I am converting a 50’s Magnavox record player amp to a guitar amp.
It has a 6av6 as preamp, 12ax7 as first stage and phase splitter, and 4 x 6v6gt as output.
I have made some mods ona knowledgeable persons advice to remove the riaa compensation. Have replaced all the tubes.
It sounds good, really clean. However it has not near the volume i think it could have.
I was thinking a larger B+ voltage (a different power transformer), but not sure.
Is the solution that and or output transformer, what other component values would need to be changed.
I have the schematic.
I am not an EE but have a lo of experience in electronics, just not audio; so you will not be talking to an idiot.
The plate voltage on this is 280, my 65 deluxe reverb its about 400, why I’m thinking about the transformer.
If anyone want to assist me with this I’m willing to pay or whatever. Just above my pay grade and I don’t want to smoke this thing. It’s older than me.
Thanks in advance
Brian
It has a 6av6 as preamp, 12ax7 as first stage and phase splitter, and 4 x 6v6gt as output.
I have made some mods ona knowledgeable persons advice to remove the riaa compensation. Have replaced all the tubes.
It sounds good, really clean. However it has not near the volume i think it could have.
I was thinking a larger B+ voltage (a different power transformer), but not sure.
Is the solution that and or output transformer, what other component values would need to be changed.
I have the schematic.
I am not an EE but have a lo of experience in electronics, just not audio; so you will not be talking to an idiot.
The plate voltage on this is 280, my 65 deluxe reverb its about 400, why I’m thinking about the transformer.
If anyone want to assist me with this I’m willing to pay or whatever. Just above my pay grade and I don’t want to smoke this thing. It’s older than me.
Thanks in advance
Brian
Well, post the schematic so we can see it too.
Power is not volume. I suspect for guitar, your preamp lacks the gain of a guitar amp.
Power is not volume. I suspect for guitar, your preamp lacks the gain of a guitar amp.
Picture of the circuitry also. Might be a good idea replacing the power supply caps given its age.
What are you using for a speaker? If you're using the original record player speaker it is going to be a lot less efficient at converting electrons to sound. Open back cabinets are louder than sealed or closed back cabinets, too, so an old speaker in a closed cabinet will not be as loud as a modern speaker in an open back cabinet.
A 1950s Magnavox phono was likely a ceramic needle and therefore no RIAA in the electronics.
A four 6V6 Magnavox into guitar-rated speakers should be a very strong guitar amp. Yes, four 6V6 working easy at 280V can be just as much power as two 6V6 working hard on 400V. I'd much rather have the quad and keep the voltage down.
Gain is not power. If gain is low, you have to work too hard to get all 20+ Watts it can do.
Post your schematic, stock and as you have modified it.
A four 6V6 Magnavox into guitar-rated speakers should be a very strong guitar amp. Yes, four 6V6 working easy at 280V can be just as much power as two 6V6 working hard on 400V. I'd much rather have the quad and keep the voltage down.
Gain is not power. If gain is low, you have to work too hard to get all 20+ Watts it can do.
Post your schematic, stock and as you have modified it.
http://www.shedradios.com/images/254_7_PG4.jpg
I don't see how to insert an image directly here.
the model is CP251M (amp-128a, b,amp-129)
the mods I've done are:
Removed C5, C12, C15.
Jumpered around R4.
This was recommended by Jim Kelly of Jim Kelley Amplifiers, so I think
he knows what he's saying.
as I said I have replaced all the tubes, filter caps should be here in the next few days.
I don't see how to insert an image directly here.
the model is CP251M (amp-128a, b,amp-129)
the mods I've done are:
Removed C5, C12, C15.
Jumpered around R4.
This was recommended by Jim Kelly of Jim Kelley Amplifiers, so I think
he knows what he's saying.
as I said I have replaced all the tubes, filter caps should be here in the next few days.
Piezoelectric cartridges generate a signal level proportional to the *amplitude* of wiggle in the record groove. Magnetic cartridges generate a signal proportional to the *velocity* of needle motion in the groove.
A constant +6 dB/octave treble boost, rising with frequency, actually makes a constant-amplitude groove in the record.
If you look at the RIAA recording equalization curve, it is a +6 dB/octave treble boost over most of the audio frequency range, except for down near 20 Hz, and a fairly narrow region around the middle of the audio range, centered on 1 kHz.
As a result, a ceramic / piezoelectric cartridge tends to produce a more or less flat frequency response on its own, with no electronic equalization, over most of the audio range, except for that region around 1 kHz. A perfect (but unequalized) ceramic cartridge playing back an RIAA equalized record would have a flat response from maybe 50 Hz to 500 Hz, a smooth drop of a dB or two, and then another flat response above 2 kHz or so.
In practice ceramic cartridges were usually of the "cheap and nasty" variety, with multiple mechanical resonances, cheap spherical needle tips (rather than elliptical or hyperelliptical), and so on. So the typical ceramic cartridge wasn't very flat, but neither is the screechy little "speaker" in a contemporary smartphone. Ceramic cartridges were still "good enough" for many people, just as those tinny screechy smartphone speakers are today. (Yeah, I know people who listen to music on their phones without earbuds or headphones.)
