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Old 23rd September 2007, 05:39 PM   #1
gerryc is offline gerryc  United States
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Default Parallel Output Transformers

I'm designing a 200 watt bass amp using a quad of KT88's in ultralinear configuration. I'm looking at Hammond output iron for cost / performance reasons, particularly the 1650W rated at 280 watts at full power bandwidth down to 30 Hz. At 28 pounds its a monster. I was also looking at the 1650T with the same PP impedance (1900 ohms) but rated at 120 watts, at only 9.5 pounds (not the 14 that is listed on Hammond's site).

I know people have pushed the 1650T to over 150 watts, and Kevin O'Conner has a 200 watt guitar amp in is Principles of Power book using the 1650T. I'd guess that would be fine for guitar, but not for bass. I was thinking of the possibility of paralleling two 1650T's (primaries and secondaries) which would keep the turns ratio (and therefore the impedance) the same but would effectively double the power handling and maintain full power bandwidth to 30 Hz. The seeming advantage? Price for two 1650T's is about the same as one 1650W, total weight would be around 19 lbs vs 28 lbs, and even though they would take up a bit more chassis space than the 1650W (not by much, as I said the 1650W is a beast), you could balance the weight on the chassis with the two.

Has anyone tried anything like this? I saw that John Broskie had a blog about paralleling output transformers from Dynaco ST70s to make a "super" Dynaco ST70 so I figure the idea is not too far fetched:

http://www.tubecad.com/2005/March/blog0041.htm
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Old 23rd September 2007, 08:34 PM   #2
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Are you able enough to lift it ? By the time the mains tranny is fitted you will have more than double the weight and the chassis be strong enough to take settling down thumps.
There is nothing wrong with the power tact except over 530V B+ with modern day KT88's is going to stretch them. Going for a triple per side push pull would relieve but one would up the heater current supply with larger chassis size but with more o/p tubes will tax the performance of the interstage driver. where quality isn't a issue then it doesn't matter.

Paralleling output transformers is fine (As Broskie points out) so long the specs on each are fairly sim. Any problems would come apparent at higher frequencies, as circulatorary leakage inductance / capacitance parasitics become a conflict pest. At the low f end it wouldn't be noticed and in fact many amp builders have used standard mains transformers frequencies under 500Hz. Other builders in this forum have done this.

Generally for hi-fi it is better to lower the primary side working impedance (i.e fit more tubes) which lowers parasitic problems which increases the bandwidth whereas your suggestion would not.

I use 2K A-A UL 43% 10Lb lump made by Majestic which can hull 175W at 20Hz but needs over 530V B+ to get it. The mid and bass range is incredibly forceful and dangerous. With KT88's running at this voltage with 60mA Iq per tube and 130mA on peaks, the remaining ability is on the power supply to hold up 0.75A on power peaks.

The other possibility is to build a normal stereo amp and power bridge mode the outputs by using a phasesplitter in the input. Anyone done this ?
Changing ideas, this enables you to simply revert to a standard stereo version at virtually nil expense.

richj
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Old 23rd September 2007, 11:14 PM   #3
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hey-Hey!!!,
Getting 100W/pair of KT88 is a bit of a stretch. I'd suggest a larger single pair of valves. 813 perhaps? How important is the 200W spec?

Multi-parallel pairs risks oscillation. Nearly all of the commercial stuff I've looked at has at least one countermeasure, most have a few. Extra care is certainly required for that sort of topology.
cheers,
Douglas
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Old 23rd September 2007, 11:48 PM   #4
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Quote:
Originally posted by Bandersnatch
hey-Hey!!!,
Getting 100W/pair of KT88 is a bit of a stretch. I'd suggest a larger single pair of valves. 813 perhaps? How important is the 200W spec?
The way most muso's treat gear, those long 813 filaments should last several days at least.
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Old 24th September 2007, 12:02 AM   #5
gerryc is offline gerryc  United States
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Actually, both the original Genalex data sheet and the Shuguang datasheet from Pentalabs both spec a pair of KT88s at 100 watts in ultralinear with a plate supply of 560 volts (I'm assuming that's under full load, so idling would probably be 600+ V for a decent power supply) into 4.5K P-P. So a quad should be good for around 200 watts with about 2.2K P-P. The 6550W is 1900 ohms P-P so it should be doable. Yes, it's pushing the tubes a bit, but that's the challenge. Rich, maybe you're right, it would just be easier to use the 1650W and deal with the extra weight.

