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 Post subject: Gutar Preamp for Amp169-AA
PostPosted: Oct Sat 07, 2017 1:54 am 
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I have a Magnavox Amp 169-AA that I would like to turn into a guitar amp. It looks to me like the amp does not have a pre amp.

Could some one recommend a preamp schematic or an amp I can copy the preamp from to add to the Amp 169-AA.

Id like to build the pre amp from scratch using tubes. I'm wondering if it would be possible to use the power transformer that is on the Amp 169-aa to power the Pre amp or if I would need another power transformer.

I'm uploading the schematic for the Amp 169-AA


Last edited by trudie on Oct Sun 08, 2017 5:01 am, edited 1 time in total.

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 Post subject: Re: Gutar Preamp for Amp169-AA
PostPosted: Oct Sat 07, 2017 2:45 pm 
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Location: South Jersey East of Philly
Looks like it's an amp from a phono that had a separate tone controls and that plugged into this unit...as well as a tuner input here also. Your guitar has too low of a signal for the existing pre-amp 6AN8 tube, which was designed for a higher output ceramic cartridge. Although not too difficult, you could just add another tube on the amp chassis (if there's room) in the circuit like a 6AV6 for the x-tra gain before the 6AN8. Just for a test---If you have a stompbox that has some clean gain to it, plug that into the input on this amp (probably an RCA jack, so you'll have to convert it to standard 1/4" phono jack)..and see what it sounds like first. Not all tube amps have the 'sound' you want..I know as an old guitar guy.


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 Post subject: Re: Gutar Preamp for Amp169-AA
PostPosted: Oct Sun 08, 2017 1:29 am 
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Posts: 34
Were you thinking of something like this for a 6av6 preamp?


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 Post subject: Re: Gutar Preamp for Amp169-AA
PostPosted: Oct Sun 08, 2017 2:39 pm 
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Location: South Jersey East of Philly
Yea...you could adapt that...but just start simple with one 6AV6 before the 6AN8 stage and see how it sounds....depends on how much gain you want, and what values you use on the resistors and caps.


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 Post subject: Re: Gutar Preamp for Amp169-AA
PostPosted: Oct Tue 10, 2017 12:51 am 
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Posts: 34
I copied another Preamp section schematic from a Stromberg Carlson 20 PA any thoughts on using this as a preamp. Would it give a different sound?


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 Post subject: Re: Gutar Preamp for Amp169-AA
PostPosted: Oct Tue 10, 2017 1:24 am 
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You can just build the op-275 opamp pre amp... or buil the whole amp as shown.
A wonderful sounding HIFI amp... and easy to build.
( to avoid Ebay / Asian counterfeit chips remember to buy the OP-275-GPZ chip only from US distributors like Mouser or Digikey etc.

Image

Image

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 Post subject: Re: Gutar Preamp for Amp169-AA
PostPosted: Oct Thu 12, 2017 3:23 am 
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PBPIX,
Would this work for a guitar Preamp?


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 Post subject: Re: Gutar Preamp for Amp169-AA
PostPosted: Oct Thu 12, 2017 3:32 am 
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trudie wrote:
PBPIX,
Would this work for a guitar Preamp?

Sure ... but remember this is HIFI and most guitar amps don't care about quality fildelity but more-so they like distortion by over-driving the signals and pushing the tubes into saturation.
That's that heavy metal stuff.
This amp will beautifully reproduce any guitar pickup signals you feed it and will make a dynamite practice amp and more. It is a great full rich amp that will fill any house with plenty of volume..

