mini D-HRM amp

[tubenit]

Got the board built today ........... really looking forward to plugging in and playing this thing about 3-4 wks out!

With respect, Tubenit

[Geezer]

I should be fire'n mine up this week...almost got all the soldering done.

I would be done tomorrow, but for going to the amusement park with the kids & grandkids. Gotta' make sure to take my Excedrin & Dramamine.... :icon_puke_l: :icon_puke_r:

[tubenit]

Oddly, my biggest concern is that the effects loop is not really practical or functional. It's a good idea to have it at the end of the preamp chain to capture the sound created by the preamp, but needs to be knocked down to about 1/200th the size (guessing 20v to about 100mV) for feeding into pedals meant to gulp guitar-signals, then will likely need to be boosted back up to drive the output tube.

OK, let's say for arguements sake ............ HBP (who is my amp building mentor) knows more than I do and that he's correct (just for arguements sake) and that a passive effects loop might not work with a pedal (you know for arguements sake ....... let's pretend he is right) ...................          Not that anything like that has actually happened but for arguements sake .............
 

Could an active effects loop like is drawn here work correctly????

AND would there be a change in the tone of the amp when it is engaged without a pedal in the loop. Assuming shorted jacks on send and return that are connected.

Any comments about this, please?         with respect, Tubenit

[jojokeo]

Hello, I'm curious why you feel the need for an effects loop w/ this kind of amp? For driving what effects? You'd be diminishing your tone that you've worked so hard for to colour it down w/ little signal destroying solid state op amps and transistors?    Just kidding you, but that's not the intent for this type of amp and why you're building it, is it? (I wouldn't be thinking while drawing the schematic, "I want to build a cascading tone monster so that I can run all kinds of effects through it!")

More questions that come to mind is how often would you actually have these in use? Do you play w/ pedals a lot through the send & returns in your other amps often enough to justify it w/ this one? What type of effects are you going to be using that are most beneficial to be placed after the preamp distortion that you wouldn't want inserted through the input? Reverb, delays, chorus, phlangers, etc... you'd want for this but I still think that you will definately change your original tone of the amp - kind of comes w/ the territory, IMHO.

Your switched out loop shouldn't affect anything. Switched in (using a jumper?) you might like the extra two gain stages w/out any of the effects? 

Having a line out is definately something that is practical and useful in many ways as I'm sure you know, wouldn't color down your tone and wouldn't necessitate the extra tube, controls & parts. (unless you need to drive your signal over long cable runs?) But, I would bet    that you will forge ahead and install these if for nothing else, your experimental need to do it anyway. Good luck T I hope it turns out well.

ps - I'm getting ready to drill the chassis for my hot/cold designed stages I described earlier using a load line plotter for each stage. I'm really curious how much I can advantageously push the grid current limiting and cut-off aspects of the design? I didn't copy the Bumble design or values to attain this but it turned out similar flow chart wise in that it goes through the first two stages and switches in another two stages sort of Bumble-like, then goes into a PA like an 18 watt.

[LooseChange]

Hey, just wanted to chime in here.  I built one like this about a month before this thread got started. I used the tweaked light version for the preamp.
The passive effects loop does not work well with pedals if you drive the preamp hard. It was intended to be used with a Dumbleator (a tube based effects loop).  I built one of those too. It does work and actually adds to the tone of the amp even without an effect in the loop.
I am working on a parallel version now.

[tubenit]

More questions that come to mind is how often would you actually have these in use? Do you play w/ pedals a lot through the send & returns in your other amps often enough to justify it w/ this one? What type of effects are you going to be using

I always use a Boss digital delay playing with the band. And I have built in reverb for 98% of the amps I've built & always use the reverb (no off switch to it).  Occasionally I use a homebuilt tube screamer for leads. I have a wah pedal that collects dust. Don't own any other pedals at this point.

I'm not real keen on playing amps with no verb or delay. Just a personal preference.

The digital delay pedal works perfectly in every passive effects loop I've used including the one on the T-Lite 5879OD. No issues & nice tone to my ears.

With respect, Tubenit

[tubenit]

OK, what about a paralleled effects loop with wet/dry mix instead of series effects loop? Perhaps that would eliminate a change in tone due to increased gain?

Would this work?

With respect, Tubenit

[OldHouseScott]

Tubenit, I would use the 4.7M if you have one. 1M is going to load the tone circuit more. 4.7M is roughly an order of magnitude higher than the tone controls and should stay out of the way better.

