Getting cathodyne PI to work well.... Maybe

[chopstuck]

I have been building tweed amps since the mid 90's and have had problems getting the cathodyne PI to sound good in overdrive mode. I guess I'm not alone.
When I tried Merlin's idea of a big grid stopper, BAM it worked better than ever before. Suddenly I had all the tone I wanted, clean and OD.

Now I expect one problem I have had is with the transformers being higher voltage than what the schematic called for. As well as house voltage being higher may all conspire to increase gain in previous stages.

I have a few big tweed repros I need to retrofit with a PI fix. They had Bassman RI PTs. (370-0-370)


Is there a way to docter the PI to work better with the higher voltages in the amp ? Is there a way to change the bias to get it into it's happy operating range without adding a grid stopper ?

I have tried a zener/transistor Voltage drop of 50v and it helps a little. It makes the amp spongier, hot and doesn't sound as good as Merlin's trick.

I have heard a few real Deluxes and I know those PIs can sound good.
Iv'e heard a few Princeton Reverbs and know they could use help.

[Phil_S]

Much of the technical aspect of this is over my head. Nevertheless, I suspect you are not approaching the cathodyne on the most appropriate basis.

The cathodyne PI is mostly about equal current flowing from the plate and the cathode and not about voltage. If you check, I think you'll find the voltage output is unequal (is that a word?). I am guessing the issue is whether you can alter the current output from the PI because the voltage isn't that important. I imagine current output changes with the type of tube and the value you select for Ra and Rk -- typically 56K for a 12AX7 and about 10K-15K for a 12AU7.

You might also have some luck tinkering with the coupling cap values.

(By contrast, the typical LTP uses a pair of triodes and takes all the signal from the plates, so you simply duplicate both triodes and that works...yeah, we all know about the 82K/100K thing to compensate for something or other.)

Maybe there is someone here who can make more sense of this and say whether I'm on the right track.

[tubeswell]

It should work reliably with any HT voltage up to the point where the h-k rating is exceeded as long as the bias is sufficient (which could be almost as low as reading zero volt difference on your meter when comparing grid-to-ground and cathode-to-ground voltages). The plate will merely sit at 2/3 to 3/4 of HT and the cathode at 1/3 to 1/4 respectively. Even load balance is important for keeping output impedance low, therefore if you want to bias cooler, then ensure that bias method leaves the loads evenly balanced. This could be achieved using the fixed bias methods in Merlin's 1st book with same value load resistors, or bootstrapped cathode biasing with a fully-bypassed bias resistor and taking the cathode output from the bias/load resistor junction methinks. I have used this method in the past with 3-4k bias resistance and 25uF bypass cap and it worked for me. YMMV

[chopstuck]

Thanks , it seems I need to do a little more reading and math.

Merlin's article is more about the dc coupled stage and I'm more interested in the ac coupled stage where the grid sees no vdc.
I wonder if the shunt to ground (with diode) is applicable to the ac stage if the grid see's no HV on start up.

I'm just bugged that most of the (non-6V6) amps I build with that PI don't sound as good as the originals when entering distortion.

[gingertube]

The other thing which can help make cathodyne phase splitter amps sound better is to go large on the output tube grid stops. This establishes the minimum load on teh PI when overdriving the output tubes. For 95% of cases when the output tubes are pentode connected then 47K grid stops will be fine.

Cheers,
Ian