bass amp 6DQ6B PP

Hello everybody. I'm putting together a tube amp for electric bass.
While I used the playmaster 117 as a base, I made quite a few changes to be able to experiment with valves that I have on hand.
In the first circuit I used a 6SQ7, which turned out to be excellent in that place, a very quiet triode. and I put together an active Baxandal with the pentode of a 6U8A. The triode of the 6U8 was like cathodin to push the 6DQ6B. After having assembled 6 or 7 tube amps, this one really surprised me. It is a very great power. Although measuring at 1KhZ continuous sine wave gives around 70W into 8 ohms, it sounds on par with my other 400W Gallien Krueger amp.
I used the cabinet and many of the components of a Panadaptor radio spectrum analyzer and after using it for about a week when connecting an overdrive pedal I saw that the volume turned up much more before saturating, which made it clear that it was missing one. amplification stage.
In this second circuit I added the triode section of an ECF80 behind the 6SQ7, as you will see in the second diagram.
According to the ECF80 datasheet, I should use the triode at about 14mA to work on the linear part of the curve, but I decided to make it work by stepping on the non-linear part to see if it would gain some more harmonics.
It currently works at about 9 mA when idle.
I would like you to make criticisms and recommendations regarding this.
Thanks.
 

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Gamar,

Welcome to Tubes / Valves!

Those are creative circuits.
The first has matched loads on the Concertina phase inverter.
Match your output tube DC currents (the tubes have to be matched), or you will have early lamination saturation (could sound good for a Guitar amp).
Reduce the Global negative feedback, for another sound.
Early Saturation of Laminations is not corrected by Global negative feedback, it actually makes it worse, could sound good on a Guitar amp (not Hi Fi).

The second has un-matched loads on the Concertina phase inverter.
There is global negative feedback which will correct some of the 2nd harmonic distortion that is caused by the un-matched concertina load resistors, versus the the first circuit.
It seems that you can individually adjust the fixed bias for the output tubes.
Match the output tube DC currents, and get one sound.
Mismatch the output tube DC currents, and get another sound.

You might also find the Instruments & Amps Threads of the diyAudio Forum to be helpful (Guitar amps).
Tubes / Valves is typically for Hi Fi and Stereo playback amplifiers.

Some Hi Fi and Stereo amplifiers Sound Different, versus Some Guitar amplifiers.

Have fun strumming!
 
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PRR

Member
Joined 2003
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> it was missing one. amplification stage.

Yes.

The cathodyne is unity gain. The Bax tone stage is unity gain. So the 36V you need at 6DQ6 grids must be 36V at Volume pot wiper. The 6SQ7 has gain of less than 70. So you need a half-Volt at the input jack. Most amps will saturate with 0.020V at the jack. So yes, you need another gain of 20 to 30.

But not before the volume control. You can also have a half Volt from a hard-played guitar. Times 50 is 25V, times another 25 is 625 Volts!! The gain control should come after the first stage.

You do NOT want to run guitar amp stages "linear". First, high current is not a lot more linear than reasonable current. You only need enough current to carry the load without strain. And dead-linear sounds like chips. Which are cheaper and cooler and more reliable, so why use tubes to sound like chips?

There are thousands of g-amp plans posted on the internet. Study.
 

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PRR

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Joined 2003
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Your NFB suggests gain of 30 but two unity-gain stages into a loudspeaker stage is much less gain than that. This power stage probably has voltage gain of less than unity. 40 Watts in 8 Ohms is 18Vrms or 25V peak. Since it takes 36V peak to do that, the 6DQ6 and OT give voltage gain of 0.7. You want to plan added gain of 50 to 100 inside the power stage NFB loop.
 
Unloaded voltage on the high rail would be close to 700 volts, with the mid rail near 350. Although technically “better”, that might be higher than was planned. Could everything handle it? Doubling up on tubes and getting more watts is certainly possible, if the OPT has enough headroom. 70 watts with the saggy supply might be safer. Going full wave with choke input on at least the upper supply would put it at the right voltage, and make it pretty stiff (once you start drawing bias current). But that means buying an expensive choke and finding a place to put it.

Most guitar/bass amp schematics I’ve seen have *two* more gain stages. one inside the feedback, and another in the front end. Around unity gain inside the feedback doesn’t really need feedback. Some instrument amps have feedback, some don’t. You may prefer the tone of your loudspeaker without NFB, even for a bass amp. Makes the fundamental resonance very pronounced. If that’s in the right place, it can sound very good.
 
Hi,
by the way, the centre tap of of the transformer is shorted to ground by the first filter capacitor of the filter chain C-L-C.
The output tube is best to operate with 450-500V at the plate and 200-250V at the screens.
I would wire it to a two way rectifier with the plates direkt to the first capacitor and the screens and the rest of the amp via Choke and R-C-R filters, similar to fender circuits. With abt. 400V and the existing opt it will be 40-50W rms output.

With a stable 700V +b is feasible with 4 output tubes if this opt come to play but with only 2 tubes need a opt with a higher inpedance obt 7-9k ohms.

73
Wolfgang
 
I understand your concerns about rectifying the power supply. In the first stage I had another transformer but it was short on power and I got this much bigger one. As you can see, it has 250v + 250v secondaries. If I rectify with a midpoint, the plate voltage is very low and if I rectify at the ends with a diode bridge it exceeds 800v in vacuum, which is too much for my capacitors. Rectifying half wave I did not like but when testing it I assure you that there is no curling in any part of the circuit. I put a very large filtering capacity. By using it like this I have the advantage of obtaining around 320v passed through an inductor for the low power stages that avoid having to waste heat in resistors by lowering that voltage and it is also more isolated from the consumption of the 6DQ6B.Yesterday I added a relay with a 30 second delay to avoid adding the classic stand-by switch. In other words, only the filaments are connected during that time and when the high voltage appears, the tubes are already operating. I didn't mention the OPT to them. I did it by hand, very oversized, for the power I expected and with several secondaries to be able to test which impedance I obtained the best performance at and it turned out that the optimal one is 6900 ohm, much more than I expected but there is little accurate data with these valves. Thanks
 
If you rectify this way, you would get 700/350V nominally. Even if you get close to 800V unloaded (due to poor regulation, with much higher unloaded secondary voltage) you can still use 450 volt caps. That may still be too high for the rest of the circuit but it DOES solve the capacitor problem if that was the only thing holding you back. The upper leg can even be choke filtered (inside the cap) to bring it down to 500V once you hit the critical current.

Using this with a trafo that was 250-0-250 without a load (say, rated 480VCT) I would get right at 700/350 with no load and 600/300 under full load. With 400 volt capacitors.
 

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Hi,

The output tube is best to operate with 450-500V at the plate and 200-250V at the screens.


73
Wolfgang

I forgot to clarify this issue. This particular valve needs to stabilize the screen voltage at a maximum of 200v. I stabilized it at 175v. Its curves are very variable with this tension, it is a world apart.
Thanks
 
All sweep tubes need the g2 regulated. When used as an audio amp, Horizontal outs want about 1/4 to 1/3 of B+, vertical outputs about 1/2. Some of the really high current ones can drive heavy loads even with 125V on g2. And you never want to drop 400 volts across the regulator unless you like blown mosfets, which take the tubes out with them because they fail short circuit.