Gibson Hawk tremolo problem

[words453]

I have a Gibson Hawk (7591 tubes, 2x10 speakers) that was missing the trem bug. I made one out of a 6838 bulb (28v/25mA) and a Radio Shack ldr and it worked but the amp volume was significantly lower with the bug installed than without. I pulled the bulb out of the heat shrink and found that the bulb stayed lit whether the trem was on or off. When I disconnect the bulb, the amp volume returns to normal. FWIW, I've also tried 6 and 12 volt bulbs but they would not light up. Any advice would be greatly appreciated.

[Chuck H]

How did you determine the bulb and LDR values for the bug component you had to recreate? Do you have a schematic you can post? Did you change ANYTHING else in the amp to accommodate the homemade bug?

If the LDR reduces resistance when illuminated and your bulb is always lit then you are always sending some signal to ground via the trem function. Why is the bulb always lit? Perhaps there is a problem with your oscillator or how it interfaces with the bug rather than a problem with the bug. Does the trem work properly? Does the volume peak at what seems the proper level when the trem is operating? Perhaps your LDR should be one that increases resistance when illuminated? Then, since your bulb stays lit with the trem off, the LDR won't ground your signal.

[words453]

How did you determine the bulb and LDR values for the bug component you had to recreate? Do you have a schematic you can post? Did you change ANYTHING else in the amp to accommodate the homemade bug?

If the LDR reduces resistance when illuminated and your bulb is always lit then you are always sending some signal to ground via the trem function. Why is the bulb always lit? Perhaps there is a problem with your oscillator or how it interfaces with the bug rather than a problem with the bug. Does the trem work properly? Does the volume peak at what seems the proper level when the trem is operating? Perhaps your LDR should be one that increases resistance when illuminated? Then, since your bulb stays lit with the trem off, the LDR won't ground your signal.

The bulb and ldr were suggested by other forums. I don't know why the bulb is staying lit but the trem does work when switched on. I would think that a bulb that stays lit all the time would burn out and have to be changed frequently, so I would hesitate to put in an ldr that increases resistance when illuminated.

[words453]

BTW, I did not change anything in the circuit and I checked to make sure the cicuit components are correct.

[Chuck H]

I can't find a Gibson "Hawk" schematic that uses a bug trem. If you've been conferring with people that were able to give you information about that trem bug, can you perhaps furnish a schematic?

Does the amps volume come up to "correct" level when peaking in trem operation?

If the components are all as stock, the trem brings the volume up correctly at peaks, the bulb remains illuminated with the trem OFF and the signal is damped then it may be that Gibson designed this amp such that the bulb remains illuminated with the trem off and the oscillator DIMS the bulb.?. In which case you need to undamp the signal with the trem off and that means an LDR that increases resistance with lamp intensity (BUT I DON'T HAVE A SCHEMATIC TO CONFIRM THAT). And yes, you may be replacing that lamp often-ish.

[bob p]

It's difficult to help you without a schematic. The best that we can do is to take blind stabs in the dark. In general, it's a good idea to post a schematic when you're asking for help on an uncommon circuit. until we have a schematic, all that we can suggest are simple divide and conquer techniques.

What kind of light source did you use? 6838? That's a 28V incandescent. Are you sure that is the right lamp for the application? If not, chances are that your oscillator output never drops below the excitation voltage of the lamp, which is why it is staying lit.

Measure the range of voltages at your lamp terminals and tell us the peak and nadir values. Chances are a 6838 is the wrong lamp to use. I would guess that your amp is intended to drive an NE-2 neon lamp or something similar. Strinking voltages are higher on neon lamps. NE-2 is about 90V.


I'd check the drive voltages and the waveform on the plate of the oscillator to make sure that you have adequate drive for the lamp.

I'd also short the LDR with a 1k resistor to verify that the audio amplifier driven by the bug is working properly. When shorted you should get full volume.

Do you have a scope? Waveforms and voltages at all of the relevant points would give a definite answer.

