Higher power Princeton Reverb w/ 4x6V6 ... a few questions

[Diverted]

Hi all,

It's been a while since I've posted. New job has me working crazy hours so I'm not around too much. Oh well.

Anyway, I have a few questions. I have a friend who recently took possession of a Princeton Reverb I built. All stock except no tremolo. He loves it — after only playing 6L6-based amps, he fell in love with the 6V6 tone and wants me to build him a larger one, with more power (25-30w or so). He was adamant that it should be 6V6-based, so I told him if he was looking for that kind of wattage, I could build him a parallel push pull 6V6 output stage instead of two 6L6s. He's very interested in that.

My questions:
The transformer I used for the Princeton has a 104ma HV secondary with a GZ34. A little higher than stock which I think was around 70 or 80. On my shelf I have a ClassicTone 40-18029, which is a 200ma transformer. According to the spec sheet, with a 150ma load the B+ will be about 435 with a 5U4 ... just about perfect. But will the over-rated secondary clean it up too much? Should I go for a smaller power transformer?

Second question: I also have a Classictone 40-18010, a 4.2K primary output transformer, on the shelf. Big and beefy, its a 50w output transformer I believe. I should probably go with something a bit smaller (I think?) but what about the primary rating? Is 4.2K high? I've read conflicting stuff on what I should use for 4X6V6 ... all the way down to 2.8 to 3K. What are your opinions?

Thanks for the help!

[shooter]

fwiw
any time I've tried "replicating that sound" only LOUDER, dynamics change.  so leave yourself some wiggle-room on expectations
both your iron seems close enough.  keep whatever PS filtering you had originally
have a handful of speakers that you can roll

[Diverted]

Thanks. Yeah, I told him it would never be exactly what he'd heard from the PR, only louder. But he wants 4X6V6, so that's what I'll do. I'm looking forward to putting it together. The 50W OT seems like massive overkill; maybe I'll try to find a 30w one with the same approx primary.

[Diverted]

Here's a layout I drew up for the power section. Obviously the wiring diagram is not complete but I just wanted to show the rectifier, bias supply, phase inverter, filter caps etc.
Is this layout workable? I'm trying to fit it on a chassis about 20.5"x 8" and this is what I came up with.
Note that the PI is a 6AV6.

Also, will I need to adjust negative voltage, screen grids, grid stoppers etc. to account for two extra tubes? Thanks!

PS The layout is sloppily drawn on the rectifier. Backup diodes going from 3 to 4 and 5 to 6 not shown. Bias supply coming off pin 5. Thanks.

[Willabe]

You might want to read this, if you haven't, while doing your layout?   

Grounding, Merlin;

http://www.valvewizard.co.uk/Grounding.pdf

[labb]

FWIW: IMO there is a lot of love for the PR. When I look at the schematic for it and the PT and OT that it uses  I just don' t quiet understand it. It is under powered. It uses the same PT and basically the same OT as a Champ. A friend asked me to build him one less the Trem and the Reverb circuit..and I did. I used the PR front end and the Deluxe Power end...Works pretty good. Took about 10 watts to up around 18. The friend, who is a guitar player, really likes it. He uses pedals for Reverb or Trem.

[dwinstonwood]

FWIW: IMO there is a lot of love for the PR. When I look at the schematic for it and the PT and OT that it uses  I just don' t quiet understand it. It is under powered. It uses the same PT and basically the same OT as a Champ...

And, it's single channel; it doesn't have power tube grid stoppers; or screen resistors (just like the 6G3). It does stand out from the other BF push-pull Fender amps.

[tubeswell]

My questions:
The transformer I used for the Princeton has a 104ma HV secondary with a GZ34. A little higher than stock which I think was around 70 or 80. On my shelf I have a ClassicTone 40-18029, which is a 200ma transformer.

you want 2 x the power rating that you'd use for 2 x 2V6s, so 200mA for the HT winding etc etc. A GZ34 should still have enough oomph for 4 x 6V6s, but it may be a little saggier.

Second question: I also have a Classictone 40-18010, a 4.2K primary output transformer, on the shelf. Big and beefy, its a 50w output transformer I believe.

4k2 50W should be fine.

[Diverted]

Hi all, most of the parts are assembled and I'm going to be building the board this week, when I have some time. I have a few questions about a detail here, a detail there. Any suggestions?

1. Grounding. I'm using a cap can for all high voltage filter caps including the one that supplies the plates of preamp and half the reverb circuit. Would it be better to move the D node off the cap can, and install a 20uf cap on the board to supply that D node? It would presumably be grounded at the preamp bus bar which runs off the input jacks.

