Motown Direct Amplifier-inspired Preamp?
- Thread starter untune
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Last stage is a cathode follower driving the transformerAnd thd etc. might be lower if that 6V6 is a cathode follower (maybe driving a global negative fb loop) which was implied when Btown said the last, transformer driving tube had zero gain
that’s one good way to hit that output mark. Never seen it in a historical product.
I was certainly not implying btown was misleading anyone.
I did however interpret you as trying to discredit his schematic, I may have been wrong about that, if so I apologize.
ruffrecords
Well-known member
Here is a 6V6 CF design from 1949:
A Direct-Coupled Amplifier with Cathode Follower by Raymond H. Bates
Cheers
Ian
A Direct-Coupled Amplifier with Cathode Follower by Raymond H. Bates
Cheers
Ian
5v333
Well-known member
As ive speculated before. there were VUs back then, calibrated so nobody could screw up the headroom.
While at the same time the 6v6 will give you headroom and linearity there was also the fact that musicians were not allowed to bring their amps.
it wount get more clean under thoose conditions.
jacomart
Well-known member
I recall seeing that one. Can anyone identify a commercially produced audio amplifier from before 1970 with a power tube cathode follower output? Closest I can think of offhand are things like the Krohn-Hite filter sets, which are cathode follower small signal tube outputs through large filter caps, and while 'audio', those were used in lab settings.
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jacomart
Well-known member
However, it will be necessary to add a little more gain, about 10/15 dB, and the input impedance will have to be around 1M ohm. Maybe an ECC88 and a good amount of NFB?
Cheers
JM
Cheers
JM
Winston OBoogie
Well-known member
The amp from the Pultec MEQ5 was mentioned early in this thread. It isn't a 6V6 on the output, but it does use the 6AQ5 which is considered equivalent to the 6V6 up to 250V. I read this as meaning - keep the anode to cathode voltage under 250V and you have a decent little power tube.
In the MEQ circuit where it's a cathode follower, dissipation will be within that safe zone.
I also like the look of the DC feedback to the first stage cathode. Given the value of the cathode resistor, my guess is that they (Ollie Summerland and/or Gene Shank of Pultec) kept that cathode R value a bit low for noise reasons, but the DC feedback pulls the bias up a little bit to the required point. Gain of that stage would lie somewhere between it being a bypassed and unbypassed cathode.
Jüst a guess so, don't shoot!
Edit: P.S. thanks Btown for the clarification of it being a cathode follower
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untune
Well-known member
I had started sketching this one out going off 1000tinyempire's suggestion of the MEQ5, based on the same observation—the 6AQ5 essentially being the same as a 6V6 within specs. Seems to closely fit the bill. I was also looking into 5:1 cathode follower/balanced line output transformers yesterday, of the ones I was checking out (namely Sowter 1010 and 1290) the 1290 is considered a near replacement for the one shown on the attached MEQ5 diagram (Triad HS-50)
Winston OBoogie
Well-known member
Oh yeah, MB-1, very similar.
That gain adjust ability looks tempting, but it's probably intended as a 'strap on the resistor you need for the gain in your studio' rather than
being an adjust on the fly thing.. There's DC at that point so, unless you have a high tolerance for loud annoying cracks while you adjust things...
But yeah, good call.
Winston OBoogie
Well-known member
Checked the throughput gain on the MEQ again to make sure and, 28dB give or take if you use a 5:1 output.
Put a 2M2 resistor on the first triode grid, plug your guitar in and, as everyone who's anyone says: 'The Rice Is Cooked'.
For adjusting gain, I'd maybe take a look at the MB-1 approach if you need adjustable gain, but put an electrolytic at the top junction of where the gain adjust resistor is shown as going, then use the negative of the cap as a means to determine the amount of bypassing happening on the lower leg of the cathode array.
Although there's something quite elegant about leaving the MEQ as is, completely at DC.
Put a 2M2 resistor on the first triode grid, plug your guitar in and, as everyone who's anyone says: 'The Rice Is Cooked'.
For adjusting gain, I'd maybe take a look at the MB-1 approach if you need adjustable gain, but put an electrolytic at the top junction of where the gain adjust resistor is shown as going, then use the negative of the cap as a means to determine the amount of bypassing happening on the lower leg of the cathode array.
Although there's something quite elegant about leaving the MEQ as is, completely at DC.
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ruffrecords
Well-known member
I am familiar with the MB1 mic pre design with its parallel 12AU7 cathode follower from when I first began designing mic pres many years ago but I was not aware of the 6AQ5 variant. I have extensively simulated the MB1 and found its performance to be exceptionally good. Its NFB path operates qown to dc which, combined with the single capacitor in the forward path makes it unconditionally stable. It was this design that inspired me to create my own design with NFB down to dc.
Given Ken Sands is reported to have said the design had lots of NFB I think this is an excellent candidate for the output section at least. We just need to decide how much overall gain we need.
Edit: Looks like Winston already did!
Cheers
Ian
Winston OBoogie
Well-known member
Well, I didn't quite
But what my personal feeling is, is that leaving it as a DC feedback stage would be my gut preference. If the 28dB through gain of that MEQ stage isn't what folks think is the best, then I'd use the schem of the MB-1, but keep it DC as it is there, and fixed.My side track stuff about putting a cap in was more aimed at if the MB-1 needed to be an adjustable gain amp.
ruffrecords
Well-known member
OK got it. this topology looks the best candidate so far. Anyone know when the MEQ first appeared?Well, I didn't quite
My side track stuff about putting a cap in was more aimed at if the MB-1 needed to be an adjustable gain amp.
Cheers
Ian
untune
Well-known member
Agreed, and I'm struggling to find anything with a date. I have two versions of the MEQ5 schematic, the one posted earlier in this thread, another probably earlier version with a tube rectifier and minor component value differences (18K in the NFB rather than 20K, a couple of 20uF filter caps as opposed to 40uF) plus different power TX, nothing major
Can't remember where I found this but I *think* CJ might have posted it in another thread
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Winston OBoogie
Well-known member
I don't see a date in the manual but the nature of the graphics on the first page say '50's to me. Definitely within the scope of being out there pre Motown.
Attaching the manual for anyone who doesn't have a copy.
Attaching the manual for anyone who doesn't have a copy.
jacomart
Well-known member
To know exactly we need to connect a guitar with single coil pickups (lower level than humbuckers) to a sound card and measure the signal level using various positions of the volume potentiometer and different energy in the picking. REW should give us the answer. We must not forget that the impedance of the input must be about 500k ohm (I checked) 1 Mohm is too much.Well, I didn't quite
My side track stuff about putting a cap in was more aimed at if the MB-1 needed to be an adjustable gain amp.
Cheers
JM
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