Premier 120 - Inoperative Tremolo

[RightLurker]

I've been working on bringing an old Premier 120 back to life. Thus far I have cleaned and tightened all the pots and jacks, installed new filter caps and power resistors, replaced the rectifier, PI and power tubes, and installed a three wire power cord. The amp is now functioning as I believe it should, with the exception that the tremolo doesn't work at all. I've tested all the components in the tremolo circuit, and they seem to be fine. I also replaced the tremolo tube (12AT7). There is a 1/4" jack for a footswitch, which is long gone. The only schematic I have for the amp is one drawn by "J. Hardy," which seems to match up with this version of the Premier 120. One oddity is a large potentiometer in the tremolo circuit, mounted so that its shaft protrudes on the tube-side of the chassis. I'm not sure what its function is. Any suggestions for troubleshooting the tremolo circuit would be appreciated.

[cobalt]

Just a heads up, but many tremolo circuits with a footswitch are open until the footswitch completes the circuit! No switch?, try sticking a wide shaft phillips screwdriver in the footswitch jack. Tremolo work now? Fender works this way.
I once had a client bring me a bfdr(ri) and said "NOS re-tube, and fix the tremolo".
He didn't bring the footswitch, so the above method is what I used to "fix" the tremolo. He brought the footswitch when he picked up the amp, and was amazed.


weirddave

[RightLurker]

Thanks, Cobalt. The same thought occurred to me, so I put a short patch cord in the footswitch jack and the jumpered the tip and sleeve on the free end of the patch cord. I don't know if that would fix the missing footswitch problem, but it seems to me like it would. It didn't fix the problem. I'm sure there is a bad resistor or capacitor in there somewhere and I'm just missing it.

[martin manning]

As shown, the J. Hardy schematic has the tip of the footswitch jack grounded, and so the oscillator should start without the footswitch. The tip is the ground for the phase shift oscillator's cathode and the 15k pot (the one on the tube side of the chassis?) which looks like it might be an intensity control to me. Have you installed a new footswitch jack? It needs to be a shorting-type or switching-type wired to short the tip to ground with nothing plugged-in. If so, and it still doesn't work, get voltages from the pins of the 12AT7 V2, and the 12AX7 V3 in that part of the circuit, and the supply node that feeds them, between the 1k and the 4.7k dropping resistors.

[martin manning]

By the way, there is an error in the J. Hardy schematic- the connection to V4a's 470k/100k input divider should come from V6's grid, not V5's.

[RightLurker]

Many thanks, Martin. I shorted the footswitch tip, which didn't do anything. I'll take the voltage readings next.

One thing I noticed that's unusual (in my very limited experience): I fired the amp up through a dim bulb tester, and the bulb didn't come on right away, as it usually does with other amps. As the amp warmed up, the bulb began to glow dimly. I don't think there is anything drastically wrong with the amp - I've played it for about thirty minutes and nothing smoked, flamed or exploded!

Thanks to all. This is a great place to hang out.

[martin manning]

One thing I noticed that's unusual (in my very limited experience): I fired the amp up through a dim bulb tester, and the bulb didn't come on right away, as it usually does with other amps. As the amp warmed up, the bulb began to glow dimly.

Unlike a solid-state rectifier, current will start to flow slowly as a vacuum rectifier warms up and begins to conduct. Since you don't have a standby switch, the reservoir charges up as this is happening and you don't get any initial high current surge through the bulb.

[RightLurker]

I just found the time to take some voltages, as suggested by Martin. For better or worse: V2(12AT7): P1 242v; P2 0v; P3 7.65v; P6 243v; P7 3.9v; P8 8.8v. V3(12AX7): P1 152v; P2 3.9v; P3 7.9v; P6 108v; P7 7.6v; P8 8.8v.

The dropping resistors in the amp were 4k7 and 100k; I am certain they were original to the amp. The Hardy schematic shows the 100K should be a 1K. I replaced those two resistors with 4K7 and 100K; perhaps this was an error on my part. The reading on the supply node between those two resistors is 289v.

Thanks in advance.

[martin manning]

One thing that occurred to me yesterday is to make sure that the footswitch jack itself is well grounded to the chassis. Maybe put a plug in the jack, jump its tip and sleeve, and ground it to the chassis. The oscillator's cathode looks high in voltage, so that might be it.

The 100k dropper will only affect the preamp tube V1, which must be running a pretty low plate voltage. That might be something to play with later, or it could just be a mistake in the schematic.

