Upgrading resistors in a 66 bassman ab165

[dragonbat13]

OK. Working on an mostly original ab165 bassman, vintage 1966.

I finally got the amp to working after redoing part of my cap job on the third/shared cathode circuit ground. The only caps I replaced are the filter caps, cathode caps, and bias cap.

Amp does have some resistor noise. So I want to tackle that. I am not planning on changing any values in the amp yet, as far as resistance goes. Resistor wattage maybe.

One area I want to do is the screen resistors. These along with the grid resistors on the output tubes.

I want to use a fusible, metal oxide type resistor on the screens to add some protection against a shorted screen grid in a power tube to hopefully waste the resistor instead of the power transformer. I was thinking of using some of these:

https://www.digikey.com/en/products/det ... 500/596196

Am I on the right track with those for some 470 ohm screen resistors? Is the wattage correct for the safety factor mentioned above or is it too high?

Regarding the grids. I plan on using metal film for the 15k grid stopper resistors and the 220k grid leak resistors.

For the plates of the phase invertor and third/shared stage plates I was thinking of using either carbon comps or carbon film, most likely carbon film.

input and tone stack stages would get metal film on the plates, perhaps throughout if necessary.

Mostly concerned with the resistors with higher voltages and the power tubes.

I plan on working my way back from the power tubes to the phase invertor and third stage first, then going back through the input stages if there is still too much noise. Or should I just go ahead and swap everything out?

There are other mods I plan on doing such as bias adj. added to the bias balance, maybe a humdinger, new diodes for sure in the rectifier, a fuse on the B+ of the power transformer.

My questions though are if I have a good plan of attack, especially regarding the wattage and choice of the screen resistors.

All suggestions will be considered. I really appreciate any help

Attached is the schematic and layout I am working with now. Would love to find an editable schematic to use later on.

[LOUDthud]

Those resistors for the screens look good. First step to lower noise is those 68Ks on the input jacks. Metal film is best. After you get done with the rest of the amp, if the noise isn't low enough, consider lowering the 68Ks as low as 10K.

Edit: Ya, I know you won't be able to drive a 20K input with a passive guitar. But the noise will be insanely low and you can daisy chain to another amp.

[dragonbat13]

These are the resistors I plan on using in the phase invertor and third stage plate load resistors

https://www.digikey.com/en/products/det ... J/13537322

And these for the other tubes plates:
https://www.digikey.com/short/rwd29jwb

[dragonbat13]

Appreciate the help. I posted the metal films I want to use.

I have a feeling both input stages are going to end up with all new resistors through. I will start with the grid resistors at the jacks as mentioned.

I'm looking at those metal film resistors and out of four, two are more expensive that the other two. The only difference is the temperature coefficient. I have no idea what the difference between 100ppm and 200 ppm. It's something I'm gonna look into.

[rooster]

Change the plate resistors, use 100K 1W CF, metal film here is a waste of time. The 470 ohm screen resistors actually benefit the sonics of the amp by using wire wound resistors. 5W ceramic block types are what I've always used in Fender builds. The carbon 3W parts that Fender used because they were less expensive at the time, but the idea that they were specifically used to avoid damage to the tube or amplifier is not valid. When the power tube goes, it's going to take the fuse (if your AC primary path is done correctly) and, the 5W resistors will still be intact so all you'll need to do is replace the power tube.

Carbon Comp resistors have a unique sound, this is something to remember. Those 1/2W 100K plate resistors in your amp were soon to be replaced (by Fender) with 1W CC resistors and this was actually quite an improvement (less noise and longer life). The only problem with sourcing these CC versions today is the current expense and, when you do find NOS pieces, you will find the 100K CC versions now measure 110K. And this still may be acceptable, but not for the Slope resistor position because it will change the midrange of the channel.

The 68K CC input grid stoppers are not causing any real noise, I would leave them alone.

The Fender amp, particularly the AB165, has become a collectable amp. Duane Allman was a user back in the day in Muscle Shoals studio for one. It's too bad yours needs to be reworked to be made operable, but I would recommend avoiding the MF resistors completely. I suggest CF resistors because they are accurate as to the resistance value and are not MF, but don't forget the unique sound of the CC resistor. Good luck with the amp

BTW, regarding the 5W resistors. You understand how the choke in that amp reduces 60 hum from the power supply, right? Well, this is the basic result of a coiled wire as it relates to AC derived electricity. These 5W resistors are made of coiled wire and they perform a similar role at the screens. Marshall figured this out as they built their amps, and, although Jimi blew plenty of EL34s, it was very rare (if at all) that the 1K 5W screen resistors were replaced as a new set of Mullard XFs were installed.

[martin manning]

The 68K CC input grid stoppers are not causing any real noise, I would leave them alone.

"...the input grid stopper adds the most amount of Johnson noise (hiss) of any resistor in the amp. (A 68k stopper generates at least four times more hiss than a typical 12AX7!)." http://www.valvewizard.co.uk/gridstopper.html Blencowe doesn't say what kind of resistor he is talking about here, but CC would be the worst.

I would recommend avoiding the MF resistors completely. I suggest CF resistors because they are accurate as to the resistance value and are not MF, but don't forget the unique sound of the CC resistor.

Anything but CC (or MO), IMO, unless you want the look of the originals, and don't mind the additional noise.

