a couple of stupid questions

[DG]

Re: using the Hammond 272 for the 35 - It is noted that Shannon suggests
keeping R40 and R41 in place and NOT using the heater CT .... is the green/yellow wire from the hammond tranny that is shown to go nowhere (left unconnected) the heater CT in question?

Also, I assume that all the yellow wires from the Hammond Power Tranny are just left unconnected?

Re: setting the negative feedback bias pots ..... it states they can just be set to 27K ..... is this a true 27K per the meter reading or would you adjust the metter to 28.3K to account for the 1.3K cathode resistors?

Thanks

[Shannon Parks]

1) Right. The green/yellow is the 6.3VAC (heater) center tap. It is
superceded by the voltage dividing resistors you mention which act as a
virtual center tap and so it is not connected. The three yellow wires are
for the 5VAC heater for a tube rectifier and are not used. Clip any ends of
the unused wires and individually terminate them with electrical tape or crimp to make sure they don't short on anything. You might
use them another time, so don't cut'em off. 8^)

2) Add 1.3k to whatever you read for a true measurement. A small bit
actually doesn't matter a whole lot. You can experiment with different
amounts of negative feedback. All these early designs heavily used NF and some prefer much less (ie this would be a higher impedance then). NF is not a bad thing at all though! More NF= less distortion, less gain, more solid state sound. Less NF= more distortion, more gain, more colored sound. Experiment.

> Briefly ... how does the standby switch work ..... if you don't use it, you
> just connect the red/yellow stripe wire from the power tranny to ground.
> If you do use it, the switch sends red/yellow stripe wire to ground when the
> switch is engaged anyway doesn't it?
> So if the red/ yellow strip wire has to ultimately connect to ground
anyway,
> what exactly does the switch standby???
> I know that I must be missing something here ....
> You had previously suggested I skip the switch, but I had already drilled
> the chassis for it so I may as well use it :-)

This is actually a good question. When you turn it on (on/off switch), the
heater taps will be engaged and the filaments will warm up (thus getting the
cathodes happily emitting), but the B+ will not be turned on. Once the
filaments are warmed up (30-60 seconds), flip the standby switch and the B+
will come up. Theoretically, a high B+ on cold cathodes causes 'cathode
strippage' (this is the key Google search phrase if you wanna know more),
possibly damaging the tubes over time. I don't worry about, but many do.

One more thing, adjust your bias voltages to .320mV (ie 32mA per tube)
instead of the .350mV in the manual. If the adjustment doesn't go that low,
don't worry, just set them all equal as low as you can go (which might even
up up being the original .350mV). Please let me know the adjustment range
you are able to get. I may have to adjust the cathode resistor values for
folks using the Hammonds.
Keep an eye on the website & forums the next several days. Many folks are at
the stage you are and I'm trying to add more info on the website. I'm just
slow and lazy!
Plus, I too am drilling a chassis and hooking up trannies - on the Eiclone monoblock! Regards,

Shannon

[EWBrown]

Would it be worthwhile to consider using 6.3 VDC regulated power for the filaments? I have a couple of nice OEM linear supplies that would be perfect for this, they would require a larger chassis, to accomodate the extra supply. I have seen this done on some home made amps, the builders are atempting to remove any last vestiges of noise and hum.

Just a thought.... If it isn't worthwhile, I'll pass on trying it. I can understand doing this for directly heated tubes, but seeing as the Dynao
PAS-3X preamps rectify the filament voltage for the four 12AX7s, I SWAG'ed that it might be worth a try.

Another thought for the "standby" function, a simple delay circuit using a 55 and a relay, to give enough time for the filaments to heat up properly before applying the B+ voltage. THis could be built on a small board also mounted inside teh chassis, use that "extra" 5VAC filament winding to power the delay timer circuit.

When I made my Mouser parts order, there were two components not immediately available, the 20 pF silver-mica capacitors, so I substituted 22 pF and the 50 Ohm 10 Wat resistor, so I substituted 47 Ohm, 10W.

Still waiting on trannies, and getting 'round to drilling and punching the chassis.

