Stiletto Deuce low frequency rumble. Motoboating?

[Rob Lockwood]

Hello all,

I have an early Stiletto Deuce Series 1. It has developed a low frequency rumble, seems less than mains frequency, in fluid drive 100watts only. It is audible but it also can be felt, anything on top of the amp will rattle and you can feel it through the floor. In that mode, it can be greatly reduced by switching from bold to spongy or from silicon to tube rectifier but it is still there a bit. However if I switch to 50Watts, or to any other mode other than fluid drive it stops dead immediately. When I switch back it doesnt start immediately, it takes a maybe 15 seconds to gradually build back up. From descriptions others have given I think it might be a mild type of motorboating. I think it is in the power amp section because turning the gains and channel masters to zero or even inserting a lead into fx return to cut off the preamp has no effect on it. Also it is still there with the master volume at zero. In fact it seems worse with the master below 9 oclock. An odd observation is that if I swap the speaker lead from the (correct for the cab) 8ohm to the 4 ohm socket the frequency of the rumble changes. This makes me think it is some sort of resonance in the power amp, but I dont know why that should be only in fluid drive.

I changed all the preamp tubes and phase inverter (which I was going to do anyway). That improved the tone a bit but made no difference to this problem at all. I have not changed the power tubes but I have measured the idle current on all 4. All seem fairly close to each other and in line with what others have posted except that in fluid drive the meter clearly fluctuates quickly by about +/-1ma on all 4, whereas its very steady in other modes. Bad power supply filtering maybe but why fluid drive only, voltage on clean modes is higher? Also swapped 1and 4 for 2 and 3 to see if the problem moved to 50w, but no change at all. If it was a power tube issue I would expect it to be worst in clean modes where the B+ voltage is highest.

An observation that worries me is that during testing with the amp on and standby in play mode (speaker connected!) although the tube idle currents are fine the power transformer gets so hot that I can't touch it for more than a coupke of seconds. I also notice that in all modes whether the rumble is there or not there is quite a loud hum from the power transformer (doesnt seem to come through the speaker though). I checked on my mark iv under exactly the same conditions, there there is no hum and the power tranny just feels slightly warm but is nowhere near being uncomfortable to touch.

Any ideas or experience of similar problems would be greatly appreciated!

 

[BostonRedSox]

Hello all,

I have an early Stiletto Deuce Series 1. It has developed a low frequency rumble, seems less than mains frequency, in fluid drive 100watts only. It is audible but it also can be felt, anything on top of the amp will rattle and you can feel it through the floor. In that mode, it can be greatly reduced by switching from bold to spongy or from silicon to tube rectifier but it is still there a bit. However if I switch to 50Watts, or to any other mode other than fluid drive it stops dead immediately. When I switch back it doesnt start immediately, it takes a maybe 15 seconds to gradually build back up. From descriptions others have given I think it might be a mild type of motorboating. I think it is in the power amp section because turning the gains and channel masters to zero or even inserting a lead into fx return to cut off the preamp has no effect on it. Also it is still there with the master volume at zero. In fact it seems worse with the master below 9 oclock. An odd observation is that if I swap the speaker lead from the (correct for the cab) 8ohm to the 4 ohm socket the frequency of the rumble changes. This makes me think it is some sort of resonance in the power amp, but I dont know why that should be only in fluid drive.

I changed all the preamp tubes and phase inverter (which I was going to do anyway). That improved the tone a bit but made no difference to this problem at all. I have not changed the power tubes but I have measured the idle current on all 4. All seem fairly close to each other and in line with what others have posted except that in fluid drive the meter clearly fluctuates quickly by about +/-1ma on all 4, whereas its very steady in other modes. Bad power supply filtering maybe but why fluid drive only, voltage on clean modes is higher? Also swapped 1and 4 for 2 and 3 to see if the problem moved to 50w, but no change at all. If it was a power tube issue I would expect it to be worst in clean modes where the B+ voltage is highest.

An observation that worries me is that during testing with the amp on and standby in play mode (speaker connected!) although the tube idle currents are fine the power transformer gets so hot that I can't touch it for more than a coupke of seconds. I also notice that in all modes whether the rumble is there or not there is quite a loud hum from the power transformer (doesnt seem to come through the speaker though). I checked on my mark iv under exactly the same conditions, there there is no hum and the power tranny just feels slightly warm but is nowhere near being uncomfortable to touch.

