My new Hoffman 5F6-A Bassman works...but why? &nbs

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PhatTele
Junior tube assistant
Posts: 10
(2/29/04 11:18 am)
 My new Hoffman 5F6-A Bassman works...but why?
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 I just finished building a Hoffman style 5F6-A Bassman using Doug's layout and some Bassman transformers from Allen Amplification. The amp works great and sounds great, but I'm not sure why.

At first, I was having trouble diagnosing a serious hum. The hum was there regardless of whether there was anything in the input or not. After doing some reading, I realized that the problem was likely tied to the filament heater connections. So, I double checked my work. Low and behold, I had not connected the two 100 ohm resisters to the right on Doug's layout. Once I connected those, the amp worked fine.

However, I left those unconnected for a reason. Down below, there is a discussion about not having both the center tap from the power transformer AND the artificial center tap (the two 100 ohm resisters) connected to ground at the same time. So, in my case, I connected the PT center tap to ground and left the two 100 ohm resisters sitting there...one end connected to ground and the other side unconnected. Of course, once I connected the heater wires like Doug's diagram describes, everything worked fine...even though I left the PT center tap connected to ground (which someone said really wouldn't hurt things too much).

So, my questions are: Why did my amp hum in the first place? Was it because the 100 ohm resisters were in place with one end left open? Should I have left them off the board entirely if I had planned to use the PT center tap in the first place? Now that it works (with both sets of connections in place), which, if any should I disconnect?

I'm glad the amp works, but I want to know why.

Thanx...
 
Andy Poupart
Senior tube assistant
Posts: 122
(2/29/04 1:17 pm)
 Re: My new Hoffman 5F6-A Bassman works...but why?
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 The high voltage secondary *must* have it's center tap grounded. Amp won't work otherwise. The 6.3 Volt filament secondary may or may not have a center tap. If you use the two 100 ohm resistor method to provide a ground reference for the heater cicuit, you must tape off the 6.3V center tap. If the PT does not provide a 6.3V center tap, then you have to use the 100 ohm resistor method. If you use the resistors *and* ground the 6.3V center tap, then the two resistors are doing nothing.

The high voltage center tap is usually red-and-yellow. The 6.3V center tap is usually green-and-yellow.

The amp hummed because it sounds like you grounded the high voltage center tap, and, by leaving the 100 ohm resistos unconnected, there was no ground reference for the heaters. That situation will hum like a mofo!
 
PhatTele
Junior tube assistant
Posts: 11
(3/1/04 2:39 pm)
 Now, for the tweaking...
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 Thanks. That makes sense. Each build has been bringing its own challenges. I definately learned a lot with this project.

...which brings me to...now I'm ready to tweak it ;-)

BTW - I'm using a 12AX7 in the first stage instead of the 12AY7 and I split the bias path so each side of the tube is separately biased.

The first stage of the Normal Channel will have a 1.5K cathode resister with the regular 220 uF bypass cap called for in the schematic. However, I want the second channel to be "cleaner" (rather than "bright"). Am I right in assuming that by increasing the cathode resister on the "bright" side from 1.5K to say 2.7K or 3.3K will increase the headroom for that side of the 12AX7? Is there anything else to consider before I do the swap?

Note: I won't be using the 100pF cap on the "clean" channel volume pot.

I guess what I'm looking for is sort of a Marshally 12AX7 sound on the Normal Channel and a Fendery 12AY7 on the "clean" channel....or maybe Ampegy is what I'm going for?

Thanx...


 
PHATamps
I only work on tube amps
Posts: 419
(3/2/04 10:15 pm)
 Re: Now, for the tweaking...
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 Hey PhatTele, I like your tag!

Here's what I would suggest:
Use the Normal channel for your clean Fendery sound, and the Bright channel for your Marshally sound.

First, swap V1 for a 12DW7 to keep the V1a side clean (that triode has a mu of 19 vs. 44 for a 12AY7). Stick with the 1.5K cathode resistor for max headroom--that value will bias it pretty much in the middle of the swing, if your plate voltage is close to stock.

V1b has a mu of 100, but to get the Marshall tone, go with 2.7K/.68uF on the cathode, and keep both plate loads at 100K.

I agree about leaving the bright cap off the pot, the V1b cathode network should roll off enough highs.
 
 
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