or oppose the distortion due to the change in rp with signal.

Quick and Dirty analysis:
More current means reduced tube internal rp, also more voltage across Ra which I think means more Ra, so I would suggest that you get partial distortion cancellation rather than more distortion -ASSUMING that the Ra change is +ve with respect to the voltage across it.

Cheers,
Ian

[Wil]

One video speak a thousand words!!!! You never mentioned you open the NFB loop, you can't even start to compare when you modify the amp already. You increase the gain by opening the loop like 4 times, your noise voltage is going to be increased by 4 times!!!

The white pot at the back of the amp is an On/Off negative feedback loop switch. The 4 times noisier noise you mention is clearly audible and most of the time, the amp is set on "normal", with the NFB wire virtually connected, so it's the stock, original configuration.

I agree which is why I said "MAYBE".

The change of tube internal rp with signal current (which is the source of tube distortion) will swamp any distortion from the change in Ra resistor due to voltage across it.
I did not do the analysis to see if the change in Ra with signal would add to or oppose the distortion due to the change in rp with signal.

Quick and Dirty analysis:
More current means reduced tube internal rp, also more voltage across Ra which I think means more Ra, so I would suggest that you get partial distortion cancellation rather than more distortion -ASSUMING that the Ra change is +ve with respect to the voltage across it.

Cheers,
Ian

I have to admit I'm quite lost. This discussion is way beyond my knowledge.
You mentionned a redesigned input stage that was really interesting. Do you have any schematic of this ? And would this alter the sounds of the original AB763 design ?
My main problem is that because of my modest technical english language skills I can't put the names you mention on the associated component.
Filter caps, grid resistors, coupling caps... I can identify the easiest ones. But I can't determine what component has the biggest impact on tone, because, still, it's what I'm for: tone, reability and quiet operation.

I really changed my mind and am after building (or rebuilding I should say) my first amp (Hoffman AB763 DR) with the highest quality components I can find, in a learning-focused perspective (does that even mean something in english ?).
The idea is going all metal film resistors, quality caps and turret board.

About the resistors, I can put my hand on RN65D, but they are quite pricey (.65$ each) and Xicon (.14$ each).
RN65D have the reputation to be great resistors and I have to admit they're really good-looking... But the cost forces me to use them sparingly but where ? What are the most importants places in a AB763 circuit to achieve the best tone, reability and quiet operation ?

About the caps, I'm thinking of going all Sozo Mustard caps.
But to cut the cost a little, I'd like to know what are the least important places (those who can accomodate with more standard without affecting the tone) where I can save a little $$$ ? (Filter caps will be F&T which are already installed)



My reference is the ultra famous Hoffman diagram. To which I'd like to add bright switches and the input design you talked about in your previous post.

Once again, to everybody, thank you for taking the time to answer this thread.

[Wil]

One video speak a thousand words!!!! You never mentioned you open the NFB loop, you can't even start to compare when you modify the amp already. You increase the gain by opening the loop like 4 times, your noise voltage is going to be increased by 4 times!!!

The white pot at the back of the amp is an On/Off negative feedback loop switch. The 4 times noisier noise you mention is clearly audible and most of the time, the amp is set on "normal", with the NFB wire virtually connected, so it's the stock, original configuration.

I agree which is why I said "MAYBE".

The change of tube internal rp with signal current (which is the source of tube distortion) will swamp any distortion from the change in Ra resistor due to voltage across it.
I did not do the analysis to see if the change in Ra with signal would add to or oppose the distortion due to the change in rp with signal.

Quick and Dirty analysis:
More current means reduced tube internal rp, also more voltage across Ra which I think means more Ra, so I would suggest that you get partial distortion cancellation rather than more distortion -ASSUMING that the Ra change is +ve with respect to the voltage across it.

Cheers,
Ian

I have to admit I'm quite lost. This discussion is way beyond my knowledge.
You mentionned a redesigned input stage that was really interesting. Do you have any schematic of this ? And would this alter the sounds of the original AB763 design ?
My main problem is that because of my modest technical english language skills I can't put the names you mention on the associated component.
Filter caps, grid resistors, coupling caps... I can identify the easiest ones. But I can't determine what component has the biggest impact on tone, because, still, it's what I'm for: tone, reability and quiet operation.

I really changed my mind and am after building (or rebuilding I should say) my first amp (Hoffman AB763 DR) with the highest quality components I can find, in a learning-focused perspective (does that even mean something in english ?).
The idea is going all metal film resistors, quality caps and turret board.

About the resistors, I can put my hand on RN65D, but they are quite pricey (.65$ each) and Xicon (.14$ each).
RN65D have the reputation to be great resistors and I have to admit they're really good-looking... But the cost forces me to use them sparingly but where ? What are the most importants places in a AB763 circuit to achieve the best tone, reability and quiet operation ?

About the caps, I'm thinking of going all Sozo Mustard caps.
But to cut the cost a little, I'd like to know what are the least important places (those who can accomodate with more standard without affecting the tone) where I can save a little $$$ ? (Filter caps will be F&T which are already installed)
(edit: I just read that SoZo in Fender amps weren't that great and that people generally prefer Orange Drop. I don't really like OD, for the moment Mallory 150s sit in my DRRI and I think they brought an improvment over the stocks caps. But I'm pretty curious and can't resist the influence of the hype and "mojo", haha)



My reference is the ultra famous Hoffman diagram. To which I'd like to add bright switches and the input design you talked about in your previous post.

