Raoul Duke wrote: ↑Sun Jan 28, 2024 2:20 am
Going through 2nd gen build threads and trying to see the different approaches. I like Erwin’s implementation of a PI trimmer. Probably “borrow” that
I seriously considered doing that on my 3rd Gen. I even built the board but after chatting with another builder, whose opinion I deeply respect, he told me it wasn't really necessary. I went without the trimmer on his recommendation and I can't say I miss it.
Hi Ian,
Just like any other ODS build, balancing trimmer settings have a big impact on sound, feel and overtones. So does the 2nd gen ODS with trimmer. I never regret implementing it and would do it again on another 2nd gen build. Hasn't #54 something like that? I will check ny pics.
Raoul Duke wrote: ↑Sun Jan 28, 2024 2:20 am
Going through 2nd gen build threads and trying to see the different approaches. I like Erwin’s implementation of a PI trimmer. Probably “borrow” that
I seriously considered doing that on my 3rd Gen. I even built the board but after chatting with another builder, whose opinion I deeply respect, he told me it wasn't really necessary. I went without the trimmer on his recommendation and I can't say I miss it.
Hi Ian,
Just like any other ODS build, balancing trimmer settings have a big impact on sound, feel and overtones. So does the 2nd gen ODS with trimmer. I never regret implementing it and would do it again on another 2nd gen build. Hasn't #54 something like that? I will check ny pics.
Erwin
Hey Erwin
Correct, #054 has a PI trimmer.
Picture 070.jpg
I hear you on the trimmer and maybe my amp would have been better with it. That being said, there are some wonderful sounding ODSs out there without it... not to mention countless other brands that never needed a trimmer (Fender, Marshall et al). Regarding the impact of the trimmer, I have had more impact on sound, feel and overtones by changing the PI tube than tweaking the trimmer. I'm not saying the trimmer is useless, I just think folks place a lot more value in it than the actual PI tube.
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Plate load values could be specified for a perfectly matched, on-spec tube, or selected to suit the one in the amp, but as soon as it drifts some, or someone changes it out, the exercise would have to be repeated. At least having a trimmer makes that relatively easy.
1. Do I need to stick with the 4 pin XLR for the footswitch, or can I use 3? Got a few high quality 3 pin cables already on hand, plus it looks like one pin goes unused in the pics and layout - so I figured I’d ask even though it seems evident that 3 pin will work (only plan on switching OD).
2. Didn’t realize what a rabbit hole transformers would be since my 102 build basically all came in a box minus boards, hardware, and some resistors - the transformers were already picked out. Looking at PTs in 325-0-325 range (with heater and bias supplies at or above 150ma) I see there’s a lot of variation. What’s the best way to determine which is best?
a. Same goes for the OT. Do I just try to ball park the primary specs of the Bassman OT I previously used or is there a margin to operate within? I understand the secondary part (I think), just trying to put it all together.
Probably really basic questions, but I didn’t dig too deep into transformers in the last build because I really didn’t need to - so now I must learn about them.
I have some pictures of more of a transitional model between a 3rd gen and 4th gen. Has the board layout of 3rd gen, but some values of a 4th gen. Also a trimmer in the PI. Trimmers are cheap and give some extra flexibility. Unless you are building a part for part clone, I suggest installing one.
1 Nothing wrong with using a 3 pin XLR if you only want to switch channels. Might limit a bit when you want to add a feature like PAB, but that isn't a 2nd/3rd gen feature. SSS001 and 002 have 3 pin XLR connectors, So they are Dumble approved .
2 Basically any known good Bassman set will do the job. Dumble used Fender 022871 OT and 022814 PT IIRC. You should have something like 440VDC on the plates. E.g. a Hammond 290EEX would work as a PT. 660V CT loaded. I'd say anything between 660 and 690 V CT @ 300mA would do the job. A small adjustment in the dropping string can always be made. OT should be for a pair of 6L6's, 4k2 primaries and your choice of secondaries. The originals only had 4 Ohm secondaries.
Edit: I sometimes use a 5k OT with good results, as in: it works and sounds good to me. I'm usually sticking to the recipe though: a Bassman clone OT.
ijedouglas wrote: ↑Sun Jan 28, 2024 4:05 am
I seriously considered doing that on my 3rd Gen. I even built the board but after chatting with another builder, whose opinion I deeply respect, he told me it wasn't really necessary. I went without the trimmer on his recommendation and I can't say I miss it.
Hi Ian,
Just like any other ODS build, balancing trimmer settings have a big impact on sound, feel and overtones. So does the 2nd gen ODS with trimmer. I never regret implementing it and would do it again on another 2nd gen build. Hasn't #54 something like that? I will check ny pics.
Erwin
Hey Erwin
Correct, #054 has a PI trimmer.
Picture 070.jpg
I hear you on the trimmer and maybe my amp would have been better with it. That being said, there are some wonderful sounding ODSs out there without it... not to mention countless other brands that never needed a trimmer (Fender, Marshall et al). Regarding the impact of the trimmer, I have had more impact on sound, feel and overtones by changing the PI tube than tweaking the trimmer. I'm not saying the trimmer is useless, I just think folks place a lot more value in it than the actual PI tube.
Is this a 3rd gen Ian? Not only a trimmer, but also 100k plate loads. So 12ax7 PI? But there are some 47n 6PS caps in the PI. Transitional values going from 3rd gen (as we know from schematics on this board to (early) 4th gen?
