Bassman Micro: one input / two channels wiring check?

[Lancer X]

Hi folks. Finalizing my Micro Bassman build and need a quick review. I created a second channel to accommodate @robrob 's Plexi Lead channel mod. I would like to use one input jack only, and then control the level of each channel with a pot - just as if I was using a patch cable to jump the channels together in a full size amp. Will this input wiring work okay? Thanks in advance.

 

[Lowerleftcoast]

That should work.

It is your choice but you have the option to change the value of the 33k grid stoppers. They are there to keep RFI at bay. 15k will work.

The 270k mixing resistors are used a little differently than in other designs. These are screen resistors now. The volume pots are adding to this resistance as the volume is turned up. Typically these are 220k in other circuits. Again your choice. You have options.

 

[Bitsleftover]

By way of disclaimer….I am by no means claiming any expertise. My amp builds are simply a collection of other peoples ideas built by following the god advice of the likes of LLC, Rob and countless others. Having said that, the end product works well and achieves many of the goals you’ve set yourself.
Given that you are tuning the first gain stage (most susceptible to noise) for maximum gain, I’d begin trying to minimise that noise right at input. So I’d use shielded cable from the jack to a single point on my board. Then split the signal in 2 from there. If the board is close enough to the tube socket I’d mount the grid stoppers between the 2 as in my first doodle below. (I did it that way on my recent tweed Princeton build and it works well.
If you don’t like that idea, I’d mount the resistors on the board with a short cable up to the tube socket. (2nd doodle)
Both approaches minimise that vulnerable input signal’s exposure to interference.
Also, on the subject of noise.
I had a huge problem with my build due to noise when I turned the gain up high. (With 12ax7s and similar gain mods to the ones you’re planning) It was unusable and resulted in me tearing the whole thing apart bit by bit to track it down.
The solution was to elevate the CT as per the version Rob has on his amp mods page. I think I went the whole hog and fitted an elevated

humdinger pot which made it quiet as a mouse.
So much so that I now use the same approach in anything I tinker with.
Other people have built similar things without, but to me, the addition of one or two resistors or a hidden pot at the planning stage is well worth the peace of mind. And never hurts.
I really should take the back off my amp and have a look at what I actually settled on. There was so much trial and error that I’ve forgotten where I actually called it finished.
Anyway….Hope that helps. As I said, when in doubt, listen to the clever fellas here rather than me any day of the week!

 

[Lancer X]

Thanks so much, @Lowerleftcoast and @Bitsleftover! Really appreciate the review and suggestions!

 

[Lancer X]

It is your choice but you have the option to change the value of the 33k grid stoppers. They are there to keep RFI at bay. 15k will work.

The 270k mixing resistors are used a little differently than in other designs. These are screen resistors now. The volume pots are adding to this resistance as the volume is turned up. Typically these are 220k in other circuits. Again your choice. You have options.

So, the 33k grid stoppers and 270k mixing resistors are the original values from the 5F6A Bassman, as well as Mr. Rob's Micro version. Is there any specific reason you'd recommend altering those? My main goal is to replicate the Bassman tone when all the mods are defeated.

 

[Lancer X]

Given that you are tuning the first gain stage (most susceptible to noise) for maximum gain, I’d begin trying to minimise that noise right at input. So I’d use shielded cable from the jack to a single point on my board. Then split the signal in 2 from there. If the board is close enough to the tube socket I’d mount the grid stoppers between the 2 as in my first doodle below. (I did it that way on my recent tweed Princeton build and it works well.
If you don’t like that idea, I’d mount the resistors on the board with a short cable up to the tube socket. (2nd doodle)
Both approaches minimise that vulnerable input signal’s exposure to interference.

Will do, thanks! I know I have enough room on the board, because I had it wired the way you suggested initially (other than the shielded run). But looking at those diverging resistors, I was worried they would also act as a voltage divider and squash my input voltage.

I had a huge problem with my build due to noise when I turned the gain up high. (With 12ax7s and similar gain mods to the ones you’re planning) It was unusable and resulted in me tearing the whole thing apart bit by bit to track it down.
The solution was to elevate the CT as per the version Rob has on his amp mods page. I think I went the whole hog and fitted an elevated humdinger pot which made it quiet as a mouse.

Mr. Rob's Bassman Micro design actually has an artificial center tap which is elevated off the power tube cathode resistors, so I don't think heater AC hum should be a problem. I can always throw the humdinger on there later if that doesn't quite cut it.

 

[Lowerleftcoast]

Is there any specific reason you'd recommend altering those? My main goal is to replicate the Bassman tone when all the mods are defeated.

