5F6A Bassman total custom build - super low output

[tomgilmartin]

I don't mean to criticize your choices, but you might temporarily try your amp with some different speakers. IMO, the speakers you have chosen are intended to be a woofer in a hi-fi system built with a tweeter or even 3 way system. They are going to be much more responsive at low frequencies and while "they have some of the best mid-range performance of any woofers", the key to that statement is "of any woofers". Most guitar speakers don't have foam or rubber surrounds and are not high excursion speakers. They are a more of mid-range device. They don't do sub frequencies well. They are also less likely to have that "funny sizzle" on the top end. Being woofers, the speakers you have aren't really designed for high frequency. I'm not saying those are crappy speakers. I'm just saying that they don't really fit this application, which is probably why you like "bass and treble all the way down, mid all the way up". The right speakers might even give you that "chime" you're looking for.

Good stuff, I will have to see if I can get some other speakers. What do you recommend? I have been doing hi-fi speaker design for years, would love to compare my specs with some Jensen blues or something. I don't usually lay down money until I know what I'm buying.

When I was testing the woofers I described them as "floppy". At one point I had them sitting face-up on a towel with no enclosure and wired out of phase with each other. With a 27hz test signal through my studio amp it looked like waves were going across the speakers. With a guitar into them, striking the strings percussively made incredibly cool sounds and the speakers were trying to jump off the table.

[tomgilmartin]

Well the amp did get louder or quieter at a given gain setting when you disconnected the feedback loop. Not much different is fine, but you do need to determine what that difference is. If it got louder then your OT is wired correctly.

The squeal is likely a lead proximity issue. The whup is more likely to be a ground scheme or filtering issue. "I" might try moving the main ground point to the other end of the scheme, near the input. The chassis earth is just a safety ground. All other grounds are circuit grounds and should ideally go in the other direction, as it were. So try disconnecting the main ground point from the power amp end of the circuit and make it so all grounds lead to the input end. I would also double and triple check to be sure all the filters are wired correctly. The preamp filters are actually there to decouple signal AC from the HV rail. If anything is wired cattywhompus that can't happen and you can actually get cross talk on the supply rail. A common cause of oscillation is some models when filters fail.

Since the squeal is happening fairly high on the control settings it might be easiest to manage it with small peripheral circuits rather than rerouting all your zip tied, twisted pair goodness I might start with adding 4.7k grid stops to the power tube sockets. Some mojo minded "aficionados" think this is a tonal detriment, but it's not. I double dog dare them to tell the difference. The grid stops can be of great benefit to circuit stability. They should be placed right on the tube socket with virtually no resistor lead between the resistor body and the tube pin.

With you on all points. I have to understand the reasons, not because I think it's wrong, just because I am obsessive. :-) Thanks again.

[Chuck H]

With you on all points. I have to understand the reasons, not because I think it's wrong, just because I am obsessive. :-) Thanks again.

The reason to locate the ground nearer the input has never been obvious. Not to me anyway. It was explained to me here some time ago that ground, WRT the safety, would be earth. That's your chassis earth at the AC mains plug. But ground WRT to signal would be a 0V reference point. Since even small resistance in ground leads can allow for signal interaction it works out that tracing toward the higher current circuits forces the sensitive preamp to encounter the greatest resistance getting to 0V prior to interaction with those higher currents and is therefor more prone to interact. Flip that and it is the higher current, later amplifier stages that experience the greatest resistance while the input and preamp are relatively unimpeded to their 0V reference and less likely to interact. But not only am I not an engineer, I'm not an author or a teacher. So I hope my explanation makes some sense. I had a hell of a time getting my head wrapped around this on the premise that the AC ground connection was as close to 0V as could be "seen" by the circuit. But this isn't so because the circuit prefers that it's ground favor the input end. So even if that input end isn't as exactly 0V as the AC mains ground it won't matter significantly for circuit function as long as the all paths lead that direction. I break this ideal all the time actually because I'm still not comfortable connecting my high current grounds to the input of an amplifier. So I use two (or sometimes three) ground points to chassis. One for the high current power amp and power supply end of the circuit at that end of the chassis and another for low current signal circuits at the input end of the chassis. Which is earth, yes. This is my own misconception because as I said, our goal for the circuit isn't "ground" per se', but a 0V operational reference point.