Just curious, where did the circuit inside the blue originate? Particularly the cap and resistor to ground before the OD entrance. I've not seen it on the various D schematics at the other site.

[tubenit]

Scott,

Thanks for the feedback & I'll use a 4.7M.  The blue was in the design that Mat on AmpGarage came up with on his mini-HRM.

I probably will change the blue area and get rid of the 470k & 250p to ground.


Anybody out there successfully use an effects loop on a SE design?

With respect, Tubenit

[HotBluePlates]

Well, I gotta admit up front my ignorance on effects loops. It's simply that I've never had one in any amp that I owned or built. So if some of the guys here have a loop they've built and tested, I'd give it a shot.

On the face of it, both of the active loops look good. Again, take the word of someone who's built and used it before taking mine. But there should be some good active circuits out there that take a preamp signal, knock it down to pedal-level, and then reamplify the returning signal to drive the output stage. I have a suspicion that these usually go between the preamp and the driver stage/phase inverter to minimize how much heavy-lifting the return gain stage needs to do.

[Geezer]

OK, my amp is up & running, but there are problems.......

The output volume is very weak, not nearly as loud as 2x 5881's should be (I've built several parallel SE amps, so I know).
1st thing I did to was remove the 180k grid stopper on the power tube inputs (Mat had a 280k on his EL84 version) leaving just the 1.5k stoppers in place. That helped a good bit, but it's still anemic & weak.

So, to verify the preamp is OK, I ran from the effects send/preamp out of this amp, to the effects return of my PP EL84 version HoSo56 (just using the LTPI & power tubes of the HoSo).

It sounds fantastic, very similar to Mat's clips! The preamp is good.

So, I don't have enough signal (voltage swing?) to drive the 5881's  

What can I do? Add a 12AU7 as a driver tube?? That would "kill 2 birds with one stone" I guess, as it would also serve as a return buffer for the effects loop.

Any insight/ideas would be greatly appreciated......

Geezer

[tubenit]

VERY VERY encouraging news that you're getting that same tone that Mat got with the preamp !!!   Hooray !!!


OK, here is what I am thinking .............. Mat's min-HRM with a SE EL84 was loud enough that he played it with a band.

And a 5881 is louder than an EL84, correct?

I am thinking that maybe the master vol on the clean OR the last OD pot is acting like a dropping resistor sending too much signal to ground to adequately drive the 5881.  So use the 280k as a mixing resistor and add another 1M resistor for the grid resistor.   The mixing resistor helps seperate the 1M from the pots.

I'm thinking that may boost the 5881's to be pretty loud and adding the 1M grid resistor using the 280k mixing resistor is a quick experiment to see if my guess is correct?

Worth a try??

With respect, Tubenit

Hey, a favor (if you have time and interest) ........ can you plug the mini-HRM preamp into the 6BM8 HoSo poweramp and let me know how it sounds?  No rush but I am very curious as to whether that would work for me?

[tubenit]

Hey Geezer,

Can you run your mini-HRM preamp into the Super SE poweramp to see if there is more volume?

Also I'd try the HoSo56 preamp into the mini-HRM poweramp just to see what happens?

I sure would be interested in hearing how the mini-HRM preamp sounds thru 6BM8 HoSo56 poweramp.

Remember Bruspeed did a version of a Champ D'Lite?? If you look at post #70 (second page of thread), he comments that the amp is really loud using a 6V6.  So something seems wrong with not enough volume with two 5881's?

http://www.el34world.com/Forum/index.php?topic=7950.50

With respect, Tubenit

[Geezer]

I've already tried a 1M ground ref from the 5881 grids.....no improvement.

I won't be able to try the different combinations until tomorrow (Friday), but will be glad to do so.


Any other ideas on where to look?
Again (for those just tuning in), the preamp is fine, but there doesn't seem to be enough grunt to drive the 5881's.
I'll post some voltages around the power section as soon as I can......

G

[tubenit]

Did you try a mixing resistor between the 1M grid resistor and the pots?

If you have a pot with 100k to ground & 1M resistor to ground then you'd have 90k to ground and be bleeding off alot of signal (for example).

I have a VibroChamp that I thought was pretty loud using a 10" speaker & Bruspeed's Champ D'Lite was reportedly loud. And your Super SE is pretty loud.
And Mat played his mini-HRM with a band so it must've had some volume to it?
So what's different in this amp's poweramp compared to those others?

And the reason I mentioned trying the Super SE power amp with min-HRM preamp is that might tell you that the mini-HRM preamp does have enough power and the mini-HRM power amp section is possibly the problem.

