Mr. C-Verb SL (Steve Luckey)

[Swampertech]

Hey Steve, RobRob, 2deaf, I want to submit the schematic of my next (2nd) build - the Mr. C-Verb SL - built on the PR chassis and cabinet platform. I used part of the Dual 50 idea Steve, the Tremor-lator mods from 2deaf, and some of RR's mods. I hope there are no glaring errors, but if so please correct me. I'll have to add one tube to the chassis, but don't see a problem. I've specked the PT, OT, and filter choke to handle the high power KT 66 config (GZ34 and KT 66's), and hopefully the low power 6V6 config (5Y3 and 6V6's) will also sound good. Any input will be appreciated. And as Tubenit says, "With respect"!

[PRR]

The 220K R15 and the 50V tap are not what you want.

The 220k may be good if you only have the 350VAC available. You need much drop.

Since you show a 50V tap, which will make 70V DC, which is only a little more than you need, R15 should be smaller than the 15k+50k+47k leg. Perhaps zero. Perhaps 220 Ohms (not k-ohms).

[Swampertech]

PRR, thanks! Looks like if I leave that resistor (220K) completely out, I should have at least -43 V for those KT 66's.

[Swampertech]

Schematics of Mr. C-Verb SL. Fixed bias, tremolo.

[Swampertech]

Pre amp section

[Papa Jim]

What do you all use to open files of this type .DIY ?   Is there an app that I need to buy or something?

[Swampertech]

Sorry PapaJim.

[Swampertech]

PI, PS, PA schematic

[PRR]

What do you all use to open files of this type .DIY ?   Is there an app that I need to buy or something?

DIYLC
http://diy-fever.com/software/diylc/
"DIY Layout Creator (DIYLC in short) is freeware drawing tool..."

[Papa Jim]

Thanks PRR, I will check that out.  Swampertech thanks for the pdf.

[Papa Jim]

What does Mr. C-Verb mean? I'm smart enough to figure out "SL" but stumped by the first part. Also what genre will this amp favor?

[Swampertech]

Papa Jim, I'm building this amp for my grandson (and son-in-law). When he was a baby we affectionally called our grandson "Mr. C".  His name is Christian. He is taking guitar lessons, and I thought he might appreciate something especially built for him. The concept for the amp sprang from my first build, a 5E3 modded using some of Rob Robinette's suggestions like the lead channel mod, fixed/cathode bias switching, low power (6V6 and 5Y3) and hi power (6L6 and GZ34) capability, master volume, mute switch, TB tone stack, send/receive jacks, and coupling cap changed to .022uF. And no, it's not much like a stock 5E3. I built it (the Madzar Deluxe 5E3 RR - RobRob mods) for my son-in-law for Christmas last year. After much help from sluckey, RR, Tubenit and others on this site, the amp turned out quite well and sounded incredible. Super quiet, great crunch and overdrive. Very versatile. The speaker was a Celestion Gold. The lacquered Tweed cabinet was built by Peter Mather. I contacted Peter and asked him which of the BF, BF, SF cabinets were closest in internal dimensions to the Tweed and he said the PR. I wanted to have a bit more room inside the chassis to work after the 5E3, I loved the cabinet/speaker resonance, and I wanted the controls on the front. So, that's how I envisioned the new platform. Most of the Mr. C-Verb circuits are SL's, thus the name "Mr. C-Verb SL". The genre is, well, hopefully most anything a player would need. It has the Fender AND Marshall preamps, TMB tone stack, 1 tube reverb, one tube tremolo, master vol, adjustable FB, 10% switch for quiet practice without giving up tone, lo power/hi power capability (12W/50W), fixed/cathode bias, and presence control. A player should be able to play jazz, country, rock, or flaming hair through this thing. Although I'm an analog guy, I'm considering adding a SS send/receive buffer circuit. Most players, I figure, will want a pedal at some point. Anyhow, thanks for the inquiry on the Mr. C-Verb.

[Papa Jim]

That is really cool how it has all transpired, and transpiring still. Best of luck with it. Can't wait to see the final design and pics. I might even build one someday. Do you have any sound samples of the first one?

[Swampertech]

I don't have any sound samples, but I'll see if I can get some. Thanks for the interest.

[Papa Jim]

that would be great.

[Papa Jim]

Doesn't it need a reverb tranny?

