Re: Doubling my Paramount pleasure...another 45 conversion

[galyons]

“I hope everyone understands that the Paramount was designed for the 300B and 2A3; the 45 modification is not a standard thing. “ PJ

Acknowledged!  But, the Paramount design is so good, that, like many other Bottleheads…..I want to do it all the same!!
 
So here is the system configuration:
Speakers:
Electro Voice Sentry IV-A’s ( 3 way  horns, 102dB/1w/1m  400Hz, 3500Hz crossovers), will be bi-wired bass bins & mid/treble. The bass could use a little boost, but don’t want to fatten up the lower mid-range too much.

Line stage:
Smash with Smash-up.  2uF PIO output caps.

Current amp:
Paramount V1.0 300B 6CG7 driver, 4.0uF PIO output caps, stock iron.


Game Plan:
Keep current Paramounts, use for bass bins
Build V1.1 Paramounts, use for mid/treble

Available Iron:
BH-7  27H/76mA  Plate chokes
BCP-15  50H/40mA  Plate chokes (2 pairs)

TFA 2004 Jr.  nickel pinstripe
EXO-45 all nickel

Line voltage is pretty consistent at 120-122VAC.

I can mix & match the iron between the old and new Paramounts.  I can convert the old Paramounts to V1.1.    I can take the old Paramounts back to 12AT7 for greater sensitivity/gain. Maybe replace the input load with a stepped attenuator to control/balance with the mid/treble amp. Lotsa’ output cap choices from 2.0uF to 12uF.  PC-2’s in the old Paramount to do CLC?


Way too many options to decide empirically by listening to each iteration.  So the thoughts of the collective experiences would be greatly appreciated!

PJ  has made several posts about the Paramount V1.1 conversion, beyond just changing the cathode resistors from 2K to 3K ohm.  Some of the recommendations were to specific circumstances, (Jim R & Paully). Some of his posts are linked in other discussions, but, since the forum upgrade, they lead to the dreaded “404 - Page not found”.  Sadly I could not find much more with the search function.  If there are other changes please post!

Can the cathode cap value for the V1.1 45 amps be reduced from 47uF?

What is the minimum power rating that could be used for the cathode resistors?  Would two 3k ohm 12 watt Mills resistors be sufficient?

Anyone's thoughts and experiences would be greatly appreciated!

Cheers,
Geary

[Paul Joppa]

* Gain control on at least the higher-gain pair of amps is a good idea. I would put it to the left of the driver, symmetrical with the hum pot on the far right - that's mostly just for looks.

* To add a choke to the power supply, the best location is on the top side, sharing mountiing bolts with the FC-1 filament choke. You'll need something with wire leads - the choke is at very dangerous high voltage, so no exposed solder terminals! The old reliable C7-X choke (10 henries at 90mA) is a reasonable choice. You may want to experiment to choose which amp gets the choke, before making a final decision.

* The cathode bypass can be smaller for a mid/hi amp; it is there mainly to maintain bass response as the plate choke impedance becomes smaller than the cathode resistor.

* IMHO the minimum rating for the cathode resistor is 30 watts (two 15-watt 3K resistors for example). The price of direct coupling is lots of wasted heat in that resistor.

[galyons]

Thanks PJ, that is most helpful! I will follow your advice on all four points. I will go with 2 3K ohm 25 watt resistors to replace the 2K ohm units I received in the kit.  The input load resistor is 249K. Would a 100k attenuator be OK? Since the choke should go topside, I have some nice Transcendar 10H 200mA chokes that might work.  They are, at least, a little easier on the eyes than the Triad's! Using a 33uf or 22uf film cap for the cathode resister bypass will be easier than the bigger, higher capacitance units. I have Solen 47uf's in the Paramount 1.0 and they take a fair bit of real estate.

Still looking for experiences and thoughts on the iron. My initial thoughts are to use the BCP15/EXO-45 combo on the V1.1 driving the mid/hi horns.  Still, I would really appreciate input as to using the TFA2004Jr for the 45.  I understand that it is not a text book match-up.  But I  have seen comments on how good it is when used with 45's at compatible operating points.

Cheers,
Geary

[2wo]

The original SR-45, used the pinstriped BH-5 (I think it's 5) "miss-taped" To increase the reflected load. It sounds awesome. I replaced it with the EXO-45 ni and I am not sure I like it better.

