Finished my 50W Plexi... channel 1 not working :(

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mad-axeman
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Finished my 50W Plexi... channel 1 not working :(

Post by mad-axeman » Sat Aug 25, 2018 10:47 pm

Hello everyone,

Could I please trouble you folks one more time for some advice. I built this to bass specs, btw

My build is complete, and channel 2 sounds good, but channel 1 is absolutely silent (not a peep with volume all the way up, from either input)

i am including my readings which are particularly sketchy for V1. The only other noteworthy things i can add is that i can't bias to 35-40, as the pot doesn't get me past ~20mV at full rotation. Also the heater voltages are a bit higher than I'd like. Not sure about anything else to be honest.

I have went over the entire amp with a fine tooth comb and don't see anything visually wrong. I just don't have enough smarts to dig any deeper. If anyone would like the challenge, I'd be VERY appreciative.


link to my readings:
https://drive.google.com/open?id=1NAijs ... 8TJL199-V4

link to photos of amp:
https://drive.google.com/open?id=10z__U ... iBirduz_iK
https://drive.google.com/open?id=1948OA ... PXofOw0UbY
https://drive.google.com/open?id=17uFAO ... l-y0yxs7ed
https://drive.google.com/open?id=1FRGw5 ... jEpvaB-Jew


Thank you in advance to all,
Stu

mad-axeman
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Posts: 62
Joined: Mon Jan 11, 2010 11:31 pm
Just the numbers in order: 7

Re: Finished my 50W Plexi... channel 1 not working :(

Post by mad-axeman » Sun Aug 26, 2018 12:13 am

Update:

Now I figured out a way to get some noise (hiss, not guitar signal):

if i turn up volume TWO (the one that works fine) and the presence control up all the way. I do hear a hiss under these conditions, out of channel 1. Still no guitar signal, but at least there's something coming thru.

also to note, regardless of volume settings, turning the presence knob does create noise/hiss in channel one. almost like a scratchy pot sound.

i did try a different preamp tube in all 3 positions with no change

Tazin
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Re: Finished my 50W Plexi... channel 1 not working :(

Post by Tazin » Sun Aug 26, 2018 10:19 am

Why is there a yellow wire attached to pin #8 on the first preamp tube socket?...That should not be there if your running a shared-cathode setup. You should have a jumper from pin #3 to pin #8 on the first preamp tube socket instead.

mad-axeman
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Posts: 62
Joined: Mon Jan 11, 2010 11:31 pm
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Re: Finished my 50W Plexi... channel 1 not working :(

Post by mad-axeman » Sun Aug 26, 2018 10:53 am

GETTIING BETTER:

spoke to Robert at Valvestorm and he pointed out that on the bass build, I should:

"In the board at v1, try tacking a jumper from the corner eyelet (yellow wire) to the unused eyelet nearby. On bass circuit, this connection is usually mad at the v1 socket; but doing it this way will make it easier to try the lead config in the future"

and then there was sound!

my bias is better, but not where it needs to be - V4 @28 and V5 @31; i am suspecting i need to change a resistor value but still need some feedback on that

mad-axeman
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Posts: 62
Joined: Mon Jan 11, 2010 11:31 pm
Just the numbers in order: 7

Re: Finished my 50W Plexi... channel 1 not working :(

Post by mad-axeman » Sun Aug 26, 2018 10:58 am

Tazin wrote:
Sun Aug 26, 2018 10:19 am
Why is there a yellow wire attached to pin #8 on the first preamp tube socket?...That should not be there if your running a shared-cathode setup. You should have a jumper from pin #3 to pin #8 on the first preamp tube socket instead.
thank you for your feedback - i very much appreciate it. i am guessing that this was fallout from not doing the lead specs? I say this because I followed the drawing in the instructions on page 32.

i assumed (in error it appears) that the only changes b/w lead and bass were component choice and loayout on the board, and that everything else would be the same.

I think Robert noticed the same thing and suggested the jumper. does that make more sense? is there more to be changed?

mad-axeman
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Joined: Mon Jan 11, 2010 11:31 pm
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Re: Finished my 50W Plexi... channel 1 not working :(

Post by mad-axeman » Sun Aug 26, 2018 12:12 pm

i changed out the bias resistor as well and now i'm all biased up and FULLY FUNCTIONAL

thanks Robert and Tazin!!

Best,
Stu

Tazin
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Re: Finished my 50W Plexi... channel 1 not working :(

Post by Tazin » Sun Aug 26, 2018 1:59 pm

The circuit you have now is reaaly close to a '68 Bass setup with the following differences:
The negative feedback resistor is a 47K instead of a 27K. The 47K will give you a little more dirt.
You have a 500pF bypass cap on the Bright channel volume pot where as the Bass circuit doesn't.
You have a 10K/2w & a 8K2/2w for the first B+ dropping resistors where as typical '68 - '69 50w amps used two 8K2 resistors.

mad-axeman
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Posts: 62
Joined: Mon Jan 11, 2010 11:31 pm
Just the numbers in order: 7

Re: Finished my 50W Plexi... channel 1 not working :(

Post by mad-axeman » Thu Aug 30, 2018 9:30 pm

Thanks again Tazin. I really like the way it sounds but it is nice to know these things.
Stu

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