Re: Hoffman 5E3 Tweed Deluxe Turret Board

[jbefumo]

Just received my board for this project (lovely workmanship!).


Not sure yet, but am considering building it with a single-ended KT120 output, which, by my calculations, should be capable of delivering a similar power output to the two 6V6s in push-pull. (I actually prototyped the back end a while back, but with a different front end, and it sounded quite good.)  Since I'll have no need of a phase inverter, I figure I'll replace it with some kind of driver that will deliver a net signal gain similar to the stock PI.  Any suggestions?  My initial thoughts are:


a pair of 12AT7s in parallel,
1/2 of a 12DW7 (the 12AU7 side), but that will leave me with an extra triode, which I will feel compelled to use, thus complicating the circuit.
An EF86, again, configured to more or less match the gain of the phase inverter.


Any other thoughts?  (Including "don't do it at all")?


Thanks!


Joe

[jbefumo]

Oh yeah -- last possibility: Use a 12DW7 with the 12AX7 side for gain and the 12AU7 side as a cathode follower ...

Just received my board for this project (lovely workmanship!).


Not sure yet, but am considering building it with a single-ended KT120 output, which, by my calculations, should be capable of delivering a similar power output to the two 6V6s in push-pull. (I actually prototyped the back end a while back, but with a different front end, and it sounded quite good.)  Since I'll have no need of a phase inverter, I figure I'll replace it with some kind of driver that will deliver a net signal gain similar to the stock PI.  Any suggestions?  My initial thoughts are:


a pair of 12AT7s in parallel,
1/2 of a 12DW7 (the 12AU7 side), but that will leave me with an extra triode, which I will feel compelled to use, thus complicating the circuit.
An EF86, again, configured to more or less match the gain of the phase inverter.


Any other thoughts?  (Including "don't do it at all")?


Thanks!


Joe

[EL34]

Just received my board for this project (lovely workmanship!).

Thanks Joe

Moving this to the main board

Maybe the members have some ideas

[HotBluePlates]

...  Since I'll have no need of a phase inverter, I figure I'll replace it with some kind of driver that will deliver a net signal gain similar to the stock PI.  Any suggestions?  ...

The stock 5E3 circuit uses a 12AX7 for the split-load phase inverter (which provides no gain) and the stage before it. Therefore a single 12AX7 triode yields the same gain.

But we don't know the bias voltage of your KT-120 output stage. That figure sets the required drive from the circuitry before the output tube. Always design from output stage back towards the input jack.

1/2 of a 12DW7 (the 12AU7 side), but that will leave me with an extra triode, which I will feel compelled to use, thus complicating the circuit.

The 12AU7 side of the 12DW7 will be lower-gain than the stock 12AX7 gain stage plus inverter. If you know for a fact 12AU7-gain is what you need, then a 6C4 is a single-triode which is identical in performance to 1/2 a 12AU7.

An EF86, again, configured to more or less match the gain of the phase inverter.

Triode-wired EF86 has a Mu of 20, so it's no better/different than using a 6C4 or 12AU7. The EF86 will cost you a lot more, so it doesn't seem worthwhile unless it's running in pentode mode for more gain.

... a single-ended KT120 output, which, by my calculations, should be capable of delivering a similar power output to the two 6V6s in push-pull. ...

Do you have power & output transformers picked out?

[shooter]

your probably have this, but fwiw;
http://tungsol.com/tungsol/specs/kt120-tung-sol-specs-curves.pdf

I did a kt88 SE that settled in needing a grid drive around -20.  I tweaked the pre to get me about 50ish vp-p at the KT grid.  Even then, maxed, the PA tube only gets a little cranky. (by design )

HBP etal have taught me well to design the PA 1st.

[PRR]

.

[jbefumo]

Will cogitate on the technical points, but jumping to the last -- no, no iron at this point (I actually used an entirely different circuit/board with the iron I on hand, and it seems to be on its way to working well (see my "Princeton on Steroids" thread).  To be honest, I was sort of taking a WAG that the 12AU7 side of a 12AX7, with, maybe, a 10k load resistor, would provide more current to drive the power tube, while the 12AX7 side would make the same gain as the corresponding preamp stage in the original circuit, but ultimately, I just used a 12AX7, with the second triode configured as a cathode follower with a 100k load resistor.  Really need to step on the brake, sit back, and really work through all the numbers again -- it's been a while, with some major distractions inbetween.
 

...  Since I'll have no need of a phase inverter, I figure I'll replace it with some kind of driver that will deliver a net signal gain similar to the stock PI.  Any suggestions?  ...

The stock 5E3 circuit uses a 12AX7 for the split-load phase inverter (which provides no gain) and the stage before it. Therefore a single 12AX7 triode yields the same gain.

But we don't know the bias voltage of your KT-120 output stage. That figure sets the required drive from the circuitry before the output tube. Always design from output stage back towards the input jack.

1/2 of a 12DW7 (the 12AU7 side), but that will leave me with an extra triode, which I will feel compelled to use, thus complicating the circuit.

The 12AU7 side of the 12DW7 will be lower-gain than the stock 12AX7 gain stage plus inverter. If you know for a fact 12AU7-gain is what you need, then a 6C4 is a single-triode which is identical in performance to 1/2 a 12AU7.

An EF86, again, configured to more or less match the gain of the phase inverter.

Triode-wired EF86 has a Mu of 20, so it's no better/different than using a 6C4 or 12AU7. The EF86 will cost you a lot more, so it doesn't seem worthwhile unless it's running in pentode mode for more gain.

... a single-ended KT120 output, which, by my calculations, should be capable of delivering a similar power output to the two 6V6s in push-pull. ...

Do you have power & output transformers picked out?

[jbefumo]

So, there's really no value in including that cathode follower -- not sure where I got the notion that big output tubes required big driver current -- probably from some hi-fi board ...

.

[jbefumo]

Yup, do have that data sheet.  In fact, I ended up going with the "typical application" scenario, with 400V (390, actually) on the plate, and 225V on the grid, with cathode biasing, which I will probably change to fixed.  For whatever reason, I wasn't even thinking in terms of the signal voltage, but was focusing/fixating on current... Why? Too lazy, a/o drug (chemo) befuddled to think things through...

your probably have this, but fwiw;
http://tungsol.com/tungsol/specs/kt120-tung-sol-specs-curves.pdf

I did a kt88 SE that settled in needing a grid drive around -20.  I tweaked the pre to get me about 50ish vp-p at the KT grid.  Even then, maxed, the PA tube only gets a little cranky. (by design )

HBP etal have taught me well to design the PA 1st.