I was just lookin at this earlier
Amps looks to be a "Transition Generation"look Low plate classic /LNFB on V1B/Standard OD configuration PAB /w accent switch.PCB Style loaded with his old Brown Dales and Q-lines and X5F's I'll bet she sounds good!. Altec Lansing (coffee can) Alnico
The only thing strange about the amp other than the PCB is it only has 50 uF filtering on the plates. looks KILLER!!
The amp has obviously been recapped with replacement TVA Atoms.
HoytODS.jpg
I just caught CW thread. Got more pics and info.
Tony
I guess this as a good a topic as any to ask this question. I have yet to have a look in or see a photo of a Low Plate Classic that does not have the 2k7/270 ohm accent switch configuration. I have seen a few converted High Plates but not a Low Plate. One of the things that I am not loving about the Low Plate Classic I have built is the stiffness on the top end and not as round and loose in the lows and mids as I wanted. So I clipped a 10k across the 4k7 and a 1k across the 390 ohm and it sound pretty darn nice. It looks like there is a 500 ohm trimmer in series with the accent cap and then the switch. I guess having the 2k pot in series with the presence cap would not mess with it too much. I also think that the .01 V1b coupler makes more sense in this configuration. Tony or anybody else what do you think?
CW
I was just lookin at this earlier
Amps looks to be a "Transition Generation"look Low plate classic /LNFB on V1B/Standard OD configuration PAB /w accent switch.PCB Style loaded with his old Brown Dales and Q-lines and X5F's I'll bet she sounds good!. Altec Lansing (coffee can) Alnico
The only thing strange about the amp other than the PCB is it only has 50 uF filtering on the plates. looks KILLER!!
The amp has obviously been recapped with replacement TVA Atoms.
HoytODS.jpg
I just caught CW thread. Got more pics and info.
Tony
I guess this as a good a topic as any to ask this question. I have yet to have a look in or see a photo of a Low Plate Classic that does not have the 2k7/270 ohm accent switch configuration. I have seen a few converted High Plates but not a Low Plate. One of the things that I am not loving about the Low Plate Classic I have built is the stiffness on the top end and not as round and loose in the lows and mids as I wanted. So I clipped a 10k across the 4k7 and a 1k across the 390 ohm and it sound pretty darn nice. It looks like there is a 500 ohm trimmer in series with the accent cap and then the switch. I guess having the 2k pot in series with the presence cap would not mess with it too much. I also think that the .01 V1b coupler makes more sense in this configuration. Tony or anybody else what do you think?
CW
CW
A couple of things. It sounds to me like you want "less" GNFB..(loosen) I would start first by going straight for the GNFB resistor only. Although the NFB and tail resistors (390)voltage divider work together to give you the FB ratio. If you change the tail resistor your altering the gain structure along with the overall balance of the PI and could alter the sound of the amp. (If you like the sound GREAT!?) I generally leave the tail alone since Dumble has a pretty smooth PI there.But that's me?
Remember the 4K7/390 is for a 100W (I thought you built a 50W)so the GNFB resistor on the 50W amp needs to be approx 70% of the value it would be in a 100W amp to keep the negative feedback (7.7%) ratio the same as the 100 (on 4 ohm tap). If it's the frequency response you want tweaked you can always change the 1uF presence cap like he did with 102.
Too much GNFB will definitely squash the low end and an excessive amount the amp will motorboat
Does this answer your question?
Tony
" The psychics on my bench is the same as Dumble'"
Well let me put it a different way. As the only low plate classics I have seen have the 3.3k/270 ohm I have to assume that those are the values Dumble intended for that circuit. Yes I much prefer this configuration with this circuit. Top end is hard and kind of ugly with the 4k7/390. I just am not sure why it has been assumed that a low plate classic has the 4k7/390. OK a little more on this. The 100w or 50w thing just sunk in but the only problem with that is the amp in this thread IS a 100w(Borderland is a 50w with 2k7). As is the amp in the attached photo. If you change just the 4K7 you mellow out the top end but loose some of the gain in the mids and bottom. Changing the 390 to 270 rounds things out a bit. To my ears, everything makes more sense with the 3k3/270. The PAB is rounder and nicer, jazz/rock sounds nicer, and the deep switch sounds nicer less boomy. To me, putting the more raw, aggressive GNFB with this circuit is like putting a PAB boost with a 2ND gen. I guess some people like that. I also notice that none of the Low Plate Classics I have seen have a high frequency taper. Now I just have to decide whether I like it with the existing presence cap set up or am going to modify the presence pot to be the 500 ohm trimmer and add a switch.
