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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 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

Replies:

pumpstein I just thought of another possibili... -- 8/31/2000 10:22 PM
Winnie Thomas Pump,Having just go... -- 9/1/2000 2:51 AM