Continental Frankengrand Revision 2!

[Moto_TJ]

On the old site, I had a write up on how I modified my '58 Continental to the specs of the '58 Concert grand with the dual amp setup of the Amp-150 and amp-179 (same basic amp, just some different capacitor and resistor configs, Magnavox did this for some reason from the factory on the Concert Grands). I had also upgraded the woofers to those from a 1961 Imperial and had to install a tuner that had a filament transformer. My goal was to hopefully find a full fledged Concert Grand, but that has not happened.

A few weeks ago while browsing eBay, I stumbled across an Amp-148, the 40 watt output predecessor to the 50 watt output 82-01 amp in the later concert grands. I was able to purchase it, for more than I should have paid, with the intent of using it, with the amp-148 that was in my Magnavox Brittany, to make a full fledged big Concert Grand.

I put an Amp-150 in the Brittany, which the early Brittanys had from the factory. I then went and modified the two Amp-148s to the same specs as the 82-01 amps to get them to the full 50 watt spec. There was not a lot to modify, just a couple of capacitor values, and some resistors changes. Magnavox may have been underrating the original amp-148 units as well, I have thought this about many of their amps.

I also had to get a 54-02 tuner for the unit, as I needed a tuner that had it's own rectifier and power transformer. All of the other tuners got their B+ from the amp, some got their filament power from the amps, depending on the amp. I also upgraded the woofers, again, this time to the Ultra-Sonic woofers from a late-'60s Imperial. I wanted to make sure the woofers could handle all the power that this was creating.

This is how I wanted to do the original Frankengrand project from the get go, but the Amp-148 is somewhat of a rare bird, there are lots of Amp-150/179 units for sale (these are great sounding too!) just rarely ever see a 148 come up.

To say that the Frankengrand sounds great would be an understatement; It sounds phenomenal! Not being able to find a Concert Grand within a reasonable distance and a sane price didn't deter me from making my own! This thing also pulls quite a bit of power from the wall, 36 tubes pull 480 watts, or so the Kill-a-watt shows it pulling. Heats the room nicely!

Currently, to work on it and to make room for my Christmas tree in the Living room, the Frankengrand is living in my Den, which is quite small. I can't wait to get it back into the living room to do some full demonstrations.

Pictures and videos are in the link: https://photos.app.goo.gl/vugpCdAoms54YpZv7

[William]

That's awesome Trevor, and, it's good to see you sharing your latest project. We missed you on the forum. I think I would get confused trying to keep track of all you are doing. But, that does not take much for me. Me thinks you are going to need a bigger house soon.

Bill

[electra225]

Welcome back, Trevor! This sounds like quite the project with a successful outcome. If you don't mind, could you share the changes you made to the AMP-148 to bring it to more like an 82 series? Did any of your modifications have anything to do with bass response? My personal opinion is that if Magnavox had put the 93 series amps in a Concert Grand, that would have been quite the setup. I'm interested in your thinking on this project.

[William]

One of these days, I need to learn what amps went into the different Magnavox consoles. I get confused between 150's, 148's, 82's, and all the rest of them. Has anyone have, or, now where such info can be found?

Bill

[electra225]

Perhaps someone can fill us in on the mono stuff, but, starting in 1959 with the stereo instruments, the 81 series amp went into the big Imperial, it was a stereo amp. The 82 series went into the Concert Grand, two of them, both mono. The 88 series was the smallest push-pull stereo amp and it went into the smaller series, Concerto mainly. The 93 series went in everything else from the 1961 model year. Symphony, little Imperial and Stereo Theaters. 1962 and 1963 saw continued use of the 88 and 93 series, although their use varied from earlier years. The 81 series allegedly died after the 1961 models, but we have at least one example of a 1962 model. The 1960 Symphony models used the AMP-176, so, technically the 93 series replaced it. Since we are dealing with Magnavox, the best way to tell which models a certain amp was used in is to look on the chassis and see what it written in heavy black ink. With a Magnavox, you never know anything for sure.

The 88 series amps and the 93 series amps are quite similar. The 88 series used 6V6 output tubes and a 6CA4 rectifier, the 93 series used 6BQ5 output tubes and a 5U4 rectifier. Plus, the 88 series did not have the sweet sound of the 93. The 88's were no slouches in their own right.

[Moto_TJ]

Welcome back, Trevor! This sounds like quite the project with a successful outcome. If you don't mind, could you share the changes you made to the AMP-148 to bring it to more like an 82 series? Did any of your modifications have anything to do with bass response? My personal opinion is that if Magnavox had put the 93 series amps in a Concert Grand, that would have been quite the setup. I'm interested in your thinking on this project.

