5F6A Ghost Notes & B+1 Filtering: 40uf @ 500v vs. 50uf @ 730v?

[gfarina55]

Hello again,

I built a 5f6a Bassman with plenty of help from this forum last year. I toured w/it this summer and it's perfect in every respect but one: There are ghost notes when I play OD leads, especially when I use the neck pickup. (Both with and w/out an OD pedal.) I’ve inquired about this here and elsewhere, and I understand it has something to do w/the light 5F6A power filtering and “comes with the territory” on some vintage-style amps. That said, I'm wondering if I can improve it by using BF-style filtering.

My B+1 filter stage currently consists of 2 x Xicon 20uf / 500V in parallel for 40uf @ 500v total. I want to try the 2 x 100uf caps in series with 220k balancing/bleeder resistors for 50uf @ 770v as seen here:

https://el34world.com/charts/instruc_reissuecaps.htm

I purchased the F&T (better quality than Xicon?) caps to try this, but before I do the invasive maneuver, I thought to ask here if anyone has experience with this Tweed-to-BF "upgrade"? Will +10uf and +230v of filtering help ghost notes or have other impacts? Am I barking up the right tree?

[Latole]

I purchased the F&T (better quality than Xicon?)
-gfarina55

IMO yes, but do we hear difference ?
I always buy F & T or Sprague.  I sometimes use Xicon ( easy to find in my local electronics store) on no name or low budget amps or for tests.

[sluckey]

I suggest using two 47µF@500V in parallel. No resistors or rewiring needed.

[WiderGates]

GZ34 has a maximum rating of 60µF after the rectifier.

[pdf64]

The 5F6A has an awful, hot switching standby, I suggest to avoid using it.
If you find standby useful, I suggest to modify it to a non hot switching type, eg https://el34world.com/charts/Schematics/files/Fender/Fender_bassman_aa165_schem.pdf

[sluckey]

GZ34 has a maximum rating of 60µF after the rectifier.

Oops! Overlooked that. Sorry.

[gfarina55]

I suggest using two 47µF@500V in parallel. No resistors or rewiring needed.

So the GZ34 won't handle this, correct? I might try the 50uf @ 770v and see if it makes a difference. Thx!

[Latole]

Where do you find a 50uf @ 770v ? I never saw that in DC
Any link ?

[Bieworm]

I would suggest trying Sluckey's idea , but temorarily snap in a SS rectifier. That way you could check if it's überhaupt the filter stage that's problematic...

[sluckey]

Where do you find a 50uf @ 770v ? I never saw that in DC
Any link ?

My B+1 filter stage currently consists of 2 x Xicon 20uf / 500V in parallel for 40uf @ 500v total. I want to try the 2 x 100uf caps in series with 220k balancing/bleeder resistors for 50uf @ 770v

[Latole]

Thank's ; it two caps not only one.

[pdf64]

Why not a 47uF 500V cap?
Or just swap one of the 22uF for a 33uF?

Is there a recording of the ghost note issue?

Are there any mods to the 5F6A circuit?

Regarding cap quality, my understanding is that the term gets used incorrectly, people seem to use it when a product’s specifications are being referred to.
A product’s quality refers to its conformance to specification.

Cap manufacturers generally have several or many product ranges within each category, with different product lines having different specifications.
Hence it seems wrong to lump all xicon etc caps together.
There will be cheaper product lines with a lower spec, and more expensive product lines with higher spec.
With modern production practices, all should be of high quality.

[gfarina55]

Why not a 47uF 500V cap?
Or just swap one of the 22uF for a 33uF?

Is there a recording of the ghost note issue?

Are there any mods to the 5F6A circuit?

1) I could, but the 50uf @ 770 700v w/the bleeder resistors setup is in the reissues (https://el34world.com/charts/instruc_reissuecaps.htm), and I also understand that some techs do this as an upgrade.

2) It sounds like an out-of-tune note pitched below the lead note that I'm playing when the amp is overdriven (starts around 7 on the volume pot). It's most noticeable on the neck p'up. It's difficult to record, but it's the same as the isolated Clapton Bluesbreaker tracks posted on YouTube.

