Direct coupling and tube choice limitations ?

backtomono · 3006

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Offline backtomono

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on: December 06, 2012, 11:31:19 AM

Hello there, all Experts onboard !

It appears to me with some help
that the directly coupled grid of my 2a3 Paraglow amplifier limits greatly
the choices regarding a new tube in replacement of my stock ValveArt.
because of high floating Voltage at the amp's start, i've heard.

So here is my question,
And i hope i'll get some help here from the Experts
even if the Paraglow has been discontinued a few years ago:

if mesh plates and single plates won't make it... (tell me if i am wrong)
that means no JJ Tesla, and no Sophia electric !???

so what are the tubes capable of handling such a high starting voltage :
RCA ? Shuguang? Sylvania ? Sovtek ? others ?

Any recommendation and advice will be greatly welcome!






Offline Paul Birkeland

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Reply #1 on: December 06, 2012, 01:48:38 PM
Depending on your flexibility, you may be able to attempt to retrofit the Paramount soft start board to your Paraglows.  There would be some part changes from the stock kit, and some good understanding of how the circuit functions would help to aid in success.

As far as I know, the EML's are the only problematic 2A3's so far.

-PB

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline Paul Joppa

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Reply #2 on: December 06, 2012, 05:03:03 PM
Well, I finally took the time to look for my manual archive. It makes a difference!

The original Paraglow (my manual is dated 1998) used a single section of the 5965 as a driver, fed from the 2A3 cathode with a single C4S. This configuration is completely immune to the high voltage at start-up problem, since the grid of the output tube cannot go higher in voltage than the cathode. If you have real original ParaGlows with only one C4S per channel and no other circuit boards, then you can use any tube you like.

There was a later "Paraglow II" which was John Tucker's modification of the original and added the shunt regulator circuit using the second section of the 5965. I am proud of having suggested it, though it was Tucker who actually built it. And I must in the same paragraph confess that I was so surprised that it made such a difference, I spent some time turning his amps over (one was modified, the other not) to make sure it was actually true. I may even have insisted we swap the amps between channels before I believed it.  :^)

That ParaGlow II circuit ultimately morphed into the Paramount 2A3, with a re-designed shunt regulator design.

Paul Joppa

Offline backtomono

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Reply #3 on: December 06, 2012, 08:25:41 PM

Thank you Paul (and Paul) for the help !

i just looked at my Paraglow's circuitery,
and recognized the small square PC board of the c4s
with its LEDs and transistors,
and... there is just ONE of it !

so i guess i can move to any tube i'd like ?

Thanks again for your help !

PS : i built my Paraglow some 10 years ago,
didn't come back for a while... (it is still sounding glorious!)
But while reading the posts here on BH and looking at the new amps,
I realize how strong is the knock-on effect
and i have to be very careful not to have a recurrence of soldering audiophilia !


 


Offline Paul Joppa

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Reply #4 on: December 07, 2012, 06:29:20 PM
If you are likely to use the EML or another expensive tube, I'd check the actual filament voltage at the tube socket, and your power line voltage as well, to be sure you are within 5%. This will ensure longer life from the tubes. In fact, you might repeat the original voltage checks to be sure nothing has drifted out of spec while you are in there.

If I recall correctly, some of these amps were built using a different section of the driver triode (they are dual triodes) in each amp, so that you could swap tubes from one side to the other when the first side started to lose emission. If you haven't, then 10 years' use seems like a convenient time to "rotate the tires".  :^)

Paul Joppa

Offline backtomono

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Reply #5 on: December 07, 2012, 09:33:35 PM

Yep, 10 years sounds like the good time to find some new lights to put on... :)
But i am running mono so i just cannot exchange the drivers!
Anyway i got a nice Mullard e180cc on its way home, so i guess it will do
as well as a brand new JJ Tesla 2A3 i just fished in the e-bay...
nothing fancy! so i guess that will do ?
And I just can't put a hand on the construction manual so voltage verifications
are out of reach just now... or is there a place i can download it ?


Offline RPMac

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Reply #6 on: December 12, 2012, 09:12:22 AM
A word of caution, the Paraglow II also only has one PC board, but it is a larger board with a "large" red cap and a blue trim resistor box. I got my upgrade from John and they came with 6n1p tubes. I had problems with my RCA biplates arcing. The tubes I used without arc problems were the ValveArt, Shuguang 2A3C, and Sovtek single plate.

Haven't used my PG II's for some time because I pick up a pair of Parabee's for less than half the cost of the TFA-2004 Ni outputs. My Paraglows have EXO-036 Ni outputs. Doc sure put some good iron in those early kits.

I have high line voltage that was causing problems with the shunt reg boards. I have soft start kits to retro fit, but haven't found the time to get them done.(too busy listening).

I have JJ 300B in the Parabees that I really like. Have read many good things about the JJ 2A3-40...should be a good one for you to try.


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