Voltage on 5E7 seems way off....

[playonit]

I got the voltage on the preamp tubes tamed down. I found that my dropping resistor was 1k and not the 10k it was supposed to be. After replacing, the voltages dropped substantially.
My voltage on my output plates is still high @ 424v instead of the charts spec of 388v ..... I meter all my resistors and one got by me

[Phil_S]

Good that you got it done!

I own an apology for two bad reads on this one. 1st for the choke, and second for not paying attention to the bias question. I read the 45mA as 45 volts and thought you needed to get down to something in the mid 30's. I'm really sorry and didn't intend to mislead. I think I'll lurk for a while and keep my pointer off the reply button.

[playonit]

Good that you got it done!

I own an apology for two bad reads on this one. 1st for the choke, and second for not paying attention to the bias question. I read the 45mA as 45 volts and thought you needed to get down to something in the mid 30's. I'm really sorry and didn't intend to mislead. I think I'll lurk for a while and keep my pointer off the reply button.

Phil.... keep that pointer aimed at the reply button you have been a great help and I have learned a lot from you sir.... I actually looked back at my posting history because I had thought I had asked one of my questions a couple of years ago.. you were replying to my questions way back then.. always willing to help... I don't do well with books and the ability to post here and ask questions and then drill down with further questions when I am not understanding something is worth it's weight in gold and it's because of guys like you that I get to understand some of this...

Now with that said.... I still can't get my bias lower then 45mA ? Funny thing is I have two identical 5e7's one a ceriatone kit and one a scratch built with that same circuit.. only difference is The transformer manufacturer's ..... one amp will bias adjust lower even below the 35mA I'm looking for and the one I'm working on its bottomed out and won't get below 45mA ??? I will go back and see what I can figure out.. But I have to say so far I'm scratching my head on this one...

[martin manning]

I got the voltage on the preamp tubes tamed down. I found that my dropping resistor was 1k and not the 10k it was supposed to be. After replacing, the voltages dropped substantially.
My voltage on my output plates is still high @ 424v instead of the charts spec of 388v ..... I meter all my resistors and one got by me

The 5E7 schematic shows 410 for the voltage right after the choke, so you are not too far off, really. On the bias, you can reduce the size of the 6k8 resistor, and it would be easy to clip a pot across the 6k8 and dial it to what you need, with the bias pot you have in the amp set at mid range. It could just be that the power tubes you have installed need a little more negative voltage to hit the idle current.

[rdjones]

I got the voltage on the preamp tubes tamed down. I found that my dropping resistor was 1k and not the 10k it was supposed to be. After replacing, the voltages dropped substantially.
My voltage on my output plates is still high @ 424v instead of the charts spec of 388v ..... I meter all my resistors and one got by me

Your voltage is high by the same amount that my wall voltage is high !
424/388 = 1.092, 9.2% high
128/117 = 1.094, 9.4% high

rd

[rdjones]

Phil.... keep that pointer aimed at the reply button ...

Agreed, Phil.
Thanks, and apologies to all those who tolerate my occasional sidetracking and banter.

rd

[rdjones]

What kind of range do you have on the bias pot ?
Can you pull the output tubes and measure at the socket grid connection's bias voltage from one extreme to the other, as well as the voltage at the center point ?
Something Martin hints at is to have the 'normal' bias voltage occur at the center of the bias pot travel.
This allows for both reset-ability and a suitable range above and below the normal value to accomodate tube variance and even alternate tube types.
Also, what's the bias supply voltage at it's cap ?

rd

[playonit]

I got the voltage on the preamp tubes tamed down. I found that my dropping resistor was 1k and not the 10k it was supposed to be. After replacing, the voltages dropped substantially.
My voltage on my output plates is still high @ 424v instead of the charts spec of 388v ..... I meter all my resistors and one got by me

Your voltage is high by the same amount that my wall voltage is high !
424/388 = 1.092, 9.2% high
128/117 = 1.094, 9.4% high

rd

I measured my wall voltage where I plug it in and it coming out at 122v......

