I'm using these amps AS guitar amps... That means cranking them into distortion... That's why I'm worried about blowing up the P15R. I'm not worried about a pair of 12s as the ones I'm using are rated at least to 15-25 watts each. I wouldn't go plugging just two of them into one of my Fender Bassmans though...
Mystery Motorola
- Thread starter swjanitor
- Start date
I'm using these amps AS guitar amps... That means cranking them into distortion... That's why I'm worried about blowing up the P15R. I'm not worried about a pair of 12s as the ones I'm using are rated at least to 15-25 watts each. I wouldn't go plugging just two of them into one of my Fender Bassmans though...
cademan
Lunatic Member
If that's the case, the distortion will still burn the voice coils of speakers rated at 15-25 watts. Some people like to use old PA amplifiers as guitar amps as they overdrive quite nicely. They use their guitars with these old PA amps mic inputs. The hi-fi amps are just not made for guitar playing, plus you will have to add another gain stage as the line level or ceramic inputs on your mono Motorola is not high enough for your guitar input.
Well, if that's the case I will punt on using the P15R with either of the Motorola chassis I have coming and use one of the pairs of 12" speakers I have instead. I've been running the two Motorola 6V6 PP amps into a pair each of 12" speakers cranked out and they show no signs of having a problem handling it. If I don't use the Jensen with one of the Motorolas, I will probably build a 1x15 cabinet for it and use it with an Epiphone Valve Jr. That amp is a single ended EL84 and only 5 watts so it shouldn't kill the P15R.
I'm not sure about the gain on this EL84 based amp, but the modified (as I mentioned earlier) Motorola 6V6 amps have enough gain for guitar. One of the main mods is removing the "loudness compensation" circuit from the first gain stage and the other is changing the value of the feedback resistor. I also greatly increased the gain in those by swapping a 12AU7 for a 12AX7 For the EL84 based amp I also may not need to add a gain stage as I've seen variations of the Marshall 18 watt that only use 2 12AX7s like the Motorola, but I may need to change some other components as I've done with the 6V6 based amps. I will know more once I get a hold of the schematic and can compare it to some of the Marshall 18 watt schematics.
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cademan
Lunatic Member
I'm kinda confused at your mod as most hi-fi amps stock just don't have enough gain for a guitar pick-up unless there is a microphone or mag phono stage, or you physically added another 12AX7 tube (or similar) to the amp, or you are using a seperate preamp with a mic or guitar input!
I have built many many stereo and mono hi-fi amps but the first IF amp/phase inverter stage does not have enough gain for a mic or guitar, but only enough gain for a 3 volt line level input, and another 12AX7 tube for the 1st AF amp stage to drive the amplifier easily with a 1 volt input.
I know the 12AU7 to the 12AX7 swap, but I am not seeing how you are getting enough gain from a hi-fi amp, to use with a guitar. With your mods then, it sounds like or seems like you would not be able to use that motorola 6V6 amp ever again with any line level inputs lest you get terrible distortion just like you get when you connect a CD player (line level) into the phono jacks of an integrated amplifier. You do know that microphone and guitar pick-up impedances are darn near the same, where as line level impedances don't even come close to mics or guitar pick-ups.
Although I do not play a guitar (strictly hi-fi stereo guy), you have piqued my curiosity so let's see this motorola 6V6 amp in question.
I am looking at a mono PA amplifier schematic where it uses a 6SJ7 tube for the mic input, then goes to a 6SC7 tube (like a 12AU7) for the first AF amp, and then goes to the second 6SC7 for the second AF amp/phase inverter then to the pair of push/pull 6V6 tubes for the output. If you remove the first 6SJ7 from the circuit, and inject a signal into pin 4 of the first 6SC7, then it becomes the same as a line level input, and not enough gain for use with a mic or guitar pick-up.
You say it works for you, but I am just not seing it in my head LOL.
I have built many many stereo and mono hi-fi amps but the first IF amp/phase inverter stage does not have enough gain for a mic or guitar, but only enough gain for a 3 volt line level input, and another 12AX7 tube for the 1st AF amp stage to drive the amplifier easily with a 1 volt input.
I know the 12AU7 to the 12AX7 swap, but I am not seeing how you are getting enough gain from a hi-fi amp, to use with a guitar. With your mods then, it sounds like or seems like you would not be able to use that motorola 6V6 amp ever again with any line level inputs lest you get terrible distortion just like you get when you connect a CD player (line level) into the phono jacks of an integrated amplifier. You do know that microphone and guitar pick-up impedances are darn near the same, where as line level impedances don't even come close to mics or guitar pick-ups.