3) It's Robrob's 5F6A: https://robrobinette.com/images/Guitar/Bassman/Bassman_Pre-Build_Mods.pdf3  Original build thread: https://el34world.com/Forum/index.php?topic=27769.msg307683#msg307683 I've altered the grounding scheme so that the B+1 filter stage is in the chassis close to the PT, among other grounding changes suggested by Slucky and others here. The amp is dead quiet. I think the only other difference is that I put a resistor across the standby switch instead of the cap to ground, safety diodes on the rectifier tube, and 1 ohm resistors to ground from pin 8 of the power tubes for biasing.

Regarding cap quality, my understanding is that the term gets used incorrectly, people seem to use it when a product’s specifications are being referred to.
A product’s quality refers to its conformance to specification.

Cap manufacturers generally have several or many product ranges within each category, with different product lines having different specifications.
Hence it seems wrong to lump all xicon etc caps together.
There will be cheaper product lines with a lower spec, and more expensive product lines with higher spec.
With modern production practices, all should be of high quality.

Interesting, thanks!

[sluckey]

1) I could, but I've seen the 50uf @ 770v

You keep saying 770v but the caps you have are most like rated for 100µF@350v. Two in series would give you 50µF@700v, not 770v.

[gfarina55]

I would suggest trying Sluckey's idea , but temorarily snap in a SS rectifier. That way you could check if it's überhaupt the filter stage that's problematic...

Great troubleshooting idea. Will my rectifier safety diodes (one across pins 4 & 5, one across pins 6 & 7) conflict with one of those plug in SS rectifiers? I've wanted to try one anyway just to see how it changes the feel of the amp.

[sluckey]

Great troubleshooting idea. Will my rectifier safety diodes (one across pins 4 & 5, one across pins 6 & 7) conflict with one of those plug in SS rectifiers? I've wanted to try one anyway just to see how it changes the feel of the amp.

No problem, but your B+ will be considerably higher.

If you just want to try SS rectifiers all you need is to add two more diodes to your safety diodes. Connect one diode between pin 4 and pin 8 (cathode on pin 8) and connect another diode between pin 6 and pin 8 (cathode on pin 8).

[PRR]

... it's the same as the isolated Clapton Bluesbreaker tracks ...

Am I missing something? If "it's same as Clapton", how can it be wrong?

There ARE sources of sour IM distortion in an overdriven Bassman (any amplifier). Until we build infinite-power amplifiers, that will always be true.

[gfarina55]

... it's the same as the isolated Clapton Bluesbreaker tracks ...

Am I missing something? If "it's same as Clapton", how can it be wrong?

There ARE sources of sour IM distortion in an overdriven Bassman (any amplifier). Until we build infinite-power amplifiers, that will always be true.

Right. As I said in my OP:

“I understand it has something to do w/the light 5F6A power filtering and ‘comes with the territory’ on some vintage-style amps. That said, I'm wondering if I can improve it by using BF-style filtering.“

My question is if this BF filtering setup might help, or if there are other ways to reduce intermodulation distortion w/out dramatically changing the character of the amp.

[HotBluePlates]

... As I said in my OP:

“I understand it has something to do w/the light 5F6A power filtering and ‘comes with the territory’ on some vintage-style amps. That said, I'm wondering if I can improve it by using BF-style filtering.“

My question is if this BF filtering setup might help, or if there are other ways to reduce intermodulation distortion w/out dramatically changing the character of the amp.

No, I do not believe "blackface filtering" is anywhere near enough to truly have an impact.

And No, I do not believe you can reduce the intermodulation with Power Supply Ripple and still maintain the character of the amp.


I've had extensive discussions about these topics with a player on another forum, who makes heavy use of a Fryette Power Station to record silently while cranking the snot out of his amps.  And he keeps objecting to his vintage-style amps having the sour IM when pushed hard into power-stage distortion.  Doesn't matter that the sour-stuff isn't heard so much in the context of a mix; he hears his guitar solo'd a lot in the DAW and dislikes the sound.

