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Old 20th June 2013, 04:36 AM   #41
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I feel your pain. That's why I'm exited about sweeps. Just the output tubes for those big amps are $400. An 8 pack of 6CB5s was $35 shipped

I'm really anxious to learn screen drive.
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Old 20th June 2013, 04:39 AM   #42
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i think there are enough threads here about the subject, David Berning is the king of screen drive amps afaik...
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Old 25th June 2013, 09:05 PM   #43
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Back on topic, I am looking at the Hammind 1650T which is 1.9K primary and rated at 120W.

Maybe a quad of 6CB5s per channel would work better into 1.9K?

What screen voltage would be optimal for this arrangement? Do you select the screen voltage based on the anode load?

I am considering using a doubler for ~650-700V B+. I have some spare 180V transformers that I can use for the screen supply depending on the screen current demand. Maybe a choke type input for ~162v on the screens?

Or is my original plan to use a lower B+ better?

Thanks!
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Old 26th June 2013, 04:59 PM   #44
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Quote:
Nice looking OBs!.........They are fully passive and dynamic as almost any box speaker I've heard. The bass drivers are Acoustic Elegance IB15, midrange Peerless 830883, and the BG Neo3PDR with back cup removed for the tweeter.
I use these mounted in a pair of 1941 vintage Zenith console radios.

Silver Iris OB 15 Coaxial Unit [SI-OB-15] - $175.00 : Zen Cart!, The Art of E-commerce

The big difference is the efficiency. My speakers are about 96db. They will work with a 2 WPC 45 based DHT SET, but you can't play loud bass heavy music. Feeding them with a 15 WPC triode wired KT88 SET allows Pink Floyd and Depeche Mode, and the bass can be heard inside the house across the street.

I have fed these things with all sorts of amps, SE tube, P-P tube, a 100 watt Marshall, and even a little 15 WPC class D chip amp, but the absolute killer for bass transients and "rock concert in the living room" realism is a 125 WPC sweep tube amp based on Pete Millett's big red board. This amp used 6HJ5's with a 3300 ohm load and 600 to 625 volts of B+, conventional G1 drive class AB1.

The term "effortless" best describes the system since there is about 20 db of headroom at loud listening levels where the average power is about 1 WPC. Big bass transients don't even slow the amp down. It has been operational for over 2 years and at least a dozen clones have been made.

This has prompted me to design a new amp, but time has been very sparse lately. It is still in the early breadboard stages.

Quote:
What screen voltage would be optimal for this arrangement? Do you select the screen voltage based on the anode load?
I have built a lab setup with variable power supplies for just about every voltage in an amp. I have a two sets of OPT's that allow me to test at 1250, 2500, 3300, 5000, 6600, and 13200 ohms. Impedances in between these values can be tested by using a load resistor different than 8 ohms. I have a big collection of 100 and 250 watt Dales, and a pair of 500 watt 8 ohm loads.

I simply wire up a set of tubes and turn knobs to find the optimum trade off between power, distortion, harmonic distribution, and efficiency. Maybe this is a bit emperical for a true engineer, but it has been working for me for far longer than I have had an engineering degree! Whazza load line??????

I have found that almost all sweep tubes behave the same as long as the tube in question has the peak cathode current capability to slam those 15 inch cones around with the same authority that the hammer puts into the bass drum head, AND the plate dissipation capability to avoid meltdown at your chosen power level.

Watch the idle dissipation at high B+ voltages too. 40 mA at 650 volts is 26 watts! I run my 24 watt tubes at 30 mA on 630 volts for 19 watts. This is where the low idle current of screen drive shines.

Just about every true horizontal sweep tube (not the early tubes based on conventional audio tubes like the 6BG6) will be happy with 150 volts on the screen. Trying to hang too low of a load impedance on a small tube (below 2K ohms on a 13GB5 comes to mind) can require a bit more voltage like 175 volts. The screen grid on a sweep tube needs to be fed from a regulated supply, so design your regulator to cover 150 to 175 volts and you have it covered. The regulator doesn't need to be a uber regulated feedback system with .001 ohms output impedance, a VR tube or zener string and a mosfet or cathode follower is fine.

Tube dissipation capability requirements can be figured by knowing the maximum power output level, and the plate efficiency in the output tubes. The plate efficiency can be determined by testing. You measure the output power, the plate current in each tube and the plate voltage.