[Swampertech]

This is a transformerless, constant current source, reverb tank driver. I found the circuit here:

http://www.channelroadamps.com/articles/reverb_driver/

I haven't tested it yet, but was looking for something like this where the impedance of the input coil could be driven directly by the tube. It states that the input coil can be driven by a 4 mA source, which is available with the 6gh8a. Keeping my fingers crossed.

[PRR]

> Doesn't it need a reverb tranny?

Every transformer-coupled amplifier "can" be re-designed transformer-less.

Transformer-coupled can be very efficient, DC power to audio power. However here we want maybe 0.1 Watts of reverb drive and the same chassis has a 5+ Watt loudspeaker power amp. The vast majority of the DC power is for the speaker-amp. Some inefficiency in the reverb driver won't make a big difference.

Wound coils over a few hundred Ohms require hyper-fine wire winding which is trouble. Tubes have internal resistance of a few thousand Ohms. This is a mis-match, bad for moving power through tube to load. But again, the power for reverb is so small that mis-match is tolerable.

Mis-match is *also*, for reverb, a simple way to compensate the mass (treble loss) of the mechanical mechanism. If you drive with a high impedance, the falling response of the mechanics is compensated by the rising response of the coil inductance and frequency response is nearer-flat.

Even the big transformer jobs do this. The stand-alone reverb with 6F6 power pentode connects a ~~50K plate to a 5K transformer. At the "8 Ohm" secondary it looks more like 80 Ohms. Driving an 8-10 Ohm tank we get a happy rise to 8KHz, the top of the real audio band.

The real reason Fender "always" used transformers is A) Hammond did it that way, B) the Fullerton surplus shop was full of small OTs for 300V communication radios needing <1W outputs. The 6F6 standalone used a 2W OT, but the 12AT7 used in "reverb amps" got a sub-Watt OT. These parts were going out of style in the period Fender was adding reverb, so the supply and price was good, it worked, why change?

On the other coast, Ampeg may not have had a similar surplus shop or just wanted to KISS. They ordered high-impedance tanks and ran a triode pretty hot to get sufficient reverb drive. That Channel Road Amps design is similar but beefier.

[DummyLoad]

> Doesn't it need a reverb tranny?

Every transformer-coupled amplifier "can" be re-designed transformer-less.

Transformer-coupled can be very efficient, DC power to audio power. However here we want maybe 0.1 Watts of reverb drive and the same chassis has a 5+ Watt loudspeaker power amp. The vast majority of the DC power is for the speaker-amp. Some inefficiency in the reverb driver won't make a big difference.

Wound coils over a few hundred Ohms require hyper-fine wire winding which is trouble. Tubes have internal resistance of a few thousand Ohms. This is a mis-match, bad for moving power through tube to load. But again, the power for reverb is so small that mis-match is tolerable.

Mis-match is *also*, for reverb, a simple way to compensate the mass (treble loss) of the mechanical mechanism. If you drive with a high impedance, the falling response of the mechanics is compensated by the rising response of the coil inductance and frequency response is nearer-flat.

Even the big transformer jobs do this. The stand-alone reverb with 6F6 power pentode connects a ~~50K plate to a 5K transformer. At the "8 Ohm" secondary it looks more like 80 Ohms. Driving an 8-10 Ohm tank we get a happy rise to 8KHz, the top of the real audio band.

The real reason Fender "always" used transformers is A) Hammond did it that way, B) the Fullerton surplus shop was full of small OTs for 300V communication radios needing <1W outputs. The 6F6 standalone used a 2W OT, but the 12AT7 used in "reverb amps" got a sub-Watt OT. These parts were going out of style in the period Fender was adding reverb, so the supply and price was good, it worked, why change?

On the other coast, Ampeg may not have had a similar surplus shop or just wanted to KISS. They ordered high-impedance tanks and ran a triode pretty hot to get sufficient reverb drive. That Channel Road Amps design is similar but beefier.

take that methodology and "cold bias" an EL84 or 6V6 in triode mode and skip the need for the OT and the funky out of production tubes.


--pete

[Papa Jim]

Have you tried it DummyLoad?

[2deaf]

The Channel Road design approximates a constant-current source by having the tube output impedance significantly higher than the tank input impedance.  I don't think you would want to run a pentode in triode mode for this because triode mode has a way lower internal plate resistance which will result in a much lower output impedance.   

[sluckey]

If this doesn't work out for you or even if you just need some more ideas, there are plenty of nice sounding Ampegs and Magnatones that use a cap driven reverb tank. The Gemini's and M10A come to mind right off the top of my head.