2. Biasing.
I'm thinking about the realities of biasing four output tubes given the crazy cost of matched quads of 6V6GTs. I have plenty of strong, very good pulls, but not matched likely. Would it be advantageous to run two bias pots, one for each side of the push pull circuit, or even four, one for each output tube? Is this necessary?

Odds and ends to note:
The only changes in the circuit are a 470K phase inverter grid stopper, bias pot and a mid pot. I'm reducing the value of the PI input coupling cap, per suggestions I've read online that it will give the circuit a better fit with the 12" speaker I'm planning on using. And after looking around at the best options for a power transformer, I'm going with a Hammond 290DX. With secondary rated at 325-0-325@230ma, it should provide plenty of current. The output transformer is 4.2K to 8 ohms, rated at 42w.

Others elsewhere have suggested employing the Stokes mod, which moves the PI plate to the unused C node. I have never tried this but from what I've read, not sure I want to put this in the amp so I'm leaving it stock as is for now. The rectifier is an old stock Mullard GZ34, which should be able to handle the current demands no problem (I hope). Rectifier backup diodes will be installed in case it ever goes poof.

Anything else you all would do? Thanks!

[Willabe]

Read these on lead dress and grounding.

Others here will say a single cap can will be fine, but I wouldn't do it if I didn't have to. A single cap can could be a problem with grounding. It's your amp build it how you think best.

http://www.geofex.com/Article_Folders/lead_dress/lead_dress_in_tube_amps.htm

http://www.valvewizard.co.uk/Grounding.pdf

Too late, I see you have already cut a hole in the chassis for the cap can. 

[Willabe]

1. Grounding. I'm using a cap can for all high voltage filter caps including the one that supplies the plates of preamp and half the reverb circuit. Would it be better to move the D node off the cap can, and install a 20uf cap on the board to supply that D node? It would presumably be grounded at the preamp bus bar which runs off the input jacks.

I would. I would also wire up 2 of the 20uF caps in parallel for the B+ A node for the power tube plate feed and the other 2 20uF caps for the B+ B node that feeds the power tube screens. Then get a separate 20uF @ 450v or 500v for the PI, and then do like you said for the preamp tubes, another 20uF or even 10uF or 8uF would now be fine at 450v or 500v. Yes, the last B+ filter cap for the preamp tube(s) gets grounded to the preamp buss that's grounded at the input jacks.

2. Biasing.
I'm thinking about the realities of biasing four output tubes given the crazy cost of matched quads of 6V6GTs. I have plenty of strong, very good pulls, but not matched likely. Would it be advantageous to run two bias pots, one for each side of the push pull circuit, or even four, one for each output tube? Is this necessary?

Being able to bias the 6V6's in pairs would be nice. You could use a bias/balance, for each pair, needs 4 pots. Or just use 4 -bias pots, 1 for each tube. That way you can use matched tubes that aren't very well matched. 
 

Odds and ends to note:
The only changes in the circuit are a 470K phase inverter grid stopper, bias pot and a mid pot. I'm reducing the value of the PI input coupling cap, per suggestions I've read online that it will give the circuit a better fit with the 12" speaker I'm planning on using.

That PI grid stopper should be good for preventing grid blocking. I love having a mid pot. You can use a 25K pot for it, or use a 25K pot and put the 6K8 R Fender used to set the mids between the pot and ground, that way setting the pot to full off, it's stock, and as you turn up the mid pot you get more mids than stock, taking the mid scoop out of the amp. A lot of guys like the 25K or even 50K mid pots.   

Others elsewhere have suggested employing the Stokes mod, which moves the PI plate to the unused C node. I have never tried this but from what I've read, not sure I want to put this in the amp so I'm leaving it stock as is for now. The rectifier is an old stock Mullard GZ34, which should be able to handle the current demands no problem (I hope). Rectifier backup diodes will be installed in case it ever goes poof.

The stokes mode is very good, I'd do it. It not only raises the PI's B+, it gives it it's own B+ filter node, both very good things.

I've never heard of decreasing the value of the PI cap on an amp to work better with a single 12" speaker? 

A GZ34 should work with 4 x 6V6. It's getting close to it's limits but, it should work, I think.

Backup diodes on the rectifier tube is very good to do.

[sluckey]

1. Grounding. I'm using a cap can for all high voltage filter caps including the one that supplies the plates of preamp and half the reverb circuit. Would it be better to move the D node off the cap can, and install a 20uf cap on the board to supply that D node? It would presumably be grounded at the preamp bus bar which runs off the input jacks.

