LTPI ?

[catnine]

As many here know the tweed fenders for the most part had split load PI's There is another name for them too but I forget Cathodine perhaps ?

I have done some research of the two types and I'm not entirely sure but I think some fender amps even BF and perhaps SF amps like the princetons mainly only used spit load PI's .

I just don't really understand the reason fender changed to the LTPI so perhaps someone could clue me in. I don't know one tenth as much about amps as many here do , I have only been at it since 2004 and basically only built simple fender designs like champs or 5E3 's or princetons SE and P-P .

At one point I thought the LTPI might change the way my build sounds and I have read that a split load with a gain before it like the 6G2 has has more gain than a LTPI . I see some fenders that have the LTPI and no gain before it just the preamp like the 5G9 tremolux yet that amp has a simple tone control without a lot of loss most BF and SF fenders had the BF stack with either a vol, treb and bass and some had a vol, treb, mid and bass and all have the preamp then a second gain to recover the tone stack loss .

So what will I gain installing a LTPI in my 6G2 build ? A less gain sound or a different tone . I like the sound of the amp as is just thought since fender leaped on the LTPI perhaps it's better in some way.

[loudthud]

The LTPI can supply more signal to each power tube than the split load or Concertina can. 6L6 amps need the extra drive to get to their limit around 40 or 50 watts. The early tweed amps with 6L6s only made about 25 to 30 watts.

[catnine]

The LTPI can supply more signal to each power tube than the split load or Concertina can. 6L6 amps need the extra drive to get to their limit around 40 or 50 watts. The early tweed amps with 6L6s only made about 25 to 30 watts.

I don't know if I have enough room on the eyelet board since the LTPI requires more resisters and the eyelet board I have in this build is a weber 5E3 board , My build has 6V6's in it now The Allen Amps PT TP25 can run both 6V6's and 6L6's and the Allen amps OT is a TO25 which can run either 6V6's or 6L6's that OT is basically for a deluxe only beefed up to use 6L6's . I have no idea what this amp puts out in watts Allen amps makes a version like the fender princeton 6G2 and he uses the same tranny's I have and claims 15 watts with 6V6's and 25 watts with 6L6's and it uses a split load PI .
A fender BF deluxe reverb states 22 watts on the fender field guide and 15 watts for a 5E3 and 12 watts for a 6G2 brown princton. 15 to 18 watts for the 5G9 tremlolux which uses a LTPI as does the AA763 deluxe reverb . All use 6V6's . Basically I don't want more gain or less gain my amp does not break up until at 7 and by then it's very loud through one 12 " speaker . I have no idea how they come up with watt ratings. What even mine is it is louder than a fender blues Jr . I also has a real strong bottom end possibly because of the OT in it. I did have this build with the 6L6's in it and ran two 8 ohm 12's with a GZ-34 rect and it was real loud . I built a taller cab for it and slid the chassis right in and had one twelve above the other for a 4 ohm load. Trouble is with this OT I can run 6V6's with a 8 ohm load or 4 ohm load . with 6L6's it's a 2 ohm load or 4 ohm load and I only have 8 ohm speakers so I can't run the one twelve with the 6L6's @ 8 ohms . The tall cab is just to heavy and more than I need unless I was gigging and I'm not. I guess If I did use a LTPI I would use the specs from the 5G9 LTPI since the rest of the amp is real close to mine.

I just like to tinker yet the two builds I have sound pretty good as they are and another build is not in the cards right now. My build only had 2 9 pin sockets no room for more and I use V1 as the preamp 12aY7 and V2 is the second gain after the tone stack and the PI , I left V1 by itself so I can use a 12ay7 by itelf but I only use on triode.