[Swampertech]

Hey guys, thanks for all the input. One of my requirements for the reverb section was for a one tube reverb. That's because I'm using the PR chassis, and didn't want to add more than one tube to the original layout.

sluckey, I had studied the Ampeg and Magnatone reverb circuits and knew that transformer less was possible, so when the ChannelRoad design was discovered I took notice.

2deaf, I hoped the ChannelRoad designs' use of the 6GH8A pentode section as the tank driver would provide both a safe driver for the input coil and impedance matching. And, it has the triode section for the recovery amp.

PRR,  that was quite an education on Leo's design reasoning, having all those tfmrs in surplus. I know he used them for Z matching, but do you think his use of the trfmrs was because he found a "safe" method to drive the tank input coil? I mean, it's a whole lot cheaper to replace the trmr than a reverb tank. And if Leo did it, it must be right!!  Right?

So again, thanks for all of your valuable input.

[2deaf]

2deaf, I hoped the ChannelRoad designs' use of the 6GH8A pentode section as the tank driver would provide both a safe driver for the input coil and impedance matching. And, it has the triode section for the recovery amp.

That driver works.  I never tried it in an amp, but I built it on the bench and analyzed it extensively.  I couldn't replicate the author's results, but it is what would be considered constant-current in the reverb world.

No comprendo safe driver for the coil nor impedance matching.  Comprendo triode section for recovery.

[DummyLoad]

2deaf, I hoped the ChannelRoad designs' use of the 6GH8A pentode section as the tank driver would provide both a safe driver for the input coil and impedance matching. And, it has the triode section for the recovery amp.

That driver works.  I never tried it in an amp, but I built it on the bench and analyzed it extensively.  I couldn't replicate the author's results, but it is what would be considered constant-current in the reverb world.

No comprendo safe driver for the coil nor impedance matching.  Comprendo triode section for recovery.

perhaps he was using a resistor load?

--pete

[Swampertech]

2deaf, when you say you couldn't replicate the author's results, did you mean the 4mA drive current needed for the tank input coil, or Z matching? Or something else? By "safe driver" I meant that the output of the pentode wouldn't overdrive the tank input coil and cause it to open. Maybe Leo used the transformers for isolation to protect the input coil of the tank.

[2deaf]

2deaf, when you say you couldn't replicate the author's results, did you mean the 4mA drive current needed for the tank input coil, or Z matching?

I couldn't get a current across the frequencies that was that constant.  I think the difference may be that the simulation used an ideal tube and I used real ones. 

I put 1.0V_p in with no input capacitor and got 5mA @ 100Hz, 5.7mA @ 1KHz, and 4.7mA @ 4KHz.  Pretty good for a reverb driver, but no where near as flat as the simulation.

My rig idled at 9.2mA plate current and 2.7mA screen current.  That's something to keep in mind when designing the power supply.  One of the drawbacks with pentode drivers is that they draw more current due to the screen current.  This driver depends on a high internal plate resistance, so the pentode cannot be configured as a triode because that would dramatically reduce the internal plate resistance. 

As far as Z matching goes, the idea is to get the biggest mis-match possible.  My results were in accordance with the Channel Road design in that respect.

By "safe driver" I meant that the output of the pentode wouldn't overdrive the tank input coil and cause it to open.

Those little coils can take an astounding licking.  I've put ten times the saturation current through them with no apparent damage.

[Swampertech]

Thanks 2deaf, always! The freq response vs. current draw isn't exactly constant current, but do you think it is an acceptably small enough deviation to be usable? Thanks for the tip on the needed PS current draw. I think the Heyboer TP40, which I've initially specked, will work but I'll definitely go back and add up all the tube currents before I order. I figured that the pentode would have enough to drive the tank, and wasn't going to wire it as a triode. Doesn't the tube Z need to be MUCH larger than the tank Z? Thanks again for your "real world" tests. I was urged by Randall Aiken to learn LTSpice, but that would take some time, especially for this non-software guy!!

[PRR]

> I couldn't get a current across the frequencies that was that constant.

Thumb-count, and idiot assistant, both say it is flat within a couple dB over the audio range.

The bend at 30KHz puzzled me until I worked out that Z(coil) equals plate resistor about there. It can't be constant forever. Trying is a power-drain. It is reasonably "constant" only as far as we should need.

[2deaf]

I'm never amused by a severely logarithmic Y axis.

[PRR]

> I'm never amused by a severely logarithmic Y axis.

We can't have that.

[2deaf]

A logarithmic X axis, on the other hand, just plumb tickles me to death.