Collins 6Q-1

[DaveP]

I've been reading the old posts on this pre-amp and looking at the tube charts and something doesn't make sense.

The bias point of the 6C4 seems very unusual, from the spec sheet posted by Doug here:-

1956 catalog:

The current draw for both amps is 8mA, there will be about 0.8mA going to each 6AQ6 which will bias each tube at ~1.4V which is what we would expect.  The problem is with the 6C4 which will have ~3.2mA/tube making the bias point on the early model 150 x 0.0032 = 0.48V, or the later model 220 x 0.0032 = 0.7V.  These bias points are way too low and will overload with the slightest voltage from the 6AQ6.

Incidently, the 6AQ6 is not half a 12AX7 it is half a 6SC7 or 6SL7.

Any ideas?

best
DaveP

[emrr]

I consistently got more like 11mA on the bench at the stock 140V B+.   

The 6C4 does overload very easily, really the output stage headroom is about the same as the input stage headroom.  You have to think about it from the perspective that expected signal input was human speech, and hardly anything louder.  Current draw and heat on a whole console was likely also a part of the design decision. 

I never ran B+ up any higher than that which gave double the current, 220V maybe.  You definitely gain headroom there. 

[DaveP]

I've just discovered PRR's posts back in 2006 on the :-

6SN7 / 6CG7 preamp
« Reply #28 on: January 12, 2006, 03:08:51 am »

There is something there about "Read's Plan" which I believe refer's to Oliver Read, but I have not found out what this is about yet.

Its something about biasing a tube close to 0V.

best
DaveP

[emrr]

The voltage chart gives 65V B+ and 0.7V cathode.  B+ is actually higher on the 6AQ6, at 92V. 

I might call this purposeful headroom limiting in the age of unreliable broadcast limiters, especially given that it's open loop gain and difficult to distinguish the transition into distortion.   This creates a headroom bottleneck before the program amp, which can stay clean rather than pass on an overload, 2 amps in series farting, etc. 

[DaveP]

I might call this purposeful headroom limiting in the age of unreliable broadcast limiters, especially given that it's open loop gain and difficult to distinguish the transition into distortion.   This creates a headroom bottleneck before the program amp, which can stay clean rather than pass on an overload, 2 amps in series farting, etc. 

Well that makes sense Doug, I shall re-read PRR's posts in detail and see what i can come up with.

best
DaveP

[lassoharp]

The current draw for both amps is 8mA, there will be about 0.8mA going to each 6AQ6 which will bias each tube at ~1.4V which is what we would expect. 

I'm showing 0.75V for V1 bias on my schematic.

The problem is with the 6C4 which will have ~3.2mA/tube making the bias point on the early model 150 x 0.0032 = 0.48V, or the later model 220 x 0.0032 = 0.7V.  These bias points are way too low and will overload with the slightest voltage from the 6AQ6.

I just built a pair of semi-clones and rebiased V2 to 4V.


What is also unusual, in terms of general rules of thumb for triodes is the loading on V1.  It appears to be just below what the expected rp is for V1.  Apparently not a major problem but not ideal.  6AQ6 predate 12AY7 or AT7?

[emrr]

Should predate by a little.  The console does.  12xx7 line arrived in '48, from memory. 

[DaveP]

I've just found the circuit they were referring to in the 6SN7/6CG7 thread.

http://www.groupdiy.com/index.php?topic=12637.20

Its attached.

The output tube has a 10M grid resistor and no cathode resistor to get the rp as low as possible to drive the TX load.  I guess it was the same principle with the 6Q-1, you get the low rp with the low cathode resistor but have to suffer the low headroom.

best
DaveP

[emrr]

Interesting.  Makes sense for the 12AY7, but for the 6C4?   The Collins console bus following the 6Q (from memory) is a 600/600 T feeding a 620 series resistor to the bus.  I remember trying to cypher out the bus Z years ago, and being somewhat puzzled.  I haven't thought about it in awhile, and my scribblings are buried away, but I remember something in the 200 ohm range, if I was anywhere near right (I easily wasn't).  The program amps are resistively loaded to have a 2K4 input Z to the bus.  So.....the paralleling of input channels into the bus presents a pretty low source Z to the program amps, and it's really a bridging input relationship with the program amp.  Anyway, I'm riffin' here and easily missing something obvious.  Suffice to say the 6Q when wide open into the bus is driving a pretty low load, given all the parallel channels.  There's some blah blah for ya!

[lassoharp]

Should we assume the "Read's Plan" was based more on compactness and economy of the day?

Anything more AY7 offers outside of that,  that would override the other design obstacles?

The early UA console pres ran that tube on the output stage too.