I also wondered about trying your amp with a  single 6V6 to see what happens since it's an easier tube to drive than the 6L6?

The only other thing I can think of right off besides a wiring or component value that's incorrect might be if the OT wires need to be reversed?  I built an amp maybe 1-2 yrs ago and posted about the problems.  It turned out that it was oscillating without squealing and reversing the OT wiring fixed it.  But that might not be applicable with your SE & no negative feedback?

Any chance you have a bleeding resistor on the power rail that has the wrong value?

I am confidant you'll get it fixed and sounding good. Gotta be something rather simple, I'd think?

With respect, Tubenit

[Geezer]

Yeah, the problem right now is that I only have a few minutes at a time to work on it (very busy), but tomorrow I will have more time free to do an in-depth investigation........

G

[tubenit]

Yeah, the problem right now is that I only have a few minutes at a time to work on it (very busy), but tomorrow I will have more time free to do an in-depth investigation........

What  !!!!      You are taking your real life job and career waaaayyyyy tooo seriously!   This is outrageous!  You're gonna keep us waiting til tomorrow?   Boy, talk about a Hoffman forum cliffhanger.  Gee whiz!  The suspense is killing me.

 

With respect, Tubenit

[Ritchie200]

Nit,
Some of us have REAL jobs and don't have time to be posting at...for example, 12:35 in the middle of the DAY!!!!!  Oh wait....
Jim

[HotBluePlates]

So, I don't have enough signal (voltage swing?) to drive the 5881's 

What can I do?

Measure and think!

What is the bias voltage on your output tubes? What is the measured output voltage when you send that preamp signal out to the other amp? Please note if your meter reading of the signal level is peak or RMS. If you were able to apply a steady sine-wave signal, that would be great, but you might also have a meter that can save an average reading (some Fluke and other DMMs have a button that records and displays maximum peak, minimum, and average voltages for a certain span of time).

My fear is that the preamp generates a big enough signal for driving EL84's (easy to drive), but can't muster enough output for 6L6's and their large(r) bias voltage.

One other off-the-wall idea: have you tried not using the jacks at all, but simply running a wire from the switch to the output tube grids? Sometimes a wiring error on a shorting jack, or just getting carried away with using shorting jacks will lead to problems.

[tubenit]

My fear is that the preamp generates a big enough signal for driving EL84's (easy to drive), but can't muster enough output for 6L6's and their large(r) bias voltage.

I thought about that also ......... but DaGeezer built the Super SE and it worked. And other guys have built a 6L6 SE amp on the forum?  So what would be different?

With respect, Tubenit

[Geezer]

One other off-the-wall idea: have you tried not using the jacks at all, but simply running a wire from the switch to the output tube grids? Sometimes a wiring error on a shorting jack, or just getting carried away with using shorting jacks will lead to problems.

The same thought occurred to me today.......I'll try a jumper straight to the grids & see if there is any improvement.

[Geezer]

Well, I'm stumped!

Running the preamp into the 2xEL84 PP poweramp yields a very loud (& absolutely fantastic sounding) output.
Running the same preamp into the 2x6L6GC (or 5881 or 6V6, doesn't matter) poweramp is weak & anemic........the EL84's are at least 3 times louder!

I swapped out the OT for another known good OT......no change.
I ran the output of the preamp directly to the input(s) of the output tubes.....same results.

Triple & quadruple checked the power amp wiring...all looks good.

Plates & screens are @~~360v, cathodes (each with it's own separate 300Ω resistor) are @~~24v.

I'll look more tomorrow, but tonite I'm whipped!   

Geezer

[HotBluePlates]

Plates & screens are @~~360v, cathodes (each with it's own separate 300Ω resistor) are @~~24v.

So your bias is about 24v.

I don't know that Tubenit's first schematic is representative of the original circuit, but I took it on faith that it was and arrived at a guess.

The first schematic showed 270v B+, and a 130 ohm cathode resistor for an EL84. 12w/270v = 44mA, and guess a couple extra milliamps of screen current; how 'bout 50mA of cathode current? 130 ohms * 0.050A = 6.5v on the cathode.

We don't know that the EL84 in the original design ran at 6.5v on the cathode, but 6-8v is believeable in SE. If it's 8v, then 7-8v peak would drive the EL84 to full output power. You have 24v of bias on your output tubes. Therefore, you need 23-24v of peak signal to drive your output tubes to full power. So you potentially have 1/3 the needed drive voltage!