Although I do not play a guitar (strictly hi-fi stereo guy), you have piqued my curiosity so let's see this motorola 6V6 amp in question.
I am looking at a mono PA amplifier schematic where it uses a 6SJ7 tube for the mic input, then goes to a 6SC7 tube (like a 12AU7) for the first AF amp, and then goes to the second 6SC7 for the second AF amp/phase inverter then to the pair of push/pull 6V6 tubes for the output. If you remove the first 6SJ7 from the circuit, and inject a signal into pin 4 of the first 6SC7, then it becomes the same as a line level input, and not enough gain for use with a mic or guitar pick-up.
You say it works for you, but I am just not seing it in my head LOL.
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Horn Head
Super Member
I've built a similar 6V6PP Motorola for guitar use as did SWJ. Turned out very nice - very good gain.
But that model has 6AU6 as the phono amp, 12AU7 for tone, and 12AU7 for 2nd AF and inverter (stock). Did same mods as SWJ and performs extremely well for a low powerd amp.
About the Jensen....I'm using a C15N in a 6L6PP Sears amp (perhaps 20-30W) and it takes the power quite well. I think the first letter denotes magnet, C=ceramic, P=ALNICO. 15 is inches and don't know what the last letter is for though.
I'd take a stab at that Jensen especially since you'll likely get 20 watts or under with those EL84s.
Very much the novice here but I think that amp will do well in guitar amp service.
But that model has 6AU6 as the phono amp, 12AU7 for tone, and 12AU7 for 2nd AF and inverter (stock). Did same mods as SWJ and performs extremely well for a low powerd amp.
About the Jensen....I'm using a C15N in a 6L6PP Sears amp (perhaps 20-30W) and it takes the power quite well. I think the first letter denotes magnet, C=ceramic, P=ALNICO. 15 is inches and don't know what the last letter is for though.
I'd take a stab at that Jensen especially since you'll likely get 20 watts or under with those EL84s.
Very much the novice here but I think that amp will do well in guitar amp service.
The Motorola chassis I've used probably had the old style cartridges which I think needed more gain than the newer stereo ones.
The pre-amp/driver on the HS-475 chassis is: 6AU6 -> 12AU7 -> 12AU7. The 6AU6 I believe is being used in triode mode although it is a pentode tube and has a mu of approx 36. The 12AU7s have a mu of approx 17-20.
One of the mods is replacing the middle tube with a 12AT7 which has a mu of 60. That alone is a significant boost in gain. I also remove the "loudness compensator" circuit from the back of the volume pot, which also increases gain and balances out the tone more properly for guitar.
Actually I only went to 12AT7, which seemed to be enough. That was a typo above when I said 12AX7.
That is true, this amp would probably not work well for Hi-Fi anymore. But its a mono chassis, so I would never use it for that purpose anyway.
Weren't the old 1950s mono record player cartridges higher impedance than the newer 1960s stereo ones? I don't really know for sure, but it might explain why these 1956-1958 Motorola chassis work so well for guitar.
You can find the schematic for the stock chassis here:
http://techpreservation.dyndns.org/schematics/Motorola.htm
Its chassis HS-621 (or HS-475, HS-544 or HS-619, they are all pretty much the same except for input/speaker layout and pilot lamp configs). It is also fairly similar to a 1950s tweed Fender Deluxe.
http://www.recproaudio.com/diy_pro_audio/diy_files/tweed_deluxe/deluxe_5e3_schem.gif
Here's what the bare chassis looks like:
Here's it in the tweed covered cabinet with 2x 12" Oxford speakers I built:
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The last letter is the power rating. The closer to A the more power it can take. a C15N can take a lot more power (like 40 watts I think) than a P15R (only 15 watts).
Also you can't compare the letter codes between vintage and repro Jensens. The newer (Italian import) production are all rated for considerably more power than their vintage counterparts.
I don't think I will take a chance on running a vintage speaker anywhere near its rating with a guitar signal.
cademan
Lunatic Member
Once I saw that amplifier, its the 6AU6 that most hi-fi amps don't have, and it is used as a phono stage in that model Motorola. This is why you get the gain for guitar use.
I should have known as I have that schematic in my collection of Sam's FotoFacts. I have actually been wanting to find that particular amplifier for a long time now, but I have always been able to add a phono stage (minus the RIAA network) to any of my little mono amplifiers, and use them as portable PA amplifiers.