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 Post subject: Re: Gutar Preamp for Amp169-AA
PostPosted: Oct Thu 12, 2017 3:59 am 
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You want something with a tone stack ( tone controls )

http://www.instructables.com/id/Portable-Guitar-Preamp/

http://www.till.com/articles/GuitarPreamp/


Lots of tube guitar amp schematics , in the one below

http://el34world.com/schematics.htm#All ... p_project_

Don't forget to recap the 169AA , the output transfomer is 4 ohms out so your speaker should 4 ohm to get the most power out with that amp .

http://el34world.com/Hoffman/images/Ranpre.gif tube amp preamp

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 Post subject: Re: Gutar Preamp for Amp169-AA
PostPosted: Oct Thu 12, 2017 4:42 am 
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Howdy Folks,

I was on D-Lab Electronics channel on YouTube, and I was wondering if Terry (great guy, and a very knowledgeable guitar-amp dude) had ever removed the tone controls out of the amplifier, making it linear as possible, and just adding all of the tone and distortion stuff in a preamplifier. Doing things this way, it keeps all of the amplification chain very linear and happy, and the final tubes last much, much longer, of course. Not only that, but clipping in the final causes excessive current draw that can not only shorten tube life, but can also destroy speakers and crossover networks. We've all heard this happen when the bartender is really drunk and has the amplifier turned up WAY too much and blows the speakers :D.

I also suggested using something like a laptop, cell phone, or other digital and analog devices to get that "distortion tone" players seek, and I was abruptly told by some kid that I didn't understand what "real players" wanted in a guitar amplifier :D. Never mind that I've been playing since I was 8-years-old, gigging for 40 years, and recording in studios for a long time as well :D.

Anywho, the reason I bring all of this up is that you can also use a notebook, laptop, or even a cell phone to act as a preamp and digitally manipulate the waveform before it enters the amplifier, getting just about any tone you'd want, as long as you're willing to spend time learning the software to do this stuff. Hell, even using VLC Media Player's equalizer and turning on its software preamp and other controls should give you something semi-decent, plus you'll have the added advantage of full equalization.

73,

Randy AB5NI

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 Post subject: Re: Gutar Preamp for Amp169-AA
PostPosted: Oct Fri 13, 2017 1:48 am 
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I have a 152-AA here that I was considering doing that with. Small amp chassis mounted on a panel with two speakers. Obviously pulled from it's original case. It is all there and functional. However it is AC DC and series string filiment, and needs the level of input from a crystal or ceramic cartridge, apparently. I tried a guitar into it and it lacks gain. All I could think of was to take out the tube rectifier and put in a solid state rect, then use the freed up tube socket to add a gain stage. The catch is the filiments, so I would need to find a tube with a 35V filiment to use for a preamp(unlikely) or change the 35V outputs to the 50V filiment version, thus allowing maybe a 12V tube of the same current to work in the string. Then install an isolation transformer to make it safe with an instrument connected! So far it is in the R and D stage. Waiting for the R & D time. (Retirement and doldrums) i see yours has a transformer for isolation, savings there. And it should have more gain than a triode input would have with the pentode, but maybe not enough. You could maybe build a little FET preamp box running from a 9V battery for a real long time, or one of those boxes for guitars that adds extra gain to the input of an amp. You will presumably be adding effects and such to the guitar anyway, I assume? Don't know but I bet alot of those add gain.


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 Post subject: Re: Gutar Preamp for Amp169-AA
PostPosted: Oct Sat 14, 2017 4:36 am 
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Attachment:
TONE AMP 2.png
TONE AMP 2.png [ 78.64 KiB | Viewed 182 times ]
trudie wrote:
Attachment:
The attachment TONE AMP 2.png is no longer available
I have a Magnavox Amp 169-AA that I would like to turn into a guitar amp. It looks to me like the amp does not have a pre amp.

Could some one recommend a preamp schematic or an amp I can copy the preamp from to add to the Amp 169-AA.

Id like to build the pre amp from scratch using tubes. I'm wondering if it would be possible to use the power transformer that is on the Amp 169-aa to power the Pre amp or if I would need another power transformer.

