- Feb 12, 2006
- 61,053
- 5
- 221,030
- Disclosures
- Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
I would identify the cause of the problems before changing things around.
Enjoy the benefits of a premium membership!
Upgrade Account
-
TalkBass has been independent since 1998. Add your voice.
Create a free account to reply to discussions, view embedded media, and browse with fewer display ads.
Join free Log in
Want zero display ads or expanded classifieds tools? Compare plans.
Amp Guts Photos - Official Thread - Lets see those pics
- Thread starter svtb15
- Start date
I don’t know if this matters, but when I checked my GE 7027As in my V4, they were not very closely matched. BUT , the amp runs smoothly and sounds great. It makes me think that those older circuits are very hearty. Maybe try the 7027As back in there?
Mixed blessing: the Ampeg's speaker/cab seems to be the culprit. I might have guessed sooner if my 4-pin XLR breakout connectors hadn't taken the whole month of April to ship out via Amazon.
If it's voice coil rub, I suppose I'll swap the unobtanium speaker with a baffle ring + Beta 15A. Like a big ol B15.
Anyway, the speaker output faithfully resembles the the signal source up to ~50W @1Khz (19V RMS @ 6.6R speaker load), ~10V on the speaker at 100Hz. No artifacts when plugged into an external cabinet via ext. speaker jack or via main XLR-1/4" breakout cable. Scoped the PI outputs and they compare favorably up until the cathodyne gets nipply at severe overdrive levels. Pretty sure that this picture is with a 1kHz signal.
Thanks for the thoughts and suggestions, all!
...As penance, here are some pictures of a Plush 3000B.
The 0C3 series dropper wouldn't strike with only the teeny trickle of idle screen current through it, and when it did strike, would make a horrible sound. Even a 100k bleeder wouldn't keep it steady, although the 10K resistor shunted across the gas tube explains the failure to strike. Maybe they put it there because 6x6L6GC of full-tilt screen current is well beyond the 45mA max rated current of the gas tube. 6x15V 5W zeners in a socket with a 100k bleeder brings the screens down to a steady ~465V. I'd simmed and re-simmed the "diabolical" choke-input supply used in a different revision, but this is where it's at now.
This one has had some type of explosion and repair on the Nelco PCB. This one hass the Northlake OT4000 & PT4000 common the the Plush 4000 amps. Full-wave rectifier and 0C3 as a series dropper for the screen supply. LCLC filtering; first choke seems to be in the ground leg of the rectifier. Second choke feeds the screens and PI/preamp.
[edit: the choke-input version from O.T.B. Plush 3000B ...This example uses the PT3000 power transformer]
If it's voice coil rub, I suppose I'll swap the unobtanium speaker with a baffle ring + Beta 15A. Like a big ol B15.
Anyway, the speaker output faithfully resembles the the signal source up to ~50W @1Khz (19V RMS @ 6.6R speaker load), ~10V on the speaker at 100Hz. No artifacts when plugged into an external cabinet via ext. speaker jack or via main XLR-1/4" breakout cable. Scoped the PI outputs and they compare favorably up until the cathodyne gets nipply at severe overdrive levels. Pretty sure that this picture is with a 1kHz signal.
Thanks for the thoughts and suggestions, all!
...As penance, here are some pictures of a Plush 3000B.
The 0C3 series dropper wouldn't strike with only the teeny trickle of idle screen current through it, and when it did strike, would make a horrible sound. Even a 100k bleeder wouldn't keep it steady, although the 10K resistor shunted across the gas tube explains the failure to strike. Maybe they put it there because 6x6L6GC of full-tilt screen current is well beyond the 45mA max rated current of the gas tube. 6x15V 5W zeners in a socket with a 100k bleeder brings the screens down to a steady ~465V. I'd simmed and re-simmed the "diabolical" choke-input supply used in a different revision, but this is where it's at now.
This one has had some type of explosion and repair on the Nelco PCB. This one hass the Northlake OT4000 & PT4000 common the the Plush 4000 amps. Full-wave rectifier and 0C3 as a series dropper for the screen supply. LCLC filtering; first choke seems to be in the ground leg of the rectifier. Second choke feeds the screens and PI/preamp.
[edit: the choke-input version from O.T.B. Plush 3000B ...This example uses the PT3000 power transformer]
Last edited:
- Feb 12, 2006
- 61,053
- 5
- 221,030
- Disclosures
- Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
Those radial caps suspended in mid are are a disaster waiting to happen, but there's more than enough disasters for one amp already 
680uF/400V & 220uF/600V snap-ins and horizontal-mount clamps are also in the mail, along with some inrush limiters & UF5408.Those radial caps suspended in mid are are a disaster waiting to happen, but there's more than enough disasters for one amp already
...This amp certainly seems to have earned a reprieve from fiery ill fortune, but it got stuck with me. Series 400V caps seem safer w/r/t line surges & spikes, but a single 600V cap is potentially more "stimulating"....This thing still has a little magic smoke left to share.
