5F6A Bassman total custom build - super low output

[tomgilmartin]

Just want to verify this. The feedback connection point is not ground. I hope I'm misreading and you mean the ground connection of the presence pot.

Because the path for the feedback current is from the speaker, I used a twisted pair back from the speaker terminals, but there is no ground connection to chassis at the speaker terminals. You are exactly right, the negative side of the speakers is grounded through the feedback negative at the presence pot negative side. You can't see it in the pics because it's behind the orange drop capacitor. Those 0.1uF orange drop caps are huge!

If you look at the schematics you will see the careful connection of ground path from input to output. There are NO internal ground loops at all. There is only one chassis connection, right at the plug cord input. If external effects are used they can make a loop through the wall power cord ground.

[Chuck H]

I hope I'm wrong and you don't experience squealing or odd overtones at higher gain or treble settings. Honestly, if you report back and say there are no problems I'd be happy for that.

[tomgilmartin]

I hope I'm wrong and you don't experience squealing or odd overtones at higher gain or treble settings. Honestly, if you report back and say there are no problems I'd be happy for that.

Thanks Chuck. It has a funny sort of sizzle at high treble. Still have the bias super cool. I'm not really blown away with the "chime" I was hoping for.

[Chuck H]

Well building an amp is just the first part It's an unconventional build and now you've got to dial it in. The sizzle could be crossover distortion as you're clearly guessing. Easy enough to start there. The chime could have a lot to do with your speakers. 4-10" Jensens are one thing, What you've built is another. Certainly the speakers you built will do some things better than the 4x10 cabinet, but maybe one of them isn't "chime".

[tomgilmartin]

At least it's making some noise now. Set the bias to -48V, sounds better than -53V, just a little more hum. I've been playing through it, some nice clean tone. I like bass and treble all the way down, mid all the way up, with neck/mid single coils. Unplugged the guitar and cranked volume to check the hum. SQUEAL! Yikes. I think I may need to reverse the OT secondary leads. Screws up my color scheme, now white to orange and orange to green (grounded side). Dammit. Thanks for all your help.

[tomgilmartin]

Do you like dovetails? There are 48 of them here on the back of the speaker cab.

[g1]

Don't let the drummer see those.

[tomgilmartin]

Forgot to note, when I received the speakers, I couldn't believe how heavy they were so I put one on the scale.


Speaker spec if anyone cares - www.aespeakers.com

The Dipole10 woofers are designed specifically for open baffle/dipole use. In open baffle applications, drivers are required to operate at much higher excursions than in typical boxed applications to compensate for the rolloff of the baffle. In applications where woofers are continually called upon to operate at higher excursions, Bl and Le linearity are extremely critical. To achieve this linearity, the Dipole10 woofers were designed with a large underhung motor and Full Copper Faraday Sleeve covering the entire pole. Full details on what makes this motor design different can be found here: The Lambda 001 Motor Design. This combination utilizing the Faraday ring and underhung motor provides the most possible linearity and low distortion during high excursion use. The high Q and low Fs allow the Dipole10 woofers to be very efficient at low frequencies and have very good low end extension. While EQ is typically needed to correct for the baffle rolloff itself, no additional EQ is required to flatten the response of the driver.

As with all of the TD drivers, the Dipole woofers have extremely low and linear inductance. The ideal Qts and soft suspension means the Dipole woofers are very efficient at low frequencies and can be pushed to high excursions with very little power required. The low power requirements and non-reactive load make the Dipole woofers an ideal match for use with tube amplifiers as well. In addition, they have some of the best midrange performance of any woofers Acoustic Elegance offers, extending up to 2KHz or higher. They are available in dual 8ohm and dual 16ohm options for a variety of wiring configurations.

Dipole10-D8
Fs: 25.9 Hz
Qms: 11
Vas: 118 L
Cms: 0.7 mm/N
Mms: 54.6 g
Rms: 0.81 kg/S
Xmax: 12 mm(peak)
Xmech: 18 mm(peak)
Sd: 345 sqcm
Vd: .82L (p-p)
Qes: .73
Re: 3.08 ohm (parallel)
Le: 0.04 mH (parallel)
Z: 4 ohm (parallel)
Bl: 6.1 T/m
Pe: 100W (cont.)
Qts: 0.68
1WSPL: 86.5 dB
2.83V: 90.6 dB

[Chuck H]

That is really special. Nothing I even have tools for. Nice.

You can just disconnect the feedback loop and see if the amp gets quieter or louder. That will tell you right away if you have a positive feedback or a negative feedback and maybe save you the trouble of going after the OT/tube pin connections.

[Chuck H]

Don't let the drummer see those.

Unless you might like to make drums!?! Some guys get really exited about special snare drums. A little thinner stock and you might have 4 $1000 hoops in one of those tubes.

[tomgilmartin]

I had a theme all along. A tube amp with tubes inside tubes. Now I have to drill about a million holes in the aluminum tubes and then polish them like chrome. Or maybe have them anodized blue like the backing plates for the vacuum tubes.

My son has a lovely drum kit. Maybe I will make some shells. People like laminate for that usually though.

[g1]

Actually I thought a drummer might just stand them on end and think 'these speakers will make great skins!'

[tomgilmartin]

Disconnected feedback, not much difference really. Squeals like mad with normal vol at about 8. Oscillates with bright at about the same point, whup-whup-whup.

[The Dude]

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.

[Chuck H]

Disconnected feedback, not much difference really. Squeals like mad with normal vol at about 8. Oscillates with bright at about the same point, whup-whup-whup.

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.