Paraglow Output Transformer to 2A3 Cathode?

[Bryon]

Hi Paul

I think I've got all the parts to go ahead with the rebuild.  You mentioned that the output transformer should be moved from ground to the 2A3 cathode- could you be more specific in terms of which pin it should connect to?  I'm a bit confused [not unusual when I'm trying to figure this stuff out] I think I'll be connecting the 3.3uf capacitor to the cathode of the 2A3 - although I'm still not sure which pin and looking at data sheets for the 2A3 didn't help?

Any other suggestions about what I should change?

thanks

Bryon

 

[Paul Birkeland]

You will have a hum pot between A1 and A4, the wiper of that hum pot is the cathode of the 2A3.  I recommend connecting the parallel feed capacitor to the plate of the 2A3, feeding the top of the primary of the output transformer, then connect the bottom of the output transformer primary to the 2A3 cathode (wiper of hum pot).  There will be a number of connections to the wiper, so plan accordingly.

 

[Bryon]

thanks for the response - so to make sure I understand let me reword your instructions.

the parafeed cap connects to A2 and then to + side of the primary [the side closest to the edge of the chasis, is this what you mean by top?], then the - side of the primary is connected to the wiper of the hum pot; whereas now A2 is connected directly to + on the primary and the parafeed cap is connected from the - on the primary to terminal 1T on the PGP 8.1 or to the ground buss.

Please correct me if I've misunderstood.

thanks again

Bryon

 

[Paul Joppa]

Crap! I posted a long, long white paper but it got lost before it showed up.

Lots of things are unclear, especially about what you have already. This makes it hard to say exactly what you should change! Here are the biggies:

* 2A3 cathode bias resistor should be 4K 50 watts. You probably have 2.5K or 3K, 10 watts. Even the 50 watt unit will be dissipating 15 watts, same as the 2A3. It's smaller than the 2A3 and will get HOTTER.

* The driver plate voltage must be raised to 200 volts from the original 100 volts. The shunt regulator must provide another 100v of headoom (regulated 300v or more.)

* The optimum parafeed capacitor value is 10uF to 15uF, 630v or 800v if you have the EXO-03 choke and EXO-35 or -36 output transformer.

The original design had the output transformer to the plate with the other end grounded through the parafeed cap; as PB pointed out we now do it the other way around to reduce the voltage on the winding. That's a good idea since the transformer is exposed. Even so, remember the transformer and choke terminals can kill you if you tough them, be very careful about insulating them!!!

 

[Bryon]

Thanks Paul

Your post while informative brings up some questions

The 2A3 cathode bias resistor is no problem, I'll just order some new resistors. When I changed from the original resistors to the combination of the 1.8k and 1.3k 10 watt resistors for Tucker's mod, the 3k ohm 12 watt resistors that were in there had literally started to melt and had dripped a brown substance onto the chassis plate.

Is the suggested change in the voltage of the driver plate based on a change from the 6n1p to the 6080?  would I need the new shunt regulator board?  Did you have a chance to finalize and evaluate the new board and is there any hope it will be available soon? 

The parafeed capacitor suggestion brings up yet another problem.  I'm using the Paraglows to feed Lowther DX4's in 204hz Azura Horns and so have limited the parafeed capacitor to .22uf in order to have the amp roll off naturally at about 200hz.  I thought it would allow the Paraglows to focus on the frequencies they were being asked to produce and also allowed me to be able to afford V-Cap Teflon caps which sound glorious.  In a previous post you carefully avoided telling me that I was off my head for doing so, but I'll ask again anyway.  Do you think it would be better to go with the larger caps in the Paraglows and then limit the frequencies to the speakers through my active crossover?

thanks again

Bryon

 

[Paul Joppa]

Sorry about that - I knew my post would raise more questions than it answered, just not enough energy to write the full white paper.

>> Is the suggested change in the voltage of the driver plate based on a change from the 6n1p to the 6080 5670? 

No, it's based on setting the 2A3 grid 300v below its plate, which you said is at about 500v in your amp.

>> would I need the new shunt regulator board? 

The new board uses that voltage (for the Paramount 300B driver) and will do the job. I don't know whether, or how, the Tucker board can be made to do that.

>> Did you have a chance to finalize and evaluate the new board

It is still in evaluation. Both versions sound very good; we are not yet sure whether one is better than the other.

