180W Studio Bass

[Smokebreak]

Got one of these in for repair with complaints of "it's nowhere near as loud as my bassman 100" and "doesn't seem quite right". Apparently a brand new sextet was put in 4 years ago but something wasn't right, so the frustrated owner put in on a shelf until now.
I was gettin ready to crack it open and figured I should look at a schematic
Interesting with the PI then the CF driver before (6) 6l6GCs and no bias pot but what looks like a possibly useless "output balance" or "output tube matching " on another schematic.
Oh yea and the handful of tube driven inductor based tone controls ala Ampeg.
Anyways, maybe something new to look at for you fine folks.

[Phil_S]

LOL, if you lift the grid input to the 12AU7 to disable it, I'll bet you can close it up and tell the guy it's fixed!

Seriously, that amp looks like a PITA to fix. Good luck!

[Smokebreak]

I was just gonna throw an ax7 in there and make it an extra stage

[Mr. dB]

Isn't the "Output Tube Matching" the same as the "Bias Balance" control on many silverface amps?

[Smokebreak]

It's kinda a mess in there. Lotsa burnt stuff on 1/2 the sockets, on the same side. Some original elytics, some replacements. At least the last tech installed sense resistors for me. Dig the giant 39K balancing resistors for the series filter caps...that aren't in the doghouse.

[Smokebreak]

Isn't the "Output Tube Matching" the same as the "Bias Balance" control on many silverface amps?

Yes same concept, but this pot is 400r and I think the SF were 10K. Proportionally though, if you look at the rest of the circuit on each, they are equals. But with 6 tubes, this would mean if you had 3 tubes that were "close" and then 3 others that were close, you could dial it in, but I can't see that happening. I guess it was the first step in the matched tube craze. With 2 tubes, I could actually see it being useful, if you are into that kinda thing.

[Smokebreak]

I'd be interested to hear some thoughts on the circuit design of the PI and driver. Why such a high value tail and then low value (33K) grid leaks. Why not just a good ol classic fender LTPI?

[Lauri]

I'd be interested to hear some thoughts on the circuit design of the PI and driver. Why such a high value tail and then low value (33K) grid leaks. Why not just a good ol classic fender LTPI?

Maximum recommended grid 1 resistance for 6L6GC with fixed bias is 100k. So for 3 tubes in parallel it's about 33k. You could change the the PI resistor values to a more traditional Fender style as long as you have the cathode followers after it to drive the 33k load. High value tail resistor gives better balance in LTPI.

I have a 180W Super Twin that I completely rebuilt and it's pretty much the same circuit as Studio Bass. On my amp I changed the phase inverter to a split load PI.

[Smokebreak]

Thanks Lauri and very interesting. How did you decide on the split load?

[Smokebreak]

BTW this thing is very loud now and will not break up. at all.

[pdf64]

I'd be interested to hear some thoughts on the circuit design of the PI and driver. Why such a high value tail and then low value (33K) grid leaks. Why not just a good ol classic fender LTPI?

The LTP looks to be running quite cold, with the 220k plate and 2k7 bias; so the 120k tail resistor is in line with those.
Note that the cathode voltage is reasonable, <100V.
My guess is that they found it was more linear when operated like that, especially given the following.
My guess is that the 12AT7 cathode followers that buffer the LTP and drive the power tube grids are intended to achieve a degree of linear operation in the grid current AB2 region.
The lack of transformer or direct coupling will prevent true AB2 operation on a continuous drive basis (as the coupling caps will probably charge up and cool the bias) but may facilitate it on signal peaks.

What have you done to get it running properly?

[Stevem]

Those amps are heavy as is , but they are begging for a choke !!

[pdf64]

Aren't 6 chokes enough?

[Smokebreak]

Thanks for the analysis. Someone had literally done a half-assed cap job at some point. The 220u reservoir caps were changed and some preamp bypass, but the Mallory metal caps were still there.
I replaced those, but the reason the amp had such low out put was that a couple of 1ohm sense resistors were burnt out, as well as a couple 1K5 grids. When they gave it up, they burnt a bunch of adjacent wires. One of the socket pins broke halfway off, but I was able to salvage enough to grab onto. I replaced the bias caps while I was in there and changed out all elytics in the power supply. The Ruby sextet had about 4 hours on them, and I would guess 3 of them had zero hours
Turns out someone converted the 400ohm balance pot to a bias pot that was quite effective once I paralleled something across the existing 2K7 in the bias supply, to get less neg voltage. 500V on plates and all 6 were close in current, I think I set them around 38mA.
Amp is reeeeaaallly loud and won't break up. The inductor based controls are good at making a guitar sound like a cocked wah at various states of "cocked". I don't have a bass, so I'll just assume they are useful for that instrument.
A slip of the meter probes upon initial fireup took out the 6.5amp fuse, so there is a 5a in there now, I'll need to get the correct one.

[pdf64]

Sounds like you've done some good work - well done!

Have you verified the power output?

Consider putting some sleeving over the meter probes, just leaving the very tip exposed, it helps to reduce such 'doh' moments.
Their drawback is that it them becomes a faff to slip them into aligator clips etc.
To get around that I keep a separate set of leads for use with clips etc.