The way I look at circuits, everything is a choice and I question the choices made in the past.

I have always thought of the two 68k resistors on the input as an arbitrary choice by Fender so, I suggested a smaller value.
The 270k is not a critical value. The grid leak bleeds off the small amount of DC found at the control grid. Imo, chances are you will use both volume pots most of the time and the grid leak resistance will almost always be more than 270k. It is not a problem. It will work.

You have a reason to use the 5F6A values. You have made that choice. Don't let me dissuade you from chasing that dream.

 

[Bitsleftover]

Just to correct something I said in your other thread…
I did keep the V2A gain boost .68cap mod. I put it on the same switch as my Fender/Marshall tone stack mod so they both engage at the same time.
I just whipped the rear panel off to remind myself what I’d settled on.
I initially had a .0022 coupling cap on the Bright Channel but reverted back to .022 because it sounded too thin to me. Especially with the gain boost mod and given that mine is a 1x10 combo not a 4x12.
I used 470k mixing resistors with a 470pf treble peaker (or Marshall emphasis cap) on the bright channel mixer.
I put a 100pf bright cap on the normal Bassman channel vol pot and 220pf cap in the Marshall side amd got rid of the switches once I’d settled on what I like.
So if I plug into the Normal side I’ve got Bassman. Bright side I’ve got JTM with the option to switch JMP.

 

[2L man]

Mr. Rob's Bassman Micro design actually has an artificial center tap which is elevated off the power tube cathode resistors, so I don't think heater AC hum should be a problem. I can always throw the humdinger on there later if that doesn't quite cut it.

Those low power power tubes cathode voltage comes only few volts above the zero and that low filament elevate does not help much!

You could install three component circuit, two resistor voltage divider and an electrolyte where filament CT is connect. It also serve as a capacitor bleed circuit. Something like 390k dropper and 100k reference connected to 300V will elevate filaments to 60V and it is close to where the Cathode Follower cathode here comes which is most prone to filament leak based hum.

 

[sds1]

will elevate filaments to 60V and it is close to where the Cathode Follower cathode here comes which is most prone to filament leak based hum.

Could you elaborate here plz? I know heater elevation is potentially most beneficial when CF's are present in the circuit, but seems like you are not just arbitrarily elevating the heater voltage. You say there is a target elevation voltage for max benefits?

 

[King Fan]

Could you elaborate here plz? I know heater elevation is potentially most beneficial when CF's are present in the circuit, but seems like you are not just arbitrarily elevating the heater voltage. You say there is a target elevation for max benefits?

Good question; it's a subtle point. Merlin recommends an elevation voltage of 30-60V. As @2L man says, the cathodes on these micro output tubes only reach a few volts; ~4-5V on my EF80 Bassman Micro, for example.

Merlin also shows a possible solution here, to put a voltage divider on the HT to create the elevation voltage, and gives these parameters:

"The elevation voltage can be taken from a potential divider across the HT (it doesn't matter where you position the divider), and an elevation voltage around 30 to 60V is typical. The divider should have a fairly high resistance so as not to waste current, although the lower arm (R2) should not be excessively large or Rhk(max) may be grossly exceeded, so it is advisable not to make it greater than 100k. The elevation voltage should be decoupled/smoothed with an arbitrarily large capacitor (C1), say 10uF or more."

As @2L man also says, we can also make this divider a bleeder for the filter caps. I combined Merlin's guidelines with a little RC-time-constant calculation and came up with this approach -- on my 295V B+, this creates an elevation voltage of ~50V and bleeds all the filters in less than a minute. Other resistor values could be used, as per @2L man ; just do the voltage divider and RC math. Green/yellow goes to my artificial heater CT...

 

[sds1]

Merlin recommends an elevation voltage of 30-60V.

Yeah, actually he says that range is "typical", like it's an observation. Maybe the fact that he included it in his writings implies recommendation. Either way, I've always wanted to know more -- he skips over how that range was determined. Given what was said above, seeing if maybe @2L man can shed some additional light.

 

[King Fan]

Yeah, actually he says that range is "typical", like it's an observation. Maybe the fact that he included it in his writings implies recommendation. Either way, I've always wanted to know more -- he skips over how that range was determined. Given what was said above, seeing if maybe @2L man can shed some additional light.

