2A3 conversion for ParaSEX anyone?

[ToolGuyFred]

Hi All

Just bought one of the last two pairs of ParaMounts. Unfortunately, these are not destined for my system, they will reside  one floor above  in my son's  bedroom. The price for building them is that I will have the use of them until I complete the final upgrade to my ParaSEX amps - the 2A3 conversion.

This was suggested in the original SEX manual, so the question is - has anyone else tried this?

I am thinking of keeping the SRPP stage, replacing 6DN7s with 6SN7  or 12AU7 then  capacitor coupled to the 2A3 with traditional cathode "auto" bias. The wood base will be deepened to give me room for film/foil PSU caps, filament choke etc. Any opinions on whether I will need a new filament power transformer or will  the original work with  the FC1 filament choke?

Some of you will be asking why bother instead of just buying new and better amps. My ParaSEX amps are old friends and have the Magnequest nickel outputs and lots of other goodies so this will be the  final upgrade.

[braubeat]

You could always install the paramount soft start upgrade with the 2A3 tube. That should get you about as far as you can go with a parasex amp.

Michael

[Paul Joppa]

I have been working (slowly) on a history the original SEX amp, including some options for updating the various versions. This exercise has reminded me how many versions there were ... Here is a paragraph from that history, describing things from the original manuals:

"27. If you have the deYoung or PGP8.1 transformer, convert to 45/2A3/300B output tubes. Basically throw away everything but the chassis plate and power transformer, and start over. The suggested target [was] the Afterglow, and for a short while a kit was offered. Afterglow has since been succeeded by the Paraglow and then the Paramount 2A3. There is a documented 300B conversion in VALVE magazine as well. It's a lot of work, and you wind up with something we have replaced ten years ago with something better. Not recommended but if you really want to do it, consult the Forum."

My section on updates that I can actually recommend today does not include a 2A3 version, but if there's interest I might add something. In any case, further comments and recommendations are highly dependent on what power and output transformers you have now.

There were two power transformers when ParaSEX was produced, the deYoung with leads and the Magnequest PGP8.1 with solder tabs. The Magnequest puts out significantly more voltage. (There was also an older SEX power transformer but I don't think it was still in use when the ParaSex came out.) The original output transformer had a 4K impedance; Magnequest never made a transformer at that impedance. If you will share details on what you have for iron, I will offer some notes on practical conversions that use what we've learned in the last 20 years. Photos will help, especially if you don't see suitable labels on parts!

[ToolGuyFred]

Hi Paul

Thanks for your response.

I think I have the de Young mains transformer as it has leads. On power up I measure 500V on the HT which drops to 450 when the cathodes warm up. (I also have mains varying between 240 and 250 VAC @ 50Hz.)

The outputs are an MQ upgrade currently listed on the "products" page as BH-1 outputs and BH-2 plate chokes. I have the version of the outputs with all nickel laminations which were a stunning replacement for the stock items.

I will look through my copies of Valve for the 300B version and ponder whether this is for me.


Hi Michael

Good thought on the soft start.

[braubeat]

You know that the soft start upgrade includes the fully shunt regulated active loaded driver stage with tube and socket included. It's a bargain gusher.

Michael

[Paul Joppa]

Excellent! The deYoung has two 2.5v windings (blue, grey, blue-white, and grey-white), not used in the ParaSEX. They can be paralleled to power a 2A3, or used in series for a 300B. You have quite a rarity; the ParaSEX only used the deYoung transformer for a very short while, and only on export (240v) versions.

The output components are designed for the stereo SEX amp (one tube per channel). The primary impedance is 8K ohms, and the rated power is 2 watts; the choke is 50 henries at 40mA maximum current. These are lovely components; I have them in my own stereo SEX amp, but they are not well suited to a 2A3. Here are a few options:

* Use the output iron you have, to obtain maybe 2.5 watts, but very clean  below that power limit.

The following possibilities use new output transformers and plate chokes:

* A 4000 ohm transformer and 50mA choke is used in the Stereomour. It would call for a modest reduction in the PSU voltage. We rate the amp 3.5 watts but you can usually get at least 4 watts from this arrangement.

* A 3000 ohm transformer such as the TFA-2004 or very similar BH-5, along with a choke rated 55-60mA such as the EXO-003, should give you 4 watts. My article "SEX and the single 300B" was in the November 1997 VALVE, v4n11. It can drive a 2A3 as well as a 300B but the plate dissipation is 18 watts - too much for an old-stock 2A3 but fine for the Sovtek and other "super-2A3s". I used it for several years, and preferred the 2A3 - I used cheap Chinese biplates which were cheap but lasted 1000-2000 hours before becoming noisy. They sounded better to me than the Sovtek 300B. YMMV. You can reduce the plate voltage with an added resistor/capacitor to get to 15 watts dissipation.

