Bottlehead Forum
Bottlehead Kits => Legacy Kit Products => Paramount => Topic started by: johnsonad on February 15, 2014, 02:52:12 PM
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This project started with a pair of 300B's I couldn't use in the BeePre, the EML 300B mesh plate. They were too nice not to use so I decided to build an amp around them. I had previously owned Paramounts and knew how good the amp was but always thought it could be pushed a little more than the original design. In corresponding with PJ over the years there where areas to explore without grossly changing the original design. I'm not sure what to call them but they are on the theme of Paramount.
The chassis plate is built for mods sizing up at 12"x18". There is plenty of room to work and room enough for mods in the future. The larger plate also allows for greater distance from the power supply (PS) to the output stage. The top plate and the transformer bell ends were power coated black wrinkle in keeping with my other BH projects. The PF cap was rattle can painted in a similar black wrinkle. The bases are an extra long kiln dried maple giving them a smoked appearance with a semi gloss polyurethane finish. A big thanks to John at Harmonic Design Inc who made the bases and makes the BH bases. He really went the extra mile to get these right.
The power supply design is detailed in another thread, a simple CLC using the stock capacitance and a lot of induction (BH iron). The power supply ripple is less than 1mV according to sims. The added inductance gives the needed voltage drop for the operating point.
I chose the 350v, -76v, 50mA, 5k operating point from the WE data sheet for a couple reasons. First, to hear a 300B with a 5k output transformer (OPT) and not just any but the Magnequest EXO-050. It has the headroom to run the 300B into the lower bass without giving up the go and has one of Mikey's more advanced windings. Second, to take advantage of another piece of MQ iron, the Dowdy plate choke. In this amp they are gapped for 60mA with 80H of inductance. And lastly, this operating point has one of the lowest second and third harmonic distortion measurements of those listed by WE and at 50mA, I'm ensured long life from the EML's.
The driver stage uses the soft start board with a simple mod for top mounted bias adjustment. The 10k pot is super glued to the top plate and wired to the board. The test point is also wired to the top plate via a tip jack. There is another tip jack at star ground (thank you Grainger). In this configuration I'm able to swap driver tubes and quickly bias them without flipping the amps. I started with the 5670 and listened to my entire stash of tubes (a dozen different pairs). There was just something missing and in looking over old posts and with the help once again from PB and PJ I switched to the 6SN7 and haven't looked back. There are other variants out there that I may try in the future but for now, I'm very happy with the 6SN7.
The output stage uses a MQ high nickel grid choke instead of the resistor. The cathode bypass resistor is a Caddock thick film mounted to a heat sink (PJ's idea). The cathode bypass capacitor is one that Steve (Ironbut) posted about and is 100uF film to possibly get a little more bass. There is a V-Cap Copper Teflon coupling cap and the parafeed (PF) capacitor is a Russian KBG PIO. The hum pot is a ten turn type and much easier to find the sweet spot with. There is a switch between the PF cap and OPT to ground. It's set to ground as the amp warms up and after 30 seconds or so I switch it to the OPT to prevent magnetization. Otherwise it's a nearly by the numbers Paramount build.
Operation-wise the circuit seems to be idling. The cathode bypass resistor heat sink stays at 100F and the power transformer is cool enough to continually hold. There are zero hot or warm spots on the chassis plate.
So how do they sound? Well they sound great. I've been listening to them for a couple of months now and have a good feel for them. Needless to say but these amps aren't going anywhere.
More to follow in future posts and thank you again to everyone who has helped me with this build.
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Under the hood
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More under the hood
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A few glamor shots
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Nice, I was just working on my 300b's. Maybe I'll post a shot
What is the potted bit next to the grid choke?...John
edit, I see, that's the cathode bypass cap
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It's hard to see in that photo but in order from left to right is the grid choke, then in black, the cathode bypass cap then the heat sink and cathode bypass resistor. The large round black shape below the cap next to the 4 pin socket is the large 10 turn pot.
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Today the grid chokes were removed and they are never going back. We found as if a fog had been lifted once they were gone. There was an increased clarity and ease to the music. Decay and resonance feel natural now. I noticed an improvement when I removed them from my SEX amp but hadn't gotten around to removing them from these amps. Many people love them but they didn't work for me. At least I gave them a try.
