firefly & hammond 125A question

[ncc]

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

[Aharon]

Go with the chart,maybe he made a mistake when he wrote the schem.
Aharon

[Alex C]

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

[Joep]

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

[ncc]

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

[Joep]

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

[Mike Burgundy]

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 )
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.

[Doug H]

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

[ncc]

Guys,

Many thanks for the replies. Looks like
we have the answer now.

ncc

[Doug H]

I fixed the schematic.


Doug

[ncc]

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

[Paul Marossy]

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.

[Doug H]

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

1/2 W resistors are fine for the 100 ohms. I overdid it on the first rev.

Doug

[Ansil]

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.

i got some tan grill cloth if you want it.

[Paul Marossy]

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.