SWR Interstellar Overdrive (Bass preamp) Drive issue

[BramP84]

Hi everyone, so my bandmate is a bassplayer (he can't help it) and asked me to check his preamp out because it didn't sound (and work) as good as it used to. I have some experience building guitar pedals, tube amplifiers and recently finished an LA2A and Pultec build, so I know how to solder and troubleshoot a bit, but the issue I encounter with this preamp leaves me a bit puzzled:

I put in some new tubes and biased the thing properly, clean tones sound great, but when using the drive knob, it does a whole bunch of nothing for about 85% of the taper and then the last 15% has all the distortion you would ever want and more.

Luckily there's a schematic floating around so I've been checking all the components but I'm not sure what gives, my suspicion originally was with that vactrol VTL5C1 in the drive circuit, but it doesn't seem to be broken (but perhaps out of spec?)

I unsoldered the photoresistor side of the vactrol and replaced it with a 100K pot , this gives a perfectly useable taper for the drive, 150K would probably be a bit better even. So can we conclude from this that it's the vactrol that's causing the issue, or might it be another component that hampers it's performance?

With the LDR side still unsoldered, I measured the LED side which has about 7.4VDC 'before' the LED and 9VDC 'after' the LED with Drive completely off (hope this makes some sense).With Drive fully open this goes to 4.7mVDC and 1.3VDC respectively. Pin 1 of the opamp driving the LED goes from 9VDC to 1.3VDC. Are these conditions that should make the vactrol behave normally?

Current to the LED goes from 22.5mA when drive is off to 14.5uA (microAmps) when fully open. Resistance on the (unsoldered) LDR side goes from 127.5ohms with drive off to about 20M ohms when turned open but then it drops gradually to about 5M ohms (this might be normal?)

Hope you can help me out! Apologies if the above information is totally irrelevant

Bram

 

[Matador]

This is LDR1? The whole point of this is to allow a voltage (coming from an external expression pedal) to control the resistance of that pot. If you don't use that function, you can remove the LDR, and just use the 500K pot to control the resistive section of the LDR.

 

[BramP84]

Thanks for your reply, yup LDR1, I suppose that's a fix too, might try that. But I'm also still curious why it isn't working the way it should. So it seems that something in the structure for that expression pedal is messing up the drive knob then?

 

[Khron]

Could those normally-closed switches in the expression pedal input jack be ever so slightly corroded?

I've encountered that in my Randall RG50TC combo, where the ring of the input jack triggers a muting of the preamp output. The muting was stuck permanently on, until i opened the thing up and gave both sides of each pair of contacts a good but gentle scrub with a little piece of fine-grit sandpaper / emery cloth.

 

[BramP84]

Could those normally-closed switches in the expression pedal input jack be ever so slightly corroded?

I've encountered that in my Randall RG50TC combo, where the ring of the input jack triggers a muting of the preamp output. The muting was stuck permanently on, until i opened the thing up and gave both sides of each pair of contacts a good but gentle scrub with a little piece of fine-grit sandpaper / emery cloth.

Good call! I checked the continuity and gave the contacts a scrub with some sandpaper just to be sure, could be my imagination but it seems there's a little more signal coming through at the start of the drive taper, but still pretty much the same situation with distortion only kicking in in the last 15% or so.

Judging by the manual there should be some grit coming up sooner:

"At the 10 o'clock to about the 2 o'clock position of the Drive control, the power amp will begin to reach maximum headroom and start to distort or be overdriven. If you like a slight "edge" to the peaks of your notes with a small amount of compression, this is that area. The best example of this sound we can give you is to listen to Jack Casady's bass parts on the album "Bless It's Pointed Little Head" by the Jefferson Airplane. After the 2 o'clock position to full maximum, the power amp will be heavily overdriven and saturation of the output transformer will occur. Heavy distortion, sustain, HARMONIC feedback and compression may be incorporated into your sound under these conditions."

I'll check some bassforums to hear some experiences from other owners as well.

 

[frans]

I got a SWR interstellar overdrive. If i can help, pm me. Of course the distortion should set in at 10 or so - depending on the input, it can get too much after 12 or 14 already, mushy pffflurbphssssss......

 

[Matador]

I would disconnect the resistive element, and sweep the drive control from min to max, and plot the resistance curve. Stock, it's roughly 'linear-ish', moving from about 100 ohms at ~20mA to about 100k at 0.1mA.

I would confirm that the taper you get is approximately linear to see if something happened to the LDR.

 

[BramP84]

I got a SWR interstellar overdrive. If i can help, pm me. Of course the distortion should set in at 10 or so - depending on the input, it can get too much after 12 or 14 already, mushy pffflurbphssssss......

Thanks Frans, PMed you!

 

[BramP84]

I would disconnect the resistive element, and sweep the drive control from min to max, and plot the resistance curve. Stock, it's roughly 'linear-ish', moving from about 100 ohms at ~20mA to about 100k at 0.1mA.

I would confirm that the taper you get is approximately linear to see if something happened to the LDR.

I took the following measurements (the knobs on this thing go to 11 )

 

[Matador]

That doesn't seem too far off from the datasheet specs: was this unit always doing this, or did it change suddenly?

 

[BramP84]

That doesn't seem too far off from the datasheet specs: was this unit always doing this, or did it change suddenly?

Well it hadn't been used for a couple of years, I think it's best if I compare my unit to the one Frans has to see if we find differences in the performance. Thanks for your help!