Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => EZ Board postings => Topic started by: Geezer on June 17, 2005, 08:59:22 pm
-
Hoffman Amplifiers
> Tube Amp Building - Tweaks - Repairs
> 5F6A heater question ?
<< Prev Topic | Next Topic >>
Author Comment
JTM
Junior tube assistant
Posts: 83
(1/29/04 6:19 pm)
5F6A heater question ?
--------------------------------------------------------------------------------
A friend of mine bought an amp off ebay that someone never finished .It's a home brew 5F6A board with transformers from Doug.
My question is:
The amp has the two 100 OHM resistors in the heater circuit.
He also has the the Green-yellow center tap grounged to the tranny bolt.
Shouldn't it be one or the other, not both?
I may have answered my own question from reading this page, but just wanted to check with you guys.
www.hoffmanamps.com/fenderservice4.htm
JTM
Craigzbrand
Junior tube assistant
Posts: 76
(1/29/04 6:36 pm)
Re: 5F6A heater question ?
--------------------------------------------------------------------------------
Correct, either the center tap OR the resistors to ground, not both. CB
GroundhogKen
Forum Moderator
Posts: 2235
(1/29/04 7:40 pm)
Re: 5F6A heater question ?
--------------------------------------------------------------------------------
I vote for resistors over center tap.
Ken
es347
Junior tube assistant
Posts: 9
(1/29/04 7:58 pm)
Re: 5F6A heater question ?
--------------------------------------------------------------------------------
Why resistors over center tap? Does that reduce hum?
bluesbear
Hey get your own solder
Posts: 310
(1/29/04 8:22 pm)
Re: 5F6A heater question ?
--------------------------------------------------------------------------------
The 100 ohm resistors are an artificial center tap. I don't think it really makes any difference one way or the other as far as hum goes. Think of the resistors as a fuse. If the cicuit develops a short and the center tap is grounded, you lose a transformer. If the 100 ohm resistors are grounded, you lose a resistor or 2. I have put a .5 amp fast blow fuse in line with the center tap. That was my first amp. Now, I just use the resistors. It's easier and cheaper.
Make sure NOTHING is grounded besides the resistors. I'd desolder them from ground and check every point in the filament circuit for infinity ohms to ground to make sure, THEN resolder the resistors to ground. Again, your transformer will thank you for it.
Dave
Rotting27
Junior tube assistant
Posts: 101
(1/29/04 10:35 pm)
Re:
--------------------------------------------------------------------------------
Personally, I would use the centertap at ~60VDC and fuse them for the current rating of the heaters. The reason is this: The HT secondaries should be fused as well, so buying and installing a four fuse fuse block for a couple bucks, and fusing both heater and HV secondaries seems the best choice.
Relying on a resistor to be a fuse is risky, at best.
JTM
Junior tube assistant
Posts: 84
(1/30/04 6:19 am)
Re: Re:
--------------------------------------------------------------------------------
Thanks for all the advice.
I'll let him know his options and he can decide what he wants to do with the amp.
JTM
Tiny Daddy
I will work on all amps
Posts: 452
(1/30/04 9:09 am)
Re: 5F6A heater question ?
--------------------------------------------------------------------------------
I know resistors are cheaper but there is no way to know at what current they will open. Some resistors are plenty tough and others (carbon comp) will flame. Instead of using resistors as fuses why not install a pair of pigtail fuses? Just asking, not being retorical.
bluesbear
Hey get your own solder
Posts: 311
(1/30/04 9:31 am)
Re: 5F6A heater question ?
--------------------------------------------------------------------------------
"I know resistors are cheaper but there is no way to know at what current they will open. Some resistors are plenty tough and others (carbon comp) will flame. Instead of using resistors as fuses why not install a pair of pigtail fuses?"
That makes perfect sense to me. That's why I did my first build that way. That was before I found this place, though. The consensus here seems to have always been that resistors are better. If they're not, I have a lot of work to do!!
More opinions, please?
Dave
mrr3000gt
I will work on all amps
Posts: 358
(1/30/04 10:53 am)
Re: 5F6A heater question ?
--------------------------------------------------------------------------------
I worked on a JCM900 a while back that had fuses on the heater lines. It seemed like a good idea, but its too bad they put a good idea in such a crappy amplifier.
PhatTele
Junior tube assistant
Posts: 3
(1/31/04 10:33 am)
Re: 5F6A heater question ?
--------------------------------------------------------------------------------
Is there a danger in having both the center tap and the resisters hooked up? I know that it should be "one or the other." There's an amp that a friend has been playing that has both hooked up (a Hoffman AC30 with a Matchless transformer set from West Labs). I need to get it from him and disconnect the center tap ground. Is it safe for him to play in the meantime? Thanx...
GroundhogKen
Forum Moderator
Posts: 2236
(1/31/04 10:46 am)
Re: 5F6A heater question ?
--------------------------------------------------------------------------------
There's no danger with having both connected. If the center tap is connected to ground, then the 100 ohm resistors are just sitting there wasting of few milliwatts of power.
Ken
PhatTele
Junior tube assistant
Posts: 4
(1/31/04 2:44 pm)
Re: 5F6A heater question ?
--------------------------------------------------------------------------------
Ken...Thanks. That's good to know. I'll wait then.
JTM
Junior tube assistant
Posts: 89
(2/13/04 6:02 pm)
Re: Update on the Ebay amp.
--------------------------------------------------------------------------------
We had to rewired the whole amp and replaced every tube in it.Some how he destroyed all of them. The heaters didn't even light up.
I built a pine cabinet for him and mounted the 15 inch Eminence speaker that came with the deal in it.
It sounds great,even better when I hooked it to my Celestions.
I made the cabinet big enough so he could change to two 12 inch Celestions in the future.
It sounded so good I have to build one for myself.
Here's a picture of it.
www.geocities.com/retsej26/DSC00600.JPG
Tim
Edited by: JTM at: 2/13/04 6:04 pm
<< Prev Topic | Next Topic >>
Topic Commands
Click to receive email notification of replies
jump to: Tube Amp Building - Tweaks - Repairs (New)New Product Suggestions (New)Praises - Complaints - Suggestions (New)Buy - Sell- Trade (New)Archives (New)
- Hoffman Amplifiers - Tube Amp Building - Tweaks - Repairs - Hoffman Amplifiers -
Powered By ezboard® Ver. 7.32
Copyright ©1999-2005 ezboard, Inc.