#124 started

[norburybrook]

Lars see above erm last page.......

[larsvictor]

Yes, cool.

Stand by "on", otherwise there is no voltage on the plates.

[norburybrook]

OK so I have 424vDC measured directly across the plates. Is that about 20v to low?


Also Looking at the pre amp voltages now the amp is working.

V1 I have about 185vDC and V2 around 176v DC, the layout says V2 should be about 10v higher than V1 . Do I need to adjust my dropping string?

Marcus

[martin manning]

B+ depends on line voltage and your PT... I wouldn't mess with the dropping string just yet. Try swapping V1 and V2.

[larsvictor]

Adjusting the dropping string will of course have no effect on the power tube plate voltages. What is the secondary voltage (HT) of the PT ?

[larsvictor]

I suppose you have a 150k resistor instead of the FET circuit. You can lift up the voltages in the string by changing the value of that resistor.

[norburybrook]

swapping the valves gives me now

V1 175vDC and V2 now 190vDC


the power transformer is a fender twin with an extra secondary for the relays so 320-0-320 that's giving 420vDC loaded which is spot on.


what should I set the bias to?

[martin manning]

6L6 is 30W Pa max, so for 60% (a little on the cool side) 30*0.6/420=0.043

[norburybrook]

excellent, that actually made sense....for once....

OK I'm at the limit of my Bias pot with the 27k resistor and I'm only at 39mv

what should I change it to?

M

[martin manning]

excellent, that actually made sense....for once....

OK I'm at the limit of my Bias pot with the 27k resistor and I'm only at 39mv

what should I change it to?

M

Need to go closer to ground. Try 22k. Or, increase the 3k3 a bit.

[norburybrook]

22k did it. got 43mv now.


on the PI trim I've got a 5k trimmer I can only get a 6.7v difference with the trimmer at full.

normally the trimmer isn't fully at one end so you can mark it and then tweak by ear. Is this why the later amps had a 10k? is it worth changing?


the deep switch has more effect on jazz setting than rock, is that as it should be? I've had neither on my previous builds.


tone controls are more subtle than on the skyliner tone stack at first glance.

M

[martin manning]

I'd go to a 10k trimmer. If the two triodes in the PI tube are not matched very well that might not even be enough, but in that case I'd look for a better-matched tube.

I have no real experience with the classic tone stack to comment on that but I could look at it analytically later.

Did you see my post above on the definition of a circuit node?

[norburybrook]

Martin, I did, however I need to go back and re read it as I was concentrating on getting the amp working. It will be another little bit of knowledge gained from a build. I must say this one was a bit of a journey but from it my understanding and comfort factor of schematics has been greatly enhanced which can only be a good thing. I still struggle with the language at times , hence 'nodes' etc but doing the trouble shooting makes you dive straight in the deep end

I'm going to swap out the PI trim.

whats the deal with the OD trim? people seem to set it very low ...edit just found an old post and it seems with a low plate classic 100k to ground when on is OK. I've got mine set at 110k and with a telecaster it's OK.


My amp seems quieter than my 50w 102 however I've used different pots on the volumes, I used these

http://www.mouser.co.uk/Search/ProductD ... HRT02105A2

so they may have a different taper from the Alphas.
Would the slightly lower voltage make the amp quieter? if I removed the FET 150k from the string how much would that up the voltages?


I must say the amp is whisper quiet when it's on hardly any noise at all and I'm sat next to it here in a very quiet studio.


M


p.s. I always buy balanced tubes for the PI

[martin manning]

I think your voltages are fine, but if you removed the 150k FET sim the preamp voltages would go up 30-40V, and the power amp voltages much less than that. Most of the change is caused by a couple of extra mA flowing through the 22k in the dropping string. I don't think a few 10's of volts B+ would make that much difference in overall loudness, but it could be that and some other things such as you mentioned in combination.

Of course we all learn something from these threads, and I think I can see you are catching on to the schematic view of the amp as opposed to just copying the layout. The layout is the practical implementation of what's intended in the schematic, and sometimes the circuit will not function as intended in the schematic unless the proper layout is followed. Precise language is important as well when trying to debug over the web.

[norburybrook]

Indeed Martin, and I can see that just from a schematic there would be many physical ways of wiring up a circuit , things like lead dress and component positioning are not taken into account with a schematic it would seem.

I'm going to make a start on the head cabinet so it has nice clothes to wear. Taylor from amplified Nation sent me a piece of brown Vox cloth and I have just taken delivery of some tan suede fabric so I'm going to embrace the whole suede/vox Dumble thing here

I'll keep this thread going with any other questions I come up with and of course keep up to date with the cabinet making....which will be a first.

with a bit of luck and some tweaking it may end up sounding as good as my kemper


Marcus