[Chuck H]

And to add... Not taken into account by many inexperienced builder/designers with this sort of question is that a split load PI SOUNDS different from a LTP. Lemme qualify this. Provided there is adequate swing voltage for the power tubes to generate their expected output, the type of phase splitter shouldn't matter. BUT, some types of guitar amps are overdriven at least back to the stage feeding the PI. Split load PI's don't overdrive well in a way that is musically pleasing. There are things that can be done to make them less bad. But the LTP has the split load whipped when it comes to OD quality and clipped signal integrity. That doesn't explain why Fender would have changed the design. Since at that time amps still weren't expected to be overdriven. But from a modern design standpoint, if you are planning a build, it's something to consider. Will the amp be overdriven? Will it be a strictly clean amp? Or will it be a high gainer that cascades preamp tubes for distortion and the power amp won't ever be clipping? If the amp will be clipping the power tubes, and likely the PI, then the LTP is the way to go IMHO. If the amp will be clean only and will have high Vp on a pair of big bottles then the LTP is again the one to use. If the amp will be clean only and has lowish voltage on the power tubes then a split load would be fine. Some could argue that lowish voltages and overdriven still accomodates a split load PI. That's how the 5E3 is. Not my thing, but valid anyway. My point is just that the PI type is best chosen to suit the overall design. And I think that Mr. thud is spot on with the reason Fender would have switched.

[Merlinb]

AFAIK, Fender adopted the LTP in 1957 when it swapped from 6L6s to 5881s, although the 5881s are not really any more difficult to drive, so Leo's design change remains an interesting puzzle!

[catnine]

And to add... Not taken into account by many inexperienced builder/designers with this sort of question is that a split load PI SOUNDS different from a LTP. Lemme qualify this. Provided there is adequate swing voltage for the power tubes to generate their expected output, the type of phase splitter shouldn't matter. BUT, some types of guitar amps are overdriven at least back to the stage feeding the PI. Split load PI's don't overdrive well in a way that is musically pleasing. There are things that can be done to make them less bad. But the LTP has the split load whipped when it comes to OD quality and clipped signal integrity. That doesn't explain why Fender would have changed the design. Since at that time amps still weren't expected to be overdriven. But from a modern design standpoint, if you are planning a build, it's something to consider. Will the amp be overdriven? Will it be a strictly clean amp? Or will it be a high gainer that cascades preamp tubes for distortion and the power amp won't ever be clipping? If the amp will be clipping the power tubes, and likely the PI, then the LTP is the way to go IMHO. If the amp will be clean only and will have high Vp on a pair of big bottles then the LTP is again the one to use. If the amp will be clean only and has lowish voltage on the power tubes then a split load would be fine. Some could argue that lowish voltages and overdriven still accomodates a split load PI. That's how the 5E3 is. Not my thing, but valid anyway. My point is just that the PI type is best chosen to suit the overall design. And I think that Mr. thud is spot on with the reason Fender would have switched.

See that's the thing. My build is not the same as a 5E3 because I do not like the high gain sound a 5E3 provides and it does not take much to over drive a 5E3 the way the tone stack is set up . I just took the tone stack from a 6G2 princeton and used the two inputs I had and after V1 a 12ay7 I spliced in the 6G2 tone stack and let V2 be the second gain and split load PI . All I did was change the power section of the 5E3 to a fixed bias and between the 6G2 tone stack and fixed bias and larger PT and OT I ended up with a loud clean amp with plenty of bottom end , you really have to crank it up to get it to break up. My build has much higher voltages than a 5E3 and I use 6V6's. I can't say the power tubes clip or not. I am not into high gain amps that clip the preamp tubes or PI. In fact I'm not sure if my amp breaks up at all . My build is really a more powerful 6G2 without the tremlo with voltages close to what a BF deluxe has without the BF tone stack loss.

I know fender was after cleaner sounding amps when the BF came along and I haven't played many larger fenders except a 67 super reverb and a 67 bandmaster that I used to own . I did have a 1994 tweed fender blues deluxe that had two channels and a OD circuit but I didn't care for the OD part much . These were all 40 watt amps with 6L6's . But the SR and BM didn't break up all that much unless they were cranked up .

[Chuck H]

It would be interesting to scope your amp at different points when it begins to clip. I think it's possible that your actually clipping a stage prior to the power tubes first. Although you say your amp is loud, without knowing the details you can't know if it's possible to make it louder and cleaner. And, FWIW. Don't confuse clean watts with where the volume knob is set when an amp begins to break up. They are two different things. For example, putting the 12ay7 into your amp didn't make it cleaner. Not at all. It only made your amp have less gain for a given volume knob setting. That doesn't equal cleaner due to greater head room. And it doesn't equal cleaner at a louder setting. It means exactly that your amp is cleaner because it has less gain. Your amp was just as clean at the same volume before you swapped in the 12ay7. The only real difference is that now you are using a higher knob setting.