Part numbers are listed on the layout. Hammond shows 290EX as a replacement for Fender 022814 PT, and 1750L for the 022871 OT. These are Blonde/Black Bassman.
Mojo has transformers for Bl/Bk Bassman too, 774 for the OT, but the PT's they offer are the higher voltage 355-0-355 with 5V for a vacuum rectifier. Theirs are typically made by Heyboer, and are of good quality.
Hammond 194B is the equivalent choke.
Triad F-313X is available from Mouser.
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Last edited by martin manning on Mon Jan 29, 2024 12:05 am, edited 1 time in total.
Just like any other ODS build, balancing trimmer settings have a big impact on sound, feel and overtones. So does the 2nd gen ODS with trimmer. I never regret implementing it and would do it again on another 2nd gen build. Hasn't #54 something like that? I will check ny pics.
Erwin
Hey Erwin
Correct, #054 has a PI trimmer.
Picture 070.jpg
I hear you on the trimmer and maybe my amp would have been better with it. That being said, there are some wonderful sounding ODSs out there without it... not to mention countless other brands that never needed a trimmer (Fender, Marshall et al). Regarding the impact of the trimmer, I have had more impact on sound, feel and overtones by changing the PI tube than tweaking the trimmer. I'm not saying the trimmer is useless, I just think folks place a lot more value in it than the actual PI tube.
Is this a 3rd gen Ian? Not only a trimmer, but also 100k plate loads. So 12ax7 PI? But there are some 47n 6PS caps in the PI. Transitional values going from 3rd gen (as we know from schematics on this board to (early) 4th gen?
Here's another one with a PI trimmer and some nice updates on a 3rd gen eyelet board. Also 100n PI caps it seems. Cool to see Dumble did all these updates or built these amps with some new insights. Also looks like on the boards of #45 and this one, the trimmer is only attached to the board with the wiper and the resistors are simply soldered to the other legs of the trimmer. Apparently modding it this way is also Dumble approved.
Bone stock 2nd gen (at least AFAIK): no trimmer, but also 47k/51k plate loads for a 12at7.
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martin manning wrote: ↑Sun Jan 28, 2024 10:18 pm
Mojo has transformers for Bl/Bk Bassman too, 774 for the OT, but the PT's they offer are the higher voltage 355-0-355 with 5V for a vacuum rectifier. Theirs are typically made by Heyboer, and are of good quality.
Hammond 194B is the equivalent choke.
Yes, this is the iron in my 102. Seemed pretty high quality and I’m loving the way the amp sounds The higher voltage might explain the dropping resistor situation?
I understand now; blonde or black Bassman (usually) but not tweed (or SF/UL - figured that out myself, lol).
As usual, I was probably overthinking it.
Rootz - great pic. Both updates I’m considering on the same board. Very helpful
Are there any resistors on the main board that should have a higher tolerance than 1/2 watt?
Also, looking at Tony’s 2nd gen layout as the pattern for my main board fabrication, I’m trying to figure out how best to insert the OD trimmer. Would the 1.2m and 33k each feed a leg with the out going to the V2B grid?
Raoul Duke wrote: ↑Tue Jan 30, 2024 4:20 pmAre there any resistors on the main board that should have a higher tolerance than 1/2 watt?
That would be "power rating," right? And no, 1/2W will be fine.
Raoul Duke wrote: ↑Tue Jan 30, 2024 4:20 pm
Also, looking at Tony’s 2nd gen layout as the pattern for my main board fabrication, I’m trying to figure out how best to insert the OD trimmer. Would the 1.2m and 33k each feed a leg with the out going to the V2B grid?
I would leave it out. It's the PI trimmer that you want to add, no?
Oops, meant power rating. Forgot tolerance is the percentage of accuracy indicated by the last color band.
As for the OD trimmer, I could go either way - just trying to:
1. make practical mods that increase tweak-ability toward best performance (like the PI trimmer)
2. ensure the OD is “enough” - which I’m fairly certain it will be - but considering it “just in case” I guess
As for the PI - I guess I’m hedging a little there. If I had the technical skill/knowledge/experience of some of the talented builders here, I’d probably be less inclined to add it - but very likely will just to provide me a way to adjust out anything that experience might have helped me avoid through previous lessons learned (and give me a little leeway with tube selection).
Raoul Duke wrote: ↑Tue Jan 30, 2024 6:30 pm
As for the OD trimmer, I could go either way - just trying to:
1. make practical mods that increase tweak-ability toward best performance (like the PI trimmer)
2. ensure the OD is “enough” - which I’m fairly certain it will be - but considering it “just in case” I guess
As for the PI - I guess I’m hedging a little there. If I had the technical skill/knowledge/experience of some of the talented builders here, I’d probably be less inclined to add it - but very likely will just to provide me a way to adjust out anything that experience might have helped me avoid through previous lessons learned (and give me a little leeway with tube selection).
If I were adding an OD entrance level trimmer, I think I would replace the 33k to ground with a 100k wired as a variable resistor.
The PI trimmer is easy: Just do it. 12A_7 tubes rarely have identical sections, and if you go out of your way to select for that, standard plate load resistor values might not result in a balanced output, and the degree to which the power tubes are matched (and remain so) is also involved.
.