Modified AB763 circuit

[TheTinMan]

Warning #1: please don?t read this stupidly long post if you frequent the Hoffman forum and it starts to sound familiar.

Warning #2: this post is stupidly long.

I would really appreciate some input on my Super Reverb build. The basic idea is to have the "Vibrato" channel be stock except for bias-vary tremolo. The player wants the "Normal" channel to have reverb and tremolo and be good for use by his son with a harp & Green Bullet mic. The player does classic jazz and some old time blues (not very dirty). Guitars include an acoustic, a hollow body, and sometimes a Strat. He wants "clean and fat" tone. His harp-totin' son wants reverb and something closer to Tweed tone. They want to able to play through the same amp simultaneously sometimes with the harp on the Normal channel. Currently they are using a vintage BF Super Reverb.

The bias vary tremolo is from the 6G16 Vibroverb circuit as implemented by Doug Hoffman.

Father & Son like the idea of a "Brown" tone stack as an option for guitar and for the harp, so the "Normal" channel in the attached schematic shows the 6G16 tone stack with the tapped Treble pot. Seems like this will be easier to tweak for more bass & low-mids for the harp. I also bumped up the cathode resistor on V1-B to reduce gain in that stage (subject to change. The "Bright" switch is for guitar when "on" (contacts closed) and for harp when "off" (contacts open). One side of the Bright switch adds a 50uf cathode bypass cap to V1a and the other side is like a "Raw" switch under the bass/mid portion of the tone stack.

The part that I had the most trouble with is that I'd like to get the plate voltages on both V1-A and V1-B down to about 175 volts or so, versus the 270 VDC which is stock on the BF Super Reverb. Every "good" harp schematic I've seen seems to have preamp plate voltages below 200 and Tweed seem to be pretty popular among harp players.

There are two additional nodes on the power rail. I added one between the screen node & the PI node so that the reverb driver would have an isolated power supply and ground. The second added node is for the two triodes of the Normal channel. My target is about 175 plate voltage for those two triodes, so the 56K dropping resistor is a SWAG. 10uf for each of those last two nodes may actually be too high in terms of excess "stiffness" and bass response.

Tying the plates of V1b and V2b together is the tried & true way to get reverb on both channels, but that would require having the same plate voltages on both channels.

I added unbalanced "isolation resistors" between the coupling caps and the reverb/clean split of the circuit (10pf/3.3M). If I'm looking at the voltage dividers this approach creates correctly, this should send relatively less of the Normal channel to the reverb circuit and mildly reduce the Normal signal going to V4-B (330K + 3.3M vs. 220K to ground). The values and even existence of these resistors are subject to change. The thought is less "Dwell" less gain would be a good thing for harp in terms of reducing feedback somewhat.

The input jacks are wired a bit differently for the harp too. The Green Bullet needs to see at least 100K between it and ground (I forget the technical term for that... duh), so I tweaked the "Lo" input to still have about a 2:1 divider but a 101K drop to ground. Thought it might be useful for the harp to be able to use the "Lo" input. A 5751 in the V1 spot will be an early experiment as well.

There's lots more tweaking to work out and/or experiment with like cathode bypass caps in the Vibrato channel, tuning the tremolo to push the 6L6s, etc.

FWIW this build will have a Mojo cab, Mercury Magnetics iron (customer preference), and two 10A125 plus two 10F150-T speakers from Weber.

Thanks for any ideas and assistance!

Chip

[TheTinMan]

I checked back on this thread and realized that the thumbnail link doesn't seem to work right. Hopefully this format for the schematic will work better.

schematic page 1

schematic page 2

ARGH! is right!!! I can't delete the attachments even though they are worthlessly small...

Chip

[MWJB]

Arrrgh! Evey time I close the thumbnail, that I can't read anyway because it's to small, I get kicked off the forum & lose the response I'm posting.

So, here are my thoughts, irrespective of what might be on the schems you've posted....

Harp channel 170-200v at pin1 of the 12AX7, use 1.5K to 2.2K cathode bypass at pin 3, use 820ohms at pin 8, V1 & V2 have their own cathode resistors. For the harp channel 100-220uf bypass caps.

I wouldn't recommend on board reverb for harp, big gain boost & subsequent drop in volume before feedback...a good delay(lone Wolf Boss DD2 or DD3) /reverb pedal would be better.

Raw switch - is this a mid/tonestack disconnect switch? Forget it for harp, 10-12K max for mid resistor or 25K pot. Use 0.022uf mid cap.

10uf at each triode of V1 will be fine, so would 20uf. Use 2x220uf 350v in series at main B+(each bypassed with 220K resistors), use 2x100uf/350v at the screen supply (similarly bypassed).

