6L6GC usage in Princeton Reverb kit

[SuperSix]

I received a PR Mojotone kit for Christmas and have built some of their kits before with pleasing results. In this case, I wanted the PR so I would have a very light amp since I am 71. I have used a Blackstar 60 and Super Six Reverb converted to a head cab (Still massively heavy). I am used to having excellent clean headroom. I just recently sold a non-reverb Princeton from 64 I'd used since 1968 and realize how they are treasured for their breakup, but in my case, it was overdriven much too easily and I want only as much clean as I can get out of an amp. I want no breakup. (obviously the value of the vintage amp meant no mods since it was all original and mint beyond recap and AC cord).

As I've embarked on this build, I'm making changes to meet that need. The mods I'm doing are a solid state rectifier, an adjustable bias, and 6L6GCs. When I received this kit, I contacted Mojotone to see if I needed to upgrade the PT and OT. They stated that as long as I have the right bias, their transformers could handle this (these are Mojo760EX PT and Mojo722 OT. Thus I'm headed down this road. It's not necessarily to increase output wattage much, really just make the princeton as clean as possible with it's current power and possibly a little more.

My next research before putting it all together is involving using a speaker that matches the goals without being overly heavy. I've contacted Weber on that.

Is there anything else you can think of that I should be looking into for this configuration. I recognize that a common response to this would be to just get a Deluxe Reverb, but I really like the size of the PR so wanted to pursue this for the learning, and then potentially replace my monster (Super Six) in gigs.

BTW, the reason clean is so important, is I use Zoom G11 (amp and cab modeler off) into the clean amps, even for overdrive. It really sounds great and is so configurable, but the amp has to be clean like my two larger amps.

Thanks in advance for any ideas and warnings.

[Carlsoti]

The one thing that comes to mind is putting a sizeable grid-stop resistor on the phase splitter, which I don't think was stock. It might only matter if you crank the amp most of the way/all the way up.

[SuperSix]

Thanks for that. My brother (radio tech) had mentioned that but I forgot about. I'll check the Bassman schematics for a similar config.

[SuperSix]

I did think of something that might be necessary with this build to get the most out of it. If I were to place a choke inline between output transformer and screen supply, it sounds like it would reduce sag and possibly support more clean headroom better. Is this ill-advised or a good idea?

[SEL49]

You would need to replace that 1K 1W resistor with the choke.

[SuperSix]

Thanks for that. That was what I was thinking, and it appears that the standby switch would go right in there too. I have heard I need the Standby when using the SS Rectifier.

[SuperSix]

Here is the schematic with modifications to support increased clean headroom. I apologize for how hideous it looks, the drawing tool I used only allowed me to produce resistor and choke graphics that came from a Kindergarten electronics class ;-). The values on the 6L6GC resistors are just what I took from my other large fender. If possible please slaughter it so anyone coming along and seeing this will not think it's a valid prototype.

I borrowed much from the Hoffman and Robinette pages along with several others.

I put a giant disclaimer on it since some guitar player might browse it up in images and think he/she has found the brass ring

[DummyLoad]

Here is the schematic with modifications to support increased clean headroom. I apologize for how hideous it looks, the drawing tool I used only allowed me to produce resistor and choke graphics that came from a Kindergarten electronics class ;-). The values on the 6L6GC resistors are just what I took from my other large fender. If possible please slaughter it so anyone coming along and seeing this will not think it's a valid prototype.

I borrowed much from the Hoffman and Robinette pages along with several others.

I put a giant disclaimer on it since some guitar player might browse it up in images and think he/she has found the brass ring

100mA PT HV secondary won't last long. 200mA part would have best survivability. APD sells an upgrade Princeton PT (Heyboer part) that's rated at 150mA, taller but same mounting footprint. Link HERE. IMO a better choice than 100mA for 6L6 tubes. 
Vibroverb OT (Hammond 1760J or MOJO770) fits with minor mod. 

--Pete

[SuperSix]

The same mounting footprint sounds very good. I will check the link and probably buy one.

[SuperSix]

@dummyload.

Purchased. I had just purchased from them but didn't look at their transformers. I'd rather given up on trying to find something with the PR footprint that would be larger than the mojotone.

Really appreciate the input.

[SuperSix]

I've finished the initial build of the PR stock with only 5 minor mods (bias pot, mid pot, standby, power tube screen and grid resistors, resistor on the PI V2B grid). It works perfectly and I'll be ready to start the 6L6GC mods tomorrow when the new PT, 6L6 Choke and parts for all the other minor mods arrive.

I've been troubled about the OT. As I got into the math on the impedance for the 6L6s versus the contention that the OT from Mojotone (Mojo772) will be fine with the 6L6GCs in there. I can't see how the 6L6s expecting to see ~4K will be fine with an 8000 ohm OT though admittedly, it exceeds my knowledge to make that call. I found a reasonably priced bassman OT with the specs shown below. Would it be better for me to install this from the getgo or is it really acceptable to use this 8K ohm OT?

Specifications

Audio Power   50 W (RMS)
Primary Impedance   4,200 Ω CT
Secondary Impedance   4 Ω
Mounting Centers   2" x 3.5" vertical
Weight   4.5 lbs.
Bobbin Material   Paper

[DummyLoad]

That 8K part is rated at 20W. Use the Vibrolux or Bassman part. With 6L6 you want a part with better Z and power rating match. If you're using a PR chassis, the Vibrolux/Tremolux part would probably be a better fit. YMMV.

