• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

audioXpress - tube amp project

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Classic design


The design of the amplifier you mention is a classic design and is similar to designs I have seen in the early to mid 60's.

I personally would not use tubes with 12 volt filaments for this application. But to change this would require a redesign of the DC filament power supply.

I noticed that the circuit diagram does not provide a ground for the filaments. This may or may not be beneficial.

All and all it would be a good starter project to learn from.

John Fassotte
Alaskan Audio

[Edited by alaskanaudio on 12-02-2001 at 01:42 AM]
I haven't seen the circuit in question, but filament supplies should always be grounded. Otherwise, they will tend to drift, sometimes to alarmingly high voltages which can exceed the heater/cathode voltage specifications for the tube. It's easily done, just attach either side of the filament supply (better still, a center tap, if the filament winding happens to have one) to ground. Even if the filaments are run from AC, this will prevent excess voltage.
Depending on the gain, it's often a good idea to use DC filament supplies for preamps--particularly those with phono stages--but it's not necessary for amplifiers. AC will work just fine. Twist the filament supply wires tightly and keep them as far away from the signal leads as is practical. For all the pains and troubles I went to in order to max out my tube amps, I didn't bother with the filaments. They're AC. Hum? Nope. They're absolutely silent.
I see no problem with 12V filaments, per se, and at any rate many 12V tubes (e.g. 12AX7) have a center tap for their filament so that you can run them from either 6 or 12V.

Clarification of my prior posting


The reason I mention the 12 volt tubes is due to potential availablity of the 12SL7 and 12V6 tubes used in the circuit. They are likely not to be as easy to find as 6SL7 and 6V6 tubes. Tubes with 12 volt filaments used in low level stages are of course very common, as is running them from DC power sources. It is just somewhat unusual to see this in a power amplifier.

The circuit is basically a 12SL7 stage configured as a paraphase phase inverter driving a push pull self biased 12V6 output stage. The 12V6 output stage is configured in ultra linear mode with the screen grids tapped at 40% into the Hammond 1608 output transformer primary.

A link to the transformer specifications is here:


There is a error in the diagram at the secondary of the output transformer. It is shorted out as drawn.

John Fassotte
Alaskan Audio

[Edited by alaskanaudio on 12-02-2001 at 08:42 AM]
Volume control

Adding a volume control is a simple matter.

For all partical purposes you can replace R1 with a volume control. A audio or LOG taper pot would be best.

Break the connection from J1 to the top of R1.

Replace R1 with a 500K pot. (Counter clockwise connection to the ground side and clockwise connection to the grid of tube V1A).

Hook the center (wiper) connection of the pot to the J1 center conductor.

Thats about it.

John Fassotte
Alaskan Audio

[Edited by alaskanaudio on 12-02-2001 at 08:57 AM]
Personally I did have a problem with the 12 volt tubes. I used to scrounge a lot and I have only scrounged 1 quad of 12V6s vs I don't know how many 6V6s. Buying NOS might be cheaper for the 12v but the 6V6 is being made today. 12AU7s are as common as dirt as far as tubes go so I don't get why they weren't used instead of the 12SL7s other than the fact of the 12v. A 6V6 and 12AU7 amp would be better in my book just because there are so many of them that they don't cost as much as others for the $/quality of sound they reproduce.

I am just scratching around my old Audio Xpress back issues and noticed this PP amp and this thread on diyaudio. I don't have the schematic to post (I do have it in the magazine, but posting it might be a violation) and cannot find it elsewhere on-line.

I was wondering if anyone has since built this amp from December 2001 and what do their thoughts are about the sound, the ease of build, and other than the 6V6 tubes suggested above, were there any improvements that can or were made to the circuit...

I just need one more thing to work on right now and a this would be my first PP tube amp (versus the SE build that I am tackling).

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