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Vintage threads from the first ten years

5F6A pics & build tales

6/19/2005 7:15 PM
joe6v6	5F6A pics & build tales Another one in the can! This time a scratch built 5F6A Bassman. For the most part I followed the original Schematic & Layout but I also referenced Dougs layout & weber. Two problems occured at start up, the first was an extremly idiotic mistake on my part. I initaly checked voltages with no tubes then with pre-amp & rectifier tube installed. I pulled them all out and looked things over one last time and decided it was ok to put all the tubes in and fire it up, in my haste I put a power tube in the rectifier sockett. there were a few tubes laying there & I didnt pay attention. When I threw the switch the fuse blew. I imideatly realized what I had done but what I didnt know was if I had destroyed any components ie; transformers,capacitors,diodes you know the usual stuff. Turns out everything was OK. After I put the GZ34 in and started it up there was a high pitched squeall that changed in pitch with the presence controll. Thanks to me paying attention to all the knowledgeable people on this forum & others I imideatly knew what to do to correct it. I couldnt find the post but I remembered reading about the same exact problem and I cured it by reversing the brown & blue output transformer wires to the power tubes. I dont know how it fixed it but I know it did! Other than that this amp is dead quiet even at full voulme. Its nice to know Im learning from past experiences with lead dress, sheilded wire and the importance of a proper ground scheme. There are a few pictures here http://photobucket.com/albums/v234/joe6v6/Bassman/ The Bias is fixed per the original schematic but if it would benifit me to put an adjustment in there I will do it, Im not sure what value pot to use or even where to put it so if someone will let me know the advantages of haveing one and how to install it I will consider it. The silver thing between the power trans & the boared is a home made cap can & it houses the 4 - 20UF capacitors for the power supply filtering. I think these were originaly outside the chassis. I built the cabinett with an angled front, stained it dark & cleared it with Urethane. I dont know if you can see in these pics but I made the feet for it out of 3/4" aluminum bar that I turned on a lathe. Im really happy with this amp , Thanks for all the help along the way . . . JOE

6/19/2005 7:15 PM

joe6v6

5F6A pics & build tales

Another one in the can! This time a scratch built 5F6A Bassman. For the most part I followed the original Schematic & Layout but I also referenced Dougs layout & weber.
Two problems occured at start up, the first was an extremly idiotic mistake on my part. I initaly checked voltages with no tubes then with pre-amp & rectifier tube installed. I pulled them all out and looked things over one last time and decided it was ok to put all the tubes in and fire it up, in my haste I put a power tube in the rectifier sockett. there were a few tubes laying there & I didnt pay attention. When I threw the switch the fuse blew. I imideatly realized what I had done but what I didnt know was if I had destroyed any components ie; transformers,capacitors,diodes you know the usual stuff. Turns out everything was OK.
After I put the GZ34 in and started it up there was a high pitched squeall that changed in pitch with the presence controll. Thanks to me paying attention to all the knowledgeable people on this forum & others I imideatly knew what to do to correct it. I couldnt find the post but I remembered reading about the same exact problem and I cured it by reversing the brown & blue output transformer wires to the power tubes. I dont know how it fixed it but I know it did!
Other than that this amp is dead quiet even at full voulme. Its nice to know Im learning from past experiences with lead dress, sheilded wire and the importance of a proper ground scheme.
There are a few pictures here http://photobucket.com/albums/v234/joe6v6/Bassman/
The Bias is fixed per the original schematic but if it would benifit me to put an adjustment in there I will do it, Im not sure what value pot to use or even where to put it so if someone will let me know the advantages of haveing one and how to install it I will consider it.
The silver thing between the power trans & the boared is a home made cap can & it houses the 4 - 20UF capacitors for the power supply filtering. I think these were originaly outside the chassis. I built the cabinett with an angled front, stained it dark & cleared it with Urethane. I dont know if you can see in these pics but I made the feet for it out of 3/4" aluminum bar that I turned on a lathe. Im really happy with this amp , Thanks for all the help along the way . . . JOE

