RCA 307 conversion

[AlNewman]

This has been a fun project.  I'm dealing with low voltages, so I went (loosely) with a princeton reverb ab1270 preamp, into a cathode biased pair of 6v6.  I have a 6x5 rectifier, so I dialed the 6v6 to around 55mA to be safe.  They're running at 265V, so around 7 or 8 watts per tube.  The preamp and phase inverter voltages are within spec to fender's design.

I deleted the first 2 tubes, and added a 12ax7, and also replaced the TMB tone stack with a Baxandall, because I only had 1M pots, and I couldn't find a TMB that used them exclusively. 

This amp is a monster, it sounds amazing, but has next to no clean headroom.  Which isn't actually a bad thing, it does what it does, but I would like some options.  I've removed all the bypass caps, which didn't really affect gain as much as I thought they would, but it did affect tone, not necessarily to my flavour.  I've also played with grid leak resistors, and nothing seems to really dial back gain without losing that wonderful tone.  I could try a 3.3m resistor between V2 and V3 like the original design to replace the reverb circuit, but I don't want to go down that path.

So what I'd like to do is have a switchable gain stage by removing the second half of the 12ax7 immediately after the tone stack.  Then I can add bypass caps as needed as spice.  The other option would be to have V3 switchable, but I'm hesitant to try that because that's where the NFB is injected.

Any thoughts?  does anybody see an issue with making V2 switchable?  Would I be better off switching V3, NFB or not?  Just looking for ideas before I start yanking things apart.  Schematics attached.

Thanks

https://el34world.com/charts/Schematics/files/Fender/Fender_princeton_reverb_bf_ab1270.pdf

http://pacifictv.ca/schematics/rcav307data.pdf

[HotBluePlates]

... a princeton reverb ab1270 preamp, into a cathode biased pair of 6v6. ... added a 12ax7 ... This amp ... has next to no clean headroom. ...

The RCA schematic shows 14.5v across the 6V6 cathode resistor.  What do you have?

The Princeton Reverb schematic shows a bias voltage of -34.5v.  Which means it accepts a ~2.4x bigger drive signal before the 6V6s distort.

Said a different way, the stock Princeton Reverb circuit throws more signal at the 6V6s than the RCA circuit can handle.  And then you added a 12AX7?  Sure, that sounds like a recipe for "no headroom."


Look at the Princeton's phase inverter, which has a 1MΩ between grid (Pin 7) and the cathode's load resistor (56kΩ).
   -  Try replacing that resistor with a 1MΩ pot whose input still comes from the 0.022µF cap, whose "ground" lug goes to the 56kΩ resistor, and whose wiper goes to grid/Pin 7.
   -  Turn that new volume pot down until you get your desired signal-level reduction & headroom.
   -  Consider replacing the temporary pot with 2 resistors that match the resistance from wiper to each outer lug.

[tubeswell]

Can you post a schematic of your actual build?

[AlNewman]

B+   268V
Voltage drop across 270R cathode resistor 14.75V
Screen voltage  261V
Voltage drop across 470 ohm screen grid resistors  .675V
Pre amp/phase inverter node  252V

I'd have to hand draw the schematic, so that's probably something I'll do when I'm finished with it.  Basically the pre amp and phase inverter are a very close replica of the princeton, except for the tone stack and no reverb or tremolo.  I used a 470k grid leak resistor and a 13k grid stopper where the reverb traditionally is injected.  I also reduced the dropping resistors at the filter caps to 1.2k rather than 18k.

I'll try playing with the input impedence at the phase inverter, that may be a simple solution if it works.  I worry that the preamp itself is creating too much gain before it even hits the power section.  The amp would probably be happier with 6k6's, and I could run an 8ohm speaker off the 4 Ohm tap, the OT I used has an 8k primary, but I don't have any 6k6 on hand ATM.

Like I said, the amp sounds great for what it is, for a one trick pony, it does the trick very well.  It's SRV with a strat, and Angus Young with a Les Paul.  But there ain't no Mark Knopfler at any decent volume.  I am quite happy with the overdriven sound, but I'd like to have a switchable clean sound as well.

