5902 design problems

[UR12]

dana i actually emailed you about the transformers you had built. i modeled my first pi and power tube section off the schematic you said you used the trainwreck schematic. i found it to not work for me. but i might be making newbie mistakes. i am aiming for a 2 channel build with a fender clean and a mesa style lead. i actually worked my bias off yours, where you said you set your bias to 30 31vdc-. i am going to rebuild the pi tonight and see what i come up with again. i have been making careless mistakes in my past tries ex. not grounding my pi and output section properly, measuring my bias off the pot and not at the tubes which was about 10 volts or so more. im very glad that you guys have chimed in i was starting to think i would be sailing alone on this thread. will post some updates as soon as i get the time to work on this one. i have some mixers that have been eating up my bench time. one thing im curious about is how can i lower my screen voltage more. i am using a 5 henry choke at 90 ma with a 103 ohm resistance. i figured that would be all i could do unless i took out the choke or got another one with higher resistance.

The PI values I used are the same as used in the TW Liverpool. I had to adjust the resistor value in the feedback circuit for the presence control from the normal 100k found in the express to a 47k because there was not enough negative feedback with the 100k to make the presence work properly. As far as the choke, I used a 2k resistor instead of a choke for the screen supply. The rest of the power supply nodes are identical as the express 20k, 10k, 10k.... Dropping resistors. Both the Lil Devil and the Microplex are using fixed bias.

[Disabled_shredder]

hmm. i used the 90 kelly schematic. one side of the pi was coming out ok and the otherside was completely wrong. i was thinking of using the super six fender schematic pi but that went bunk too. so far ive come up with trying eiher 33k or 47k on the plates of the pi and 470k on the screens and then 1k and 18k on the cathode and leaving the negative feedback off until i get a clean signal. also using a .01 and .1 caps on the blocking caps. im not really sure why things are coming out bonkers for me. i will try the liverpool values and see what i can get. i have repaired a large number of amps but this is my first design and build i expected some problems but i feel like im being brick walled.

[Disabled_shredder]

one thing does come across my mind. every pi ive built the pi input side comes out ok but the other side is absolutely wrong. any thoughts?

[Disabled_shredder]

one thing i have had a problem with is my output transformer one side resistance is 115 and the other side is 153. which means that my b+ is 164.8 on one side and 160.3 on the other and i have a negative dc of 31.5 and -48 on the other. i think this ot is bad or very mismatched im gonna give ce a call and see if they can make an exchange

[Disabled_shredder]

well ce is going to set me up with hammond so i can send it back to them to get another one sent to me. im sure i have more problems than just the resistance between sides but its a start on that section. am i wrong for thinking the transformer is off by too much?

[Disabled_shredder]

ok. so i swapped the lines going to the control grid and that made a big difference in voltages. now my b+ is equally 170.9 but my screens are 171.6 but i have a 470r resistor. guess i need to change that to a larger value obviously. the bias evened out and is -29.9 on both. but one time i took a reading and it was -15 and -48 then i turned the amp off powered back up and a reading of -29.9. im getting very frustrated with this amp fast. i still have a cleanish signal with the input side of the pi but not on the opposite. im going to rebuild the pi with the liverpool values and take some voltage readings. i need to get a little more organized.

[UR12]

I'm using a 1k and 39k on the cathode of the PI and 100k resistors for the PI plate resistors and this is on the 6112 PI. Also use 1 meg from the PI tail to the grids of the 6112. That's how my PI is set up. I would think that the lower value resistors on the plates and grids may be throwing things off a bit. Your tranny may be a little mismatched but I think most are a little but not as much as you are seeing. I'm just not a big fan of the off the shelf Hammonds. Unfortunately I don't have any OTs like I use in stock that I could sell you at the moment.