A typical G1 driven class AB1 sweep tube amp can achieve 65% plate efficiency at just below clipping, if it sees an optimum load impedance. This means a 100 watt amp will consume 154 watts of DC from the power supply to make those 100 watts of audio. This burns 27 watts in each output tube, which is just above the 6CB5 spec, but you aren't going to listen to a continuous tone at 100 watts. I have found that any of the 25 watt class sweep tubes live well in 125 WPC amps. I use 6HJ5's which are rated for 24 watts.

The peak current capability requirements can be estimated by looking at the impedance curve of your loudspeakers and the ratio of your OPT, and the B+ voltage. We assume that a sweep tube can pull its plate down to near zero. It can in screen drive, possibly blowing the screen grid, and it can get to 30 volts or so in G1 AB1. So if your speakers are exactly 8 ohms for all frequencies and dynamic conditions (NOT!), a 3300 ohm OPT will draw 727 mA from 600 volts. 600 volts / 825 ohms = 727mA. The max peak current for a 6CB5A is 850 mA. This limit will be reached when you speakers go below 6.8 ohms with a 3300 ohm OPT on 600 volts.

These numbers assume that the amp is driven to the edge of clipping at the moment of the transient. Attempting to push the amp harder will not cause overcurrent, just distortion since the amp will clip.

Reducing the load impedance below 6.8 ohms AT FULL POWER will cause over current, but you will be clipping and distortion should be audible. Operating the amp where no audible distortion is present will prevent overcurrent failure. Use bigger tubes when building a tube guitar amp.....or select speakers with a resonance peak just below 80 Hz and you won't blow anything!

The manufacturers usually publish an impedance VS frequency curve for loudspeakers. It is generated under static single frequency tone test conditions. This curve does not cover dynamic conditions where the cone could be moving in one direction and a dynamic transient instantly tries to reverse its direction. My attempts to measure this have been futile, but I believe my old Yamaha NS-10MS monitors can go below 4 ohms where the published curves show values from 8 to 25 ohms.

I believe that the peak current capabilities of a sweep tube are responsible for its dynamic capabilities. The 6CB5A is an old design with 825 mA ratings. There are sweep tubes with 1400 mA peak current ratings. Compare that to a 6L6GC with 400 mA ratings.
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Old 17th October 2013, 03:19 AM   #45
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Since summer is over, my kids are back in daycare, and winter is coming up, I decided today to resurrect this project. I tried to sell the output iron, but to no avail.

Click the image to open in full size.

If I use a separate power transformer for screens, does it need regulation, or just good filtering?

Thanks!
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Old 17th October 2013, 05:41 AM   #46
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Glad to see you back on the project, I thought you wanted the tubes below deck for safety? If you want max output power without sag, the screen probably should be regulated.
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Old 17th October 2013, 12:03 PM   #47
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Hi,

Yes, I did want to build the amp like that. I ran into space issues. If it were a UL circuit with KT or EL tubes, I would have just gone with it.

Is Zener string regulation sufficient? The reason I restarted this project is that I have about 95% of the parts including spare tubes. This chassis is also trimmed out with all the hardware, pots to control level, etc. along with being pretty large to help play around with layout.

I went with the 6GF7 as a concertina type phase inverter per my schematic a few posts back. Then just used a single 6DJ8 as the VA. Hopefully I can get enough swing out of the front end to get some decent power. I plan to knock out holes today and mount the parts. I'm not going for pretty here
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Old 17th October 2013, 02:03 PM   #48
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One thing I am curious about is, if the 6CB5 can be run triode on ~350VDC, then wouldn't it be fine in UL also? How could the plate pull down to the screens if they are connected to the same winding?

Thanks!
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Old 29th October 2013, 05:02 PM   #49
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I got the chassis fully cut and the filaments wired up. Pulled it up on my variac to make sure I did not short anything with the heavy gauge wire. With just a bridge (no cap), I see about 310VDC. I think I may need to go with a doubler to get a B+ high enough to make any decent power. I fear that, after filtering, and a load is placed on the secondary, the B+ will be below 300V and decent power will be unobtainable.

Click the image to open in full size.

Thoughts?
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Old 29th October 2013, 05:36 PM   #50
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With a cap in place your B+ will raise. Theoretically by 1,41 (sqroot of 2), but in practice this will be less due to ripple, RDC of transformer, filtering, so maybe 1,2 to 1,25. So your 310VDC will result in about 380VDC with low ripple.
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