-Gnobuddy
On top of that you want to take out C6 and R6, Then put a 2.2 - 22mF cap across the 1.5k resistor. 2.2 adds some gain to the high end, moving up to 22uF brings in the bass. Even 10uF should get you the full guitar range.
Check to make sure of the resistance values of the other resistors.Other than that I would put a boost pedal in front of the input and see what it will give you. If you wanted to keep it as is, you could put a SS gain stage up front, take out the 6AV6, open up the hole a little more to fit in a 9 pin socket and put a 12AX7 in there. Pick what kind of amp you want to sound like and add parts to the 12AX7 to taste. I would not touch the inverter to output section other than take out the NFB cap as noted above.
> The RIAA equalization is introduced in the vinyl mastering process
Well, no. NAB/RIAA is a playback standard for magnetic playback. The disk mastering does all sorts of EQ for cutter and other mastering issues; so-that it will playback right on a RIAA EQed magnetic needle.
As Gnobuddy says, there is a fundamental difference between magnetic and piezo. Given constant groove wiggle-width (a fair approximation to the limits of the medium), a magnetic needle gives a *rising* response. The fall of the published RIAA curve is not for the record, but for the magnetic transducer.
The kinks at 500Hz/2KHz are mostly historical artifacts. The needs for bass and treble EQ arose separately as narrow-band recording improved. Different companies (both recorders and players) did things different. The NAB committee picked a split-difference round-number compromise. Somewhere in there a 50Hz corner was written in.
With a piezo, output voltage is proportional to groove wiggle width, not rising like a magnetic needle. With no EQ, the response is "nearly right". The remaining error is the 500Hz/2KHz joggle. In many ceramic players, the total error (cheap circuits, junk transformers, crap speakers) is greater than the error of ignoring the jog. Or a screechy speaker can approximate the required rise. However most ceramic makers could pick some rubber compliance to set the <500Hz some dB down from the >2KHz.
Well, no. NAB/RIAA is a playback standard for magnetic playback. The disk mastering does all sorts of EQ for cutter and other mastering issues; so-that it will playback right on a RIAA EQed magnetic needle.
As Gnobuddy says, there is a fundamental difference between magnetic and piezo. Given constant groove wiggle-width (a fair approximation to the limits of the medium), a magnetic needle gives a *rising* response. The fall of the published RIAA curve is not for the record, but for the magnetic transducer.
The kinks at 500Hz/2KHz are mostly historical artifacts. The needs for bass and treble EQ arose separately as narrow-band recording improved. Different companies (both recorders and players) did things different. The NAB committee picked a split-difference round-number compromise. Somewhere in there a 50Hz corner was written in.
With a piezo, output voltage is proportional to groove wiggle width, not rising like a magnetic needle. With no EQ, the response is "nearly right". The remaining error is the 500Hz/2KHz joggle. In many ceramic players, the total error (cheap circuits, junk transformers, crap speakers) is greater than the error of ignoring the jog. Or a screechy speaker can approximate the required rise. However most ceramic makers could pick some rubber compliance to set the <500Hz some dB down from the >2KHz.
I don’t know what the deal is with this web site. I have posted things and seen replies via email but they don’t show up here. Wtf
I did what you suggested. Interestingly r7, the 1.5 k cathode resistor, did not exist.
Anyway, I replaced r6 and c6 with a 1.5k resistor in parallel with a 1.5k resistor.
Only got the 10 uf filter caps, the 30’s backordered.
Result: no increase in volume, tone is worse, muddier.
Anyway, I replaced r6 and c6 with a 1.5k resistor in parallel with a 1.5k resistor.
Only got the 10 uf filter caps, the 30’s backordered.
Result: no increase in volume, tone is worse, muddier.
Any update?
You know the drill, make one change at a time, document that change, then test, measure, listen. If a change makes things worse, you'll know what to undo to get back to where you were before. If a change makes things better, you'll know which one.
Things can get murky quickly if you make more than one change at a time, so it's best to avoid doing that unless you're on well-known territory, i.e., building a popular circuit that's already been built, tested, and listened to many times by other people.
-Gnobuddy
No...im gonna try to make a new preamp with 12ax7.
Was gonna gave to rebuild it anyway because of the way orig was built (separate little enclosure that isn't going to work if/when I build a cabinet.
Was gonna gave to rebuild it anyway because of the way orig was built (separate little enclosure that isn't going to work if/when I build a cabinet.
If you upload a jpeg it should show up as an image. A pdf is handled differently. Looks like the first time you used a link to a jpeg rather than uploading it. The second time was an upload, but pdf.
I also rotated the image using Irfanview before uploading.
Not sure if you are having any other problems.
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Hard to read image
Heyoh, any chance you can upload a higher-resolution image of that schematic? Would be super helpful.
Heyoh, any chance you can upload a higher-resolution image of that schematic? Would be super helpful.