A tale of 2 tweed champs

[Shack]

OK, so I built a tweed champ, with a power tranny for a princeton....like 325-0-325 , and used Dougs layout and parts.....thing sounded amazing.

So I decided id use another tranny to build one for my other guitar player.....this tranny is 250-0-250

I smoked my output tranny during practice, and I suspect it was a bad ground connection on the speaker jack, as the chassis was powder coated, and I didnt get bare metal.

I installed the new OT, and did measurements....and this is what I get...

bias resistor- 459 ohm
voltage drop from bias resistor- 20.5V
Plate voltage-338V
plate current-.044ma
and plate dissipation is at 14.8 W

Does that sound reasonable, and should I use the same value bias and dropping resistors with his too to start with? AND.....could that current cook the OT, or was it probably the poor ground from speaker jack?

I love these amps

[sluckey]

You need to subtract the cathode voltage from the plate voltage to do the calculation. P_diswill only be 14W.

Hard to say if 44mA could cook that OT since we don't know the ratings for the OT. Did you have the OT common lead soldered to the speaker jack? If so, the poor ground would not be a factor for burning up the OT, but it would be a factor with the negative feedback. More likely cause... playing the amp wide open long time trying to be heard over the drummer.

[Shack]

hmmm, yes the common was soldered to the jack, and also yes, I tried to play over a drummer.....so why do you think I cooked it, since it has a new one soldered in, and im building a second amp? So ya say that maybe we shouldnt use these amps at our loud rehearsals?

[Shack]

the output tranny is a classic tone champ transformer with the 8k and 5k primary and the 8 and 4 ohm secondary.....uncle doug aid he preferred the 5k primary, but everywhere else says 8k, so thats what I used

[Shack]

and it is rated for 50 ma DC

[sluckey]

6V6 would likely prefer the 8K. If you played that little bedroom amp long and loud, that's likely the cause for the smoked OT. Maybe you need a super reverb for band practice. Or mic the champ.

[Shack]

Sluckey, I have higher powered amps....you guided me through them lol... this little guy just sounded so good and it wasnt even dimed and cut through ,....oh well..

Well onto the second champ....first one is back in business. With the different power tranny, should I at least try to match voltages to preamp or use the same dropping resistors to start with? Still waiting on OT.

Also, is the tone circuit the only difference between the tweed champ and princeton? I may put the tone circuit into my buddys amp if it is, and it appears that way to me.

[shooter]

fwiw;
I know some of the tweed mojo comes from a saturated OT, but I've found after building a couple, using an "oversized" OT gets me  better results, they run cool, keeps sound tight, and the players have enough addons in front to make up for a properly oversized OT

[Shack]

I know that this amp sounds great but the 2 trannys get pretty warm....and I definitely do not want to replace this one again. The 6V6 doesnt get too hot , but if it runs 40ma at idle, and I like to crank it up , wouldnt the the tube be exceeding the 50 ma rating of the OT by alot at times?

I will see what the second one does with same OT, but lower voltage PT, when its finished.

[shooter]

If you have a single tube, SE, then the tube is running about as hot as it gets at idle no signal, but if you're driving so hard it's basically a squarewave, then increasing B+ might be the better option, you'll be less likely to square out the signal, and keep current somewhat tamed

[PRR]

If correctly loaded(?), a SE audio amp runs the *same* DC current idle or maxed-out. (This is in fact one test for "correct loading".)

BTW, the tube runs *cooler* when maxed-out. On one old bottle, on dummy-load, I could hear it creak like the pipes after you turn-off a car, and could watch numbers drop on my IR thermometer.

However the Output transformer in audio amplifiers normally does not run much hot. Is it too close to a tube? The hot PT?

[Shack]

OK, this is where I am at.....the first Champ is back together with a new Classic tone OT, and sounds awesome, so I suppose I wont try to use it at band practice anymore. The third OT never came , and I dont even know how to contact Triad because they never answered my email. So, i ordered a tranny from antique electronics supply, that is 8W, and a 5k primary to 8 ohm secondary......installed in the second Champ which is now a 5F2 Princeton, since I added a tone ontrol.

Since I used a diffferent power tranny....250-0-250, and a GZ-34, with the same dropping resistors as the Champ , I have only 85 volts on the plates of the 12AX7....amp works and sounds ok, but not even close to my first build.

the Plate voltage is 332 VDC
Plate current is 35ma
and looks like 11.7W dissipation

I clipped the bypass cap out of the first stage and the amp sounds pretty good, but gets fizzy when I turn the volume or tone up all the way, and the headroom is nowhere close to the amp that I have already built.....altho that amp cant be played cranked without smoking the OT


What do yall think would cause low plate voltage on the plates of the preamp? crap tube or wrong dropping resistor in power supply?

