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 Post subject: Re: 807 Williamson amp
PostPosted: Nov Sun 04, 2018 6:26 am 
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john8750 wrote:
I need to knows so I can finish your chassis. Do you like my heat sink bolted to the under chassis support? The rear panel ok with you? Front panel changes.
Oh. Well, I need some pics of all that then.


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 Post subject: Re: 807 Williamson amp
PostPosted: Nov Sun 04, 2018 7:14 am 
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Flipperhome wrote:
john8750 wrote:
I need to knows so I can finish your chassis. Do you like my heat sink bolted to the under chassis support? The rear panel ok with you? Front panel changes.
Oh. Well, I need some pics of all that then.




Well, allrightythen :arrow:

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 Post subject: Re: 807 Williamson amp
PostPosted: Nov Thu 08, 2018 5:35 am 
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OK. Got the trim pots in the mail today. Installed them. Using R20 and R18, I adjusted both cathodes to 212mv. Then turned the new trim pot all the way from one end to the other with no real change on the vu meter. What. I disconnected one trim pot and read it at 50ohms, not 500. 500 is marked on the case. So now need to wait till I can get the correct ones. :mrgreen: :mrgreen:

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 Post subject: Re: 807 Williamson amp
PostPosted: Nov Thu 08, 2018 10:13 am 
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john8750 wrote:
OK. Got the trim pots in the mail today. Installed them. Using R20 and R18, I adjusted both cathodes to 212mv. Then turned the new trim pot all the way from one end to the other with no real change on the vu meter. What. I disconnected one trim pot and read it at 50ohms, not 500. 500 is marked on the case. So now need to wait till I can get the correct ones. :mrgreen: :mrgreen:

I wonder if there's a coding mistke... like 50 and 0 zeros ... when it should have been 50 and 1 zero ?
Green blk blk vs green blk brn

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 Post subject: Re: 807 Williamson amp
PostPosted: Nov Thu 08, 2018 5:50 pm 
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Pbpix wrote:
john8750 wrote:
OK. Got the trim pots in the mail today. Installed them. Using R20 and R18, I adjusted both cathodes to 212mv. Then turned the new trim pot all the way from one end to the other with no real change on the vu meter. What. I disconnected one trim pot and read it at 50ohms, not 500. 500 is marked on the case. So now need to wait till I can get the correct ones. :mrgreen: :mrgreen:

I wonder if there's a coding mistke... like 50 and 0 zeros ... when it should have been 50 and 1 zero ?
Green blk blk vs green blk brn





Good point Peter. I dont know. Got a refund instantly, so must have been a common mistake. I ordered another 10 and should be here by the 15th.

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 Post subject: Re: 807 Williamson amp
PostPosted: Nov Fri 09, 2018 1:44 am 
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john8750 wrote:
OK. Got the trim pots in the mail today. Installed them. Using R20 and R18, I adjusted both cathodes to 212mv. Then turned the new trim pot all the way from one end to the other with no real change on the vu meter. What. I disconnected one trim pot and read it at 50ohms, not 500. 500 is marked on the case. So now need to wait till I can get the correct ones. :mrgreen: :mrgreen:
I take it you've decided that the 3.92 Ohm resistors are 3.92 Ohm.

Common error with places that have no clue what they're selling. The code is two digits for value and the third is the decade multiplier so 500 Ohm would be marked 501 (50 x 10^1). But those who don't know read 500 (50 x 10^0), meaning 50 Ohm, as 500 Ohm. Same with any sub 1000 trimmer. You'll often erroneously get a 25 Ohm for a 250 Ohm, a 10 Ohm for a 100 Ohm, etc. Apparently 1000 and up confuses them enough (102 ???) to look up the code but "100" is "obviously" 100 Ohm... except it's 10 Ohm.


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 Post subject: Re: 807 Williamson amp
PostPosted: Nov Fri 09, 2018 4:06 am 
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Flipperhome wrote:
john8750 wrote:
OK. Got the trim pots in the mail today. Installed them. Using R20 and R18, I adjusted both cathodes to 212mv. Then turned the new trim pot all the way from one end to the other with no real change on the vu meter. What. I disconnected one trim pot and read it at 50ohms, not 500. 500 is marked on the case. So now need to wait till I can get the correct ones. :mrgreen: :mrgreen:
I take it you've decided that the 3.92 Ohm resistors are 3.92 Ohm.

Common error with places that have no clue what they're selling. The code is two digits for value and the third is the decade multiplier so 500 Ohm would be marked 501 (50 x 10^1). But those who don't know read 500 (50 x 10^0), meaning 50 Ohm, as 500 Ohm. Same with any sub 1000 trimmer. You'll often erroneously get a 25 Ohm for a 250 Ohm, a 10 Ohm for a 100 Ohm, etc. Apparently 1000 and up confuses them enough (102 ???) to look up the code but "100" is "obviously" 100 Ohm... except it's 10 Ohm.