[Luthierwnc]

playonit; what is the bias voltage at the grids? How about the VAC on the tap? You can stick a 100k pot in series with the existing bleed circuit and just dial it back until you get where you want. You won't hurt anything since you'll be going from current-starved to ideal.

I've built this circuit many times and have always included a cathode/fixed bias switch. You don't need a ground switch anymore so that is a great place for it. That works really with this design and the break-up is more like 4 1/2 than 5 1/2 if you need it to get a little dirtier for a small club.

[Phil_S]

Both of you are very kind to me. The lesson here is to exercise more restraint. I may have, without intending, thrown a bunch of stuff against the wall to see if something sticks. This sort of work is more scientific than that. We'll see if I learned anything

[playonit]

"I've built this circuit many times and have always included a cathode/fixed bias switch. You don't need a ground switch anymore so that is a great place for it. That works really with this design and the break-up is more like 4 1/2 than 5 1/2 if you need it to get a little dirtier for a small club.

Can you explain this switch for me... Thanks


Also my bias voltage at the grid is -41

[Luthierwnc]

Couple ways (at least) to do this. The one on top uses a SPST switch. I've usually done this but it does ground the bias voltage so use stout resistors. The one below uses both halves of a DPDT on/on switch and doesn't ground the negative voltage.

One advantage of the top one is if you are tight on space you can use a heavy duty on-off-on DPDT switch. Wire one side as your standby (with the throws tied together) and the other side as your F/K switch. The middle position is standby. The cathodes always reference ground through the cathode bias circuit so you don't have to worry about runaway current.

There are some amps this doesn't work well for. I tried it on an EL84 build and the fixed couldn't hold a candle to the cathode biased side. The higher-end tweeds, OTOH, just love this. sh

[playonit]

I am losing my mind..... I have tried to track down the source of my bias problem and have struck out and don't have any more ideas... My bias will not come down below 45mA .. My pot is adjusted as low as it will go..

checked all solder joints
changed out choke to a more robust unit (from 90mA 4HY to 200mA 5-8 HY)
checked all resistor and cap values
measured voltage on the PT 50V tap (47v)
checked pot resistance ( 0-25kL)
mapped out circuit

I appreciate the recommendations to change resistor values to tame voltage on bias but I am a Root cause kind of guy ( what is the cause).. as I stated above I have 2 identical amps except the transformers used Weber in one and Magnetic Components in the other amp... the bias adjustment works fine and gets me down to the lower levels.. I have included my updated voltages in red..

I'm pulling out what little hair I have left... this makes no sense to me any longer.. there is a root cause... it's there somewhere I just can't find it...

[Phil_S]

Lack of restraint noted...

Your bias voltage is -40/-41. The schematic I'm looking at says -40V on it. IMHO, there is nothing wrong with your circuit. Unless I'm missing something (and that's possible), I think you have a pair of power tubes that just want to do what you are observing. I take it that you don't have a spare pair of power tubes to verify that it's the tubes? Then the cure for this is to modify the bias circuit to give you a little more tweakable voltage.

You need to go more negative, maybe to -45V (not really sure). Increase the voltage supply by tacking in another 6.8K in parallel and see what that does, or decrease the bleeder resistor by adding a like value in parallel on that side of it. Do it and see what happens!

[Luthierwnc]

47VAC at the tap is on the low end of the range. If decreasing the first resistor won't get the volts high enough, just pull your bias off one of the HV taps and you can make the bias voltage as high as you need. You'll need a couple new resistors and caps but chances are good you can cram it into the space provided.

FWIW, the O'Connor schematic that shows the fixed/cathode bias switch on the net also has an HV tap bias set-up on the same picture.

http://www.londonpower.com/GRAPHICS/pwr-fig3.gif

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