norburybrook wrote: ↑Tue Apr 10, 2018 12:25 pm
I had similar thoughts. I think it's not just a voltage thing, as I've had amps before where changing the pre voltages didn't make as much difference as I'd thought.maybe with a lot of these things if you're running the amp flat out it might do but who in reality ever does that?
I could swap the first dropping string resistor and try and get another 20v overall and report back.
Anyone care to tell me what I'd need to change the first resistor to from 2k2 to get a higher overall voltage down the line?
M
Running flat out has IMO little to do with it. If your v1 on the anode is 160V with the 12AT7 installed on v3, and it's 175v on the anode on v1 with the 12AX7 on v3, you're gonna hear that. I'm not saying you should alter anything it's more a curiosity thing.
Is it possible for you to do a measurement with the 12at7 on v3?
For the dropping string you could try a 1k8 resistor.
Edit: when doing this you probably have to bias again.
Maybe going from 160 to 175 is enought to hear in the past i remember going from say 190 to 205 and not hearing any difference. Maybe the sweet spot is quite wide say 175-205 and inside that there's not a lot of difference.
Is this the Ceriatone layout? If so, it's not compatible with yours. Look at the #124 layout, at the right there's also a footswitch wiring as I recall correctly.
I'm not touching anything at the moment as it sounds good. Unfortunately I'm using my EVM 12 cab...i don't want to put an EVM12 in a combo....argh!! let's see when I build the cab what works best. it's a shame there isn't a 'neo' EVM12 L
When this amp was designed, HAD was mostly using Altec 417 speakers, so maybe it's voiced for something kind of "lively"?
Paralleling something across the 2k2 is a good way to test the effect of higher preamp voltages. Swapping a 12AX7 for a 12AT7 with no other change results in very different operating points for the PI triodes due to the plate load and bias resistor values being about half a typical 12AX7 LTP, so this is confounded with the voltage change.
martin manning wrote: ↑Tue Apr 10, 2018 3:43 pm
Paralleling something across the 2k2 is a good way to test the effect of higher preamp voltages. Swapping a 12AX7 for a 12AT7 with no other change results in very different operating points for the PI triodes due to the plate load and bias resistor values being about half a typical 12AX7 LTP, so this is confounded with the voltage change.
The difference was quite substantial sonically Martin, I was pleasantly surprised.
Amp has been on all day now, it's very quiet, with a guitar plugged in even on high volume settings there's very little noise at all, unless you hit the OD switch then there's some hiss, but it's not made a pop or a squeak or a crackle all day sat there idling.
I'm just letting it cool down then I'll give it a go with a higher voltage.
martin manning wrote: ↑Tue Apr 10, 2018 3:43 pm
Paralleling something across the 2k2 is a good way to test the effect of higher preamp voltages. Swapping a 12AX7 for a 12AT7 with no other change results in very different operating points for the PI triodes due to the plate load and bias resistor values being about half a typical 12AX7 LTP, so this is confounded with the voltage change.
Yes correct and I assumed Marcus did change the PI resistors(you mentioned it a few posts back). Confounded that's a new english word for me; mixed up?
Could go either way I guess on the plate loads and bias resistor. I see there is a note on Tony's layout that says a straight swap is an option some prefer. Yes confounded is mixed together in such a way that the two effects can't be separated.
adding a 9.1k in parallel to make a 1.77k has raised the v1 and v2 plate voltages to up around the 175v area, the PI is still running around 315vdc.
Can't say I noticed any tonal changes but I'm going to leave it like this for now as I feel 175 is a good ballpark voltage for V1 and 2.
It will be interesting when it get put in a combo how it will sound. You generally loose bottom end in small cabinets so speaker choice will be interesting.
Just ben looking at the Altec 417, it's an Alnico speaker! Anyone tried a series 2 ODS with say a Celestion gold/blue high powered Alnico, is it a good modern alternative to the Altec?
martin manning wrote: ↑Tue Apr 10, 2018 3:43 pm
Paralleling something across the 2k2 is a good way to test the effect of higher preamp voltages. Swapping a 12AX7 for a 12AT7 with no other change results in very different operating points for the PI triodes due to the plate load and bias resistor values being about half a typical 12AX7 LTP, so this is confounded with the voltage change.
Yes correct and I assumed Marcus did change the PI resistors(you mentioned it a few posts back). Confounded that's a new english word for me; mixed up?
Erwin, I didn't change the resistors as the layout indeed said they could be left.
What would swapping them for say 100k or so do to the sound? would you just change the plate resistors or the whole thing? wouldn't that then make it something else?
If you change the plate loads you'd change the bias resistor too, making it a typical Dumble 12AX7 PI. If it sounds good as-is go with it, unless you are keen to experiment. I agree 175 or better is a good place to be for the V1 plate voltages.
Here's the wonky foot switch....I actually measured and marked everything out but should have perhaps drilled pilot holes as the centre punch didn't hold the large drill steady...hence the wonky LED distances. All functional though so I doubt I'll change it.
DSC_1667-01.jpeg
Sent from my Nexus 6 using Tapatalk
You do not have the required permissions to view the files attached to this post.
Lower voltages for V1/V2 could help smoothness (it could feel a bit stiff at higher voltages as the amp is more raw overall).
That amp really shine with NOS tubes, even more so than later gen, it was night and day for me.
Keep trying the AT, good quality NOS are available at a good price, I like both after running the amp for a while.
I didn't change anything regarding plate PI resistors (47k/51k) whith a 12AX. 70's owners seems to change that tube according to taste but the amps (and 13/40/etc) were designed around 12AT7.
I would lower output stage feedback (or use a pot).
Those amps need to be open a bit more volume wise than a 102 to do their thing, and I like closed back cabs better.
Look at Tony Talbany 13 build, lots of info on that thread and another tonestack. I went for a classic TS after a while.
13 is less responsive, 40 a bit more but act strangely, classic was a bit better and more flexible and PAB option was nice too on OD.
You could remove one of the two caps on the bass pot (3rd gen tonestack).
I would ground the OD input on the OD relay like later amps (better cleans).
I would bias the amp a bit hotter than later amps.
And last but not least you could tweak the funky OD entrance or the output network (1M/180k). My amp was not balanced properly between clean and OD. Check the Kittyhawk amps FR exemple.
Last edited by mojotom on Tue Apr 10, 2018 6:39 pm, edited 2 times in total.
Just ben looking at the Altec 417, it's an Alnico speaker! Anyone tried a series 2 ODS with say a Celestion gold/blue high powered Alnico, is it a good modern alternative to the Altec?
Could be interesting, the Celestion Alnicos have a sort of "hifi" quality where you can hear little details of pick attack and such.
I did a search a few weeks ago for comments on a modern substitute for the Altec 417, hoping that Weber or WGS or Skumback or Eminence or someone might have a replica. Only info I found was that the guy who makes the Sebago Overdrive Deluxe thinks that the Eminence EJ1240 is the closest thing.
in the past i remember going from say 190 to 205 and not hearing any difference. Maybe the sweet spot is quite wide say 175-205 and inside that there's not a lot of difference. 