BTW, regarding the 5W resistors. You understand how the choke in that amp reduces 60 hum from the power supply, right? Well, this is the basic result of a coiled wire as it relates to AC derived electricity. These 5W resistors are made of coiled wire and they perform a similar role at the screens.

I measure ~1.8 uH for a 470Ω 5W Xicon wire wound cement resistor. Being attached right at the screen pin, it can't hurt, but it amounts to almost nothing in impedance. My LCR meter doesn't show any difference between R and Z at 10kHz, 467.2 for both (and it shouldn't, it's way below it's 0.1Ω resolution). Even up at 100 kHz the calculated reactance is only 1.13Ω.

[pdf64]

… The carbon 3W parts that Fender used …
… Marshall figured this out as they built their amps, and, although Jimi blew plenty of EL34s, it was very rare (if at all) that the 1K 5W screen resistors were replaced as a new set of Mullard XFs were installed.

Fender schematics show 1W 470ohm screen grid resistors, not 3W eg https://el34world.com/charts/Schematics ... reverb.pdf

Suppressor grid pentodes such as EL34 will tend to draw more screen grid current that beam pentodes such as 6L6 or 6550. Hence 5W screen grid resistors are necessary to support normal fully overdriven operation in EL34 amps. Whereas 6L6 Fenders can manage with 1W 470R.
The 5W 1k resistors in Marshalls typically don’t blow when a valve shorts because there tends to be a HT fuse that blows first.

I don’t think it’s a good idea to uprate the Fender 1W screen grid resistor rating unless the HT is fused somehow (fusible screen grid resistors being great idea).

[rooster]

Anybody installed a HT fuse in an Express? I'm haven't. Did Ken? Was this science beyond his scope?

As to the shot noise of the 68K grid stopper pair (34K/136K) in a Fender amp, whether adding shot noise 4X greater than a 12AX7 or not (which I doubt in the guitar amp application), I would have to ask you, what about the carbon traces in your guitar's Volume and Tone pots?

I think there is noise and then there is *noise*. Tell me, since the typical guitar speaker produces audio up to say 6K, are the measurements that Blencowe has taken here referring to the frequency range we are dealing with? Or has he borrowed these numbers from the testing result of some range of *noise* that only a HiFi enthusiast or dog might appreciate? This is important to know, right?

My experience tells me that a guitar amp is an instrument in itself when used by the right artist. Someone someday might somehow remove all the component and circuit noise from an electric guitar amp, OK. But what would we have as a result? You don't have to look any further than the Kemper ppl to understand that creating the sound of a 'perfect' amp is not going to sell Kemper modeling amps. They're being realistic here.. and getting richer than you and I.

[bepone]

My experience tells me that a guitar amp is an instrument in itself when used by the right artist.

correct, this need to be underlined, most folks forget that when speaking about guitar amplifiers.. this is another instrument..thats why we are playing with resistors, capacitors, output transformers!

[dragonbat13]

Thank you for all the good info. Great thread. All these points of interest are what I want to learn about.

Regarding the value of a vintage amp. In my opinion, unless an amp is pristine, clean, untouched perfect, museum piece, it's not of a concern changing certaint parts to be playable. The amp must be restored to a nice playable condition. My 54 wide panel Princeton wasn't a museum piece, so it was upgraded with new caps. It was righteous afterwards.

Regarding the screen resistors, I'm using fusible metal oxide. I want to find the correct wattage so the resistors can function properly regarding their safety feature. I have been told or have read everything from 1,2 and 3 watt.

At this point I'm considering the two watt,
https://www.digikey.com/en/products/det ... 500/596196
That's the same resistor as above.

Regarding carbon type, all I will use is CF. Most likely 1 watt at the plates.

[Reeltarded]

I also want to argue!!

Tolerance and robustness are more important than noise. The waterfall that is my fully amped rig is tamed by signal when the knob cracks.

Amps are supposed to shhhhhh

I use a lot of CF on the board. Most of my amps get sand cast 5w screens, but some get big fat Pihers or Welwyns. The Welwyns are my swellest and best pronounced like Bob S would say.

[Reeltarded]

Regarding the value of a vintage amp. In my opinion, unless an amp is pristine, clean, untouched perfect, museum piece, it's not of a concern changing certaint parts to be playable.

lol snobs on a spectrum. I will mod anything to make it funner. I will drill metal!

[pdf64]

…
Regarding the screen resistors, I'm using fusible metal oxide. I want to find the correct wattage so the resistors can function properly regarding their safety feature. I have been told or have read everything from 1,2 and 3 watt.

At this point I'm considering the two watt ….

I’ve not yet tried fusible resistors.
Even with heavy power amp overdrive, with 6L6 types in typical Fender type circuits, I’ve only known 1W 470R MO to blow if the valve shorts.
I can’t make sense from the datasheet of the fusing characteristics of those fusible resistors, so I’d get 1W and 2W, and see how the 1W cope.

[dragonbat13]

I'll try the one watt,

I couldn't make sense of those graphs either.

[WhopperPlate]

2 cents

swapping out those CC 470 ohm screen resistors are going to change the sound significantly, for better or worse. I specifically will install these in a new build for that added character . I find it is sweeter in many ways. Never underestimate the power supply effect on tone, especially when CC resistors are involved .

Wirewound ohmites and welwyns are my favorite after. The typical white cement variety a bright and zingy to my ears .

I could say the same thing overall about changing most original parts , but I will choose one hill to die on today .