RCA Classic tube preamplifier help - Page 1
Forum section: electronics / projects

author: darkness_electro
author_rank: Contributor
timestamp: August 24, 2015, 07:06:12 pm
content: http://diyaudioprojects.com/Schematics/7025-12AX7A-Tube-Phono-Preamplifier-Schematic.htmI have built the circuit and I have some practical questions:Tubes are 6N2PThe Power Supply is ~300V DC unloaded. Loaded is, mysteriously still ~300VPower supply is 2 2X12 transformers back to back. Rectified with 4x1n4007 (temporary) and a 180uF cap.Heaters are In series.I have not tested yet any sound. At the first output (second grid) there is no voltage. At the output (after the cap) there is some noise (ac) about 500mv RMS1. I measured 305V at imput (instead of 250). First plate is at 173VDC. Current is at about 8ma.2. The second plate is at 193VDC. Current is at about 7ma.3. 14ma is the total current on b+Are these normal parameters? I see in the original with 250v b+ there should be 193v on first plate and 210v on the second. Why do i get less voltage on the plates and so much current consumption? I knew that 12ax7 draw about 2~3 mA max.

author: darkness_electro
author_rank: Contributor
timestamp: August 24, 2015, 07:40:22 pm
content: I've made a terrible mistake. By removing the first tube, the second draws 0.66ma. (66v drop on the anode resistor). By removing the second tube, there is 0.75ma trough the first tube. (75v drip on the anode resistor).Still, the plate voltage is lower, but the supply is higher.

author: Fank1
author_rank: Regular Contributor
timestamp: August 24, 2015, 10:30:14 pm
content: Measure the cathode voltage of each section and compare the current draw to the data sheet.Keep in mind that the data sheets are for "typical" tubes and some variance is to be expected.Also if you didn't apply a signal and have a load resistor on the output you have not "loaded the amplifier" , impossible without input.

author: Quarlo Klobrigney
author_rank: Super Contributor
timestamp: August 24, 2015, 11:18:51 pm
content: Is it a fact that the 7025 is identical electrically to the 6N2P? Have you looked at the datasheets for both? And difference in gain and biasing will give you the symptoms that you are having. What are the grid voltages? Are the tubes in cutoff or are they conducting. Without pulling up the data sheet, I would say cutoff. What are you using for R1? It must be a high value to amplify.All this is of course that the circuit was built correctly. Check those coupling caps for leakage. A picture is worth a thousand words. Can we see?

author: vk6zgo
author_rank: Super Contributor
timestamp: August 25, 2015, 02:29:22 am
content: This circuit,if built correctly,should be pretty much foolproof.I built one of these when I was 16 (many,many long years back), for our next door neighbour who was building a "Radiogram".He made a good job of the box,but the old radio he used didn't have enough gain at the "pickup"terminals.The preamp was horrible to look at------the chassis was a bit of aluminium bashed into shape over a piece of "3 x 4" timber.I didn't have a 7025 or a 12AX7,so I used a 12AT7 instead.Filaments & HT were supplied by the "host radio".It went first go!Fank1has a good idea,as the circuit shows the voltages to expect at each cathode.If you really have anode currents of around 7mA,you would be seeing around 21volts across each cathode resistor.This would equate to 21v negative bias at the grid,which should cause cutoff.but isn't------check that your 2700 Ohm resistors are really that value (or close to it).P.S: I'm worried about this comment in your original posting---" Heaters are In series."The main difference between the 12AX7,!2AU7,12AT7,7025,family & the 6N2P is that the latter tube has its two filaments in parallel,& thus needs 6.3v ,not 12.6v.(pin 9 is not the filament centre tap on these tubes,it goes to an internal shield.)If you run them at the higher voltage,the characteristics will be completely different,plus you will very quickly kill the tube.

author: Quarlo Klobrigney
author_rank: Super Contributor
timestamp: August 25, 2015, 03:42:15 pm
content: "P.S: I'm worried about this comment in your original posting---" Heaters are In series.""That one scared me as well. I was hoping it was a typo.