I'm uploading the schematic for the Amp 169-AA

In the RCA receiving tube manuals are two circuits that I used for this exact purpose and had great, neutral sound. The first is the Low Distortion Preamplifier using a 5879, and the Bass and Treble Tone Control Amplifier which uses a 6EU7. The 6EU7 is a very low noise 12AX7 with different pinout. Either will work fine, you can transpose the pinout for a 12AX7. Take the 5879 preamp out directly into the tone stage input. You can use a pot on the output if desired to match the tone control output level to the input of your existing amplifier.

The 5879 although a pentode is not noisy, unless it's a bad tube. They were used in a number of instrumentation amplifiers where lack of noise would be paramount.


Attachments:
TONE AMP 1.png
TONE AMP 1.png [ 119.05 KiB | Viewed 182 times ]
6EU7.png
6EU7.png [ 128.13 KiB | Viewed 182 times ]
12AX7A.png
12AX7A.png [ 124.65 KiB | Viewed 182 times ]
TONE AMP 2.png
TONE AMP 2.png [ 78.64 KiB | Viewed 182 times ]

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 Post subject: Re: Gutar Preamp for Amp169-AA
PostPosted: Oct Sat 14, 2017 3:52 pm 
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Or you could just go to one of those websites that had guitar amp schematics and steal the input preamp circuit from one of those. I had a Bogen amp that I modded. 35 Watt version tube. It was a PA not instrument amp. It had dual triode input mic circuit but it was an odd configuration. So I rebuilt that first stage to be the input circuit of a Fender amp. Then it was a better match for guitar work. You could just build from scratch a 12AX7 type input circuit robbed from Fender or maybe Marshall or whomever. And power it from the main amp it is being piggybacked onto. 6EU7 tubes are now costing as much as 12AX7 anyway, hi mu twin triodes are pretty much inflated in cost nowadays. The low/med mu like the AU7 not so much, as they do not give the gain you want in the circuits. I have a dozen 12AU7 and equivalent for each AX around here. Mostly no one wants them. Some of the subminis like the 6112 dual triode can be had cheaply if you want to work with them. If you think only one triode gain stage is enough, the 12AV6 is equivalent to half an AX7 and they are cheap as dirt, used in AA5 radios. Well they say they are alike. The plates are round instead of box like an AX but they are still 100 mu. Well I have never used one as a simple preamp, but if it works there are tons of NOS ones and good used ones floating around. You could get one from a junked table radio.


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 Post subject: Re: Gutar Preamp for Amp169-AA
PostPosted: Oct Sat 14, 2017 6:34 pm 
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Location: Lafayette, LA.
Hey Wazz,

I've often thought of saying to hell with it and adding a small audio transformer from an old pocket transistor radio and jacking that into the volume control of an AA5 and using that as a guitar amp :D. I guess you could just ground both sides of the transformer and add a coupling cap (as an added safety measure). These days, 90% of the time, you'd mic the thing up to a house PA anyway. I guess you could yank the guts out of the AA5 and put it into a guitar-like enclosure, and nobody would be the wiser. LOL!

BTW, did you guys hear that Carvin is going out of business after 70 years?! They are running a 25% closeout sale on all amps. I'd love to purchase a Legacy 2 or 3 before they disappear and become outrageously expensive. Sad situation when you see a company like that leave the market, and now I'm wondering what in the heck Steve Vai is going to do. I'd imagine Fender is trying to get him in on their bandwagon as we speak.

Oh. Just out of curiosity, I was wondering what kind of credentials a professional guitar tech has to have to do his job well? I have a friend that might be looking for one soon. I told him: "A guy that not only can play well, but loves your music, and he also needs to know electronics decently and keep your gear aligned properly and repair it, too." I didn't know if all that was too much to ask of a guitar tech, though. Now I'm wondering if there is a "guitar tech" network out there or something. (Shrug.)

73,

Randy AB5NI

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 Post subject: Re: Gutar Preamp for Amp169-AA
PostPosted: Oct Wed 18, 2017 11:19 pm 
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Posts: 34
Con-Nav_EVM., Which RCA manual did this come out of? I have 30 and 19.
But I can get others online.


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