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PostPosted: 24 May 2016, 00:46 
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Hello everyone. I recently completed a build of Alex Gendrano's KT88 SET amp on the chassis of an old Baldwin organ. The chassis originally was set up with four 6L6s that were run push pull parallel with one 6SN7. It also had two 5U4 rectifier tubes run in parallel. I kept the two rectifiers setup, although the amp will run on one, in order to keep that part of the chassis original and avoid having to fill in holes. I used the oversized power supply transformer, the choke and the filament transformer from the chassis and added all new film capacitors to complete the power supply. With two Russian 5U4s in the rectifier sockets, the amp produced a B+ of 443.9 volts. With one 5U4 it produced 402.9 volts. With one 5AR4 rectifier it produced 432.8 volts. With one 5u4 and one 5AR4 it produced 461.6 volts. With one 5AR4 and one 5Y3 it produced 451 volts. This amplifier can take a number of different tubes including 6550, EL34, 6L6 and the KT88s that are in it currently. Being able to vary the B+ so effectively by having the option to use one rectifier or two of the same or different types allows a great deal of flexibility with the tubes that can be used by this amp. With the exception of having to use two 5Y3s at a time, or a 5Y3 and another tube together, the amp will run on a single rectifier tube without issues. The best part of this arrangement for me is being able to get a higher B+ with two 5U4s than I can get with a single 5AR4. I only have the one 5AR4, but I'm guessing that two would probably give a B+ of 470-475 volts. The driver tube is a 6N1P-EV. Is there any downside to this arrangement that I am missing or unaware of?


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PostPosted: 24 May 2016, 14:01 
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Hi tizman, Glad to see you re-purposing the amp. Others might disagree with me on this but I have found that changes in B+ of less than about 15% on large power tubes don't make a lot of difference. This is particularly the case IMO in class AB amps. In class A amps (the only kind I design) it can very slightly reduce the output power, but the amount is not audible under 99% of circumstances. (remember that sound loudness is a logarithmic function of power and not linear) I find it advantageous to run the larger power tubes (anything from 6L6GC to KT120) in the 425-450 B+ range. In that range most are quite linear and with careful choice of the operating point will last a long time. The higher the B+ the more critical the operating point gets as you can easily reach or exceed the maximum dissipation ratings. I like to keep class A amps at or below 85% of max dis. A second issue (might not be so in your case with the B+ values you listed) is that filter components for voltages over 500 get costly quickly. If your rectifiers warm up faster than the power tubes the drop in the filter chain will be less than in steady state and thus the voltages higher. With say a 475 volt steady state output the B+ under light or no load would easily go past 500 perhaps as much as 100 volts. Sure most modern components will put up with it for a while....but it shortens their life and many times will result in failure. Murphy's law will guarantee it will occur at the worst time...probably when you are showing it to some friends.

I would use the two 5U4s and not mix and match the tube types. First it will put the B+ in a good range, second if you mismatch the tubes one or the other will have an effective lower impedance and tend to hog the current. It might be safe but it is poor practice and can cause failures.

Good listening
Bruce

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PostPosted: 25 Aug 2016, 00:57 
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Hi Bruce. Thanks for your reply and sorry for the extremely delayed response. I must have forgotten to request that I receive a notification when I get a reply. I have been running this amp with a variety of power tubes and have usually used either one or two of the same type of rectifier to bring voltages up and down to better suit the power tubes that I'm using. I have a switch on the amp that allows me to choose between two cathode resistor values as well. The ability to vary voltage and cathode resistor values allows me to use everything from EL34 to KT120. I haven't fully tabulated all the voltages that I get with each rectifier/rectifiers and each type of power tube, so I have been running the tubes conservatively. It should, however, be reasonably easy to install two or three switchable cathode resistor values that combine with voltages to put most octal power tubes in the sweet part of their their respective curves. I'm pretty excited by the Swiss Army Knife quality that this amp has. If anyone is interested, I could post the modified schematic for this amp. Thanks for your response, and once again sorry for my delayed response. Your input is much appreciated.


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PostPosted: 25 Aug 2016, 00:59 
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Thanks Bruce!


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PostPosted: 25 Aug 2016, 07:50 
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Certainly a 5v winding designed for 6A operation will run high with a 2A load. Running a 5v rectifier at 6v is 20% over and will shorten tube life.


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PostPosted: 05 Sep 2016, 22:53 
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A very good point. I've had to go through all the amps that I have made with oversized repurposed iron to check the filament voltages. There doesn't appear to be any pattern to when the voltages are too high or in range. It certainly is not predictable. I've had to reduce the filament voltage on only one amp so far as it was 6.9 volts instead of 6.3. Most of the others were within range. I have not thought to check the 5 volt though. Thanks for the heads up.


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PostPosted: 12 Sep 2016, 03:59 
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I checked the heater voltages, and they are 5 with two rectifiers and 5.15 with one. With two 5AR4s I get a B+ of 495v with KT88s and 465v with EL34s. My dissipation is fine for both tubes as I use a switch that gives me 470 Ohms for KT88s and adds 220 Ohms for a total of 690 Ohms with EL34s. I usually run 6L6s the same as EL34s. I was thinking about a lower value resistor for the KT88s, like 330 or so, to get the dissipation up, and to add a 360 Ohm on the switch to add up to 690. Or some variation on those numbers. Also, I recently picked up a pair of KT120s, which have an additional 18 watts of dissipation available. Different rectifiers in singles or pairs put out a wide variety of voltages. This combined with switchable cathode resistors allows for many tubes to be used. It requires a chart to keep track of settings that are ideal for each tube. it would kill an EL34 pretty quickly to be run at KT120 settings.


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PostPosted: 12 Sep 2016, 16:24 
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Hi, The EL34s (and 6L6s) would make for a nice light show though. The bright red glow of the anodes being driven that way would be quite cheery on a cold winter night....that is until the glass shattered. :hot:

Good listening
Bruce

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