My first tube amp... I mainly mess around with SS car audio. I Just saved a little 1962 Admiral tube amp from a console that was getting thrown out. The owners manual and schematics were tucked away inside the console. Its a stereo tube amp model 3G2- it has one 12AX7A and two 50EH5 tubes on it. No power transformer? The output transformers stick off each end of the casing. After studying the schematics and nervously adding a switch and power plug I was able to fart around with it and get it going on the test bench with a couple of bookshelf speakers and an ipod. It doesnt have much output, but the sound is perfectly listenable. I think I will clean it up, and upgrade it some and just enjoy it for what it is. It has a big ol honking dry electrolytic cap sticking up off the top right next to the tubes that will have to go somewhere else to make it presentable. Anyone have anything similar or have any suggestions as to what would be reasonable upgrades? I can already see that a larger tube amp is somewhere in my future. This little thing is really neat.
Mine wasn't so up-market (only had 2 x 50EH5) Note that it has an isolation transformer.
5-Buck Amp
dave
5-Buck Amp
dave
Im listening guys- please educate me about the isolation trans. Im not real knowledgeable about the tube amps but I did notice there did not seem to be a third transformer anywhere in the console or on the schematics. The way I wired it duplicated exactly the original power feeds from the schematics... unless I'm missing something.
yeah, well, electrons are not too concerned about your care - or the ignorance of others. Fit a power transformer to isolate the circuit from the line supply. You don't have the economic imperative excuse of the manufacturer.
isolation trans
Looked thru all my parts boxes/salvage stuff and I dont have a suitable transformer. Where do you guys get them? Is there a common type of equipment (other than tube amps) I can look for to salvage one, or would I be better off ordering it from somewhere? My schematic says the power input is 117 VAC/55 Watt- what iso trans specs should I be looking for?
Looked thru all my parts boxes/salvage stuff and I dont have a suitable transformer. Where do you guys get them? Is there a common type of equipment (other than tube amps) I can look for to salvage one, or would I be better off ordering it from somewhere? My schematic says the power input is 117 VAC/55 Watt- what iso trans specs should I be looking for?
It's an AC/DC single ended (must use Class "A" operation) amp, with a 150 mA. series heater string. What you described is as cheap as can be.
Look for what appears to be a stacked collection of fins. That's a selenium rectifier, which is a ticking toxic time bomb. Replacement is mandatory.Please post the a scan of the schematic. It will help the members make useful suggestions.
The 50EH5 data sheet suggests that a B+ power supply with a capability of 100 mA. or so will be sufficient. The inexpensive 50 VA rated Triad N-68X isolation trafo has the cojones needed to energize both the heater string and a proper B+ PSU.
Construct a bridge rectifier from 4X UF4007 diodes. CRC filter the "raw" B+ with 2X 220 μF./250 WVDC caps. and a 3 W. rated 100 Ω resistor. That setup will be a SUBSTANTIAL improvement over the factory original.
Back in the day, the 112.6 V. total for the heater string was reasonable. Today's higher average mains voltage requires a small amount of resistance at each end of the string. A couple of 0.5 W. rated 3.9 Ω parts will do the job.
Photos will be helpful too. We need to see how massive the O/P trafos are. Given the cheap construction, I suspect the trafos will be rather small. That does note bode well for adding a GNFB loop to the circuitry. Magnetic headroom is needed for that tweak and switching to triode mode "finals" brings the power O/P under 1 WPC. Can you say 103+ dB. sensitive horn speakers?
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.