* The Paraglow direct-coupled circuit is also a possibility; it ran at 2500 ohms and was rated 3.0 watts. You need an EXO-35/36 output transformer and a 60mA choke as above. The "soft-start" Paramount upgrade is a place to start; it can be used with many dual triodes including the standard 5670 and the 6SN7.

So, some food for thought.  :^)

[ToolGuyFred]

Have been giving this some thought. First it might be an interesting exercise to re-bias the existing setup for my "wrong"  output transformers. I have access to an oscilloscope and signal generator so I guess I could just reduce the standing current until I get something like symmetrical clipping at 1KHz. Any suggestions welcome.

For the upgrade rebuild, the driver stage with soft start from the Paramount upgrade kit looks good. For the output stage, I could either use parafeed iron intended for Paramount from MQ or maybe one of the many conventional outputs from Audionote intended for a 300B (I will be doing the 2A3 directly coupled thing). For filament heaters I'm going with the DC kit from Tentlabs, probably with a little dedicated torroidal transformer.
   
All comments and opinions welcome.

[2wo]

First off, what output transformers do you have in there now?...John

[ToolGuyFred]

As noted above, I have the Magnequest pure  nickel outputs for SEX Parafeed. I wasn't aware when I purchased these that they were intended for the current SEX amps (a single 6DN7) and not the original ParaSEX amps I have (parallel pair of 6DN7).

[Paul Birkeland]

As noted above, I have the Magnequest pure  nickel outputs for SEX Parafeed. I wasn't aware when I purchased these that they were intended for the current SEX amps (a single 6DN7) and not the original ParaSEX amps I have (parallel pair of 6DN7).

That would be about an 8,000 Ohm primary transformer.

Other than half a 6DN7, you could probably use said transformer with a #10, or a low power #45 amp.

[ToolGuyFred]

Just finished building one of the last pairs of Paramounts but have to pass them on to their new owner (my son) tomorrow. These are the way to go so...

OK, here's the plan.

New (extra) set of wooden bases to double the height/volume available.
Paramount soft-start upgrade for the driver stage.
Direct-coupled as per 2A3 Paramount.
Magnequest EX003 plate chokes.
DC filament supply from Tentlabs - I'm having 4V torroidal transformers wound to supply the modules.

I have researched the topic of output transformers long and hard. Very tempted to just go for the Magnequest Paramount upgrade kit but in the end have gone for a more experimental route. Audio Note seem to be very big on C-core transformers these days but that is pushing the budget a little too far. Lundhal produce more affordable C-core transformers, so I had a look at their range. No parafeed outputs listed but they have a zero air gap/push-pull listed with 3K plate-to-plate. As an option, these can be specified with amorphous iron. This stuff is cooled so fast that there is no time for grain boundaries to form, so we have a metallic "glass". It is said to be a good choice for high resolution audio output transformers but has twice the iron loss of regular core materials so you need twice the iron cross sectional area. (Over 30 years ago our college Materials Science lecturer described how amorphous iron is cast by pouring liquid iron on to a rapidly rotating water-cooled roller to give cooling rates up to a million degrees centigrade per second. At the time no-one had found applications for the material. I guess I'm just a sucker for exotic materials...) The final thing which swings it for the Swedish transformers is that I pay less tax for stuff in Europe than anything I have to import from the US.

[Paul Joppa]

Push-pull transformers are designed on the assumption that the center tap is AC-grounded, whereas SE transformers are designed on the assumption that one end is AC-grounded. The parasitic capacitances are very sensitive to these assumptions about grounding, leading to poor high-frequency performance when not used as designed - specifically, resonances among the capacitances and the leakage inductances.

They do NOT in general substitute well for each other. The wider the bandwidth, the worse this effect will be.

[2wo]

The Lundhal data sheets can be confusing, they use the same number for a transformer to be used for P-P or gaped for single ended use. Unless I am missing something it is the same core and winding geometry. I have a pair of 1623 90ma that work very well in a non Parafeed amp. I would want to talk to an expert at your distributor before I would try a 0 gap in Parafeed...John       

[ToolGuyFred]

Thanks Paul, thanks John.

Will investigate further.

[ToolGuyFred]

Have exchanged e-mails with Per Lundahl today. I am looking at the 1664 transformers. They have dual primary windings which can be used either push-pull or single ended. Standard core gaps are for 100mA, 50mA or zero DC current. I have been assured that the zero DC variant is suitable for Parafeed. I have ordered a pair and will post listening impressions when the rebuild is done.

Thank-you all for your input.