Plexi in a Princeton - One input, other input to bypass preamp wiring question.

[jasonvilla]

Hi all, Im building my last amp EVER and I had a few questions and some general "please check my work" type stuff.

The input - The Princeton chassis has a spot for two inputs. I want one input for guitar and the other input to bypass the preamp. Like an FX return. I've been using my Pod Go a lot lately and it sounds pretty good and opens up a lot of options when I run it into the FX return of my store bought amp. I have no desire to build a true FX loop. Ill never use. But I thought it would be cool to have the option to use it has a powered guitar speaker box, for a Helix type set up, or a stand alone guitar amp.
So, I could use the other input hole for a switch, to switch between V1 and where ever I stick the "fx return". Or maybe I could use a second input jack to wire straight. Im using isolated/switched cliff jacks. I wasn't sure the best way to go about this.
How would you guys do it? Thank you!

[tubenit]

I would do a passive FX loop with the FX send input on the back and the FX return on the front as you're suggesting.  That way you have both worlds of an actual FX loop AND using the FX return as you're suggesting.

IF on your Marshall, you used a mosfet as the cathode follower, and then used the available triode as an FX return ……. you could then have an FX loop without adding any tubes. *  see attached schematic as example

With respect, Tubenit

[jasonvilla]

Thanks for the reply tubenit, im actually putting in a one tube reverb in this build as well! :D It worked pretty well in a 2204 type build I made years ago.

Would inserting a signal work how I have it pictured? I dont think id be able to have the jack switched like a normal input jack or else the signal would have a path to ground when the "return" wasn't used... I can always build it and see if it worked or makes noise if i try to insert a signal before the MV.

I found a schematic from a thread you wrote back on that is very close to what Im building!

[sluckey]

That's where I'd put it.

[Latole]

Yes IMO

[jasonvilla]

The amp is working, so Im happy about that. I still need to make some adjustments -  waiting for the speaker to get here.

But in the mean time... Can I reduce the "dwell" or drive of the one tube reverb by replacing the 1M resistor (used a 1M resistor instead of a 1M pot) to 500k? Or would I need two resistors to form a voltage divider as if the 1M pot was turned down?
Thank you

[shooter]

if i understand, your dwell pot is a fixed 1M R?


If so, just put another in parallel and see what you think


If it's a pot be happy on 2, white out the higher numbers 

[jasonvilla]

Haha thank you shooter. The quality and amount of the reverb is good, i just dont want it to hang around for so long.

if i understand, your dwell pot is a fixed 1M R?


If so, just put another in parallel and see what you think


If it's a pot be happy on 2, white out the higher numbers 

[jasonvilla]

500k seemed to either do nothing or make it worse. I have a 12dw7 in there now. Ill try a 12ax7 next. If Im still not happy I guess I can take the cathode bypass cap off the driver stage or just get a short decay tank?

[AmberB]

I'm assuming that you don't want to add another pot to the amp for a dwell control?

[jasonvilla]

I'm assuming that you don't want to add another pot to the amp for a dwell control?

I dont have room for it even if I did! ... I suppose I have room for a trimmer pot, though. The board is installed so taking it out to install turrets is a no go. I would rather just have one setting I like.

[ac427v]

I did not find a "dwell" control to change the length of the reverb signal. I think it is mislabeled in popular usage. It does help reduce distortion if the driver tube or tank are being driven too hard. My advice is to try a 1M "dwell pot" in a temporary installation before drilling any holes. If it works you can install fixed resistors on terminal strips or tweak the 150k mix resistor to a smaller value.

Short or medium decay tanks give me the length of reverb that I prefer. YMMV.

[jasonvilla]

I kind of remember that to be the case - Its been years since Ive screwed around with this one tube reverb circuit but I know I had one in my last amp and took it out and wired it into a resonance control. I think a short decay tank is what I need! :)

I did not find a "dwell" control to change the length of the reverb signal. I think it is mislabeled in popular usage. It does help reduce distortion if the driver tube or tank are being driven too hard. My advice is to try a 1M "dwell pot" in a temporary installation before drilling any holes. If it works you can install fixed resistors on terminal strips or tweak the 150k mix resistor to a smaller value.

Short or medium decay tanks give me the length of reverb that I prefer. YMMV.