[tubeswell]

AFAIK, Fender adopted the LTP in 1957 when it swapped from 6L6s to 5881s, although the 5881s are not really any more difficult to drive, so Leo's design change remains an interesting puzzle!

And AFAICT the 5G9 (circa '57?) was the first 6V6 amp to have an LTP.

[catnine]

It would be interesting to scope your amp at different points when it begins to clip. I think it's possible that your actually clipping a stage prior to the power tubes first. Although you say your amp is loud, without knowing the details you can't know if it's possible to make it louder and cleaner. And, FWIW. Don't confuse clean watts with where the volume knob is set when an amp begins to break up. They are two different things. For example, putting the 12ay7 into your amp didn't make it cleaner. Not at all. It only made your amp have less gain for a given volume knob setting. That doesn't equal cleaner due to greater head room. And it doesn't equal cleaner at a louder setting. It means exactly that your amp is cleaner because it has less gain. Your amp was just as clean at the same volume before you swapped in the 12ay7. The only real difference is that now you are using a higher knob setting.

At first I tried a 12ax7 in V1 and the amp had a grainy sound at even low levels so I tried a 12at7 and it was better but still broke up at low levels then the 12ay7 at the same level was clean and strong and clear. I tired all three tubes at the same vol level just to test it out . I tried a 12ax7 again just the other day and set the vol at 2 when the amp begins to offer bedroom vol and it had that same grainy sound , turned off the amp put the 12ay7 back in left the vol at 2 and it was just as loud but clean sounding. Certainly if I raised the vol with the 12ax7 the amp had more gain and breakup yet at the same level with the 12ay7 it was still clean and I could not hear less volume just less breakup. I understand that more gain sounds louder the thing is I had to keep the vol at 2 or the amp was to loud and it was the same with the 12ay7 I think a 12ax7 is 100% and a 12ay7 is 40% or 60% in gain . I built two other amps just likem this build only they had less powerful PT and OT and they all sounded gainy and harsh with the 12ax7 and 12at7 but in them I never tried a 12ay7. As soon as I dialed the vol past 2 they were harsh sounding but not noticably louder . In this build I even added a switch to add the 6G2 NFB loop and take out the 25 uf bypass cap on the second gain stage and tried the 12ax7 like a 6G2 uses and it still sounded harsh but then I had to turn up the vol to 4 to equal what it sounded like at 2 with no NFB and the bypass cap in circuit. The only way this build sounds clean at any level is if I use a 12ay7 whether I have the NFB or not . I added the switch to make the amp a 6G2 which uses a 12ax7 preamp yet it still sounded harsh more than louder so for some odd reason the 12ay7 works great with or without the NFB loop but I still have to turn the vol up if I use the NFB even with the 12ay7 . perhaps it's because my build has a higher plate and B+ than a stock 6G2 I can't say my plate on the 6V6's is 417 VDC which is much higher than a 6G2 so maybe that explaines it I can't say. I am no expert I just know what I hear . Yes more gain = more breakup yet I do feel more gain is the illusion of more volume . You can have an amp with a two channel or one channel with a gain control , turn up the gain stage and it's certainly louder but at the same time if you lower the master vol the gain remains the same . True? I do know what you are saying and respect that. I know the 12ay7 has less gain than a 12ax7 and it is the preamp then this singal is sent to the second gain stage one triode or a 12ax7 after the 6G2 tone stack then the same tube second triode split load PI and it would make sense that since the 12ay7 offers less gain that the amp would have a lower volume at the same vol setting yet isn't it possible because with less gain and more clean headroom and more bottom end that with the 12ay7 that the amp would sound just as loud at the same vol setting because what you lose in gain you get back in headroom and bottom end ie a fuller sound. On my build I have no way to control gain other than to change the preamp tube and I have not idea if it breaks up before the power tubes but it has to be cranked past 7 to get any breakup at all with the 12ax7 at 2 it has that harsh sound that I would not call gain or breakup just harsh and brittle.

