Hi,
I am just about finished building this and I was comparing the
rev.3 schematic to the hammond transformer information sheet.
I noticed that Doug's schematic has output 2 & 5 for the
8ohm secondary but the hammond info I have indicates the use of
output 2 & 4 to get the 22500/8 ohm ratio.
See images below (hammond indo & Firefly schematic).
(hammond 125 spec: http://www.hammondmfg.com/125.htm)
What am I missing, please help me understand this before
I get to the smoke test? :)
thanks,
ncc
(http://www.magma.ca/~nccyr/images/hammond-125a.jpg)
(http://www.magma.ca/~nccyr/images/Firefly_sch_R3b-OT.jpg)
Go with the chart,maybe he made a mistake when he wrote the schem.
Aharon
I think maybe Doug mentioned something about this, but maybe not. I used 2 and 4 on mine, and it's working fine (I'm getting some oscillation on the boost setting, but I'm pretty sure it's not from that). I'd say the chart is probably correct.
Alex
I'm pretty sure Doug changed to 22k recently. It may be so the schem is wrong.
Anyway if you connect 2-4 or 2-5 it should work either way.
Bye,
Joep
Thanks Aharon, Joep,
I think you're right.
I went back to check rev 1. and back then, the ratio was different.
I assume the pin out was right on schematic. I think Doug changed the
circuit a little in rev 2 and 3 and could have missed the update on the
output pin.
This is just an assumption and I just would like to be sure before
I put the power on this puppy. The other issue is that I have
no idea what the side effect would be if we interchange the
pin out, 2-4 vs 2-5. Maybe someone with more experience can expand on this.
ncc
The transformer changes the load the circuit "sees" from the speaker, from a high load (8ohm) to a (relatively) low load (8k-22k).
So with 8k instead of 22k you are loading the circuit a bit more. It will work with both connections, but I think Doug found out 22k is prefeerd over 8k.
Bye,
Joep
Exactly. I think he actually talks about this on the new site (which looks great).
The tubes used are fairly rugged - there's probably quite some leeway to play with as far as impedances are concerned. I'd still be careful, but probably anything from 1 or 2 k to sveral hundred will work just fine. Enter the Guru's for comment on this (I'd love to do some theory on this but am desperately pressed for time and basically a lazy bastard ;D )
Playing around with imp-edances like this is fun - it alters the way the (power)amp/speaker combo filters, and how much the speaker is dampened. This has a lot of influence on sound and especially feel of the amp.
The "2 5" on my schem is a typo. Use 2 and 4 like the hammond chart. I originally had it set up for 8200 (2 and 5) and didn't change the numbers on the schem when I increased the impedance. I will update the schem.
Thanks for the catch!
Doug
Guys,
Many thanks for the replies. Looks like
we have the answer now.
ncc
I fixed the schematic.
Doug
Thanks Doug,
One more thing: on the rev 1 schematic you specified the use of
5W resistors for R11 & R12 (100ohm) connected to the 6.3 VAC power.
On the rev 2 and rev 3, you did not indicate 5W requirement. So the general rule for using 1/2 watt resistors also applies to the two 100 ohm.
Was it your intention to drop from 5 Watt to 1/2 Watt?
ncc
I've tried the output transformer hooked up w/ 2 & 5 and 2 & 4, and I didn't notice a huge difference in tone, although using 2 & 4 seems to give a little better bass reponse.
It's been a couple of days since I played it last, and I just switched the connections on the OT tonight since I had my soldering iron warmed up, so take this too literally...
I also stuffed my 2x8 speaker cab tonight with a little bit of fiberglass batting, so I'm sure that factors in, too. All I need now is my grille cloth and it will be complete.
Quote from: nccThanks Doug,
One more thing: on the rev 1 schematic you specified the use of
5W resistors for R11 & R12 (100ohm) connected to the 6.3 VAC power.
On the rev 2 and rev 3, you did not indicate 5W requirement. So the general rule for using 1/2 watt resistors also applies to the two 100 ohm.
Was it your intention to drop from 5 Watt to 1/2 Watt?
ncc
1/2 W resistors are fine for the 100 ohms. I overdid it on the first rev.
Doug
Quote from: Paul MarossyI've tried the output transformer hooked up w/ 2 & 5 and 2 & 4, and I didn't notice a huge difference in tone, although using 2 & 4 seems to give a little better bass reponse.
It's been a couple of days since I played it last, and I just switched the connections on the OT tonight since I had my soldering iron warmed up, so take this too literally...
I also stuffed my 2x8 speaker cab tonight with a little bit of fiberglass batting, so I'm sure that factors in, too. All I need now is my grille cloth and it will be complete.
i got some tan grill cloth if you want it.
Ansil - I have some on order, but if you're looking to get rid of that tan grille cloth, let me know. I'll take it off of your hands. 8)