Princeton Reverb with 6AQ5 tubes

[Bieworm]

Hi y'all

I intend to build a Princeton Reverb with 6AQ5 power tubes (cathode biased)
Of course the plate and screen voltage would be too high with the Hammond 291AEX when using the 6AQ5. Out of concern to affect the preamp in a negative way when I would drop the overall B+ voltage to accommodate the 6AQ5 max Va an VG2 I was thinking of branching off the B+ at node A and drop the voltage with zener diodes, supplying only the power tubes, while keeping the voltages for the preamp higher to spec.
Any trouble with this approach?

thanks

[Latole]

Google : The 6AQ5 is a miniature 7-pin (B7G) audio power output beam tetrode vacuum tube with ratings virtually identical to the 6V6 at 250 V.

_________________

Why a 6AQ5 if it's going to cause you so much trouble?
What are the advantages?

[HotBluePlates]

I intend to build a Princeton Reverb with 6AQ5 power tubes (cathode biased) ... I would drop the overall B+ voltage to accommodate the 6AQ5 max Va an VG2 I was thinking of branching off the B+ at node A and drop the voltage with zener diodes, supplying only the power tubes ...

250v / 1.414 x 2 = ~354v

I would grab the Hammond 269EX, which is 380v CT.
380v / 2 = 190v ----> 190v x 1.414 = 269v

I would then added maybe 50Ω between each B+ lead and the solid state rectifier, to give a touch of sag.  I wouldn't even bother with trying to get higher preamp voltage.

   - Look at the 6AQ5 data sheet.
   - You could be running these tubes at only 12-13v of bias, instead of 34v of bias as in the original circuit.
   - The preamp can afford to lose some supply voltage, since it only needs to deliver about 1/3 the output.


I'd also use an audio-taper pot for the Reverb control and a medium-decay tank so I'm not always stuck at setting the Reverb at 2-3.

[Bieworm]

I intend to build a Princeton Reverb with 6AQ5 power tubes (cathode biased) ... I would drop the overall B+ voltage to accommodate the 6AQ5 max Va an VG2 I was thinking of branching off the B+ at node A and drop the voltage with zener diodes, supplying only the power tubes ...

250v / 1.414 x 2 = ~354v

I would grab the Hammond 269EX, which is 380v CT.
380v / 2 = 190v ----> 190v x 1.414 = 269v

I would then added maybe 50Ω between each B+ lead and the solid state rectifier, to give a touch of sag.  I wouldn't even bother with trying to get higher preamp voltage.

   - Look at the 6AQ5 data sheet.
   - You could be running these tubes at only 12-13v of bias, instead of 34v of bias as in the original circuit.
   - The preamp can afford to lose some supply voltage, since it only needs to deliver about 1/3 the output.


I'd also use an audio-taper pot for the Reverb control and a medium-decay tank so I'm not always stuck at setting the Reverb at 2-3.

Thanks HBP!
FWIW(1) I already have the 291AEX and would like to use that one. There is an additional 280V tap on the HT winding that would help me to knock down the B+ to some extent. If (with zeners) I can get in the area of 300V on the plates I will be a happy man. I have another amp running on that plate voltage without issues. I don't know if I'm wrong, but won't the preamp lose clarity on lower voltages?
As for the reverb, I always let the cathode of the gain stage after the recovery unbypassed. That still gives plenty reverb and better sweep on the 100kL pot. YMMV
FWIW(2) the 6AQ5 is merely an experiment. I drilled the chassis for 6V6 tubes and have the adapter plates for noval tubes installed (I'm using 6P1P/EL90 tubes, SED Winged=C=) If I don't like it I can always go to using 6V6 without a lot of hassle...

[Bieworm]

Google : The 6AQ5 is a miniature 7-pin (B7G) audio power output beam tetrode vacuum tube with ratings virtually identical to the 6V6 at 250 V.

_________________

Why a 6AQ5 if it's going to cause you so much trouble?
What are the advantages?

just an experiment. You can get NOS SED =C= 6P1P for 5€. That's 5 to 6 times less than the price of an average current production 6V6.. so why not?

[Latole]

Google : The 6AQ5 is a miniature 7-pin (B7G) audio power output beam tetrode vacuum tube with ratings virtually identical to the 6V6 at 250 V.

_________________

Why a 6AQ5 if it's going to cause you so much trouble?
What are the advantages?

just an experiment. You can get NOS SED =C= 6P1P for 5€. That's 5 to 6 times less than the price of an average current production 6V6.. so why not?

You ask why not ?

 1- Less headaches for the voltages.
2- More chance for the amp to work well

But by trying things out, you learn

[HotBluePlates]

FWIW(1) I already have the 291AEX and would like to use that one. ... If (with zeners) I can get in the area of 300V on the plates ...

I can understand wanting to use a transformer that's already in-hand.