[pyjaman]

I've ever seen this (very high grid resistor and no cathode resistor) in first stage of old guitar amps, described as "ultra-high mu" stage . I can't remember wich amp it was right now.

[emrr]

12AY7 as found in the UA and RCA output stages was a higher step down ratio than we are talking about here.  More like 25K-30K, if I recall.    And 12AY7 a very different beast than the 6C4. 

[DaveP]

I've ever seen this (very high grid resistor and no cathode resistor) in first stage of old guitar amps, described as "ultra-high mu" stage . I can't remember wich amp it was right now.

It was used in Fenders Champ 5C1 and the Deluxe 5B3 & 5C3 models and probably many others from that era.

Doug,

I guess that the 6C4, even though biased close to the limit, nevertheless drew substantially more current when passing AC, which kept the voltage up to acceptable levels.  Its an interesting way of running a tube, they have used two 150mA heater tubes to keep the heat down and then run the 6C4 to the limit to get the most from the tube.  That keeps the rp low and reduces the output  impedance to the lowest possible for that tube/transformer combination without feedback.

You said that it was one of the most euphonic amps you have tried and I guess the 2nd harmonics produced by this tube setup are what is responsible for that.

Lassoharp, did you try the 0.75V bias before settling on 4V?

best
DaveP

[lassoharp]

Lassoharp, did you try the 0.75V bias before settling on 4V?

I did, many months before when I built up the circuit on the bench as close to stock as possible.  It ran out of headroom too quickly for the project I used it on.  But, with stock conditions you also have a relatively low B+ figuring into the mix.  I did not try it with an increased B+, but instead just moved the bias up to what made more sense for the higher supply voltage.

That keeps the rp low and reduces the output  impedance to the lowest possible for that tube/transformer combination without feedback.

Ok, I'm confused here.  I'm seeing bias of 0.75 on 150r = about 5ma.  For lowest rp of V2, would you not be going up to 9-10ma or higher?  Without looking at data sheets I'm guessing 5ma = around 12K for that particular tube, and that it could achieve as low as 8K rp under higher current conditions.  If it turns out to actually be 10K effective rp under stock conditions, then wouldn't we still be at slightly less than matching load on V2?  (10Krp into 10K plate load// 15K OT pri.) = slight shift towards more distortion relative to higher loading plan.

[DaveP]

Ok, I'm confused here.  I'm seeing bias of 0.75 on 150r = about 5ma.  For lowest rp of V2, would you not be going up to 9-10ma or higher?  Without looking at data sheets I'm guessing 5ma = around 12K for that particular tube, and that it could achieve as low as 8K rp under higher current conditions.  If it turns out to actually be 10K effective rp under stock conditions, then wouldn't we still be at slightly less than matching load on V2?  (10Krp into 10K plate load// 15K OT pri.) = slight shift towards more distortion relative to higher loading plan.

Alan,
I've checked out the data sheets and 5mA gives an rp of around 8.5k at 65V with gm about 2.3.
I guess they didn't want to run the tubes too hot, so if 5mA gave them sufficient output, they left it at that.  When I said run them to the limit I meant close to 0V grid voltage not maximum current for the tube, sorry for he confusion.
best
DaveP

[lassoharp]

Ah, yes,  6C4 closer to half 12Au7 than any of the octal equivalents.  So that makes sense to use that tube for numerous reasons - size, heater power, and ability to run low supply voltages and have lower current draw at a lower rp than what came before it.

[DaveP]

Ah, yes,  6C4 closer to half 12Au7 than any of the octal equivalents.  So that makes sense to use that tube for numerous reasons - size, heater power, and ability to run low supply voltages and have lower current draw at a lower rp than what came before it.

It is virtually exactly the same spec as a 12AU7, I went over the charts today.  You're right, it's a very good low voltage tube that is rarely used in that way nowadays.

best
DaveP

[Dr Gris]

Hi,

I hope it' s ok with you Dave, that I post this in your thread, if not, let me know and I will delete this post.

This is an quote from Doug a couple of years ago: "Collins 6Q-1 is a no/very little feedback circuit that's pretty easy to reproduce with other parts." That together with the comment about it' s euphonic sound makes me think that this is something I want to build some day. I' ve searched and read almost every thread about tube amps but this one seems to fit the bill best. I already have fixed gain tube amps so I don' t consider that a problem.

So my question is this; can anybody point me to what parts to get and from whom to get it?
Is it even doable??

Kind regards
Magnus

[DaveP]

No problem Dr Gris,

Did you mean just the transformers?  I have no idea of what electronics suppliers you have in Sweden.

Regarding the transformers, they may come up on Ebay, or otherwise you will have to use modern near equivalents.

best
DaveP

[emrr]

lots of transformer options.  change V2 grid to a pot; volume control.