[ampdoc1]

First thing I'd do is replace the caps in the tremelo circuit. I worked on 1000 amps or more when I did repair work, and almost all oscillator circuits straighten up after a re-cap. Ya don't need special caps and to replace 3-4 caps takes very little time.

a'doc1

[martin manning]

Pin 7 of the 12AT7 looks odd- both grids should be close to zero, I think, but the schematic doesn't show any ground reference for the one that gets the input from the volume control. Is that consistent with your amp? Where is the 15k pot set now?

Same with the 12AX7 grid measured at 3.9V... it should be zero. Perhaps the caps need to be replaced as ampdoc suggests.

[mike356]

This is my first post here, I hope it is of some value to someone. I came across this forum while searching for information on the Premier 88N, which I am currently restoring.

A while back, I did a Premier 120. My own hand-drawn schematic appears below. I believe it's an accurate representation of the amp that I had, but I don't guarantee it.

When I got it back together, everything worked, but I wasn't satisfied with what I thought was an excessive amount of hum.

The first thing I discovered was that, while the low level tubes were shielded, the shields themselves weren't grounded. This was because the sockets were mounted on rubber grommets for isolation, but whoever did this never actually completed the ground path to the shields. I don't know if this amp left the factory this way (with the rubber isolation) or was a later mod to deal with a microphonics problem (more on this later).

That helped, but not enough. I eventually realized that throughout the amplifier, ground connections were made through rivets to the chassis. This is a frequent source of trouble, especially if there is any corrosion. Drilling out these rivets and replacing them with nuts and bolts helped the hum problem too. Bad grounds can cause all sorts of strange problems.

I also discovered that the last filter cap (originally 10uf), which feeds the preamp tube, needed to be increased. I used 40 uf.

Finally, since this amp didn't have a hum balance control, I ungrounded the filament circuit, and added two 100 ohm resistors from either side to ground. That pretty much killed what was left of the hum.

I found that the amp had a microphonics problem, even with the rubber mounted tube sockets. The culprit was the 12AT7. I tried a number of tubes from various manufacturers, US and foreign, and found that the best tube for this was a Sylvania triple-mica 12AT7WA.

The tremolo should work without the foot pedal (I didn't have one either).

The resistors in the power supply should be 4.7K and 100K. 1K is not correct.

The footswitch jack is NOT grounded and shouldn't be. If it is grounded for some reason, that would kill the tremolo.

[martin manning]

Thanks for posting your experiences and schematic. I guess we don't know if thw OP solved his problem or not.

The footswitch jack is NOT grounded and shouldn't be. If it is grounded for some reason, that would kill the tremolo.

Are you saying that the footswitch jack should be isolated from the chassis? The LFO would not work at all if that were the case.

[mike356]

Thanks for posting your experiences and schematic. I guess we don't know if thw OP solved his problem or not.

The footswitch jack is NOT grounded and shouldn't be. If it is grounded for some reason, that would kill the tremolo.

Are you saying that the footswitch jack should be isolated from the chassis? The LFO would not work at all if that were the case.

All I can say is that he foot switch jack WAS insulated from the chassis in the amp that I restored. If it was grounded, the 47K resistor from the wiper of the 15K pot to ground would have been shorted out, and would therefore have no reason to be in there in the first place.

I admit that I don't fully understand how that circuit is supposed to work (or how that 15K pot is supposed to be set).

But it would seem that the oscillator output appears across that 47K resistor, and that in turn varies the bias on the 12AX7 (the section driven by the 12AT7), and hence the gain of that tube.

In retrospect, it would have been interesting to poke around in there with a scope, but it worked fine, and I don't own the amp any more.

I should do that when I get to that point with my Premier 88N, I would assume that the circuits are similar.

[martin manning]

Here's the schematic I've been looking at. Note there are some differences relative to yours, and there is an obvious error in the phase inverter. It's not clear whether the footswitch jack is grounded or not, and the input jacks don't show a chassis ground either. I thought perhaps the 47k you mentioned (looks like its 6k2 in the attached schematic) might be there to lift the oscillator ground when the trem was switched off. As it turns out the oscillator will work either way and still produce a tremolo effect in the output. I modeled the tremolo circuit to see how it worked, and have now tried it with all the grounds associated with that node in the schematic lifted to the top of the 6k2. The output waveform has a sinusoidal tremolo the first way, and a more complex shape when done in the second way. Did it sound any different from the more usual tremolo circuits?