I received my A50 2 days ago and started playing with it. Here are my initial impressions and progressive improvements (I have not started on the Skunkie modifications yet). The initial sound of the amplifier was very "thin" and not at all dynamic. I replaced the rectifier with a 5AR4 and heard a significant improvement overall. I then switched out the 6SN7s for USA Sylvania and the 300B for a Sophia mesh plate. Each of these swaps resulted in a much cleaner, crisper, dynamic and fuller sound. Overall I would say it made a 300% improvement to the sound to my ears at least. I did no THD testing yet as I was more interested in the sound and did not want to be swayed by numbers.
I will do the Skunkie recommended mods one at a time and listen to the results. Once I am satisfied with the amps I will run thru a full series of performance tests and publish the results on this forum. Prior to this, the other thing I will be looking at is converting the tube supply to a solid-state design. I know this will raise many eyebrows and I do not wish to get into an extensive debate about the virtues of one vs the other. I will share how I feel about the sonic difference. There are at least four highly regarded amplifiers, the McIntosh 240 and 275 as well as the Marantz Model 8B and 9, all use solid-state rectification. I have not read any suggestions regarding converting them to tube rectification.
Joe in Virginia
So far completed just two modifications.
47uf 500v ripple cap and bypass that other cap.
waiting on coupler caps.
notice the top end is lacking and using my Schiit Lokius fixes the lack of liveliness.
What stinks I didn't watch the entire series and purchased the first set of .33uF coupler caps.
For the front end I had to teach myself how to read the diagram and draw a mock representation of the sockets wired up.
I became confused about the two resistors that look connected to the left tube I think they are little 100 ohm units attached to the black cables which come from rear and connect each tube and guessing that's power? Figured out from the other diagram that's where they are exist.
I swapped tubes and that improved the the performance by fixing the noise the drivers made and the GZ33 I am using did something but not sure how to describe it.
Waiting on Parts Connexion
If I upgrade the couplers from the Mundorf .33uF MKP's I purchased, are Jantzen Superior Z-caps any good for the price? If not is there a sub that can be recommend for less?
I want to use my GZ34 NOS Philips can I before modding?
I have done all the mods to the A50 apart from the Cascode - this is due to my lack of knowledge about the LED used. Local suppliers (Australia) do not seem to have an inline LED so I wonder if a simple diode would suffice? Wayne
I have all my parts ordered but have an unexpected problem finding single-stranded hook-up wire at local stores due to shortages, so I may have to put an order in to get some. As our amps generate some heat, is there any preferred type of sheathing for the wire? Is 16AWG OK? Otherwise I can get some house wiring and strip out the single core strands but it may be a bit thick to bend easily (1mm dia wire). House wiring is rated 250V 10A and does have a fire rating so let me know what you think.
Do you have any particular preference for the 100uF cathode bypass cap? I have in my stash some speaker crossover types, I think electrolytic, 100uF 100VDC 85degC non-polarised axial and was wondering if this would be OK?
Maybe I missed something, but what was the verdict on trying the .22uF coupling cap vs the .33uF? Any better or worse? I see the Skunkie schematic still calls for the .33uF, so deduced that it is preferred?
I am a total noob to this tube amp endeavor. Thank you so much for such thorough explanations and step-by-step mod. Your videos are fantastic and filled with wealth of information. I was able to purchase most of the items on your list for the 50A mod. however, I am having a really hard time finding the the 0.22uf 150V cap. I searched the recommend site and google but can't seem to find them anywhere. any other place you would recommend looking?
Thanks in advance,
Yes, I fully intend to do the Cascode front end as well as the other mods you suggest. They and the tube swap should be enough to make a very nice sounding 300B amplifier on my Klipsch horns.
Joe in Virginia
The 300B tube needs 140-160V peak to peak of drive. And honestly, the cascode is the first thing to do, everything else is just very minor improvements until you have a frontend that can actually drive the output tube. That cascade doesn't even come close to being able to do that. It would be much more interesting to see what these other tweaks do after you have a frontend capable of providing the drive the 300B needs but this is your project :)
I will be doing the cascode modification last and report back as to my impression of the sonic response improvement. I am also playing around with the idea of tearing out the entire preamp front end section and driving the 300B tubes directly from a very clean tube preamplifier. I have not looked into this too deeply yet to see what the downside will be. The tube preamp I may use is capable of providing about 10 volts of very clean power. Any thoughts?
Joe in Virginia
I think you will be pleasantly surprised by the mods. Until the front end is changed from a cascade to a cascode, any mods and tube swaps are very minor improvements compared to what this does. The cascade just doesn't have enough drive/voltage swing for a 300B tube. It's a night and day transformation of the amp.