[jasonvilla]

I just ordered this:

https://www.amplifiedparts.com/products/reverb-tank-mod-9ab1c1b-short-decay-3-spring

[ac427v]

That one should do the trick. You will have to decide how to ground or insulate the send/return cables as discussed in the thread on "Reverb tanks that are grounded on both ends".

[jasonvilla]

Hmm, how come?

That one should do the trick. You will have to decide how to ground or insulate the send/return cables as discussed in the thread on "Reverb tanks that are grounded on both ends".

[ac427v]

ERRR BECAUSE I'M WRONG.

I misremembered the code for tank connections and thought you ordered a "grounded in, grounded out type A" tank. I am sorry. Next time, I'll check before posting.

[jasonvilla]

Haha, it's alright. Im just glad I didnt read the code incorrectly.


I have a single input thats internally jumpered at V1 with a 33k resistor. The normal channel was REALLY bass heavy and muffled. I just realized that the 470k/500p mixing resistor/cap going to the bright volume control is bleeding some highs (at least I think that is my problem). Does anyone remove the treble peaker cap here? I might just disconnect the normal channel all together or wire the preamp up as a 2204 style if I cant get these to play nicely...

[jasonvilla]

Well, the "plexi" preamp turned into a 2204 type and I like it way better. This is the last amp building and Im actually happy how it turned out.  Everything seemed to come together. It's a good balance of weight, size, volume, and tonal options. I took a chance on the Eminence Legend EM12N speaker and Im super happy with it. And its only 6lbs! The oversized Princeton cabinet turned out great and has plenty of bass. The last step is designing the faceplates, and then Im done :)

I don't love the reverb, but it sounds fine. I think the MOD tanks are just a bit too much for my taste.

I left the older style presence control alone when I switched the preamp over. I also have a resonance control - 1M pot with a .0047cap. I used a 22k NFB resistor coming off the 8ohm tap. I have two output jacks and a selector to switch between 4ohm and 8ohm, incase I want to run an extension cab.

I added two switches on the back. One adds a 10k across the 10k cathode resistor on V1b. This REALLY helps the amp sound more like a plexi (imho) when the pre amp gain is turned down and Im playing cleaner. The other switch adds a .68uF across the 820ohm cathode resistor on V2a. I played around with diode clipping but couldn't get it to work right.

The reverb is a standard "one tube reverb" driven by a 12ax7. I had a 12dw7 in there but just couldn't get it to play right and it sucked a lot of voltage from the amp. I was pretty surprised by that. I ended up with a 470k/680k voltage divider instead of a 1M dwell pot - those were the values I had on hand.

The "FX RETURN" jack is inserted on the master volume input lug. So this input has reverb and volume control. It works just fine using the Pod Go as an amp preamp and effects unit - basically making this amp a tube powered guitar speaker. I only have one amp that has an fx loop to compare it to, a Fender Super Sonic 22. The Pod Go into the fx return of that amp is quite a bit louder. But the Pod Go has a gain setting for the output, so I got it to work just fine.

I could be doing this wrong... But I think I am getting 37watts clean and 72watts cranked. 17.2v rms and 24v rms, respectively.

Ill post pics when its done done. Thanks always for the help.

[shooter]

17 squared divided by 8 i get ~36w
test at 100 hz and 4khz to get a "sense" of dynamic range

[jasonvilla]

17 squared divided by 8 i get ~36w
test at 100 hz and 4khz to get a "sense" of dynamic range

At 17.2 I got 36.98. Great idea to test at different frequencies... I wish I had thought of that when it was all hooked up :( I just wanted to have a ballpark idea of what it could put out.
Does output voltage go down with a lower input frequency?   

[PRR]

...Does output voltage go down with a lower input frequency?

Obviously a transformer passes zero power at zero frequency, and any amplifier passes zero power at infinite frequency. So it is a "hump".

Look at this page. That's for a pretty good hi-fi transformer. It is odd for a "guitar" transformer to go 15Hz (means it is about 5X the cost and weigh t it needs to be). Little G-iron may fall below 100Hz. Cheap iron may fall above 6kHz, though I have seen moderate-price iron flat well past 15kHz (I don't bother to go higher for a g-amp).

37 clean Watts sounds like a lot for "a Princeton". What is it really? Is it correctly loaded?