5751 might be cool for some players, but it rounds the edges off the sound a lot, EVERY harp player I am aware of who uses a Brown Concert uses 12AX7 in V1 & PI.

If the green bullet is an old CR/CM Shure then use 5.6M input load, if it's a new 520DX then 1meg max...I have no idea why Shure suggest 100K, ignore it.

For harp you'll want a larger PI coupling cap, 0.022uf may be the largest you can go without making the guitar channel too thuddy...if this happens use 2.7K/15uf at pin 3 of the guitar channel preamp tube, this will restore 'chime' to the guitar channel whilst using 12AX7 as PI tube.

Also for harp SS rectifier would be good with the lowered preamp voltages, plate voltage in the region of 500v +/- 10v.

Power tubes - Sovtek 5881WXT will be best for harp in a 4x10", problem is, to keep feedback under control you'll want to back off plate current...I usually aim for 15mA in 4x10" for harp, less current if the player wants more crunch & less highs/fidelity. for guitar you will want more current, so you'll end up with a compromise...20-25mA? Basically minimum current that allows guitar to sound OK but keeps the harp channel useable.

6G12A presence control would be a good idea.

Normal & reverb channels will be out of phase with each other, the players will just have to get used to this. It's workable as long as they realise that one channel is going to affect the other and they don't keep trying to drown each other out.

[TheTinMan]

Please note that I have finally posted links to readable versions of the schematic above.

MWJB - wow! Thanks for the long post. It'll take me a while to digest all of your thoughts. I "get" almost everything except the comment about the two channels being out of phase with each other. Does the reverb transformer flip the phase of the signal?

Thanks again,

Chip

[MWJB]

It's the difference in the number of gain stages (every time signal goes thru a triode phase is flipped), brown pros/concerts/bandmasters all have the 2 channels out of phase due to additioonal gain stages in the trem circuit.

[MWJB]

Your schem shows speaker load to be 8ohms? Why not 2ohms (4x 8ohms in parallel - this will work better for harp)?

[TheTinMan]

It's the difference in the number of gain stages (every time signal goes thru a triode phase is flipped), brown pros/concerts/bandmasters all have the 2 channels out of phase due to additioonal gain stages in the trem circuit.

Sorry, the tremolo circuit and tone stack are from the 6G16 Brown Vibroverb. There isn't any gain stage in the trem circuit - it just jiggles the bias voltage. That Brown Bandmaster tremolo circuit is just plain weird to my ignorant eyes.

Looking at a stock AB763 circuit, there are three gain stages between the Vibrato channel's V2 and the point just before the PI where the Normal channel is mixed in. Assuming that the two channels are in phase in a vintage Fender, it looks like the reverb trannie (or tank) must flip the phase a fourth time.

I'm wondering about a back-panel switch to select whether or not to have reverb on the Normal channel. Noise worries me though...

Chip

[MWJB]

"Looking at a stock AB763 circuit, there are three gain stages between the Vibrato channel's V2 and the point just before the PI where the Normal channel is mixed in. Assuming that the two channels are in phase in a vintage Fender, it looks like the reverb trannie (or tank) must flip the phase a fourth time." AB763 (& the 6G16) with reverb have the 2 channels out of phase with each other, it's the additional reverb recovery triode that causes this. Taking the output from both preamp tubes and running reverb on both channels will put them back in phase with each other. However, I may be muddying the water a little here, if they're used to playing through a SR simultaneously there shouldn't be a problem...I know guys who have shared Twin Reverbs in this manor, as long as they give each other space it's fine.

If you want to have functional on-board reverb for harp & guitar you are probably going to have to do a fair bit of tweaking to optimise for harp & compromise with guitar - e.g. many harp players prefer the short decay 3 spring tank, you may want to experiment with caps to bleed off highs for the harp application, fit a dwell control, you may find you want to generally reduce gain in the reverb circuit - all of which may have a knock on effect for the suitability for guitar.

Yes, a lot of harp players who use AB763 amps with reverb do use that function, but in amps with no on-board reverb the majority of players opt for delay over reverb (a tight, more distinct delay doesn't get the harp signal so lost in the mix as a reverb does), even if I was going to use reverb (it wouldn't be a spring effect through choice either), I would still go with a pedal in front of the amp to keep gain much closer to unity compared to the on-board reverb method...I really think you're making a rod for your own back with reverb on the harp channel.

[TheTinMan]

Gotcha! Sounds like it may be worth exploring the idea of a Reverb switch on the back to bypass reverb for the Normal channel. Despite negative comments from others, the Dwell switch I added to my recent Princeton Reverb build is a great addition from my perspective. May have to talk my customer into having one too...

MWJB - I really appreciate your insights, especially regarding harp amps.

Chip