What we've found is pushing the power section hard saps the power supply's capability to keep AC ripple to a minimum, which is the hum-source for intermodulation with the notes played.

He has experimented with increasing power supply filter capacitance, after I repeatedly cautioned that it wouldn't make a difference.  Ultimately, he found you have to go as far as Soldano did in the SLO amps, with a solid-state rectifier, a couple-hundred µF in the main reservoir, and another 100µF in the screen filter node.  And the amp sounds super-stiff unless you crank the snot out of it.

Sag & Ghosting or Stiff & Clean are the options.  Most serious amp designers skip the whole problem, and move to generating distortion in the preamp one way or another.

[gfarina55]

... As I said in my OP:

“I understand it has something to do w/the light 5F6A power filtering and ‘comes with the territory’ on some vintage-style amps. That said, I'm wondering if I can improve it by using BF-style filtering.“

My question is if this BF filtering setup might help, or if there are other ways to reduce intermodulation distortion w/out dramatically changing the character of the amp.

No, I do not believe "blackface filtering" is anywhere near enough to truly have an impact.

And No, I do not believe you can reduce the intermodulation with Power Supply Ripple and still maintain the character of the amp.


I've had extensive discussions about these topics with a player on another forum, who makes heavy use of a Fryette Power Station to record silently while cranking the snot out of his amps.  And he keeps objecting to his vintage-style amps having the sour IM when pushed hard into power-stage distortion.  Doesn't matter that the sour-stuff isn't heard so much in the context of a mix; he hears his guitar solo'd a lot in the DAW and dislikes the sound.

What we've found is pushing the power section hard saps the power supply's capability to keep AC ripple to a minimum, which is the hum-source for intermodulation with the notes played.

He has experimented with increasing power supply filter capacitance, after I repeatedly cautioned that it wouldn't make a difference.  Ultimately, he found you have to go as far as Soldano did in the SLO amps, with a solid-state rectifier, a couple-hundred µF in the main reservoir, and another 100µF in the screen filter node.  And the amp sounds super-stiff unless you crank the snot out of it.

Sag & Ghosting or Stiff & Clean are the options.  Most serious amp designers skip the whole problem, and move to generating distortion in the preamp one way or another.

Fantastic, this is exactly what I've been trying to understand. Thanks for this detailed and direct answer. But if "Sag & Ghosting or Stiff & Clean are the options", why does my 5E3 have no IM distortion w/both volumes on 12? Is it b/c a 5E3 is cathode biased? I've been chasing a 5E3 tone and feel (cranked for distortion w/no pedals) in an louder amp that works in a 1000+ capacity venue. High gain amps like the SLO don't really work for my classic rock/blues style, but I was toying with the idea of building a Trainwreck Express. Are there better choices for a simple, loud amp that I can crank for power amp distortion w/out pedals, or should I look towards a design w/more preamp gain stages?

[sluckey]

I've been chasing a 5E3 tone and feel (cranked for distortion w/no pedals) in an loud amp that works in a 1000+ capacity venue.

So just use your 5E3 for your tone. Mic it into any high-powered clean PA, or even your own Twin Reverb to produce the desired loudness.

[gfarina55]

I've been chasing a 5E3 tone and feel (cranked for distortion w/no pedals) in an loud amp that works in a 1000+ capacity venue.

So just use your 5E3 for your tone. Mic it into any high-powered clean PA, or even your own Twin Reverb to produce the desired loudness.

I've done many gigs like that, also w/a Champ or an old single-ended Masco. I have every flavor of OD pedal that I can run into a rented BF Twin and have everything I need (particularly now with Kingsley pedals), but I love the idea and the challenge of doing a rock set with one guitar and a loud overdriven amp. I often use a Matchless C-30 or Clubman for this and they work great. I guess the 5F6A is simply not the right amp for this, and I'm really just pursuing this for fun and for the learning experience.