Dont know. I read 212mv across the 3.92 ohm. Dont really matter, its close enough. But was able to balance the two 807's perfectly. The resistors measure close to 4ohms, the best I can tell.

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 Post subject: Re: 807 Williamson amp
PostPosted: Nov Fri 09, 2018 4:19 am 
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john8750 wrote:
Flipperhome wrote:
john8750 wrote:
OK. Got the trim pots in the mail today. Installed them. Using R20 and R18, I adjusted both cathodes to 212mv. Then turned the new trim pot all the way from one end to the other with no real change on the vu meter. What. I disconnected one trim pot and read it at 50ohms, not 500. 500 is marked on the case. So now need to wait till I can get the correct ones. :mrgreen: :mrgreen:
I take it you've decided that the 3.92 Ohm resistors are 3.92 Ohm.

Common error with places that have no clue what they're selling. The code is two digits for value and the third is the decade multiplier so 500 Ohm would be marked 501 (50 x 10^1). But those who don't know read 500 (50 x 10^0), meaning 50 Ohm, as 500 Ohm. Same with any sub 1000 trimmer. You'll often erroneously get a 25 Ohm for a 250 Ohm, a 10 Ohm for a 100 Ohm, etc. Apparently 1000 and up confuses them enough (102 ???) to look up the code but "100" is "obviously" 100 Ohm... except it's 10 Ohm.

Dont know. I read 212mv across the 3.92 ohm. Dont really matter, its close enough. But was able to balance the two 807's perfectly. The resistors measure close to 4ohms, the best I can tell.
It's easy enough to find out. Put the VOM in series and measure the current. Then measure the voltage. R = E/I


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 Post subject: Re: 807 Williamson amp
PostPosted: Nov Fri 09, 2018 4:46 am 
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That would work better If I had two meters.

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 Post subject: Re: 807 Williamson amp
PostPosted: Nov Fri 09, 2018 4:54 am 
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john8750 wrote:
That would work better If I had two meters.
True, but it shouldn't make enough difference to matter.


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 Post subject: Re: 807 Williamson amp
PostPosted: Nov Fri 09, 2018 6:29 am 
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Flipperhome wrote:
john8750 wrote:
That would work better If I had two meters.
True, but it shouldn't make enough difference to matter.



I know. Just a hassle to connect meter into one circuit, then reconnect. It would take a few cycles.
But I have it pretty close. Just need to trim the VU meters.
I should a just ordered 1K trim pots. BTW-those multi trim pots are dirt cheap nowa days.

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 Post subject: Re: 807 Williamson amp
PostPosted: Nov Fri 09, 2018 6:38 am 
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john8750 wrote:
Flipperhome wrote:
john8750 wrote:
That would work better If I had two meters.
True, but it shouldn't make enough difference to matter.



I know. Just a hassle to connect meter into one circuit, then reconnect. It would take a few cycles.
But I have it pretty close. Just need to trim the VU meters.
I should a just ordered 1K trim pots. BTW-those multi trim pots are dirt cheap nowa days.
Yeah, they're cheap. Same price for a 500 Ohm 10 turn (give or take a dime).


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 Post subject: Re: 807 Williamson amp
PostPosted: Nov Fri 09, 2018 11:43 pm 
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I have a question and I have looked for the answer to no avail.
How was the 3.92 ohm resistor value arrived at?

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 Post subject: Re: 807 Williamson amp
PostPosted: Nov Sat 10, 2018 1:50 am 
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kc5gym wrote:
I have a question and I have looked for the answer to no avail.
How was the 3.92 ohm resistor value arrived at?
Fair question. The short version is I screwed up but that doesn't really answer the question.

Before I got the meters I just guessed at the sense voltage required to hit "100%" on the meter's secondary scale (which is less than full scale). Then divided by 54.6 mA (which is '100%' of bias) to get the 3.92 Ohm. But I didn't put a lot of effort into the 'guess' because I had planned to measure the meter when it arrived. I just had to put in 'something' as a placeholder.

When the meter arrived I put a resistor in series and measured the voltage needed to hit the 100% mark, then calculated from the (resistor) divisor what the mV was across the meter. Again using the 54.6 mA I calculated 2.74 Ohm (instead of 3.92) for the sense resistor. However, I neglected to update the "build" schematic with the new value.

That may have been fortuitous because the 'exact' resistor value was always a bit in question. For one, there was potential error in my resistor divider measurement and then there was the choice of the 'closest' standard value. And I wasn't sure the meters themselves were accurate from one to the other (not to mention different manufacturers). And then, what if 54.6 mA (for a number of reasons) didn't turn out to really be the desired bias point? A fixed resistor just doesn't allow any fine tuning. It is what it is.