Cut 'n Paste can cause headaches.

But you didn't give a signal level measurement, so we don't really know of the driving signal is deficient. You really need to measure that.

[darkbluemurder]

Running the preamp into the 2xEL84 PP poweramp yields a very loud (& absolutely fantastic sounding) output.

Well then, THAT could be a working solution.

Cheers Stephan

[Geezer]

you didn't give a signal level measurement, so we don't really know of the driving signal is deficient. You really need to measure that.

I'll get that measurement later this morning.

Running the preamp into the 2xEL84 PP poweramp yields a very loud (& absolutely fantastic sounding) output.

Well then, THAT could be a working solution.

Yes, but I'm trying to avoid adding the tube for the LTPI (& all the associated circuitry). However, that may be where I end up......

G

[LooseChange]

From my experience building one... the preamp (especially the clean) does not have enough drive. It's funny how you jumped it over to a PP EL84 power amp. I did the exact thing when I first powered the amp up and noticed the low output. AND it did sound great.

[tubenit]

I still don't understand why the Super SE would work & this sounds anemic?

Would it work out for you to have a dual single ended EL84?  Maybe just "cap" the 8pin holes with sheet metal and repunch for 9pin EL84's?

Man sorry to hear it's not working out with the 6L6's?  Maybe the answer is a buffered effects loop in series using a 12AU7?

Not sure what I'm going to do now? I got a brand new SE OT.  

I might consider a dual SE 6BM8 amp since I'm not real keen on EL84's?

With respect, Tubenit

[darryl]

...poweramp is weak & anemic...

Can the 5881 amplifier be driven to overload with its own preamp, or does it remain clean until the preamp runs out of gain? If it can be overdriven, then the problem may be in the output stage - although you seem to have covered all the possibilities there. On the other hand, if the output remains weak but still undistorted even with the preamp driven hard, then an overall lack of gain may be the cause.

Do you have another amplifier with a buffered line out that could be used to drive the line in of the 5881 amp a bit harder, to verify that it does have adequate output when driven to its maximum?

[Geezer]

OK, here's some actual voltages.......

Plates = 380v
Screens = 380v
Cathode = 23.5v Each tube has separate Rk, both read ~~the same)
G1 voltage (peak) = ~~6v avg (if I crank everything up, full blast, & hit power chords, I can get a max voltage of ~~17v....the amp is pretty loud at that point, but is way too distorted/ugly sounding)

I guess that G1 voltage is the problem, so I guess I'm either going to have to install a driver stage, or go PP.

Ideas, opinions?

[Geezer]

How about a single triode type (7-pin) as a driver?

I have some nice:

6C4 - http://www.mif.pg.gda.pl/homepages/frank/sheets/093/6/6C4.pdf

6AT6 - http://www.mif.pg.gda.pl/homepages/frank/sheets/093/6/6AT6.pdf

6AV6 - http://www.mif.pg.gda.pl/homepages/frank/sheets/093/6/6AV6.pdf

I'm thinking I've seen the 6C4's used as drivers (?)

[darryl]

An extra gain stage before the 5881's is probably necessary. The original push-pull design from which this is derived would ( presumably ) have had some extra gain provided by a long tail pair phase inverter. A relatively low gain single triode section is all that should be necessary. This could be half a twin triode ( eg 12AU7 ), or a single triode as Geezer has already suggested.

[tubenit]

I am thinking if you used a 12AU7 in series you could do a buffered effects loop that might work?  I would be tempted to try that if sticking with the 5881 dual SE idea.

Another idea would be to parallel a 12A_7 tube if component space on the layout board is an issue.

With respect, Tubenit

[Geezer]

An extra gain stage before the 5881's is probably necessary.

I tacked in a 6C4 triode & it worked.....it's louder, but doesn't sound completely *good*. I think the 5881 grids are now being pushed too hard.

On the 6C4, I have a 100k to the plate from node "C".
On the cathode, I tried 1.5k, 3.3k & 10k Rk's, both bypassed (2.2uf) & unbypassed.
I tried both .02 & .001 coupling caps to the 5881 grids.

Any suggestions on values to try, or any tricks to get it to behave?

[tubenit]

Maybe a large grid resistorfor the 5881?  Do you have a resistor substitution box?  What about a trimmer pot as both a grid resistor and sending some signal to ground (5881)?

or what about a trimmer pot going into the 6C4?

Are you using the 220uf cathode cap for the 5881's?  Maybe a smaller value like
47uf or 22uf might help the tone?

