Express build bias - strange outcome

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MikeR670
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Express build bias - strange outcome

Post by MikeR670 » Fri Apr 21, 2017 4:26 pm

Hi all, oddball situation here. I used old parts of a pa and constructed an express circuit to the Kelly 90 schematic. The old pa had a small turret board with its ss rectifier and bias circuit already on it, so I used it stick. The issue was that it was set up for 6550C output tubes and the bias circuit with no load measures -74 vdc. Figuring I'd want about -33 for 6v6 tubes, I added a variable screw type pot to adjust it down. Worked great and I could wind it down easily to -33, or anything between 0 and -74.

The problem is that it now only shows voltage on one leg at -33, but 0 on the other output leg. I wired the original bias output straight to the meeting point of the two 220k resistors, and I added the vr by wiring one side into that same point, and the other side to ground.

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DaveWell
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Re: Express build bias - strange outcome

Post by DaveWell » Sat Apr 22, 2017 12:23 am

Post some Picture :)
easyer to visualized
Last edited by DaveWell on Sat Apr 22, 2017 4:14 pm, edited 1 time in total.

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M Fowler
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Re: Express build bias - strange outcome

Post by M Fowler » Sat Apr 22, 2017 12:49 pm

TwreckKellyFrancesca build.jpg
Is this 6550 amp using 150K/150K resistors rather then 220k/220k for EL34's?
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pompeiisneaks
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Re: Express build bias - strange outcome

Post by pompeiisneaks » Sat Apr 22, 2017 2:00 pm

Yeah I agree on pictures and/or the modifications you made to the above schematic, I can't visualize exactly what you're saying.

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MikeR670
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Re: Express build bias - strange outcome

Post by MikeR670 » Sat Apr 22, 2017 5:44 pm

So this is my rough prototype, on the eyelet board there's the two yellow .22 caps and their top ends join to the 'v' shape of the two 220k resistors. The bias circuit output wire comes underneath the board into the meeting point of the 220k resistors. This produced -74 dc. So i added the pot by wiring one lead into that same 220k meeting point, and it's other lead to ground. You can see the two yellow wire going from each of the tops of the meet points between the other end of the 220ks and the yellow caps. Those wires then go to a little board with a 1k5 resistor on each and on to the power tubes. Hope this makes sense....:)

The left side produces -33 at the power tube but the right has 0 volts. I can't understand it because they're both split and I've checked all joints and continuity.....
rps20170422_104926_215.jpg
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Re: Express build bias - strange outcome

Post by pompeiisneaks » Sat Apr 22, 2017 6:04 pm

It seems to be a bit covered with the wires. Is the potentiometer wired as a variable resistor? The joint where the two 220k meet should be a single joint with identical voltage anywhere on that joint. If you're doing it more like a balance pot, then you may have the pot done too far one way so most if not all of the current is going towards the other half, or you could have the pot wired wrong so that the wiper is on one side only, and therefore current isn't flowing to one side at all. Are you trying to use it as a variable resistor? or is this a bias balance pot? Schematic of what you're trying to do would help too.

Usually you want the wiper connected to the junction of the 220 and to the next resistor before going into the transformer etc. See this picture of a modded bassman to allow the variable resistor wiper to act as the adjustment on how much resistance is provided. (sorry if this is already clear to you, but I'm not understanding what you're doing).
Bias2.png
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MikeR670
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Re: Express build bias - strange outcome

Post by MikeR670 » Sat Apr 22, 2017 8:57 pm

Thanks for that breakdown, much appreciated! I'll check tonight, I might have the pot wired wrong. From your reply, I guess I could adjust the pot in different directions to also see if it then sends voltages across the 220k legs differently....

MikeR670
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Re: Express build bias - strange outcome

Post by MikeR670 » Sat Apr 22, 2017 8:59 pm

Yes as per that Modded bassman, that's exactly what I wanted to do, although I don't have the other resistor after it going to ground.