>> and is there any hope it will be available soon? 

Yes, I certainly hope so!

>> Do you think it would be better to go with the larger caps in the Paraglows and then limit the frequencies to the speakers through my active crossover?

You must use an upstream crossover if you wish to free the amp's tubes (and the speakers) from the deep bass. If you wish to free only the speakers from that duty, then the small parafeed capacitor is a good solution. Since you are driving horns, the total (acoustic) roll-off will be pretty steep - you might consider doing both, perhaps a highpass at 100Hz before the amp and leave the 200Hz one in the parafeed capacitor. The larger capacitor optimizes the deep bass performance into a resistive load such as a speaker below its impedance resonances.

 

[Bryon]

thanks for the response and the clarification

Sorry about the 6080/5670 mix-up - I haven't even ordered one [yet] but I must have crack on the brain.

Thanks for the suggestion for limiting the frequencies to the Paraglows and speakers - I'll definitely try it when I get the Paraglows back up.

I've decided I'm going to revert [more or less] to the original Paraglow set up with a few of the changes you've suggested and see if I can get the amp up and running.  I'll wait until your new board comes out and then rebuild the Paraglows with that upgrade in mind.  In the mean time I'll start the search for some 5670 or 396A tubes or has that been decided yet?

getting sick of mono in Beautiful BC and about to resort to solid state for a spell

Bryon

 

[Grainger49]

Byron,

Right now one lead of your transformer will measure ~0 ohms to ground (common) with your meter.  

The output tube has a resistor from the cathode to ground with a capacitor in parallel with that resistor.  One wire of the transformer is connected to the plate of the tube through the Parafeed capacitor.  The "return" for the transformer, the other lead of the primary, is that grounded lead of the transformer.  This is the one that Paul is suggesting you move.

What Paul Birkeland was suggesting was that the return for the transformer, now grounded, lead be moved "up" to the cathode end of the cathode resistor and capacitor.  Where the wiper of the hum pot is connected.

I think that VoltSecond describes this on his site as A Different Way To Parafeed.  When I bought upgrade iron I did this.  I didn't try it at ground first so I don't know the sonic improvement it made but electrically it is a very good idea.

 

[Nutube]

I am trying to understand the rewiring as well......

In my original wiring . One end of my Parafeed cap goes to 1T lug the other is attached to the red wire of my primary which nconsist of a red and a blue lead the Blue tranny primary lead went to  A2.

My understanding  is that the cap/Tanny sequence will be reversed for this new wiring.

I have removed the output parafeed cap from 1T; I have soldered one of the cap lead  directly to Plate (A2) of the 2A3.
The other end of this  3.9mfd 600V cap I need to solder to either the RED or the BLUE  primary wires of my output Tranny.

Which color to hook to my cap? I am not certain that  I chose the right color even back when I first did it several years ago.

I plain don't know the top or the bottom insofar as the conventional color code for OPT tranny.

So is the BLUE the top of the tranny where my cap feeds to - Or should I hook it to the RED. My understanding is that once the color ( top or botoom of the winding) is selected to connect with the cap to the tranny- the remaining wire should be soldered to the center  wiper of my POT which is the 2A3 Cathode as well.

Done !....right?

 

[Paul Joppa]

The convention is, the blue lead goes to the plate of the tube; in this case, through the capacitor. The mnemonic goes back at least to the thirties, a "blue plate special" was the special of the day at diners and small restaurants. It's not that current an expression these days, but then SE amps are not that current either except among us cognoscenti/nutcases.

The difference is a matter of minimum capacitive coupling and maximum self-shielding between the transformer and the external environment; it will probably make little difference in an output transformer but it's still good practice just in case.

 

[Nutube]

Thanks for the input . I am waiting on a couple 4 k ohm 50 watters . My current value is 3.k3 ohm 10 watt.
Yeah I remember we used to hear the siren for the Kmart Blue Light Special : ) usually no Tubes on sale after the mad rush : )
By the way did Eileen forward the email to You?
Regards
Tony

 

[Paul Joppa]

Paraglow came with Magnequest iron from the start. It's only the Paramour (I and II) and the SEX amp that use(d) the unrated output transformer stock.

(The new Stereomour has new iron that is rated for DC voltage. I still hope to use that iron on the SEX amp too, but that revision is further down on the priority list.)