Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TIMBO on June 04, 2015, 12:07:41 am

Title: Phillips 10w PA Amp
Post by: TIMBO on June 04, 2015, 12:07:41 am

Hi guys, I hoping this will be a QUICK fixup.
I don't think that at this stage there is much wrong with it, other than the usual SAFTY items.

The one thing with PA amps is the OT secondary taps aren't really general speaker friendly.
With help from the local boys, we dug into the OT to see if the 50,70 and 100v line taps could be used. These were not usable.
The OT also has a separate FB winding and sometimes this can be used, this had a possible 3ohm impedance, but I would rather have an 8ohm to be a more usable tap.

Some smart guys here nutted out that 100v line transformers work well as standard OTs.
  http://www.ozvalveamps.org/optrans.htm (http://www.ozvalveamps.org/optrans.htm)
I removed the old OT and installed the new line transformer and as you can see has 4,8 and 16ohm taps.
These are quite cheep at $20 and are from the local store.

Title: Re: Phillips 10w PA Amp
Post by: trobbins on June 04, 2015, 02:36:36 am

If only you could inject Botox in to that OT  :icon_biggrin:

Title: Re: Phillips 10w PA Amp
Post by: PRR on June 06, 2015, 01:14:59 am

Your un-used primary tap has 300V DC on it, and should be insulated against touch.

Title: Re: Phillips 10w PA Amp
Post by: TIMBO on June 06, 2015, 05:08:43 am

Thanks PRR, A bit of heat shrink as well it has a metal cage that sits on top

Title: Re: Phillips 10w PA Amp
Post by: Willabe on June 06, 2015, 10:07:13 am

Quote from: trobbins on June 04, 2015, 02:36:36 am

If only you could inject Botox in to that OT  :icon_biggrin:

     :laugh:

Title: Re: Phillips 10w PA Amp
Post by: TIMBO on June 06, 2015, 04:04:09 pm

Thanks guys. The original circuit had a smallish (maybe 2w) dropping resistor between nodes A and B that was a bit crispy, I in creased this to 5w.
With a few mods to be able to plug in the guitar, has a decent sound and then there was a smell
The 5w dropping was red hot  :think1:
I checked the FILTER caps prior and they measured OK for value
When doing a quick measurement down the B+ chain I was only getting about 50v at node C, thinking that the 16uf filter cap was cactus replaced with new and the voltage increased to a reasonable value.
Still the dropping resistor between A and B was very warm.
It has a 50/50 cap can and it was measuring about 65/65uf so maybe this is also on its way.
The data sheets for the EZ81 has the MAX input cap as 50uf and even the new brands are a bit over their marked value, Am I better to get a 32/32 as a replacement or will the EZ81 handle a few extra uf.
At this stage I can't see any reason for the dropping resistor the heat that much. Thanks