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Paul - I have kids myself, and I remember how when they were little that even the simplest things like electrical outlets were fun to play with...

and with these amps - the voltages here can KILL. Never mind the innocent "Daddy I burned my hand on your project" ER visit.

Sorry - this isn't meant to be condescending in any way, probably just a knee jerk reaction to your post.

I know that it is possible to inadvertently put high volts on things like power, triode/feedback switches etc... corral that amp in a place where they can't touch it.

probably an over reaction - but just had to post it...
 
Thanks, yes, I understand. I'm definitely planning on a complete - probably wooden - enclosure, no tubes or anything at all sticking out aside from the jacks in the back, and some sort of vented top with small holes that no fingers can go in. (Or paws, we have cats, too...)

Hmm... maybe I need to rethink my approach to the whole volume control question, I might be making it too complicated. Oh well, this is a small detail compared to everything else!
 
PaulyT said:

Hmm... maybe I need to rethink my approach to the whole volume control question, I might be making it too complicated. Oh well, this is a small detail compared to everything else!


You can certainly build it with or without a volume control, your choice. If you choose to omit the vol pot, you will need an appropriate R for loading the stage before the power amp.
 
PaulyT said:
It's too easy for it to get changed accidentally (especially since I have 2 and 5 yr old kids who are very curious).

You can put the volume pot inside of chassis without the knob. Put a hole so that you can control it with a screw driver (volume pot w/ split shaft end). In an active bi-amp, it is essential to be able to vary the input signal level at the amp for low noise floor.
 
Evenharmonics said:

You can put the volume pot inside of chassis without the knob. Put a hole so that you can control it with a screw driver (volume pot w/ split shaft end). In an active bi-amp, it is essential to be able to vary the input signal level at the amp for low noise floor.

That's a good idea, thanks. Now, what should the resistance of the pot itself be? 10k? 50k?
 
More noob questions:

1) Volume control pot - part #? I'm thinking something like this Alps, but that may be overkill?

2) How do I determine the specs for C4, given that I'll be using the Triad C-14X choke at R4? The manual talks about a chart for this, but it's not there in the linked page...

3) C5 - in addition to the external motor run cap, should I go with the C5 from the parts list, or is there a reasonable upgrade I should do here to C5 itself?

4) C6 and C7 are the same capacitance on the schematic, but different in the parts list. Which is right, or does this depend on the other configurational details? Upgrades here?

5) C8 and C10 - is there some good upgrade that's not as expensive as a tantalum cap ($150 each!)?

6) For the motor run cap at C5, would this one from eBay work ok? Not sure what "SH capacitor" means... George talks about paper in oil, polypropylene, ... not sure if it matters?
 
There are two flavors of Alps volume pots, nicknamed "blue velvet" (nice) and "black beauty" (nicer). The auction is for a black beauty. I believe Alps does not mfr these any longer and there are knock-offs out there. The one in the auction appears authentic to me, but I am no expert.

Since the PS design is a CLC (C4-L1-C5), you can tweak the value of the first cap (C4) to dial in your B+ voltage. Start small (like 1 uf or less), and increase to get B+ where you want it at steady-state operating conditions. (it usually takes some time for B+ to settle out upon start-up) Increasing the value of C4 will increase B+.

EDIT: I just noticed that Tubelab Gearge is calling out a 47uf for C4, so starting with a 1 uf will probably yield a too-low B+ voltage


For C4 and C5 high voltage electolytics with low ripple are are good choice, such as Panasonic TS-HA, TS-HB, TS-UP series or the motor run cap that you have found on ebay. The TS-HA caps are available up to 400V rating and the TS-HBs to 450V. The 370V motor run cap will handle the volts also (they can handle more DC volts than AC) I'm not sure what "SH" means but as long as it's a motor-run cap it should work fine.

Use a good quality electrolytic cap for C8/C10 (4700 6.3V). Black Gates are highly regarded and (reasonably) affordable for this location at about $14/ea for 16V std quality caps. Michael Percy Audio sells them if you are interested. You can also use Panasonic FC's or similar in this location; they are very affordable. Another trick is that you can bypass electrolytic caps with high quality film caps for improved performance, typically at a 100:1 ratio (this is a rule of thumb), so you would bypass the 4700 uf with a 47u film cap. If you have the real estate, higher voltage rated electrolytic caps usually have a lower ESR than lower voltage rated caps for the same uf value but they are physically larger and more $$.
 
