EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: mp4207q on November 11, 2022, 02:29:37 am
-
Hello all.
I’m building a mosfet-tube stereo preamp using a transformer (EI core – not torodial) with a single 230V primary and 2 secondaries. The secondary windings are 260V @ 200mA and 12V @ 3A. The preamp uses a single 12AU7A twin triode (wired in parallel) for each channel. The total HV current draw for the two tubes plus solid state regulators should be less than 30mA.
I’m seeking your kind advice concerning only the HV (B+) supply.
(HV = 370V raw dc, 300V dc regulated)
I would like to include a 40 sec delay for when the regulated 300V dc is applied to the tubes. The delay can be controlled using a PiC micro. I was considering using a Solid State Relay with Zero Crossing Switching to switch the 260V ac secondary before the rectifiers. Rectifiers used are Vishay VS-HFA08PB120-N3 Hexfreds. The SSR will be off at power on, then after 40 odd seconds it will switch on and stay on continuously.
A couple of questions please:
Do I need to use a SSR? Maybe a heavy duty traditional relay with a snubber (cap plus resistor in series) would suffice?
Secondly, when the SSR receives the control voltage (after say 40 secs) and turns on, does the Zero crossing character persist or is it a “single shot’? Does the triac Zero Crossing function pulse the output of the relay while the ac voltage swings through zero?
Finally, because the SSR will be turned and remain on, does the minimum current for the device matter? I’ve read data sheets which suggest the use of an additional parallel load to reach the minimum current draw. The leakage current may trigger a reset. I’m a little lost here and my comprehension is lacking.
Thanks for reading and I look forward to receiving your comments.
Mark.
-
maybe a daft idea,but instead of switching and diodes combine the 2 and use thyristors,turn the gates on from your time delay,maybe even slowly ramp it up.
-
TMH, thank you for your reply.
I'd like to persist with the Ultrafast Soft recovery hexfred diodes because they're remarkably quiet in the power supply.
Mark.
-
I did that in the past but with a soft start.
The main smoothing cap in the HV supply was connected in series with a big resistor, so it charges up slowly.
Then, from the filament supply an RC delay fed a transistor that turned on a relay to short the HV resistor.
Maybe something simple would suffice for your application.
-
Tektronix used a thermal delay tube, I don't know how hard those are to find but it would be period correct for a tube amp.
Otherwise I'd probably use a standard mechanical relay. You can even buy time delay relays for industrial systems.
-
I have used (surplus) time-delay relays, with mechanical contacts rated for 120 V, on transformer primaries in tube amplifiers.
(But not with transformers supplying both HV and heater voltages.)
I think Amperite tube-shaped thermal relays are still available, but quite expensive.
With toroidal transformers, I haven't used zero-crossing switches, but used NTC thermistors to limit surge current.
-
My recollection is that while not intuitive, using a zero crossing switch on an inductive load results in turning on at peak current and that random trigger is preferred.
-
I think pardo-bsso's idea is good and simple.
Does the relay switching minimise the thumps at both power-on and power -off?
Thumps can be bad for expensive speakers and headphone users.
Originally soft start was handled by the thermionic rectifiers' 5V heaters being last to get hot.
There is a guitar amp here that uses parallel GZ34 rectifiers.
They waste power, but I did not change them to silicon for the startup reason. That amp has inaudible transients at switch-on and switch-off.
When silicon diodes started to be used in mid- 1950's , the approach was to apply B+ at switch-on when the tubes were cold, so the plates see the full B+.
There is an old receiver here with that, it starts quietly but with some distortion until warmed up.
The 12AU7A (Phillips ECC82 data) has limit Va0 = 550V. I think that is the limit of plate with no current. Va=300V
Same datasheet remarks that hum and noise will be 60dB below 100mV with some conditions.
Also the long tail pair preamp is stated as having total distortion of 2% at 15 ~ 25V rms out.
However the one stage preamp distortions are a lot higher.
http://frank.yueksel.org/sheets/010/e/ECC82.pdf (http://frank.yueksel.org/sheets/010/e/ECC82.pdf)
-
Looking at the sequencing, I'd place a normal electro-mechanical relay in the HV DC line. This will give the HV caps time to stabilize.
Switching AC is tricky. both for inductive and capacitive loads.
-
This is one case of switching HV DC where things work out in your favour. With proper sequencing, the contacts close DC but by the time they open, there is no longer DC current flowing.
P.S. The same applies if you switch AC at the transformer secondary.
-
We use Amperite thermal delay relays in our HV designs and SIGSALY 1942 Quantizer.
115NO30, avail on epay and ham sites in both Octal and miniature tube envelopes. perhaps $5..35
The inductive load of a plate xformer will be an issue with any solid state stuff, triac, SCR,etc.
Typically CPUs, PICs do not mix well with tube stuff.
Jon
-
What is the reason for this delay?
Many authors suggest that applying high voltage to a tube whose cathode has not yet warmed up is bad for the cathode surface.
This is why lots of stuff back then used indirectly-heated-cathode rectifiers (e.g. GZ34) to apply high voltage to the other tubes after the cathodes were warm.