[shooter]

I worry that the preamp itself is creating too much gain before it even hits the power section.  The amp would probably be happier with 6k6's,

i built a 6K6 then modified to 6V6.  the 6V6 provided less OD sound and gave me a couple more #'s on the vol before breakup.


without much change, going by HBP's analysis and your comments, change out the AX tube to AU to see if this moves you closer to what you're looking for.  The AU's do want to be re-biased from AX but for testing they will do fine.

[AlNewman]

So I plugged in a dummy load and listening amp, and by the plate of V2 it was reacting how a fender normally would, where it was starting to break up when it was dimed.  On the plate of V2, it was into pretty heavy distortion at noon or so.  At the plate of the phase inverter, it was downright juicy by 9-oclock.  So I think that dialing back the grid leak on the PI might work to shape the gain at the 6v6 input, but I'm probably looking for another solution for a clean tone.

Another thing I never mentioned was I have 220k grid leaks on my 6v6's and 1.5k grid stoppers.  Would increasing the grid leak values give me more clean headroom on the output?  I initially thought that adding a bypass cap would give me more headroom, but it didn't seem to.  That being said, I was already at full distortion by the back of the PI.

The 12 Au7 was a good suggestion, I managed to dig one up from an old hammond chassis, and the results gave me a couple extra clicks on the volume.  Maybe as an end result they would be a good stand in, but I would still like to have a switchable clean channel.

So nobody is in the camp of bypassing the second half of the 12ax7 directly into V3?

[PRR]

So nobody is in the camp of bypassing the second half of the 12ax7 directly into V3?

Can you post a schematic of your actual build?

[AlNewman]

Ok, you asked for it.  No whining about your eyes getting crossed looking at it.   

[sluckey]

Ok, you asked for it.  No whining about your eyes getting crossed looking at it.   

Lower right... You wrote 470K value but resistor symbol is missing.   

[PRR]

Oh, I see.

This is a snip and a clip-lead, to test, isn't it?

Since "clean headroom" is ambiguous and always situational, I'd think it would be quicker to just try it than take a poll.

Collecting and cleaning the docs:

[tubeswell]

You could try split load on one or more of the 120k plate resistors if you want to lose some gain. Try 47k/47k

[AlNewman]

Snip n clip, that's my middle name.  Alfred E. snip n clip Neumann.

I did try switching out V2, I found a switch that would work, figured it out and tacked it in.  It definitely made things clean, but no volume at all.  Plus the switch was noisy.  It was worth a shot, if only to realize it was a dumb idea.

I also tried HBP's suggestion, and I did gain some more volume, which is always a good thing.  Not necessarily clean headroom, and not in the way I had invisioned, I would have expected to raise the value of the tail to ground, but ended up lowering it.  So I have a 20k tacked in rather than the 56k.  Must be how it reacts with the 1k to cathode.  Anything much below 20k, it started motorboating.

I also tried changing the grid leaks on the power tubes to ground, up to 1M with no discernible difference, at least to my immediate memory, which I found surprising.

I think I'll try the potentiometer between the grid leak and grid stopper on V3, see if I can find a happy place, maybe put my switch there if it works.

You could try split load on one or more of the 120k plate resistors if you want to lose some gain. Try 47k/47k

Split load...?  Would that mean daisy chaining the plates together in series?  Like run 47k to V2 plate, then jump another 47k to V3 plate?

[tubeswell]

Split load...?

Look at the 2nd stage on 6G3 Deluxe schematic (which uses 100k/15k) - what a split load looks like.  Adjust to suit

[AlNewman]

So how does it work?  Does it create the same amount of gain and dump off a percentage of signal?

[tubeswell]

So how does it work?  Does it create the same amount of gain and dump off a percentage of signal?

It’s a voltage divider between the plate and the B+

[HotBluePlates]

... a princeton reverb ab1270 preamp, into a cathode biased pair of 6v6. ... it sounds amazing, but has next to no clean headroom. ...

Can you post a schematic of your actual build?

Ok, you asked for it.  ...

O.I.C...

I bet you looked at something like the Deluxe Reverb and said, "Dry Path is 12AX7, Tone & Volume, another 12AX7, then 12AX7 mix-stage, then phase inverter & output," and thought your amp is the same thing.

Except it isn't.

