Express Question

joeboo88 · Post by **joeboo88** » Wed Mar 04, 2015 2:40 pm

Hello: I was looking over Colossal's Karen build - what work of amp art.
I noticed the on V1 pin 2 he used a resitor on pin 2?
From the Express layout, I dont see one.
I looked on the schematics and some layouts used a 820ohm resistor and on some schematics that resistor is missing.....
my question because I'm new what is the purpose of that resistor?
Thanks

Post by **Colossal** » Wed Mar 04, 2015 2:56 pm

There is a letter Glen K posted in Ken Fischer's own hand regarding the use of 820R as a tuning option on the entrance to the second stage. Tames a bit of the extra wild hair in the amp. A grid resistor is used to minimize oscillation as well as radio interference.

Thanks for the nice comment.

joeboo88 · Post by **joeboo88** » Wed Mar 04, 2015 3:52 pm

thanks for the explanation: I will try that out on my Express

M Fowler · Post by **M Fowler** » Wed Mar 04, 2015 5:22 pm

Ceriatone has the changes on their layout.

joeboo88 · Post by **joeboo88** » Wed Mar 04, 2015 6:23 pm

Thanks Mark. I used the layout in the files section here. But this will definitely help.
The reason I was wondering, I have scratchiness on the volume pot.
I have been reading on whether to measure with the standy on standby
to measure for DC, if there is any there.
When I chopstick around pin1 on the pot to ground makes noise. I'm going to redo the grounding scheme around the input jack and check to make sure everything is clean.
I kinda wanted to do the grounding like Colossal's Express and not have the buss on the pots but floating near.
The amp still sounds great but I want to make it correct
(well as much as my ability allows) And I really like reading the posts on how guys solved their issues, gives me good insight
Thanks for the help guys

Mark · Post by **Mark** » Thu Mar 05, 2015 1:18 am

I have scratchiness on the volume pot.

Have you measured the wiper of the pot (set midway to full volume and with PI tube removed so you don't deafen yourself) for millivolt DC voltages?

I would eliminate that as a possible fault first.

joeboo88 · Post by **joeboo88** » Thu Mar 05, 2015 1:40 pm

Hi: sorry to sound dopey: I am new to troubleshooting, as I have been lucky with my other 3 builds.

Where do I measure the DC with what you suggest?

I was thinking maybe I have a bad filter cap, and I will be ordering some more stuff this week, but your test would be helpful.....thanks for any info

Mark · Post by **Mark** » Thu Mar 05, 2015 2:11 pm

Hi Joe

Regarding having a scratchy volume pot I would first measure DC volts from the wiper (centre lug of the volume pot) to ground. You should have zero volts there, if you have 10 or so mIllivolts then I'd try a different 12AX7.

joeboo88 · Post by **joeboo88** » Thu Mar 05, 2015 6:40 pm

thanks for the info, Mark

Mark · Post by **Mark** » Fri Mar 06, 2015 10:34 pm

How did you go?

joeboo88 · Post by **joeboo88** » Sat Mar 07, 2015 11:20 pm

Sorry Mark.... I've had to work OT during the last few nights so sleep is off. I pulled v3 and as I turned up the volume I had DC increase?
With no volume no DC. I did pull V1 and no scratchiness....I shouldn't do this stuff with lack of sleep....it seems the tone pots are the cause. With all the tone pots off, the volume doesn't scratch, then I broke a lug. Needless to say I hve no spares, so I did put in an order,but because I live in Canada, I will probably have to wait 2 weeks,so I have lots to read about, about what voltages on the pots or not. Thanks for the advice,I'm gonna take a nap

joeboo88 · Post by **joeboo88** » Sat Jul 25, 2015 1:02 am

Well, it's been a long time, but I managed to fix this.
The problem was I had scratchy volume pot on my express build.
I measured (thanks to Marks advice), 5 volts DC on pin 2 of the pot.
after goofing around a lot.....I tracked the problem to pin 3 of the treble pot.
So the 500pf cap that goes to pin 3, was bad, replaced it and now the amp is the way it should be........

Next problem.
I did the math to get bias on my el34s and to get 17.5 watts I am getting 388 volts on the plates. = .45
My problem I can't set the bias with the 22k trimmer to the right bias, 26mv is all I get to the end of the trimmer.
I have all the right parts and so, on....
I was wondering what I should be doing, or what I'm doing wrong?
Thanks in advance

Littlewyan · Post by **Littlewyan** » Sat Jul 25, 2015 7:41 am

What bias voltage have you got when the EL34s are running at 26mA? Are you measuring the bias with 1R resistors on the cathodes of the EL34s?

joeboo88 · Post by **joeboo88** » Sat Jul 25, 2015 12:18 pm

Hi Ryan. I get 40mv on the 1R resistors.

Littlewyan · Post by **Littlewyan** » Sat Jul 25, 2015 1:14 pm

What voltage do you read on Pin 5 of the EL34s?

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