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TUT3 Portaflex
Hi All,

New guy here.

I'm just testing a TUT3 Portaflex.
I've built some simple valve pre-amps and done some repair work on valve amps previously.

The Portaflex build went well (I'll post some pictures) and it sounds very good.  Loud, dead quiet, as well.

I have two questions,
1) The preamp seems to have quite high gain.  Putting the volume above 2 starts to get pleasant fat distortion.  Seems a little early.  Any advice?
2) What sort of voltages should I be testing for in the circuit?  Most of the published Ampeg Portaflex schematics have voltages indicated at the relevant points.

Woo Hoo!

Hi Evan

Congrats on a successful build Smile

None of the voltages in the projects of TUT3 and TUT5 are critical. The 270FX has a low-ish plate voltage and you would expect to see <400V with the tubes in place.

The only important voltage would be across Rk for the output tubes, which is 250-ohms. Since the tubes self-bias, there is no specific voltage that you want to see at their cathodes, however for each individual circuit there will be a maximum voltage you want to see. First, you let the tubes settle, which will take a minute or so. If they red plate turn the amp off and make sure there is not a short across Rk or a missing connection between the grid (pin-5) and ground. If everything is okay, measure the voltage across Rk and divide Vk by Rk. This gives you the total current through the tubes.

Measure the supply voltage to the output tubes.

Subtract Vk from Va to have the voltage across the tubes.

Multiply Vak x Ik to find the total power dissipated by the two tubes. Half of this is for one tube - hopefully! - and should be equal-to or less-than the plate dissipation rating for the specific tube.

So, as an example. The target plate voltage for this amp would be 388V based on the 270FX specs. We are using 6L6GCs rated at 30W each. As a first approximation, we simply divide 30W / 388Va = 77mA. Since both tubes share Rk, the current through Rk is 2 x 77mA = 154mA. This current through 250R generates just over 38V. This is what we consider the maximum voltage to see across Rk.

Many will quibble that the actual dissipation will now be less than 30W per tube as the whole Va is not across the tube. That is try, but we need an easy to derive maximum. The other point is that we do not need the tubes fully to their limit to achieve full audio power as Rk is bypassed by Ck.

For the second approximation we subtract Vk from Va and have about 349V across the tubes. 30W at that voltage allows there to be 86mA per tube, but that pushes Vk up to 43V. You can see that we are chasing our tail here as the tube does its own thing finding a happy place to settle, which we cannot predict accurately.

Before we turn the power on with the tubes in place, we connect the voltmeter across Rk to see that voltage rise and where it settle.


Always use the Power Limiting Safety Socket with a low-wattage bulb for the first power-up. Check around to make sure there is voltage where it should be and that it is the correct polarity. This is especially important in fixed-biased amps where you want to make sure there is negative voltage right at the tube socket. This verifies the entire grid path and bias supply. In a fixed-biased amp, sweep the bias pots to see that there is voltage variation, then set them to the most negative voltage.

Portaflex is cathode-biased, so the voltage at pins 1,2,3 will show as zero with no power tubes.

Follow the procedure above to safely track the output stage idle condition.

Regarding the gain of the amp. The preamp is fairly typical but is actually a bit on the low side compared to a Fender preamp. The main difference is that the Fender has the Level control between the gain stages and Portaflex has it after the second stage. This change makes the two stages look like one in a sense, and lets / forces the player to use the instrument volume control to tap into preamp compression and then set the power amp to play clean or to go further into compression. Backing off on the instrument volume lets the preamp operate cleanly.

Many players forget that that knob on their guitar or bass has other settings Big Grin

The real difference in the Portaflex is that the power amp runs at a gain of 46 or so, where a Fender PA is usually 9 and a Marshall is 20. The PA gain in Portaflex is set by the 10k series-R from the speaker jack and the 220R shunt to ground, which is part of Rk for the PA input stage. To reduce PA gain, you can reduce the 10k or increase the 220, although changing the latter will impact the symmetry of drive to the output stage. Reducing the 10k will tighten the sound, which you should try, but do not go too low on this value or the amp will have a bit of a constipated tone like a Fender PA.

An alternative is to add a resistor to ground where the channels mix at the PA input.

Another easy (but expensive these days) way to reduce the amp's gain is to swap in a lower-mu tube Were these 9-pins, we could go from 12AX7 to a T to a Y to a U for ever-reducing gains. With octatls, there are fewer choices, so it is easier to make circuit value changes or mods to get where your requirement decides.

Have fun
Hi all,

Thanks for this fantstic explanation!
It is.. well, explaining so much!
From the volume knob to the gain of the PA, and how to avoid constipation!


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