I am planning for a VT25 SET amp. I intend to regulate the B+ using Maida Regulator to be around 420V.
Fixed bias should be between -35 ~ -40V for a bias of 18~20mA on the VT25 tubes.
Can the LM337 or even the LM317 (+ve output to ground , and the -ve side as output to be the fixed bias voltage) be used to regulate this voltage?
I have not seen anyone posted such application for the three pin regulators. Is it a very bad/silly/technically wrong idea to do it this way?
Fixed bias should be between -35 ~ -40V for a bias of 18~20mA on the VT25 tubes.
Can the LM337 or even the LM317 (+ve output to ground , and the -ve side as output to be the fixed bias voltage) be used to regulate this voltage?
I have not seen anyone posted such application for the three pin regulators. Is it a very bad/silly/technically wrong idea to do it this way?
In that case I recommend using LM337 - negative voltage regulator.
It works like LM317.
https://www.onsemi.com/pdf/datasheet/lm337-d.pdf
It works like LM317.
https://www.onsemi.com/pdf/datasheet/lm337-d.pdf
Using an LM337 to regulate the negative bias supply is one way to keep control of things.
However, a novel approach to maintaining bias that has additional benefits, and is quite simple, is discussed in detail
on David Gillespie's site, and called Enhanced Fixed Bias (EFB).
I've implimented this system on my own tube amp, and I'm sure it's adaptable to a SET design, perhaps with some changes.
What makes it beneficial is that it 'follows' or tracks any B+ fluctuations due to any 'sag' in voltage, and instantly adjusts/regulates bias to maintain output tubes biasing in their best area of operation.
My EL83 PP amp, under normal cathode biasing, put out 12 watts, but now with the EFB modification, can output 17 watts, and I can clearly notice an improvment in 'punch', clarity and performance that the EFB modification has brought about.
Do a search of David Gillespie - EFB, and do some reading about it.
I use this and like it very much. https://www.audioamp.eu/module-ab-2...he-amps-circuit-with-ttl-error-signal-ouptut/
An unregulated bias supply will track changes in line voltage which means when the B+ goes up, the bias gets higher, too. No fancy solid state parts required.
That's why John Broskie recommends against regulating C- if you don't also regulate B+.
My design works well enough but like @TR00 I generally use automatic grid biasing -- AB-4 or AB-Q (I only make push pull amplifiers) from Pavel at audioamp.eu so it handles those changes for me - Like a Ronco product: Just set it and forget it.
If you design your stuff to run hot, I'd change the 25v 100uF caps to 12000H 105°C ratings. I've actually had some of the stock capacitors fail - I found the jelly-roll part of the cap still in the board, but the can had fallen off.
That's why John Broskie recommends against regulating C- if you don't also regulate B+.
My design works well enough but like @TR00 I generally use automatic grid biasing -- AB-4 or AB-Q (I only make push pull amplifiers) from Pavel at audioamp.eu so it handles those changes for me - Like a Ronco product: Just set it and forget it.
If you design your stuff to run hot, I'd change the 25v 100uF caps to 12000H 105°C ratings. I've actually had some of the stock capacitors fail - I found the jelly-roll part of the cap still in the board, but the can had fallen off.
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To be honest, I didn't expect so much sharing on this topic from members here. I have thought that I would be deemed as asking a newbie question that have answers that could be found anywhere. I didn't imagine there will be many different ideas to share. Thank you fellows.
Yes, the 1N5374B would work. It needs the leads to be left long to handle it's rated power in my experience. 0.1uF (100nF) is enough capacitance, too.
The VR75 (SG2S is the Soviet version - cheaper) looks cool though 🙂
25$ for ten of them shipped from Ukraine.
https://www.ebay.ca/itm/385101927348
The VR75 (SG2S is the Soviet version - cheaper) looks cool though 🙂
25$ for ten of them shipped from Ukraine.
https://www.ebay.ca/itm/385101927348