PaulyT said:
1) Volume control pot - part #? I'm thinking something like this Alps, but that may be overkill?

I've used Blue Velvet in a preamp. I used a cheap Alpha pot in my SSE. I can't tell the difference.

2) How do I determine the specs for C4, given that I'll be using the Triad C-14X choke at R4? The manual talks about a chart for this, but it's not there in the linked page...

You only need to worry about this if you are trying to use a transformer that is not within the specs George talks about in the instructions. Otherwise the 47uF works fine. What power transformer are you going to use? You can also use PSUD to do some simulations and get a rough idea. You can use it to see how dropping C4 affects the work that the choke has to do.

3) C5 - in addition to the external motor run cap, should I go with the C5 from the parts list, or is there a reasonable upgrade I should do here to C5 itself?

I wouldn't spend any more money on this cap if you are going to bypass it. I originally wasn't and so I found the biggest cap that would fit in that spot and not be any taller than the others. It's a 220uF of the same type: Digikey P7457-ND.

4) C6 and C7 are the same capacitance on the schematic, but different in the parts list. Which is right, or does this depend on the other configurational details? Upgrades here?

Looking at my parts list, it appears that I went with a couple of caps from Mouser that I had on hand: 647-UPT2W101MHD. They are both 100uF. It's not very critical at all because it is the bias supply and it dumps more power as heat through R6 than it uses in the amp (I'm making that up...I never measured it...but I wouldn't worry too much about it).

Speaking of R6...make sure you upgrade that sucker to a 7W or higher! He mentions this on his front page and in the instructions, but his parts list still specs a 5W. I put Digikey PPC7W10.0KCT-ND
there, but even it is being pushed hard. It's fine in open air, but if you are planning to flush-mount this board, I would consider what I am doing...mounting a chassis mount 10W, 1k resistor to the chassis.

Also, make sure you take note of the changes to R14 and R25 if you are going above 320V B+. The 20k biases the newer Toshiba FETs a little hard and he recommends 30k (P30KW-3BK-ND) for 320-360V B+ and 36k (P36KW-3BK-ND) for >360V B+.

6) For the motor run cap at C5, would this one from eBay work ok? Not sure what "SH capacitor" means... George talks about paper in oil, polypropylene, ... not sure if it matters?

That is a film-in-oil cap and it will work fine. A regular polyprop film cap or a paper-in-oil cap will also work, but is waste of money IMO. You just want a cap with really low ESR here and these motor run caps fit the bill and are cheap.
 
PaulyT said:
Thanks guys, I'll go with the "blue velvet" from alps. What should the overall pot resistance be (10k, 50k, 100k)? Linear or logarithmic?
I use 100K and it works fine for me. You can also go with 50K but lower the value, further you have to turn down to get the same attenuation in which case "audio taper" will be ideal because of more fine tuning it allows towards the lower volume setting (left turn) due to its attenuating ratio.

There has been some discussion on it here.
 
Apologies for being dense, but I'm still a bit confused about C4:

rknize said:

You only need to worry about this if you are trying to use a transformer that is not within the specs George talks about in the instructions. Otherwise the 47uF works fine. What power transformer are you going to use? You can also use PSUD to do some simulations and get a rough idea. You can use it to see how dropping C4 affects the work that the choke has to do.

I'm using your PT. ;) George's manual says: "Some users will substitute an external choke for R4. This will require modifying the value of C4 to get the proper voltage, which is now dependent on the value of the choke and the value of C4."

I am indeed using the choke. I'd rather not have to go through too many rounds of soldering/desoldering different caps... this is my first amp project so it's not like I have a lot of different ones lying around, I have to order each one.

I realize maybe there's no definite answer here, but I'd like to make the best guess possible. Should I still just start with the 47uF? Can you give me more detail about what "PSUD" is?
 
DougL said:
There are many people who would be happy to help with PSUD.

I recommend it as part of the experience.

Doug


Absolutely. It's easy to use, useful, quite accurate and free.

Download it, play with it, and ask questions; totally worthwhile. First recommendation, use a current load instead of a resistive load, it's more intuitive (to me, anyway), and allows stepping the load once you've got the model up and working.
 
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