Collins 6Q-1

[emrr]

If you can find a cheaper tube than the 6AQ6 that does the same thing, you might try it.  Of course the diodes do nothing. 

right on the pad. 

You can use all kinds of things for the output, or not.  They may all sound different, and you'll affect total gain.  I recall having the output unloaded causes a rise in the top end.  Any different transformer ratio will have it's own loading you'll have to find (or ignore). 

 

[Matt C]

thanks Doug.  This stuff is obviously new to me and I'm still trying to sort out all the differences between one type of triode and another, so the design choice to have the 6AQ6 in there seemed strange.

 

[emrr]

6aq6 makes perfect sense:  low filament draw, console uses 12 of that type alone. 

 

[Dr Gris]

Slowly getting the few parts I need to finish this racking project.
When arranging them in my case from Collective Cases ( a really nice case) my 6Q#1 stops
passing signal. Everything is connected as before on the bench. B+ on the turrent shows 200V
but all the plates show -0.820mV with or without the tubes in??

This makes me embarrassed again...anybody knows what my stupidity has caused?
Two days of reading on the internet and still no clue  :-[

Best
Magnus

 

[emrr]

Where is the last place you see 200?  Where is the first place you see incorrect voltage?  Just one component in between?  If so, thats the problem.  The wire wound metering resistor is probably the easiest to damage mechanically and not have a visual indicator, and I'm sure you aren't using it, you could short it out and see. 

Back to your bench tests, if you move tubes around and voltages change reasonably close with the tube movements, the resistors are probably fine.  You can of course confirm with cold resistance readings too.  For classical work, I might be tempted to replace the 6AQ6 cathode and plate resistors with metal film to get a little better noise performance.  I don't think any other properly working carbon resistor elsewhere will make much difference. 

 

[Dr Gris]

Hi Doug,

Thanks again for chiming in, got back to the racking mess and the meter resistor is kaput.

And for some reason the B+ PSU now only puts out about 72V??? What the ......
Disconnected the heater transformer from mains and disconnected the B+ PSU from the preamps.
A new measurement show a rippling ≈200mV from the B+ out from the PSU

First I got mad, now I only feel sad... maybe something is terribly wrong and my fusing wasn't enough
to save the B+ PSU.
I better take all this apart and start over again.

No classical work here, although I like it. Mostly Blues/Jazz/Singer Songwriter/Rock

Best
Magnus

 

[emrr]

I misunderstood the classical comment. 

You have an International Power unit? If it was fused as recommended, it should be fine.  Really, you could fuse it more to the actual range of the preamps, maybe 1/2 the suggested value, and be ok.  Improper load might make it latch up to a funny output voltage, I haven't seen that happen.  If it's latched up, letting it rest sometimes resets to proper voltage.  Play with the voltage adjustment too.  B- is tied to ground somewhere, right? 

 

[Dr Gris]

I gave it a rest and went to bed yesterday, I was tired. When I did that last reading B- had lost ground,
it' s grounded through a 10 ohm resistor to chassis star. The heater transformer secondary CT is also
grounded there through a 100 ohm wirewound resistor.

Anyway, the International Power is ok. I' ll get back to it where I left off when I get some spare time,
something rare these days.

Thanks for your support Doug, it really means a lot!

Best
Magnus

 

[Dr Gris]

As usual, it's baby steps over here.
Ordered tubes from Vacuum Tubes Inc (thanks Dave P) so I have spares.
Prepared the case from Dan and gave it a nice painting job.

Because of the different voltage readings and the way too high V2 cathode resistor in my 6Q#2
for my taste (1k) I'm replacing all resistors in both of them.

Wiring the pots I'm thinking shielded cable for input/output, connecting the shield only at the pot.
Ground wire from pot unshielded. Does that sound ok? Overkill??

Could someone please explain what the 100 pF (100MMF) cap does and what's it called,
grid leak bypass cap? Is it there to block RF?? Just guessing here...
In a manual from 1954 it's omitted (thanks Ruffrecords)

And again, thanks to Doug for making me pursue this.

Hey Matt C, any progress? Let us know!!!

Best
M

 

[Matt C]

got mine all put together a couple days ago but haven't had much time to test it or fine tune it.  right now it's just a single channel prototype built within an Akai M8 chassis. maybe i'll post a couple pictures.  I'm getting a bit of buzzing when the gain is above 12 o'clock or so.  not sure why, might just be sloppy layout on my part since this one isn't meant to be the "finished" unit.  the noise is not overwhelming but maybe bad enough to make it unusable for an actual recording at the moment.  so it's mostly working but I need to spend some time finding ways to reduce noise. 

I also didn't order "proper" transformers for it yet, so I'm using a Cinemag CMMI-7C on the input and a Cinemag 1:1 steel core transformer on the output.  still not sure if I want to keep the 1:1 on the output or order a step-down like the original circuit specifies. I don't know much about how each option would affect the sound. 

I'm also planning on adding an attenuator on the output so that I can drive the circuit harder, but haven't put that in yet.  i'm not sure if i'll be able to get away with just strapping a pot across the output transformer's secondary and using that as an output volume control, or if i'll need to do something fancier like a t-pad.

 

[emrr]

You don't need an output pot.  Try it and see.

 

[Dr Gris]

Finally everything is in place, sort of...
Two 6Q's in one rack with internal PSU, crowded indeed!
4 channels total, one channel per module has mains hum 
Well, since the other two channels are ok it must be doable to get them all hum free.
I wish I didn't have to go in there again. To keep leads short it' s a pita to disassemble/assemble.

How about a crappy iPhone pic...

 

[Seamus]

Nice job!

We had to go external with the power to get our hum to stop.

Seamus

 

[Dr Gris]

Thanks!!

I'm gonna wiggle around things for starters to see if I can keep the PSU inside the case.
Like I said, channel 1 and 3 (counting from the back panel) are hum free so I'm hoping it can be done.

I feel I just have to show a pic of my whacky front with the oversized knobs.
To be honest I think the knobs look better with the skirts off.
What can I say, I'm a Hammond B3 player, BIG IS BEAUTIFUL!!!

// M

 

[Seamus]

Awesome!

Here's ours.
Also a 4 banger.

 

[Dr Gris]

Wow!!!

Very cool retro look! A lot more professional looking then mine. A piece of art.

I think it sounds fantastic so if you ever in the future consider selling it I'm here 
For what I do I could use these on almost everything, or maybe it's just a honeymoon feeling...

I'm so glad that Doug shared info about these over the years and are really thankful for all his help.
A big thank you to all of you that inspired and helped me to do this!!!

Best regards
Magnus

 

[Dr Gris]

I'm back!!

I've done some measurements using Fuzzmeasure. I used the info from here:
http://www.benlindell.com/2010/01/how-to-make-sweeps-of-gear-using-fuzzmeasure-pro/

I have no idea if it's accurate but it corresponds well to what I hear in the higher frequencys.
My soundcard for this test is a RME UFX set to 96kHz.
Fuzzmeasure puts out a sine sweep and I calibrated it to do so at -10dB using the long (sweeptime) setting
to be on the safe side.
When racking them I came up with the idea to make a U-pad from matched 500ohm series resistors
and a 169ohm shunt. Don't know if thats ideal but it pads the amount I want.
They are also terminated with 620ohm on the output.
As I've written before one of the 6Q's don't have the V1 Cathode bypass cap but the second does.
The second also has the feedback arrangement.

So here we go, pad/no pad:

 

[Dr Gris]

and....

 

[Dr Gris]

third...

 

[Dr Gris]

last channel...