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 Post subject: project help
PostPosted: Apr Thu 06, 2006 12:45 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 6283
Location: Portland Oregon
I think Im going to build this amp and I have all the parts except for the output transformer, so I sure could use some help choosing the correct one. The original transformer part numbers are in the parts list below. Also should I make any changes to the amp as shown in the diagram.<BR>Ed<BR> <IMG SRC="http://antiqueradios.com/albums/temp/amp_diag_002.sized.jpg"> <IMG SRC="http://antiqueradios.com/albums/temp/parts_list_001.sized.jpg"> <P>------------------<BR>


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 Post subject: project help
PostPosted: Apr Thu 06, 2006 1:08 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 21215
Location: Detroit, MI USA
I have used this particular transformer from Triode Electronics in Chicago with very good results in 6V6 circuits. <A HREF="http://www.triodeelectronics.com/tf65wscta48o.html" TARGET=_blank>http://www.triodeelectronics.com/tf65wscta48o.html</A> <P>I don't think you can do better for anywhere close to that price. It only has 4 and 8 ohm taps, you will need to connect the feedback to the 8 ohm tap and possibly alter the resistor and cap value associated with it. You also have the option of connecting the screen grids for ultralinear operation using the primary taps. <P>You could also look on eBay for output transformers by Stancor, Triad, Thordarson, Merit, or others. A primary impedance in the range of 6.6K which is specified on the schematic, up to a max of 10K, which is more commonly seen on P-P 6V6 circuits would be needed. You can find good data on 6V6 circuit design in many tube manuals, or follow other published schematics for commercially built amplifiers. <BR> <BR>As far as the circuit, it looks basic enough. Depending on what you intend to use the amplifier for, you may want to add another stage with bass and treble tone controls. I think those are very important although some people feel they should not be included. This amplifier should be good for about 12-14 watts with the power supply voltages given. <P><BR>------------------<BR>Dennis


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 Post subject: project help
PostPosted: Apr Thu 06, 2006 2:34 am 
Silent Key

Joined: Jan Thu 01, 1970 1:00 am
Posts: 34328
Location: Sandpoint, IDAHO 83864
I wonder why they didn't use a 6SN7 in place of the two 6J5 tubes? Only thing I would want to change is to put a DC blocking capacitor right at the input. The reason for this is that some of the solid state devices we use nowadays have direct coupled outputs with appreciable amounts of DC showing in the outputs. That would upset the bias on the first stage which is no good. A .1 or .22 uF cap should provide this blocking action.<BR>Curt<P>------------------<BR>Curt, N7AH<BR>(Connoisseur of the cold 807)<BR>QCWA# 25085 AMI# 242<BR>CW forever


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 Post subject: project help
PostPosted: Apr Thu 06, 2006 3:06 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Latham NY
What will you be using the amp for? The input impedance might not be right for your application. What is the unit this schematic for?<P>------------------<BR>


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 Post subject: project help
PostPosted: Apr Thu 06, 2006 3:39 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 6283
Location: Portland Oregon
Hi and thanks for the responses. I'm going to use it as a power amp to feed by a turntable, CD, and tuner into. I don't need much power and I just found this one in my records and I don't know what it was used for. It was made by Webster and the reason I was going to go with it is that I already have most everything already. How would I go about adding a stage that would include tone controls and also why is there a feedback circuit from the transformer seccondary back to the 6SJ7?<BR>Ed<P>------------------<BR>


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 Post subject: project help
PostPosted: Apr Thu 06, 2006 5:28 am 
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Posts: 27254
Location: Pocasset, Cape Cod, MA
That bleeder resistor from pin 8 of the 5Y3 to ground is just wasting power.<P>------------------<BR>


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 Post subject: project help
PostPosted: Apr Thu 06, 2006 6:29 pm 
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Location: Detroit, MI USA
I suspect they included the bleeder resistor because the B+ turned out to be a bit too high to please the engineers. It certainly does waste power as Alan noted, and is not usually found in consumer equipment of this type.<P>Negative feedback is commonly used in audio amplifier designs and reduces distortion and helps add stability.<P>You might want to look at the schematic of the Motorola chassis HS475 which is about the same vintage as the one you posted. It will be found in Sams 282-10 and is also in Beitman. If someone could make that schematic linked to below come up in this discussion I would appreciate it. <A HREF="http://techpreservation.dyndns.org/schematics/56/56_Motorola_HS475_62.djvu" TARGET=_blank>http://techpreservation.dyndns.org/schematics/56/56_Motorola_HS475_62.djvu</A> <P>It is somewhat different in certain respects from the Webster, and has bass and treble controls. You can even operate it at a slightly lower B+ and it will work fine, but the same power transformer used for the Webster should be good here. I have one of these Motorola amps that came out of a junked console, and it sounds great. You probably also have most of the parts needed to build it, or you could build a unit using features of both designs. If you are using all octal tubes for appearance, then a 6SJ7 would sub for the 6AU6, and 6SN7's for the 12AU7's.<P>If you wanted to stay with the Webster, then tone controls could very easily be added to it. You would likely need to change the first 6J5 to a higher gain tube (6SQ7, 6AV6, 6SL7 or 12AX7 with corresponding increase in plate load resistor) as there is some loss going through the tone controls. The RCA tube manuals show a bass and treble tone control stage which could be substituted for the first 6J5.<P>------------------<BR>Dennis


