Hi all,
Compliments of the season to all of you great folks. I hope all of you had a fantastic festive season and I wish each of you all of the very best for 2013.
Some of you will recall, I finished building my Express clone, Tarryn, late last year. The amp was stable and well behaved from the start, although it was a little on the buzzy side at low VVR levels at the stage when I recorded my now infamous sound clips. I have subsequently fixed that buzziness as well as generally tightened up the amp by installing new DC blocking caps, thanks to some advice from Richie, so all in all I am very happy with the amp.
Having said that, I'm also bored now with no other projects on the horizon, and so I thought it was time to learn some more and possibly fiddle a bit.... which got me to thinking about my amps voltages. I recalled from my build process, that a number of my voltage readings were quite different to the "Expected" voltages that were included on the voltage chart I downloaded from this site. I've attached my voltage chart below.
What you will see when you look at my chart is that my plate voltages particularly in the preamp and PI are a fair bit lower than the "Expected" voltages.
My questions in this regard are as follows:
1.) How important are these "Expected" voltages, particularly the plate voltages, and how closely should I be attempting to be matching these "Expected" on the chart ?
2.) Reading the chart, are any of you experienced guys able to predict what tone characteristics the amp is likely to have? ie, is it likely to be breaking up sooner due to the lower than expected plate voltages in the preamp, or what effect are these lower voltages likely to have, and what is likely to happen to the amps tone by somehow increasing these plate voltages?
3.) Is there anything in particular you would change in this build, for instance, change the values of any of the dropping resistors, plate resistors etc?
4.) Does this chart tell you anything else I should be considering looking at, and if so, what?
Thanks guys,
Shaun
Tweaking an Express Clone
Moderators: pompeiisneaks, Colossal
Tweaking an Express Clone
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Re: Tweaking an Express Clone
No takers? 
Re: Tweaking an Express Clone
I did have a look at it:
The expected voltages imply an expected 15mA through the 1k B+1 -> B+2 dropper resistor and 4.8mA through the 2x9k1 B+2->B+3 dropper resistors. Therefore the B+2 node should be sourcing ~10mA (5mA per screen). The expected screen & B+2 voltages imply 8mA per screen which is inconsistent with the previous, but to be expected if these are 'aggregate' voltages from several builds.
Your voltages (**assuming std. circuit**) imply 21mA through the 1k & 5.3mA through the 2x9k1, so your B+2 node is sourcing ~ 16mA, but the remainder are nominally the same.
Your screen & B+2 voltages imply a screen current of 6-7mA - probably within the limits of accuracy consistent with your B+ voltages.
At this point I meant to look back to see if you had made any mods to the amp, but never did.
In short, I don't see anything untoward...
hth!
The expected voltages imply an expected 15mA through the 1k B+1 -> B+2 dropper resistor and 4.8mA through the 2x9k1 B+2->B+3 dropper resistors. Therefore the B+2 node should be sourcing ~10mA (5mA per screen). The expected screen & B+2 voltages imply 8mA per screen which is inconsistent with the previous, but to be expected if these are 'aggregate' voltages from several builds.
Your voltages (**assuming std. circuit**) imply 21mA through the 1k & 5.3mA through the 2x9k1, so your B+2 node is sourcing ~ 16mA, but the remainder are nominally the same.
Your screen & B+2 voltages imply a screen current of 6-7mA - probably within the limits of accuracy consistent with your B+ voltages.
At this point I meant to look back to see if you had made any mods to the amp, but never did.
In short, I don't see anything untoward...
hth!
Re: Tweaking an Express Clone
hmmm.. do you have the PT primaries on 240? Since your wall voltage is 236. Your heaters are a little high.. best is to be on 6.3 or lower imho.
If voltages are a little less in the preamp stage, doesn't it mean less headroom and a smoother sound? Isn't that what Eddie van Halen did with his plexi's to get the brown sound? Might not be a bad thing.
If voltages are a little less in the preamp stage, doesn't it mean less headroom and a smoother sound? Isn't that what Eddie van Halen did with his plexi's to get the brown sound? Might not be a bad thing.
Re: Tweaking an Express Clone
Thanks for the replies guys.
Re the question around what primaries I am using. Officially, voltage in South Africa is 220V, but I thought it best to use the 230V tap, but I left the 220v and 240v taps long enough, just shrink wrapped the ends, in case I needed them at some stage.
Re mods, most of the circuit is standard. I have a different bias resistor in place to get me into the correct bias range. I also have a VVR fitted, along with DC blocking caps on V1, otherwise no other mods.
Re the question around what primaries I am using. Officially, voltage in South Africa is 220V, but I thought it best to use the 230V tap, but I left the 220v and 240v taps long enough, just shrink wrapped the ends, in case I needed them at some stage.
Re mods, most of the circuit is standard. I have a different bias resistor in place to get me into the correct bias range. I also have a VVR fitted, along with DC blocking caps on V1, otherwise no other mods.