Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Jack_Hester on December 01, 2014, 11:08:16 am
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I keep finding stuff that has followed me home. I'm finding it, while digging through my other electronic jewels (junk?).
I have a nice old (looks like new) Magnavox 175 FF Power Amp chassis, that I had no intentions of converting to anything else. But, I have every intention of using it for a HiFi Stereo music amp. Yesterday after Church, I warmed it up with the lamp limiter, until I could get it on full line voltage/current. It has 4 x 6V6 and 2 x 12AX7 tubes. Being cathode biased, I had to dig around, until I could come up with 4 tubes that were close. So now, the bias readings range from 37mA to 43mA. I did not write the plate voltages down, though I think I remember them being around 315vDC. I'm thinking that this is setting right up there at max plate dissipation.
It has a balance pot on top of the deck, for changing the level of Channel 2. And, it has a Hum Balance pot on the top of the deck, as well. I have a couple of these nice little Dayton speakers to use with it:
http://www.parts-express.com/dayton-audio-b652-air-6-1-2-bookshelf-speaker-pair-with-amt-tweeter--300-651 (http://www.parts-express.com/dayton-audio-b652-air-6-1-2-bookshelf-speaker-pair-with-amt-tweeter--300-651)
I connected my little MP3 player to the inputs, volume of player at almost full. I did that because I figured that it wouldn't have enough push, to get much out of the amp. Man, was I wrong. Had to cut it back to about 1/4-1/3, to have a comfortable level. Tweaked the balance to bring up Ch2. What a nice sounding amp. Killer quiet, with nothing connected to the inputs. And, I was impressed with my little Daytons. Great big sound in a small package.
After listening for a few minutes, scrolling through a variety of songs (just to hear how different genres sounded), I decided that I needed an external pre-amp with Tone Controls, as well as Volume. For my listening, I don't need much in the way of additional gain. So, I began looking at some active Tone Controls. Here's one that I'm considering:
http://www.angelfire.com/electronic/funwithtubes/Amp-Tone-A.html (http://www.angelfire.com/electronic/funwithtubes/Amp-Tone-A.html)
I'm tending toward the one if Fig. 14. But, I'm only in the thinking mode, for now. As I stated above, mine is the 'FF' model. I found quite a few schematics, of other configurations of this model. The schematic that I have for this one is 1.5M+ in size. Seems all the schematics that I have to offer, lately, have been too large to post. Again, send an email, and I will reply with the correct schematic for my amp.
I'm interested in opinions of a suitable pre-amp. Again, I don't think that I will need more gain, though I could have a Hi/Lo input for each. With maybe a 6C4 or 6AU6 on the front end of one input of each channel. Or, one dual-triode, and split them to use one for each. Probably the better choice.
Anyway, if one of the moderators feels that this needs to be moved to another heading, then please do. Thanks.
Jack
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Ok, here's my first draft of a potential pre-amp for the 175. Comments or suggestions, please. Thanks.
Jack
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build a dynaco pas-2. you don't need all the fancy schmancy input switching, just a couple of line inputs... maybe for CD and another for SS media player.
http://www.the-planet.org/dynaco/Preamplifier/PAS2.pdf (http://www.the-planet.org/dynaco/Preamplifier/PAS2.pdf)
another alternative is the eico HF85 or marantz 7C preamp.
--pete
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build a dynaco pas-2. you don't need all the fancy schmancy input switching, just a couple of line inputs... maybe for CD and another for SS media player.
http://www.the-planet.org/dynaco/Preamplifier/PAS2.pdf (http://www.the-planet.org/dynaco/Preamplifier/PAS2.pdf)
another alternative is the eico HF85 or marantz 7C preamp.
--pete
Thanks Pete, for some more sources. At this point, my main interest is a turntable amp. As I'm far removed from the HiFi world, I'm not up on whether most have their own pre-amp, or not. Either way, I need a means to adjust volume and tone.
In the morning, I will begin to research the examples you gave. I've just downloaded the Dynaco file. Thanks a bunch. Have a good one.
Jack
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Pete -
The Dyanaco link reminded me of this source:
http://store.triodestore.com/dynacoupgrades.html (http://store.triodestore.com/dynacoupgrades.html)
Their stuff is rather pricey, but good for more Dynaco info.
Jack
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I downloaded a very large version of the PAS-2 manual, but very high-res. I copied and pasted the two schematic halves to Paint, stitched them together, and printed to a PDF file. I've attached that schematic. The special potentiometers in the Tone Control are alien to me. I don't believe that I could duplicate this pre-amp very easily. Maybe there's an easy way to simplify it, for my application. Thanks for any and all help. Have a good one.
Jack