OT impedance mismatch, HRD is it safe?

[Tonegeek]

I have always read that you can mismatch the impedance that a Fender OT secondary sees by a factor of 2 and be Ok. Example: A Twin can safely push a 2 ohm or 8 ohm load as well as the nominal 4 ohm load.

I have a Hot Rod Deville in for repairs and by inserting a blank 1/4" plug in the extension jack you force the OT to switch to the 2 ohm tap. This to me sounds much better than when it is on the 4 ohm tap pushing a 4 ohm load. This actually puts less stress on the tubes than at the 4 ohm position but I just want to make sure it won't arc. Any comments?

BTW this amp is a different animal when pushing 2 G12H-65 Celestions in a THD cab than its own Eminence speakers. The Celestions warm it up quite nicely especially for some solid lead tones!

[pdf64]

2 ohm tap. ..sounds much better than when it is on the 4 ohm tap pushing a 4 ohm load. This actually puts less stress on the tubes than at the 4 ohm position

See RG Keen's take on this
http://www.geofex.com/tubeampfaq/TUBEFAQ.htm#mismatch
In addition, my findings are that with your arrangement (1:2 mismatch) the screen grids dissipate more power than with a matched load, especially when overdriven. Because the power output is lower (about 70%) overdrive is more likely. Pete.

[Tonegeek]

2 ohm tap. ..sounds much better than when it is on the 4 ohm tap pushing a 4 ohm load. This actually puts less stress on the tubes than at the 4 ohm position

See RG Keen's take on this
http://www.geofex.com/tubeampfaq/TUBEFAQ.htm#mismatch
In addition, my findings are that with your arrangement (1:2 mismatch) the screen grids dissipate more power than with a matched load, especially when overdriven. Because the power output is lower (about 70%) overdrive is more likely. Pete.

I do understand how arcing might happen if you have too light a load on the secondary, but I am not sure I understand what you are saying about the screens. It seems to me the tube would dissipate less current with a lighter load (although voltages might be higher). I had read the Keen article years ago and after looking at it again it still seems that even a 1 to 4 mismatch (4 ohm setting with 16 ohm speaker would likely not hurt the output section. I would not really want to try even that.

[Gaz]

I cannot explain why either (however I think drawing a load line would explain it), but I have read many times that increasing the reflected impedance (running into a larger load) is much harder on the screens as pdf64 noted. As you probably know, screen grids are already stressed in a typical guitar amp, and they are a common point of failure. It can't sound that much better, or surely there's another mod perhaps in the preamp to achieve something similar to what you're hearing running into a larger load - maybe lowering preamp voltages??

[pdf64]

The theory of why >1:2 mismatch results in greatly increased screen grid dissipation is beyond my current understanding to explain. But with a good meter (something that will measure true rms ac+ dc heat power like a Fluke 87) try measuring how much your screen grids are dissipating when overdriven, with various loads.
Screen to cathode voltage x (V across screen grid resistor / screen grid resistor) = screen grid power.
Be prepared for some big numbers, way above the 5 watt rating - Pete.

[martin manning]

If you look at a good set of characteristic curves, like GE's, you can see the screen current increases dramatically at low plate voltages (when the signal drives Va-k toward zero), about where the knee in the plate current occurs. In general, if the load line crosses Vg-k =0 below the knee (i.e., the primary impedance is high), and the stage is overdriven, more time is spent in that high screen current area. A low primary impedance will rotate the load line clockwise and reduce this tendancy. However, low primary impedance will also increase the maximum plate dissipation along the load line in direct proportion, so there is the possibility of red-plating- if the power supply can provide all that current without the voltage sagging. Besides the power supply's capability, it also depends upon where the B+ places the load line; higher B+ voltage and lower load impedance both mean higher max plate dissipation.

The HRDeville has a high B+, 480V, and if I use the AC voltages given on the schematic, the primary impedance looks to be a little higher than usual at 4k6. Fender was concerned enough about maintaining the correct load to provide the switching jacks (it switches from 8 to 4 Ohms, BTW, the 2-Ohm tap is NC), which wasn't done in the old days. Maybe that's a reflection of the capabilities of modern tubes.

I think I'd see if it red-plates to determine whether this is safe or not.

[Tonegeek]

If you look at a good set of characteristic curves, like GE's, you can see the screen current increases dramatically at low plate voltages (when the signal drives Va-k toward zero), about where the knee in the plate current occurs. In general, if the load line crosses Vg-k =0 below the knee (i.e., the primary impedance is high), and the stage is overdriven, more time is spent in that high screen current area. A low primary impedance will rotate the load line clockwise and reduce this tendancy. However, low primary impedance will also increase the maximum plate dissipation along the load line in direct proportion, so there is the possibility of red-plating- if the power supply can provide all that current without the voltage sagging. Besides the power supply's capability, it also depends upon where the B+ places the load line; higher B+ voltage and lower load impedance both mean higher max plate dissipation.

I think I see what you are talking about with the screens. Need to mull over that some more...

The HRDeville has a high B+, 480V, and if I use the AC voltages given on the schematic, the primary impedance looks to be a little higher than usual at 4k6. Fender was concerned enough about maintaining the correct load to provide the switching jacks (it switches from 8 to 4 Ohms, BTW, the 2-Ohm tap is NC), which wasn't done in the old days. Maybe that's a reflection of the capabilities of modern tubes.

I think I'd see if it red-plates to determine whether this is safe or not.

The HRDeville schematic I have shows 2 and 4 ohm connections for the secondary with an unused tap for 16 ohms. I believe a Hot Rod Deluxe shows 8 and 16 ohms.

[martin manning]

The HRDeville schematic I have shows 2 and 4 ohm connections for the secondary with an unused tap for 16 ohms. I believe a Hot Rod Deluxe shows 8 and 16 ohms.

The one I'm looking atr came from Fender's site today (drawing dated '96)...
http://support.fender.com/schematics/gu ... ematic.pdf
Maybe there's been a change?

[Tonegeek]

The HRDeville schematic I have shows 2 and 4 ohm connections for the secondary with an unused tap for 16 ohms. I believe a Hot Rod Deluxe shows 8 and 16 ohms.

The one I'm looking atr came from Fender's site today (drawing dated '96)...
http://support.fender.com/schematics/gu ... ematic.pdf
Maybe there's been a change?

I have that same drawing and now see what the confusion is about: There is an alternate output section on that drawing, right above the main one. My eye was going to that one, I guess your eye went to the other! (I have the 2x12 version). I guess Fender makes versions for other speaker configurations also. It gets confusing when Fender makes The HR deluxe/Deville and the Blues Deluxe/Deville! When I first got this amp in I was using the Blues Deville schematic and was scratching my head till I realized my mistake.

[martin manning]

Yes you're right, my mistake. I'm going to blame this dinky screen I'm looking at...When it's big enough to read, I can only view a portion of the scheme.