Help needed with additions to new design

[Howlin' Mad Mac]

Just drew this up... kind of a single channel Vibrolux without the vibrato. Very happy with it so far (could always use some sharp eyes proofreading tho, hint hint) but I'd like to make a few additions if not too difficult (+ a little help how to accomplish or where & what to mine for information.)

1.) Add a presence control... I've some idea how to accomplish this one but can"t decide between 2 places to locate tap & not quite sure what would be a good value to start with so assistance very welcome.

2.) I'd like to add a master volume control, after the P.I. if possible... again ideas but confusion as well

3.) Could I add a deep switch to the stack as well? I already have a bright switch. Any thoughts?

In advance mant thanks for any knowledge, examples, circuits, etc. you can provide.

New 6V6 Class AB.pdf

[Chuck H]

I can't modify the image with my computer. Can you repost it as a gif or jpeg?

[Howlin' Mad Mac]

Hi Chuck, Here's the file saved in a JPEG format. I don't know what everybody else uses to do schematics, somehow I'm guessing few use MS Publisher like I do but hey, it's what I have lol. I've made only a minor change so far to the filter section in the power supply after running more simulations in Duncan's PS design tool. Tranny is kicking out 280v/120mA, C1 & C2 - 47u, C3 & C4 - 16u, choke - 4h/50mA. Anticipated B+ voltages are: 371v @ C1, 368v @ C2, 250v @ C3 & 192v @ C4. OT is 8.4k:16R/9.2k:8R otherwise all other values straight from the Fender playbook at this point. Hope this helps.

[Chuck H]

Would those anticipated voltages be with current taps at the nodes? Otherwise they're unloaded voltages and you would expect significantly lower B+.

I've never learned to modify PDF files. With my current software it seems like no matter what I tell it to do nothing happens. Including saving images to different formats. I actually draw my schematics on MSPaint. It goes pretty well if you save files for all the components you've drawn. Then it's just click, paste and lines.

Thanks for converting the image. I'll play with it and repost.

[Chuck H]

OK... Fun.

Corrections in red and additions in blue:

Deep switch: Since most builders fight to remove bottom end from vintage style preamps to avoid "fartiness" I figure you could use a 2.2uf V1 cathode bypass cap and then switch in a larger cap to boost bass frequencies about 6db.

The feedback line was moved from the jack to a specific impedance tap. Switching impedance on the FB line drastically changes the FB ratio.

The presence control is similar to the standard 5f6a type. Values will depend on other component values in the FB loop.

I added grid stop resistors to the power tubes... You'll want them in there.

The PPMV circuit is similar to the "larmar"(?) type circuit but instead on having the power tube grids on the wiper I moved the PI outputs to the wiper. This just makes sense because when pots fail it's usually the wiper and in the typical version of this circuit that would deprive the power tubes of bias causing meltdown.

I moved the main filter to the other side of the standby switch. This is needed for the rectifier tube to have a load and charges the cap before B+ is applied to the tubes. Better. I also removed the bleeder cap from the second filter node. You could put one in there but it's not really doing anything.

I moved the pilot lamp. As it was the lamp would remain lit n the event of an open fuse. Unless that's what you wanted?

[Howlin' Mad Mac]

Sorry I was away for a bit but I did carefully look over the corrections & additions you were kind enough to provide. Since then I've changed things up a little bit however I've tucked the other revised schematic away for future reference. I decided to keep this design as "minimalistic" as possible, thus scrapping both bright & deep switches as well as the master volume. I also chose to use fixed bias for the 6V6 pair instead of simple cathode bias. Please review my altered schematic & component list/values & let me know if you think any changes are needed before I build this puppy. In particular I'm concerned if I've got the PI, NFB & bias correct as like most of my "projects" it's a bastard compilation of several "classic" circuits, in this case a Blonde Bassman, a Brown Concert & a Blackface Deluxe. I'd also be very interested in hearing from anyone good at designing "tagboard" layouts willing to lend their talent to this endeavor.

P.S. That wasn't a pilot lamp in my other drawing, it was meant to indicate a 120v/60Hz ac source, the lamp & heater filaments were not shown. Also if it helps, PT is a "Deluxe" model from Ted Weber (340-0-340v, 120mA), OT is an "18 Watt" model & design uses the 16 & 8 ohm taps with NFB on the 8 ohm & Choke is a 4H, 50mA similar to those used in several Fender Tweed/Brownface/Blonde 20-40 watt designs.

[overtone]

...The PPMV circuit is similar to the "larmar"(?) type circuit but instead on having the power tube grids on the wiper I moved the PI outputs to the wiper. This just makes sense because when pots fail it's usually the wiper and in the typical version of this circuit that would deprive the power tubes of bias causing meltdown...

