Firefly - transformer/power supply question

[JamesS2]

Hi, I'm in the middle of ordering parts for an AX84 Firefly build;

I'm planning on using a toroid power transformer to save a space, with THIS ONE being the one i'm thinking of using.
(0-240V at 30mA , typically produces a B+ of around 320V)
My question is, how do alter the power supply to get b+1,2 and 3 voltages close to the ones on the schematic? Do I just add one resistor before the first filter cap? How do I calculate it's value? I've been reading up specifically for this for days and there's something i'm just not understanding... I guess I need to know the current draw of the plates and the B+ coming straight from the transformer.
Can anyone point me to somewhere with a simple, basic explanation of power supplies?

Thanks alot,
James

[P Rails]

Your best source for knowledge for the firefly will be at AX84.com website. However, there are many members here that are on that site too. That PT in your link will work voltage wise and as it states in it's description you could use a zener at the center tap or in the PSU schain to drop voltage if needed. However, it states the max mA (current) draw is 30mA which would be barely sufficient after PSU and 2 preamp tubes drawing current. A lot of Firefly user's end up putting in an ECC99 http://www.jj-electronic.sk/pdf/ECC99.pdfas the output tube which list 400v as max plate voltage. The 12au7 list 300v as max plate, so you would most likely end up needing to drop voltage with a zener to get under 300v. The spec PT (Hammond 269EX) for the firefly will net about 300 to 310v after rectification and has 71mA max current. So, in summary IMO, I would not use that PT you linked.

As far as altering voltages in the PSU chain, you could use a zener for less current draw, which will lessen sag a.k.a "stiff" supply or use higher than spec dropping resistors in the PSU filtering chain that will use more current draw and create sag (compression). Both have the advantage and disadvantage and basically correlate to the type of music you play. Sag (classic fender or vox), stiff ( marshall,orange, or most modern amps).

I'll give you a recommendation based solely on my own experience.
a) Avoid using PCB boards as traces can easily lift if you want to mod
b) For a few more bucks get a transformer that has output of 325 to 350v and 100mA
c) Get a good size chassis and make room for two 9 pin holes and two 8 pin holes and fill them in with the respective tube sockets. (only wire in the sockets you plan to use)
d) make room for an additional preamp tube and fill it with a 9 pin socket
e) get a Hammond 125E or similar multi-tapped Output Transformer.

If this is your first build, you will probably get the building bug and if you follow my
suggestions, while more expensive initially, you will save yourself money in the long run by cutting down on shipping cost, various PT's and OT's. The rest of the components are cheap and can be generally be found locally.

You may tire of the Firefly sound-wise or output wise.

[JamesS2]

Hi, thanks for the reply and info. Having someone highlight the important numbers is very helpful. It's my first non kit build, I made an SE-5A from ampmaker in the UK, which is similar to an AX84 P1. I'll do it on turret board or tag board.
Okay, so it seems that transformer won't be suitable, I wasnt aware that the ECC99 would require different voltages. One (more) thing I don't understand is; If i were to use a 269EX (or equivalent, such as THIS ) would I need to change the PSU to drop the voltage? I think I will use a zener for any voltage dropping that I need. I've had a bit of a go on the Duncan Amps PSU designer and tried to replicate the power supply shown on the schematic but get very different values, which is due to me having no clue how to use it.

I'm still quite keen to keep the physical size down, although it is not essential, so what about This transformer? . Lower voltage, so I suppose using an ECC99 would be out of the question, but it has a higher current rating of 40mA. I saw somewhere that someone had used one for a firefly inspired build, but couldn't find any more details.

Thanks,
James

[P Rails]

The PT from Amp Maker that you linked would work spot on, and the 80ma gives you room to go to a true push-pull (within tube specs) design if you ever feel inclined to do so.The other torodial you linked would be very limited voltage and current (40mA) wise. You don't have too, nor would you want to run a tube at it's max voltage, but with 185v most typical output tubes will sound dark, and leave much less headroom (cleans).

I would lean towards a PT with higher voltages that can be dropped, rather than
lower voltages that need to be multiplied. If you are using a tube rectifier you will drop voltage, if you use solid sate diodes you can multiply voltage by 1.2 (eg 100 X 1.2= 120v).

As far as the OT make sure you use a mult-tap to achieve the 22,500 to 8 ohm the schematic calls for using a 12au7. If you use an ecc99 you will have to use a different cathode resistor to achieve proper bias.Most people use a a switch to make it more convenient to switch bias resistors for the respective tube.

[JamesS2]

Okay, thanks, i'll go for the 269 style tranny, seems most sensible for a 'noob'... but i'm still unsure about whether i'll need to change the power supply. How do you work out the voltage coming from a transformer? In this case is it 190V x 1.4? 266V, which fits in with the voltages given on the schematic, but then what about the change from the diodes? Does this figure need multiplying by 1.2? to give 319v? I'm guessing from the layouts i've seen I won't need to change anything if I use this tranny, but the voltages don't make sense to me, maybe they never will

When looking at specs for an OT, is the 'Max DC Bias' the same entity as the max current for the power transformer? i.e could I get away with using a 125B (with Max DC Bias of 45mA)?

Thanks for your help P, it's much appreciated, and i'll remember that bias switch. Is that the only change that i'll need to make if I swap output valve?

[P Rails]

Originally Posted by JamesS2

Okay, thanks, i'll go for the 269 style tranny, seems most sensible for a 'noob'... but i'm still unsure about whether i'll need to change the power supply. How do you work out the voltage coming from a transformer? In this case is it 190V x 1.4? 266V, which fits in with the voltages given on the schematic, but then what about the change from the diodes? Does this figure need multiplying by 1.2? to give 319v? I'm guessing from the layouts i've seen I won't need to change anything if I use this tranny, but the voltages don't make sense to me, maybe they never will

When looking at specs for an OT, is the 'Max DC Bias' the same entity as the max current for the power transformer? i.e could I get away with using a 125B (with Max DC Bias of 45mA)?

Thanks for your help P, it's much appreciated, and i'll remember that bias switch. Is that the only change that i'll need to make if I swap output valve?

Depends on your load line: tubes,PSU,OT, maybe this link will help http://www.ax84.com/drums/bbs/dm.php?thread=141902

As far as the OT, basically the bigger it is, the more bass to be had. I would go with a 125C which max's at 8 watts, and better bass than the 125a or b

As I stated before, you can get a lot of help at AX84. The Firefly's designer Doug H is a regular there. You can scan through old and current post and find out about as much as you probably want to know about that amp.

[JamesS2]

Okay, I think I've got all the info I need to get started. I've had a few looks at the ax84 forum, will post there if if I have any more questions. Thanks P