Using 813's while interesting, wouldn't be practical for a portable amp becuase of their size and power supply requirements. The 200 watts isn't a spec as such, but a goal. As I said the challenge is what makes it interesting. A sextet would easily provide the 200 watts conservatively except for the cost of two more tubes and the chassis area to accomodate them (the datasheet recommends 4 inch center to center spacing which is kind of largish but doable). Six would require a lot of area, unless I cheat a bit. Maybe that's OK if running them conservatively and/or using a fan? Have to think about that.
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Old 24th September 2007, 12:23 AM   #6
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Quote:
Originally posted by gerryc
Actually, both the original Genalex data sheet and the Shuguang datasheet from Pentalabs both spec a pair of KT88s at 100 watts in ultralinear with a plate supply of 560 volts (I'm assuming that's under full load, so idling would probably be 600+ V for a decent power supply) into 4.5K P-P. So a quad should be good for around 200 watts with about 2.2K P-P. The 6550W is 1900 ohms P-P so it should be doable. Yes, it's pushing the tubes a bit, but that's the challenge. Rich, maybe you're right, it would just be easier to use the 1650W and deal with the extra weight.

Using 813's while interesting, wouldn't be practical for a portable amp becuase of their size and power supply requirements. The 200 watts isn't a spec as such, but a goal. As I said the challenge is what makes it interesting. A sextet would easily provide the 200 watts conservatively except for the cost of two more tubes and the chassis area to accomodate them (the datasheet recommends 4 inch center to center spacing which is kind of largish but doable). Six would require a lot of area, unless I cheat a bit. Maybe that's OK if running them conservatively and/or using a fan? Have to think about that.
Been here, done this. The 100W/pr in P or UL is really stretching the envelope. They'll sound a hell of a lot better at 80W/pr.

Forced air cooling works well, allowing the c-c distance to be less but requires some thought on layout, airflow and thermal transfers.
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Old 24th September 2007, 02:42 AM   #7
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Have a look at this project:

http://www.dogstar.dantimax.dk/tubestuf/dzindex.htm
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Old 24th September 2007, 04:02 AM   #8
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Quote:
Originally posted by Bandersnatch

Multi-parallel pairs risks oscillation. Nearly all of the commercial stuff I've looked at has at least one countermeasure, most have a few.
Umm....I've done alot with multi-parallel p-p pairs without instability problems. A KT88 is more likely to oscillate than others.
What I intended to mention is that B+ 530 V in UL is pushing it with todays KT88's. Nachbaurs design uses direct coupling the output stage to driver and running from neg supply risks Iq drift problems if driver tubes get fusty. It is this that I still prefer separate tube setup bias pots with coupling caps. This is entirely desgners preference.
Any method of running tubes at lower voltages which prolongs life is a must in these tube expensive days. The annoying problem of trying to use a single electrolytic for the B+ instead of series/parallel arrangement would make psu things simpler. Still, simple design using a single 500V electrolytic running at 450V with a parallel pair of 6550's in UL 2K A-A gives touch over 100W, so it isn't so difficult to get more output with an additional pair.
I'm for it.

richj
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Old 24th September 2007, 12:44 PM   #9
mrwhy is offline mrwhy  United Kingdom
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The Fender Twin Reverb with either 4 or 6 6L6 tubes gets 120 W rms (or more) in UL form.
Might be worth looking at their transformer and schematics.
Their transformer cost is about 30 from excellent German firm ($60)

In generasl the bigger the transformer core the better. Inductance is essential for low bass. Using 2 in parallel lowers this. Placing them in series might be better.

Back in the 50s we built a TRANSFORMERLESS amp using 16 KT66s in cathode follower mode. These days you MIGHT be able to get someone to wind a speaker coil for say 100 ohms - that would be the way to go. No weight, no distortion - and the KT66s would keep you warm!
John
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Old 24th September 2007, 02:09 PM   #10
gerryc is offline gerryc  United States
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Quote:
Originally posted by DigitalJunkie
Have a look at this project:

http://www.dogstar.dantimax.dk/tubestuf/dzindex.htm

Yes, I have looked at that project. Pretty complex and Fred used the tetrode configuration (have to for 807's since they have such a low screen voltage rating). Looks fine for guitar, but I'd guess that it would be a little lacking for bass, at least at full power.

My goal is to get 200 watts in UL using either KT88's or 6550's using an off-the-shelf output transformer. A quad looks possible for KT88's, albeit pushing them a bit hard. A sextet of either should easily be able to deliver the goods, however the 6550's might need a lower impedance output transformer (maybe around 1200 - 1300 ohms? The lowest Hammonds seem to be 1900) since it doesn't have as high a screen voltage rating as the KT88, so the B+ would have to be somewhat lower, probably under 500V. Yeah, tetrode operation would solve this issue, but I'm stubborn.
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