The 249k Ohm resistor I used in their place was a nice dale MF type. Is this a resistor location that could potentially improve with a Tantalum version?
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That resistor is indeed in the signal path. I tend to find that tants can help when there is a standing DC current, but there's nothing wrong with giving them a shot regardless.
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Thanks Paul, I'll give them a try.
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I think (technical/theoretical thinking, not listening experience!) the virtue of grid chokes is in overload management. When the power tube draws grid current on transient overloads, the interstage cap and grid choke together recover more quickly than a cap-resistor combination. So if you are pushing the power limits, it will sound more dynamic. Few people realize how often we do push those limits, because SETs overload more gracefully than other topologies.
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It's just me or the pics cannot be seen anymore?
Ciao!
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Marco, the photos were dropped when they switched the website over. I'll see what I did with them and reload the pics.
For PJ/PB, the grid resistor, the 249K, that is a soft value right? I'm good to use a 250K in the same spot?
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Yes the grid resistor is not a precision value. 250K is the maximum value as specified by WE (in my copy of the 1950 data sheet). significantly lower values will load the driver more heavily, leading to less gain and greater distortion, but this effect will IMHO be extremely small with values 150K or above.
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Thank you for the explanation Paul!
I tried removing the cathode bypass capacitor again and it lasted all of one day without it. Even though the signal is crossed at 80Hz before reaching the amps, that cap still carries some weight across the frequency range. Without it music looses weight and definition. Everything feels thinner. Maybe there is a magic value I could reduce it too but the present cap sounds great if that is possible and I have little reason to change it out.
Next up, a different PF cap. More to follow.
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The Parabee's got me thinking about getting a little more life out of the driver tube. Am I crazy to want to run a separate shunt reg tube, like the 6J6 that PB is using and use one half of the 6SN7? I could run different halves for the left and right amp and swap in the future like the Parabees do. If this is advisable, can you run half a heater on the driver tube or are both halves always in use?
Thanks,
Aaron
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Something like a 6SN7 doesn't have a center-tapped heater, so there's no option to only heat half of it.
FWIW, I would expect an old stock 6SN7 to last well over 30,000 hours.
On the other hand, you could migrate to a 6J6 regulator and 6J5 driver tube if you are interested in getting more glass onto the chassis.
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Thanks Paul. I've got a stash of 6J5's though I'm so happy with the 6SN7 I find little reason to change it. With a lifetime like that, my stash will last longer than I will.
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Yesterday thanks to the local tube guys I finalized my selection of 6SN7's to roll through these amps. I've been using a pair of Sylvania 6SN7WGTA'a 90% of the time but with the recent changes to the system as a whole they are starting to sound a little lean. Here is what I walked away with:
one Tung Sol VT-231 round plate, black glass (I had one already)
two Ken Rad VT-231's black glass
one Raytheon 6SN7WGT (had one already)
four Sylvania CHS 7N7 tubes in boxes marked 1942.
If your calculations hold true PB, I've got tubes for more than one lifetime ;)
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I swapped the KBG PF cap for a Rike Audio cap a week or so ago. I really like the KBG's and they offer a lot of bang for the buck. The Rike in these amps sounds great. Strings sound better and the music seems to have more feeling and weight to it. The attack is not as good as it was with the KBG's and the decay isn't as airy. This may improve with break in. We overall like the results and they are going to stick around for a while.
There were a few rapid changes to the system. The first being the removal of the grid chokes. In talking this morning, with their removal the sound was more precise but lost some body We also noticed the music could be painful at times. Though it was more precise we weren't sure if it was more correct or not. Next was the PF cap swap. The music sounded more correct but thin and painful at times. The strings were better but still lacked some body(not metallic sounding anymore). This was throughout the mid range. Today I swapped drivers from the Sylvania 6SN7WGTA (short bottle, brown base) to the Ken Rad VT-231. The mid range depth and weight returned in spades. We were both impressed by the improvement overall to the entire frequency range. The sound became more intimate on our usual test recordings. The attack is still not perfect but the sound is wonderful.
I've said it many times but everything makes a difference to the overall system. What works for one person may not work for everyone. In my case, the Sylvania tubes I think helped compensate for the grid chokes. When we had used RCA grey glass drivers, the mid range was over emphasized. When the chokes were removed (with the Sylvania tubes) the sound thinned out some and we compensated with a different driver.