Nice job on yours BTW! :thmbsp:
I should have known as I have that schematic in my collection of Sam's FotoFacts. I have actually been wanting to find that particular amplifier for a long time now, but I have always been able to add a phono stage (minus the RIAA network) to any of my little mono amplifiers, and use them as portable PA amplifiers.
Nice job on yours BTW! :thmbsp:
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I really love these Motorola console amp chassis. They are really nice. The amp pictured above is actually the 2nd one I built like that. I've got another one that I'm going to be building soon, and another I won on eBay the other day that should be getting here pretty soon. I've also got a couple of chassis that have problems which are on the back burner for the time being. All those are the 6V6 based models. I'm pretty excited to see what the EL84 based chassis is like.
Scuzzer
Fixed Bias
Triode strap the EL84s and then it won't be anywhere near the speaker rating. But IIRC guitar guys like the distortion from an overdriven pentode don't they?
We live for it...
So there is nobody that knows what the chassis model number for this amp is?
I've been resorting to going through and opening up the schematics one by one looking for it... but I've looked at dozens and dozens so far and not found it yet...
I was hoping someone had one and could tell me what the number was right off the top of their head...
I've been resorting to going through and opening up the schematics one by one looking for it... but I've looked at dozens and dozens so far and not found it yet...
I was hoping someone had one and could tell me what the number was right off the top of their head...
Well, the chassis came today... it is a HS-701B. Unfortunately techpreservation.dyndns.org appears to be down at the moment so I can't look up the schematic! Dohh! Radiomuseum.org looks like they have it... but I don't really want to pay $25 just to join that to get one schematic... Dohh!
Anyway, it looks really nice...
Anyway, it looks really nice...
jeffhamman
Active Member
schematic?may be in luck!
I havent run across a sams for these older sets that my local library does not havefrom the 40s to 70s. I can copy and possibly scan it to you,I have your model number I will look it up and see if its avail here.I have several motorola amps like the 6v6 type above but have not seen yours before,maybe from a mono large portable record suitcase style or something simular.hve to be 58-61 era.looking for a HS-701B my cross says its in sams folder 464-13.
I havent run across a sams for these older sets that my local library does not havefrom the 40s to 70s. I can copy and possibly scan it to you,I have your model number I will look it up and see if its avail here.I have several motorola amps like the 6v6 type above but have not seen yours before,maybe from a mono large portable record suitcase style or something simular.hve to be 58-61 era.looking for a HS-701B my cross says its in sams folder 464-13.
Thanks, I appreciate any help you can give. According to radiomuseum.org the HS-701B chassis was used in the SK-33 series of record player consoles which appear to have been full sized rather plain consoles w/o radio built by Motorola in the 1959 model year.
This chassis looks pretty straight forward. It appears to use the same kind of loudness control circuit module on the back of the volume pot and I think I've identified the feedback resistor.
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I tried it out last night... I haven't done the full conversion yet, all I did was crimp on some spade lug connectors to the speaker out wires and unsoldered the RCA jack and soldered on a 1/4" jack...
As I expected in stock form it is pretty much totally clean all the way up to max volume and not very loud, which is similar to what the 6V6 based Motorolas are. For grins I tried it out with the Jensen P15R I mentioned before. Nice sounding speaker. I expect that when I get to the point where I start modding that the volume, tone and gain should improve. First will be to remove the "loudness" circuit from the back of the volume pot, 2nd will be to try removing the feedback resistor and if that isn't satisfactory, try a few different larger values for it.
The tubes that came in this one are a Motorola labeled 5Y3, pair of Tung-Sol 6BQ5's and the pre-amp tubes one is labeled 12AX7 and the other 7025, I forget the brands but they are all vintage. I put the 12AX7 as the first tube and the 7025 as the driver/inverter (the tubes came packed separate).
As I expected in stock form it is pretty much totally clean all the way up to max volume and not very loud, which is similar to what the 6V6 based Motorolas are. For grins I tried it out with the Jensen P15R I mentioned before. Nice sounding speaker. I expect that when I get to the point where I start modding that the volume, tone and gain should improve. First will be to remove the "loudness" circuit from the back of the volume pot, 2nd will be to try removing the feedback resistor and if that isn't satisfactory, try a few different larger values for it.
The tubes that came in this one are a Motorola labeled 5Y3, pair of Tung-Sol 6BQ5's and the pre-amp tubes one is labeled 12AX7 and the other 7025, I forget the brands but they are all vintage. I put the 12AX7 as the first tube and the 7025 as the driver/inverter (the tubes came packed separate).