6AQ5 Magnavox Conversion- it's a hummer.

[shaun]

Hi All,
I've converted a cute Magnavox radio/turntable into a guitar amp/ MP3 player. I'm surprised to say it actually works. I couldn't find a schematic for it - Model CR740AA - so I'm flying by the seat of my pants.

Original line up was: 5Y3GT; two 6AQ5s in PP mode; 12AX7 inverter; 6BE6 and 6BZ6 in the radio/turntable preamp section.

I traded the two preamp tubes for a single EF86 preamp, kept the power amp configuration. Added two 1/4" jack inputs, one with low impedance for the MP3 player, one with hi imp for the guitar.

However, I'm experiencing considerable hum, as well as a temporary motorboating sound for the first 10-15 seconds after turning the amp on - it sounds almost like a tremolo has engaged for a few moments, except it has no tremolo:).

I have a couple of suspects. I changed all the electrolytics, including the cap can (old one dead). The problem is I inadvertently got an EC cap can with high + tolerances, so instead of one section being the required 40uF, it actually reads 50uF on my meter. The other sections read 25uF instead of 20, and 13uF instead of 10. I'm wondering if the collective increase is causing problems further down the line.

The other thing that's interesting is the way the filter system is designed. I've attached a very crudely drawn schematic. The main difference between this and a typical guitar amp is the way this one has a B+ second node that runs direct from the rectifier cathode instead of the other side of the first filter cap. I wonder if that's causing the filtering to behave oddly. I'm also loading up a pic of a Skylark schematic that I found in an earlier 6AQ5 post on this forum, which uses 6AQ5s, just as a comparison - the second node is connected to the first node, not direct to the rec cathode. But I'm sure you know this already.

Shielding does not seem to be the problem; I've used shielded cable for most signal lines, and when I cover the open chassis with shielding, the hum is not reduced in any way.

Thanks for your help - always greatly appreciated.

[sluckey]

I don't see anything unusual about the power supply. Pencil is a little wonky. Did you work for Sears back in the '60s? 

[shaun]

Ha! However did you guess? So I guess it makes no difference when the 2nd node comes straight off the cathode. Guess I've been overthinking things lately.

[sluckey]

Ha! However did you guess? So I guess it makes no difference when the 2nd node comes straight off the cathode. Guess I've been overthinking things lately.

What are you talking about? Node A is the only node connected to the rectifier cathode. Look at you own drawing. Yeah, it's drawn a little wonky but surely you can see that electrically, it's the same as thousands of other amp power supplies.

[shaun]

Hmmm. Okay.

[shaun]

My ignorance overwhelms me sometimes, but I learned about an unusual transformer filament output in this unit.

One filament lead measures 4.6 vac at 120v, and the other measures 10.75vac at 120v. When I place the leads across them, I get 7vac at 120v, which is a little high but about right for 115v.

I don't understand how to wire this filament system, and I can't find anything online other than the typical trans with a 6.3v filament output.

It's running two 6aq5s, a 12ax7 PI, and an EF86 pre. 5Y3 rec. I suppose I could buy the appropriate trans, but I like to keep things as original as possible. And I like saving $$ of course.

Thanks for any help - much appreciated. (I still haven't found a schematic for this.)

[sluckey]

One filament lead measures 4.6 vac at 120v, and the other measures 10.75vac at 120v.

In reference to what? IOW, one meter probe is connected to a filament lead but what is the other meter probe connected to?

When I place the leads across them, I get 7vac at 120v, which is a little high but about right for 115v.
I don't understand how to wire this filament system,

The two leads should connect to the two filament pins of a tube. But you said the amp was working, so can't you just look at the working amp and see how the filaments are wired?

[PRR]

> One filament lead measures 4.6 vac at 120v, and the other measures 10.75vac at 120v.

And there's no CT, so NO connection to where your other meter probe goes?

Then you are just measuring leakage. Put a resistor across your meter, both readings will fall.

Try it the way you will use it. Two 100R to ground. I bet now you get 3.5V both sides. The 100rs swamp-out leakage.

You expect 10% sag from no-load to full load. From 7V you will end up very near 6.3V.

[shaun]

Very sorry for my unclear process here. My questions are an indication that I've lost my marbles. Thank you for your patience. I was reading the filament voltages with reference to chassis ground. Why? I don't know, unless I was confused by the fact that the filament wiring going to one 6QA5 (fil pins are 3 & 4) was also connected to the cathode, pin 2. I guess I started trying to understand why the filament would connect to the cathode on only one power tube. From there, I extrapolated all sorts of incorrect ideas, then I went completely off into some extraterrestrial daydream.

I'm gonna blame the aging process. Also, at my level of inexperience, I find it much easier to build straight from a proven schematic than to try and modify an existing amp without any schematic, when my limitations are sorely tested.


Obviously, if the filament voltage reads 7vac through the meter, at 120v into the PT from the variac, I can simply wire it like any other typical heater wiring, then add 100 ohm resistors to float it. I was creating problems where none existed. Doh. Thanks once again for your patience.

[sluckey]

Obviously, if the filament voltage reads 7vac through the meter, at 120v into the PT from the variac, I can simply wire it like any other typical heater wiring, then add 100 ohm resistors to float it.

If you decide to do this then you should disconnect the filament reference to pin 2 cathode of 6AQ5.

[shaun]

Yep, did that. Now I've got big motorboating issues again. I'm in way over my head, which is why I should probably stick to tried and true schematics for a year or two more. I'm okay with monkey see monkey do, but monkey do math? Not yet:).

[shaun]

Heyo. I found the required schematic for the Magnavox amp151AA which is apparently the same - or very similar to - amp151BB. It sure makes things easier to understand. I'm posting it here for other interested parties, along with the data sheets. From Sams for $15.

The connection btwn pins 2 and 3 of one 6AQ5 seems to provide a ground for the heater. One other item to note is the amp was designed with a 6X5 rectifier, which, according to the schematic, is wired differently from the 5Y3 that came with the amp. The amp I have is wired for a 5Y3, but there were also heater leads attached to the filament pins of the rectifier socket, I suppose just in case someone wants to switch it back to a 6X5 - almost like they got half way through building it and then modified to the 5Y3. The 5Y3 has Magnavox printed on it and seems to be original, so I'll roll with that for now.