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 Post subject: project help
PostPosted: Apr Fri 07, 2006 4:01 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 8192
Location: Latham NY
Is your turntable equiped with a magnetic cartridge or ceramic? I guess you are using this in a mono set up? I was looking through some Sams I have but a lot of amps have switching for different types of phono equalization and that can get complicated. I would look for an old Heathkit on Ebay for a mono set up with a preamp and tone controls that you could restore. Or if you have a decent console you could add an input to it for your other sources.<P>------------------<BR>


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 Post subject:
PostPosted: Jan Thu 11, 2007 7:51 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 314
Location: Denver, Co. USA
Looks good, although you might think of separating the output tubes cathodes w / separate resistors and caps to gnd. ( 1 pair per tube ) ( better bass response, I think ). 1 RC network per tube. And less work on the components. Just a thought. OT.


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 Post subject:
PostPosted: Jan Thu 11, 2007 8:12 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 314
Location: Denver, Co. USA
Looks like a Sams schematic. Good luck. You could wind your own x-former.


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 Post subject:
PostPosted: Jan Thu 11, 2007 12:44 pm 
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Posts: 11538
Location: Warner Robins, GA
You could use this transformer http://www.tubesandmore.com/scripts/fox ... em=P-T1620 The 6600 ohms primary impedance is very close to the 6.7k impedance of the origional transformer


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 Post subject:
PostPosted: Jan Thu 11, 2007 11:10 pm 
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Joined: Dec Sun 03, 2006 8:16 pm
Posts: 1255
Location: Pittsburgh PA
IMO, it's hard to beat the sound of vintage UTC iron. A UTC HA-136 output transformer would be a good choice. Probably find one on fleabay. Good luck.


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 Post subject: Re: project help
PostPosted: Jan Fri 12, 2007 2:34 am 
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Posts: 10717
Location: Baltimore, MD, USA
Mr. Detrola wrote:
I suspect they included the bleeder resistor because the B+ turned out to be a bit too high to please the engineers. It certainly does waste power as Alan noted, and is not usually found in consumer equipment of this type.<P>Negative feedback is commonly used in audio amplifier designs and reduces distortion and helps add stability.<P>You might want to look at the schematic of the Motorola chassis HS475 which is about the same vintage as the one you posted. It will be found in Sams 282-10 and is also in Beitman. If someone could make that schematic linked to below come up in this discussion I would appreciate it. <A HREF="http://techpreservation.dyndns.org/schematics/56/56_Motorola_HS475_62.djvu" TARGET=_blank>http://techpreservation.dyndns.org/schematics/56/56_Motorola_HS475_62.djvu</A> <P>It is somewhat different in certain respects from the Webster, and has bass and treble controls. You can even operate it at a slightly lower B+ and it will work fine, but the same power transformer used for the Webster should be good here. I have one of these Motorola amps that came out of a junked console, and it sounds great. You probably also have most of the parts needed to build it, or you could build a unit using features of both designs. If you are using all octal tubes for appearance, then a 6SJ7 would sub for the 6AU6, and 6SN7's for the 12AU7's.<P>If you wanted to stay with the Webster, then tone controls could very easily be added to it. You would likely need to change the first 6J5 to a higher gain tube (6SQ7, 6AV6, 6SL7 or 12AX7 with corresponding increase in plate load resistor) as there is some loss going through the tone controls. The RCA tube manuals show a bass and treble tone control stage which could be substituted for the first 6J5.<P>------------------<BR>Dennis


Dennis,

I have been searching for this amplifier for quite a while. And, what you suggested is exactly what I want to do, use all octal tubes. I have saved the schematic and I am trying to round up the parts. I have an output transformer that is made by Merit and I remember using it for a Motorola amp in the 60s. When I build the amp, I will post pictures. Thanks!

_________________
Don


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