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 Post subject: Magnavox AMP 169 conversion to guitar amp
PostPosted: May Tue 11, 2021 6:53 pm 
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Location: Culpeper, VA
I got an amp off eBay that appears to be a AMP-169. If I'm right, it came out of a small Magnavox mono console with a tuner and phonograph.

I want to convert it for a guitar amp. I'm attaching a schematic and a pic of the chassis wiring.

For a preamp, all it has is a 6AN8A tube which is a combination pentode/triode. The pentode is an amplifier and the triode section is used as a cathodyne phase splitter feeding into 2-6V6s. The rectifier is a 5Y3. I've also got the Loudness/Bass/Treble "control module."

The 6AN8A "preamp" looks weak to me and I'm wondering if anyone knows this model? I've seen pentode pre-amp sections before but never as the sole preamp amplifier.

Anyone know this model? Did it have an upstream tube pre-amp chassis associated with the tuner/phonograph?

I'd like to insert another dual triode, maybe a 12AX7 or even a lower mu 6SL7 to beef up the front end. Then I'll feed that signal into the control module and on through to the 6AN8A and the rest of the existing amp circuit.

Does this make sense or not?

Does anyone have any experience with a 6AN8A? I've never run across it before. I'll attach a data sheet.

Thanks in advance,

Rich


Attachments:
AMP-169 Schematic.jpg
AMP-169 Schematic.jpg [ 545.89 KiB | Viewed 797 times ]
GutView_1.JPG
GutView_1.JPG [ 2.77 MiB | Viewed 797 times ]
6AN8_data.pdf [473.3 KiB]
Downloaded 32 times

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 Post subject: Re: Magnavox AMP 169 conversion to guitar amp
PostPosted: May Wed 12, 2021 2:49 am 
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Location: Rochester NY USA
Heath, Dynaco, others used the 6AN8 for audio - pentode can have plenty of gain, more than any triode, which can be used with negative feedback to reduce distortion and flatten frequency response. Magnavox used R-C networks to "shape" a non-flat response to compensate for speaker, cabinet, simple tone controls. The bass control does nothing unless it's connected to a ceramic phono cartridge, treble is "cut" only. Look at a Fender or other guitar circuit for suitable tone controls.

Oh, that phase inverter circuit may cause low-frequency instability - C10 should be smaller (.0047 - .01) like in most other Magnavox amps.

Reduce or remove feedback by increasing or removing R21. One more stage is probably enough, even with a Fender style tone setup added. 6AV6 or 6AT6 is approximately half a 12AX7, could do. Two stages might work, but would likely have to be on a separate chassis, noise and oscillation likely anyway.

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 Post subject: Re: Magnavox AMP 169 conversion to guitar amp
PostPosted: May Wed 12, 2021 12:38 pm 
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Joined: Nov Sun 22, 2020 7:57 pm
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Location: Culpeper, VA
Tom, thanks for all that. Exactly what I was looking for. I hope you'll answer a couple more questions for me.

This is only my second amp build. The first conversion I did also had a pentode front end and it seems to work well enough. But almost all guitar amps (Fender, Marshall, etc) have dual triodes in that position. One reason I've learned from reading is that triode voltage amplifiers provide a big initial boost to the ~20-50 mV guitar signal.

None of the books I have on guitar amps discuss pentodes as first stage voltage amplifiers. I don't see the reasoning to put a pentode in the first stage instead of a high-mu triode. Actually I don't understand exactly how pentodes work as voltage amplifiers at all. It seems to me they're main use is to serve as current amplifiers in the output stage. But again, I haven't found much material explaining pentodes as preamp tubes. Is there a quick way you can explain why, or can you point me to a resource that discusses it?

Also the tubes you recommended have a single high-mu triodes with a diode in the same bottle. Is there some use for the diodes that I don't see? It seems to me it would be better to use a dual triode 6SL7 or 6SN7 there and I already have them on hand. Would they work as well?

Thanks again,

Rich

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 Post subject: Re: Magnavox AMP 169 conversion to guitar amp
PostPosted: May Wed 12, 2021 2:58 pm 
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Location: Rochester NY USA
The 6AV6 and 6AT6 were used as diode detector and audio amp in AM radios. Just leave the diodes unconnected. Could fit on the existing chassis.