Sounds like you continue to make progress blazing this trail to your tone!

With respect, Tubenit

[HotBluePlates]

A couple of things come to mind to get the drive level needed.

You already have 2 gain stages for clean and 4 for dirty, so it seems like you should be able to get all the drive you need with the existing preamp.

Tubenit has a schematic on the first page of this thread with your name on it; does the schematic accurately reflect what you have? Supply and plate voltage are likely different throughout... so what do you have?

I think this is the moment where the "design with theory" method will beat the "design by ear" method, until the issue of driving the output tubes is solved. Once that's licked, design by ear has to take over again to make sure the resulting sound is enjoyable.

Also, there's a vloume trim pot of some kind between every gain stage. Until you've played with each of those to know where you like them set to shape the signal through the preamp, giving a good idea of the amount of amplification the last stage will need to make, it will be tricky to know what to change.

It will take me a few days to ponder and do some math (because I'll only be able to work on it a little at a time), but I can probably give some good suggestions on how to proceed if I know that Tubenit's schematic shows the parts values you used, what the power supply node voltages are, and maybe also the plate/cathode voltages for each stage. If you also want to strum your guitar and measure the average signal output of your insturment (peak or RMS), that'll prevent errors from me guessing an input voltage.

[tubenit]

HBP,

Always appreciate you weighing in on these projects!  Thanks!  DaGeezer and I have been communicating some off line and he confirmed this schematic reflects his current efforts (as of yesterday) prior to the 6C4 being added.

I am wondering IF adding an effects loop would boost things enough to drive the 6L6?? Any thoughts about that?

With respect, Tubenit

[HotBluePlates]

DaGeezer and I have been communicating some off line and he confirmed this schematic reflects his current efforts (as of yesterday) prior to the 6C4 being added.

Sweet! I can infer the supply node voltages from the voltages that are given. It would be helpful to know what a "guitar-level signal" really looks like, instead of just guessing 100mV like we always do.

I am wondering IF adding an effects loop would boost things enough to drive the 6L6?? Any thoughts about that?

Well, let's figure a 12AX7 will have a gain of about 50-60, after considering the effect of plate load resistance and the following grid reference resistor. Let's also figure a guitar can make a 100mV peak signal (maybe his guitar will do more or less, which is why a real-world measurement would be helpful).

2 stages at a gain of 60 each have a total gain of 3600 if all controls are full up and we ignore the tone circuit. 0.1v * 3600 = 360v peak. Except that it doesn't... the supply voltage is close to 360v, and the tubes have to have voltage across them to amplify, so we must wind up at a voltage driving the output tubes well below 360v peak.

However, he has said that his bias voltage is 23.5v (call it 24v), and if you drive the grid so that the grid-to-cathode voltage momentarily reaches -1v or 0v, you will start drawing grid current and have gotten all the clean power the tube is capable of in the particular circuit. We don't need 360v peak, we only need 22.5v peak, or a bit more. Again, we'll use 24v peak as a handy number.

Again, if each stage had a gain of 50, then 24v / 50 = 0.48v peak needed to drive the last stage before the output tubes (whether clean channel or dirty channel), and 0.48v / 50 = 0.0096v peak 2 stages prior to the output tubes if there was no loss between stages. That's only 9.6mV needed to drive the clean channel!

Except... We do have tone circuits and we do have volume pots. So the question is more about are the right supply voltages available and the right operating points chosen to enable each stage to contrinute its effect, but also to drive the output tubes. We know it works fine with EL84's, but they have small bias voltages and only need a driving signal maybe 1/3-1/4 that of a 6L6. I'm just guessing a circuit tweaked to worked great with EL84's without slamming them, now needs to be tweaked a different way to drive 6L6's.

I am hesitant to add more gain stages to this amp, because we know from other 6L6 amps that they shouldn't be needed, and I would be concerned about oscillation.

[Geezer]

I just took readings & updated any component changes.
Here's the actual amp as it stands now:

[tubenit]

Just a thought and perhaps headed down the wrong road ............

The D_mble amp has most of the overdrive in the preamp and the power amp is relatively clean .........correct?

Any chance to use an opamp to boost the preamp into the poweramp? Maybe that would be a clean boost meaning not adding tone or color to the preamp and maybe the opamp would not be prone to oscillation like a tube might be?

Having said all that, I would have NO idea how to go about it. Nor do I know if that would work?     

With respect, Tubenit

[tubenit]

This is what I was wondering about as being a possibility?