[HotBluePlates]

... if "Sag & Ghosting or Stiff & Clean are the options", why does my 5E3 have no IM distortion w/both volumes on 12? ...

Filter caps are "buckets of charge" (or "current waiting to move").  When those buckets are full, our power supply is perfectly-clean DC.  When those buckets are depleted, the AC Ripple from the rectifier is a greater % of Total Voltage in the power supply.

6V6s suck small current; 6L6s suck big current (when each is making its max output power).  The filter capacitance of the 5F6-A Bassman is larger than that of the 5E3 Deluxe, but probably not in proportion to the bigger 6L6 power section's ability to "drain the filter buckets."  So playing the Bassman at high power is more likely to result in a larger AC Ripple.

The guy I'm talking about mostly gripes about his vintage-style Marshall & Vox amps.  The modern amps he plays mostly have much stouter power sections (and he doesn't complain about IM with those), but he bemoans the "lack of vintage feel."  It's a hard trade-off, or you go elsewhere looking for your dirt.



You may already know this, but intermodulation distortion ("IM") is not just when ripple hum gets mixed with audio & makes odd subharmonics.  It's just that is a particularly-noticeable form of IM distortion.  Your Deluxe absolutely has IM distortion happening when it is cranked, but perhaps mostly between various notes played (and their harmonics).

... Are there better choices for a simple, loud amp that I can crank for power amp distortion w/out pedals, or should I look towards a design w/more preamp gain stages?

I don't own any master-volume amps, but I'm also not afraid to use pedals for distortion.  Outside of a very narrow range of volume, power section distortion made be overrated or not usable (due to the hum-IM, if that matters to you).

Jim Kelley occasionally makes an appearance on another forum, and his latest amps mimic the behavior of a distorting power section with a special preamp stage, which gets the dirt at any volume & sidesteps the issue of hum-IM.  That's because the player generally isn't pulling big current from the power section, and never experiences an increase of ripple AC, so there's no hum-IM.

So it's not that power section distortion or preamp distortion is better/worse, but that they both have limitations & trouble areas.  And the #1 trouble area for a lightly-filtered vintage-style non-master-volume amp is that it sounds forgiving & problem-free while clean, but could have a lot of hum-IM when driven to distortion; distortion is also at/above a single loudness level.  Preamp distortion circuits have their own trouble areas, and some designers handle these well while others do not. 

It is wise to remain open to other approaches of circuit design that mainly evolved to work around the problems as they were discovered through the decades.

[gfarina55]

Thanks HotBluePlates! This is quite a helpful explanation, and I never thought about the difference btwn. power tubes and how that might impact IM distortion. I've always thought my amps that have some power tube distortion (Matchless, 5E3, BFDR, etc.) were the most "real" or 3d when overdriven compared to pedals, so that's the road I've gone down, but since I've started building a few vintage replicas I can see the trade off. In any case, my Bassman sounds absolutely fantastic, so I'm going to let it be what it is.

[pdf64]

Consider trying a 4ohm cab. That should halve the peak anode current, so reduce HT ripple and hence reduce ghosting. Consider that JTM45 use a 6k6 or 8k OT (compared to the 4k of the 5F6A).
The 470 ohm screen grid resistors may need uprating from 1W to 2W or 3W, maybe increase the value to 1k.
Ht fusing is a great idea, especially as you're cranking the power amp.

[gfarina55]

Consider trying a 4ohm cab. That should halve the peak anode current, so reduce HT ripple and hence reduce ghosting. Consider that JTM45 use a 6k6 or 8k OT (compared to the 4k of the 5F6A).
The 470 ohm screen grid resistors may need uprating from 1W to 2W or 3W, maybe increase the value to 1k.
Ht fusing is a great idea, especially as you're cranking the power amp.

Unfortunately my OT just has the 2 ohm tap, but I'll try the screen resistors. Thx!

[pdf64]

Sorry, doubling the intended load impedance is how my suggestion works.
So a 4 ohm load on the 2ohm output is the idea.
Doing that will lose a dB or 2 of power output, so all else being equal, its response may seem a fraction less punchy.