My preference is to use "the part that delivers the desired voltage" rather than use a kludge like zener to attempt to lose 100-150v.

... but won't the preamp lose clarity on lower voltages? ...

No.  The biggest change is maximum AC output voltage that the preamp can deliver.

I have some outstanding vacuum tube mic preamps that only have a B+ of 250v.  "Clarity" can be achieved (or discarded) with EQ, or by "indirect-EQ" in the form of output impedance based on your choice of operating point & plate load resistor.

... the 6AQ5 is merely an experiment. I drilled the chassis for 6V6 tubes and have the adapter plates for noval tubes installed (I'm using 6P1P/EL90 tubes, SED Winged=C=) If I don't like it I can always go to using 6V6 without a lot of hassle...

... You can get NOS SED =C= 6P1P for 5€. That's 5 to 6 times less than the price of an average current production 6V6.. so why not?

6P1P and 6AQ5 share a max voltage rating of 250v.  So it's easier to build a safe environment with the lower-voltage transformer.

   -  6V6 (with a Princeton's 8kΩ OT) delivers about 15 watts.
   -  If you lower the plate & screen to 250v (and use a 5k-6.6kΩ OT) you get 10w of output.
   -  10 log (10w/15w) = -1.76 dB SPL

The output power reduction is so minor, you might miss it.  OR, you will definitely fail to notice the power-reduction if you experience a "correct voltage" Princeton Reverb with an original-style Jensen C10Q or C10R, then use a Celestion or Eminence speaker for your "250v amp."  The higher sensitivity of the latter speakers will make the 250v amp louder than the "higher-powered Princeton."

[mresistor]

use 6BQ5s   there is also a russian equivalent

[Latole]

use 6BQ5s   there is also a russian equivalent

Answer #4

Why 6AQ5 ?

" That's 5 to 6 times less than the price of an average current production 6V6.

[HotBluePlates]

use 6BQ5s   there is also a russian equivalent

Ideally those also stay below 300v (though some amps push much higher, often with higher failure rates).

[PRR]

use 6BQ5s   there is also a russian equivalent

6BQ5 _IS_ EL84, the "18W Marshall" tube.

6AQ5 is a cheaper 6V6 (already a cheaper 6F6-size version of 6L6).

[mresistor]

Yes I know what they are..  and they work fine at 330 or so   and also I've always wanted to build a PR with EL84s   
guess a lower watt 6V6 aka 6AQ5 would be cool but that pt isn't really suited as you all have stated


what about a zener on the center tap to ground of the HV winding?

[HotBluePlates]

... what about a zener on the center tap to ground of the HV winding?

A zener (or series-string of zeners) on the center-tap can reduce effective output voltage.  The 275v tap on the 291AEX would deliver about 389vdc, to be knocked-down by whatever method is chosen.

But as I said, my preference would lead me to get a different transformer and/or sell the one on-hand to avoid the need to bypass some characteristic of a part.

[Bieworm]

As mentioned previously it's an experiment. I wouldn't mind the lower output anyway. At some bar gigs my current Princeton clone is too loud already. If the 6P1P turns out to deliver a cool sounding Princeton I will probably change this one to 6V6's and build another one with the appropriate power transformer. I was just wondering if any of you guys saw a potential problem with the schematic in the original post. Since nobody had any remarks on that one I assume it'll be ok to execute?

[tubenit]

HotBluePlates comment about using the right PT for the right voltages is excellent advice. And that is what I would personally do.


Having said that, quite a few yrs ago .......... I used a VVR to lower the voltages on the power tubes and LTPI on an amp to use 6AQ5 tubes. 


Here is some VVR info in ARCHIVES     VVR and cathode biased amps (el34world.com)


With respect, Tubenit

[HotBluePlates]

... I was just wondering if any of you guys saw a potential problem with the schematic in the original post. ...

If you were to use a zener to drop voltage for the entire amp, it would typically go between the power transformer center-tap & ground.  The effect is to move the center-tap "lower-than-ground" so that the voltage between Ground and B+ is reduced.  You might still do it this way if you don't mind dropping B+ for the entire amp.


In your case of wanting to lower just the output tube plates & screens, you would place the zener between the filter cap negative terminals & Ground for the plate & screen filter nodes.  Then provide a 2nd output from the rectifier to a completely separate power supply for the phase inverter & preamp only.  You will want a cap-to-ground and a series resistor prior to your "normal preamp power supply" to replace a little of the filtering you'll no longer be getting from the plate & screen nodes.  However, this may not be crucial for preamp tubes and their small current draw.

[PRR]

Any form of loss enough to use that PT with those tubes is going to be 7 to 10 Watts of waste heat. And high-voltage hot-parts so not exposed outside the chassis.

https://www.google.com/search?q=fallacy+of+sunk+costs

[Bieworm]

Maybe it’ll be better to wire it up for 6V6 from the start. And keep the experiment for an appropriate transformer type…