Very cool. I just had one of those "now why did I not think of that" moments! Thanks Chuck.
Best, tony

[Chuck H]

Thanks for the props. I came up with the idea on another thread where a guy wanted to use the Larmar type master with a bias vary trem. Obviously the bias vary trem can't work with the typical Larmar type circuit so I just changed the connections. I realized later that another advantage was the power tube grid bias would not be dependant on a panel mount type pot wiper. I've never tried either circuit to be honest. If there is any objectionable tonal difference another idea pioneered here by another poster is to use the standard Larmar type wiring but add a pair of high value resistors (2.2M) parallel to the power tube grid bias feed so if the wipers fail there won't be a catastrophic failure.

[Chuck H]

Actually your circuit looks good topology wise. There are some values I think might be different for the final design. I didn't go over it complete because it's a PITA jumping back and forth between the schem and component value list. It would be much easier if you rendered a version of the schem with values in place. Your NFB looks fine.

With that PT and a fixed bias your going to have some pretty high plate voltages. On the preamp also since you have a choke and low value droppers in the B+ rail. That said I think the value on V1B cathode resistor and the PI tail resistor are low. I might change the 3.3k first dropping resistor in the rail to 10K or 22k. Believe it or not your topology is going to have pretty high gain. If that's what your after that's fine. But all the clipping will be in the PI and power tubes and too much clipping of any one stage might not sound good. High gain is usually best done by clipping at least three stages moderately rather than two stages heavily.

If your not looking for high gain I think a 1.5k unbypassed cathode resistor on V1B and a 22k tail resistor on the PI will balance things out better. Or, if you plan to possibly use the V1B bypass cap in a smaller value for voicing you could design in a split plate load for V1B and keep the cathode resistor bypassed. This also reduces that stages output impedance which is a good thing in general IMHE.

Could you mock up a schem with values? I bet you'll get more opinions from other posters that way.

[Howlin' Mad Mac]

Thanks again Chuck! I revised the schematics with values listed this time. Rail voltages were estimated using Duncan's PSU Designer 2. I'd include the .psu file except it is not a file type I'm allowed to upload here. If anyone's interested in seeing it PM me with an email address & I'll forward it. Anyway, based on that the rail voltages are similar to those from Blackface "AA763" Deluxe. I'm guessing that perhaps because the AA763 uses a 12AT7 PI & my design is using a 7025 PI from the Brownface Concert Amp might be where most of my "gain" issues lie. As for the "tone" or "feel" I'm after, as close as possible to a Blonde or Brownface Bandmaster or Bassman. My personal taste in musical styles tends toward country, rock-a-billy, blues, southern rock & classic rock so your assumption that I'm not after tons of gain is right on the money I do believe. I look forward to your suggestions to balance this design although your comments got me thinking... might a "better" solution lie in redesigning the front end with an additional 7025 in there (bright/normal inputs - volume - 2nd stage w/ cathode follower - stack - volume - PI - power amp)?

Blonde Classic 20 pg 1 with Values.pdf
Blonde Classic 20 pg 2 with Values.pdf

[Howlin' Mad Mac]

Chuck, I'm re-posting my schematics, all component values drawn in, revised according to your suggestions. On page 1 I split the V1b plate load 82k/18k which should, if I implemented correctly, reduce gain by approx. 20%. Is that what you had in mind? Too much? Not enough? I wasn't sure regarding changing V1b's bypass capacitor value not knowing how this will affect its sound. Any thoughts regarding this? I play mostly single-coil equipped guitars (Tele, Strat, P-90 SG, MelodyMaker) & like my tone on the bright side, a full round bottom end & a scooped mid. For my design I relied heavily on early 60's Fender models with Treble/Bass stacks but still incorporating a Presence control, which to my ears seem to allow you to keep that classic Fender scooped mid sound but adds a bit of punch in the upper midrange. Not sure if that's what it's supposed to do, just my perception, please enlighten me if I'm way off here. Also on page 1 the PI tail has been increased from 6k8 to 22k. Please note voltages have also changed due to increasing the 3k3/3W resistor in B+ rail to 10k/3W on page 2. These voltages are from model of the PS in Duncan's PSU Designer II with a 50k load. I guessed at that value, however the default value of 5k just seems way low to me. Is this wrong? Should it be higher? I anxiously await your reply.

Blonde Classic 20 pg 1 with Values - revised.pdf
Blonde Classic 20 pg 2 with Values - revised.pdf

[trobbins]

You could move the standby switch to the CT - that way you switch only AC - and a bypass resistor across the switch can provide a soft-start to the cap charging.

You could put a failsafe resistor across the bias setting pot (wiper to top end).

Ciao, Tim