Aha, I shoulda realized you were all over the concept, and yeah, good question about ‘how high’. I’ll look forward to insight from @2L man. As you’ll know, Merlin does mention elevation is required for CF tubes to avoid exceeding Vhk(max)… FWIW I found a useful discussion of that spec here, but I’m not smart enuf about CFs to apply it:

https://www.diyaudio.com/community/threads/vhk-max-rating.363084/

 

[2L man]

Yeah, actually he says that range is "typical", like it's an observation. Maybe the fact that he included it in his writings implies recommendation. Either way, I've always wanted to know more -- he skips over how that range was determined. Given what was said above, seeing if maybe @2L man can shed some additional light.

I have read technical articles which made sense but I can't explain it without looking them. I just follow that principle because I have read it is good principle.

Another benefit is Cathode Follower tube reliability against sudden death and increased operating hours when max cathode-filament voltage cap comes smaller. I recall 12AX7 max Vc-f is 160V. If filament is at zero volt and CF cathode idle point is 100V the signal peaks can approach the range which accelerate tube wear. Perhaps high gain amps are more prone to this.

Anyway it is so easy initially to make and components are cheap when compared to 12AX7 price and it tube it fail it might happen when you play to audience and dime the amp for final

 

[sds1]

Anyway it is so easy initially to make and components are cheap when compared to 12AX7 price and it tube it fail it might happen when you play to audience and dime the amp for final

Maybe this will keep me interested in heater elevation, I unfortunately haven't personally experienced any noise benefit from elevation in the several times I've implemented it.

 

[King Fan]

I also wondered if 4-5V was even enough to do its basic job of heater hum reduction (*if* your amps have any heater hum to reduce ). Merlin notes it does this by "reducing or saturating the leakage current between heater and cathode.*" (The asterisk says: "See: Cooper, C. E. (1944). Valve Hum. Electronic Engineering, (July), pp72-5.) -- hmmph, I misplaced my copy...).

But casting about for more info, I see our smart friend @tubeswell had a general (20V) and a more specific guideline on how high in this 2015 thread:

Depends why you want the heater elevation. As a guide, somewhere between 1/5 and 4/5 of h-k voltage rating of the tube(s) you are wanting to elevate the heaters of. Check the datasheets. Bear in mind most current production tubes have pretty unreliable h-k insulation, so keep the elevation lowish if you're just wanting to minimise hum from the filaments (you only need about 20V on pre-amp tubes for that).
If you're wanting to elevate heater voltage to relieve stress on a CF filament where you've got a highish (say) 170-180V cathode voltage, then elevate to about 1/2 way between the cathode voltage and ground.

Reading both this and the DIYaudio thread above, it's clear the risk of too big a heater-cathode voltage gap is it can stress the insulation between the two and damage the cathode. Now if only I understood how a CF works, I could figure out why the H-K gap there is bigger or more critical. Next year....

 

[Lowerleftcoast]

Could you elaborate here plz? I know heater elevation is potentially most beneficial when CF's are present in the circuit, but seems like you are not just arbitrarily elevating the heater voltage. You say there is a target elevation voltage for max benefits?

The 12A_7 heater element heats the cathode. It is in very close proximity to the cathode. The datasheet gives a Max rating for the difference in voltage. (It is kinda like a voltage rating for insulation on wire.) Datasheets for 12AX7 show 180v and 200 Vhk. 12AT7 show 90v. 5751 show 90v. 12AY7 show 90v.

Cathode followers and some phase inverters push those limits. Elevating the heater can ease the stress of running near or over the max Vhk saving the degradation of the heater insulation. As the insulation is compromised, the tube may start to hum, develop other noise, and/or have a shorter life.

As you can see the target elevation voltage, the designer chooses, may vary due to the type of tube. 30 volts is often mentioned as a target but 50 to 60volts may be desirable. The higher elevation voltages make a larger Vhk difference in the other preamp tubes so the designer should keep that in mind when making the choice.

 

[Lancer X]

Man, I am learning so much from you guys and from going through this planning process. This amp is going to be so much better for it - really appreciate it!

@King Fan I had originally intended to add your bleeder circuit, but forgot about it. Now that I know it's also a source of effective elevation DC, it's a no brainer. This PR board is gonna be crowded...

 

[jchabalk]

Nothing to add but i'm going to follow your build.

I built one of these a couple of years back and i love it. I have it driving a 2x12 cabinet, it sounds fantastic and i can play it whenever i want at volume. The only thing i'd change if i were to do it again is to leave out the effects loop. I made a concious decision at the time i was planning to leave it in and i don't use it at all. There's still plenty of space for it but it's just taking up space.

 

[King Fan]

That's interesting. Just for those planning builds in the future, I'll respectfully say my experience is the reverse -- FX loop is great for my reverb, delay, trem pedals. Not saying you're wrong, it totally depends on pedals, style, and taste, but for the effects I use the loop is excellent.