The whop whop is a low frequency oscillation. Like a tremolo oscillator circuit. Like any oscillator circuit it is caused by positive feedback. It just happens that in a guitar amp circuit environment this is most likely to happen in the ground scheme or HV rail interactions due to impedance/frequency relationships (short of writing a book here on the matter).

The squeal, a higher frequency oscillation, is more easily manifested in the higher impedance environment of the signal chain and is typically, but not always, caused by radiant field interaction between signal leads. Although sometimes component location and/or ground scheme and HV rail interactions can also cause them. Since we don't have a scope I think we have to hedge our efforts toward the most common likelihood of lead length and proximity.

Ok... Going for the aspirin now.

[potatofarmer]

Merlin has a chapter in his preamp book about grounding and goes into detail about the ground-to-chassis connections. You can read the grounding chapter here: The Valve Wizard

[tomgilmartin]

The reason to locate the ground nearer the input has never been obvious. Not to me anyway. It was explained to me here some time ago that ground, WRT the safety, would be earth. That's your chassis earth at the AC mains plug. But ground WRT to signal would be a 0V reference point. Since even small resistance in ground leads can allow for signal interaction it works out that tracing toward the higher current circuits forces the sensitive preamp to encounter the greatest resistance getting to 0V prior to interaction with those higher currents and is therefor more prone to interact. Flip that and it is the higher current, later amplifier stages that experience the greatest resistance while the input and preamp are relatively unimpeded to their 0V reference and less likely to interact. But not only am I not an engineer, I'm not an author or a teacher. So I hope my explanation makes some sense. I had a hell of a time getting my head wrapped around this on the premise that the AC ground connection was as close to 0V as could be "seen" by the circuit. But this isn't so because the circuit prefers that it's ground favor the input end. So even if that input end isn't as exactly 0V as the AC mains ground it won't matter significantly for circuit function as long as the all paths lead that direction. I break this ideal all the time actually because I'm still not comfortable connecting my high current grounds to the input of an amplifier. So I use two (or sometimes three) ground points to chassis. One for the high current power amp and power supply end of the circuit at that end of the chassis and another for low current signal circuits at the input end of the chassis. Which is earth, yes. This is my own misconception because as I said, our goal for the circuit isn't "ground" per se', but a 0V operational reference point.

The whop whop is a low frequency oscillation. Like a tremolo oscillator circuit. Like any oscillator circuit it is caused by positive feedback. It just happens that in a guitar amp circuit environment this is most likely to happen in the ground scheme or HV rail interactions due to impedance/frequency relationships (short of writing a book here on the matter).

The squeal, a higher frequency oscillation, is more easily manifested in the higher impedance environment of the signal chain and is typically, but not always, caused by radiant field interaction between signal leads. Although sometimes component location and/or ground scheme and HV rail interactions can also cause them. Since we don't have a scope I think we have to hedge our efforts toward the most common likelihood of lead length and proximity.

Ok... Going for the aspirin now.

Incredible diatribe Chuck!!

I read the reason to connect to chassis at the inputs is because when FX pedals are connected there with power supplies, the ground loop created will not include the low-level signal ground in its current path. Great article as I recall but can't seem to find it again. I will keep looking.

So based on what you have said, I'm going to separate some wires. I have a copy of the Bassman re-issue circuit which gives typical AC signal voltages throughout the amp.

The inputs from the guitar to V1 show signal voltage as 0.00385VAC typical, while the outputs of V1 show as 0.213V typical, implying a gain of 55. These could couple and create oscillation as suggested, notwithstanding any twisted-pair goodness.

In and out of the CF tube we have 0.213V and around 5V, again a gain around 50. These could also couple and create oscillation as suggested.

Cables in the same AC voltage range (say *10) will have minimal coupling.

