S.O.B. Limit control

[zjdrummond]

I have been researching old boogie amps. Kind of like the idea of grabbing one for that loud, mean, clean thing that they can do. Came across the S.O.B. and wondered about the limit control. What was that about? Is it an actual limiter dynamics processor, or just an attenuator aimed at reducing output level to keep your super distorted sounds from blowing your eardrums out?

 

[bgh]

I bought an S.O.B. back when they first came out. So, my memory may not be all that well.

The limit control was kind of weird. I left it on 0 most of the time. If I recall, it's purpose was to starve the PI (Phase-Inverter). At 0, it doesn't do anything. As you turn it up, it starves the PI more and more - kind of like a "reverse gain".

I don't think I ever got it sound good with the limit value off of 0.

Hope this helps.

 

[redrabit1]

No technical data here, but as a previous owner I can agree that nothing useful came from the LIMIT setting. I would state that the later version with a PRESENCE instead was more musical to me.

 

[woodbutcher65]

I have an SOB. The limiter was useless. It was based on a Triac. Yank that out, bypass it, or replace it with a post phase inverter master volume control. (There's already a master on it, too. But a PPIMV is a better type so maybe bypass the first master.)

There's a .003 cap in the preamp that makes the SOB very harsh. Remove it. You'll never want to put it back, believe me. It's located near the master volume pot, bypassing a 220K resistor near the plate of V2B.

I modded mine by changing around some capacitor values so it now sounds much better than it used to. Mine is now voiced like a Marshall JTM45 but with more gain and there's a pull gain boost as well. The thing is a real terror now.

 

[dexter.sinister]

Better late than never…but…

IIRC, the “Limit” pot introduces variable resistance in series with a FET acting as a diode that clamps the current flowing through the driver/PI cathodes. As the resistance rises, it swamps the effect of the FET and starves the driver/PI so that it becomes overdriven by the preamp at lower voltages, for an effect similar in concept to a master volume.

My S.O.B. has the Limit pot labeled “Presence” but others indicate it is a “Limit”.

I love that amp. I run it turned up a good bit with a couple pedals up front (wah, phaser, drive, compressor), then add a Fryette PS1 attenuator to dial the overall volume to the environment, using the effects loop of the attenuator to introduce some volume pedal controlled delay and verb. I don’t use the Limit pot…

 

[mark1406]

I have an SOB. The limiter was useless. It was based on a Triac. Yank that out, bypass it, or replace it with a post phase inverter master volume control. (There's already a master on it, too. But a PPIMV is a better type so maybe bypass the first master.)

There's a .003 cap in the preamp that makes the SOB very harsh. Remove it. You'll never want to put it back, believe me. It's located near the master volume pot, bypassing a 220K resistor near the plate of V2B.

I modded mine by changing around some capacitor values so it now sounds much better than it used to. Mine is now voiced like a Marshall JTM45 but with more gain and there's a pull gain boost as well. The thing is a real terror now.

Agreed, the Triac does lower the volume but it comes with some other nasty side effects.

SOB circuit diagram

There is no negative feedback so the bass isn’t tight, the P.I. Is easily loaded down for some reason. I think that’s the reason for the 330K grid leak resistors. The circuit is a bit different and there is a resistive network to reduce the signal prior to the master and P.I. stage.

I put a Mark 1 P.I. stage in and a post master volume.

You state you pulled the 0.003uF cap out, what did you replace it with?

Regards

Mark

 

[dexter.sinister]

The TCR 5305 in the schematic is the current limiting diode I mentioned. In my amp, “Presence” was wired as “Limit” as I mentioned, with a potentiometer wired in series with the diode.

A service hack that Mesa is said to have endorsed is swapping the diode for a 1k5 resistor on the cathode of the driver/PI. In my case this frees up the pot to be actually used for the “Presence” control.

See here: MK1,SOB,MK2 Constant current Diodes

 

[mark1406]

The TCR 5305 in the schematic is the current limiting diode I mentioned. In my amp, “Presence” was wired as “Limit” as I mentioned, with a potentiometer wired in series with the diode.

A service hack that Mesa is said to have endorsed is swapping the diode for a 1k5 resistor on the cathode of the driver/PI. In my case this frees up the pot to be actually used for the “Presence” control.

See here: MK1,SOB,MK2 Constant current Diodes

I don’t follow what you are saying. I understand the bit where you say replace the TCR5305 with a 1.5K resistor, but the bit about freeing up the pot doesn’t sound right. Will the AC content be on the cathode?
If it were then the grid attached to the pot would have to be grounded.

TAG thread on TCR5305

Regards

Mark

 

[dexter.sinister]

The pot on the limit control is a variable resistance. As the pot adds resistance, the current limiter is made less relevant. If the diode and pot are replaced by a fixed resistance, the pot can by used for another function, such as presence.

 

[woodbutcher65]

If you want to preserve the functionality of the limiter, but in a better way, without adding a second master volume control, there is a way to do it. I'm a bit rusty on the details but I clearly remember repairing a VHT combo amp that had a variable power circuit in it which worked extremely well and it was simple. It worked by varying the plate voltage to the power tubes.

 

[mark1406]

The pot on the limit control is a variable resistance. As the pot adds resistance, the current limiter is made less relevant. If the diode and pot are replaced by a fixed resistance, the pot can by used for another function, such as presence.

Okay, I understand that the Limit control is a variable resistor, but more importantly the pot is attached to the secondary winding of the output transformer. This surely must be important.

Why does adding resistance make the current limiter less relevant?

I would think the pot is introducing negative feedback which is cancelling out the original signal.

 

[dexter.sinister]

The TCR508 FET is acting as a diode in this implementation. It is there to provide a constant current source through the nominal range of the device. The Limit is a pot in series which reduces the regulated current to starve the tube.

Not uncommon to use a FET with two legs joined for this very purpose in HiFi, except without the variable resistance as the SOB is not meant to be HiFi and the Limit allows some semblance of cranked amp tones at lower output. There are better ways to do this. The Mojave amps use a similar to SOB limit in the Scorpion and Peacemaker.

 

[woodbutcher65]

I remember now, it was the power scaling circuit in the VHT 12/20. Pretty easy to implement. I'll probably put it in my SOB.

 

[mark1406]

The TCR508 FET is acting as a diode in this implementation. It is there to provide a constant current source through the nominal range of the device. The Limit is a pot in series which reduces the regulated current to starve the tube.

The signal from the secondary of the output transformer must surely be out of phase with the input signal to reduce the output of the P.I. stage?

I say this as the pot goes to the grid of the other triode in the P. I. stage.

 

[dexter.sinister]

The signal from the secondary of the output transformer must surely be out of phase with the input signal to reduce the output of the P.I. stage?

I say this as the pot goes to the grid of the other triode in the P. I. stage.

Not in mine.

 

[mark1406]

Not in mine.

So what is the circuit in you amp?

Regards

Mark

 

[mark1406]

The other SOB and more common I suspect circuit. I have included the circuit as it’s useful and links disappear from the internet.

Regards

Mark