[Chuck H]

If you are overdriving a stage prior to the power tubes then it's possible that different preamp tubes, since they will bias differently when plugged into the same circuit, could change the actual headroom of the amp as it is. BUT... If the amp were operating in a more typical way, that is, the power tubes are the first to clip and each previous stage in order, then the effect you hear shouldn't happen. There is no way to know if this is the case without testing the amp.

Another thing that could be happening is that the frequency response of different stages is being affected by the different tubes different operating points and characteristics. The PERCIEVED volume may not represent the actual watts being produced. Again, there is no way to know if this is the case without testing.

As to "grainy" sound, I think this is probably indicative of different bias conditions for the different tubes tested. You could see exactly what is happening on a scope. In which case, if each tube tested were adjusted for bias before evaluation, your listening results may have been very different.

[catnine]

If you are overdriving a stage prior to the power tubes then it's possible that different preamp tubes, since they will bias differently when plugged into the same circuit, could change the actual headroom of the amp as it is. BUT... If the amp were operating in a more typical way, that is, the power tubes are the first to clip and each previous stage in order, then the effect you hear shouldn't happen. There is no way to know if this is the case without testing the amp.

Another thing that could be happening is that the frequency response of different stages is being affected by the different tubes different operating points and characteristics. The PERCIEVED volume may not represent the actual watts being produced. Again, there is no way to know if this is the case without testing.

As to "grainy" sound, I think this is probably indicative of different bias conditions for the different tubes tested. You could see exactly what is happening on a scope. In which case, if each tube tested were adjusted for bias before evaluation, your listening results may have been very different.

I biased the preamp the same meaning the 6G2 witha 12ax7 is biased the same as a 5E3 with a 12ay7 . The 5E3 useds a 820 ohm bias resister and a 25 uf bypass cap so since I used one triode of the 12ay7 I raised the bias resister to 1.5K and still used the 25 ud bypass cap . this is the way the 6G2 12ax7 is biased a 1.5K cathode and a 25 uf bypass cap . The V2 I used is the same as a 5E3 for the second gain stage and the split load PI . Now perhaps just increasing the 5E3 's 820 to a 1.5K ohm for one troide of the 12ay7 makes a difference I can't say yet from what I've seen and read this is the proper way to do it and it may also have something to do with the make of the tubes I used. I know a 12ay7 draws more current than a 12ax7 per triode. Plus my build is adj fixed bias which a 5E3 is not so perhaps that comes into play.
Would a wiring diaghram help with voltages?

[Chuck H]

Would a wiring diaghram help with voltages?

Positively. Better would be a schematic. Following wiring diagrams is a PITA. Further, what usually gets posted in these cases is something like a standard 5E3 diagram and vague explaination of what's different from what's posted. I don't ihink I'm alone in saying that in these cases I don't even bother to look it over. It's an hour I'm never getting back! a schematic that's accurate to the discussion is really the only way to get a considered response.

The bias point I was discussing isn't the resistor used in the circuit, but the actual bias. The different tubes will bias differently. Even if you make considered changes to the circuit you can't know that the 12ax7 wasn't biased cold or hot compared to the 12ay7. A scope is the best way to check for assymetrical clipping. It's possible that the 12ay7, in the circuit you adjusted for it, was closer to center bias and therefor capable of more headroom before clipping. But this is probably a moot point since your not likely to get any clipping from the first gain stage anyhow. But the other considerations of how the different operating characteristics can change the frequency response of the circuits they exist in still holds. And again, the only way to really know what's going on is to test with bench gear. Not that it matters much. Guitar amps are about the sound as much as the science. All my final tweaks are done by ear. Not with meters and scopes. So if you like the way it sounds and performs it's great. But I'll stand by what I implied. Which is simply that there is no reason a correctly functioning amplifier should produce more clean watts with a lower gain tube in as the first amplifier.

[tubeswell]

I biased the preamp the same meaning the 6G2 witha 12ax7 is biased the same as a 5E3 with a 12ay7 . The 5E3 useds a 820 ohm bias resister and a 25 uf bypass cap so since I used one triode of the 12ay7 I raised the bias resister to 1.5K and still used the 25 ud bypass cap . this is the way the 6G2 12ax7 is biased a 1.5K cathode and a 25 uf bypass cap . The V2 I used is the same as a 5E3 for the second gain stage and the split load PI . Now perhaps just increasing the 5E3 's 820 to a 1.5K ohm for one troide of the 12ay7 makes a difference ...