Thousands of successful PRs have been built using only a cap can. Even Leo built a few!     I would just use the can. Connect the PT center tap directly to the can then use a short wire to ground the can near the PT. Call this the power ground. Look at Hoffman's Princeton Reverb Video 1 to see how the pro does it. Then connect the preamp ground buss to chassis near the input jack. Some people like to use the input jack to provide the preamp ground point. You can always add a separate cap for node D later if you think it will be better.

2. Biasing.
I'm thinking about the realities of biasing four output tubes given the crazy cost of matched quads of 6V6GTs. I have plenty of strong, very good pulls, but not matched likely. Would it be advantageous to run two bias pots, one for each side of the push pull circuit, or even four, one for each output tube? Is this necessary?

You can make the bias circuit as complicated as you want. But I would use a single bias pot. With four tubes you can probably shuffle them around to find a combination that provides matched current for each half of the OT.

[sluckey]

Looks like you have 470Ω 1/2 watt screen resistors mounted on the sockets? If so, better up the rating. Fender used 1 watt resistors. Hoffman and I like to use 3 watt metal oxide for screen resistors.

[Diverted]

1. Grounding. I'm using a cap can for all high voltage filter caps including the one that supplies the plates of preamp and half the reverb circuit. Would it be better to move the D node off the cap can, and install a 20uf cap on the board to supply that D node? It would presumably be grounded at the preamp bus bar which runs off the input jacks.

Thousands of successful PRs have been built using only a cap can. Even Leo built a few!     I would just use the can. Connect the PT center tap directly to the can then use a short wire to ground the can near the PT. Call this the power ground. Look at Hoffman's Princeton Reverb Video 1 to see how the pro does it. Then connect the preamp ground buss to chassis near the input jack. Some people like to use the input jack to provide the preamp ground point. You can always add a separate cap for node D later if you think it will be better.

2. Biasing.
I'm thinking about the realities of biasing four output tubes given the crazy cost of matched quads of 6V6GTs. I have plenty of strong, very good pulls, but not matched likely. Would it be advantageous to run two bias pots, one for each side of the push pull circuit, or even four, one for each output tube? Is this necessary?

You can make the bias circuit as complicated as you want. But I would use a single bias pot. With four tubes you can probably shuffle them around to find a combination that provides matched current for each half of the OT.

This is what I was initially thinking, with respect to the cap can. I have worked on a few blackface princeton reverbs, all with the cap can. All have been dead quiet at idle, no issues .. no hum, no nothing. Where as I have carefully laid out and wired three or four otherwise stock Princeton Reverb builds, all using split grounds ... power stuff at the center tap, preamp stuff on the buss. All of them have had some slight bit of noise above what I saw on the originals.
Reading about all the things one can do can be like mental gymnastics!

[Willabe]

And you're doubling the power amps current, needs more B+ filtering.

Deluxe Reverb ~22w had 32uF at the 1st B+ node. Although the Deluxe, without the verb and trem tubes used 16uF.

But a little more B+ filtering wont hurt, it can only help at these values. 

[sluckey]

In case you missed Hoffman's video, here's a screen capture from that video...

[Willabe]

This is what I was initially thinking, with respect to the cap can. I have worked on a few blackface princeton reverbs, all with the cap can. All have been dead quiet at idle, no issues .. no hum, no nothing. Where as I have carefully laid out and wired three or four otherwise stock Princeton Reverb builds, all using split grounds ... power stuff at the center tap, preamp stuff on the buss. All of them have had some slight bit of noise above what I saw on the originals.

Then you did something wrong.

Like all the 'power stuff' doesn't go to the PT CT, just the PT CT and the 1st filter caps ground lead, then you run a wire over to the other power ground connections. It makes a difference.

And this amp will have ~double the current, than a Princeton. So double the ripple and charging currents and that's where this amp will generate extra noise from it's PUP.

The red arrow points to the PT CT, the purple arrow points to a yellow wire that isolates the charging/ripple ground currents from the 1st filter cap and the power grounds. The red wire with the blue arrows shows the charging loop. It lets the currents loop by themself and not modulate the other filter cap grounds, which would cause the hum.

Without that yellow wire, the charging currents through the 1st filter cap can/will modulate the grounds for the screen cap, the power tube K, the power tube grid return R's which are connected to ground through the -grid bias supply.   

[sluckey]

Without that yellow wire...

...the entire amp is dead.

[Diverted]

Ok, thanks. Also I watched the video. That is some amazingly beautiful wiring. Wow.

OK, so the bias cap positive, 6.3v ct and hv secondary ct all go to the transformer lug, along with a jumper wire to the can ground. Got it.