[Enzo]

The wall wart type power supplies usually used for pedals have no ground/earth connection and so don;t create ground loops.

[tomgilmartin]

The wall wart type power supplies usually used for pedals have no ground/earth connection and so don;t create ground loops.

^^^^^^^^
Good point. Conceded.

[tomgilmartin]

For those interested parties - Comparison AE Dipole 10 to Jensen Vintage Alnico
Not exactly sure the magnet weight on the AE, took a guess. It's probably more because the spider is aluminum and weighs nothing.

[eschertron]

I see the resonant freq (fs) is much lower than on any guitar speaker. What I didn't see on their website is a graph to show the speaker's loudness over a frequency range. Typical guitar speakers have a peak around 2k. I'll wager the woofer does not.

Also, I'm not a speaker enclosure expert, but doesn't the relatively low Q indicate the need for a closed and/or ported box to tune it up?

[tomgilmartin]

I see the resonant freq (fs) is much lower than on any guitar speaker. What I didn't see on their website is a graph to show the speaker's loudness over a frequency range. Typical guitar speakers have a peak around 2k. I'll wager the woofer does not.

Also, I'm not a speaker enclosure expert, but doesn't the relatively low Q indicate the need for a closed and/or ported box to tune it up?

I think the low Q and high excursion are intended to allow them to achieve loudness in an open baffle with no speaker box loading to assist. That's why I chose them over the TD series. It is counter-intuitive because they are so "floppy" one would think loading them with a sealed box would be better.

[Leo_Gnardo]

I see the resonant freq (fs) is much lower than on any guitar speaker. What I didn't see on their website is a graph to show the speaker's loudness over a frequency range. Typical guitar speakers have a peak around 2k. I'll wager the woofer does not.

The stats regarding efficiency aren't too impressive either. Maybe OK for a hi fi woofer where you have 100+ watts to drive it. Most guitar speakers fall into the 95 to 103 dB/Watt range.

It's a good idea to keep ears open, when you hear something outstanding coming out of somebody's guitar rig in a live setting, find out what it is & put it on your tentative shopping list. "In a live setting" is important, on recordings you have no idea what was done to the signal once it came out of the speaker as far as mic, preamp, compression/limiting, eq etc. Half the people I give this advice to land on Celestion Vintage 30, which is a fine guitar speaker. But it's definitely not for everybody. So listen, audition & pick one you like. Or more - nobody says you have to use multiples of the same speaker.

[tomgilmartin]

The stats regarding efficiency aren't too impressive either. Maybe OK for a hi fi woofer where you have 100+ watts to drive it. Most guitar speakers fall into the 95 to 103 dB/Watt range.

It's a good idea to keep ears open, when you hear something outstanding coming out of somebody's guitar rig in a live setting, find out what it is & put it on your tentative shopping list. "In a live setting" is important, on recordings you have no idea what was done to the signal once it came out of the speaker as far as mic, preamp, compression/limiting, eq etc. Half the people I give this advice to land on Celestion Vintage 30, which is a fine guitar speaker. But it's definitely not for everybody. So listen, audition & pick one you like. Or more - nobody says you have to use multiples of the same speaker.

Yeah, once I saw a show and the guy had a 1x12 tweed that sounded fantastic. So many people play giant loud rigs in tiny venues. I don' t play out, so efficiency and loudness are not really important to me. That's why I have the big soak resistor in there. Even with that, this thing is blowing my head off at volume 4.

[The Dude]

It's difficult to recommend a speaker. Everyone has different tastes and a speaker that sounds great with one amp may not sound so good in another. That said, I just put one of these in a little Fender amp I worked on for a customer. Both he and I were very happy with how it sounded.

https://www.ebay.com/itm/Celestion-T...QAAOSwp-RaX-hc

[Chuck H]

Ok, this ones for Tom (just good natured ribbing). Speculating on the inspiration for his design:

[tomgilmartin]

Ok, this ones for Tom (just good natured ribbing). Speculating on the inspiration for his design:

LOL!