It's push-pull, cathode bias, like a 5E3, with basically the same power supply. You'd need to add another 9-pin, but that shouldn't be hard. Got a shot of the inside?
It's a push-pull design. The V1a drives the first 6V6 directly and then that signal is tapped-off and sent to V1b to invert the signal and drive the 2nd 6V6.
If I had that amp, I'd probably put it to use as a stand-alone poweramp to use for experimenting with stand-alone preamp designs but I would somehow try and get a volume pot after the input jack.
What you have is a power amp. It's a paraphase PI driving a push/pull power amp. I would fix it just as it is. Then you could plug in almost any guitar amp voiced preamp into the input jack. Maybe even build a preamp in a small chassis, bolt it to that chassis and use the B+/filaments supplies from the power amp to power the add on preamp.
Give us a gut shot. I suspect you can drill a couple of holes in the chassis for volume, tone, and input on the front, an output jack on the back, and a 12AX7 on the deck. A gut show will allow an informed opinion. It might be too tight for all that, in which case, the separate pre-amp chassis would be a great plan. Right now, my thinking is leaning towards a 5F2A-like preamp. That's a great project platform!
martin manning wrote: ↑Sun Mar 19, 2017 6:36 pm
It's push-pull, cathode bias, like a 5E3, with basically the same power supply. You'd need to add another 9-pin, but that shouldn't be hard. Got a shot of the inside?
Yes, here it is:
Magnavox_164-BA_inside.JPG
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I am wondering about your level of experience with:
-amplifier build/repair
-guitar amplifiers specifically, which can be a bit different from hi-fi
-schematics
-vacuum tubes
-recycling/repurposing old amp chassis
The reason I ask is the content of your original post. It seems you didn't know if it was a push-pull amp and you suggest there is no room for volume and tone controls. There is a concern out the size of the transformers and their adequacy for the intended purpose.
We established that the amp is PP. Looking at the gut shot, I know I could add a tube, a couple of pots, and a couple of jacks in that chassis. It might get tight in there and it would probably help to rearrange a few things. Also, I don't see a fuse or a switch, both of which are essential and will require two more holes and I think there's plenty of room for those near the power transformer.
As for the transformers, it is reasonable to expect they are up to the task or Maggie wouldn't have used them. Also, sometimes the magic in a guitar amp is smaller iron.
If this is a project you'd like to undertake, I think you've come to the right place. There are people here who will help you. To do that well, we need to understand who we are working with and the scope of the project. Please tell us about your experience level. The folks here are generally a welcoming and forgiving bunch.
Phil_S wrote: ↑Sun Mar 19, 2017 10:08 pm
I am wondering about your level of experience ...
Good point. I have some varied electronics knowledge from the military & school (both long ago), but not a lot specifically about tube amps beyond general understanding of electronic principles - and a healthy respect for safety issues. I've recapped a few old amps and cleaned up obvious problems, but never really built from scratch or designed anything ... and I'm out of practice, the last such project being over 20 years ago. Other than that I'm an expert
I think this will be a good project based on the input I've received here - I can play around with some preamp circuits without having to redesign the rest of it.
OK then. It will take a bit of effort. It might take a bit for you to ramp up. I suggest going for something relatively simple and with a low parts count. There are many possibilities. I should tell you that I'm an amateur, not a pro, so take everything I say with a grain of salt. I've done a few projects like this. If others get involved, opinions about what to do will vary. I suggested the 5F2A front end because it has a low parts count and it will get the job done. You can look around for something else, too. I suggest that you look for something that will work with one 12AX7, so you don't have to add more tubes. We don't know if the filament supply in that amp will give you the 0.3A you need to support the filament for one more tube, but it is probably not too much of a stretch. The thing to do is get the socket installed and wire the filament harness and then see what the filament voltage is with all the tubes in sockets.
I think the 5E3 is too ambitious and the available space is going to be a real challenge. It's too many pots and too many parts for the available space. It involves reworking the phase inverter -- not a problem, but one more thing to do. I might build the 5E3 with a second separate chassis and an auxiliary filament transformer, but that seems like a lot of work to me and more complex.
I'd pass on the Gibson circuit. Many of the circuits from that era have an annoying notch filter that today's guitar players seem to dislike.
Here's how I think I'd approach this. Remember, my idea isn't the only idea. It's just one person's idea.
Remove the RCA jack from the deck. Open up this hole for a noval socket. It's an acceptable location. Remove the terminal strip. If you need those caps and resistors, they have to be relocated and that's something to deal with later. Doing that will open up a corner to set either a small turret board or a few terminal strips to build the preamp you need. At that end, on the front, the side marked T-7, first remove the plug that's there. Hopefully that hole can be reused. It looks close to being OK for an input jack. You may need to close it with a plate. Hopefully, on the front, you can find enough space to drill holes for an input jack, and two pots. It's going to be tight. If it's too tight, move the input jack to the back side in the corner near the new 12AX7. Since that's going to take the input from the jack, it's OK if they are close together -- actually its a good thing.
(As an aside, I think I'd look at replacing all the caps in that amp. Modern caps won't leak DC and they will be much smaller physically.)
On the rear panel, drill for an output jack on one side of the back 6V6. Exact location depends on measurements. I can't tell real well from the pictures.
Over on the power transformer side, you'll need to drill for a strain relief for the 3 prong power cord. I'd put this in the rear corner. (Get a surplus IEC cord from the computer tech guys at work -- they have a bin full they will never use -- and cut off the IEC end, or go to Target where you can buy a 16 or 18AWG extension cord for about $3.) Also, drill for a fuse holder and a power switch. The power switch can go on the front or back and the fuse on the rear. It will be tight, but it looks to me like it can be done. If I wanted to install a power indicator lamp, I'd use the 120V type to avoid taxing the filament winding further. That can go in the front corner opposite the fuse.
A project like this is 80% planning and 20% doing. It is really important to think through where everything will go. Measure, measure, measure. Then measure again. Think in 3 dimensions. Take your time. If you move forward here, this is likely to be a long thread. That's OK.
How this plays out exactly depends on what you build into it. There's nothing wrong with the existing circuit from what I can see. The paraphase or see-saw inverter was used by Ampeg and others to good effect.
Phil_S wrote: ↑Mon Mar 20, 2017 1:43 am
OK then. It will take a bit of effort. It might take a bit for you to ramp up. I suggest going for something relatively simple and with a low parts count ...
Thanks Phil, all good info. This sounds like it's going to be a fun project.
OK, now I'll demonstrate my ignorance. Have a look at this diagram: I just plugged in the Champ pre-amp circuit to see how that might fit.
So we have the existing 12AX7 there acting as both an AF (Audio Frequency) amplifier and Phase Inverter, the start of the power section.
Then the new 12AX7 works as a preamp & driver, and hands the signal off to the power amp.
Does that make sense or am I way off base?
164_BA-schematic_New_Preamp.jpg
I'm also guessing some of those resistor & capacitor values may need adjusting due to a difference in the voltage between the Champ circuit & this amp.
BTW, I like the idea of using a lower value tube as Kagliostro suggested, but I can almost get my head around this example since I'm used to thinking in terms of Champs from reading the Dave Hunter book. So, this is mostly just trying to make sure I understand how adding a preamp section might work.
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Something like this. You are wasting half of a 12AX7. You might see if you can find a 6AV6, which is going to be cheap and is just one triode and equivalent. It uses a 7 pin socket, which is slightly smaller than a noval socket. Every little bit will be helpful in that small chassis.
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