Only other things going to the chassis are the green earth wire from the power cord, the transformer shielding wire and the 6v6 cathodes. Should I isolate the 1 ohm resistors from ground at the sockets, daisy chain them all together and then run one wire from them to either the cap can negative or power transformer lug? Or will it be bad if I just leave them grounded at the socket, as Fender did.

The only other grounds are the reverb tranny which I'm grounding at the tank jack, and the preamp bus that comes off the first input jack.

Thank you for clarifying this!

As for B+ filtering. The first cap in this build is 40uf. I can always parallel another 10 or 20 across any and or all of the other three caps if it sounds like it needs it. Appreciate the tip.

Ted

[Willabe]

Without that yellow wire...

...the entire amp is dead.

 

[Willabe]

... and hv secondary ct all go to the transformer lug, along with a jumper wire to the can ground. Got it.

No. Look at the drawing again.

The PT HV secondary CT goes to the 1st filter caps ground lead, just those 2. Then, from there, that 1st cap ground lead and PT HV CT, you run a wire over to the rest of the power amp grounds.

But you have a 4 x cap can that the 4 caps share the same ground. So you run the PT CT to that cans ground lug. That's the reason I suggested using that cap can as 2 x in parallel and 2 x parallel. (I was thinking you have a 4 x 20uf cap can?) That way that 4 x can would supply the OT CT for power tubes plate with 2 x of the caps in || and the other pair of caps in || would supply the power tube screens. Then put in separate single caps for the PI and preamp close to their circuitry.   

Should I isolate the 1 ohm resistors from ground at the sockets, daisy chain them all together and then run one wire from them to either the cap can negative or power transformer lug?

You can ground the 1 ohm K R's by the power tube sockets. Ideally you want to get rid of as many chassis grounds as possible. That way there's less chance of them interacting through the chassis. So I'd probably at least daisy chain buss those 4 x 1ohm R's together on the ground end and connect that buss to the chassis at only 1 ground lug. The PT internal shield can go anywhere close to the PT, but all by itself. The heater CT goes to the power amp chassis ground. The power cord safety ground wire, 3rd wire, goes to the chassis by itself, no other grounds on that chassis ground lug.   

The only other grounds are the reverb tranny which I'm grounding at the tank jack, and the preamp bus that comes off the first input jack.

It gets a little tricky here. The problem can be that the B+ that feeds the verb OT that drives the tank comes from the screen node. So the verb OT secondary ground, 12AT7, the verb driver tubes K and grid R should all get grounded together, then run a wire from those 3 wires/leads back to the screen grid caps ground lug.

And the verb recovery/mixing tube, 12AX7, 1st 1/2, verb recovery, 2nd 1/2, dry/verb mixing are fed their B+ from the same node. So they should be grounded together. You might/should be able to ground that tubes grounds at the verb tank jacks. But if you ground some of those verb recovery/mixing tube grounds to the preamp buss, and some to the tank chassis jacks, it could be a problem.

The thing you want is to gather all the tube grounds together and ground them at the B+ filter caps ground lead that feeds that tubes B+. That's why to be able to control the ground wiring it needs separate caps. You don't have to do it that way. 

It's all in that link from Merlin.

As for B+ filtering. The first cap in this build is 40uf. I can always parallel another 10 or 20 across any and or all of the other three caps if it sounds like it needs it.

Like I wrote above, I thought you had a 4 x 20uF can? By doubling the amps power you could probably use a little more B+ filtering/decoupling.

[sluckey]

Just do it like Hoffman did. I'd follow him anywhere... except a bike trail!

[Willabe]

Company man. 

Here's something on reverb fixes;

https://el34world.com/Forum/index.php?topic=17645.msg308973#msg308973

[sluckey]

Hoffman speaks from experience and rides the true path.

[Willabe]

More than 1 path of truth and enlightenment.

Especially on the bike path. 

[Willabe]

Now a days, with the quality, hi-temp/hi-ripple, long life, size and price of radial caps, $2 to $3 a cap, under $2 when you hit the price break at 10 or 20, why not add a few more B+ nodes? They seem to be as good or better than the standard axial caps or cap cans we use.

If you don't trust them at the highest current stage, like the power tubes plate, then just use standard caps for the 1st B+ node and the small radial caps for the other stages. 

There small enough to put them right on the board by the circuit they feed. Win, win.

[Diverted]

I’ve actually used Tons of Nichicon 500V caps. Never had any problems with them and I love their size.

Not sure which which way I’m going to go but that’s a possibility. I want to keep this as quiet and we’ll-grounded as possible.

[Willabe]

Use your cap can if you like, or maybe use 2 double || sections for plate and screen and small caps for the rest?

What is your cap can, 4 x 20uF? 