I've still got a few mods to try. Once we get a good long feel for these Ken Rad tubes there are Tantalum resistors on standby along with quite a few other driver tubes to roll.
Overall I'm very happy with these amps and as we continue to refine the system they get better and better!
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It's been a while since I updated this thread. The system was sounding like crap. I started with the BeePre and got it to a point where it sounded great and today I updated these amps. I replaced the Rike Audio parafeed caps with Auricap 5uF bypassed with a V-Cap 0.1uF. This sounded better than it did with the Rike's. The icing on the cake was swapping the Ken Rad drivers for Tung Sol's. This made a significant improvement in the detail and highs. I'm happy with this sound as it stands. I'm going to play around with smaller valued cathode bypass capacitor values and see if that does anything once I'm used to this new sound.
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A small update. I purchased a pair of Elrog 300B tubes after reading many favorable reviews. They haven't much time on them yet (two days) but I thought people would like to hear about them. If you haven't read anything about them, they are not really a 300B in the technical sense but work in place of a 300B. On Paul's recommendation, I dropped them in replacing a pair of EML 300B mesh plates (another none true 300B). They needed a few hours to wake up but after a couple of days of listening my wife and I are most impressed by them. Sonically, when compared to the mesh, they have a fuller mid range. When listening to piano works, notes have a more realistic tonal range. The decay has improved as has the spacial perspective of the sound. They are not as holographic as the mesh plates yet but this pair is still breaking in. They are noisier than the mesh plates. I attempted to measure the difference this morning but I didn't get any appreciable numbers. It's not a significant difference and this may dissipate as burn in some.
Overall I'm very pleased by the purchase. They were expensive but if they last, they will pay for themselves in sonic improvement.
I wonder though if my amps can be better optimized for them. My operating point is 350v P-K, -76v, 50mA, 5k. Thoughts from PB or PJ?
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My rule of thumb target impedance is 4858 ohms - I'd call that pretty close to 5000 :^)
Thanks for the update, will be curious to hear how they progress.
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Thanks Paul! A question, in looking at the curves, do you feel that 50mA is enough? I could work it up to 60mA before needing to swap out PF chokes.
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60mA would call for a 3700 ohm transformer. At 70mA, it wants a 3000 ohm transformer. These options will give you a bit more power, but to get REAL power from these tubes you need more voltage. They really are closely related to the 845 from which they were derived!
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Thank you sir! I seem to be in an optimal area and the sound shows as much. Power isn't needed and the sound is sublime. Thank you for the reality check Paul!
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A short update on the ER300B tubes. That have been in the amps a week now with my wife getting more play time than I have. She loves them for classical piano, strings and choral voices. With the EML mesh plates, she wouldn't listen to the system. Now she is playing music every day.
I've had two short listening sessions with the new tubes. With my music, I find that they are a little thick in the mid and lower mid range if that makes sense. The mesh plates have a certain clarity to them that the Elrog's don't. It could be my operating point or system synergy as all the reviews I've read so far are favorable. As they continue to break in I'll keep you updated.
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Another update. These tubes change with time! We are both happy with them on most recordings. The meshies sound great but these are better, more life like if you will. It's nice having the best of both worlds!
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Another update. These tubes change with time! We are both happy with them on most recordings. The meshies sound great but these are better, more life like if you will. It's nice having the best of both worlds!
Thanks for the update Aaron! I'm still waiting for mine. I pre-ordered them from True Audiophile in early or mid February with delivery expected to be late February or first week of March.
Now it's mid March and still no word on the next shipment. That's the way it usually goes with items like this and hopefully our patience will be rewarded soon!
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A little something that came in the mail today. They work for the BeePre so why shouldn't they help with the power amps. The Elrog tube disapated heat from the base or so it's written. So far no melted rings.
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The silicone rings are very likely to melt in a power amp (others have melted them); we don't recommend them in that position.
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PB, they work just fine with this tube. They don't get as hot as others and after eight hours on the tube they didn't melt. Now, I'm running them very light at 50mA. Btw they do reduce microphonics which is why I bought them.
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Is the coating silicone or vinyl? Very different melting temperatures ... :^)
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They are the standard BelArt weights so I will assume vinyl though I'm not sure. They look to be of the same material as that used on the larger version. Two full days of use and no issues.