I've been pondering ways to achieve an acceptable level of 30-watt Voxishness with cheaper/lighter iron. This is my 1st attempt, an AC-30 non-tremolo amp based on the Hoffman AC-30 layout/schematic. It uses a $35 ANTEK AS-1T230 230v PT with two 230v and two 6.3v taps, and a $42 40W Fender Hot Rod Deluxe OT. TAG member rock_mumbles previously reported that the Anteks deliver lower than spec voltages and the 100VA models were not sufficient for 4-el84s, but on paper they look like they should work, so I decided to try. In this amp B+ is indeed quite a bit lower than expected from the specs (296v at idle) but the PT does not seem to be getting particularly warm so I'm thinking it can in fact do 4-el84s. Antek makes a 250v version that may yield more correct AC30 voltages, and they also make a slightly heavier more expensive 200VA 230v that might deliver higher voltages under load.
As for tone the amp sounds pretty damn good with a decent amount of headroom and chime and a very nice overdrive. It is very reminiscent of though not quite as loud as my Hayseed 30 which is in turn reminiscent of a "real" AC-30 (whatever that is - there are so many versions). While I'm fine with the sound I'm getting I am still puzzled by the plate and screen voltages I'm seeing. With a B+ of 296v I am seeing about 289 on the plates and 294 on the screens so the screens are about 5-6v higher than the plates. There is 8v at the 50r cathode resistor. I am running the HT taps in parallel and am using FWB rectification, otherwise I followed the schematic. The question is whether the high screen voltage needs to be addressed if the amp otherwise sounds good. I would rather not have it eat tubes if that's an issue, even if they are cheap Russian 6P14Ps
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joeboo88 wrote:Lord, that's a real beaut. Can you take picture showing the pot side. I'm very nosy. I like looking to see how builders ground their stuff. Great job.
Here you go. Note I later clipped off the excess bus wire.
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xtian wrote:
And is it true that the toroid does not induce hum in the OT? Or just less?
I positioned the OT using the headphone trick and there was definitely hum to be minimized. I can't really say that it is less. In operation the amp is not especially quiet at idle - if it was a bit more I would feel compelled to do something about it. rock_mumbles reported that the toroids he used were "leaky" but they were different models and I believe Antek has since added more shielding. In this amp guitar proximity doesn't have much affect on the hum if any.
Yes, what is the choke resistance?
The higher the better really, within reason, to keep the screen grids happy.
If it's down ~ 100 ohms, consider using a series resistor to make it up to at least 500 ohms.
matt h wrote:I'd think running them in series is how you'd end up with a Ct.
anyway-- the 290ish plate voltage is pretty delightful for el84's. yay.
it's not surprising, however, that your screens popped up over the plate voltages. when you lower the voltages as a whole, current draw drops, most notably in screens. (this is why running voltages too high tends to kill amps fast-- the screens start drawing more of the current and hit the dissipation limits under load way more often. gets ugly.)
i'd definitely recommend getting your screens at least to plate voltage, if not below.
use a shared, flying dropper resistor rather than increasing screen resistors on all four to keep the response more similar .
Yeah, the "parallel" thing was a brainfart - I meant "series".
I guess I will add the dropper as you suggest. It's a PITA to add it to this layout - I'll have to pull the board to do it Time to suck solder.
pdf64 wrote:Yes, what is the choke resistance?
The higher the better really, within reason, to keep the screen grids happy.
If it's down ~ 100 ohms, consider using a series resistor to make it up to at least 500 ohms.
I'm using a Fender 90ma 4hy choke which I think is about 100R. Series resistor it is.
I've used Antek PT's on several builds over the years. Never had a problem with them. I like the fact they have dual 6.3V taps. I usually use one for the power tubes, and the other for the preamp tubes.
matt h wrote:You shouldn't need to pull the board--just add the "flying" resistor between the board and the first screen that gets fed (assuming they're all currently in parallel).
(edit as afterthought: might also be a good candidate for a chassis-mount, say, 1k, resistor for the "flying" resistor load. that keeps the heat off your board, and gives you a good anchorpoint for other stuff. otherwise, toss in a terminal strip)
(second edit for further clarity: the resistor I'm talking about adding isn't series in line with your whole power supply, but goes between the filter cap and the actual screen resistors... this keeps the rest of your voltages unaffected, but gives your screens greater drop. values from 470-1k will work well, I'd guess)
I have to change the under board wiring to do anything because of the Hoffman layout, but check on not adding the resistor in line with the rest of the power supply, and mounting it off board is a good way to go. Thx .
billyoung wrote:they are supposed be inherently alot quieter, could hum have been more
from heaters you think ? Very suprised to hear about hum issue with
torroids. Thought glass snobs used em for low noise.
Possibly the heaters, I haven't done more than a bit of moving wires to track it down yet. Keep in mind that this a $35 transformer. I could be wrong but I doubt hifi fanatics use cheap iron of any kind and there are much more expensive toriods out there that might be different (if in fact the PT really is the culprit.) Anyway, it's not like it's as bad as a Pro Jr.
Time to suck solder.