/ed brown in NH

[Shannon Parks]

Ed,
I considered both of these options, but the added complexity almost violates the spirit of the amp in the first place. The original ST-35 was bone simple, and a tube newb can even look at the schematic and see what is going on. Just the addition of the individual cathode biasing on mine muddies up the schematic, somewhat. A timer circuit and a regulator (for the 12AX7 & 12AU7) just didn't seem to reap the necessary rewards and troubleshooting problems grows exponentially.
That said, I definately am experimenting with regulated current & voltage designs that will be used later and a simple timer circuit is something worth looking at as well. I think I will just do some small mods on the next revision, but down the road (after I get the next two projects out the door) I may do a gussied up 6BQ5 amp as you suggest.
It does come down to what folks want. :wink:

Shannon

[DG]

I noticed that Mouser was low on the 20pf silver micas but they had a few when I ordered. They are always low on the Cornell Dubilier silver micas. I don't know much, but I'm sure that 22pf should be fine to substitute. They were out of the 50 ohm, 10watt sandcast power resistors. I ordered a 15 watt 50 ohm. But I also ordered a 10 watt 50 ohm from AES when I ordered my trannies and sockets. The 15 watt would have fit but you would have had to move the resistors adjacent to it a little to the sides.

[EWBrown]

I ordered some 22 pF SM caps from AES, with my latest parts order for the Dyna 70 and SE amp parts. I had a couple of 22 pF 500V on hand, from a long ago uncompleted ham radio project, so they went on the first board, but other than those and the 47 ohm 10Watt resistor, I stuck 100% to the original parts list. Maybe on the next (3rd) unit, I'l start experimenting with some "fancy" caps for the .1 uF 400V coupling caps, just to see if there really is a worthwhile difference between the Orange Drops and Hoyland Musicaps (at about $ 8 - 10 each).
I have been using the ceramic / gold 9 pin PC mount tube sockets from AES ($3.50 each).

/ed brown in NH

[DG]

I thought about the extra $6 for the GP sockets but figured if the only difference was the Gold plating (and I think that is the only difference) I would save a few dollars there .... trying to keep it a semi budget project anyway and I really don't believe the $6.00 is worth any sonic benefit ..... plus, you don't get to see virtually any of the gold plating when you plug the tubes in anyway :-) ..... maybe they'd last a little longer because of the plating? ..... maybe a gold pin tube would like the gold plated socket better? .... I don't know .... I just figured that 6 bucks was 6 bucks :-)

[gogzhad]

First, on the gold plated stuff - I live in a marine climate, and the gold plating is helpful for reducing corrosion - don't know if you can hear the difference, but after a few years, you can see the difference - standard connectors get oxide on them, the gold fares a little better.

Second item, I'm looking for some teflon hook up wire for another project, and I notice that there are a couple of choices in, for example, a 22 guage wire - 7 strands, which are 30 ga each, or 19 strands, which are 34 ga each (numbers might be a little off, but you get the idea). So, does size matter in this case? The stuff with fewer but larger conductors is slightly more (5 cents a foot) expensive than the other.

tia,
dave

[Shannon Parks]

OK, someone jump in here and correct me if I'm wrong, but here's the scoop on the gold sockets.
First off, they have a lug attached to the center that probably would need to get trimmed off to fit on the PCB. I was shipped a couple by accident and was suprised by the little 'lug'.
Second, the gold on the pins should be scraped off (!) and then the pins be tinned with solder, prior to soldering to the PCB. This is pretty important, otherwise embrittlement will occur over time (ie crappy socket connection - the opposite of what you want!).
The gold will give you the corrosion free sockets you want, as Dave says, but if you don't prep them right, you'll get a <worse> connection than the plain sockets. :evil:

To clarify myself, where the gold meets the tube pins, is very good. But where the socket meets the PCB, it must be scaped & tinned as stated above.

Hmmm, as to the wire question, I'll have to check that out myself!

Shannon

[EWBrown]

These are the relatively cheap GP sockets at AES, white ceramic, GP PC pins, no center lug (the ceramic upper and lower parts are held together with a rivet in the center. I've dealt with enough old radios, etc that developed "scratchy" 7 and 9 pin tube sockets, gold doesn't tarnish
and shouldn't wear off unless you go changing tubes on a daily basis.
I have seen "fancy" GP sockets at upwards of $20 each, but that is way beyond my requirements (and cheapskate habits).

The teflon wire that has more, smaller strands will be somewhat more flexible, one could possibly argue the points of "skin effect", but I'm not going to go there...


/ed brown in NH