Selmer Concord 10 Rebuild/Conversion Issues

[BungleSim]

What's more likely than a whisker short is a failing component as this thing is very old. At this point I wish I had just decided to rebuild the whole thing from scratch. Probably would've taken me less time.

[BungleSim]

In the stock PSU setup (250R/40uF, 22k/40uF, 68k/40uF) the quiescent current is 88mA and when I really dig in it pulls as much as 132mA! This baby cooks!

[BungleSim]

I've tried replacing the 250R with a 3k and a 2k and the current draw stays the same. I've also discovered that the OT CT is drawing the lion's share of the current. The preamp only pulls about 12mA while the OT is pulling the rest (76-120mA).

[Phil_S]

I've tried replacing the 250R with a 3k and a 2k and the current draw stays the same. I've also discovered that the OT CT is drawing the lion's share of the current. The preamp only pulls about 12mA while the OT is pulling the rest (76-120mA).

How are you reaching these conclusions?

Earlier you posted the voltage on the B+ ladder drops from the screen node to V2 from 297 to 233 over 22K. 64Vdrop/22K = 3mA. That should be for both V1 and V2. I think "only12mA" seems excessive and is not "only." 12AX7's are rated for 1.2mA per triode, so x4 you should not see more than 4.8mA and 3mA certainly seems reasonable.

A pair of EL84, according to the spec sheet, at Va =300, VG2 = 300, will pull 72-92mA for the pair.

I am wondering about your power tubes or your method of measuring, trying to understand what you are actually saying as the reported 120mA isn't making sense to me nor is the 12mA.

[BungleSim]

I inserted my DMM in line with the power supply, right at the output of the rectifier, measuring DC current. I connected the OT CT to B+0 and measured after this node but before the 2k resistor that replaced the 250 ohm resistor. I was only seeing 10-12mA. This was the current draw of the preamp and the inverter (the tremolo is still disconnected). Sorry I misspoke.

I then moved the OT CT from before my meter to after it and that's when I was measuring a max 132mA draw.

The fact that the rectifier is only rated for 90mA is another concern.

[Phil_S]

The EZ80 rectifier might just be the problem. Somehow we missed this as it is appropriate to the schematic. The original design was for 6AQ5's which have the same spec as 6V6. A pair of 6AQ5's plus a pair of 12AX7's should be at or under about 80mA. This amp's been modified for pair of EL84's. EL84's pushed hard plus a pair of 12AX7's might put the demand up as high as 120mA. You report 132mA, though hard to imagine, might actually be happening. To meet this demand you really should be using a 6CA4/EZ81.

A cheaper and quicker solution is to pull the tube and configure a FW rectifier on the socket pins with two 1N4007 or similar diodes. These are 10 cents each and will do the job nicely. If these fix your 30 sag problem then you know the answer and can decide whether you want a tube rectifier or not. Really, you'll be fine with the silicon diodes. Give this a try.

Aside from this, I still think your reported current draw isn't making sense. You reported plate voltage of 308 and idle current draw at 88mA, which is 27W. I'm thinking the most you can expect from a pair of EL84's is 20W. At 27W, I think you should see them red plating. So, IMHO, there is something wrong with this picture and I don't know what.

Try the silicon diodes. See what happens.

[BungleSim]

Well, I had high hopes for using the the diodes as a full wave rectifier but that seemed to make the problem even worse. Quiescent current is now at about 100mA and when I crank the amp and dig in I can pull as much as 166mA! I didn't do this for long at all as I didn't want to damage the transformer as the HT is only rated at 150mA.

I swapped out all of the tubes to make sure none of them had been damaged in this process and the readings stayed the same. I am just as bewildered by this as you are, but there must be a simple explanation/solution. At this point I have measured and verified the values of every resistor in the circuit and replaced those that I felt were too far out of spec. I think it is time to go through every capacitor, one by one, and replace them to see if any of them are providing a path to ground. It seems to me, and correct me if I'm wrong, that those of particular interest are the cathode capacitors.

[Phil_S]

How about publishing a voltage chart for us for all tube pins, but omit the heaters, and for all B+ nodes. Not sure what your B+ ladder is right now, either. I feel quite blind here.

[BungleSim]

OK. What I've been wanting to do is completely redraw the schematic as the one I found online isn't totally accurate and at this point I've made a good number of changes. I'll try to get that done tonight.

[BungleSim]

Here's the schematic of the amp in its current configuration (i.e. not stock). I actually have the tremolo disconnected as well. The tremolo footswitch is completely disconnected and the wiper of the depth pot is disconnected. I have connected the node between R17 and R18 to ground.

[hans-jörg]

Hello,
I´m quiet sure I will not solve your current problem, but isn't it better to raise the first (reservoir) cap to let say 40µF and lower the following ones to 20µF?
That's what I read usually in schematics. Maybe there is the weak cap hidden?
And yes, you need a EZ81 or SS rectification, but this you know now.

Good luck

Hans-Jörg

PS: do I read right, your coupling caps C12 and C13 are 0,25µF?
shouldn´t be there 0,022µF, or 22n?

[Phil_S]

Schematic looks nice. Voltage chart?

This amp, as drawn, I continue the have difficulty with some of the numbers you report as they don't make sense to me. How the heck can a pair of EL84's with plate voltage around 300, draw 166mA without red plating? That's 50W. If voltage sagged to 200, that's still well over 30W. This just doesn't seem right for this tube type.

This amp is a low gain affair into a cathodyne inverter. I"m thinking 15W max and likely close to 12W.

In addition to a voltage chart, if you can get the DCR of the OT primary, each outer leg to the CT, that will also be helpful. This information will give us more than one way to compute power current.

[BungleSim]

I'll post a voltage chart and the DC resistance of the OT's primary tonight after having the amp in front of me again.

Obviously the amp isn't operating properly, hence the wonky numbers.

[hans-jörg]

Hello,
I´m quiet sure I will not solve your current problem, but isn't it better to raise the first (reservoir) cap to let say 40µF and lower the following ones to 20µF?
PS: do I read right, your coupling caps C12 and C13 are 0,25µF?
shouldn´t be there 0,022µF, or 22n?

??

[BungleSim]

Hello,
I´m quiet sure I will not solve your current problem, but isn't it better to raise the first (reservoir) cap to let say 40µF and lower the following ones to 20µF?
That's what I read usually in schematics. Maybe there is the weak cap hidden?
And yes, you need a EZ81 or SS rectification, but this you know now.

Good luck

Hans-Jörg

PS: do I read right, your coupling caps C12 and C13 are 0,25µF?
shouldn´t be there 0,022µF, or 22n?

I don't really see any reason why the caps won't work just fine in the current configuration. The original configuration did not have the 22uF cap, I just added that to get rid of the ripple I was seeing on the B+. I had that cap as high as 47uF and there was no discernible difference so I dropped it back down to 22uF. I also did that because the EZ80's capacitor reservoir limit is 50uF.

As for C12 and C13, these are the values as I could read them from the original schematic. The actual values may differ.