   -  The 3.3MΩ, 470kΩ, 220kΩ and 100kΩ Reverb pot in the Deluxe Reverb conspire to toss so much signal in the Dry path that those plus the 12AX7 Mix stage yield a total gain of about 4x (yeah, like 1/3 as much as just using a 12AU7).  Your 3rd gain stage will have a gain of something like 60x, so 15x more than Fender had.

   -  Of course, the Princeton Reverb's 3rd gain stage is also the "pre-gain" ahead of the split-load inverter, which is also inside the feedback loop.  That means it naturally develops somewhat less gain than the Deluxe Reverb.  So you're over the "Princeton Reverb level of gain" again.

   -  The use of the Baxandall/James tone stack means you get more signal-level out in most settings compared to Fender's tone stack.  Nearly 6dB more (which is 2x signal voltage), according to the Tone Stack Calculator.  So you're again "more-more gain."

   -  Your bias is 14.75v while the Princeton's bias is around 34v.  The Princeton-plan would already throw 34v/14.75v = 2.3x too-much signal to your output tubes.  "More-more-more gain."


Gathering amounts:  15 x 2 x 2.3 = 69x ---> lose a 12AX7 and/or have a voltage divider drop signal to 1/69 = ~0.014x = 10% audio taper Volume control at only "1" out of "10".  You built way too much gain into the preamp for the drive-needs of the power tubes.

Which is a reminder that amp-design works from speaker (or output tubes) backwards towards the input jack.

[AlNewman]

Sort of, but not really.  I had a transformer, a rectifier and a set of tubes in a chassis with no cabinet, so I wanted to make use of it.  I knew I had low voltage, so I picked a pre amp which operated at low voltage to put in front of it.  I needed long shaft pots, and only had 1 meg pots, so I picked a tonestack which could utilize them.

I wasn't really chasing any certain type of sound, and I knew it would be a high gain amp.  I already have 4 fenders, I don't really need another one.  For what it is, it sounds amazing, and for the amount of gain on tap, I'm actually quite happy it doesn't howl like a coyote when I turn it up past 4.  The Baxandall tone stack actually does a really good job of reducing the gain, in fact the more I play with it, I'm discovering it provides a lot of the effect I'm looking for, the treble knob almost works like a gain pot.

I'm happy with the amp, I'm just tweaking it to get the most out of it and some extra versatility if possible.  If it can't be done, then I'll leave it as it is. 

I tried bypassing one of the 12ax7, and it castrated the amp, so it's a no go.  I'll try adjusting the voltage divider in V3 to get the best results there.  I'll try Tubeswell's suggestion using a split load at the plates, maybe that will have a good result.  I'm also thinking maybe I could squeeze more current out of the output tubes, but I am leery because of the limitations of the 6x5.  But I could maybe get them between 60-65 mA safely, that would likely get me another watt per tube.  I dunno, I'll play around. 

I do appreciate the suggestions and advice, even the criticism, especially of the constructive variety. 

At the end of the day I'm just having fun dude, building cool shit for practically free and learning as I go.  These are just grown up toys after all.

[AlNewman]

Well, the voltage divider at the grid of V3 worked as intended.  69:1 was a little off, more like 10:1.  So I went with 220k/220k for the drive "channel", with a 20k in parallel to the grid leak into a spst switch for the clean "channel".  So the clean starts to break up pretty nicely at the end of it's sweep, and the gain is full on saturation.

I reverted the PI tail back to 51k, as the amp was motorboating again after I added the 220k to V3.  I left the 1m grid leaks on the output tubes, and added in a 220uf cathode bypass cap, and also a 10 uf cathode bypass cap on V2.  I am very happy with the results, I'm glad I tried something different.  It is a sweet sounding amp on both channels.

I have a little cabinet, I think it's maybe from a mono price or something amp, with an 8" baffle that I'm going to drop it into, it fits just perfectly without any mods to the cab besides new chassis bolt holes and I'll have to build a wooden faceplate.  I have an old 8ohm R&A alnico speaker that I'll drop in, and run it off the 4ohm tap, with a 8ohm resistor tucked away in the cab somewhere in parallel, I think that should work to properly reduce the power to the speaker? 

