Hello to all.
Has any one used a split load PI in the TW's like in the deluxe or the 18w marshalls have.You would only need 2 12ax7's (twin triodes).I was also wondering about NFB(If needed).Any and all input would be cool.
thanks
johnny riff
TW PRE W/ SPLIT LOAD PI ???
Moderators: pompeiisneaks, Colossal
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johnny riff
- Posts: 72
- Joined: Wed Aug 08, 2007 7:47 pm
- Location: N.H. USA
Re: TW PRE W/ SPLIT LOAD PI ???
The Cathodyne or split load provides unity gain or less. The LTPI used in the Marshalls and in the TWs do provide some gain and you would be changing the gain structure of the amp if you did that. The gain of the PI is adjusted accordingly in the each of Ken's amps by changing the values of the tail resistors in the PI. He does this to provide the right drive to the different power tubes. Having said that, I don't know of anyone who has tried it. Give it a shot and let us know how it turns out.johnny riff wrote:Hello to all.
Has any one used a split load PI in the TW's like in the deluxe or the 18w marshalls have.You would only need 2 12ax7's (twin triodes).I was also wondering about NFB(If needed).Any and all input would be cool.
thanks
johnny riff
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Jackie Treehorn
- Posts: 236
- Joined: Thu Jan 20, 2005 4:54 pm
- Location: New Orleans, LA
Re: TW PRE W/ SPLIT LOAD PI ???
Yes, I've got an amp with a wreck style preamp and a cathodyne inverter. But, I added separate drivers. Usually with the cathodyne, you add an extra triode to drive the power tube before or after the PI. For instance, you'd use a 7199 with the pentode stage driving the pi ala Ampeg Gemini 1/Dynaco ST70. The negative feedback is returned to the cathode of the driver stage preceeding the PI, which has an advantage in that the impedance is well matched to the output transformer secondary.
In the case of using only 2 tubes for a wreck style amp, you may have to change the bias of the third stage to make it swing more voltage, or play with the voltages on the preamp to get more swing. You could try using a more sensitive output tube like a 7591, so that the drive requirements are reduced. At express voltages, you would probably only need to swing 40-45 volts to reach full output. [Using the larger tail/cathode resistors in the Express still gives enough voltage swing to get the grids to zero. I measured this, actually. So, I think Ken chose the different values for Liverpool/Express for purely tonal reasons.] The third stage, being the clipping stage, probably wouldn't be the best place to return global negative feedback. You might try ultralinear instead.
There are a lot of cathodyne based amps I like; they have a great color to the midrange. They seem focused whereas often a LTP, depending on how it's setup, can give this white noise type of diffused sound. I think you could end up with a very nice sounding amp should you pursue this direction. Post clips!
In the case of using only 2 tubes for a wreck style amp, you may have to change the bias of the third stage to make it swing more voltage, or play with the voltages on the preamp to get more swing. You could try using a more sensitive output tube like a 7591, so that the drive requirements are reduced. At express voltages, you would probably only need to swing 40-45 volts to reach full output. [Using the larger tail/cathode resistors in the Express still gives enough voltage swing to get the grids to zero. I measured this, actually. So, I think Ken chose the different values for Liverpool/Express for purely tonal reasons.] The third stage, being the clipping stage, probably wouldn't be the best place to return global negative feedback. You might try ultralinear instead.
There are a lot of cathodyne based amps I like; they have a great color to the midrange. They seem focused whereas often a LTP, depending on how it's setup, can give this white noise type of diffused sound. I think you could end up with a very nice sounding amp should you pursue this direction. Post clips!