The use of slightly higher sensitivity speakers could compensate for that.

eg kinda pricey, but Matt Schofield got an incredible sound out of his Super Reverb loaded with these https://celestion.com/product/celestion-g10-gold/

[gfarina55]

Consider trying a 4ohm cab. That should halve the peak anode current, so reduce HT ripple and hence reduce ghosting. Consider that JTM45 use a 6k6 or 8k OT (compared to the 4k of the 5F6A).
The 470 ohm screen grid resistors may need uprating from 1W to 2W or 3W, maybe increase the value to 1k.
Ht fusing is a great idea, especially as you're cranking the power amp.

My 470 ohm screen grid resistors are already 3w but I'll up them to 1K. A suggestion from another forum was "drop coupler capitance between PI outs and power tube grids". Is this the .1uf coupling cap in the PI section of the board? (The one parallel w/the neg. feedback resistor)? Could this help, and could a different (balanced?) PI tube help?

Thx for the speaker info...I had a vintage-style tweed combo cab built for this and purchased 4 Webber alnico's after much research, so I don't think I'll change those at this point, but thanks for the tip!

Also, today I figured out that the 50uf/700v setup that I describe in my first post comes from Gerald Weber's second book. He writes that 500v is not enough voltage for the plates on a Tweed Bassman, and suggests this 50v/700uf setup for a recap of vintage Tweed Bassmans. I know there's a lot of disputed info from his books, but if this is the case, perhaps I fried my plates filter caps w/too much voltage, hence more ghosting? I might try the 50uf/700v setup just to see if that's the case.

[pdf64]

My 470 ohm screen grid resistors are already 3w but I'll up them to 1K.

There’s not much point unless moving to the high load impedance.

A suggestion from another forum was "drop coupler capitance between PI outs and power tube grids". Is this the .1uf coupling cap in the PI section of the board?
 (The one parallel w/the neg. feedback resistor)? Could this help.

Possibly but no more so than just turning the bass down a tad. Rather it’s more to deal with a different issue (blocking distortion) which you haven’t described / mentioned.

could a different (balanced?) PI tube help?

Almost certainly not.
The long tail pair circuit of V3 just really needs both sections of the valve in it to be functional, it’s the circuit itself that does the balancing bit.
The balanced valve thing for this application seems like BS for the golden ears fraternity to me.

So provided the valve is good, a balanced valve offers no benefit.

Thx for the speaker info...I had a vintage-style tweed combo cab built for this and purchased 4 Webber alnico's after much research, so I don't think I'll change those at this point, but thanks for the tip!

The point was that if you disconnect one or 2 speakers, the load impedance will go up, but the total sensitivity and power handling will go down. Hence suggesting the Gold, as they have good sensitivity and power handling. Sorry I didn’t make that clear.

Also, today I figured out that the 50uf/700v setup that I describe in my first post comes from Gerald Weber's second book. He writes that 500v is not enough voltage for the plates on a Tweed Bassman, and suggests this 50v/700uf setup for a recap of vintage Tweed Bassmans. I know there's a lot of disputed info from his books, but if this is the case, perhaps I fried my plates filter caps w/too much voltage, hence more ghosting? I might try the 50uf/700v setup just to see if that's the case.

Rather than speculating and stressing about that, why not measure your amp’s HT voltage, and the AC ripple voltage on it, at idle and when putting out full power?

Regarding the HT fuse suggestion, see the far right side of https://el34world.com/charts/Schematics/files/Marshall/Marshall_1964_jmp_lead_and_bass.gif
Edit: Fixed quotes. (?) Willabe

[gfarina55]

Rather than speculating and stressing about that, why not measure your amp’s HT voltage, and the AC ripple voltage on it, at idle and when putting out full power?

Thx for the info PDF64. I'll try disconnecting the speaker setup even tho I want to keep the 4 Webers. To measure AC ripple, do I just measure the AC across a filter cap node and compare it to the HT voltage?