That's the biggest Plush I've ever seen! Thanks for sharing!!!
Mid 90's Carvin R600. It looks as crappily built as it sounded. (in my opinion)
- Feb 12, 2006
- 61,053
- 5
- 221,030
- Disclosures
- Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
How is that crappily built? Specifically?
I guess I shouldn't have said it that way. It was a bugger of an amp with problems and it ended up failing.
Last edited:
A local friend and TB member swears by these amps. However, he has to address issues with ribbon cable connections at least annually.
Last edited:
If I may, this may be purely a matter of taste, but I don't like how the wiring is dressed. Looks like the wires going to the power switch are a bit too short. Also, some of the caps on both boards don't look like they're seated very well to the board, and might vibrate or wiggle loose over time. Likewise for those power resistors standing on their ends. But those things might be purely aesthetic.
- Feb 12, 2006
- 61,053
- 5
- 221,030
- Disclosures
- Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
I don't particularly care for how the wire is dressed either, but it's probably as good or better than average. The film caps are installed pretty much average too IMO. The stand-up power resistors actually work quite well provided the PCBs are double sided, the holes are plated and the solder chemistry up to snuff. I have seen very few of these fail over a wide variety of products, and where they do fail it's generally traced to poor workmanship.
Aesthetics do matter though, I agree, when folks are going to be looking at them. I consider it part of the art of design.
Fender Super Twin
Being converted into a
Studio Bass
Being converted into a
Studio Bass
The mighty Bogen M120(plural):
4x8417 off a 230V HT primary into a voltage doubler. Screen supply and preamp comes off the mid-tap. At idle, about 615VDC on the plates and the screens at ~310VDC. Twitchy high-gain 8417 sits at 9-14VDC bias, compared with 30-40VDC for 6550 at the same operating point. 47R anode stoppers are a must for the 8417.
The 3 mic inputs run 1/2 6eu7 gain stages with grid-leak bias, so I just pipe the signal in through the AUX IN. That skips all that and runs it through a 330k into a resistive mixer; effectively a 330k/330k divider.
[EDIT: SIGNAL CHAIN MISTAKE]
That signal sits on the 1M master volume pot (330k effective impedance is from the total of three paralleled MIC IN volume pots), then into a 1/2 6EU7 driving the tone stack. Then into the driver circuit after that....
[EDIT: SIGNAL CHAIN MISTAKE]
Driver circuit is a 7247 dissimilar dual triode, with the 12AX7 section as a voltage amplifier; global feedback into the cathode. The 12AU7 section is next, as a concertina PI into the power tubes. 22k ground reference on the output tube gride (44k effective per tube): this is one amplifier that does not violate the grid-leak recommendations. Doing so would probably be promptly discouraged by the twitchy 8417 running anywhere near 100W/pair.
The first one arrived with a popped/cracked anode stopper. Clipped it out and rebuilt an adjustable bias circuit to reach -50V. Set the bias, and have been running it with a single pair of 6550 (and the output transformer load mismatched; 8R load on the 5.2R tap.
It came without any tubes, and lacking a 7247 in my stash, I rebiased the 12AX7 section for a 12AU7. It takes a poop-load of driving signal to get full power out of the amp, but the overload characteristics of the amp are much smoother. This voltage amplifier stage is inside the beginning of the poweramp's global feedback loop, so when it clips, all bets are off.
By subbing a lower-gain 12AU7 as the driver voltage amplifer, the feedback loop is changed, so the feedback resistor was drastically raised to ~270k.
[EDIT: the 6550 has much lower gain than the 8417, as does the 12AU7 wrt the 12AX7; each substitution reduces the open-loop gain of the global feedback circuit. The NFB signal is drastically reduced accordingly. Double negatives are not not confusing....]
The feedback signal in these amps comes off a spare 70V tap on the OT secondary. Combine that with the main 70V tap and you have symmetrical feedback windings, albeit windings with potentially limited high-frequency response (not much above 15kHz, allegedly).
This overload dynamic is gone into in depth in a thread about the (mightier) Bogen MO-200, which is effectively the same poweramp, albeit in dual-parallel. The overload seems to result from the limited input headroom of the 12AX7 section of the 7247, combining with the impedance of the 47R anode stopper.
With 3 preamp tube sockets and 3 extra noval sockets for the mic transformers, there's plenty of room to add some preamp flexibility in this thing.
...I eventually ran across a second M120 loaded with original tubes, and it does work. Overload sound like poopie on this one though. Sound nice before clipping, but graceless when pushed.
