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5998/421A build and schematic feedback wanted
5998/421A build and schematic feedback wanted
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Old 28th March 2017, 11:16 AM   #31
tizman is offline tizman  Canada
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Default Are my operating points okay?

I finished building the amp last night. I ended up with a B+ of 300 volts. The 5998 has 250 volts from plate to cathode, and 47 volts across the 1000 Ohm cathode, which puts me at 11.8 watts of dissipation. This is 3 watts less than expected and desired. I will play with 800, 850 and 900 Ohm cathode resistors to see if that gets me closer to my target of 14.5 or so watts.

The 6SL7 has 162 volts from plate to cathode and 1.27 volts across the cathode resistor, for a dissipation of .2 watts. Is this a good place for this tube to be operated? Its seems low to me. I haven't checked the heater voltages, which I find are often higher than they are supposed to be when I use oversized vintage power transformers.

It sounds very good as is. It's the first time that I've used the Hammond 125ESE, and I like it so far. I ended up using a pair of .47uF Hovland Musicaps that I had in inventory as coupling capacitors. The amp has a nice clear, open quality and sounds dynamic and fast. I'm wondering if giving the driver and output tubes a bit more juice will improve things. I am going to let it play overnight to break it in a bit, as all the parts other than the power transformer were unused, and listen again tomorrow. Here's a picture.
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Old 30th March 2017, 09:08 PM   #32
tizman is offline tizman  Canada
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Old 11th April 2017, 08:22 PM   #33
tizman is offline tizman  Canada
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I guess that's why it's called DIY....
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Old 11th April 2017, 11:15 PM   #34
PRR is online now PRR  United States
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Yup.
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Old 12th April 2017, 05:21 AM   #35
tizman is offline tizman  Canada
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I'm pretty sure that my question has already been answered in this thread. My problem is that I lack the experience and knowledge to use some of the advice that has been given. I'll figure it out. Thanks to everyone, and especially PRR, for the help so far.

Last edited by tizman; 12th April 2017 at 05:45 AM. Reason: Careful consideration of unedited post
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Old 29th April 2017, 10:59 AM   #36
tizman is offline tizman  Canada
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Default Final 5998 amp schematic

Hello All. This is the final schematic for my 5998 amplifier with voltages added for reference. After careful consideration of all prior posts in this thread and more reading and research, I did numerous load lines for both tubes. I then experimented with different component values and subsequent listening tests. This is my favourite version so far. I added resistors in line with the heater supply to drop the voltage from 6.9 to 6.2. I think it sounded a little better with the higher heater supply. I am very happy with the Hammond 125ESE OPTs. They sound great and are very reasonably priced. I may try the 125FSE or 125GSE next time and do a parallel SE version of this amp. This is my favourite amp so far, and it has displaced everything else that I have built.
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Old 8th May 2017, 06:58 AM   #37
tizman is offline tizman  Canada
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Default Sorry! This is the right schematic

Sorry, I attached the wrong schematic to the above post. The following is the correct one. I ended up using a 5U4 recover as the 5Y3 was not able to handle the current, and I replaced the small chokes with one large one. I also reduced the amount of capacitance which made for a faster supply in PSUD2, but without increasing audible ripple. The previous supply also had issues with ringing in PSUD2 that presented audibly as midrange harshness. Now it is dead quiet with no signal going through it, and very dynamic and lively sounding when playing music. Anyway, as I mentioned above, this final design sounds great to me, and is the amplifier that I most listen to. Thanks again to everyone who helped me out by actually helping at first, and then later by not helping and thereby getting me to help myself. I have a much improved understanding of the process of designing an amp because of it. If you have any further comments or advice on the design please feel free to comment. Cheers, Tiz.
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Old 11th July 2018, 09:04 AM   #38
tizman is offline tizman  Canada
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A year and two months later....

This is the latest version of my 6SL7/5998 amp after a recent revisit to its innards, spawned by a rereading of this thread. I made changes to it a few days ago based on the reread. It uses the component values discussed in posts number 2,5 and 6 in this thread, written by PRR. The attached schematic reflects what is in the amplifier today, including the change to a potato masher 5R4. I would probably simplify the power supply somewhat if I was going to do it again from scratch, but left it as is since doing so seemed like a lot of trouble. It sounds very nice on my La Scalas (noticeably better than before the changes), and is also very quiet through them.
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