I don't have my notes but I think you use the 8 and 16 ohm taps. Maybe Paul will remember...John       

[galyons]

The original SR-45, used the pinstriped BH-5 (I think it's 5) "miss-taped" To increase the reflected load.

Thanks John!  I am not near the Jr.'s, but IIRC they are tapped for 8 & 16 ohms. My speakers are 8 ohm.  If there was a 4 ohm tap, I could hook the speakers to that tap and get an impedance of 6K ohms.  But I don't think there is a  4 ohm tap.  I'll look when I get home.

Cheers,
Geary

[Paul Joppa]

The original SR45 was designed for the 3K transformer. It's not an ideal impedance load, but the available voltage is also not ideal, so the operating point was adjusted to work with the available voltage and as close to optimal as possible for the 45.

The best load for a 45 is about 4K ohms - this gives the most output power at the highest rated voltage. Many people use a 5K because they are more widely available; it makes for a slight reduction in both power and distortion.

For the OP's situation, the larger transformer (the TFA) should handle the bass, no question. Bass needs bigger transformers, and so does power.

It seems to me that I have posted before about operating points for 45s in the Paramount, specifically how they vary with load impedance. Fortunately the v1.1 allows some adjustment. If you post voltages when it's done I can look at suitable tradeoffs - the details depend on the power line voltage so it's hard to generalize. It may be that the cathode resistor shold have a different value; again I need to see the voltages before I can make a useful comment.

[galyons]

The original SR45 was designed for the 3K transformer. It's not an ideal impedance load, but the available voltage is also not ideal, so the operating point was adjusted to work with the available voltage and as close to optimal as possible for the 45.

The best load for a 45 is about 4K ohms - this gives the most output power at the highest rated voltage. Many people use a 5K because they are more widely available; it makes for a slight reduction in both power and distortion.

For the OP's situation, the larger transformer (the TFA) should handle the bass, no question. Bass needs bigger transformers, and so does power.

It seems to me that I have posted before about operating points for 45s in the Paramount, specifically how they vary with load impedance. Fortunately the v1.1 allows some adjustment. If you post voltages when it's done I can look at suitable tradeoffs - the details depend on the power line voltage so it's hard to generalize. It may be that the cathode resistor shold have a different value; again I need to see the voltages before I can make a useful comment.

Thanks Paul.  I changed my search parameters and finally found a couple of posts with a great deal of your operating points for 45's.  But most were for Paully's 5K OPT configuration. (Perhaps those were the  posts Paully linked, but the links are now dead?)   I would like to run the 45's conservatively, maybe dissipate only 7-8 watts.  (All but one pair of my 45's are old stock,  the one pair is Sophia meshplates, but I have liked the OS better in my Paramours).

My line voltage runs pretty consistently around 120-122VAC.  I can measure the voltages on the 4 pins on my current Parmounts if that will help. (They were pretty close to target, last check up.)

The new V1.1's will be used for the mid/high horns, so bass is less of a concern.  The focus on these V1.1 Paramounts will be, obviously the mid range and treble.  Based on the posts that I read, seems you prefer the BCP-15's  for the plate chokes with 45's. My conundrum is whether to use the EXO-45Ni or the TFE2004Jr Pinstripes for the OPT's and then get the operating point dialed in to match.  Since neither is the 45 ideal 4K, I am trying to pick the best for this duty.  I could take the easy way out and put the EXO-45Ni's into the V1.1 for mid/treble and the Jr. PS into the V1.0 for the bass bins. My concern is that the Jr. is probably wasted in that application and would better the EXO-45 in the Paramounts. Your thoughts! (Plus there is the issue of top mounting the Jr verus bottom mounting the EXO-45, so not a simple "try both ways!")

I ordered 4  3K ohm 25 watt resistors and will dial in the voltage by adding additional resistors in series, as you recommended. So the voltage tweaking should be easy.

Cheers,
Geary

[Paul Joppa]

The TFA pinstripes have a majority M6 (or M4, sometimes Mike changes them up) along with the minority nickel. That gives them an extra advantage in handling large low frequency signals - the TFA will handle twice as much power.

