Did you mean transformer instead of rectifier? That transformer, 360V one, does not have the 5V tap for tube rectifier. Go with SS rectification.
ST-70 Tube Amplifier DIY Kit
- Thread starter wdecho
- Start date
Give them a call. Not to late to change. See if they will sell you the EI transformer they are selling in the EI kit they are now selling.
the 5AR4 is overloaded in an ST70 anyway so no great loss.
Roy Mottram sells a solid state power supply board that mounts nicely underneath of where the stock bias pots go, using 2 stock hole locations. I think one was the original selenium rectifier, other is maybe a cable clamp? I made a tweak to his parts list, the 3 amp CL80 that is specified as a thermistor doesn't see anywhere near enough current to warm up and drop resistance. I changed it to a CL120 (I think, have to look) which is still overkill but less-so. Also higher cold resistance so its a slightly softer startup.
I changed the specified diodes too, it calls for a UF4007 which is 1KV PIV. I had something else on hand that I can't recall the number of which are good for 1.5 KV PIV, close to the original 5AR4 rating. If I didn't already have whatever those are, I'd have done a pair of 4007 in series and called it a day.
my setup is rather different since its Eico iron and not even an EL34 amp, but its in a Dynaco chassis and using some Dynaco bits so I could shortcut on the design process somewhat.
Roy Mottram sells a solid state power supply board that mounts nicely underneath of where the stock bias pots go, using 2 stock hole locations. I think one was the original selenium rectifier, other is maybe a cable clamp? I made a tweak to his parts list, the 3 amp CL80 that is specified as a thermistor doesn't see anywhere near enough current to warm up and drop resistance. I changed it to a CL120 (I think, have to look) which is still overkill but less-so. Also higher cold resistance so its a slightly softer startup.
I changed the specified diodes too, it calls for a UF4007 which is 1KV PIV. I had something else on hand that I can't recall the number of which are good for 1.5 KV PIV, close to the original 5AR4 rating. If I didn't already have whatever those are, I'd have done a pair of 4007 in series and called it a day.
my setup is rather different since its Eico iron and not even an EL34 amp, but its in a Dynaco chassis and using some Dynaco bits so I could shortcut on the design process somewhat.
Another problem going with a Dynaco PS transformer is you will have to cut the chassis to fit the Dyanco transformer. Tubes4hifi sells the transformer for $225. I considered all the options when buying the AS-3TD360 transformer and chose to stick with a toroidal transformer for appearance and price.
Re: the feedback, I built some Mk2's a couple of years ago with a 6SN7 driver board. It also uses a quite low level of feedback and appeared to be stable but I found that when I disconnected the feedback for a test that the output decreased instead of increasing. The drive signals to the output tubes grids were reversed on the schematic from what was ultimately the correct phasing. Point being that with low feedback your amp could work apparently ok but still be wrong. Just a quick test by disconnecting the feedback will tell you.
Lavane
Lunatic Member
You can get this vented cover for that hole you punched. See link below.
Also these from Ted Weber.
CP8 Octal Cover Plate – Weber Speakers
www.tedweber.com
must be a very low level of feedback to not turn it into an oscillator.
Seems like it was only about 3dB or so. Not much. I was surprised at how little it was. Once I got the phasing corrected I bumped it up a bit but there isn't enough gain to go too far with it. Instead of 2 6SN7's, it would be better if the first tube was 6SL7 or 6AQ7. I may change it some day and see what happens. It still sounds a little wooly in the bass which I think would be better with more well tuned feedback.
Thought of that and tested. No problem
6AQ7 looks like single triode, twin diode? should be a good fit for a mono amp, just tie the diodes to cathode. 6SL7 has the advantage of same pinout as the 6SN7 though, makes life easier dealing with a PCB. Just have to change plate and cathode resistors to make it operate sanely.