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Pulling 2 Tubes on a 210 HD 130

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Mike (mike_h)
Username: mike_h

Registered: 06-2006
Posted on Thursday, May 17, 2007 - 06:07 am:   

On a typical push pull amp, that has cathode to ground on the pwr tubes, pulling a pair of tubes, as long as they are the correct ones, is a good way to reduce headroom for the rock 'n' roll tone. Is this OK with MM amps? The schematic for a 65 watt amp compared to a 130 watt amp is about the same except for four tubes rather than two tubes and a different OT, so, I can't see why it wouldn't be OK. Of course there might be some concern with the correct match of the tube to OT. But, otherwise? Anyone do this? Any opinions? Thanks.
Mike Kaus (mm210)
Username: mm210

Registered: 05-2006
Posted on Friday, May 18, 2007 - 04:39 am:   

Each matched pair goes through a 10 ohm 1 watt resistor to ground. It's not a good idea to pull one of the pair out without re-biasing. And if you have the SS driver, It may overdrive the driver transistors. I've never tried it because I like mine the way they are. It would, if successfulwant to see twice the output load but the MM's will take a 100% mismatch without too much trouble. I still don't suggest it though. JMHO.
Mike (mike_h)
Username: mike_h

Registered: 06-2006
Posted on Saturday, May 19, 2007 - 08:34 pm:   

Well, I pulled two of the tubes to try it. It does reduce the headroom a bit. Plus, I can leave the amp on the high setting to enjoy the sweet juiced-up tubes. The voltage across the 3.9 ohm resistors is the same as with four tubes, plate voltage the same, etc. So, I guess I can consider this an option? Anyway, that's my experience with this.
Ingo Raven (ingo)
Username: ingo

Registered: 07-2007
Posted on Tuesday, July 24, 2007 - 01:57 am:   

I did the same with one of my amps (an 212HD-130 with 12AX7 driver), no problems so far.

I first thought about replacing the 10 Ohms cathode resistor to 5 Ohms, but then thought it shouldn't matter much. When biasing, of course you must set it to 0.25 volts instead of 0.5 volts over the resistor. Or do I miss something?