A third M120 with no tubes is in the mail. I figure I'll keep playing the 6550/12AU7 #1 and use #3 as a candidate for a total teardown. #2 may stay stock-ish until I can find someone to buy these 8417. Top-cap super-6BG6G/7027-oids are an option if I want to use all four sockets and no OPT mismatch.
Squeeze in a 230VCT isolation transformer and some MOSFETs source followers, and 75-90W/pair should be possible from the tubes. The OPT seems high-quality, but 180W might require some bandwidth limiting down to ~ 50Hz-15kHz. Careful layout may allow the addition of a choke for CLC filtering of the screens + preamp. The octal sockets are too close to run 4x6550, but a single pair of them sound juicy & delicious and requires me to nail everything in the apartment to something heavier or squishy...
Much thanks to the contributors to this AK thread: Invalid Link Removed
edit: added [Invalid or Expired Link Removed]:
4x8417 off a 230V HT primary into a voltage doubler. Screen supply and preamp comes off the mid-tap. At idle, about 615VDC on the plates and the screens at ~310VDC. Twitchy high-gain 8417 sits at 9-14VDC bias, compared with 30-40VDC for 6550 at the same operating point. 47R anode stoppers are a must for the 8417.
The 3 mic inputs run 1/2 6eu7 gain stages with grid-leak bias, so I just pipe the signal in through the AUX IN. That skips all that and runs it through a 330k into a resistive mixer; effectively a 330k/330k divider.
[EDIT: SIGNAL CHAIN MISTAKE]
That signal sits on the 1M master volume pot (330k effective impedance is from the total of three paralleled MIC IN volume pots), then into a 1/2 6EU7 driving the tone stack. Then into the driver circuit after that....
[EDIT: SIGNAL CHAIN MISTAKE]
Driver circuit is a 7247 dissimilar dual triode, with the 12AX7 section as a voltage amplifier; global feedback into the cathode. The 12AU7 section is next, as a concertina PI into the power tubes. 22k ground reference on the output tube gride (44k effective per tube): this is one amplifier that does not violate the grid-leak recommendations. Doing so would probably be promptly discouraged by the twitchy 8417 running anywhere near 100W/pair.
The first one arrived with a popped/cracked anode stopper. Clipped it out and rebuilt an adjustable bias circuit to reach -50V. Set the bias, and have been running it with a single pair of 6550 (and the output transformer load mismatched; 8R load on the 5.2R tap.
It came without any tubes, and lacking a 7247 in my stash, I rebiased the 12AX7 section for a 12AU7. It takes a poop-load of driving signal to get full power out of the amp, but the overload characteristics of the amp are much smoother. This voltage amplifier stage is inside the beginning of the poweramp's global feedback loop, so when it clips, all bets are off.
By subbing a lower-gain 12AU7 as the driver voltage amplifer, the feedback loop is changed, so the feedback resistor was drastically raised to ~270k.
[EDIT: the 6550 has much lower gain than the 8417, as does the 12AU7 wrt the 12AX7; each substitution reduces the open-loop gain of the global feedback circuit. The NFB signal is drastically reduced accordingly. Double negatives are not not confusing....]
The feedback signal in these amps comes off a spare 70V tap on the OT secondary. Combine that with the main 70V tap and you have symmetrical feedback windings, albeit windings with potentially limited high-frequency response (not much above 15kHz, allegedly).
This overload dynamic is gone into in depth in a thread about the (mightier) Bogen MO-200, which is effectively the same poweramp, albeit in dual-parallel. The overload seems to result from the limited input headroom of the 12AX7 section of the 7247, combining with the impedance of the 47R anode stopper.
With 3 preamp tube sockets and 3 extra noval sockets for the mic transformers, there's plenty of room to add some preamp flexibility in this thing.
...I eventually ran across a second M120 loaded with original tubes, and it does work. Overload sound like poopie on this one though. Sound nice before clipping, but graceless when pushed.
A third M120 with no tubes is in the mail. I figure I'll keep playing the 6550/12AU7 #1 and use #3 as a candidate for a total teardown. #2 may stay stock-ish until I can find someone to buy these 8417. Top-cap super-6BG6G/7027-oids are an option if I want to use all four sockets and no OPT mismatch.
Squeeze in a 230VCT isolation transformer and some MOSFETs source followers, and 75-90W/pair should be possible from the tubes. The OPT seems high-quality, but 180W might require some bandwidth limiting down to ~ 50Hz-15kHz. Careful layout may allow the addition of a choke for CLC filtering of the screens + preamp. The octal sockets are too close to run 4x6550, but a single pair of them sound juicy & delicious and requires me to nail everything in the apartment to something heavier or squishy...
Much thanks to the contributors to this AK thread: Invalid Link Removed
edit: added [Invalid or Expired Link Removed]:
Last edited:
Latest posts
-
-
Software drums recommendation for practice Latin, Cumbia styles preferred
- Latest: bass player 53
-
-