6/19/2005 11:50 PM
Casey Sackett	joe6v6, Nice build!! Please post sound samples when you get a chance. A bias pot will give you the oportunity to dial in any 6L6G, WGC, GC, 5881, or 7581A and rectifier combination to the sweet spot and do some tube swapping and easy bias adjustment to find what you like the best. Also, remember that when Fender chose the value of the bias resister wall voltages were about 10 volts less than they are today. All that being said, as long as you check the bias and change the bias resistor accordingly, your expensive TS 5881's should have a long and happy life. Hope this helps, Casey

6/19/2005 11:50 PM

Casey Sackett

joe6v6,

Nice build!! Please post sound samples when you get a chance.

A bias pot will give you the oportunity to dial in any 6L6G, WGC, GC, 5881, or 7581A and rectifier combination to the sweet spot and do some tube swapping and easy bias adjustment to find what you like the best. Also, remember that when Fender chose the value of the bias resister wall voltages were about 10 volts less than they are today.

All that being said, as long as you check the bias and change the bias resistor accordingly, your expensive TS 5881's should have a long and happy life.

Hope this helps,

Casey

6/20/2005 7:11 AM
joe6v6	Hello - Could someone please explain which resistor should be swapped to change the bias on a 5f6a & which way to go to increase or decrease the voltage? Also what is the voltage supposed to be & where, what about putting a pot in to make it easier? Where would it go & what value. Maybe there is some info on this subject in another post or somewhere on the net. Thanks : JOE

6/20/2005 8:37 AM
Mike Y	Joe, nice looking amp. The pot I used for the bias was a 22K in place of the 15K resistor. Mike

6/20/2005 2:51 PM
Carl Gigun	The easiest way to do it is to replace the 15k and 56k resistors in the bias supply with a trimmer. Multi-turn is nice if you want lots of accuracy when you set the bias. Connect the 220k's and the cap to the wiper, the ends go to the bias supply or ground. To that basic setup you can add a couple resistors for cheap insurance, a resistor between the trimmer and ground puts an upper limit on how hot the tubes can be biased and a resistor from the wiper to the negative bias supply makes sure the bias voltage will still be there even if the trimmer breaks. You can do two of these so each tube can be biased separately if you like. -Carl

6/20/2005 2:51 PM

Carl Gigun

The easiest way to do it is to replace the 15k and 56k resistors in the bias supply with a trimmer. Multi-turn is nice if you want lots of accuracy when you set the bias. Connect the 220k's and the cap to the wiper, the ends go to the bias supply or ground. To that basic setup you can add a couple resistors for cheap insurance, a resistor between the trimmer and ground puts an upper limit on how hot the tubes can be biased and a resistor from the wiper to the negative bias supply makes sure the bias voltage will still be there even if the trimmer breaks. You can do two of these so each tube can be biased separately if you like.

-Carl

6/20/2005 7:14 AM
David	Is that a steel hammond chassis? If not what is it? David

6/20/2005 8:28 AM
joe6v6	David - Yes its a steel hammond blank that I bought from Mouser. I forget the exact dimensions, but as you can see it was a pretty good fit, I did have to weld tabs on the sides to bolt it to the top, so the holes wouldnt be too close to the edge of the wood. Unfortunatly I realized I needed to do this after I had it painted & labled. I could have bolted angles to the sides but I didnt want it to rattle loose & I have a Mig welder. JOE

6/20/2005 8:28 AM

joe6v6

David - Yes its a steel hammond blank that I bought from Mouser. I forget the exact dimensions, but as you can see it was a pretty good fit, I did have to weld tabs on the sides to bolt it to the top, so the holes wouldnt be too close to the edge of the wood. Unfortunatly I realized I needed to do this after I had it painted & labled. I could have bolted angles to the sides but I didnt want it to rattle loose & I have a Mig welder.
JOE

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