CW
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CW
I don't know if there is a question in there you want me to address so I'll just say that the feedback factor is a combination of both tail and NFB resistors and is calculated 390/(390 + 4,700) = 0.077. So, the circuit basically feeds back 7.7% of the output at the 4-ohm jack @100W. Using the same calculation the 270/3,300 H = .075 or 7.5%
Now the closed loop gain can be calculated Acl = 41/(1 + 41 * 0.077) = 9.8 for the 390 and 10.0 for the 270. as you can see not that much different so it's hard for me to draw any conclusions as to what you could be hearing based on the different NFB circuits? Could be the tubes/output transformer/Plate voltage/PI balance
I don't know?. It sure sounds to me like you are hearing more NFB with the 270/3k3 amp. Were your tests done on different amps?
As far a the treble bleeder in the Low Plate "Classic" AFAIK Robbens 102 had one in it's original LPC form had one installed at some point?. I think Dumble removed it later when he did the high plate Skyline update (according to a tech I know who has worked on the amp) So I would guess it would depend on the customer's needs as to if one was installed. I remember seeing a few scattered here and there in some LPC amps over the years.
Actually I never really noticed the different presence set-ups so thanks for pointing it out. I will for sure keep that in mind when I go over any classics I come across perhaps we can get some kind of consensus going and mention it in the LPC build thread layout.
Thanks
Tony
Last edited by talbany on Fri Jun 21, 2019 3:23 am, edited 1 time in total.
" The psychics on my bench is the same as Dumble'"
Thanks Tony, yeah I get what you are saying with the math. It is just to my ears, that small change in GNFB circuit really changes the vibe of the amp. Just know, I am listening to these values by clipping in resistors, so I can go back and forth pretty quickly. I guess a question would be, Have you ever seen a Low Plate Classic with the 4K7/390? I see it in Skyliners and High Plate classics, but never a LPC. I only bring this up because it may be something someone wants to try if they build this circuit and find the high end a bit to brash and pushed. None of this is intended to question your layout, I am grateful for your effort in putting that together.
CW
Charlie Wilson wrote: ↑Fri Jun 21, 2019 3:23 am
Thanks Tony, yeah I get what you are saying with the math. It is just to my ears, that small change in GNFB circuit really changes the vibe of the amp. Just know, I am listening to these values by clipping in resistors, so I can go back and forth pretty quickly. I guess a question would be, Have you ever seen a Low Plate Classic with the 4K7/390? I see it in Skyliners and High Plate classics, but never a LPC. I only bring this up because it may be something someone wants to try if they build this circuit and find the high end a bit to brash and pushed. None of this is intended to question your layout, I am grateful for your effort in putting that together.
CW
No problem and thanks for bringing that up useful info for sure!! Always interested in your findings 124 was is a strange amp!
Tony
" The psychics on my bench is the same as Dumble'"
Thanks Tony. Yeah I was just thinking that with the 3k3/270 the 2k presence pot doesn't do anything at all until just before half way up and that a 1k that measured a bit high would be perfect. I guess Dumble had the same idea. I am curious about the math in the previous post. You stated the 4k7/390 combo would feedback 7.7% of the output and the 3k3/270 would feedback 7.5%. Does this mean that there is more negative feedback with the 4k7/390 combo? That seems kind of reverse of what I would assume. Also, your calculation for changing the tail resistor(closed loop) assumes the feedback resistor stays the same(4k7). I am assuming that if the tail and feedback resistor are changed then the tail resistor gain is the same. It looks small on paper but that small amount of gain increase does make a sonic difference(rounder, more mids and lows).
CW
Charlie Wilson wrote: ↑Fri Jun 21, 2019 7:58 pm
Thanks Tony. Yeah I was just thinking that with the 3k3/270 the 2k presence pot doesn't do anything at all until just before half way up and that a 1k that measured a bit high would be perfect. I guess Dumble had the same idea. I am curious about the math in the previous post. You stated the 4k7/390 combo would feedback 7.7% of the output and the 3k3/270 would feedback 7.5%. Does this mean that there is more negative feedback with the 4k7/390 combo? That seems kind of reverse of what I would assume. Also, your calculation for changing the tail resistor(closed loop) assumes the feedback resistor stays the same(4k7). I am assuming that if the tail and feedback resistor are changed then the tail resistor gain is the same. It looks small on paper but that small amount of gain increase does make a sonic difference(rounder, more mids and lows).
CW
CW.
Heading out of town and don't have much time but will try to answer your question as best I can.