What I did is followed the schematic of the 82 series, and whatever was different on the 148, I changed to the 82 spec. Magnavox did have the ohm readings for the output transformers on the 82, and the 148 readings matched the 82. There were not a ton of changes between the 148 and the 82 amps, especially between the !48CB that was originally in the Brittany. I think the biggest changes were in the treble circuit, and some capacitor values in the bass circuit. There were also more changes between the 148AA and the 148CB that I expected.

[electra225]

The reason I'm asking about specifics. is, that when I was going thru my big Imperial with the 81 series amp, I had a recommendation to change certain caps and resistors in the input to the amp, ahead of the tone controls. The theory behind these recommendations was to change the frequency that the treble "crossed over" so it would enhance the bass. I received the recommendation after I already had the chassis back into the cabinet, so never made the changes. I'd certainly appreciate a discussion on what you have found concerning Magnavox bi-amps that will enhance bass. You claim the performance is excellent. I'm not sure I can say that about either of my Magnavox bi-amp instruments. Adequate, perhaps, but nothing to write home about. There has to be an oversight somewhere on my part, or some component that is bad that I'm unaware of.

[Firedome]

I'd submit that enhancing bass response may be better served by looking closely at speaker enclosures or drivers rather than amp modifications as amp bass response is largely a matter of output transformer design as much or more than circuit design. Scott amps are known for their excellent bass response chiefly because they wound their own in-house to Chief Engineer Dan von Recklinghausen's specs due to his dissatisfaction with those available from subcontractors, and they're way oversized compared to those used by others. My understanding is that console mfrs. typically intentionally limited deep bass in order to avoid acoustic feedback.

[William]

Thanks, Greg, that helps me to understand what goes with what.

Bill

[danrclem]

Some and maybe all of the Big Imperials had enclosed speaker systems. I'm pretty sure that some of them had a port on the bottom with a wooden plate to block it off that could be removed. If this plate was removed it very well could improve the bass. If they don't have that port and plate I'm guessing that a pretty good size hole could be bored in the bottom of the enclosure to enhance the bass. It might be hard for me to do that to a console unless I knew for sure that it would work.

[Moto_TJ]

The '67 imperial that I redid has the sealed woofer chamber with the tunnel between the two of them. It also has an amp rated at 150 watts output (supposedly) with 2 of the ultra-sonic woofers and 4 horns. It has a ton of bass, too much at times.

The '61 French Provincial Imperial that I redid had the sealed cabinets with the cut-out in the bottom, there was no evidence of them ever being blocked of, or the possibility to block them off. That had an 81 series amp and had a nice amount of bass, but not overpowering or earth shaking.

[Hydrolastic]

Hello, Just love this.

[William]

You are correct, Danny and TJ, the Big Imperials have enclosed speakers. And, there is a plug in the bottom of the speaker compartment that could be removed. Mine has it! I never thought about doing that as my Big Imperial has plenty of bass. But, my Big Imperial is the mystery Imperial that should not exist. It's the one Greg referred to in this thread being a 1962 model.

Bill

[electra225]

The speaker enclosures on my Imperial are open. Bill, if you don't mind, could you describe the plugs that are in your Imperial speaker enclosures? Maybe I need to make some for mine. Mine appear to be like TJ described that his were.

[William]

There is not much to them. If memory serves, each speaker enclosure has a square opening in the bottom of the cabinet. If my memory is telling me correctly, front corner near the sliding doors. All there is to it, is a larger square piece of wood screwed down to cover the hole. I think the wood plug is 5/8" thick, maybe 3/4". I am not sure if it was glued down or not. I hope this helps?

Bill

[electra225]

That's what I need to know. MIne doesn't have covers over the holes. Doesn't appear like it ever had them.

[William]

I'm glad I could help. Now, I wonder if that is part of the reason mine has more bass than yours?

Bill

[electra225]

That and there are different value components in the input to the amplifiers. Do you have a copy of the schematic for mine? It is dated a couple years older than the schematic for yours. If you get them side by side and compare values, they are different. The best way to determine how these things sound in comparison to each other would be to have them side by side in the same room, under the same conditions. And being listened to by more than one person. I don't see that happening, given the distance between the two instruments. Second best is to have you bring a record you played on yours, come out here, and play it on mine, then have you determine how they each sound. Mine needs a good tune up, no question about that.

[William]

Before I can do that, I need to make my Imperial work. It's stuck in limbo between radio and phono. That phono switch which is part of the function switch is bad. That switch is what allows the phono to shut everything off after the last record is played. I have the new, and I hope correct, switch. I just need to take everything apart again and try and get it installed. It's on my list of things to do.

Bill

[Hydrolastic]

Hello Guy's, Learning more about the magnavox's this had me confused as there were all the different amps out there. Looking at the photo's the amps look the same as the 1958 CG amps. (3 capacitor towers) Its making more sense now. couple of questions, Are the 148 amp biamped as the 182 series were ? The woofers are unlike any i have seen. Are they the later astro sonic imperial speakers? If so how did you deal with the passive crossovers? Hydro