In the big scheme of things tremolo is not that complicated and you should have it fixed in a few mintues. The key is to divide the circuit into it's 3 components and verify each one. You need to have an oscillator that functions properly, an optoisolator that functions properly and an audio amplifier that functions properly. Figure out which of the three isn't working and focus on that circuit.

[Jazz P Bass]

Hawk schematic with LDR.

Hawk-7591.zip

[Justin Thomas]

If "bug" = LDR, then there's a Hawk schem with 7591s over at bmamps.com

I mean, it's better than nothing, right? I'd link it, but am working from a tablet right now.

Edit: simulpost. JPB's is better quality, too.

Justin

[Tom Phillips]

Words453,
Reference the schematic posted by Jazz P Bass and note the trimmer pot R56 connecting the cathode of V4B to ground. That is a factory adjustment which sets the idle current through the bulb in the light bug. It is possible that yours is set such that too much idle current is flowing through the bulb and that is why the bulb is too bright all the time thus shining on the LDR which is then kept at a low resistance and therefor attenuating the audio signal. I'd try adjusting R56 to a higher resistance. If I'm correct that will get the tremolo working properly if everything else is in order. I cannot find the factory adjustment instructions but the goal is to set the bulb current so the bulb is just below coming on but not actually putting out light when the tremolo is off.

[Steve A.]

See this thread...

http://music-electronics-forum.com/t23156/

Steve Ahola

[words453]

Thanks for the quick replies and thanks for posting a schematic, JPB. I have adjusted the trimmer pot and it does affect the bulb's brightness but it still stays lit. I even put a 470 ohm resistor in between the cathode and the trimmer pot which made the bulb dimmer but made the trem much weaker. Unfortunately, I do not have a scope. FWIW, the cicuit voltages are about 20v higher than schematic voltages. When the trem peaks it is still not as loud as when the bug is disconnected.

[Tom Phillips]

...When the trem peaks it is still not as loud as when the bug is disconnected.

The test I would do is disconnect the power to the bulb in the roach, leave the LDR in the circuit and make sure there is no ambient light reaching the LDR. If the signal is still low amplitude under those conditions then the LDR itself is probably bad. You can also measure the resistance of the LDR out of the circuit under dark conditions. It should measure well over 1 MegΩ.

Another thought...Did the trem bug look like it had been previously messed with? Maybe the bulb or the LDR (or both) are not stock/correct. Can you post photos?

[words453]

When the LDR in in circuit and the bulb is out of the circuit, the volume of the amp is normal which tells me that the LDR is okay. I think that because the bulb is staying lit, it has the LDR at low resistance, bleeding signal to ground. The bulb and LDR are not stock. The trem bug was missing and someone had wired a vibrochamp trem cicuit in which added parasitic ocillation to the signal. The bug I built works and adds no PO but greatly reduces the volume of the overall output. The LDR I'm using does measure well over 1 MEG in dark conditions.

[olddawg]

When the LDR in in circuit and the bulb is out of the circuit, the volume of the amp is normal which tells me that the LDR is okay. I think that because the bulb is staying lit, it has the LDR at low resistance, bleeding signal to ground. The bulb and LDR are not stock. The trem bug was missing and someone had wired a vibrochamp trem cicuit in which added parasitic ocillation to the signal. The bug I built works and adds no PO but greatly reduces the volume of the overall output. The LDR I'm using does measure well over 1 MEG in dark conditions.

What happens if you wave a small flashlight over the LDR. Does the volume dissipate and then return to full volume. I would bet, as said before that you need a neon bulb. But I bet you could cobble something together with an LED and a voltage divider with a little ingenuity.

[Chuck H]

How "stays lit" is the bulb when the trem is switched off? There must be some floating point that is affecting this. Is there any DC on the grid of v4B? And I wonder why the 6V and 12V bulbs wouldn't illuminate. Do you have a 12au7 in the V4 socket?