Amperite's website does show some stock for, e.g. 115NO30, thermal relays at distributors, but they are obsolescent if not yet discontinued.
-
Alternative: "Use Brain".
Back in my rock guitarist days, we mostly used Marshall stacks. They had two power switches:
"Standby", which switched on the filament supply, and "Power" which turned on the main/HV supply. "Power" would only work if "Standby" was on. I can't remeber if there was a timer.
SOP after setting up the stacks was turning on "Standby", and when we went on stage "Power" was operated.
-
Just don't leave it in that state, otherwise cathode poisoning will set in.
-
Just don't leave it in that state, otherwise cathode poisoning will set in.
Could be, but 10 years of playing gigs with the Marshall stack, I never had a defective tube. Quality gear. But OK, it was only on standby for an hour or two per gig.
Similarly, CRT colour TVs kept the picture tube filament warm all the time, albeit at lower power on standby.
-
I remember it used to be a problem in 'instant start' valve TVs. I can't remember now whether dropping the heater voltage was the workaround.
-
Not sure why you want a delayed H.V.? When the H.V. does kick on you will probably get one hell of a thump on the output!! 12AU7 / 12AX7 / 6AL5 tubes have been used in V.T.V.M.'s for 50 years or more. Those meters were on service benches that were switched on every morning and off every night and sat dormant on weekends and I can't say anyone ever got less than about 20 years of service out of a set of tubes. You are making your life to complicated just to 'feel good' about your tubes. Eico, Hickok, Heathkit and other makes of oscilloscopes never bothered with delayed B+ and I think most of those scopes saw 40+ years of service without a tube change. Granted, some used B+ rectifier tubes but by the late 60's most had solid state rectifiers and were transistor / tube hybrids. Think about the transistor / tube hybrid color televisions, their B+ was almost instant on and you still got at least 5 years of daily use out of the absolutely brutalized sweep and video output tubes. (I really liked the old Sylvania / G.T.E. 'D12' chassis!! Tubes in all the right places back then. The all solid state 'K' and 'L' series were fantastic after replacing a couple of resistors and the flyback that always failed!!) Cheers!!
-
In my tube preamps, I used conventional time delay (with mechanical relays) to switch the HV, and then to unmute the output after a short delay (with a reed or mercury-wetted relay).
No thumps.
-
I have implemented soft-start by putting a damping diode in series with the B+ (6AX4 or 6AU4 for example.) The damping diodes that I've seen all have a heater-cathode that warms up fairly slowly (though faster than 40 seconds.) Once warm, the diode is shorted with a toggle switch. Depending on the amp, the voltage drop might be low enough it can be left in circuit without shorting - let your ears decide! I considered a timer+relay but decided that I had more important things to think about.
-
My choice for time-delay relays resulted from finding some surplus Potter & Brumfield CUF-42-70120 relays (solid-state timer with unijunction transistor and sensitive mechanical relay) that use an external timing resistor.
Otherwise, such relays tend to be expensive.
-
I like the vacuum tube relay thing
saw it in ancient stuff and it had no problems, undoubtably if it was a different switch I would be looking for parts
-
I had a hybrid amplifier with PCB and tube that uses HV.
On the PCB every single diode was cracked in half.. I could not find the problem until I use tweezers to nuddge things and the diodes separated like broke spagetti. (and this is with overrated big boy parts, not SMD)
the slightly older amplifier with the vacuum tube thermal relays was fine however.
SSR (toaster oven mod chip) for highly non resistive load is IMO not reliable. If a heater has too much capacitance it can go (so these things can get taken out by the wrong coil geometry), and probobly similar problems with inductance. If you want to try with a SSR, at least get a panasonic brand one from digikey.
-
My choice for time-delay relays resulted from finding some surplus Potter & Brumfield CUF-42-70120 relays (solid-state timer with unijunction transistor and sensitive mechanical relay) that use an external timing resistor.
Otherwise, such relays tend to be expensive.
wow the UJT appears :o
-
My choice for time-delay relays resulted from finding some surplus Potter & Brumfield CUF-42-70120 relays (solid-state timer with unijunction transistor and sensitive mechanical relay) that use an external timing resistor.
Otherwise, such relays tend to be expensive.
wow the UJT appears :o
An informal survey of engineers at my previous employer (who were all 20 years younger than I) indicated that none of them recognized the symbol for a 2N2646 UJT in a large system to which we had to kluge a synchronizing connection.
The later version of that system used a 555 (larger footprint).
-
actually I saw the UJT used in a time delay circuit for heaters in a tube before, i think its the same idea
-
UJTs were common in time-delay relays years ago. I found this article useful when trouble-shooting some of my surplus relays that had failed:
http://www.rfcafe.com/references/electronics-world/time-delay-relays-apr-1967-electronics-world.htm (http://www.rfcafe.com/references/electronics-world/time-delay-relays-apr-1967-electronics-world.htm)
-
Thank you to everyone who contributed to my original post.
Your input is kindly appreciated and has given me much to consider.
Best to you all.
Mark.