[SripBoard Design] Starving Student Millett Hybrid Vacum Tube Amp - Page 8

I stand corrected on the caps being part of the original design. Must be the starving part that reduced the parts count.

That being said, those caps are still a good idea. There is a signal being formed across the cathode bias resistors that is in phase with the input signal to the grid. This is a form of negative feedback that the capacitors eliminate. This is also why the gain goes down when the caps are eliminated (introduction of negative feedback reduces overall gain).

Perhaps it might be worth experimenting to see which sound is preferrable.

You could, but you probably would not want too without making additional component value changes.

Paralleling the triode sections doubles gm (current gain) while keeping mu (voltage gain) the same. The rp (plate resistance) is also halved. So, you would want to account for all of this in the final parts selection.

As others have said, be sure to adjust the voltage divider resistors (the resistors from 48V to ground, with the signal in the center) to accommodate the new heater voltage. 

 

The output caps are still hooked up to the gates of the mosfets - they should be hooked up to the source.
 

Just nit-picking, but Gm is not current gain, it is transconductance - the change in current through the tube or transistor when the input signal is changed by 1V. It does double when the sections are paralleled though :)

 

http://en.wikipedia.org/wiki/Transconductance

 

Current gain is literally output current/input current. 

i dont speak french so im not sure what some of that stuff means, but they look like a suitable replacement. can someone confirm this? i dont have my bom in front of me im on vacation

 

It looks similar in most ways, except about 3 times the Gm(transconductance) and about a third of the Rds on resistance. The transfer curves are not the same, but both the BUZ20 and IRF510 show most ratings in the amps of current, when we are dealing with about 150mA here. I would say if you can get a pair of them cheap, go for it. I would love to have one to test. 

 

Let me look at the datasheets for a few more minutes.

I like the way the BUZ20 looks on paper, but recent experience has shown me that these mosfets can vary alot in how much heat they can dissapate from one part to another. 

 

Without having worked with these devices, I am going to have to recommend that you stick with the IRF510 or IRF610. 

 

Can you give a link to the BUZ20 parts that you are looking at?

 

If you are having a hard time finding the 510 parts, then I will help you out there, and mail you whatever parts that you need.

My offering to send parts is no different than I have done for many others

For C7/C8 I would definitely use the parts on the schematic, which is 220uF 16v.

You could use a higher voltage rated part if 16v is not available at your supplier.

For R16/R17 you could use the 47k that you had before or the 51k, it would not matter at all.

The BOM that you linked to above is for the 19J6 amp. There are a couple of differences in parts values.
Edited by digger945 - 8/14/11 at 5:24am

you could order your c7 c8 from beezar. they have 220uf 16v nichicon muse es's 

Is this your final shopping cart?

 

Are the resistor amounts still 100 pcs, or the Qte to the left of the page? ... if so then you will need to change...

2 pcs of the 1k resistor

2 pcs of the 220k resistor

2 pcs of the 390k resistor

2 pcs of the 33k resistor

 

 ... and add...

2 pcs of the 220uF 16v. These can be Nichicon Muse ES, or Sanyo OSCON. If those are not available then we can take a look at something else.