--Pete 

[SuperSix]

Thank for the reply. I will check out the Vibrolux OT. It's unfamiliar to me and I didn't realize it was a 6L6gc amp.

[pdf64]

... I've been troubled about the OT. As I got into the math on the impedance for the 6L6s versus the contention that the OT from Mojotone (Mojo772) will be fine with the 6L6GCs in there.

Could you explain what you think the problem is?

I can't see how the 6L6s expecting to see ~4K will be fine with an 8000 ohm OT though admittedly, it exceeds my knowledge to make that call. ...

Marshall JTM45 / reissues do fine with 6L6 and an 8k OT.

Bear in mind that 4k OTs are generally used with beefier PTs, HT winding rated for >150mA.

[SuperSix]

I will accept that because I am really not sure on all this. I have the Vibrolux OT in my mojotone cart right now but haven't purchased it and it would be better if I didn't have to. It does have 4 and 8 ohm taps which sounds good and would only require movement of one hole in the chassis.

My confusion came from calculating the impedance. The calculation I found was (B+)Squared / (Power Tube wattage * 2). Taking the wattage as 30, I would have to have a B+ of 650 to achieve 7041 or close to the OT's 8k ohms. If I used a rational B+ for this amp, I would be way down there in impedance. Of course the biggest confusion I have is just what will happen if I mismatch that far. If Marshall can do it, and Mojotone says it's not a problem, then I guess I shouldn't worry so much. The PT I'm using (Princeton upgrade) is 150 Ma if that is pertinent.

Thanks for the input and I will hold on the purchase.

[SuperSix]

I have what may be a silly question. I am mounting the choke for this build. I am assuming a choke is a device that can generate a magnetic field. I have heard that transformers like the PT, the OT, and the Reverb Transformer should be mounted perpendicular to one another. Yet on my Super Six (Twin), the choke is mounted in parallel with the OT as in the first picture below. Should it be mounted as the first photo shows (convenient because the line up with the board lines up perfectly in the same manner as the OT). Or, should it be mounted perpendicular as I have laid it out in the second photo?

Edit: photos load backwards to how I describe them in the text. The second photo is the one with perpendicular placement.

[pdf64]

...
My confusion came from calculating the impedance. The calculation I found was (B+)Squared / (Power Tube wattage * 2). Taking the wattage as 30, I would have to have a B+ of 650 to achieve 7041 or close to the OT's 8k ohms. If I used a rational B+ for this amp, I would be way down there in impedance. ..,

That kinda calculation is very rough and ready, and assumes you're intending on pushing the output valves hard, trying to get very high audio power output from them.

I got the impression from your previous posts that you didn't want to do that?
You are just looking to get a 15-20W amp as clean as feasible, rather than turn the 15-20W amp into a 30/40/50W amp, right?

[SuperSix]

I don't think with the PT I have I would be pushing it hard, so 20 would be great. If it can't remain clean at that, then I would want to try and increase the wattage. I assume it will be clean.

I went over it again and I think I may have doubled the wattage. It show 31 but that was for two tubes and I shouldn't have gone 2x. Using that with a lower B+ comes close to 4000 ohms. Still kind of low, but better.

Thank you.

[SuperSix]

Replying to myself on the choke orientation. I found on a hi fi tube amp forum that it was the PT that was the main worry, and both choke and OT should be 90 degrees to the PT. That would place them as Fender did with choke and OT parallel.

[SuperSix]

Success! I brought it up with the new transformer, solid state rectifier, and 6L6GCs (plus an alteration to the Standby so it would charge the first cap, I messed that up sort of like a very early fender). The amp gives me nice power up to 8 on the volume with no breakup. This is with treble, middle (I added that as it seemed like the PR really needs it) and bass set about half way. Put reverb at about 4. All seems well with a three mods left I didn't implement yet (Fan, NFB as shown on RobRobinettes site as the second type that increases NFB, and one other that slips my mind). It's has a nasty hum that completely disappears when I get my guitar over 3 feet away, I tested this with Tele, LP, Strat and Modulus Graphite with EMGs. The latter was far better but all showed this hum if close. I assume it's just shielding needed. I am running two 16 ohm speakers paralleled at the back jacks for 8 ohms. I was going to put another cab together tonight with 2x10 75W 8ohm speakers and see how it runs at 4 ohms and hopefully won't burn up tubes or OT. I did read that to run 6L6GC in a PR, I should have the impedance mirrored to the OT. I have no idea whether that is true or not.

I have one more question since as PDF pointed out, I had a wrong calculation. In this case, I'm calculating the bias to use and currently running the  6L6GCs at 50%. I used (30/408 plate voltage)*.5. This gave me 36 Ma and I adjusted them to that. If this was the right calculation, would I be better off running it at 60% as it seems others do, or is the 50% the best for clean headroom? (BTW, looks like I need a different bias resistor since I'm just about biased as cold as I can get it, but that's another issue I won't worry about unless my calc was faulty and I should be running even colder).

One other question concerning my test equipment. If put a BNC in my dummy load, would I be able to use the o-scope to calculate actual wattage? I picked one up and I the schematic originally called for it. Just not sure that would be important, since for breakup, I can hear it quite clearly.