Don't forget the effect of the load on the bias point. A single triode needs 2 x the load of paralleled triodes to get the same operating point with a 'doubled' Rk.

[catnine]

Don't forget the effect of the load on the bias point. A single triode needs 2 x the load of paralleled triodes to get the same operating point with a 'doubled' Rk.

What would I go by to set the proper bias point on one triode of the 12ay7 . When I first built this amp I used both triodes of the 12ay7 and left the 820 ohm cathode resister ie I wired the two triodes in parallel yet I did not like the sound . It's difficult to recall the details since it was an amp in progress at the time because it started out as a 5E3 but I didn't have a 12ay7 at the time and only knew about the different gain valuse each 12a-- had so I tried a 12at7 and that didn't sound very good and a 12ax7 just made the amp breakup even more . I was at that time I realized it was the way the two volumes and one tone pot were wired in the 5E3 so at that point since I only had one vol and one tone that I chose to use the tone stack that is used on the 6G2 princeton which is common to the 5F2A princeton and then I had more control ie the amp did not start out loud and the more the vol was turned up really all I got was more breakup which is what the 5E3 is known for . Yet no having a 12ay7 it was still not quite what I was after so I got the 12ay7 and still had the same bias on both triodes which may have been the wrong bias point for a 12ax7 . Later on I took the 12ay7 and removed the parallel hookup and changed the bias resister to 1.5K and the amp was where I wanted it . After that I thought well since the 12ax7 is used in a 6G2 princeton and the only difference in my build at this point since I had added an adj/fixed bias like the 6G2 all I had to do was add on switch so I could in one position add the 56K NFB resister and remove the 25uf bypass cap across the second gain stage just like the 6G2 has and flip the switch the other way and no NFB and bypass cap back in circuit .

I did that and once I stuck the 12ax7 in place of the 12ay7 it was back to more gain and early breakup. Below are the schems of the 5E3 and 6G2 .


I tried to hand draw out my build yet it was not good enough . My build is set up tube wise ie V1 V2 V3 V4 and rect like the 5E3 .
Now take the vol and two tone pots away and 4 input jacks right up to the grid of V2 , take the input jacks of the 6G2 to V1 .V1 with a 12ay7 with one triode wired in and a 1.5k cathode bias resister and 25uf bypass cap . From the plate of V1 use the exact setup of the 6G2 and after the tone stack point of vol wiper and tone stack goes to the grid of V2 (5E3) meaning the second gain stage of the 5E3 . The 6G2 uses one tiode of V1 for the preamp and the other as the second gain stage . Leave out the trem circuit mine is just like the 5E3. Remove the cathode bias of the 5E3 and insert the fixed bias of the 6G2 (my PT has a 50 volt bias tap ) .Use the node resister values of the 5E3 but a 30 UF then 16uf and 16uf . That is my build other than the switch I added which removes the 25uf bypass cap across the second gain stage bias resister 1.5K 5E3 has 6G2 does not and add the 56K ohm NFB resister per 6G2 . or flip switch back to 5E3 . Don't know what the voltages are on a 5E3 but to compare to the 6G2 mine has 410 VDC B+ instead of 315 VDC .

[tubeswell]

What would I go by to set the proper bias point on one triode of the 12ay7 . ...My build is set up tube wise ie V1 V2 V3 V4 and rect like the 5E3 . ... From the plate of V1 use the exact setup of the 6G2 and after the tone stack point of vol wiper and tone stack goes to the grid of V2 (5E3) meaning the second gain stage of the 5E3 . ... mine has 410 VDC B+ instead of 315 VDC .

'Proper' bias point is a matter of subjectivity. I sorta worked out that the HT for V1 on the thing you described is ~365VDC give or take. A 12AY7 modelled load line for that HT with a 100k load and a 1k5 Rk is as follows:



You got about 6V P-P of grid swing before it starts grid-current limiting/clipping. (Each one of those grid curves on that chart is 1V apart). That is more than enough to keep most guitar p'ups clean through V1 without a pedal in front. However if you go to Rk = 2k7 you got about 8V P-P grid swing clean:



Is that any help?