[Disabled_shredder]

I'm gonna go breadboards the pi and see what comes up. And I agree with off the shelf transformers but I couldn't justify the almost 100$ for heyboer to wind me one without proof of concept. Once I guarantee that my design will work and sound good then I wil order some custom transformers. I've already got buyers but I can't afford to invest on a maybe. I've got way more things I need first. Like a power amp and speakers to go with my dummy loads and scopes and function generators. Would be nice to hear as well as see. I appreciate all the info and help. Just wondering, 4 5902s would be about 3k on the primaries correct?

[katopan]

I'm sure you'll work it out. As Dana said the 5902 are great little valves and I also had no problem with them. The amp I linked is a submini version of the 18 Watt Superlite TMB. The only issue I had was that the 6112 preamp valves didn't bias anywhere near right with standard 12AX7 resistor values. Hence the changes I made as described on my website.

Yes, a quad of 5902s in push-pull would need around a 3K primary.

Looking forward to your next update.

[Disabled_shredder]

I think I'm gonna have to lower some values somewhere I am getting double on the non inverting side as to inverting side. So I've got about 90-100v on the plates 1-2 v on the screens and 30-40 v on the cathode. I do not like bread boarding but it was alot closer to what I wanted as compared to the perf board I did. And I think the mini 18 watt values might work best I did the load line math from aikens white pages and that's what I got was ur values. Ailken actually only lives 30 minutes from me.

[katopan]

I'm having trouble understanding what you're finding is wrong with the PI. Can you describe it a bit more? If every one you've built (I'm assuming with subminis, not ever one forever?) doesn't work right, then it's not the resistor values. Double check the pinout? Are you using a 6111 or 6112 for the PI?

[Disabled_shredder]

ok. let me start by saying i am not very scope savvy. i dont use them often and when i do i rarely know what im doing. but what i mean is when i put the scope on the scope on the non inverting side with a 1k signal i get about 2v ptp full signal form with .5-1vpd? and on the inverting side i get a cut off signal that raises to about 1-1.5v top half only on the same setting and i get similar views from the cathode too. now this last try was alot better mostly because i think i grounded the scope this time. i am taking very close consideration to the pinout. i have never looked at a pi on the scope before so im not sure what im looking for i mainly use my scope for looking at mixer channels for noise when im done servicing them. another issue is i am hesitant to build any more of the amp until i guarantee that this section works and i dont have a way to listen to the actual power section, or i am to scared to for fear its a bust. i am going to bread board your pi today katopan because you do not have nfb loop i did the load line math and i came up with what you did except for the plate resistors mine were lower. another question should i use a 6112 or should i use a 6111. i figured the 6112 would be too much gain. but it looks like im having better results since i tried the 6112. i can also take some photos of my build thus far and post so you can actually see what kind of mess im making if that would help.

[dorrisant]

A little bit of help here: http://ampgarage.com/forum/viewtopic.ph ... sc&start=0

[Disabled_shredder]

my scope is set for 2v p2p my generator is set for a 1k signal i have the whole thing running through a variac and have it dialed with 120v input. and that is a dummy load on top capable of 300+ watts. just a chance for me to brag and show off my shop swag. haha anyway i have the pi set up exactly like ur 12 said with 100k plate resistors, 1 meg grid resistors 1k and 39k by way of potentiometer for the cathode with a 1k to ground from the pot. i guess that makes since. as you can see my inverted side is about half my non inverted side on the plates. also i have my scope set up for ac coupling not tv line like in the picture. my second channel is connected to the dummy load to see the output of the amp but i dont have it connected to the 1/4 in connection

[Disabled_shredder]

here is a picture of the 2 signals together. and how the scope is actually set up. thanks again to everyone who has chimed in with knowledge it has been overly helpful. i have a mentor who is actually retiring and who has taught me the past few years but he doesnt go into great detail about scopes for some reason and ive never had a problem that came up with that involved him putting a scope on the pi so never saw how that was done. he i guess expects me to know it already but then again 3 years ago i had no idea what most components were or did and ive been running a successful repair shop for close to 2 years so hes not leaving that much out.