Bass amps w/ preamp tube

[lowell]

Hey all,
can someone please provide some knowledge on this subject? I'm working on an Eden valvetech WT300. The output transistors check fine, I get some signal out speaker jack... however it's distorted the amp will eventually blow the fuse. I'm caught up here at the preamp tube... I put a brand new JJ 12ax7 in there, but there is nothing coming out pins 1 or 3 of this thing. I then noticed there's no glow so thought I'd check the filament supply when I shockingly realized that isn't one... unless I'm overlooking something. Can a tube be run w/o heater supply??? I'm at a loss here. There is no heater supply shown in the schematic... however I know it's pins 4 and 9. Pin 4 goes through a 1ohm to ground and pin 9 goes nowhere. Oh actually pin 5 is also heater... hmmm... I'll check that tomorrow.

[Enzo]

4 and 5 are the heater. Pin 9 is just the center tap. We usually run 12AX7 tubes with 4 and 5 wired together with 6v across 9 to 4/5. But the tube is considered a 12v tube, with 12v across 4 to 5 and 9 unconnected. I'll bet that is how yours is wired.

If it blows fuses, I'd be less suspicious of the tube and more worried about the power amp. The outputs may not be shorted, but do they run hot before the fuse opens? Remove the mains fuse and connect an AC ammeter in its place. (Or if you have a metered outlet, plug the amp into that.) Now power on the amp. WHat current flows with the amp at idle? I would expect maybe half an amp tops. Pull the tube. How much difference? Not much I would hope. Does the reading slowly rise?

Disconnect any speaker. The amp does NOT require a speaker load, it will be happy without. Did the mains current drop? Is there any DC voltage on the output?

We would love to see the schematic. Could you post it or link to it? Or send it to me and I will share it with all. My address is:
enzoamps at att dot net

As to the tube, what do you mean nothing "comes out" pins 1 and 3? Signal? DC? There should be some Positive DC on pin 1 and a smaller voltage on 3. Signal should appear on one or the other. Look at the schematic you have. either the tube is a gain stage, so there would be the higher voltage on the plate - pin 1 - as well as the signal. And the cathode - pin 3 - would have a low DC voltage and probably no signal to speak of since it would be bypassed. On the other hand they might have made a cathode follower of it, and the plate is connected right to the + supply with a relatively high cathode voltage and all th signal appears there.

[lowell]

Enzo,
Thanks for taking time to share some knowledge w/ me on this. I will try some of your troubleshooting techniques and let you know how it goes. On the other hand I just discovered that the filament supply filter cap for the 12ax7 has blown and oozed that brown coca-cola like fluid onto the board. I'm in the process of replacing it... hence the "no filament" lighting in the tube and hence the no signal on the plate OR cathode... yes it is wired as a cathode follower. Here is the schematic for the WT300. Maybe this cap was causing a current surge of sorts? I keep you posted.

[lowell]

Well I fixed the filament circuit and the tube is back up and running but the amp still sounds like it's suffering... distorted even at low volumes and sounds quite choked. I put the ammeter in series w/ the power and it started at 1.9amps and worked its way to 2.2 amps over about 5 minutes.

[lowell]

Enzo... or anyone? Anyone provide me with a direction here? I'd be so very thankful for any thoughts.

[Enzo]

Wait, this is just a preamp? No power amp? That would change things. Oh, I see power supply but no power amp drawings.

Within the preamp:

I'd start looking for power supply trouble. Fire it up without the tube. I doubt it is involved, but it is an easy check.

Before it wants to popp a fuse, but after being on a moment, feel each IC chip. ANy getting hot?

D13,14 are 15v zeners. they establish +/-15VDC for the op amps and circuitry. Are both 15v rails present and up to voltage? If not, check those diodes. Possible bad C10,11.


But mostly we suspect the power amp. As the current rises, is the heat sink getting warm or hot?

Is there a bias adjustment control? Your mains draw of 1.9A is high. At 120VAC mains, that means the amp is using 228 watts just sitting there. And at 2.2 amps that has risen to 264 watts. Way too much for an amp at idle.

Watch the mains current and turn the bias control so as to reduce the current. Turn it all the way to the coldest setting. Now does current fall into line and stay low? I'd expect a quater amp to a half amp tops, myself.

It could be a simple adjustment issue.

if the outputs are overheating and won't bias out, I still would suspect the bias circuit.

A schematic of th power amp would help.

[lowell]

Yes the heat sink and output area gets quite hot...

ok yeah... will check those zeners and see if there is a bias supply and report back... thanks very very much!

[paulh]

Just a thought. These amp have a fuse holder that selects 120v or 220v input ac. I seen these amp where the fuse holder got put in wrong and 220v was selected which is easy to do as Eden does go by the little arrow on the ac plug but goes by if the markings are right side up. wiht the amp being starved for voltage I v'e seen them fail all sorts of ways.

[lowell]

YES... fixed! Man these solid state circuits are always quite difficult for me. I tested all the zeners in the +15v supply as well as the 2 filter caps for that circuit and all was well and good. I then disconnected the clip with 6 wires going to the power amp and measured the mains current. With the clip unclipped the mains dropped to 300ma. So now I knew it was the power amp for sure that was shorting somewhere. I could not really figure out the bias circuit because I don't have the correct schematic for that section of the amp, but it made sense that this one transistor was the bias transistor. I had a schematic of a similar but different Eden output section to compare and use as a rough guide. the transistor was labeled 8099. when i tested it the emitter-base diode tested a 100mv drop. This was far from what the other transistors in the circuit of the same type were measuring. So I pulled it out to test it out of circuit... same reading. I found a radio shack transistor in my bin that was a close fit, threw it in and the amp came alive! I checked mains current and it was down to 250ma. The schematic said bias to 4mv across emitter resistors... I did it and all is good. Just wanted to let everyone know the outcome here. Thanks Enzo very much for your help! One more q... what made you think that it was the bias supply and not a shorted output transistor? I guess I told you I tested them all and they were good. Would you have said to check those otherwise?

cheers