Still working on my 120W KT88 PPP. A number of PI (LTP) tubes have crossed my mind. The 12AT7 and 12BH7 have stuck for their proven worth and specs. I had a JJ 12AT7 and EH 12BH7 laying around, so i could make some comparison regarding drive and tone. Here's the thing; the JJ 12AT7 is more harsh in the highs and quite 'middy'. Don't really like it. The 12BH7 sounds much better to my ear. Softer highs, and more oomph, the kind you feel in your underbelly 
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Easy choice right? Wrong. The 12BH7 has to be driven way too hard itself (past 20Vpp) to achieve the full 120W output, being a low/medium tube. The 12AT7 achieves full output more easy, but with given audible negatives. I've tried both tubes with 18k and 22k anode resistors, 370V B+ and of course the appropriate bias resistor.
So, could my dislikes be due to the type (12AT7 vs. 12BH7), would a different brand fair better (like the EH 12AT7 long plate), or is there a golden anode resistor value, etc. I really would like to stick with the 'one-tube-input' and want to get some input before just ordering a bunch of different tubes.
. Easy choice right? Wrong. The 12BH7 has to be driven way too hard itself (past 20Vpp) to achieve the full 120W output, being a low/medium tube. The 12AT7 achieves full output more easy, but with given audible negatives. I've tried both tubes with 18k and 22k anode resistors, 370V B+ and of course the appropriate bias resistor.
So, could my dislikes be due to the type (12AT7 vs. 12BH7), would a different brand fair better (like the EH 12AT7 long plate), or is there a golden anode resistor value, etc. I really would like to stick with the 'one-tube-input' and want to get some input before just ordering a bunch of different tubes.
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Tried EH-12AT7's, didn't like them. Mullard CV4024 are very nice, and so are GE 6201 (5-star). Lots to choose from.
jeff
Does the amp have feedback from the output transformer back to the phase inverter? Was it connected when you made the comparison? Since the gain of the two tubes is so different, it's not a fair comparison with the feedback connected.
What do you mean by Anode resistor? Do you really mean the tail resistor of the typical Long Tail Phase Inverter? The phase inverter typical of Fender Blackface amps has 82K and 100K plate (anode) resistors. The imbalance of these resistors corrects an inherent imbalance in the design. The tail resistor also plays a role in that imbalance, the bigger the tail resistor, the less imbalance needs to be corrected by offset plate resistors. Experiment with the plate resistors.
What do you mean by Anode resistor? Do you really mean the tail resistor of the typical Long Tail Phase Inverter? The phase inverter typical of Fender Blackface amps has 82K and 100K plate (anode) resistors. The imbalance of these resistors corrects an inherent imbalance in the design. The tail resistor also plays a role in that imbalance, the bigger the tail resistor, the less imbalance needs to be corrected by offset plate resistors. Experiment with the plate resistors.
With 18k or 22k anode resistors you probably won't have a large enough signal to drive the kt88s to full power, especially if you are using negative feedback from the output to the phase inverter. I would suggest using a 12ax7 with larger plate resistors an d12at7 or 12bh7 cathode followers to drive the kt88s.
Thanks all for the input. I've purchased another 12AT7 (EH with large plates) and will compare it to the JJ. I agree comparing the AT and BH are a bit apples and oranges. I'm not set on the 18k or 22k either, so I will also experiment with these values and see how this affects the tone... but that's going to have to wait, since I've got a defective KT88 and the replacement in the mail.
When everything is back in order, I'll report back.
When everything is back in order, I'll report back.
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Well I really like some other JJ's as well. KT88 and 12AX7 for example. That's why I bought the JJ 12AT7 in the first place. I'm not writing it of just yet. Once my amp is fixed, I will experiment some more with the surrounding circuitry.
The concertina with it's massive local feedback will be very linear indeed and therefor have less impact in the overall tone than a actual amplifying stage.
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