Working 13GB5 Sweep Tube Amp

[tomlang]

Attached are pics of an amp I just finished (well almost) that uses a pair of 13GB5 sweep tubes in push pull configuration, a long tail pair phase inverter using 8GU7 tube and 4EJ7 tube triode connected as the preamp. The output transformers are Atlas HT327 line transformers that reflect a 1200 ohm plate-to-plate impedance with 8 ohms connected to the 4 ohm tap.

I'm running 220vdc on the plates of the 13GB5's and 170 vdc on the screens (via two series connected #5651 gas regulator tubes). Bias is zener diode regulated, presently set at 80 ma per tube.

The Phase Inverter and preamp supply are run off a voltage doubler to make enough voltage for direct coupling between the preamp (4EJ7) and PI tube (8GU7).

Amp is built on a Dynaco chassis with Geek's "Dynamull" driver board (but 4EJ7 triode connected, not pentode as Geek designed it).

A quick check (needs to be verified), seems I'm getting about 25 watts per channel. Negative feedback is connected but I need to go back and calculate the amount I'm using. I also need to add the feedback cap.

I'll clean up and post the schematics later.

[DeathRex]

I have a pair of 13GB5s and a 18GB5 and a 25GB5. I couldn't seem to get all of them at the same heater voltage.

[Geek]

Nicely done, Tom!

[tomlang]

I have attached the power supply schematic. The higher voltage is developed by FWB, not doubler as I said earlier. The loads for the output tubes, phase inverter and preamp are represented by resistors.

Anyway, I'd like to use a TIP50 or the ilk to "filter" the supply to the phase inverter and preamp stages. I tried a TIP50 in the configuration circled above the circuit but the TIP50 blew, probably due to charging the cap in its base. All I want to accomplish here is to use a TIP50 in the same manner that works for the gas regulators but without regulation. Is it a simple matter of a voltage divider -- simply add a resistor across the cap connected to the base?

[Switch]

Burn'em hot.

Best linearity on the 6GB5/EL500 comes with 17W plate dissipation and stiff cathode current, but I couldn't red-plate them till 24W. Screen drive also allows VERY good results, if you can push AB2!

[hilldweller]

Nice job! How does it sound?

[tomlang]

This amp (as many of us do) has morphed. I now have the 13GB5's installed into the Pete Millett Red Board and I must say is the best sounding yet.

http://www.diyaudio.com/forums/tubes-va ... ost3703870 see post #130.

Since that pic was taken I reinstalled the 70v line output transformers. While they saturate below 60hz, their upper end square wave response is superior to the big Hammonds. I think I am going to keep the amp that way, high pass it and drive some satellite speakers with a gasp solid state amp driving some subwoofers.