[Diverted]

Looks like you have 470Ω 1/2 watt screen resistors mounted on the sockets? If so, better up the rating. Fender used 1 watt resistors. Hoffman and I like to use 3 watt metal oxide for screen resistors.

Those are actually 2w resistors believe it or not.

[sluckey]

High power in a small package. What's not to like. 

[Diverted]

Use your cap can if you like, or maybe use 2 double || sections for plate and screen and small caps for the rest?

What is your cap can, 4 x 20uF?

Yep, it’s 40/20/20/20.
I like your idea of using two 40s on the can and putting two 20s on the board for the preamp and pi.

I’m going to play around with a layout  for that and report back, plus with a ground scheme.


If I were using a SS rectifier I could double up that 40 and 20 for even more filtering🤔🤔

[Willabe]

Yep, it’s 40/20/20/20.

Ok, so it's 1 x 40uF and 3  @ 20uF.

If I were using a SS rectifier I could double up that 40 and 20 for even more filtering

Yes, maybe.

But it might raise your B+ too high for the 6V6's, depending on the PT's B+ wind. And it will stiffen the amps sound and feel, pick response, you might like that you might not. 

With a SS rectifier set up you could go with paralleling the 40uF with 1 of the 20uF's for 60uF total on the B+ CT plate node and || the 2 - 20uF's that are left over for 40uF total for the screen supply B+ node.

Then a series B+ 1K dropping R then add a 20uF for the verb B+ node, then another 1K series B+ dropping R for the PI, then another series dropping R, you'll have to play around with the value of that R, and then a last B+ cap, maybe 10uF or 8uF? Those 3 could be small radial caps placed on the eyelet/turret board.

[Diverted]

I'm not sure how I'm going to go yet. May still build it stock. But I'm curious about running an extra filter node and breaking up the supply feeds so each has its own. THe pic below would be easy to do by using the first 40 on the can, then paralleling the first and second 20ufs for 40, and then putting two 20uf caps on the board.

Any suggestions/critiques? Not sure what numbers I'd settle in on with the dropping resistors. But if I were to build this power supply, I'd also run various circuit grounds back to the ground they're being fed by. Thanks!

[Willabe]

Use the big Fender choke for between the plate/screen nodes, better filtering and it drops less dcv.

Then ~1K R going to the PI node, then ~1K R going to the verb node, then you have to see what's left and how much you want to drop to the preamp node.

You can always raise those 1K's if you need to. 

[Diverted]

Use the big Fender choke for between the plate/screen nodes, better filtering and it drops less dcv.

Then ~1K R going to the PI node, then ~1K R going to the verb node, then you have to see what's left and how much you want to drop to the preamp node.

You can always raise those 1K's if you need to.

I have a few of those chokes laying around, but no room for one on this tight chassis. Gotta stick with a resistor under the hood. Thanks though!

[Diverted]

Powered up the amp this evening for the first time after spending the day wiring it all up and going over it three-four times. I ended up going with the stock Princeton grounding scheme. I figured if it was noisy I'd rework the power supply and board to break up the grounds as some suggested. Only thing non-stock I did with grounds was run the 1 ohm 6v6 cathode resistors together and over to the power ground.
What an amp! Quiet quiet quiet, negligible reverb hum, and sounds fantastic so far, only three hours in. I need to start swapping tubes and really home in on the right quad but for now they're running at around 65 percent, 420 on the plates overall. I may fool around with the Stokes mod tomorrow and see what it does. I would love to get just a bit more clean headroom. But man it sounds like a nice loud Princeton!

Thanks for all the help so far.

[72Blazer]

Looks fantastic-congrats

[Diverted]

Thanks!

I had to change the rectifier ... Hammond power transformer rated at 325-0-325@230ma is putting out 358-0-358 loaded. This pushed all the voltages way high, around 450-460 on the plates with a GZ34 rectifier. With a 5U4GB voltages are spot on.
Only change here is with the PI B+ feed. I moved it up one node (Stokes mod). Amp sounds fantastic, no issues as of yet.

B+ 423
B+1 406
B+2 323
B+3 262
Bias voltage: -41
AC: 122

V1A
P 180
C 1.2

V1B
P 178
C 1.25

V2 (12AU7)
P 396
C 19.7

V3A
P 172.5
C 1.33

V3B
P 176
C 1.3

V4 (6AV6)
P 252
G 16
C 70

V5
P 421
G 405
C 23.6ma
9.9w (71 percent)

V6
P 421
G 405
C 21.4ma
9w (64.4 percent)

V7
P 421
G 405
C 21.5
9.1w (64.7 percent)

V8
P 421
G 405
C 20ma
8.4w (60.1 percent)