Magnavox 9300 series PP 6BQ5 console amplifier

[mike stehr]

I picked up the Maggotbox puss-pull amplifier back in 2003...the retired EE and I noodled with the amplifier back then. Some input resistance was used to make up for the balance pot missing, (in the pre-amp/tuner) which is tied in with the overall feedback. I listened to the amplifier for some time, but eventually shelved it.

The amplifier had good midrange I suppose,but the bass was a bit lacking.

Being a PP 6BQ5 amplifier, the amount of power didn't seem all the great...meaning once you start to turn things up a bit, the amplifier would just sound a little worse.

Having single-ended amplifiers that sound pretty much as loud, with more perceived sound quality, and eventually finding and rebuilding a Scott LK48B integrated 7189 PP amp, the Maggotbox 9300 was pretty much forgotten.

 

I've been reading through this thread on AK, in which Dave Gillespie experiments and does the necessary modifications to get the 9300 circuit more of a stand-alone amplifier that can be used with modern external speakers...and away from the open baffle stiff cone console stereo design that Magnavox (voiced?) the amplifier for.

http://audiokarma.org/forums/index.php?threads/more-fun-with-magnavox-the-9300-series.687735/

 

I just spent 30 bucks for the parts needed, I have a 40/40/40uF 450volt Sprague multi-cap that formed up rather quick and nice. "Don't be vague, ask for Sprague"

Tore down the chassis, and gussied it up the best I could along with the multi-cap, and replaced the lame phenolic sockets for 6EU7...I may paint just the frames of the OPTs and choke.

Maybe the Maggotbox can be resurrected...

The image is from 2003

 

Mike

[avguytx]

I know these aren't the be all-end all of tube amps but they are enjoyable for what they are. It seems the output transformers are their weak link...as they are in most console pulled tube amps. They were built and installed for a design/cost goal. But I still have two of them...the Maggie 9304-20 and the Stromberg Carlson ASR-120. I may replace the OPT's or just buy something newer and sell these two off. But I've enjoyed them for about 8 years now. For what they are. Haven't tried Dave's circuits but all caps and resistors on mine have been replaced. 

[JL Sargent]

The only thing lacking for me in these type amps is the OPTs. Not enough iron there for any real bass.  For a long time I used an amp like Mikes for the top half of a big two way horn setup. I was pretty happy with that. I have a few of those here. Still love my rebuilt SC ASR-120. Listen to that one all the time. 

 

 

[avguytx]

It's surprising the amount of low end they do actually have given the OPT's. They both sound pretty good on my CF-3's but could definitely have a little more. Maybe it wouldn't be as noticeable on a pair of Belle bass bins given their LF rolloff point 

[JL Sargent]

20 minutes ago, wdecho said:

Put a good sub into the mix without changing the OPT's. 

 

Seems like a good idea on the face of it. But, unless your using a pre/pro before the tube amp or active crossover, this is kinda tough to do. For instance, how to go from a pc to this little amp AND a sub and keep it simple?

 

PC to tube amp (full range) to speakers = simple

PC to DAC to Pre/pro to active crossover (with sub out) to amps to speakers and sub = not so simple.

 

 

[davesans]

Good luck with your project will be exciting to hear about the results. I have a amp project also coming soon. 

 

Regards

Dave 

[avguytx]

I use a highly updated Carver C-1 preamp in front of whatever amp I use plus each digital source has a DAC.  Actually, the only source I really use anymore is my media server.  I've not played an actual CD in over 2 years.  I spin some vinyl once in awhile and that's why I like the C-1 with its dual phono inputs with upgraded IC's.  Seems like most of the Chinese made tube amplifiers are integrated and I don't really need that...if I ever get one.

[mike stehr]

If one reads through the Gillespie/Magnavox thread, you realize the amplifier design is centered around the output transformer's resonant characteristics.

Using square waves, this resonance shows ringing within the upper audio band, which can't compensated with a fdbk compensation cap across a fdbk resistor in a typical way.

Too much compensation, creates an unstable speaker load with regard to higher frequencies, not enough creates distortion, too much midband response, etc...

 

Magnavox used 12db of uncompensated feedback, and limited the HF response at the input of the 6EU7, the RC loading of the tube along with the HF roll-off of the inherent imbalance of a paraphrase inverter stage. Basically choking off the high-end response of the amplifier, to get away from ringing/resonance of the OPTs.

 

After reading all this I would wonder why would somebody buy higher quality output transformers when the amplifier topology is designed around an OPT with terrible HF ringing/response characteristics. One would have to get rid of the HF compensation within the AF/phase inverter stage, along with a different or more common feedback loop/scheme. Then the common paraphrase inverter works in theory, but as mentioned it has a frequency imbalance issue. May as well use a better known type of phase inverter circuit.

Now a guy is down to a 50 henrie choke, a chassis, and a PS transformer. May as well buy some nice 6BQ5 PP OPTs and start a scratch built amplifier, centered around better amplifier topologies.

