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Help with motorboating?


 
8/31/2000 9:42 PM
pumpstein
Help with motorboating?
Last weekend I had some time, so I decided to rebuild a small Alamo champ-type amp that had been lying around for too long. I wanted to keep it simple, and have the amp done by Sunday night (well, it took me until last night to finally fire it up, but you gotta have a dream, right?)  
 
I poked around in my junk boxes and settled on the following design elements:  
 
- 1 - 12AX7 preamp  
- 2 - 6BQ5/EL84 p-p output stage  
- OPT is from a 1951 RCA film projector, originally supported 2 – 50L6 in p-p, sec = 4/8/16 ohms  
- PT from a ‘50’s record player - originally supported 1 - 6X5, 1- 6V6 & 1 – 6SL7  
- SS rectifier (full-wave)  
- Interstage Transformer Phase Inverter - pulled from a 1956 Hammond Chord Organ; this was used in the solo voice preamp. The secondary winding was feeding a pair of 6SK7’s in p-p, with the primary winding grounded on one side and connected to the coupling cap of the preamp’s 2nd triode’s plate. The SEC is center-tapped. I chose to use this out of curiosity – I worked on a Gibson GA-5T that used an interstage tranny PI, and really loved the sound it had.  
- No NFB loop  
- Features: Treble/Bass/Volume (this circuit was copied from a later-period Vox amp schematic – I forget which one now – the last one in the Vox section of Gerry’s 1st book – it’s pretty similar to BF Deluxe tone circuit).  
 
I used a 12g copper buss bar for the main ground, but didn’t have a means to isolate the input jack, so both stages of the 12AX7 are grounded to the input jack. I soldered the AC mains green wire to chassis ground right at the point of entry. I grounded the PI and output stage grounds to the buss bar in order of the signal, with the PT CT tied directly to the (-) terminal of the (100uf/450v) 1st filter, then ran a lead to the where the buss bar is soldered to chassis. The 2nd filter is grounded on the buss bar before the PI sec, but after the PI primary. The OPT sec is grounded to the buss bar between the PI sec and where the bar is soldered to ground. The 3rd filter supplies B+ to the 12AX7, and that is grounded to the input jack. So the entire preamp is grounded through the input jack, and the rest of the amp goes to the buss bar, except the AC mains green wire, which as I said, has it’s own ground solder connection to chassis.  
 
I used a pair of 100-ohm resistors to CT the filament voltage, and referenced the faux-CT to the top of the cathode resistor on the 6BQ5’s.  
 
Anyway, I fired it up last night, and was pleasantly surprised to find that it had a VERY unique sound! Kind of like a Princeton Reverb that stays clean up to 8. The amp seems quite loud and clear into a generic 20-watt 10” ceramic magnet speaker I use for bench tests. In fact, I didn’t know this goofy little speaker could sound this good! The onset of distortion is very smooth, and with my Ibanez Artist small-body, semi-hollow body w/the neck humbucker, it sounded like Larry Carlton’s tone on steroids. This is really a nice little discovery - it sounds great; very unique, ballsy but even-tempered tones. Sounds "potent" - has nice round bass and low-mids through that 10" speaker. Woody without being airy.  
 
Voltages (from memory):  
 
OPT CT - 390vdc  
6BQ5 plates - 387vdc  
6BQ5 screens - 385vdc (470-ohm screen resistors)  
12AX7 plates - 220vdc (100k Rp - 820-ohm/22uf for both cathodes)  
 
I forget right now wha the voltage was on the 6BQ5 cathodes - I used a 125-ohm 10-watt here, bypassed w/50uf.  
 
There are a few bugs to work out…  
 
- 6.7VAC on the heaters. What do I need here? A pair of .5-ohm series resistors?  
- Motorboats when the bass is up past 7-8, and/or when the volume is up around the same setting.  
- Spurious audible high-freq “weird ham radio-like sounds – as if a signal were being honed in on like in the old WWII submarine movies? – that whistling, modulating sound?)  
- Fairly gnarly hum  
 
I’m hypothesizing that the motor-boating & weird squall noises may have something to do with the fact that due to space limitations, I mounted the interstage PI tranny on the inside of the chassis. Could it be coupling with the preamp signal?  
 
Not sure where to start with the hum, but would getting those heater voltages down help? I was hoping to not have to ream out the input jack hole for isolation purposes, but I may go that route, since everyone touts it as such a tried-and-true method.  
 
I would really appreciate any t-shooting tips any one might offer. I read through RG’s t-shooter’s guide, but the section on motor-boating was somewhat brief.  
 
Thanks,  
- P
 
8/31/2000 10:22 PM
pumpstein

I just thought of another possibility re: the motor-boating...  
 
One of the OPT pri plate leads is routed at an angle perpendicular to the PI sec plates leads, and the former is also essentially laying flat on the chassis, BUT it's underneath the PI sec leads, which lay flat on the chassis.  
 
any comments are welcome and appreciated,  
- P
 
9/1/2000 7:11 AM
Steve A.