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Re: Express build bias - strange outcome

Post by sluckey » Sat Apr 22, 2017 9:16 pm

MikeR670 wrote:
Sat Apr 22, 2017 8:59 pm
Yes as per that Modded bassman, that's exactly what I wanted to do, although I don't have the other resistor after it going to ground.
You may want to put a resistor between the pot and ground before you burn something expensive. That resistor prevents you from adjusting the bias voltage to zero, which is a very bad thing.

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Ron Worley
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Re: Express build bias - strange outcome

Post by Ron Worley » Sun Apr 23, 2017 12:41 am

sluckey wrote:
Sat Apr 22, 2017 9:16 pm
MikeR670 wrote:
Sat Apr 22, 2017 8:59 pm
Yes as per that Modded bassman, that's exactly what I wanted to do, although I don't have the other resistor after it going to ground.
You may want to put a resistor between the pot and ground before you burn something expensive. That resistor prevents you from adjusting the bias voltage to zero, which is a very bad thing.

It's also known as an "idiot resistor" by some. But an extremely good idea.
Ron

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M Fowler
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Re: Express build bias - strange outcome

Post by M Fowler » Sun Apr 23, 2017 1:48 am

Some examples from old posts on bias circuits.
biastaps.pdf
Bias_Circuits.jpg
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Mark
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Re: Express build bias - strange outcome

Post by Mark » Sun Apr 23, 2017 10:28 am

The problem is that it now only shows voltage on one leg at -33, but 0 on the other output leg. I wired the original bias output straight to the meeting point of the two 220k resistors, and I added the vr by wiring one side into that same point, and the other side to ground.
If you have the bias voltage going to where both 220K meet then you must have the same voltage present on both ends of the 220K resistors.

The only reasons for an exception is one 220K resistor is open circuit, one output valve has a grid to cathode short, or you've miss wired one of the output valve sockets. Have a look at the amp and see what has happened.
Yours Sincerely

Mark Abbott

MikeR670
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Re: Express build bias - strange outcome

Post by MikeR670 » Sun Apr 23, 2017 7:32 pm

Thanks guys, I'm hoping to get some tinkering time today - I'll report back :)

MikeR670
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Re: Express build bias - strange outcome

Post by MikeR670 » Mon Apr 24, 2017 3:42 am

Mark wrote:
Sun Apr 23, 2017 10:28 am
The problem is that it now only shows voltage on one leg at -33, but 0 on the other output leg. I wired the original bias output straight to the meeting point of the two 220k resistors, and I added the vr by wiring one side into that same point, and the other side to ground.
If you have the bias voltage going to where both 220K meet then you must have the same voltage present on both ends of the 220K resistors.

The only reasons for an exception is one 220K resistor is open circuit, one output valve has a grid to cathode short, or you've miss wired one of the output valve sockets. Have a look at the amp and see what has happened.
Ok this is perplexing. On checking with tubes out - I have bias voltage to the meeting point of the 220k resistors. I pulled both resistors and substituted with a pair of 150k (as I'm running different tubes). They both ohmed correct before I installed them. Measured dc volts and same issue. On the other ends of the resistors I get bias dc on the left and 0 on the right. Still with tubes out. If the bias dc incoming should split at the resistor joint and still give bias voltage on to each resistor end - then I'm stumped.

The only thing I see different between the two sides is that one side has a 100k after the .022 cap, whereas the other side has an 82k - all as per the schematic...

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Re: Express build bias - strange outcome

Post by MikeR670 » Mon Apr 24, 2017 7:00 am

I may have deeper issues here, in tracing power back further I noticed that something might be wrong with the phrase inverter stage. With tubes in (except power tubes), I read b+3 to the phase inverter voltage at around 380 dc. This goes through the individual 82k one side and 100k other side resistors - and onto the phase inverter plates. Plate 6 shows 170 volts and plate 1 shows 365 volts. This is then feeding the two legs with the bias circuit input. I thought voltages coming out of the phase inverter are supposed to be about the same?

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