 

[JL Sargent]

To Mikes point, I've never replaced the OPTs on a Maggie. I simply understand there is trouble there. I rebuild with newer/bigger opts the completely different ASR 120 6V6 amp which is a whole different animal. I do know it takes some iron to make a good bottom end on tube amps in general.

 

 

2 hours ago, mike stehr said:

Using square waves, this resonance shows ringing within the upper audio band

I wonder what freqs? If it's above 15K many of us in the older than 50 crowd can't hear it anyway.

[tube fanatic]

Mike, try putting 1000 to 4700 pf across the input jack or volume control.  It will probably eliminate the ringing.

 

Maynard

[CECAA850]

On ‎12‎/‎10‎/‎2016 at 9:21 AM, JL Sargent said:

PC to tube amp (full range) to speakers = simple

Speaker level inputs to sub plate amp = simple.  You guys always make trying to hook a sub up into such a big deal LOL.

[JL Sargent]

42 minutes ago, CECAA850 said:

Speaker level inputs

And that may be ok, but that speaker level signal is divided in the plate amp by a "resistor" network to feed the amp. You know what Bob says about resistors in the circuit at speaker levels. That's a big no no. Might as well install a bunch of L-pads in our speakers too.

 

 

 

 

[CECAA850]

14 minutes ago, JL Sargent said:

And that may be ok, but that speaker level signal is divided in the plate amp by a "resistor" network to feed the amp.

Parallel the wiring at the amplifier.  Send one unmolested signal to the speakers.  Send the other to the plate amp.  There's no need to use the plate amps high level output to the speakers.

[CECAA850]

If your room dictates, you can parallel the signal at the plate amps high level input as well.  The speakers still get an unmolested signal that way as well.

[JL Sargent]

2 hours ago, CECAA850 said:

There's no need to use the plate amps high level output to the speakers.

 

I was referring to this:

The plate amp itself divides the speaker level input with a resistor network to feed the plate amps amplifier section.

If you use line level input at the plate amp, this is avoided.

[CECAA850]

I'm nor sure I get your point then. You're worried about  the signal the subwoofer driver gets?

[CECAA850]

No matter which input you use a filter of some sort will be used.  I doubt you could hear the difference using either method.

[mike stehr]

Split it at the source.

 

On ‎12‎/‎10‎/‎2016 at 2:36 PM, tube fanatic said:

Mike, try putting 1000 to 4700 pf across the input jack or volume control.  It will probably eliminate the ringing.

 

Maynard

 

I have the amplifier torn down, and in the process of putting it back together...awaiting parts.

 

When I first picked up the Magnavox amp back in 2003, I had taken the unit to my retired EE audio friend's place for a bench test.

The amplifier had too much input gain, (from the lack of resistance from a missing balance pot, located in the pre-amp/tuner) and was corrected with extra resistance at the input. 

The AF and paraphrase inverter were left unchanged as stock Magnavox, but the coupling cap values were changed from 0.0015uF/0.047uF to 0.033uF/.47uF.

The amplifier still used the same 3.9K feedback resistors, but feedback compensation capacitors were used in the common way, across the feedback resistor. 1000pF silver micas.

 

So back then Dan helped out and adjusted the amplifier on the bench for the ringing output transformer issues. I never noticed/heard any HF instability when using the amp in the past...of course, maybe I can't due to the roll-off of my hearing. It did pass the bench test when I left that day...the amplifier had a good sound in the midband, but did seem sorta rolled-off on top. The bass was good in sound, but attenuated in the overall sound.

It didn't seem much louder power-wise than the single-ended amps I have, with less or the same amount of bass, depending on the amp.

 

I figure I can try the Gillespie/Maggotbox mods, with the negative regulator used for "Enhanced Fixed Bias", along with the conversion to floating paraphrase inversion. Albeit not too much better the standard paraphrase, but still sorta falls in line with the classic Magnavox paraphrase inverter.

It doesn't cost much to do the modifications and see/hear how it goes, and I could always change the circuit back to the way it was if things sound/look worse.

 

Mike

 

[JL Sargent]

1 hour ago, CECAA850 said:

I'm nor sure I get your point then.

 

The whole reason Klipsch used the autoformer in all those speaker networks was to avoid resistors at speaker level voltages. Seems like a sin to introduce a big resistor based diving network back in to your subs signal. Not worried about it, but simply pointing out a difference between the two inputs. No doubt taking the signal to speaker level twice on its journey to your sub degrades it. Now as to me hearing the difference? 20 concerts, chainsaws, firearms, dirt bikes, and fire crackers all say its a miracle I can hear at all.

 

[JL Sargent]

46 minutes ago, mike stehr said:

I never noticed/heard any HF instability when using the amp in the past...of course, maybe I can't due to the roll-off of my hearing.

 

I took an online hearing test with my 12 year old son. I maxed out at about 14K, but my son took it to 22K no problem. I mention this because  I've never experienced the top end Maggie problem you describe.