Yes more signal. if you look at the "tail" of the PI is the load at which the feedback voltage is developed.(it's the "ratio" both tail and FB resistors). As far as the small difference goes I don't know but just to put that into perspective the Bluesmaster/Marshall PI generates .045 0r 4.5% (looser)now that I can hear!. I am thinking what you are hearing is most likely the PI balance (tail resistor) which does affect the sound but again I cannot say for sure as I would have to do more empirical testing to tell? (I am NOT saying you don't hear it)
Here is a wonderful explanation of the subject. I am pretty sure my formulas correct but feel free to double check me
BTW. I have messed around with both types a long time ago to try and improve the range on these things. IIRC they both worked about the same. Almost nothing until you get to around 1 o'clock on the pot
Charlie Wilson wrote: ↑Fri Jun 21, 2019 7:58 pm...I am curious about the math in the previous post. You stated the 4k7/390 combo would feedback 7.7% of the output and the 3k3/270 would feedback 7.5%. Does this mean that there is more negative feedback with the 4k7/390 combo? That seems kind of reverse of what I would assume.
It's just a voltage divider: 390/(390+4k7)=0.0766, and 270/270+3k3)=0.0756... virtually the same.
Charlie Wilson wrote: ↑Fri Jun 21, 2019 7:58 pmAlso, your calculation for changing the tail resistor(closed loop) assumes the feedback resistor stays the same (4k7). I am assuming that if the tail and feedback resistor are changed then the tail resistor gain is the same. It looks small on paper but that small amount of gain increase does make a sonic difference(rounder, more mids and lows).
What's happening to the presence cap? If you don't change that the upper shelf frequency will shift up a half an octave with the 270 tail. Seems like maybe this is more about tuning the presence while keeping the FB ratio constant.
Tony, your math is off on the closed loop gain. It should be very close either way as you found since the FB ratio is constant, but that open loop gain of 41 in Aiken's example is for a Marshal amp and 16 ohm load, as he explains. You would need to get that number for the specific case you are working.
OK, then would it be safe to say that the 4k7/390 with a 1uf presence cap is a specifically tuned feedback/presence network that shows up in Skyline and High Plate Classics and that the 3k3(or slightly lower)/270 with a .1 presence(accent) cap appears to be Dumbles choice for a Low Plate Classic? Given that there are at least four photo examples of amps with this configuration and none with the 4k7/390.
CW
Charlie Wilson wrote: ↑Fri Jun 21, 2019 11:32 pm
OK, then would it be safe to say that the 4k7/390 with a 1uf presence cap is a specifically tuned feedback/presence network that shows up in Skyline and High Plate Classics and that the 3k3(or slightly lower)/270 with a .1 presence(accent) cap appears to be Dumbles choice for a Low Plate Classic? Given that there are at least four photo examples of amps with this configuration and none with the 4k7/390.
CW
Yes but I am still interested in what it is you are hearing? and still want to do some more testing.
Thanks for pointing it out anyway!
Tony, your math is off on the closed loop gain. It should be very close either way as you found since the FB ratio is constant, but that open loop gain of 41 in Aiken's example is for a Marshal amp and 16 ohm load, as he explains. You would need to get that number for the specific case you are working. /quote]
Yes I know it was for 16 but thanks for pointing it out
Tony
Last edited by talbany on Sat Jun 22, 2019 12:14 am, edited 2 times in total.
" The psychics on my bench is the same as Dumble'"
Martin, if I lifted the presence cap out of the circuit(as it is with the accent switch) should I hear no difference in gain or frequency response with either 4k7/390 or 3k3/270?
CW
Charlie Wilson wrote: ↑Sat Jun 22, 2019 12:00 amMartin, if I lifted the presence cap out of the circuit (as it is with the accent switch) should I hear no difference in gain or frequency response with either 4k7/390 or 3k3/270?
I think that's correct; the difference in the PI tail resistance should be negligible.
talbany wrote: ↑Fri Jun 21, 2019 11:55 pm Yes I know it was for 16 but thanks for pointing it out
Besides the speaker load, the difference in the gain of the PI and of the output tubes is in that number.
Here is where I show how boneheaded I am at the technical stuff. It just seems to me that the feedback frequencies have to travel across either a 4k7 or a 3k3 resistor before it gets to the junction of the divider and that would have some effect on frequency response. I get that because of the divider the voltage would be the same at the junction point.
CW
I guess some people like that. I also notice that none of the Low Plate Classics I have seen have a high frequency taper. 
(I am NOT saying you don't hear it)