6SN7, 6SL7 tend to be more microphonic than the smaller ones - the higher the gain, the more this becomes a problem. And of course require more space on the chassis.

Pentodes have higher noise than triodes, but this only matters in the first stage of a very high gain amplifier. And there are lower noise triodes than 12AX7, but not as easy to design around. Pentodes can have higher gain, but the extra parts required mean more variability in mass production. A simpler circuit that's "good enough" will win out every time.

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 Post subject: Re: Magnavox AMP 169 conversion to guitar amp
PostPosted: May Wed 12, 2021 3:51 pm 
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Joined: Nov Sun 22, 2020 7:57 pm
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Location: Culpeper, VA
Tom,

Thank you for all this, I appreciate the voice of experience. I also like "simple" and "good enough."

Given that guitar amps are designed more for output watts, a little noise doesn't worry me too much, but it sounds like the 12AX7 is probably overkill driving into the 6AN8. I will look into the 6AT6 and 6AV6.

Rich

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 Post subject: Re: Magnavox AMP 169 conversion to guitar amp
PostPosted: Jun Tue 08, 2021 6:54 pm 
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Joined: Nov Sun 22, 2020 7:57 pm
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Tom,

I took your advice and added a 6AT6 triode as a new first stage to this conversion. I want to add enough headroom to handle a passive bass-treble tone circuit but less than the 6AT6's mu=70. I'm shooting for a boost in the 6dB range but my design choices are mostly copied rote from other schematics, with some guesswork thrown in. Can you critique this? And maybe answer a couple of questions?

1. On the 6AT6, I left off a cathode bypass cap on purpose, to add another restraint on the overall gain. I figure I can add it later as needed. Does that make sense?

2. Should I move the negative feedback from the pentode to the cathode of the 6AT6 or leave it where it is? Or just remove it since I'm not trying to match to any Magnavox speakers?

3. Does a grid leak of 2.2M make sense in this context or would 1M be a better choice?

Anything other comments would be welcome of course.

Thanks in advance,

Rich


Attachments:
AMP169_SchematicMOD_3.pdf [54.33 KiB]
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 Post subject: Re: Magnavox AMP 169 conversion to guitar amp
PostPosted: Jun Wed 09, 2021 5:31 pm 
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1. Yes, fine. Sometimes introduces more hum without a cap, but the heater bias you're using helps a lot.
2. You may want to remove negative feedback entirely, but at least eliminate the .047 uF cap - that will boost bass as feedback is reduced at low frequencies. Opposite side of secondary must be grounded.
3. Any value from a few megs down to 47K is fine. With no cathode resistor, 3-8 megs will give "grid leak" bias. Overloads more easily, though, so not always good if used with a pedal. But harmonica players like amps with grid leak bias.

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 Post subject: Re: Magnavox AMP 169 conversion to guitar amp
PostPosted: Jun Wed 09, 2021 6:40 pm 
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Joined: Nov Sun 22, 2020 7:57 pm
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Location: Culpeper, VA
Tom,

You've answered a lot of my guesswork. I did leave off the OT ground when I transcribed it but it will be in the build.

Beyond that, I'm kind of surprised to have gotten it into the ballpark at all.

Thanks again,

Rich

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 Post subject: Re: Magnavox AMP 169 conversion to guitar amp
PostPosted: Jun Wed 09, 2021 6:43 pm 
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Joined: Nov Sun 22, 2020 7:57 pm
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Sorry, I may have sent that too soon.

Speaking about the OT secondary ground, should it be grounded only when the negative feedback is used? And if I remove the negative feedback completely, then I should remove the ground from the OT secondary?

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 Post subject: Re: Magnavox AMP 169 conversion to guitar amp
PostPosted: Jun Thu 10, 2021 12:00 am 
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Location: Rochester NY USA
It needs to be grounded to complete the circuit for negative feedback. Makes no difference without it, though modern safety standards call for accessible metal (like the speaker terminals...) to be grounded or double insulated from dangerous voltage. An output transformer may not qualify as double insulation. So ground it either way.

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 Post subject: Re: Magnavox AMP 169 conversion to guitar amp
PostPosted: Jun Thu 10, 2021 3:22 am 
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Joined: Nov Sun 22, 2020 7:57 pm
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Location: Culpeper, VA
Got it. Thanks again for all the great tips--I have learned a lot.

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