Any ideas or experience of similar problems would be greatly appreciated!

I had a problem like this once with my Roadster. My issue was with a Rectifier tube. Seriously. I had the whole putting sound that you are talking about. I just bought one Rectifier tube, and swapped one at a time. Wasn't too difficult since there are only two to choose from though.

 

[guitaraddict]

Rob - Did you try changing one of the rectifier tubes as suggested? I am having a similar problem and am curious if it worked!
Thanks.

 

[Rob Lockwood]

Unfortunately having posted this I have not had chance to spend time with the amp to try anything out. Thank you for the suggestion of the rectifier tube. I dont have any spare tubes but looking at the schematics the rectifier tubes are powered up all the time even when running in diode mode so its possible that a tube fault would affect the amps power supply even in diode mode. I think that I can remove the rectifier tubes and switch on in diode mode. If the noise is gone then I will get some new tubes, if not I think its something else. Hoping to have a bit of time to play with it today, will let you know!!!

 

[Rob Lockwood]

OK so tried the amp yesterday, silicon diodes selected, rumbling away happily in fluid drive as always. Its not so loud that you hear it playing but its way over what I would consider acceptable. Removed both rectifier tubes and switched back on and unfortunately its exactly the same so I dont think thats the answer. Shame - I really hoped it would be! Put the rectifier tubes back in and still the same, as expected. Switched channel 2 to tube rectifier setting which I dont usually use and the rumble more or less goes, can hardly hear it now but I can feel the speaker cab vibrate if I put a hand on it so it hasnt gone. So pretty sure tube rectifiers are ok now - if it was the cause I think it should be worse in tube rectifier mode. Noticed that with my single coil pickups, the hum it picks up standing close to the amp seems excessive. An odd thing is that switching out the effect loop or turning the master up full also reduces the noise. I need to look at this more - I think there is a problem in the power supply on this amp, transformer or caps maybe. Will investigate further when I can.

 

[guitaraddict]

Thanks for the update, Rob. As I have the same problem, I will let you know if I figure out what is causing this. As of this morning, the problem seemed to have lessened for no reason, but I do have a large amount of hum when I'm anywhere near the speaker. If you figure it out, please let us know! Thanks.

 

[Rob Lockwood]

Spent some time with the Stiletto testerday, and the schematic for it that I found on here, to try get to the bottom of this problem. Some interesting findings

When the 'motorboating' is there in fluid drive there sounds to be a very fast but shallow modulation on the notes like a very fast chorus/tremolo. Its subtle but definitely there, can be heard more clearly if the guitar volume is turned down a bit. I suppose that reduces the signal relative to the noise.

There is a ridiculous amount of compression on the amp to me (even 100w, silicon diode, bold). In fat clean the notes sound just like there is a Dynacomp pedal on there.

Overall the tone and feel of the amp is awful and fluid drive is unuseable.

I fed a signal from the effects send to another amp, no noise increase at all on the second amp. Fed a signal from the slave out of the Stiletto to another amp and the second amp is rumbling away as well. Proves without doubt to me that the noise is generated in the power amp only.

Had a really good study of the schematics today and I am convinced that the cause must be that the main 4 filter caps are not effective resulting in an ac ripple on the supply to the power tubes (hence modulation and noise). The preamp tube supplies each have another decoupling capacitor which would send any ac to earth but the power tubes are fed direct from those first 4. That would also explain the compression if there is not enough stored charge in the caps to provide the initial kick to the notes.

Also looking at the schematic, I think that in fluid drive there is a resistor switched in that reduces the amount of negative feedback in that mode. Negative feedback reduces noise so thats a believable explanation for why the noise is only there in one mode. This cause would explain why only in 100W too - in 50W the load on the power amp supply is halved. The capacitance needed to smooth out the supply is directly related to the current. If I had 2 out of 4 working that might be enough for 50W but not for 100.

I am as sure as I can be now that the filter caps are the problem so as soon as I can source some replacements I will change them out and let you know whether its a success of if all the above is rubbish!

Dont know why there would be a problem on an amp of this age though?

 

[rabies]

I am as sure as I can be now that the filter caps are the problem so as soon as I can source some replacements I will change them out and let you know whether its a success of if all the above is rubbish!

Dont know why there would be a problem on an amp of this age though?

i had problems with tubes shorting (3 times) with a new duece II. it shouldn't be the filter caps, those usually last around 20 yrs or so. take it to a tech.