Big bad bass amp build report

[birt]

This is a project i started almost 2 years ago and i was working on it so now and then because i spent all of my time touring with a dance company. Anyway, i'm finally building this.

-The chassis is a Super Twin Reverb. I reuse the power amp and PI. it will have a fender preamp, a sunn preamp and an ampeg preamp
-I had a headcase made with flightcase handles on the side, something in between a Fender and an Ampeg head.
-Except for the grid resistors at the power tubes all carbon comps in PI and PA have been replaced. The big 30K 20W resistor was open, i also replaced the 5K 10W.
-I added my extra stuff (elevated heaters and adjustable bias), got rid of the hum balance pot.

the schematic: http://img840.imageshack.us/img840/514/funnpegpsupa.png
this whole schematic is now built except for the preamp power supply (point A, B and C)

I have tested what i have at this point.
-The voltages in red are from the Super Twin Reverb schematic. The voltages in blue are what i have right now.    Pretty high right? No tubes yet.
-The 27K 30W resistor is smoking hot. It really is smoking when i turn on the amp. The 68K 3W resistors are also pretty warm but not smoking.
-Heaters are elevated about 70V and are 6.9Vac.

(an old topic about this project: http://www.el34world.com/Forum/index.php?topic=9388.0 )

[Willabe]

If that R is smoking then either it's too small in wattage or something is very wrong.  

Do you have a lamp limiter that you can use to see if there is a short? If you have no tubes in, the amp/PS should not be drawing any current that would heat up that big R? Unless the 27K/30w to ground is letting to much voltage bleed off from the PS?


              Brad      

 
 

[Willabe]

I thought about this some more and since that big bleeder R is not in series (with the PS) and wired to ground as a shunt it should not be effected by current draw on the PS. Which makes me think it's alowing too much voltage/current to bleed off (too much work) and that's why it's smoking.

Schematic shows 27K/20w, you write 27K/30w, what do you realy have in there right now? I don't know how to do the math to figure out what you need in there, but a lot of the guys here do and it might help them to know it's value if someone chimes in.

birt, I went back and read thru the old topic. PRR and GAZ both said you can leave out the voltage sharing R's because of the CT connection and you already have a bleeder (27K) why did you leave them in? If there getting hot why not leave them out like they said?



            Brad      

 

    

[sluckey]

The 27K 30W resistor is smoking hot. It really is smoking when i turn on the amp.

There's 443v across that 27k resistor so it's dissipating 7.27 watts. It will be hot enough to blister your finger, but it should not be smoking. A 30W resistor or even a 20W resistor is plenty big enough for 7 watts of heat. Could it be dirt, grime, dust, etc. that's smoking? Are you sure the resistor is 27K? Does the resistor have adequate air ventilation around it?

[birt]

Just to be clear. it's 27K 20W. it has enough air around it. i actually just bought it 2 days ago so it doesn't have any dirt on it.

the reason i left those resistors in is because they were in the original amp and i want to have everything working like that before i change it. the resistor i replaced was open (so it also got too hot maybe?) but didn't show any burns. but on that place the chassis had a big burned mark so this resistor probably wasn't the original one either. now that it looks like i'm destroying the third one in there i want to know why this happens.
i saw that the 300PS uses the same setup with 2 caps in series and the voltage sharing resistors, but doesn't have the big bleeder... (and an artificial center tap but i don't think that makes much difference)

i just went over this whole circuit with a marker again and traced everything. couldn't find any wiring mistakes. so i want to find out why this resistor gets so hot and why the voltages are so high.

http://img707.imageshack.us/img707/6193/dsc04886d.jpg
http://img819.imageshack.us/img819/7383/dsc04887u.jpg

[sluckey]

Just to be clear. it's 27K 20W.

Is it the big black resistor in the pic? That looks mighty small to be 27K@20W. Are you sure of the rating?

Like I said before, your schematic shows 443V across that resistor. That calculates to 7.27 watts. A 20 watt resistor that's only dissipating 7.25 watts should be hot, but it should not be smoking. Even with 600v across it, that would only be 13.3 watts. Something just doesn't jive with your voltage reading or your resistor rating. What's actually written on the resistor?

[terminalgs]

I'm confused about the two 68k 3W R's.  in the original thread, you had these as 39K/2W R's.   PRR thought 2W wouldn't be enough,  so he suggested  two 75K/2W's at each location.  I took this to be two 75K's in parallel, to give you 37.5k for each.  you swapped them to 68k's ?   I don't know if it makes a big difference, since they are each matched, but like I said,  I'm confused.

also, your voltage is high, but with no tubes,   they  will be high.  the original circuit had 10 triodes and a pentode to supply plenty of load to bring the 443v down to 395v.

for a 27k/20W resistor to reach its maximum wattage, you'd have to have 1000v going across it, right?

+1 for something is not right about that smallish 20W R.

[birt]

i didn't understand PRR's comment about the 75K's like that. but parallel makes sense... the voltages would also be less high if i lower the value again i guess.
the 27K is the big green one on the left. it is smaller than the 4K7/5W but i thought that might be because the value is higher. they may have given me the wrong one in the shop... it says "CES D10 27K 5% 8817"

[sluckey]

the 27K is the big green one on the left. it is smaller than the 4K7/5W but i thought that might be because the value is higher.

When I see that style resistor, I usually think 5 to 7 watts. 20 watters are usually much bigger. I know that some modern style resistors can handle more power in a smaller size, but your resistor style has been around for 50 years. I could be wrong, but since it's smoking with only 443v on it, I highly suspect it is a lower wattage rating than you think it is.

Just for size comparison... See the big resistor mounted on the left side of this chassis? It's a 5K 30W resistor. It's about 3" long and 3/4" diameter. A 20 watter would not be much smaller.

[birt]

i found a 25K/25W resistor with a heatsink. i also replaced those 68K's with 39K/3W. voltages are a bit lower now, nothing is smoking anymore.

i think i'll start with the preamp boards now.

[birt]

is there anyone here that also makes layouts like this?


new heater wiring


i opened a normal pot and made it a trimpot with the dremel cutter


and a glimpse of what it will look like!

[birt]

the first (double) board is populated and installed!


and the bottom with my sloppy way of joining the 2 boards..


now i have to go on tour again for some time. the next update might be in 2 weeks or so.