With respect, Tubenit

[LooseChange]

Just add the phase inverter and get it done. 

[sluckey]

Just add the phase inverter and get it done.

On a single ended amp?      

[LooseChange]

This design already has two power tubes (parallel SE) ... Okay add the PI and change the OT.

[Geezer]

add the PI and change the OT.

Yeah, that's where I'm leaning at this point......I already have everything I need, just need to find the room for the extra tube.

[tubenit]

I think I am going to change the OT and use a pair of 6BM8's with LTPI and try that out. If it's not what I want, I'll just switch over to 6V6's.

I probably will put the SE OT in the Hoffman sales in a wk or so. And then replace it with a P/P OT.

With respect, Tubenit

[DummyLoad]

add the PI and change the OT.

Yeah, that's where I'm leaning at this point......I already have everything I need, just need to find the room for the extra tube.

just curious, but why push the 5881s so hard? your posted telemetry indicates that Pdiss is approaching 29 watts. could that be the problem? tung-sol datasheets show Pdiss max of 23 watts.

[Geezer]

why push the 5881s so hard?

Hmmmmm, good question!

I actually had 6L6GC's in there (30w tubes) & am just used to pushing them up to the max dis.

I put a pair of 60's TungSol 5881's in & lowered their Pdis to 18w (600Ω Rk per tube).....seems to be a bit better (can't really crank it, kids sleeping).

I lowered the Pdis further to 13w (900Ω Rk per tube) & can't tell much more difference @ low volume, but I'll try some different Rk's tomorrow & see what they sound like @ normal volume.

I also have some JJ 6V6's that I'll try........

Thanks!   G

BTW, I've been playing the preamp through the HoSo EL84 PP poweramp & it really smokes! If I can get the 5881 SE PA to sound anywhere close (even at a somewhat lower volume) I'll be happy, but as it is, the pre really sounds bad thru the SE PA. We'll see, when I can crank'r up tomorrow.......

[tubenit]

Given DaGeezer's positive review of the preamp into the HoSo56 poweramp, the lack of success with SE 5881 approach & my desire to have a passive effects loop that works ..............

Here is my Plan B. I am going to use a similar poweramp as my 56T with Mat's preamp. The 56T was loud enough to play with the band I sit in with since they mike the cab & it is a really nice volume for a bedroom amp.

And if that doesn't work .......... plan C will be a cathode biased 6V6 version.

Editable SCH version is here:
http://www.el34world.com/Forum/index.php?topic=9149.msg84134#msg84134

With respect, Tubenit

[HotBluePlates]

Okay, I completed a basic analysis of the mini D-HRM amp.

Bottom line up front: you don't need the added 1M to ground, because the master volume pots perform that function. Adding it just gives another place to load down the signal. The big killer to drive is the V2b gain stage plate load. You're gonna need either a bigger supply voltage for that stage, a higher plate load and/or a smaller cathode resistor.

Working backwards based on a needed drive voltage of 25v, you have a volume control and then the overdrive tone circuit. Rough guess based on the Tone Stack Calculator, call the loss in the tone circuit 10dB. If I didn't screw up the math, a drop of 10dB is the same as dividing your signal by 3.165, so we need at least 25v * 3.165 = ~80v peak from V2b.

I made some guesses and some graphical estimates, and assessed V2b as having an rp of 75.5k, a dynamic load presented by the plate load and tone stack at around 92k, and a resulting gain of about 55. You measured cathode voltage is 1.36v, so a reasonable maximum input voltage is 1.3v peak. 1.3 * 55 = 71.5, and after the tone stack you'll get no more than 71.5 / 3.165 = 22.5v peak. That would be good, except you can never drive the output tubes to distort, and you're likely used to hearing that. And that's assuming you have to overdrive master dimed, which you often want some extra at the top. I think I was even generous in guessing the load imposed by the tone stack, and it is likely more severe, resulting in less gain and output from V2b.

If you must add a tube, add a cathode follower driving the overdrive tone stack.

Quick, no-calculation suggestions: You need more supply voltage at C to get more output voltage. Lower the dropping resistor from 44k to more like 27k. You want about 300v of supply at C. Try raising V2b's plate resistor to 220k, or slightly less. Keep the same cathode resistor, or try a 1.5k.

These changes should get the output swing you need from the overdrive stage, but probably at pretty hefty distortion before the output tubes distort.

V1b should be good, as it just has a master volume after it. With a gain of about 60, 25v peak output only requires less than 1/2 volt peak input.

[HotBluePlates]

Any update on this amp?