I have no data and no adequate experience to say what happens when a signal transitions from mostly-nickel to mostly-iron, or at the high power end from high permeability nickel to saturated nickel - but transitions in mode make me worry about subtle distortions. So I would favor the all-nickel EXO-45s for the mids and treble.

If the mids/treble are high enough in frequency that there are no transitions, then the nickel will be doing all the work. And the smaller transformer actually has more nickel.

Of course that's just about the cores. At the highest frequencies, it's the coil that dominates - leakage inductance and distributed capacitance mostly. I have practically no experience to compare the EXO-45 to anything in the mids and highs. Perhaps DocB will have a thought about this...  Other than that I can only generalize and say that both parameters are easier to control in a smaller transformer.

[galyons]

Thanks Paul, that is helpful, (but a bit over my head!  )  I think you are likely correct in leaning toward the all Nickel for the mid & treble. Hopefully someone has used the EXO-45 in the  Paramounts and will chime in! 

Cheers,
Geary

[galyons]

From the "way back" machine!!

Cool!

I'd suggest you raise the cathode resistor to 7000 ohms (add 1000 ohms as discussed before) and run the 45 at 260v/30mA. The driver plate.45 grid should be about 157v since you'll have around 470v at the 45 plate. Keep that ratio: 157/470 =~ 1/3. Parafeed cap is 4uF, so the original 3.3 would work fine. (The higher load impedance reduces the cap size by the square of the ratio.)

This was for Paully running at line voltage of 120VAC and, I believe, EXO-45's, so 5K operating points. Looks nice and conservative, and if my math is correct would have the 45 dissipating about 7.8 watts.  Which is in the range I was targeting.  If I use the BCP-15 at 50H, then the parafeed cap would be about 4.0uF, as well.  If the half/double rule applies, then I could get by with 2.0uf or 2.2uF, since the crossover point for the mids is 400Hz. 

Am I on track? I appreciate your going over this "yet again"!  I am learning, hopefully!! 

Cheers,
Geary

[Paul Joppa]

Thanks for finding that - I stand by it still, and your intuition about the parafeed cap value is also good.

[galyons]

Having sorted out my Paramount 300B v1's with 7N7 driver tubes, it is on to the next project. Paramount v1.1 with 45's and 7N7's for driver tubes.  Since I started this odyssey over a year ago I have managed to procure some even nicer MQ iron from my source of, apparently, never ending esoteric goodies! 

So the iron BOM now looks like this:

Power Transformer           BH PT4   
Filament Transformer        BH FC1
PS Choke                           Transcendar  10H (nice HV choke with end bells)
Output Transformer           MQ EXO-145 Ni   5K (wound with pure copper Litz wire)
Plate Choke                        MQ Dowdy  60H 40mA (custom build to match the EXO-145)

I feel really privileged to have the rare MQ iron for the build...The PS Choke and Dowdy PC will go topside.  That will open even more real estate down under!

I will have to mill a larger top plate, projecting 9"x12", but flexible given the big hunks of iron in close proximity.  PJ I am very interested in your comments on the orientation of the PS choke and Dowdy PC.  Both chokes bells to PT-4?  Any issues with the PS choke having the same orientation as the OPT?  Should I keep the PS choke as far from the PT as possible or can I move a bit closer for better esthetic balance?

My rough component layout:

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi.imgur.com%2FdHdogFW.jpg&hash=2ebf918958e241a9de915576e72fa5d412717ac9)

[johnsonad]

Why not consider going even larger on the chassis? You have the room at home and it gives you more space to play with in the future. My Paramounts are on 12x18" plates for perspective. Not saying you need to go that large 😊

[Paul Joppa]

Rotate the plate choke 90 degrees. Imagine the axis of each adjacent coil, which should be perpendicular to each other to minimize coupling.

I assume the choke is taller than it is wide - hard to be sure from the photo.

[galyons]

Why not consider going even larger on the chassis? You have the room at home and it gives you more space to play with in the future. My Paramounts are on 12x18" plates for perspective. Not saying you need to go that large 😊

I don't mind going a bit larger.  The implied constraint is that each channel will be biamped, so two Paramounts per channel.  I will likely match the chassis size of the 45 amp when I rebuild the 300B amps.  that starts gobbling up space rather quickly unless I build a 2 shelf stacking stand for each side.

Cheers,
Geary