P:  
 
    My guess is that the motorboating has to do with the grounding of your tone stack... As for the non-isolated jack, you could just remove it from the chassis and add in a ground wire, just to see if that helps things. (If it does then enlarge the hole so that you can use insulated shoulder washers...)  
 
    But I am concerned about the voltages on the EL84 plates... I think that they are a bit high. You could use 10V/5W zeners in the ground circuit to bring the voltage down to something like 356vdc or 338vdc... Are you using a full wave rectifier or a FWB?  
 
    Also, you mentioned that your first filter cap was 100uF... that is just a single cap, not a pair of 100uF's in series- right? Having 100uF right after the rectifier could boost the lows enough to cause motorboating but I think it has something to do with the way the tone stack and 12AX7 grounds are wired up...  
 
--Good luck!  
 
Steve Ahola
 
9/1/2000 8:08 PM
pumpstein

Well, last night I did a little troubleshooting…unfortunately when I checked here for responses last night at around 8pm (out here in Seattle), you guys hadn’t posted yet, so I was on my own. : )  
 
Thanks for the help, though – always appreciated!  
 
So here’s what I tried…  
 
First I tried re-routing the OPT pri leads; no luck.  
 
I unsoldered one of the filament leads and jumpered it in again, turned on the amp, let it hum, then disconnected it. The hum faded slowly, which apparently means the hum is not in the filament circuit. So I checked out the filter caps, and they all seemed fine. I jumpered in a known-good electrolytic in parallel across each filter cap, but no reduction in hum. I de-soldered all the (+) terminals and verified resistance was similar to that of the known-good cap. I checked for shorted and open caps with my little old Heathkit cap tester. The DC voltages I’m getting are very stable, FWIW.  
 
Next I got a .1 uf @1600v cap and connected one end to ground via a jumper wire. I went through all the signal nodes, seeing which ones would reduce the hum, and found that the interstage transformer (both pri & sec) and the output tubes were all separate sources of hum. Touching each of these nodes in succession revealed that there were different-sounding types of hum being generated.  
 
This got me thinking that maybe the interstage PI needs to be better shielded. I took a look at the schematic & layout for the Hammond Chord Organ that I salvaged the PI from, and it was mounted on the outside of the chassis, whereas I mounted mine on the inside, between the preamp and power amp. It’s very near to both the tone stack and the output tubes, sandwiched in between. I’m going to try moving it to the outside of the chassis, and see if the hum disappears.  
 
Hey Steve – re: the 6BQ5 voltages…I was wondering about that, having read here and other places many times that 250-300vdc on the plates is desirable. I have the cathode at 12.6vdc w/a 125-ohm resistor bypassed by 50uf.  
 
Could you describe in more detail, or point me to a link that might give more details, about the zener method? I seem to remember reading that I would just put as many as necessary/desired in series between PT CT and ground? Is that right?  
 
Re: the 100uf in the 1st filter position, I’ll try swapping in a lower value and see what I hear. Thanks. BTW, how does that work? Why would this much filtering w/a SS rectifier cause motor-boating?  
 
Thanks again, Steve and Winnie –  
- P
 
9/2/2000 7:26 PM
Wild Bill

I wouldn't worry about those voltages - Traynor ran a lot higher than that! So did a Heathkit hifi amp I worked on once.  
Ditto the filament voltage. +/- 10% is the spec for tubes.  
Motorboating is often due to poor filter decoupling to a preamp stage. That's why amps have a string of filters with cascadingly lower voltage taps. It not only gives better filtering by the time you reach the 1st and 2nd preamps but it isolates the stages from each other. You'll notice if you compare various amp schematics how if they tap 4 triodes in the preamp they may use a larger filter value at that point than with a single channel amp with only two stages. Since bigger values cost more and in the days of tube rectifiers were harder on the rectifier with charging/surge currents designers tried to get away with as small as possible. I solved one motorboating problem just by adding one more dropping resistor and a 10 uf filter, so's I couold feed that 1st 12AX7 all on its own. I also run a separate leg to feed a reverb driver tube, if I'm running one.  
 
Hope this is of some help...  
 
--Wild Bill
 
9/5/2000 8:11 PM
Peter S

Pump,  
Wild Bill is on it! I'd also add that the location of the decoupling cap for the first gain stage may need to be changed. Grounding locations can cause motorboating as well as ground loop hum. FWIW. The last amp I built is totally star grounded to a single point next the OT. This worked wonders.  
 
Peter S
 
9/1/2000 2:51 AM
Winnie Thomas

Pump,  
 
Having just gone through an exasperating hum problem, I would advise grounding the first two filter caps in a separate place from the rest of the grounds.  
 
Secondly, I've cured motorboating by simply moving wires around. Lead dress has been my biggest problem with this. Turn the amp on, and use a small wooden dowell to move the wires about gently. I use a 1/4" one sharpened in a pencil sharpener. See if you find two that might be creating the problem by their proximity to each other.  
 
Winnie
 
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