The 3.92 Ohm being 'over' the desired target, then using a series pot in line with the meter, allows for any foreseen adjustment above and below the target value so pretty much 'whatever happens' the meter can be zeroed (to 100%).


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 Post subject: Re: 807 Williamson amp
PostPosted: Nov Sat 10, 2018 10:39 am 
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Thank you for the detailed explanation. I have my sights set on building a similar amp.
One other question. In my minds eye, I picture the VU meter after the preamp, before the amplifier..
That way zero can be set for the amplifier input and there would be significant meter movement
even at low listening levels. Is this a "bad idea"?

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 Post subject: Re: 807 Williamson amp
PostPosted: Nov Sun 11, 2018 12:14 am 
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kc5gym wrote:
Thank you for the detailed explanation. I have my sights set on building a similar amp.
One other question. In my minds eye, I picture the VU meter after the preamp, before the amplifier..
That way zero can be set for the amplifier input and there would be significant meter movement
even at low listening levels. Is this a "bad idea"?
It depends. Tube amplifiers have high impedance inputs and the 'VU' circuit (assuming you buy a cheap Chinese PCB) would be in parallel with it. That might (or might not) cause problems with a tube preamp that expects a high impedance tube power amp.

If the volume control is in the preamp (before the VU meters) then it doesn't matter to 'meter movement' whether it's on the power amp's input or output. The power amp has fixed gain (e.g. x8) so it's simply a matter of scaling (e.g. divide by 8 ).


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 Post subject: Re: 807 Williamson amp
PostPosted: Nov Sun 11, 2018 3:57 am 
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Flipperhome wrote:
kc5gym wrote:
Thank you for the detailed explanation. I have my sights set on building a similar amp.
One other question. In my minds eye, I picture the VU meter after the preamp, before the amplifier..
That way zero can be set for the amplifier input and there would be significant meter movement
even at low listening levels. Is this a "bad idea"?
It depends. Tube amplifiers have high impedance inputs and the 'VU' circuit (assuming you buy a cheap Chinese PCB) would be in parallel with it. That might (or might not) cause problems with a tube preamp that expects a high impedance tube power amp.

If the volume control is in the preamp (before the VU meters) then it doesn't matter to 'meter movement' whether it's on the power amp's input or output. The power amp has fixed gain (e.g. x8) so it's simply a matter of scaling (e.g. divide by 8 ).




I connected directly to the speaker output. There are adjustment pots on the driver board, but seem to be set properly already. Dont know about the input circuitry to the driver board. May be an optocoupling, or other high impedence.
So the volume output does effect meter movement. OK with me.

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 Post subject: Re: 807 Williamson amp
PostPosted: Nov Sun 11, 2018 4:13 am 
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john8750 wrote:
Flipperhome wrote:
kc5gym wrote:
Thank you for the detailed explanation. I have my sights set on building a similar amp.
One other question. In my minds eye, I picture the VU meter after the preamp, before the amplifier..
That way zero can be set for the amplifier input and there would be significant meter movement
even at low listening levels. Is this a "bad idea"?
It depends. Tube amplifiers have high impedance inputs and the 'VU' circuit (assuming you buy a cheap Chinese PCB) would be in parallel with it. That might (or might not) cause problems with a tube preamp that expects a high impedance tube power amp.

If the volume control is in the preamp (before the VU meters) then it doesn't matter to 'meter movement' whether it's on the power amp's input or output. The power amp has fixed gain (e.g. x8) so it's simply a matter of scaling (e.g. divide by 8 ).

I connected directly to the speaker output. There are adjustment pots on the driver board, but seem to be set properly already. Dont know about the input circuitry to the driver board. May be an optocoupling, or other high impedence.
So the volume output does effect meter movement. OK with me.
Yes, I suspect they 'plan' on the meters being an 'add on' to an existing amplifier, in which case the output is the easiest place to put them.

Btw, "optocoupling" would likely be low impedance because you usually have to drive an LED (or worse yet, an incandescent).


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 Post subject: Re: 807 Williamson amp
PostPosted: Nov Sun 11, 2018 4:48 am 
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Of course, just wonder about the speaker output impedance when coupled to the driver board input. No problem, I guess?

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 Post subject: Re: 807 Williamson amp
PostPosted: Nov Sun 11, 2018 8:28 am 
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john8750 wrote:
Of course, just wonder about the speaker output impedance when coupled to the driver board input. No problem, I guess?
Nope. The VU board is way too high of an impedance to affect 8 Ohm, especially since speaker impedance varies all over the place anyway.


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