Next Heathkit conversion question

[CitizenCain]

Can someone tell me if the cap and resistor from the grid of the PI to ground is some kind of hi-fi thing that I could remove for use as a guitar amp? I'm planning on using a 12DW7 in this spot with the low-mu side as the PI and the high-mu side as the driver. Hope to push a little more signal through to the power tubes.

Also, should I bootstrap the cathode of the PI?

http://www.visi.com/~sstolle/heath/aa-23-mod-003.JPG

Thanks!

[loudthud]

When the Heathkit was originally designed, someone had to sit down with a prototype and tweek it to try to meet the design goals of power output, distortion, and frequency response. The biggest bottleneck is the output transformer. So you get a prototype of the OT that meets a cost target and tweek on the rest of the circuit by adjusting the gain and amount of feedback. This is a process of making pretty square waves (not by driving the amp to clipping) and then checking distortion and frequency response. The square waves quickly tell you if there is any high frequency instability. To control high frequency stability, C15 and R28 limit high frequency gain to match the circuit to the OT.

These tweeks are all dependant on the open loop (no feedback) gain of the circuit and by changing to a 12DW7 you are changing the gain substantially. So for a guitar amp conversion, I would start with removing C15, R28 and C18. Increase R35 to maybe 47K and tweek on R26 to get the operating point of the PI close to the voltages on the schematic. The amp may oscillate so you may have to increase R35 or remove it altogether.

Boot-strapping the PI? Not sure exactly what you mean. The PI is direct coupled to the driver stage so it doesn't need an RC in the cathode.

[CitizenCain]

Thanks for the tips, loudthud. That's kind of where I was headed in my thinking. Strpping the cap out of feedback circuit and boosting R35. I was gonna start at 56k, but I like a looser amp sound.

I probably used the wrong term with bootstrapping. What I was thinking was setting it up like in the BF Princeton schematic. You have the PI with the split load resistors, but with a smaller resistor between the cathode and lower load resistor. What's the purpose of that smaller resistor? Is it actually a bias resistor like it looks?

Also, what's the diff or advantage of using a grid resistor on the PI like the 1M in the Princeton vs direct coupling the PI to the driver?

[loudthud]

The concertina PI in every Fender I can think of has a cap on it's input which allows it to self bias. In this Heathkit, the PI gets it's bias from the driver stage. There are advantages to each method depending on the tube used.

First a few words about biasing a concertina. For maximum headroom the cathode of the concertina should be between 1/6 and 1/4 the B+ voltage. The trick is to allow for the voltage across the tube when it approaches zero volts grid to cathode at peak signal. For a low mu triode this can be <25V @ 3ma but for a hi mu triode it can be >100V.

So, for a 12AX7 hi mu triode you need a lower cathode voltage at idle (1/6 B+) than if you used a 12AU7 (1/4 B+). In this amp 60V for the 12AX7, 90V for the 12AU7.

Here's the gotcha: if you want to use a 12AX7 direct coupled like the Heathkit, it hard to make a driver stage with a low enough idle plate voltage to properly bias the concertina. You are too close to zero bias to get the plate voltage low enough and then the driver stage doesn't have very much headroom. So for a 12AX7 it's better to self bias the concertina so you can get some headroom out of the driver stage. Increasing the plate resistor on the driver to 220K or more will help the driver but performance starts to suffer.

With a direct coupled 12DW7 like you want to use you can run a higher plate voltage on the driver because of the lo mu concertina and should be able to squeeze enough headroom out of the driver and PI to drive the 7591's.

The advantage of the direct coupled driver/PI is the reduced parts count and the removal of one coupling cap that can cause low frequency problems in HiFi amps. The advantage of the cap coupled self biasing driver/PI is more gain.

There is a variation of the Fender circuit is known as the "Paul C" mod. It puts a fixed bias on the concertina. This is a good idea if you want to swap different tubes, but IMHO it biases the cathode of the concertina too high and has very little headroom when a 12AX7 is used. This can be fixed by changing one resistor. People without oscilloscopes who measure headroom by how far they can turn up their volume control think the mod increases headroom. They are misguided.

Edit: Trying to operate a 12AX7 at too low a plate voltage will cause an unexpected increase in grid current.

[CitizenCain]

Great info loudthud! Many thanks. If you don't mind one more question...do you think there's any advantage to converting to a 12DW7 or would the existing 12AU7 setup drive the power tubes hard enough? I do plan on some gain tweaking in the earlier stages, mostly involving bypass caps on the preamp cathodes and some CC changes.

You've provided me a ton of great info already though! Thanks

[loudthud]

I would try the 12DW7 just because I like high gain amps, but with four stages of gain in the preamp, there is plenty of gain. If you can't find a bias that will work, you can go the Fender route adding 2 resistors and a cap or just go with the 12AU7. Just watch for an unexpected negative DC voltage on the grid of the driver tube. 7591's are very sensitive and don't take much drive. At least the NOS tubes are that way, don't know about the re-issues.

[CitizenCain]

I like this amp to be a high gain amp, at least higher than I normally use. I'll mess around with the 12DW7 and see what's what

Thanks loads, loudthud!