Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: joesatch on August 12, 2024, 10:48:28 pm

Title: OT schematic fender
Post by: joesatch on August 12, 2024, 10:48:28 pm

Ok i'm getting into this PA100 conversion. The back of the head has two 8 ohm speaker jacks.  I'm going to assume based on the schematic the jacks run in parallel and both jacks should be connected to an 8 ohm load for proper operation. Connecting one 8 ohm cab will result in 4 ohm OT into an 8 ohm load? 

Title: Re: OT schematic fender
Post by: jeff on August 12, 2024, 11:38:16 pm

Looks to me the switching jacks:
Short when nothing is plugged in
Connects to OT when either jack is used(or both)


OT wants 4 ohm load, so use either jack a single 4ohm load, or both jacks each to a 8ohm load.

Title: Re: OT schematic fender
Post by: Latole on August 13, 2024, 03:34:42 am

Jeff is right.

Title: Re: OT schematic fender
Post by: joesatch on August 13, 2024, 06:18:17 am

so connecting one 8 ohm speaker to this is not advisable?

Title: Re: OT schematic fender
Post by: shooter on August 13, 2024, 08:00:24 am

You have a single 8 ohm 100W RMS speaker?


mismatching at 15W doesn't bother me much, mismatching at 100W....moves things to the spooky realm.

Title: Re: OT schematic fender
Post by: joesatch on August 13, 2024, 08:09:50 am

EVM12L

Title: Re: OT schematic fender
Post by: pdf64 on August 14, 2024, 11:17:01 am

An 8ohm load will result in the loadline for the output valves being shallower, presumably below the knee, which will reduce maximum output power a bit, and probably increase the screen grid dissipation at high power output.
Overall it's probably not a big deal here.

Title: Re: OT schematic fender
Post by: jeff on August 14, 2024, 01:50:46 pm

Unless you need the full 100W of power, you could pull two output tubes(one push, one pull) and run it at 50W into your one, 8ohm speaker. The Xfmr reflects the impedance of the speaker load.
 4 tubes/100W/4 ohm load = 2 tubes/50W/8 ohm load.

Think of it this way:
Two 8ohm speakers parallel is a 4ohm load, right?
You've got 4 power tubes>Xfmr>two 8ohm speakers
Remove 1/2 power tubes.           remove 1/2 the speaker load
Now you've got 2 power tubes>Xfmr> one 8 ohm speaker

It's the equivalent ratio
2K:4ohm ≈ 4K:8ohm

Unless you need the full 100W, that's what I'd do.