[sluckey]

37 clean Watts sounds like a lot for "a Princeton". What is it really? Is it correctly loaded?

It's a Plexi in a Princeton cab.

[jasonvilla]

The faceplates I designed came into today and Im pretty happy how they turned out. Im REALLY happy that everything fit on the first try. Measuring out the front faceplate on that angle was pretty challenging for me!  Still don't love the reverb, but it's fine. Im tired of screwing around with it! Maybe ill try a 5751 in the reverb tube spot one of these days. Or, yet another tank.

Question, do different tube makes draw more/less heater current? While working on this amp I had JJ EL34s in it. Ran it for hours, played it loud, very loud, etc. The power transformer barely got warm. A few days ago I installed some Mullard EL34s (yes I re-biased) and today after the amp was on for about an hour I could barely hold my hand on the PT. It was hot!! Could this just be from having the chassis installed in the cabinet, or could it be the tube brand!? Thanks!

[jasonvilla]

.

[jasonvilla]

Well I rechecked the bias on both Mullard's and they were pretty far apart from each other. One was 36ma at 450v and 28ma at 451v. I don't know if that could make my PT run hot... BUT I put the JJEL34's back in today and biased them at 35.8ma/444v and 36.4ma/443v. A much closer match. I played it pretty hard and left it on for a good 40 minutes. The PT is MUCH cooler and I can easily hold my hand on it.
I don't know what the problem is but I much prefer the sound of the Mullard's :(   I might try out the JJ EL34-II to see how they sound.

[shooter]

they were pretty far apart

for fun;
(assuming PP 2 PA tubes)
take any set of tubes you want, matched at idle however you want
now set up to monitor PA current of each tube (1 ohm R's 2 meters)
get the guitar out and play everything from Sunday Morn to Friday head slapping
and watch the meters dance 


even more fun
set the meters to record min/max/avg and compare "range"
use your idle plate vdc and do some math and you get an "idea" of tube life, AND it's funner than YT 

[jasonvilla]

they were pretty far apart

for fun;
(assuming PP 2 PA tubes)
take any set of tubes you want, matched at idle however you want
now set up to monitor PA current of each tube (1 ohm R's 2 meters)
get the guitar out and play everything from Sunday Morn to Friday head slapping
and watch the meters dance 


even more fun
set the meters to record min/max/avg and compare "range"
use your idle plate vdc and do some math and you get an "idea" of tube life, AND it's funner than YT 

What kind of meters do you have!?

[shooter]

The RS I got for$12 new, when the store moved on.  gotta admit it's a pretty good fluke knock-off for $12 

[jasonvilla]

All done! Im really happy with how it turned out. It's pretty much exactly what I wanted. It's a littler heavier than I planned, but I dont know how I could make it any lighter.  Just your usual 2204 stuff, with a couple tweaks here and there. Plus a 1 12ax7 spring reverb. The preamp bypass input works great with my Pod Go. Thanks for all the help along the way.

[jasonvilla]

.

[jasonvilla]

..

[sluckey]

Very nice!

[jasonvilla]

Well, Im a sad boy. I wanted to be done screwing around with guitar amps, but Im back. I noticed my amp was getting REALLY warm. So warm that it warped the plastic back plate. I ordered a temp gun a started to check around...

I left the amp on for about an hour, with no signal and the volumes turned down. The power transformer (Hammond 376X) got up to around 145F. The power/stand by switches get up to around 130ish.

I then checked the bias. I left my bias probe on for an hour. My plate volts started off at 459 and over the course of an hour dropped to 455. The bias started close to 36ma and over the hour got down to about 34ma. I then swapped the bias probe over to the other power tube to make sure it was doing the same thing. I left it on for about another 30 minutes and it hovered around 33.7ma.

I then opened up the chassis and checked voltages (this was after the initial 1.5 hours).

The heater voltage is 6.8VAC... which seems a little high - the 376X says the 6.3 tap is rated for 5A.

Mains voltage is 121VAC. Im hooked up to the 120 tap on the Hammond.

Every other voltage i've checked seems OK.

The amp is basically a 2204 circuit with an added reverb (1 12AX7) and an elevated heater circuit (50vdc).


Any advice or additional things I should check? Thanks everyone.

