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hello all, I just finished my sIII v6 and I went through the startup steps and I got to putting in the rec tube and turned on the standby. When I did that, the wirewound resistor on the 32 32 can cap starting smoking. I noted the note on the layout that said something about a black dot on the can cap. There was no marking that I could tell so I wired it exactly as the layout shows. Any suggestions on what is going on?

Thanks

Tim

Found it, I double checked my wiring, but obviously I should have checked it three times. The wire that goes to the board from the can cap, I had it terminated on the turret to the left so it was a direct short to ground. oops

So I fixed the wiring mistake, tested the 1k5 resistor and all seems ok with it. The can cap I'm not sure how to test to see if I burnt it out when I wired it wrong. So I put the tubes in and took voltage readings and I am coming up with some that are not what they are supposed to be.

v1

pins
1 330

3 0

6 2.5

8 0

v2
1 246

3 2.3

6 334

8 0

v3
1 258

2 67

3 94

6 247

7 64

8 93

v4
3 13

7 360

9 355

v5
3 13

7 357

9 55

v6
1 315 ac

3 380

7 316 ac

v7 6v6
8 17

v8 6v6
8 17

Thanks for any help

Tim

It works and sounds awesome! I only played it for a short time, enough time to switch all the switches and turn controls all up and down and everything seems to work great. Thanks Stephen for a wonderful kit. Now on to making a cab for it.

Tim

so after playing the amp some more, the only thing I'm not crazy about is: There is a decent loss in volume when switching from the el84 to the 6v6. Is this normal to have that much of a significant difference between these two sets of tubes. If so is there a remedy that will get them closer together in volume?

One other question, is it ok to switch between these when amp is on? I remember reading somewhere both yes and no to this question. So for the no answer, what can this do to harm the amp. Does anyone use the soft switch to switch between power tubes?

Thanks

Tim

This is because the 6V6 tubes have a much lower gain level than the EL84s, but the 18W phase invertor is designed to drive EL84s. A mod, which I've described previously, is required to give both sets of tubes an optimal amount of drive.


I know some people do this while the amp is on, but I don't like taking the risk. At least put the amp into Standby.

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hey thanks Phil, I found this in a post of yours where your are talking about the same thing I am asking, I think.

"Instead of both sets of tubes using 470k grid reference resistors, you split each 470k into a 220k and a 250k, connected in series. So you end up with a 220k+250k (=470k) on each side. You connect each EL84's grid to the junction of it's respective 220k/250k pair, so that it's fed an attenuated version of the PI drive signal. The 6V6s are connected as before, to the top of the 220k+250k pair, via an 8.2k grid blocking resistor, just as if it was a regular 18W amp. I hope that makes some sense. It's actually simpler to do than to describe."


Now is this what I am asking for to get the two tube sections to a similar volume? If so I would like to do this, but I will need some more direction. I see where both of the 470k are, but how would I add two others in their place? Would I have to press in a couple of extra turrets or would I solder the 220k and 250k end to end? Do you have any pics with this mod done?

Thanks

Tim

Oh heck! This is one of the things I mentioned in my build thread a little further down the page. I'm not planning on changing anything at this very moment with regard to the 6V6 level, but I would like further details on the mod just in case.

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Sorry I don't have pictures of this one. Ideally, you would need to add a couple of turrets. If you can solder the resistor ends together fairly rigidly, along with the connecting wires to the EL84 grids, then you could get away without using the turrets. It might be a little tricky to solder. So turrets are the preferred way.

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Great tones come in small glass jars!

Thanks Phil, I wonder rather than setting up new turrets if it would be ok to just use a machine screw, say a #8 or maybe even a #6. Secure it on both sides so it can't move and then solder to it. Adding turrets at this point in the game would be a bit tough.

Just so I am straight on this, I am going to remove both of the 470k grid resistors and replace them with a 220k and a 250k in series with eachother. Then I am going to hook up the 6v6 just like before but the el84 grid will be hooked up to the junction of the 220 and 250 resistor. So my question is: which resistor am I using for the el84, the 220k or the 250k?

The other thing that is confusing me, is it seems by doing this we are leaving the 6v6 side exactly the same and modding the el84 side. Am I looking at this all wrong or is it by modding the el84 side, thats what is making the two sides more to a similar output level.

I hope you understand what I am asking and if I think of a clearer way of asking, I will edit this post. Sometimes I confuse myself

Tim

Use copper or brass ones and clean them with steel wool, so that the solder takes to them.


The drive ratio is better with the 220k resistors at the ground end, and the 250ks at the PI coupling cap end. Then the EL84 grids connect directly to the join of the 220k and 250k resistors on each side. No need for an 8.2k grid resistor there. The final result still won't be perfectly balanced, but it will be better than the way it is now.

Edit: Another alternative is to use a pair of 500k linear trim-pots and connect the EL84 grids to the sliders. Then you can adjust for what sounds best to you, taking resistance measurements on each one to make sure they're set the same.


You also need to mod the PI for more drive and gain. So you change the 56k/820ohm in the PI tail to 22k/470 ohms. Then the 6V6s are getting the drive level they need, while the EL84s are now being fed an attenuated drive relative to the 6V6s.

_________________
Great tones come in small glass jars!