It should make a nice little bedroom combo, but will be quite a sleeper amp.  I could see walking into a jam or something with your tiny little combo in hand, and then plugging into the house cab.   

[tubeswell]

I left the 1m grid leaks on the output tubes

6V6 (or any output tetrode or pentode) does better if there is adequate grid leak. Output tubes have much higher current than preamp tubes, and produce considerably more heat, which results in tendency for build up of excessive charge on the grids. IMO 1M is too much resistance for a 6V6 grid. If it were me I’d stick with 470k max. But in your gainy amp, you could even go as low as 100k. YMMV.

[AlNewman]

Good info, thanks.  Reverted back to 200k.

[AlNewman]

Just an update.
I ended up going with 6k6 output, and changed v3 to 6at6.  I also renovated the bypass caps and nfb loop.
I think I'm gonna call it an amp for now.  I'll still make some more changes to the cabinet, but I think it performs well as a head, but I'd like to make it perform equally as well as a 1x8 combo. 
Thanks to everybody who made suggestions and helped me out. 

[AlNewman]

I decided to revisit this old girl.

Used diode rectification, and converted the last 2 6av6's to a LTP inverter, rather than cathodyne and a gain stage.  Also removed the bypass cap on the output stage and the NFB loop.  I'm way happier with the outcome now, a lot more volume, and the overdrive is a lot smoother.  The increased voltage helped some, but really the LTP made all the difference in the world.

The 6k6's when warm are now idling at around 310 on the plates, 290ish on the screens, with about 22v on the cathode with a 330R resistor.  Screens idle @ 4mA each over a 1.5k resistor.

I'll probably sketch out a new schematic and upload it.  (I like posting the schematics here, otherwise they seem to get lost in the paper shuffle...  Should probably get a binder or something to keep them, but the internet works as well.)

[sluckey]

I'll probably sketch out a new schematic and upload it.  (I like posting the schematics here, otherwise they seem to get lost in the paper shuffle...  Should probably get a binder or something to keep them, but the internet works as well.)

Don't rely on this forum to be an archive for your uploaded files. Hoffman periodically purges old file attachments.

[Platefire]

I always made a hard copy folder on every amp project I've ever done. When in the future you make mods, add that and update your info. That way you have everything on that amp all together when you need it.I use these type folders:

https://www.amazon.com/Smead-Pressboard-Classification-Pocket-Style-14079/dp/B001L1REBU?source=ps-sl-shoppingads-lpcontext&ref_=fplfs&psc=1&smid=ATVPDKIKX0DER

[tdvt]

I have thought about putting all the "paperwork", especially the as-built schematic, on an SD card & stashing it in the chassis. For me or the next guy.  Maybe someone already does this...?

Kind of a digital version of the paper copies pasted to the service panels years ago.

A little adhesive SD card pocket/envelope would be handy for that, never looked for one though.

[AlNewman]

Yeah, that's too bad.  Computers and phones are a consumable item for me, rarely have backups, and quite honestly they are as unorganized as my pile of paperwork.  It's nice to have a thread to pull up with all the info there. 

Ah well, I guess I'll have to organize some paperwork as Platefire suggested.  Email myself some pictures, that gets lost in the shuffle as well, but at least they're there.  I suppose I could build folders in my email to store things as well. 

Anytime I've ever done modifications for anyone else, I've always given them a copy of the new schematic, and I guess if they lose it that's on them.

[shooter]

rarely have backups,

Alexandria wasn't backed up in the early centuries, we got the dark-ages   

[kagliostro]

Some years ago there was a discussion about to document with a schematic a custom build (or a mod) and to put the schematic inside the amp, if I remember correctly (may be not) one of our australian friends was involved, may be TIMBO (but a long time is from there and I can remember badly)

Other guy that can be involve (may be, that is a far thing) may be Tubenit, Dummyload and Richard

Franco

[AlNewman]

Well, I've had this amp open at least 3 times now, every time it gets a bit (or a lot) better.  I think this time I might be able to move onto something else, without having that nagging feeling that something isn't right.  Sounds really good in it's cab, sounds amazing through a 1x12 or 4x10. 

I've certainly learned a lot building it, and even more tweaking it.  I guess that was the whole idea.