[sluckey]

Return the temp gun and buy some aluminum face plates.   

I don't really see a problem.

[jasonvilla]

Lol, well...
It's still running hotter than I'd like. Is there anything I can do to lower the temps without replacing the PT?

Would reducing the heater voltage cause it to run cooler?

I guess I can drill some holes in the chassis and mount a fan. I don't know if I can find a 6VAC fan but I can probably get a 120vac fan and run it off the power switch...

Thank you

EDIT: I've also read the parasitic oscillations could be a cause but I dont know how to check for that.

[sluckey]

I would get a small 12VDC computer fan and a low voltage FWB rectifier and a 470µF cap. Power the rectifier from the filaments. The fan will run quieter because of the lower voltage but it will cool the amp.

BTW, I have an amp that will keep my coffee warm.

[jasonvilla]

I would get a small 12VDC computer fan and a low voltage FWB rectifier and a 470µF cap. Power the rectifier from the filaments. The fan will run quieter because of the lower voltage but it will cool the amp.

BTW, I have an amp that will keep my coffee warm.

I like that idea... Something like this? https://www.amazon.com/NTE-Electronics-NTE5311-Rectifier-Repetitive/dp/B008UTVH5A/ref=sr_1_11?dchild=1&keywords=low%2Bvoltage%2BFull%2Bwave%2Bbridge%2Brectifier&qid=1618459705&sr=8-11&th=1

Does the cap go between the + and - on the rectified side?

Thank you

[ac427v]

My thought is that the upper panel on the back of the cabinet blocks hot air from leaving the area of the tube bases. I always use narrow panels there so there is a 3/4 inch open slot along the tube bases to allow for air circulation. You could test this idea by temporarily removing the panel and repeating your heat measurements.

[sluckey]

Yes, but I prefer this style...

https://www.amazon.com/NTE-Electronics-NTE53016-Rectifier-Recurrent/dp/B007Z7LXVQ/

One screw to mount the rectifier to the chassis and a 470µF @ 25V radial will neatly mount to the rectifier, connected to the + and - terminals. The two AC terminals connect to the filament wires. And the fan connects to the DC + and - terminals. The rectifier will produce about 8VDC to run the fan.

<<< CAUTION>>> DO NOT CONNECT ANY PART OF THIS CIRCUIT TO CHASSIS GROUND!

I'd use a small fan similar to this...

https://www.amazon.com/Cooler-Master-Bearing-Computer-Coolers/dp/B005C31GIA/

{EDIT tracking data removed from URLs - PRR}

[jasonvilla]

My thought is that the upper panel on the back of the cabinet blocks hot air from leaving the area of the tube bases. I always use narrow panels there so there is a 3/4 inch open slot along the tube bases to allow for air circulation. You could test this idea by temporarily removing the panel and repeating your heat measurements.

I had the same thought and removed the chassis from the cabinet! Same results... I guess it's just a hot PT/Power tubes!?

[glass54]

Jasonvilla
Done this on many Boogie(s), Vox and Marshalls. See example attached pix.
Make sure you follow Sluckey's caution on reply #39. 
The result is a superquiet fan operation, just audible if you put your head near rear of cabinet in a very quiet room (I mean VERY quiet, studio control room quiet)
Kind regards
Mirek

[jasonvilla]

I ended up buying the rectifier and fan you suggested, thank you. The rectifier mounted nicely on a preexisting... pre-existing...!?...  bolt.  I am getting 8.88VDC from the new tap and it looks like the heater voltage dropped to 6.6VAC.

I designed and printed up a little duct/mount for the fan. I also didn't want to buy a 10 pack of grilles for 10 bucks so I printed out a grille - all in PETG. I think it all came out pretty good.

I played the amp pretty hard for about 30 minutes and then left it on for another hour after that. The fan is definitely doing its job. Temps seem to have dropped about 20 degrees on the switches and PT. I might get a higher cfm fan, but for now this is an improvement. Thanks for the help :)

[sluckey]

Nice job!

[glass54]

+1 (Nice job)
Just beware that a higher CFM might get you into the marginally audible range and undo you good works.
I say "might" because some 12V high volume fans are real screamers, so just be aware but should mostly be OK at nominal 8.8V DC.
Kind regards
Mirek