I want to make a good amp for my bandmate

[Shack]

So I put a different 12AX7 in and the plate voltage went up to like 100-120 volts .....helped a bit. I still think ill try different resistors in the power supply to get headroom up a bit, and maybe change bias resistor to get it a little hotter like my other one....maybe I shouldve left the cathode bypass cap in there until I tried a different preamp tube, but im gettin there. I threw the made in china 12ax7 in the trash.

[shooter]

I threw the made in china

Ya, the Russians have them beat hands down

[Shack]

those were tubes I had from forever ago, and didnt realize made a difference. I will make this amp rock like the other build.....I have learned so much here with every build ive done

[ALBATROS1234]

i dont mean to sound like an audiophool but i can tell substantial differences in different tubes of the same model. i have a yugoslav EI long gray plate 12ax7 which is super crunchy, some super nice early 50s sylvania which sound crisp but smooth, i dont like 50s ge they sound flat to my ears. ratheons seem btw ge and sylvania. i dont have any rca, amperex,mullard or telefunkens (although the EI was supposedly made on telefunk gear) the sovteks i have from 20 years ago sound not quite as good as old ge 12ax7s and the china type i have it pure shite. dont even get me started on power tubes i got several types of 6V6,6L6,el84,7189,6aq5,6G6,6F6,6K6 etc and each type has its own flavor which to my ears falls short with newer russian or china stuff. but within the same types i find rca 6v6 godlike with sylvanias a solid second place,national union and tung sol a close third and raytheons are nice but to me the ge tends to take up the rear again. i have a handful of older 6p3s soviet types that are pretty good including on which says ratheon in yellow. point is in my experience of tube rolling there is a noticeable difference . so i usually try different ones i have on hand until i find which one sounds the best in whatever circuit i am messing with. an to my ear a used but in good shape old tube sounds better than any new tubes i have tryed. but i am kinda new so i have only tube rolled hundreds not thousands and mainly good ole usa types . do yourself a favor and get a 1950s rca 6v6 i was surprised the clarity and balanced sound they have . i only have 7 of them but they are my 7 favorite.

[ALBATROS1234]

ive done the same thing but usually i dont waste a tranny with a ct high voltage winding and a 5 v winding on a bridge rectifier. not when you can find used control transformers and filament transformers for little or nothing. my first 4 or 5 amps were built this way . i reverse feed the control tranny and take the 240 winding with a bridge to get approx 330vdc which will handle more milliamps than a tube amp will ever give most are rated btw .150 and .300kva then you buy a $10 fleabay 6.3v @ 3a filament transformer and you are set.look in you closet you might find a 7 volt wallwart. i even used a small 120v to 12v transformer i got for free to run a 6SL7 and 6V6 in series. being an electrician i get free wire cutoffs and usually get slightly used control transformers for free so my first 5 tube amps cost me lest than $50 each to build. of course i used sections of heavy gauge 2x8 metal stud and perfboard which cost $10 for a multipack of boards. basically bought pots,knobs,jacks,caps and resistors.

[sluckey]

i even used a small 120v to 12v transformer i got for free to run a 6SL7 and 6V6 in series.

Did you do anything to get the voltage to split equally across those tubes? If you just simply wire them in series one tube will get 4.8V and the other will get 7.2V.

[ALBATROS1234]

i assume you mean because of the different current draws of the filaments. no, i didnt but it worked and is still working, i gave that amp to a coworker a couple of years ago who liked that particular amps tone. i have given away most of my early cobbled together amp builds to friends who wanted them. it was done in the spirit of finding the line of what is possible and what isnt. i am a tinkerer not a kit builder and enjoy experimentation. the amp pictured above even used a control transformer as an output transformer(all the way to the right behind the tubes) it was a 277v to 24v step down and compared to the other ot i had on hand at the time it sounded far better, never got hot even when played and left on for hours, so luckily the impedance was within range of what the circuit needed. i actually had a western electric se 8k to 4ohm ot in it first but i needed it for another project and when a friend wanted me to build him a practice amp i experimented with what i had on hand and the 277/24 sounded better than the western electric one which should have been appropriate. it had better bass response higher output and overall sounded better in every way so it stayed in and to this day its in that circuit and my buddy still uses that amp for practice. i got lucky on that one i guess.

btw that is actually a sylvania loktal 7F7 in the v1( a loktal version of 6SL7) and syvania 6V6 thats why i called her Sylvia.

[shooter]

i have given away most of my early cobbled together amp builds to friends who wanted them

   
I did the same, a few have got'n legend status among a small group of guitar folk 

[ALBATROS1234]

as you well know shooter even a fair sounding tube guitar amp sounds pretty damn good, they're sort of like sex and pizza right