Once again, to everybody, thank you for taking the time to answer this thread.

[AmpFix]

BUT in tube amp, you can also use snubber capacitor to reduce the noise. This is used widely.
3-7 pf between grid and plate is my preferred method. It works very well.
There are several other accepted methods also.
[ATTACH=CONFIG]26902[/ATTACH]

Even easier is FRED diodes... in theory but I never heard a difference. They are better for reducing harmful transients though.

[Wil]

Hello again,

Now that I made up my mind about rebuilding my Deluxe with a Hoffman AB763 style board, after studying schematics and layouts I have a couple of questions.
Here they are, in a badly made picture :



About components, the idea is : Polyprop for the normal channel, OD 225P for the vibrato channel, Jupiter for the two .1uf that go to the power tubes and the phase inverter caps (.001 and .1uF), the rest will be Mallory 150s.
About the electrolytics, I recently changed the filter caps in my DRRI but the filter caps board design (and caps values) aren't the same. Since they are new, I don't want to throw the big 220uF and 47uf to the bin. Is it possible to use them in a ab763 build ?
And what about all the other electrolytics ? Do they have a significant impact on tone as filter caps do ?

It's my first build (rebuild) so, some questions might seem dumb. But after all the time I studied those schematics I know feel able to rebuild my Deluxe. If you have any comment on the Hoffman layout or if you spot any mistake, I'll be glad to be warned :-)

Have a nice day!

[J M Fahey]

About components, the idea is : Polyprop for the normal channel, OD 225P for the vibrato channel, Jupiter for the two .1uf that go to the power tubes and the phase inverter caps (.001 and .1uF), the rest will be Mallory 150s.

Why that brand choice?
The schematic specifies no brands at all, Leo's parts orders specified type , capacitance and voltage.
Certainly he didn't pay more for boutique parts, even if they had been available.
And he bought "whatever was available" at F.C Hall's "Radio and TV Equipment co." in Santa Ana, California.

[J M Fahey]

About components, the idea is : Polyprop for the normal channel, OD 225P for the vibrato channel, Jupiter for the two .1uf that go to the power tubes and the phase inverter caps (.001 and .1uF), the rest will be Mallory 150s.

Why that brand choice?
The schematic specifies no brands at all, Leo's parts orders specified type , capacitance and voltage.
Certainly he didn't pay more for boutique parts, even if they had been available.
And he bought "whatever was available" at F.C Hall's "Radio and TV Equipment co." in Santa Ana, California.

[Wil]

Thank you for keeping this thread alive JM :-)

About the brands, I only have experience with Mallory in my DRRI, OD polyprop for amp applications and Jupiter for guitar caps so the choice is based on the things I read on the internet.
I want polypropylčne in channel one to have more of a hi-fi sound because I want to play my Boss RC-3 on that channel and maybe my vinyls too (knowing it still will be a guitar amp after all). Mallory 150s because they're cheap and sound good (a bit too much mids, that's the main reason of the caps mix). Jupiter because I had some and really made an improvment, and I believe the 0.001µF and 0.1µF I mention are key to get better tone. And the OD 225P Polyester because I read that they are the closest to those old blue fender caps.

In you experience, could it be a fine mix ?
According to people's reviews, my modest experience and my goals it seems to be good to me.

Plus, I'm quite influenced by all the hype and mojo, haha.

[soundguruman]

Thank you for keeping this thread alive JM :-)

About the brands, I only have experience with Mallory in my DRRI, OD polyprop for amp applications and Jupiter for guitar caps so the choice is based on the things I read on the internet.
I want polypropylčne in channel one to have more of a hi-fi sound because I want to play my Boss RC-3 on that channel and maybe my vinyls too (knowing it still will be a guitar amp after all). Mallory 150s because they're cheap and sound good (a bit too much mids, that's the main reason of the caps mix). Jupiter because I had some and really made an improvment, and I believe the 0.001µF and 0.1µF I mention are key to get better tone. And the OD 225P Polyester because I read that they are the closest to those old blue fender caps.

In you experience, could it be a fine mix ?
According to people's reviews, my modest experience and my goals it seems to be good to me.

Capacitors are like 31 flavors of ice cream, they all sound different.
Some people can hear a difference, some cannot hear any difference.

I like Silver Mica. Many old style manufacturers liked Silver Mica, and oil filled too...

There is nothing wrong with trying different ones, to arrive at something you like the sound of...

But there is nothing wrong with trying high end, hand made, hi fi caps either.
If I like the sound of them, then that's what I will use.

And specifically, I avoid Chinese capacitors. I don't like the way they are made, and I don't like the life span, I don't like the sound of them...
And I don't like all of our jobs being sent to China either.
But, that's just me.

[Wil]

Capacitors are like 31 flavors of ice cream, they all sound different.
Some people can hear a difference, some cannot hear any difference.

I like Silver Mica. Many old style manufacturers liked Silver Mica, and oil filled too...

There is nothing wrong with trying different ones, to arrive at something you like the sound of...

But there is nothing wrong with trying high end, hand made, hi fi caps either.
If I like the sound of them, then that's what I will use.

And specifically, I avoid Chinese capacitors. I don't like the way they are made, and I don't like the life span, I don't like the sound of them...
And I don't like all of our jobs being sent to China either.
But, that's just me.

I totally agree. And it's also a lot of fun to find the right part at the right place for what you consider is the right sound for you.
Appart from the caps, have you seen the photo I posted ? These are small tweaks to make the hoffman layout fit in my DRRI.